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Perspective
Grafting Polymers from Cellulose Nanocrystals: Synthesis, Properties, and ApplicationsClick to copy article linkArticle link copied!
- Sandra WohlhauserSandra WohlhauserAdolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, SwitzerlandMore by Sandra Wohlhauser
- Gwendoline DelepierreGwendoline DelepierreAdolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, SwitzerlandMore by Gwendoline Delepierre
- Marianne LabetMarianne LabetRenewable Materials and Nanotechnology Research Group, Chemical Engineering, KU Leuven, Campus Kulak Kortrijk, Etienne Sabbelaan 53, 8500 Kortrijk, BelgiumMore by Marianne Labet
- Gaëlle MorandiGaëlle MorandiLaboratoire Polymères, Biopolymères, Surfaces, Normandie Université, INSA de Rouen, Avenue de l’Université, 76801 Saint-Étienne-du-Rouvray Cedex, FranceMore by Gaëlle Morandi
- Wim ThielemansWim ThielemansRenewable Materials and Nanotechnology Research Group, Chemical Engineering, KU Leuven, Campus Kulak Kortrijk, Etienne Sabbelaan 53, 8500 Kortrijk, BelgiumMore by Wim Thielemans
- Christoph WederChristoph WederAdolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, SwitzerlandMore by Christoph Weder
- Justin O. Zoppe*Justin O. Zoppe*(J.O.Z.) E-mail: [email protected]. Telephone: +41 26 300 9563.Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, SwitzerlandMore by Justin O. Zoppe
Abstract
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Over the past 10 years, the grafting of polymers from the surface of cellulose nanocrystals (CNCs) has gained substantial interest in both academia and industry due to the rapidly growing number of potential applications of surface-modified CNCs, which range from building blocks in nanocomposites and responsive nanomaterials to antimicrobial agents. CNCs are rod-like nanoparticles that can be isolated from renewable biosources and which exhibit high crystallinity, tunable aspect ratio, high stiffness, and strength. Upon drying, the abundance of surface hydroxyl groups often leads to a degree of irreversible aggregation, as a result of strong hydrogen bonding. Moreover, their relatively hydrophilic character renders CNCs incompatible with hydrophobic media, e.g., nonpolar solvents and polyolefin matrices. By grafting macromolecules from their surface, CNCs can be imparted with surface characteristics and other physicochemical properties that are reminiscent of the grafted polymer. This has allowed the design of nanoscale building blocks whose readily tunable properties are useful for the formation of both colloidal dispersions and solid state materials. In this Perspective, we provide an overview of the morphology and surface chemistry of CNCs and detail various techniques to manipulate their surface chemistry via polymer grafting from approaches. Moreover, we explore the most common polymerization techniques that are used to graft polymers from the surface and reducing end groups of CNCs, including surface-initiated ring-opening polymerization (SI-ROP), surface-initiated free (SI-FRP), and controlled (SI-CRP) radical polymerization. Finally, we provide insights into some of the emerging applications and conclude with an outlook of future work that would benefit the field.
Copyright © 2018 American Chemical Society
1. Introduction
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During the last 3 decades, nanomaterials have emerged to play an ever increasing role in the material science field and are nowadays commonly found in our daily lives. (1) Nanomaterials display interesting and often unique mechanical, optical, and/or electronic properties, as a result of their high surface area as well as their specific structural features, that are not available in micro- and macroscale analogues. Cellulose-based nanocrystals and -fibrils have emerged as a particularly widely used class of building blocks for structural as well as advanced functional materials. (2−8) Cellulose, a linear homopolysaccharide composed of β-d-anhydroglucopyranose units, is the most abundant organic polymer on Earth and a promising renewable resource for the future production of biobased materials and fuels. (9) Cellulose is found in a wide variety of species, such as higher plants, tunicates, algae, fungi, bacteria, invertebrates, and amoeba. In nature, cellulose is found in hierarchically structured multicomponent materials, which at the lowest hierarchical level feature assemblies that contain individual cellulose in a uniaxially ordered (semi)crystalline organization. Careful disintegration schemes permit accessing cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs), whose properties (e.g., crystallinity, morphology, aspect ratio, and surface chemistry) (10) depend on the requirements for use in polymer nanocomposites, colloidal liquid crystals, hydrogels, and multiresponsive materials. (2,8,11−14)
CNCs are rod-like nanoparticles that can be isolated from renewable biosources, and which exhibit high crystallinity, tunable aspect ratio, and high stiffness and strength. Typical dimensions are lengths in the range from 50 nm to more than 1 μm and widths of 5–50 nm, depending on the source material. (5,10) In contrast to many other nanorods, CNCs appear to be physiologically benign. (15−17) CNCs are commonly obtained by strong acid-catalyzed hydrolysis, with sulfuric acid as the most commonly used acid, which selectively degrades the amorphous regions of macroscopic cellulose fibers, while the crystalline domains stay intact. Hydrolysis with sulfuric acid also introduces anionic sulfate half-ester groups on the surface of the CNCs, which are essential for their colloidal stability in water, on account of electrostatic repulsions. This effect also likely plays an important role in their spontaneous assembly into chiral nematic (a.k.a. cholesteric) colloidal liquid crystal (LC) phases, in addition to their inherent chirality. (18−20) CNCs hydrolyzed by sulfuric acid typically show, however, a limited thermal stability, which is apparent by significant coloration when processed at elevated temperatures and presents an obstacle for the fabrication of polymer nanocomposites. (21,22) This problem can be overcome by hydrolyzing cellulose pulp with phosphoric acid, which affords CNCs with a low concentration of phosphate surface groups and which offer an attractive combination of adequate dispersibility in polar solvents and improved thermal stability. (21) The production of CNCs has also evolved at the industrial scale in some countries such as Canada, the United States, Israel, and Sweden, even though it is still developing. (23)
The CNC surfaces also contain many hydroxyl (−OH) groups, which allow many types of chemical modifications and enable a broad range of nanomaterials that cover a wide spectrum of properties and functions, ranging from stimuli-responsiveness to antimicrobial capabilities. (8,12) In addition, surface chemical modifications allow transforming the rather hydrophilic CNCs into hydrophobic particles, and thus facilitate their use in hydrophobic media. (24−26) One of the primary challenges in the chemical modification of CNCs is to carry out successful reactions without impacting their intrinsic properties, notably the dimensions, crystallinity, and intrinsic mechanical properties. (7)
Surface modification by graft polymerization proved to be a robust approach, in which some of the physical and chemical properties can be tailored at will. (4,27−29) Graft polymerization of surfaces can be performed by using “grafting to”, “grafting from”, or “grafting through” methods. The grafting through method involves the functionalization of solid substrates with polymerizable functional groups, e.g., acrylic moieties, followed by polymerization in situ. While the grafting to method involves the connection of preformed polymeric chains with the surface through reactive end groups, the grafting from strategy concerns the growth of polymer chains via surface-initiated polymerization either from a substrate containing functional groups capable of initiating the polymerization reaction, or by immobilization of an active initiator species to the substrate surface. Due to the steric hindrance often encountered while utilizing the grafting to approach, the grafting from approach has the advantage of well-controlled polymer graft lengths and high polymer tether grafting density.
As the latter strategy has been widely employed to modify CNCs toward multicomponent colloids and, more recently, one component-nanocomposites (OCNs), in which no polymer matrix is required and the final materials do not usually suffer from phase separation issues, this will be the focus of our Perspective (Figure 1). Our aim is to provide a concise overview of the field of grafting polymers from the surface of CNCs, while highlighting some noteworthy contributions over the past decade, since the pioneering work of Habibi et al. in 2008, (30) who introduced ε-caprolactone grafting from CNCs via surface-initiated ring-opening polymerization (SI-ROP). In the following sections, the physicochemical properties of CNCs, various chemical modification pathways known to graft polymers from the surface of the CNCs and applications of polymer-modified CNCs (CNC-g-polymer) will be described and the opportunities and challenges that have emerged in the field will be discussed.
Figure 1
Figure 1. Schematic representation of CNC transformation into polymer-modified CNCs, which are useful for the formation of nanocomposites containing one or multiple components, display LC behavior, or exhibit stimuli-responsive behavior. (The center graphic in the bottom row was adapted from ref (31). Copyright 2017 Wiley-VCH Verlag GMbH & Co. KGaA.)
2. Morphology and Surface Chemistry of CNCs
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The semicrystalline nature of cellulose fibers, in which the crystalline regions are held together by strong inter- and intramolecular hydrogen bonds, gives rise to their unique mechanical properties and is the naturally selected main structural component of the primary cell wall in higher plants. (9) Although less understood from a structural standpoint due to their disordered morphology, the noncrystalline, or “amorphous,” regions of cellulose are thought to be advantageous in terms of water transport in plants. (32) This inherent architecture of cellulose fibers is what ultimately allows the nanoscale crystalline regions to be selectively extracted from macroscopic fibers through acid-catalyzed hydrolysis. Thus, before moving forward into methodologies to modify the surface of CNCs with polymer grafts, it is imperative to first discuss the structure of cellulose, its crystalline domains and the various extraction methods to produce CNCs, which lead to strikingly different surface chemistries and reactivity.
2.1. Morphology
Cellulose is the trivial name for the linear homopolysaccharide polymer composed of β-D-anhydroglucopyranose and linked by β(1–4′) glycosidic (ether) bonds. The repeating anhydroglucopyranose unit (referred to as AGU) (33) in cellulose is a six-membered closed ring form of d-glucose in the chair conformation with the anomeric carbon (C1) in the β-position (Figure 2). (34) The β(1–4′) glycosidic bond between the different AGUs results in a twist along the length of the chain, which can be defined by the glycosidic torsion angles ϕ = H1–C1–O4′–C4′ and ψ = C1–O4′–C4′–H4′. (35) It has to be noted that the definition of these angles is not standardized in the literature, so comparisons of values reported for different sources needs to take into account the angle definitions used. (35−37)
Figure 2
Figure 2. Molecular structure of cellulose, with the anhydroglucose repeating unit in the chair conformation, the glycosidic bond, as well as reducing and nonreducing chain ends.
While the ring structure of the AGU is confined to the chair conformation with the C–O bonds in the equatorial position, free rotation can occur around the C5–C6 bond, resulting in different conformations of the hydroxymethyl group relative to the glucose ring. The position of the hydroxymethyl group relative to the glucopyranose ring can be described by the angles χ = O5–C5–C6–O6 and χ′ = C4–C5–C6-O6. (5,38) Three energetic minima can be found when the angles are both 60° (gauche–gauche conformation, gg), 60°–180° (gauche–trans, gt), and 180°–60° (trans–gauche, tg). (5,36,37) The free rotation around the C5–C6 bond with the resulting freedom of positioning of O6 enables different hydrogen bonding patterns, including in cellulose crystals. (37,39)
Until recently, seven different crystal allomorphs of cellulose were thought to exist: Iα, Iβ, II, IIII, IIIII, IVI, and IVII. While cellulose II is thermodynamically the most stable form, native cellulose was thought to exist in polymorphs Iα (found in bacterial cellulose and algae, triclinic unit cell) and Iβ (higher plants, monoclinic unit cells). (40,41) Recent findings from solid-state NMR spectroscopic investigations suggest, however, that native plant cellulose is structurally polymorphic and differs greatly from Iα and Iβ, as evidenced by seven unique sets of 13C chemical shifts. (42,43) While five of these sets are attributed to interior cellulose chains within microfibrils, two are assigned to surface chains and such structural complexity arises from interactions with matrix polysaccharides unique to higher plant celluloses. Nevertheless, cellulose Iα has been found to be metastable and can be partially transformed into Iβ by annealing above 260 °C or treatment with gases and organic solvents. (44−46) Cellulose II is obtained by recrystallization of cellulose after dissolution, thus attaining the thermodynamic minimum. The other allomorphs are local minima in the highly regular hydrogen bonded network of cellulose chains. All allomorphs, despite some having parallel and others antiparallel chain alignment, have a pseudo 21 helical conformation of cellulose chains as a result of intrachain O3H–O5′ hydrogen bonding. (47) Disruption of any cellulose crystal structure can thus be expected to be linked to disruption of this intrachain hydrogen bond, a fact supported by the ability of cellulose to attain a random coil conformation when dissolved. (48) The various cellulose polymorphs have unique X-ray diffraction signatures corresponding to the spatial crystal arrangements in their structure. (9) The crystal packing of cellulose’s polymorphs differ in the unit cell dimensions and, may present variations in the chain polarity, which result in changes in the properties of cellulose, such as its rigidity, thermal stability, solubility, or reactivity. (49)
Cellulose chains are synthesized by cellulose synthase enzymes arranged in complexes located at the plasma membrane. This cellulose synthase complex (CSC) produces elementary cellulose protofibrils with a width of 3–4 nm. (50) These CSCs sit at the end of the growing fibrils. (51,52) The CSC can take different forms with three rows of ordered particles, as seen in the algae Oocytis apiculata, (51) other linear complexes, as in tunicates and the algae Valonia and Erythrocladia, (53) while in higher plants as well as the green algae Micrasterias a hexameric rosette complex containing six particles, each featuring six cellulose synthases, have been reported. (53,54) The protofibrils are arranged into semicrystalline microfibrils with diameters up to 20 nm. (55,56) The ratio of crystalline to amorphous domains and the ratio of Iα to Iβ found in the crystalline domains is dependent on the source of cellulose. (5,7,39,57)
CNCs are commonly represented as ribbons with either a parallelepiped or a hexagonal cross-section (Figure 3); the shape and the number of individual cellulose molecules varies greatly with the cellulose source. Native cellulose microfibrils in plants and bacteria have long been thought to consist of 36 chains, but recent advances in the structural characterization of CSCs suggest that an 18-chain model may be a more accurate depiction. (58) On the other hand, solid-state NMR spectroscopic experiments indicate that primary cell wall cellulose microfibrils are composed of at least 24 chains, which may be due to coalescence of multiple microfibrils synthesized by neighboring CSCs. (42,59) The exact number of cellulose chains within native cellulose microfibrils still remains a highly debated research topic.
Figure 3
Figure 3. Schematic representation of idealized cellulose cross sections showing the faces of cellulose nanocrystals with the crystal planes (t = triclinic, m = monoclinic unit cell) and the crystal structure. Each gray box represents the cross-section of a cellulose chain viewed in the direction of its long axis. Representations for CNC extracted from (a) wood, (b) tunicate, (c) Valonia, (d) Micrasterias, (e) reference bacterial cellulose from Acetobacter, and (f) bacterial cellulose from Acetobacter produced in the presence of mannan. Reproduced from ref (7) with permission. Copyright 2011 The Royal Society of Chemistry.
As the main differences between the Iα and Iβ structures have been shown to lie in the conformation of the AGU and the β(1–4′) linkages by NMR spectroscopy and X-ray and neutron diffraction, (57,60) one could assume that the availability and reactivity of the surface −OH groups between both allomorphs are similar. Crystal structure diffraction studies indicate the hydroxymethyl conformation to occur in the tg conformation with hydrogen bonding to exist between O3H–O5′ and O2H–O6′ of cellulose chain aligned next to each other, whereas hydrogen bonding between stacked chains is virtually nonexistent. (57) The 2-fold helical twist of the cellulose chain due to the β(1–4′) linkage (Figure 2) results in an alternation of primary and secondary −OH groups pointing out of the crystal along a single cellulose chain.
To assess surface reactivity, one needs to consider that surface groups are not as confined as the same groups within the CNCs. Indeed, 13C solid state NMR studies suggest that the hydroxymethyl groups on the surface chains pointing outward display a different conformation (gg) than the hydrogen-bonded primary −OH groups inside the CNCs (tg). (38,60) One can also envision that the ability to change the conformation of the hydroxymethyl group will depend on the level of interference of the solvent in the hydrogen bonding of the hydroxymethyl group with other groups in the crystal surface.
Experimental observations suggest that the surface reactivity of the surface −OH groups follows O2 > O6 > O3 for native cotton and cotton-like (i.e., linen, ramie, sisal and wood) celluloses, whereas it follows O6 > O2 > O3 for Valonia ventricosa cellulose as a consequence of a higher degree of intramolecular hydrogen bonding between O3H and O5′H and the higher crystalline order leading to a reduction in the absolute availability of O2H by inaccessibility in the interior of the crystalline region (Figure 2). (61) This may be considered further confirmation of the gg conformation of the surface hydroxymethyl group as the intramolecular O2H–O6 hydrogen bond is not possible in this conformation. Surface reactivity also needs to consider the reaction mechanism. After activating surface hydroxyl groups, the O2 position cannot react under SN2 conditions as the CNC crystal inhibits Walden inversion and only O6 can be expected to react. This can also be seen when surface alcohol groups have been replaced by chloride. (62) Finally, the reactivity of the −OH groups at the surface of cellulose seems also to be affected by the reaction media, i.e., the −OH groups at position 6 are the most available for reaction in homogeneous solutions following the order O6 > O2 > O3 due to steric reasons, while for heterogeneous modifications the −OH groups at position 2 become the most reactive species with the following order O2 > O6 > O3. (63)
2.2. Surface Chemistry
Chemical reactions carried out at solid–liquid interfaces differ from homogeneous reactions in that adsorption is a prerequisite for the chemical reaction to occur and functional groups on surfaces have fewer degrees of freedom as discussed above. (64) With an understanding of cellulose crystal morphology and surface −OH group reactivity, one must also be aware of the factors involved in CNC processing that affect their surface chemistry.
By far the most common method to extract CNCs from cellulosic raw materials is sulfuric acid-catalyzed hydrolysis. (2,3,5−7) After delignification of the native material and removal of hemicelluloses, this typically involves exposing high purity cellulose fibers to a 55–65 wt % sulfuric acid solution for 30–60 min at temperatures ranging from 35 to 70 °C. During this process, sulfuric acid reacts with a fraction of the −OH groups at the surface of the crystalline regions to form sulfate half-esters, which are typically present at a concentration in the range of 150–400 mmol/kg. (65) Upon purification by multiple centrifugation steps and dialysis against deionized water, these sulfate half-ester groups are negatively charged, yielding highly stable CNC dispersions due to electrostatic repulsions. As mentioned above, CNCs hydrolyzed by sulfuric acid typically display a limited thermal stability. (21,22) The sulfate half-esters can be removed by hydrolytic or solvolytic desulfation procedures, (66,67) but this modification comes at the expense of a reduced dispersibility.
It is also possible to employ hydrochloric (HCl) (68) or hydrobromic (HBr) (69) acid for the hydrolysis. In some cases, CNCs with a very low surface charge are produced; these charges likely originate from a small amount of carboxylate groups (ca. 20–30 mmol/kg). (65) Accordingly, CNCs produced by HCl- or HBr-based hydrolysis methods also have the disadvantage of low colloidal stability due to the lack of sufficient electrostatic repulsive forces that consequently allow hydrogen bonding between adjacent nanoparticles. Such dispersion aggregation is obviously problematic for subsequent surface modification. Phosphoric acid hydrolysis, introduced by Camarero Espinosa et al., (21) affords CNCs that feature a small concentration of phosphate groups (ca. 11 mmol/kg). In this case, each phosphate group at the CNC surface provides two acidic −OH groups and the CNCs display an interesting combination of good colloidal and thermal stability.
Other means to prepare CNCs include endoglucanase-mediated hydrolysis, (70) mechanochemical activation, and phosphotungstic acid hydrolysis, (71) ammonium persulfate (APS) oxidation, (72) and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-mediated oxidation followed by cavitation. (73) The latter two methods yield CNCs functionalized with a high concentration of surface carboxylate groups (ca. 1.12–1.74 mmol/g for TEMPO-mediated oxidation), (73) which not only promote good dispersibility but also offer alternative surface chemical modification pathways. (24,25)
Regardless of the method utilized to produce CNCs, it is clear that anionic surface charges provide the colloidal stability necessary to prevent a certain degree of aggregation in both water and polar aprotic solvents, such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). (74) The surface charges also affect the ease at which CNCs can be freeze-dried, and at the same time, they use up some of the surface −OH groups that could be used for later functionalization. In addition, the counterions of the surface charges strongly influence the redispersibility of dried CNCs by ultrasonication, especially on material with a moisture content lower than 4 wt % due to the presence of stronger intermolecular hydrogen bonds between CNCs. (75,76) Thus, using hydrolysis protocols that impart “just the right amount” of surface charges to promote colloidal stability appears advisable if subsequent surface modification reactions are to be carried out.
2.3. Pretreatments of CNCs for Surface Chemical Modification
The presence of surface impurities, bound water, and excess counterions can have a significant impact on the success of surface functionalization reactions and their reproducibility, notably between different types or batches of CNCs. (77−79) Furthermore, many reaction pathways suitable for the modification of −OH groups are intolerant to water, so that CNCs must either be dried and redispersed or transferred into a dry organic solvent by other means. It is possible to freeze-dry the CNCs (80−85) or to exchange the solvent. (30,83,86−89)
If freeze-dried CNCs are to be surface-modified, the nanocrystals are often washed with organic solvents such as ethanol, (78,79) acetone, (77) or n-pentane (77) to remove impurities. This is accomplished by either Soxhlet extraction (78,79) or by several cycles of dispersing the CNCs in the solvent of choice and subsequent separation via centrifugation. (77) It was found that such pretreatments improved the reproducibility between grafting experiments. (77)
Solvent exchanges usually involve mixing an aqueous dispersion of the CNCs with a water-miscible organic solvent, usually acetone, centrifugation, replacement of the solvent, and multiple repetitions of this cycle, until the solvent is changed to the organic solvent, in which surface-initiated polymerization is often performed. (30,87,88,90,91) For example, Tian et al. compared the SI-ROP of ε-caprolactone (ε-CL) from freeze-dried CNCs and from solvent-exchanged CNCs and observed that the grafting density is higher from the solvent-exchanged nanocrystals than from those that were freeze-dried. (86) From these findings, it is clear that impurities adsorbed on the surface of CNCs are generally better removed by treatment with organic solvents. These impurities are likely byproducts formed by the hydrolysis of residual hemicellulose or lignin or perhaps the acid-catalyzed dehydration of mono- and oligosaccharides. (2,7)
3. Surface-Initiated Polymerization from CNCs
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Much of the research efforts directed toward the surface modification of CNCs have focused on tuning the physicochemical characteristics of their surface, while at the same time preserving their morphological integrity and crystallinity. (24,25,92) Indeed, if the functionalization chemistry is chosen to selectively modify the surface or the reducing end groups, the intrinsic crystallinity and crystal structure of the CNCs should be retained, which can be checked by X-ray diffraction studies. (24,30,82,83,86,93−96) In the present section, the methods most often utilized to modify CNCs via the grafting from approach, i.e., surface-initiated and reducing end-initiated polymerization, will be presented.
3.1. Surface-Initiated Ring-Opening Polymerization
Ring-opening polymerization (ROP) is a common technique to produce polymers from cyclic monomers such as lactones, (97,98) dilactones, (99) lactams, (100) cyclic carbonates, (101) cyclic ethers, (102) and oxazolines (102) (Figure 4). For some of these monomers, e.g., lactones and dilactones, the ROP can be initiated by −OH groups. Therefore, they are ideal candidates for the surface modification of not only CNCs, but also cellulose fibers and nanofibrils, as they also contain a high density of −OH surface groups. In this case, the cellulosic substrate acts as a macroinitiator and the polymerization process is most often referred to as SI-ROP.
Figure 4
Figure 4. Examples of common monomers used for ROP.
CNC-g-polyesters
CNC-g-polyesters have been prepared by the SI-ROP of ε-caprolactone (ε-CL), (30,80−83,86,103−105)l-lactide (l-LA), (84,85,87,88,90,91,104,106−108) and d-lactide (d-LA) (89,105,109,110) from CNCs. The mechanism of the ROP of lactones and dilactones varies depending on the system used, i.e. the nature of the catalyst and the initiator. (111) The mechanism can be an anionic, a cationic, a monomer-activated or a coordination–insertion polymerization. In the case of SI-ROP from CNCs, the nanocrystals act as the initiator and the mechanism is only dependent on the catalyst. This reaction is water sensitive as water can also initiate the ROP of lactones. Therefore, the water, in which the nanocrystals are normally dispersed, needs to be removed as much as possible by means described in section 2.3.
Tin(II) 2-ethylhexanoate (Sn(Oct)2) (30,77,80−88,90,91,103,105,106,108−110,112) is the most commonly used catalyst for the SI-ROP of (di)lactones. The reaction is usually performed in toluene, however it has also been carried out in ionic liquids (1-allyl-3-methylimidazolium chloride, [AMIM]Cl (113)), in DMSO, (108) or in bulk. (84) Typical conditions further are temperatures between 80 and 130 °C and a reaction time of typically 24 h. The grafted polymers have also been prepared under microwave irradiation. (81,112) Benzyl alcohol is sometimes used as a sacrificial initiator to produce free polymer in solution, which is easier to characterize than the grafts. (77,108) Of course, the comparison with free polymer that is produced “in parallel” is only sensible if one can safely assume that the kinetics of solution ROP are similar to SI-ROP from CNCs, which may or may not be the case.
In general, the nature of SI-ROP from CNCs makes the accurate characterization of grafted polymer molecular weight and dispersity challenging, as the polymer must be degrafted before full molecular characterization is possible. Most degrafting schemes rely on hydrolytic cleavage of interfacial ester bonds, which, however, are also present in some of the polymer chains discussed here. This problem can potentially be addressed through the use of cleavable anchoring groups, such as those containing disulfides (114) or UV-sensitive moieties, (115) similar to the approached employed in the case of polymer-modified cellulosic substrates synthesized via surface-initiated controlled radical polymerization (SI-CRP). (28) Perhaps one could also take advantage of the high bond tension of the esters at the polymer-CNC interface, which are likely more sensitive to hydrolysis than esters along the polymer chain such that they could be cleaved by mechanochemical means, e.g., by ultrasound. (116,117)
Some researchers have attempted to estimate the length of the polymer grafts based on the melting temperature determined by differential scanning calorimetry (DSC) using a correlation curve, (89,106,110) although this method does not account for potential differences in grafting density or effects of surface-confinement. Wu et al. proposed the use of FTIR data for poly(lactic acid) (PLA) brush quantification. (89) The approach relied on measuring FTIR spectra of physical mixtures of CNCs and PLA and creating a calibration curve based on the relative intensities of a characteristic band of PLA (i.e., 1454 cm–1, corresponding to the CH3 asymmetric stretching) and a characteristic band of the CNCs (i.e., 1428 cm–1, corresponding to the CH2 symmetric bending). The calibration curve was then employed to analyze CNC-g-PLA samples. While this method does not provide information on the molecular weight or grafting density of grafted polyesters, it permits to quantify the composition of PLA in CNC-g-PLA to be 25.3 wt % and estimate the molecular weight of the grafts to be 3.4 kDa.
Although the analysis of grafted CNCs in the liquid phase is usually impractical due to poor dispersibility in typical solvents, (106) some researchers managed to make stable colloidal dispersion of surface-modified CNCs in CDCl3 (105) or DMSO-d6 (107,118) (mixed with small amount of chloroform) in order to perform 1H and 13C NMR in well-stabilized dispersions. In this sense, the synthesis of CNC-g-PLA was evidenced by the appearance of resonance signals at 1.50–1.75 ppm (methyl protons), (105,107) 5.00–5.50 ppm (methine proton), (105,107) and 1.35 ppm (terminal methyl protons) (107) corresponding to PLA brushes. In the latter case, de Paula et al. (107) dispersed CNC-g-PLA in DMSO-d6 with the addition of some chloroform and were able to directly calculate the degree of polymerization (DP) of PLA (=12) from the 1H NMR spectra. This particular study highlights the limited DP one can achieve with SI-ROP from CNCs. In addition, the grafting of poly(ε-caprolactone) (PCL) from the surface of CNCs was confirmed in another study by the appearance of signals at 1.50–1.65 ppm, 2.23–2.32 ppm, and 4.07–4.11 ppm (methylene protons), (105) however, the DP of the polymer was not reported. It is important to note that the NMR results obtained from well-dispersed CNC-g-polymers may only provide partial information, which, unless appropriate separation protocols are used, is mainly related to free polymer molecules, while the polymer chains grafted from the dispersed contribute broader signals. Thus, the quantitative analysis of such NMR data is not always straightforward.
Regarding the mechanism of the SI-ROP catalyzed by Sn(Oct)2, it is widely accepted that the reaction proceeds by a coordination–insertion mechanism. The first step is the conversion of the catalyst into a tin alkoxide to produce the active species, followed by the coordination of the monomer to the alkoxide through its insertion into the metal–oxygen bond (Scheme 1). (111)
Scheme 1
Scheme 1. Mechanism of the SI-ROP of ε-CL from CNCs Using Sn(Oct)2 as a Catalysta
aKey: (A) Conversion of Sn(Oct)2 into tin alkoxide. (B) Grafting of ε-CL from the surface of CNCs via coordination of tin alkoxide with the monomer.
Other metal-based catalysts that have been tested for SI-ROP of cyclic esters from CNCs include magnesium hydride (MgH2) (107) and zinc oxide. (108) Zinc oxide seems to give similar results in terms of monomer conversion and grafting yield as Sn(Oct)2. (108) Organic catalysts such as 4-dimethylaminopyridine (DMAP), (83,113) citric acid, (78) and tartaric acid (83) have also been used for the SI-ROP of lactones from CNCs.
In the case of organic acids (citric acid and tartaric acid), the polymerization occurs through a monomer-activated mechanism, (119,120) in which the monomer is activated by a proton donated by the carboxylic acid. An −OH group of the cellulose, acting as a nucleophile, then reacts with this proton-activated monomer, leading to the formation of a hydroxyl-terminated ester via the ring-opening of the monomer. The propagation occurs as this newly formed hydroxyl-terminated species, acting as a new nucleophile, reacts with a new proton-activated monomer (Scheme 2). A few drops of HCl are sometimes added to the reaction medium at the end of the procedure to stop the reaction. (30,80,82,88−90) The grafted nanocrystals can be recovered by precipitation (in heptane, (80,82) deionized water, (113) or methanol (81,89,90,105,106,109,112)) and purified by Soxhlet extraction in toluene, (80,82) dichloromethane, (80,83,86) and/or tetrahydrofuran (THF) (77) to remove residual monomer, catalyst, and free homopolymer, in cases where a sacrificial initiator is used or impurities are present that initiate polymerization in solution.
Scheme 2
Scheme 2. Mechanism of the SI-ROP of ε-CL Using Citric Acid as a Catalyst
Alternatively, they can be recovered by successive dispersion in methanol, ethanol, and acetone, each step followed by sedimentation via centrifugation. (87) Bitinis et al. noted that this second procedure allows the removal of the unwanted coproducts and unreacted reagents while losing fewer grafted nanocrystals in comparison to the purification performed via Soxhlet extractions. (87) However, when applying the same procedure with toluene, Habibi et al. could not recover the grafted nanocrystals via centrifugation as they remained highly stable in the supernatant. (30) This is likely due to the higher grafting yield obtained by Habibi et al., (30) which was on the order of 85% in comparison to the 70% yield obtained by Bitinis et al.; (87) however, the molecular weight and grafting density of the tethered polymers was unknown.
CNC-g-polyoxazolines
The grafting of oxazolines via SI-ROP follows a cationic mechanism, and thus cannot be initiated from the −OH groups at the surface of CNCs; indeed a modification of the CNC surface through tosylation substitutes the nucleophilic −OH groups with electrophilic tosyl moieties. (121) In this case, tosylated CNCs act as the macroinitiator for the SI-ROP of 2-ethyl-2-oxazoline. It was shown that after tosylation, the modified CNCs can simply be mixed with the monomer and the SI-ROP readily takes place under microwave irradiation (completed in ca. 13 min). The reaction can be stopped by the addition of cold water and is subsequently washed with methanol (Scheme 3).
Scheme 3
Scheme 3. SI-ROP of 2-Ethyl-2-oxazoline from Tosylated CNCs
3.2. Surface-Initiated Free Radical Polymerization
One of the most commonly used approaches for the grafting of vinyl polymers from the surface of CNCs is surface-initiated free radical polymerization (SI-FRP). The polymerization starts by the activation of the initiator species, which will, often via hydrogen abstraction, generate a reactive radical on the surface of CNCs, from which monomers add to a propagating chain. (122) This method allows the facile growth of polymers at the CNC surface, along with free chains in solution, with relatively high molecular weight, even at low monomer conversions.
A wide variety of radical initiators have been demonstrated to be effective for the surface-initiated polymerization of vinyl monomers. Among the thermal initiators, potassium persulfate (KPS) is widely employed to decorate CNC surfaces with polymer chains, due to the stability of free radicals and compatibility with aqueous solutions. The radical formation starts in the aqueous phase by the thermal homolytic dissociation of the KPS peroxide bond, at temperatures ranging between 60 and 70 °C. The free sulfate radicals thus produced are capable of abstracting hydrogen atoms from the surface of the CNCs and thus generate the surface-bound initiating species (Scheme 4). Using this initiator, 2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate (UPyMA) was successfully polymerized from the surface of CNCs, affording CNC-g-PUPyMA containing 6.9 wt % of polymer. (123) The presence of UPyMA units contributed significantly to the emulsion stabilizing ability of the nanoparticles (NPs) in comparison with pristine CNCs, which made them suitable to prepare high internal phase emulsions and macroporous hydrogels. Another example was reported by Wu et al., who used acrylic acid (AA) as the monomer. (124) The use of grafted poly(acrylic acid) (PAA) chains offered CNC-based hydrogels excellent water retention properties with a swelling ratio of 323 g/g in distilled water versus 33 g/g in sodium chloride (NaCl) solution.
Scheme 4
Scheme 4. Synthesis of CNCs Grafted with PAA via SI-FRP Using KPS as Radical Initiator
A particular case of SI-FRP was described by Anirudhan et al., who reported the grafting of poly(methacrylic acid-co-vinylsulfonic acid) (P(MAA-co-VSA)) from the surface of magnetized CNCs. (125) First, magnetic CNCs were prepared by the chemical precipitation of magnetite (Fe3O4) onto the surface of CNCs. Then, the SI-FRP was carried out in the presence of KPS as initiator, the corresponding monomers and 2-(2-methyl-acryloyloxy)ethyl 2-methylacrylate (EGDMA) as cross-linker in water to form a three-dimensional network, which ultimately had a surface area that was five times larger than that of the parent CNCs.
Ammonium persulfate (APS) is another thermal initiator that has been used for the grafting from the surface of CNCs, in water, via SI-FRP; in this case, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) was used as the monomer. (126) The final product contained 11.7 wt % of polymer chains, which imparted stimuli-responsive properties to the CNCs.
In addition to the radical formation via thermal homolytic decomposition of the peroxide bond, persulfate initiators can also effectively generate free radicals in the presence of a reducing agent, such as oxyacids of sulfur (e.g., sulfite, bisulfite, thiosulfate, metabisulfite and dithionate), organic acids (e.g., ascorbic acid), metal ions (e.g., Fe2+ or Ag+) or amines (e.g., TMEDA: N,N,N′,N′-tetramethylethane-1,2-diamine). This works by electron transfer from atoms or ions containing species that feature unpaired electrons and subsequent bond dissociation in the acceptor (persulfate) molecules. (127) These systems are referred to as redox initiators, redox catalysts or redox activators. They are often used at or below room temperature and are commonly employed in emulsion and aqueous polymerizations. This pathway has also been utilized for the formation of active radicals on the surface of CNCs by Zhou et al., where KPS and sodium bisulfite (SBS) were used as a redox initiator system to directly graft polyacrylamide (PAM) from the surface of CNCs dispersed in water. (128) The initiation reaction of this system is shown in Scheme 5.
Scheme 5
Scheme 5. Radical Formation of the KPS/SBS Redox Initiator System
In this case, both radicals, formed through reaction 1 (Scheme 5), could activate CNCs via hydrogen abstraction at −OH groups on the surface. Upon addition of acrylamide (AM) monomer and N,N′-methylenebis(acrylamide) (NMBA) as a cross-linker, these surface radicals acted as the initiating sites for grafted PAM chains. Simultaneously, free radicals were also formed in solution promoting the polymerization of AM, which were also cross-linked by NMBA. Finally, a nanocomposite hydrogel based on CNC-g-PAM and PAM network was formed and showed well-dispersed modified-CNCs and enhanced interfacial adhesion between the two components.
The main advantage of using this type of initiator system is that the polymerization can be performed under mild conditions, as no heat is needed to promote the radical formation, which decreases the possibility of undesired side reactions while still obtaining high molecular weight polymers with a high yield. APS and TMEDA have likewise been used as a redox couple for the SI-FRP of N-isopropylacrylamide (NIPAM) from CNCs in order to form thermoresponsive hydrogels. (129) The initiation mechanism proceeds via a contact charge transfer complex and a cyclic transition state, from which two primary radicals are formed, one from the amine and another from persulfate, which both in turn initiate the polymerization process (Scheme 6). (130)
Scheme 6
Scheme 6. Radical Formation of the APS/TMEDA Redox Initiator Couplea
aScheme adapted from ref (131).
Another technique, based on ceric-ion initiated polymerization and performed in aqueous solutions of nitric acid to modify the surface of CNCs, was first reported by Kan et al. (132)In this approach, ceric ions (Ce4+) chelate with two adjacent −OH groups on the AGUs, possibly those at C2 and C3 positions. This step is followed by an electron transfer from the AGU to Ce4+, leading to its reduction into Ce3+, opening of the ring at C2 and C3 positions, and the formation of a radical species (Scheme 7). The ability to graft poly(4-vinylpyridine) (P4VP) from the surface of CNCs proved the effectivity of this chemistry on the surface of highly crystalline cellulose. Using similar chemistry, poly(glycidyl methacrylate) (PGMA) (133) and poly(methyl methacrylate) (PMMA) (134) have also been successfully grafted from the surface of CNCs.
Scheme 7
Scheme 7. Mechanism of Ceric Ion Reduction, Leading to the Formation of CNC-Based Radicalsa
aScheme adapted from ref (136).
In addition, Tang et al. carried out the synthesis of binary polymer brushes consisting of poly(oligoethylene glycol) methacrylate (POEGMA) and poly(methacrylic acid) (PMAA) from the surface of CNCs. (135) These brushes offered new properties to the final material that were effectively controlled by external stimuli such as pH and temperature. Initially, the OEGMA polymerization was performed via cerium-mediated reactions, followed by the subsequent sulfate-initiated MAA polymerization. Combining these two initiator systems allowed the authors to polymerize two vinyl monomers, with different intrinsic properties, from the surface of CNCs while assuring the accessibility to binding sites. Thus, cerium-initiated polymerization generated radicals at AGUs by ring-opening at the C2 and C3 positions and transforming their secondary alcohol, but without affecting their primary alcohols, which left the latter available for further radical-initiated polymerization through hydrogen abstraction. Indeed, it is believed that the ring opening of the AGUs at the C2–C3 position and the generation of aldehyde groups yields a material, which undergoes thermal degradation at a lower temperature, thus reduced thermal stability can be observed compared to pristine CNCs. This was demonstrated for CNC-g-POEGMA or CNC-g-P(OEGMA-ran-MAA) (Td = 180 °C) (135) and CNC-g-PMMA (Td = 250 °C). (134)
In general, the modification of CNCs via SI-FRP comes with the formation of a substantial amount of free (unattached) homopolymers in solution, which represents a loss of monomer and a reduction in grafting efficiency, which thus is the main drawback of this method. In addition, the successful removal of such species is in some cases tedious and challenging, depending on the reactants and the final product. The purification of hydrogels, for example, request the immersion of the material into an excess of water for a period between 48 and 72 h in order to remove any water-soluble fraction. It is usually followed by a drying process (50–70 °C) in vacuum for 48 h. (128,137) For the removal of metals, i.e., particles released from sonication probe, the common reported procedure implies the use of glass microfiber filter paper followed by extensive dialysis and ultrafiltration. (132,134) If the extraction of CNC-g-polymer from the reaction media is required, it generally proceeds via subsequent centrifugation and washing steps with one or more solvents (according to the affinity of interested species), which could be followed by dialysis and drying processes. (94,135)
Nevertheless, inorganic peroxides, such as KPS and APS, have shown their versatility as initiator species in SI-FRP based on the large variety of polymers that have successfully been grafted from the surface of CNCs. The main advantage of using this type of initiator system is that the grafting can be performed directly from the surface of pristine CNCs, i.e., without previous modification. For cases where the polymerization should be performed at ambient or lower temperatures, a redox system involving any of these compounds can be used. In comparison to ceric ion SI-FRP, peroxy compounds have the advantage of not altering the structure of CNC surface chains, which usually results in nanomaterials with higher thermal stability, as previously discussed.
The SI-FRP methods presented above involved the formation of reactive species directly on the surface of CNCs, from which polymer chains were allowed to grow. Yang and co-workers reported a different approach, in which the CNC surface was chemically modified with reactive small molecules before polymerization. (137,138) A chemical entity containing end vinyl groups were covalently linked to the surface of CNCs by treatment with γ-methacryloxypropyl silane. Subsequently, radicals species were formed at the end vinyl groups in the presence of KPS initiator, thus creating propagating radicals, from which AA (137) or AM (138) monomers were polymerized until termination reactions occurred by recombination, indeed promoting the formation of cross-linked networks. On the basis of the high monomer-to-surface initiator ratio (137,138) employed and the final monomer conversion (ca. 99%), (137) the polymer grafted via this approach formed a dense shell around the CNC core due to either a high grafting density or a high molecular weight.
Espino-Perez et al. proposed the synthesis of CNC-g-polystyrene via reaction steps consistent with principles of green chemistry. (94) The protocol involved the ozonolysis of CNCs leading to peroxide groups at their surface, and the subsequent radical polymerization in the presence of KPS and styrene (Sty) monomers within a sodium acetate buffer solution (pH = 3.5). The thermally activated sulfate radicals reduced the peroxide groups, creating free radicals, which propagated to form tethered polystyrene (PSty) chains. The Sty/AGU ratio was estimated to be 0.24, which confirmed the low grafting efficiency on CNCs through this method due to the low hydroperoxidation efficiency during the ozonolysis treatment.
Another strategy for decorating CNCs with grafted vinyl polymers was presented by Lee et al. (139) The SI-FRP took place on the surface of CNCs containing trichloroacetyl groups in the presence of molybdenumhexacarbonyl (Mo(CO)6) as initiator and 2-methacryloyloxyethyl phosphorylcholine (MPC) and stearyl methacrylate (SMA) as monomers in ethanol to yield CNC-g-P(MPC-co-SMA), with over 90% of monomers conversion.
More recently, a new free radical strategy was presented by Wohlhauser et al. where the initiator species was covalently attached to the surface of CNCs. (93) This approach guarantees the presence of a high number of reactive sites at the surface of the substrate, although it does not avoid the formation of radicals in solution. In this work, PMMA and poly(hexyl methacrylate) (PHMA) were successful grafted from CNCs through an undemanding protocol, which involved the surface modification of CNCs with benzophenone moieties as photoinitiators and subsequent surface-photoinitiated FRP of methacrylates in DMF under UV-irradiation. Benzophenone is a Norrish Type II photoinitiator that undergoes intersystem crossing from its excited singlet state to the lowest energy reactive triplet, followed by the abstraction of aliphatic hydrogen from a nearby molecule or interface, yielding a ketyl radical and a carbon centered radical. (140) The SI-FRP is thus initiated by the carbon centered radical and can also result in the formation of free (unattached) polymer chains (initiated from a monomer or solvent molecule). Using this method, compositions containing 80 and 90 wt % of grafted polymers were prepared for both monomers. This was achieved by using DMF as solvent and variation of the monomer conversion. In comparison to previous studies, the fraction of polymer brushes on the surface of CNCs was very high, but given the mechanism, a substantial amount of polymer (between 24 and 48 wt %, relative to the initial monomer amount) was also produced as free (unattached) polymer.
In general, the polymer grafted on the surface of CNCs via free radical polymerization from covalently immobilized reactive species depends on factors related to the propagating radical, such as surface initiator efficiency, propagation rate and the monomer-to-surface initiator ratio. Indeed, additional studies are needed in the field to better understand the influence of these parameters. Another opportunity that currently remains in this field is the investigation of SI-FRP via the attachment of other conventional radical initiators (i.e., azo-containing initiators) and subsequent comparisons with polymer brushes obtained through the above-mentioned techniques.
3.3. Surface-Initiated Controlled Radical Polymerization
SI-CRP is an effective way to grow polymers when a precise control over polymer architecture, composition, molecular weight, and graft density is required. (28) In general, CRP reactions rely on the reversible deactivation of propagating chains, such that termination reactions are minimized, thus providing first order polymerization kinetics and low dispersity. Different SI-CRP methods can be performed, which depend on the type of initiator or so-called chain transfer agent (CTA) coupled to the surface, such as Cu-mediated SI-CRP, surface reversible addition–fragmentation chain transfer (S-RAFT) or surface-initiated nitroxide-mediated polymerization (SI-NMP), among others. Scheme 8 exemplifies the three most commonly used SI-CRP techniques, i.e. surface-initiated atom transfer radical polymerization (SI-ATRP), S-RAFT, and SI-NMP. In the following sections, a brief overview of each of these techniques will be provided.
Scheme 8
3.3.1. Cu-Mediated SI-CRP
Of all SI-CRP techniques, those mediated by transition metal complexes, especially Cu species, as in SI-ATRP, have been the most widely employed to graft polymers from CNCs. The control in ATRP arises from the existence of a dynamic equilibrium between dormant and propagating species, thus lowering radical concentration and minimizing termination reactions. (144) The reversible deactivation occurs through the exchange of a halogen atom (Br or Cl) between a metal complex (based on Cu in most cases) and the propagating species. As the presence of residual metal in the final polymers may be a problem for some applications, techniques were then developed to decrease the required amount of catalytic system to ppm by (re)generating Cu(I) in situ activators by electron transfer (A(R)GET) ATRP. (145−147) CRP in the presence of Cu(0) was also reported with a debate on the model of the electron transfer mechanism, i.e. single-electron transfer living radical polymerization (SET LRP) and supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP). (148−150) As the present Perspective aims at reporting the different structures obtained from CNCs and their related properties rather than debating the mechanism, techniques involving Cu species are grouped under the generic term “Cu-mediated SI-CRP.”
Conducting a Cu-mediated SI-CRP from CNC requires the preliminary introduction of initiating sites on their surface. This is usually achieved by reacting the numerous −OH functionalities on the CNC surface with 2-bromoisobutyryl bromide (BiBB) in a solvent (THF or DMF), usually in the presence of a base (triethylamine) and a catalyst (DMAP). (141,151−161) The extent of this modification can be tailored through the reaction duration and the amount of BiBB introduced. (162,163) Through a combination of active initiators and inactive “dummy” initiators, the overall grafting density can be controlled. (28) Over the years, strategies were developed to improve the CNCs dispersibility during the modification by either performing (1) solvent exchanges (rather than freeze-drying), which permitted avoiding a redispersion step of dried CNCs, (163,164) or (2) a chemical vapor deposition of BiBB on a CNC aerogel to partially hydrophobize the CNCs prior to a subsequent liquid phase reaction with BiBB. (165−169) Higher modification degrees (up to 15 wt % Br determined by elemental analysis) were then reached with lower BiBB excess. (165) Although less widely used, the incorporation of the initiating sites was also reported by reacting the −OH functions with α-bromoisobutyric acid activated with carbonyl diimidazole. (170,171) In another study, the use of 2-bromopropionic acid was proposed for the introduction of active sites on the CNCs surface with a yield of 46% and 2.86 wt % of Br content determined by elemental analysis. (95) The protocol involve a pretreatment of pristine CNCs with HCl, followed by one-step reaction, which combines Fischer esterification and acid hydrolysis.
In all the previous studies, the −OH functions were statistically and homogeneously modified on the entire nanocrystal surface. Recently, Zoppe et al. reported an approach to introduce initiating sites specifically on the reducing end groups of CNCs through their oxidation and reaction with ethylenediamine and subsequently 2-bromoisobutanoic acid N-hydroxysuccinimide ester. (172) This approach is discussed in further detail in section 3.4.
The CNC macroinitiators have been used in the Cu-mediated SI-CRP of various styrenic-, (meth)acrylic, or acrylamide-based monomers (Figure 5) with catalytic systems and solvents optimized for the various monomers employed (see Table S1). The polymerization control can be elucidated through the saponification of the grafted polymers (via either acidic or alkaline conditions) and further analysis of the degrafted polymer in solution; however the degrafting reactions require an extended reaction time (2 days to 3 weeks) and can potentially alter the polymer structure. (141,152−154,163,164,170,172) The versatility of Cu-mediated SI-CRP techniques and their high tolerance toward many functional groups has given access to CNCs grafted with a large variety of polymeric chains ranging from neutral PSty, (115,141,157,162) poly(alkyl ((meth)acrylate), (158,169,170) to cationic poly(amino(alkyl) methacrylate/methacrylamide) (155,172) or anionic polystyrenesulfonate (172) and PAA (through the hydrolysis of poly(tert-butyl acrylate) (PtBA)). (165,168) Thermosensitive polymers, such as PNIPAM, (153,154,163,164,172) PDMAEMA, (152,166,167,171) poly(N-vinylcaprolactam) (PNVC), (161) or poly[(oligoethylene glycol) methyl ether acrylate] (POEGMEMA) (159,160) have also been grafted with this approach.
Figure 5
Figure 5. Monomers used in Cu-mediated SI-CRP from CNC macroinitiators; Sty; (115,141,157,162) 4-SS, sodium 4-vinylbenzenesulfonate; (172) MA; (158,170) BA, butyl acrylate; (169) tBA, tert-butyl acrylate; (165,168) MMA, methyl methacrylate; (169) AEM, 2-aminoethyl methacrylate; (155) METAC, [2-(methacryloyloxy)ethyl]trimethylammonium chloride; (172) OEGMEMA, oligoethylene glycol methyl ether methacrylate; (159,160) NIPAM; (153,154,163,164,172) DMAM, N,N-dimethylacrylamide; (173) AEMA, 2-aminoethyl methacrylamide; (155) DMAEMA, dimethylaminoethyl methacrylate; (152,166,167,171) DEAEMA, diethylaminoethyl methacrylate; (171) EANI, 4-ethoxy-9-allyl-1,8-naphtalimide; (154) SBAM, soybean amide methacrylate; (156) NVC, N-vinylcaprolactam; (161) MMAZO, 6-[4-(4-methoxyphenylazo)phenoxy] hexyl methacrylate; (151) NpMA, naphtyl methacrylate. (166)
The use of a sacrificial initiator (i.e., free initiator species added to the reaction medium, usually EBiB, ethyl α-bromoisobutyrate) is common as it provides a simple means to monitor the polymerization process, allows targeting the DP, and also contributes to a better control by increasing the concentration of Cu(II) deactivator species in the reaction. (154,157,162,165,167−169) A good correlation between molecular weight of the free PSty chains (initiated from the sacrificial initiator) and the molecular weight of PSty grafts was demonstrated by incorporating o-nitrobenzyl photosensitive moieties between the CNC surface and the surface-grafted initiator, degrafting the PSty grafts under mild conditions (UV-irradiation) and comparing their molecular weight to the free chains (number-average molecular weight (Mn) = 8.5 kDa, dispersity (D̵ = Mw/Mn) = 1.08 versus Mn = 10.1 kDa, D̵ = 1.07 for degrafted and free polymers, respectively). (115) In contrast, Zoppe et al. showed high dispersity of surface-grown PNIPAM, however this was likely due to insufficient amounts of CuX2 deactivator species and complexation with NIPAM monomers, in addition to having been conducted in highly polar media. (163) Using a similar SI-AGET-ATRP approach with ascorbic acid as reducing agent, Wu et al. found that free PNIPAM chains had a bimodal molecular weight distribution, whereas cleaved PNIPAM chains were unimodal and had a lower dispersity. (154) In addition, a significantly higher Mn was observed for the free PNIPAM chains.
Monte Carlo simulations of surface-initiated polymerization from neutral planar substrates have shown that bulk polymers tend to grow at faster rates and with lower dispersity, however this is highly dependent on the grafting density of initiating sites. (174) Along with the importance of CuX/CuX2 ratio, (28,29) it is clear that the monomer type/concentration, solvent and bulk vs solution synthesis all play a role in the control of Cu-mediated SI-CRP. (28) Thus, the assumption that the polymerization kinetics of surface-initiated polymers is similar to that of free polymers is not always valid.
In general, Cu-mediated SI-CRP conducted in higher polarity solvents, such as water, tend to yield polymers with higher dispersity, as a result of high rates of activation. (175) Such high dispersity implies higher rates of termination and the loss of active polymer chain ends, thus low overall control. The kinetics of Cu-mediated SI-CRP conducted at high monomer concentrations in less polar solvents or in bulk, e.g., Sty, tend to resemble those of solution Cu-mediated CRP. (115) Such aspects are important considerations when more complex architectures are required, such as block copolymer brushes from CNCs, for example, in which good control and high chain-end fidelity are critical for success.
The impact of the CNC surface charges on SI-ATRP control was also investigated through the initiation of a DMAM polymerization from CNCs with various charge densities. (173) It was demonstrated that electrostatic interactions lead to an enrichment of catalytic species on the surface and increased the initiator efficiency, but at the expense of a higher dispersity. A qualitative model of catalyst enrichment for SI-ATRP from CNCs is shown in Figure 6. Since Cu(II) halide species readily dissociate in aqueous environments, divalent Cu2+/ligand complexes can strongly interact with sulfate half-ester groups on the surfaces of the CNCs, which in turn has a negative impact on deactivation of growing polymer chains. Therefore, low charge density CNCs are advantageous if low dispersity of grafted chains is required in the final hybrid materials; however, a lower initiator efficiency can be expected.
Figure 6
Figure 6. Qualitative model describing the composition of the interfacial region around CNCs prior to polymer brush growth from CNCs with high (CNC-1) and low (CNC-2) density of surface sulfate groups. (small red circle = initiator, L = ligand, κ–1 = Debye length). Reprinted with permission from ref (173). Copyright 2016 American Chemical Society.
It was also recently demonstrated that a controlled Cu-mediated SI-CRP of MA can be photoinduced from modified CNCs in a commercial UV nail box. (158) The CNCs were previously modified with BiBB, then the polymerization of the monomer proceed under UV irradiation at 360 nm during 90 min in the presence of a sacrificial initiator. The analysis of the free PMA synthesized simultaneously in solution demonstrated first order kinetics and, thus, good control. The use of this technique allowed high grafting density in short reaction times and a straightforward purification due to the significant reduction in the amounts of Cu and ligand employed in comparison with a traditional Cu-mediated CRP. The authors observed an increase of the glass transition temperature (Tg) of CNC-g-PMA compared with the free PMA, indicative of reduced chain mobility when tethered to the CNC surface. The development of such user-friendly SI-CRP techniques opens up a wide range of synthetic possibilities for grafting polymers from CNCs.
Even though the above-mentioned techniques has been widely used in the field of grafting polymers from CNCs, the removal of Cu residues after the polymerization is still challenging and needs more investigation. Several purification methods are used in solution polymerization but they are difficult to employ when the reaction involves CNCs, e.g., purification over silica or neutral alumina columns. (144) The (re)generation of Cu(I) in solution partially solved this problem, since much lower amounts of Cu are used in the reaction. Nevertheless, in case of biomedical applications, the removal of trace amounts of Cu appears to be crucial.
Dialysis has been used to remove Cu after polymerization reactions. (176) Prior to dialysis, Rosilo et al. performed several centrifugations of CNC-g-poly(N,N-dimethylaminoethyl methacrylate) in different organic solvents, i.e., MeOH, DMF, and dioxane. (167) In both cases, the approaches used may not be sufficient to remove the adsorbed Cu on the surface of the grafted CNCs. In another study, Morandi et al. proposed two subsequent Soxhlet extractions on freeze-dried samples in dichloromethane and ethanol for the removal of Cu traces. The blue color, related to the Cu adsorbed on the surface of the CNCs, was removed by this treatment. Nevertheless, no quantitative analysis of remaining traces of Cu was performed after this process. Another method to remove Cu could involves stirring the grafted CNCs with an ion-exchange resin. (177) Or, as discussed in section 3.2, the particles could be removed by filtering the modified CNCs through glass microfiber filter paper. (132)
3.3.2. Surface-RAFT Polymerization
Reversible addition–fragmentation chain transfer polymerization (RAFT) involves a reversible chain transfer process and is carried out in the presence of a CTA, in addition to a free radical initiator. Such CTAs are often based on dithiocarbamates, dithiocarbonates, or dithioesters and contain so-called R- and Z-groups. The function of the CTA is to mediate addition, fragmentation and chain transfer reactions, resulting in an overall controlled molecular weight and dispersity. During RAFT polymerization, radicals either reversibly react with the CTA or monomers. Consequently, S-RAFT polymerization is conducted by the prior immobilization of either free radical initiators or CTAs to the substrate surface. If the CTA is attached to the substrate surface, it can be tethered so as to present either the R- or Z-group, the former of which being the most commonly utilized. Scheme 9 shows (A) the two different CTA immobilization strategies and examples of S-RAFT from CNCs using (B) the R-group approach and (C) surface free radicals and free CTA.
Scheme 9
One of the first studies utilizing S-RAFT to modify CNCs was reported by Zeinali et al., in which PNIPAM and random copolymers of PNIPAM and PAA (P(NIPAM-ran-AA)) were grafted via the immobilization of the CTA with the R-group approach. (142) The CTA, 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT), was grafted to CNC −OH groups via DMAP-catalyzed esterification in the presence of N,N′-dicyclohexylcarbodiimide (DCC) in chloroform. A sulfur content of 2.1 wt %, determined by elemental analysis, confirmed the successful attachment of the CTA. S-RAFT polymerizations of NIPAM and NIPAM-ran-AA were conducted with 2,2′-azobis(2-methylpropionitrile) (AIBN) in dioxane and in the presence of free CTA, i.e., DDMAT. The authors obtained statistical copolymer grafts using different monomer feed ratios, in which the CNC hybrids were characterized by FTIR, dynamic light scattering (DLS) and thermal analysis. From the results of DSC experiments, the authors concluded that as the ratio of AA was increased, while the heat capacity variation (ΔCp) decreased. In comparison to free (co)polymers, those grafted from CNCs displayed higher Tg due to surface chain confinement.
Similar block copolymers were grafted from CNCs coated with a cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) shell by immobilization of the CTA, S-(thiobenzoylthioglycolic) acid. (178) CNCs were first coated with PHEMA cross-linked with ethylene glycol dimethacrylate (EGDMA), using a free radical approach with AIBN in solution, to increase the −OH groups content on the surface. From X-ray photoelectron spectroscopy (XPS) analysis of CTA-modified CNC/PHEMA particles, the surface S content was determined to be 1.8%. In this case, the authors likely obtained a higher grafting density of PNIPAM-block-PAA chains due to grafting from a cross-linked PHEMA network at the surface of CNCs. In other work, PMMA was grafted from CNCs using 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPADB) functionalized CNCs. (118) Here the authors determined a sulfur content of 2.9 wt % by elemental analysis and estimated that 1 out of 6 AGUs was modified with CPADB, which is one of the highest values reported so far for CTA-functionalized CNCs. The authors further utilized 1H NMR spectroscopy in DMSO-d6 in the presence of an ionic liquid to observe CPADB-functionalized CNCs.
Boujemaoui et al. investigated the grafting of vinyl acetate (VAc) via S-RAFT and the interchange of xanthates (MADIX) polymerization. (179) The xanthate-based CTA, 2-((ethoxycarbonothioyl)thio)propanoic acid, was attached to CNCs either by Steglich esterification or a two-step approach, the latter of which resulted in a lower grafting yield of poly(vinyl acetate) (PVAc). The kinetics of the S-RAFT process was monitored using sacrificial CTA and demonstrated good control over the molecular weight and dispersity. However, for all the aforementioned studies, little is known regarding the molecular weight, dispersity and grafting density of the tethered polymers.
S-RAFT of various acrylamides from CNCs has also been mediated by free macromolecular CTAs and conducted utilizing free radical initiators, such as ceric(IV) ammonium nitrate (CAN) (Scheme 9C). (96,180) In these cases, macromolecular CTAs with molecular weight between 1.4 and 2 kDa were used to mediate S-RAFT polymerization of AM (96,180) and DMAM (180) from CNCs, in which surface radicals were generated as shown in Scheme 7. PAM chains were cleaved from the surface of CNCs and determined to have Mn = 14 kDa and D̵ = 1.28, the latter demonstrating involvement of the macromolecular CTA. (96) In addition, increased S contents were observed for the polymer-modified CNCs, which indicated that at least some of the grafted PAM chains were end-capped with macromolecular CTA moieties.
In a unique approach, poly[poly(ethylene glycol)ethyl ether methacrylate] (PPEGEEMA) and PDMAEMA were grafted from a difunctional CNC macroinitiator via a combination of S-RAFT and SI-ATRP. (176) The difunctional CNC macroinitiator was obtained by first transforming the −OH groups on the CNC surface into reactive amines, and then, transforming them into bromoisobutyryl functionalities and azo initiator sites via consecutive reactions with α-bromoisobutyric acid (BIBA) and 4,4′-azobis(4-cyanovaleric acid) (ACVA), respectively. The subsequent step involved the growth of PPEGEEMA brushes from the surface of modified CNCs via S-RAFT polymerization, followed by the grafting of PDMAEMA via SI-ATRP. The polymerizations were sequentially performed in this way to avoid potential destruction of the azo initiator sites by the SI-ATRP processes. Free PPEGEEMA formed in solution via RAFT had a Mn of 4.7 kDa and D̵ of 1.2. After SI-ATRP, PDMAEMA chains were successfully cleaved from the CNCs by acid hydrolysis, yielding Mn in the range of 4.2–7.7 kDa and D̵ of 1.2–1.3, depending on the SI-ATRP reaction time between 5 and 30 min. However, under such conditions (64 wt % of sulfuric acid, 3 h, 45 °C), the reported size-exclusion chromatography (SEC) results likely represented a mixture of cleaved PDMAEMA and PPEGEEMA or their corresponding hydrolysates, e.g. PMAA. There is still much to learn about S-RAFT from CNCs, given the complexity of the RAFT process and the lack of knowledge on the kinetics of such polymerizations conducted from CNCs.
3.3.3. Surface-Initiated NMP
Similar to Cu-mediated SI-CRP, surface-initiated nitroxide-mediated polymerization (SI-NMP) mechanistically depends on the reversible activation–deactivation of propagating polymer chains by a deactivating species, in this case a nitroxide radical. In general, alkoxyamines, such as TEMPO and N-tert-butyl-N-[1-diethylphosphono(2,2-dimethylpropyl)] nitroxide (DEPN), are commonly used for SI-NMP. Roeder et al. were the first to use SI-NMP to graft polymers from CNCs. (143) The authors first functionalized CNCs with 4-(diethoxyphosphinyl)-2,2,5,5-tetramethyl-3-azahexane-N-oxyl, a.k.a. BlocBuilder, and subsequently grafted PMA and PMMA from CNCs in the presence of sacrificial BlocBuilder initiators (see Scheme 8). The grafting yields obtained were 75–80 wt %. The authors were unable to cleave the grafted PMA or PMMA from the CNC surfaces, despite several attempts. However, SI-NMP conducted in the presence of sacrificial BlocBuilder allowed SEC analysis of free polymers. While free PMA showed Mn values from 3.3 to 43.7 kDa and D̵ of 1.35–1.4, the free PMMA had a Mn in the range of 30.8–140 kDa and similar dispersity, despite similar monomer conversion. These results suggested that higher sacrificial initiator efficiency was obtained for PMA.
The same group later employed the same approach to graft a variety of methacrylamides, such as DMAEMA, DEAEMA and N-[3-(dimethylamino)propyl] methacrylamide (DMAPMAM), from CNCs. (181) In addition to elemental analysis and thermal analysis, these CNC hybrids were characterized by cross-polarization magic-angle spinning (CP-MAS) 13C NMR spectroscopy. The latter technique revealed that the composition depended on the reaction time, however molecular weight and dispersity of the grafted polymers were not reported. The highest grafting yield was obtained for SI-NMP of DEAEMA from CNCs, i.e., 65 wt %. Overall, little is known regarding the kinetics of SI-NMP from CNCs in comparison to solution NMP, which leaves plenty of research opportunities for this field.
3.4. Polymer Grafting from the Reducing End Groups of CNCs
Individual cellulose chains contain chemically distinct end groups, commonly referred to as the nonreducing end and the reducing end. The directionality of cellulose chains is a direct result of the mechanism of the cellulose biosynthesis. (58) This reducing end can be reversibly converted between an aldehyde and a cyclic hemiacetal under a variety of conditions. Thus, site-specific, or “patchy” CNCs can be obtained by modifying this reducing end through aldehyde-selective reaction pathways. CNCs with end-tethered polymers chains present an interesting alternative to uniformly modified CNCs for certain applications, in which their self-assembly can be exploited toward structural hierarchies with unique properties, including both assemblies on surfaces and in nanocomposite materials. (8) Depending on the molecular weight of end-tethered polymers, one can see such anisotropic colloidal building blocks as somewhat analogous to rod–coil block copolymers. (182)
Different procedures have been established in order to attach a small molecule or initiator to the reducing end groups of CNCs. Arcot et al. reported a method to introduce thiol groups on the reducing ends of the CNCs by reductive amination in an aqueous medium. (183) The authors synthesized thiolated cellulose nanocrystals (CNC-SH) by using 6-amino-1-hexanethiol hydrochloride (NH2(CH2)6SH·HCl), a reducing agent (Cn, sodium cyanoborohydride; Ac, sodium triacetoxyborohydride; or Pc, 2-picoline-borane complex) and an aqueous CNC dispersion at different pH (5, 7 or 9.2) values. The CNC-SH were then functionalized with silver nanoparticles (AgNPs) in order to characterize the successful reducing end group modification, qualitatively using transmission electron microscopy (TEM) and quantitatively using XPS. The authors determined that the highest yield of functionalization was obtained when using basic conditions.
The same group later introduced a second thiolation procedure with shorter reaction times, milder conditions, and higher colloidal stability of the modified CNCs. (184) Usuda and coworkers had previously demonstrated the selective oxidation of reducing ends to carboxylates with sodium chlorite (NaClO2), which gives access to a wider variety of potential modification chemistries. (185) Thus, the reducing ends were first reacted with NaClO2 to generate carboxylate functionalized CNC reducing ends (CNC–COOH). After this oxidation step, a mild activation step of the carboxyl group was carried out mediated by N-hydroxysuccinimide (NHS) and N-(3-(dimethylamino)propyl)-N-ethylcarbodiimide hydrochloride (EDC), which was followed by a nucleophilic substitution with NH2(CH2)6SH·HCl, resulting in CNC-SH.
Gruber et al. proposed a different reducing end group functionalization strategy via hydrazone linkages. (186) The authors used 4-hydrazino-benzoic acid or radical initiators 4,4′-azobis(4-cyanopentanoic dihydrazide) and 4-carboxyphenylazomethylmalonodinitrile to form the hydrazone linkage, which resulted in a phenylcarboxylate terminated CNC with a yield of 44.6%. Similar yields with the conversion of 4-hydrazinobenzoic acid were obtained for the other initiators. The authors hypothesized that the low yield was due to the poor accessibility of the reducing end groups. If one considers individualized CNCs to consist of 18-chains, (58) this would imply a maximum of 18 reducing end groups at one end of the cellulose I crystal. Given the rod-like shape of CNCs, the reducing end-groups are likely distributed heterogeneously. This may lead to varying degrees of reducing end group accessibility for chemical reactions, however this remains to be explored.
Gruber and coworkers grafted PAM by SI-FRP from different azo initiators discussed in section 3.2 and shown in Table 1. (186) The molecular weight of the polymers were determined by SEC measurements after degrafting the PAM from the CNCs. The authors observed high molecular weights for PAM, but only a small amount of the initiators reacted to yield the polymer-grafted CNCs. Initiator efficiencies for surface-initiated polymerizations are reported to be very low, e.g., 5–30%. (28)
Table 1. Polymerization of Different Monomers from the CNC Reducing Ends (172,186)
Recently, Zoppe et al. reported a route to functionalize the end groups with an SI-ATRP initiator containing α-bromoisobutyryl groups. (172) The reducing ends were first oxidized with NaClO2 and further modified with ethylenediamine (EDA), following the work of Arcot et al. (184) Subsequently, primary amine groups were reacted with 2-bromoisobutanoic acid N-hydroxysuccinimide ester (NHS-BiB) to yield the ATRP initiator-functionalized reducing end groups. Although the authors were unable to quantify the success of the initiator modification reactions with conventional techniques, such as FTIR, XPS and elemental analysis, they subsequently grafted a variety of polymers from the functionalized reducing end groups of CNCs via SI-ATRP. The monomers investigated included NIPAM, METAC, and 4-SS. Thermogravimetric analysis (TGA), FTIR, and XPS were used to qualitatively analyze the successful grafting of the CNCs. In order to directly visualize the selective modification of CNC reducing ends with grafted polymers, patchy CNC/PMETAC hybrids were labeled with gold nanoparticles (AuNPs) and observed by cryo-TEM (Figure 7). This image supports the point that the authors successfully achieved the preferential modification of the CNC ends, but nevertheless, optimization of the reaction conditions for both the end group functionalization and the AuNPs adsorption is necessary in future work, since it was clear that not all CNCs were successfully functionalized. Moreover, better characterization techniques are needed to quantify the number of initiators on the CNC reducing end groups.
Figure 7
Figure 7. Cryo-TEM micrograph of patchy CNC/PMETAC hybrids labeled with AuNPs. Reprinted with permission from ref (172). Copyright 2017 American Chemical Society.
4. Emerging Applications and Properties of Polymer-Modified CNCs via “Grafting from”
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The rapidly growing academic and commercial interest in CNCs and hybrid nanomaterials thereof has driven their suggested use in unprecedented applications. (3,5,7,8,24,26,92) These include Pickering emulsions, nanotemplates for functional materials, supercapacitors, biosensors, drug delivery, and enantioselective catalysis. Here, we will focus on some of the emerging applications of polymer-modified CNCs, in which the grafting from approach, where high grafting density and molecular weight, is a priori advantageous over other polymer grafting strategies. In the following sections, applications in hybrid colloidal liquid crystals (LC), stimuli-responsive nanohybrids, nanocomposites and hydrogels as well as their unique properties will be discussed.
4.1. Self-Assembly, Hybrid Colloidal Liquid Crystals, and Optical Properties
CNCs have the intriguing ability to form chiral nematic LC phases, which depend on several factors, notably aspect ratio, surface charge density, ionic strength, external fields and solvent conditions. (31) Above a critical concentration, CNCs undergo a phase transition from an isotropic phase to a chiral nematic (cholesteric) liquid-crystalline phase (Figure 8). (187) The rods align preferentially with their long axes along a director. Each director of every pseudolayer (defined as an infinitesimally thin layer taken perpendicularly to the helicoidal axis) is slightly rotated around the helicoidal axis for each subsequent layer. The vertical distance required to complete a 360° rotation of the director is defined as the pitch. (188,189) When the suspension is left to dry on a substrate, the chiral nematic structure that forms upon increasing sample concentration is maintained after complete evaporation of the solvent, resulting in a film whose pitch dimension decreases to the submicrometer scale. (31) By exerting control over the drying conditions, the film can be homogeneous in thickness while retaining the long-range order of the CNCs. (190) Thus, the film behaves as a photonic crystal, and only light that is circularly polarized with the same handedness as the colloidal helices is reflected, in this case left-handed circularly polarized light. (188) These attractive properties of the self-assembled CNCs give a wide variety of interesting applications, such as devices for optical encryption, sensors, mesoporous chiral nanotemplates, and security coatings. (188,189,191,192)
Figure 8
Figure 8. Schematic representation of the self-assembly of a CNC suspension upon evaporation to form a structurally organized array. (A) Phase diagram showing the transition from isotropic to cholesteric phase upon increase of CNC concentration. (B) Atom force microscopy image of individual CNCs. (C) Polarized optical microscopy (POM) image of a typical fingerprint pattern of the cholesteric phase. Reprinted with permission from ref (31). Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.
Grafting polymers from the surface of CNCs, without a doubt, will affect their assembly behavior. Self-assembly of polymer-tethered CNCs is of great interest since more complex assemblies can be targeted through entropy-driven processes. Furthermore, their self-assembly can be potentially obtained in a variety of solvents, since the dispersibility of the CNCs can be tuned by surface modification. Such grafted-CNCs could give rise to a variety of CNC-based functional materials and templates based on colloidal LC assemblies. Nevertheless, self-assembly of polymer-grafted CNCs has not been widely explored to date, since the grafts likely alter the packing density of the CNCs, where they may behave more like spherical objects upon reaching a certain graft molecular weight and grafting density. (193) However, by using grafting to approaches, Jean et al. showed that polyetheramine-modified CNCs self-assembled into chiral nematic phases, albeit at a lower critical concentration in comparison to unmodified CNCs. (194)
Zhang and co-workers investigated the thermotropic and lyotropic LC phase behavior of CNC-g-PMMAZO, (151) CNC-g-PSty, (141) and CNC-g-PDMAEMA (152) (Figure 9). The CNC-g-PMMAZO showed a smectic to nematic transition at 95 °C and a nematic to isotropic transition at 135 °C in the absence of a solvent. (151) Furthermore, for these grafted CNCs, the POM image reveals a lyotropic LC phase in chlorobenzene above a concentration of 5.1 wt %. The authors reported a chiral nematic liquid crystalline behavior for the CNC-g-PSty, in both the lyotropic (DMF) and thermotropic states. (141) In the latter case, chiral nematic phases were observed upon cooling CNC-g-PSty to 163 °C, highlighting the potential of generating thermotropic LC phases based on CNCs grafted with nonmesogenic polymers. The authors later grafted thermosensitive PDMAEMA from CNCs. (152) They observed the fingerprint pattern for the CNC-g-PDMAEMA in the anisotropic phase, indicating chiral organization in aqueous media. The spacing of the fingerprint lines in aqueous suspensions could be tuned by changing the temperature. Below the lower critical solution temperature (LCST) of PDMAEMA, the polymer brushes existed in an extended conformation and therefore increased the spacing of the fingerprint, the opposite of which occurred above the LCST. Such systems are of great potential in developing colloidal analogues of stimuli-responsive molecular mesogens. The ability to externally manipulate the LC phases of CNCs in a reversible manner opens a number of interesting possibilities, including templating chiral 3D nanostructures, (192) optical sensors, (31) nanocomposite liquid crystalline elastomers (LCEs), (195) and multiresponsive supracolloidal systems. (196)
Figure 9
Figure 9. POM images showing (A) the fingerprint texture of pure CNC in water (3.1 wt %), and the fingerprint of different polymer-modified CNCs in their zone of anisotropic phase, i.e., (B) CNC-g-PMMAZO in chlorobenzene (5.1 wt %), (C) CNC-g-PSty in DMF (5.7 wt %), and (D) CNC-g-PDMAEMA in water (4.7 wt %, after 3 days) scale bar 10 μm for A, C and D and 100 μm for B. Adapted with permission from ref (141) (A, C) and ref (151) (B), copyright 2008 Elsevier Ltd., and ref (152) (D), copyright 2009, Springer Science+Business Media B.V.
4.2. Stimuli-Responsive Nanohybrids
Biopolymers in living organisms are ubiquitously involved in dynamic systems that promote structural changes in response to the conditions in their surrounding environment. (197) These properties are based on highly cooperative interactions that provide a significant driving force for the response caused by small external changes. Similar adaptive behavior can be imparted to synthetic polymers by incorporating functional groups in their backbone capable of responding to diverse external chemical (pH, ionic factor or chemical agents) or physical stimuli (temperature, electronic or magnetic fields, light or mechanical stress) (Figure 10). These are known as stimuli-responsive or smart polymers. By grafting such polymers from the surface of CNCs, hybrid nanomaterials bearing reversible interfacial properties can be fabricated.
Figure 10
Figure 10. Schematic representation of stimuli-responsive hybrids based on polymer-grafted CNCs. (A) Transitions in solution of stimuli-responsive polymer modified CNCs from well dispersed to aggregates or vice versa as response to an external stimulus, such as pH, temperature or carbon dioxide (CO2) atmosphere. (B) Thermally triggered triple shape memory behavior is achieved with poly(soybean amide methacrylate) (PSBAM) grafted CNCs dispersed in a PSBAM matrix in the presence of 4,4′-(1,4-phenylene)-bis(1,2,4-triazoline-3,5-dione) (bis-TAD) as cross-linker, where H-bonding between PSBAM chains is also at play.
pH-Responsive Hybrids
Polymers that undergo chemical and morphological changes as a result of pH variations are known as pH-responsive polymers. They usually contain functional groups that can be protonated, resulting in a transition from a hydrophilic to a hydrophobic state or vice versa, which may induce precipitation in aqueous solutions. CNC-g-P4VP was reported to display a completely reversible pH-responsive behavior in water. (132) P4VP is a well-known polyelectrolyte with an acid dissociation constant (pKa) of around 5, and is thus present in a hydrophilic protonated state at a pH of lower than 5, but it can be rendered hydrophobic at higher pH values due to the deprotonation of the pyridyl groups. Accordingly, suspensions of CNC-g-P4VP in water show good colloidal stability in acidic environments but flocculate above a pH of 5, due to the change from hydrophilic to hydrophobic character of the grafts.
In a different study, CNC-g-PAA also showed equivalent properties due to the nature of its brushes. (178) PAA is a weak polyelectrolyte, which undergoes pH-dependent conformation transition according to the ionization state of its carboxylic groups; i.e., the chains adopt a partial globular conformation in their protonated form (at low pH) and change to a coiled conformation when deprotonated at high pH. (198) The CNC-g-PAA suspensions showed lower UV–vis absorbance at pH values higher than 7 as consequence of the ionizations of the carboxylic groups present in the grafts, which promotes the complete solvation of polymer chains. (178)
pH variations of a colloid can also be promoted by its exposition to acidic or basic gases, such as CO2 and N2, respectively. In addition, it is known that polymers containing tertiary amines are easily protonated in the presence of water and CO2, which involves the formation of carbonic acid and an increase in the hydrophilicity character of the polymer. The combination of these two concepts was applied to CNCs by decoration with PDMAEMA, PDEAEMA and PDMAPMAM either via S-NMP (181) or SI-ATRP. (171) The CO2-responsiveness of these systems was evaluated by determining the ζ-potential versus pH in alternating CO2/N2 exposure cycles (Figure 11). In all cases, high ζ-potential values were observed for stable dispersion when grafts were protonated under CO2 atmosphere (low pH), while low ζ-potential values were reported when the dispersion flocculated in the presence of N2 (high pH). From the three hybrids studied, CNC-g-PDEAEMA clearly demonstrated the pH-switchability of the system due to its sole solubility in water under its protonated form, i.e., in the presence of CO2.
Figure 11
Figure 11. ζ-Potential and pH at room temperature of aqueous dispersions (1 mg·mL–1) of (A) CNC-g-PDMAEMA, (B) CNC-g-PDEAEMA and (C) CNC-g-PDMAPMAM in alternating CO2/N2 exposure cycles. Adapted from ref (181). Copyright 2017, Royal Society of Chemistry.
Temperature-Responsive Hybrids
Thermosensitive polymers respond to a change in their solvation state upon heating or cooling. According to the way the measurement is performed, the reported value should be cited as cloud point, upper (UCST) or lower critical solution temperature (LCST). The cloud point is an experimentally measured value at which the first appearance of turbidity or cloudiness are observed due to the phase separation of the components of a mixture as a response of a stimulus (i.e., temperature). (199) When plotting these cloud-point temperatures (CPT) vs the overall polymer concentration, the cloud-point curve is obtained, where the minimum point is known as the precipitation threshold (or LCST). (200) In this sense, the term LCST only refers to the lowest temperature at which phase separation can happen.
Decorating CNCs with thermosensitive polymers can lead to hybrid materials with temperature-responsive features. CNC-g-PNIPAM has demonstrated thermal behavior due to the ability of PNIPAM to undergo a coil-to-globule transition at about 32 °C. (164,178,201) Zoppe et al. reported for the first time the thermoresponsive behavior of such materials as a function of grafting density and degree of polymerization of PNIPAM, which was prepared by SI-ATRP. (201) In this study, suspensions of CNC-g-PNIPAM were shown to respond to temperature changes between 30 and 32 °C (CPT) by a significant increase in their viscosity (Figure 12A). The CPT of the hybrid materials was shown to approach the LCST of the free polymer as the grafting density was increased. In addition to the nature of the grafts, the CPT was also influenced by the differences in the steric repulsive forces between adjacent polymer-modified CNCs. The latter was also shown to affect the size of aggregates at temperatures higher than the LCST of free polymer. In the same study, the CPT of these materials also was shown to decrease upon increasing the ionic strength (i.e., by varying the concentration of NaCl), as is the case of free PNIPAM in aqueous solution (Figure 12B).
Figure 12
Figure 12. Thermoresponsive behavior of CNC-g-PNIPAM. (A) Viscosity of CNC-g-PNIPAM with various graft densities (i.e., VHD, very high; HD, high; MD, medium; and LD, low densities) at different temperature normalized by the viscosity of the corresponding dispersion at 25 °C. (B) LCST of aqueous dispersions of medium density CNC-g-PNIPAM at various NaCl concentrations. Adapted from ref (201). Copyright 2011, American Chemical Society.
In another study, CNC-g-PNIPAM also demonstrated a sol–gel phase transition at about 34.3 °C, which may be exploited for embolization of hepatocellular carcinoma. (164) The weight-average molecular weight (Mw) of these grafts was more than 10 times lower than the one reported by Zoppe et al. (201) This indicates that the molecular weight of the grafts has a direct influence on the thermal behavior of the hybrid nanoparticles. Furthermore, Haqani et al. also reported the thermoresponsiveness of CNC-g-PNIPAM with a CPT around 29 °C. (178) Such differences in CPTs are often associated with differences in molecular weight, which is known for PNIPAM chains in solution, (202) and the grafting density of PNIPAM brushes.
CNC-g-PNVC was also reported to display a thermally induced phase transition above 36 °C from a stable colloidal aqueous suspension to collapsed PNVC brushes due to a change of the hydrophilicity of the grafts. (161) CNC-g-POEGMEMA (159) and CNC-g-poly(ethylene glycol)methyl acrylate (160) have exhibited tunable thermal behavior (between 23.8 and 63.8 °C and between 34 and 66 °C, respectively); the respective cloud points could readily be tailored by adjusting the comonomer ratio in the grafted polymers. In all cases, the transition was confirmed by changes in the viscoelastic properties of the materials.
A thermoresponsive enhanced fluorescence hybrid based on CNC-g-P(NIPAM-co-EANI) was recently synthesized. This nanomaterial reacted to temperature variations while displaying a simultaneous increase in its fluorescence intensity. (154) The latter was attributed to the thermoresponsiveness of PNIPAM brushes, which altered the distance between the dye groups.
Dual pH- and Temperature-Responsive Hybrids
Contributions of polymeric chains bearing different stimuli-responsive properties can result in copolymers with multiple response functions. In the domain of polymer-grafted CNCs, the first example of hybrids reporting such dual-responsiveness was CNC-g-P(NIPAM-ran-AA) containing different amounts of AA, which responded to pH and temperature variations. (142) The CPT of this system was tuned by three factors: polymer tethering to CNCs, AA content, and pH (Figure 13). The CPT increased from 38 to 45 °C at high pH (ca. 7–8) while it decreased from 30 to 18 °C at lower pH values (ca. 3–4), when the AA content of the grafts was increased from 5 to 30 mol %. The reduction observed in the CPT was mainly attributed to the hydrophobic behavior of AA in protic media. In addition, a comparison between free and CNC-grafted copolymers demonstrated that tethering copolymers to CNCs resulted in an increase up to 6 °C in the CPT, with respect to the LCST of free copolymer.
Figure 13
Figure 13. LCST diagram of 0.2 wt % aqueous suspensions of P(NIPAM-ran-AA) as free copolymer (red) or grafted to CNCs (blue) bearing different AA content at low and high pH values. Reprinted with permission of ref (142). Copyright 2014, Royal Society of Chemistry.
Using the same monomers, albeit with block rather than random distribution of comonomers and varying the inner and outer block, Haqani et al. also reported thermoresponsive systems. A dual-sensitive characteristic was only observed when the PAA was at the outer block, i.e. CNC-g-PNIPAM-block-PAA. (178) Furthermore, Tang et al. developed a hybrid, containing POEGMA and PMAA grafted-CNCs, in order to control the stability of particle-stabilized emulsions manipulated by pH and temperature. (135) PMAA brushes formed hydrogen bonds with POEGMA at low pH values (<4.5), leading to the formation of aggregates, while the presence of electrostatic repulsions at higher pH disrupted hydrogen bonding to yield stable dispersions. Additionally, POEGMA chains exhibited temperature-dependent conformational changes that influenced the stability of the emulsion.
Shape-Memory Hybrids
Shape-memory polymers (SMPs) are those capable of being deformed and returned to their original state through a specific stimulus, for example, via thermal transitions or activation–deactivation of supramolecular interactions, often in elastomeric systems. Wang et al. proposed a shape-memory nanocomposite derived from biomass, i.e., soybean oil and CNC. (156) The system was composed of CNC-g-PSBAM (synthesized via SI-ATRP with a theoretical Mn = 22 kDa) and bis-TAD using PSBAM as a matrix (Figure 10B). The network of the hybrid was formed by using two types of cross-links within the material: one covalently linked by bis-TAD and another dynamic physically cross-linked via hydrogen bonding with the plant oil functional groups (and also the bis-TAD moiety). Here, the covalently bonded network was responsible for the permanent shape of the material, while the physical network provided multiresponsive and multi shape-memory properties. The material showed triple-shape-memory with two recovery steps performed at staged temperatures of 40 and 80 °C, respectively. Additionally, water and THF induced the shape-recovery of the material within 30 and 5 min of immersion, respectively.
The properties exhibited by the above-mentioned stimuli-responsive nanohybrids are mainly attributed to the type of polymer grafted on the CNC surface. Thus, the main function of the CNCs is to provide either mechanical reinforcement (156) to the material or enhance their solubility (142) in polar solvents due to the large amount of −OH groups at the surface of CNCs. Most of these investigations have been focused on the synthesis of stimuli-responsive materials and their behavior upon application of different stimuli, but little is known about the minimum amount of grafted polymer required to observe such stimuli-responsive behavior. Furthermore, it is in many cases unclear how the grafting density of the polymer influences the responsiveness of the material.
4.3. Nanocomposites
Nanocomposites are materials composed of two or more components, often a matrix and a filler, in the solid or gel-like state, the filler being on the nanoscale. There are different processing routes reported in the literature for the production of cellulose nanocomposites, such as solution casting, melt extrusion, compression molding, injection molding, ball milling, or even precipitation methods. (4,5,7,203) The outstanding mechanical properties and the high specific area suggest that CNCs would provide good reinforcement to other polymeric materials. The reinforcement potential of CNCs was first reported by Favier et al., who incorporated tunicate-derived CNCs into a copolymer of Sty and BA (P(Sty-co-BA)). (204,205) The reinforcement effect of CNCs is often explained by the formation of a rigid percolating network in which the filler nanoparticles interact via hydrogen bonding. (5) The experimental evidence and statistical models stipulate that the matrix, the filler characteristics, such as aspect ratio, dispersion and volume fraction, and their interactions (filler–matrix and filler–filler) all influence the mechanical properties of the nanocomposites. (3)
Even though unmodified CNCs have been reported to offer reinforcement in some materials, (204−206) the hydrophilicity of the CNC surface makes their dispersion difficult in polymer matrices, especially those that are nonpolar or hydrophobic. In addition, the numerous −OH groups at their surface promotes the agglomeration of CNCs via hydrogen bonding limiting the extent of mechanical reinforcement. In that regard, surface modification of CNCs is a viable approach to create good interfacial adhesion between CNCs and a (hydrophobic) polymer matrix, ideally without compromising the intrinsic properties of the CNCs and their filler–filler interactions. Many efforts have focused on the modification of the CNC surface in order to enhance their compatibility with hydrophobic media. (4)
The first nanocomposite containing polymers grafted from the surface of CNCs was reported by Habibi et al. in 2008, based on CNC-g-PCL dispersed in a PCL matrix. (30) Nanocomposites of PCL-grafted CNCs incorporated into PCL, (30,80,88,207,208) PLA, (81) PCL/PLA 50/50 w/w %, (104) poly(butylene succinate-co-adipate) (PBSA), (82) and segmented polyurethane (PCL/PU) (209) matrices have also been investigated. The above-mentioned systems generally reported good filler–matrix compatibility due to the hydrophobicity of modified CNCs, with the exception of the PCL/PU matrix, (209) where microphase separation was observed. For example, the incorporation of 2 wt % of CNC-g-PCL in PCL matrix showed an increase on the storage modulus (E′: going from 1.95 to 2.6 GPa and 0.52 to 0.74 GPa at 30 and 80 °C, respectively) attributed to the gain in rigidity of the matrix through restriction of the mobility of the brushes (Figure 14A). (207)
Figure 14
Figure 14. Reinforcement on the mechanical properties of nanocomposites by the incorporation of polymer-grafted CNCs. (A) Pictures of neat PCL (top left) and 1 wt % CNC-g-PCL/PCL (bottom left) and the storage modulus of CNC-g-PCL/PCL nanocomposites with different wt % of the hybrid (right). (B) Scanning electron microscopy (SEM) images of electrospun mats of neat PCL (top left) and 5 wt % CNC-g-PCL/PCL (bottom left), and the elastic modulus (E) and tensile at break (σb) of samples bearing up to 5 wt % of CNC-g-PCL. Reprinted with permission from references (207) (A) and (80) (B) Copyright 2016, John Wiley and Sons.
In another study, the reinforcement effect of CNC-g-PCL was confirmed by 140% increase of Young’s modulus (E) when 2 wt % or a higher amount of modified CNCs were added to PCL/PU matrix. (209) The incorporation of homopolyester (PCL or PLA) or diblock copolymer (PCL-block-PLA) grafted-CNCs into PCL or PCL/PLA matrices enhanced the shear storage modulus (G′), recorded at low-frequency, by a factor 10–100 relative to the PCL/PLA blend loaded with unmodified CNCs. (104)
Electrospun nanocomposites, based on CNC-g-PCL in PCL matrix, also demonstrated an improvement in stiffness (from 4 to 7 GPa) and strength (from 1.6 to 2.3 MPa) upon an increase in modified CNC content from 0 to 5 wt % (Figure 14B). (80) The improvements in mechanical behavior were attributed to the addition of modified CNCs in the nanofiber and the higher molecular weight of the grafted PCL chains, which perhaps cocrystallized with the PCL nanofiber matrix.
The effects of incorporating polymer-modified CNCs in polymer matrices were not only observed by mechanical testing, but also by DSC. For PLA reinforced by CNC-g-PLA, a reduction between 3 and 5 °C in thermal stability, with respect to neat PLA, was observed and attributed to either a plasticizing effect of short PLA chains (88) or residual catalyst from the polymerization step. (90,91) However, this was not the case when d-LA was used as the monomer and loaded up to 5 wt % CNC-g-PLA in a PLA matrix. (109) The incorporation of CNC-g-PLA in poly(β-hydroxybutyrate) (PHB) resulted in a substantial reduction of the crystallization temperature of PHB varying from 2 to 8 °C with respect to the neat polymer, while the lamellar structure of the latter was unaffected. (113) In another study, the grafting of P(MMA-co-BA) from the surface of CNCs resulted in self-reinforced thermoplastic elastomers, whose Tg increased by a remarkable 19 °C in comparison to the free copolymer, as a consequence of the restricted mobility of the copolymer brushes on the surface of CNCs. (169)
Polymer-grafted CNCs used in the preparation of nanocomposites have been mainly synthesized via SI-ROP. Nevertheless, some examples have also been reported for SI-FRP (94,133) and SI-ATRP. (157) PGMA (133) and PSty (94) brushes showed a contribution in the increase of compatibility with PLA matrices. In addition, CNC-g-PGMA also showed a slight increase in the tensile strength (from 47.8 ± 0.6 to 52 ± 2 MPa) compared to equivalent nanocomposite made with unmodified CNC in PLA matrix. (133) In another study, CNC-g-PSty was incorporated in PMMA matrix exhibiting better dispersibility and thermal stability (50 °C higher than CNC/PMMA nanocomposite). (157) It is worth mentioning that the reinforcement effect in nanocomposites based on polymer-grafted CNCs cannot always be explained by percolation theory, due to the disturbance of hydrogen bonding interactions between chemically modified CNCs, which may hinder their percolation. (5) Therefore, the reinforcement may involve other factors, such as better interfacial adhesion between filler and matrix or cocrystallization of grafted polymers with the polymer matrix.
With the aim to fabricate viable alternatives to petroleum-based composites, PLA-modified CNCs have gained interest due to their biodegradability, renewability and nontoxicity. Biodegradable CNC-based nanocomposites containing CNCs grafted with homo- (88,90,91,104,106−109,133) and random copolymers (105) dispersed in PLA matrices have been reported. As common characteristics, these all showed good interfacial adhesion between modified-CNCs and the matrix, stereocomplexation from cocrystallization with the matrix, efficient nucleation induced by grafted CNCs, as well as enhancement in the mechanical properties.
A new class of nanocomposites based on polymer-modified CNCs, referred to as one-component nanocomposites (OCNs) or cocontinuous composites, has recently started to attract attention. In contrast to conventional composites these materials feature (as the name implies) only one component, i.e., CNCs from which a significant amount of polymer chains have been grafted that serves as the “matrix”. As the polymer is covalently attached to CNCs, these systems do not typically suffer from phase separation or mixing problems, like conventional nanocomposites, while beneficing properties to which both components contribute.
The first example was reported by Chen et al. through the grafting of PCL chains from the surface of CNCs, via SI-ROP, as a thermoformable material. (112) The molecular weight of the grafts was unknown, however the content of the polymer brushes was calculated to be between 92 and 96 wt % for CNC-g-PCL, depending on the reaction time and the two different microwave irradiation powers (160 and 320 W) employed. In addition, a nonlinear dependence was observed in the mechanical properties of these OCNs, indicating that the reaction conditions may influence the number and length of the grafted chains.
Recently, another study showed that OCN materials based on CNC-g-PMMA displayed properties superior to equivalent conventional two-component nanocomposites for two different compositions, i.e., 10 and 20 wt % of CNC. (93) For example, the E′ at 25 °C of CNC10-g-PMMA was 3168 MPa versus 3018 for 10% w/w PMMA/CNC conventional nanocomposite. The authors suggested that the stress-transfer is mediated by the polymer chains attached to the CNCs. Indeed, OCNs are promising materials due to their easy-to-process characteristics, which also offer a unique combination of high stiffness, strength, and toughness. (93)
4.4. Hydrogels
Hydrogels are soft cross-linked three-dimensional networks mainly based on hydrophilic polymeric materials that have the capability of holding large amounts of water or biological solutions. Such soft materials have many potential applications, especially in the biomedical field. (128) The hydrophilic character and the intrinsic properties of CNC make them an attractive candidate for the formation of reinforced or structured hydrogel nanocomposites. (210) Hydrogels based on polymer-grafted CNCs have generally reported enhanced mechanical properties due to the critical role of CNCs acting as multifunctional cross-linkers and bridges for the neighboring polymer chains.
Zhou et al. reported the synthesis of hydrogels from CNC-g-PAM in the presence of NMBA as cross-linker. (128) The grafted CNCs accelerated the formation of hydrogels and increased the effective cross-link density of the material. The interfacial interactions between PAM and CNCs were significantly improved and the reinforcement effect was confirmed by an increase in the compression stress of 2.5-fold for hydrogels containing 6.7 wt % of CNC-g-PMA. Later, a three-component supramolecular nanocomposite hydrogel showing high stiffness, rapid sol–gel transition and self-healing properties was reported. (166) The system was composed of poly(vinyl alcohol) (PVA) containing methyl viologen functionalities, CNC-g-P(DMAEMA-ran-NpMA) and cucurbit[8]uril (CB[8]) supramolecular cross-links, where CB[8] (host) selectively and simultaneously binded methyl viologen- and naphthyl-based guest units under stoichiometric molar ratio (Figure 15).
Figure 15
Figure 15. Three-component CNC-reinforced hydrogel based on (a) PVA containing methyl viologen functionalities as first-guest units, (b) CNC-g-P(DMAEMA-ran-NpMA) bearing NpMA units as second-guest and (c) the CB[8] host motif. The latter shows simultaneous binding of the guest entities resulting in the formation of (d) a dynamic hydrogel with supramolecular cross-linking. Reprinted with permission from ref (166). Copyright 2014, John Wiley and Sons.
Even though the use of external cross-linkers is a known practice in the synthesis of CNC-based hydrogels, it can be avoided by designing a CNC-based system containing building blocks capable of physically or chemically cross-linking in three-dimensional networks. The first example of such systems was based on CNC-g-PMA. (138) This hydrogel presents elastomeric features, due to the properties of the PMA brushes, and high toughness, thanks to a unique core/shell network structure, where CNCs behave as multifunctional cross-links, resulting in unique mechanical properties.
Recently, an elastic macroporous composite hydrogel was prepared via aqueous phase polymerization of CNC-g-PUPyMA containing acrylamide and gelating methacrylate. (123) Their swelling ratio, pore size and mechanical strength were tunable by controlling the incorporated concentration of CNC-g-PUPyMA. The material achieved a swelling ratio of 70% within 10 min, while the pore size was reduced from 62 to 30 μm by increasing the concentration of modified CNCs from 0.1 to 0.5 wt %, respectively. The mechanical strength also significantly increased at higher CNC content, about 50 kPa when the concentration of CNC-g-PUPyMA was varied from 0.1 to 0.5 wt %. In addition, these materials exhibited good biocompatibility and promising cell adhesion and proliferation under in vitro cell culture.
Furthermore, a fluorescent nanohybrid hydrogel for water detection was developed by Wu et al. (Figure 16). (124) The structure was composed of a cross-linked CNC-g-PAA network in which Eu2+/Dy3+-doped SrAl2O4 NPs were fixed by the polymer chains through coordinate bonds preventing the aggregation and quenching in water. The fluorescence intensity of the material showed a direct linear relationship with water absorption and an excellent fluorescence stability, making them a suitable sensor material for water detection. In the domain of injectable hydrogels, CNC-g-PNIPAM could be potentially used for wound dressing due to the good drug-loading capacity at room temperature and a burst drug release at 37 °C, as shown by Zubik et al. (129)
Figure 16
Figure 16. Hydrogel based on phosphorescent Eu2+/Dy3+-doped SrAl2O4 and CNC-g-PAA (top) showing fluorescent properties just after water absorption (bottom left), which get reduced with time (25 min later, bottom right). Adapted from ref (124). Copyright 2017, Springer Nature.
5. Summary and Outlook
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Since the seminal work of Habibi et al. in 2008 on SI-ROP of ε-CL from CNCs, (30) the scientific interest in utilizing surface-initiated polymerization from CNCs as a means to tailor their properties has manifested itself in over one hundred publications. Most investigations have dealt with the synthetic protocols to modify CNCs with high density polymer brushes, an objective appropriately reached using grafting from techniques. Upon a review of the literature, it appears that each of these approaches has distinct advantages and disadvantages regarding number of reaction steps, conditions and polymer brush characteristics.
SI-ROP, for example, is advantageous in that it does not typically require the prior immobilization of initiators on the CNC surface, as SI-ROP can be conducted directly from the CNC −OH groups albeit in a dry organic solvent. Although there is only very limited information regarding the molecular weight and dispersity of such polymer grafts, relatively short polymer chains have been reported, e.g., DP = 12. (107) In contrast, high polymer contents (up to 96 wt %) were reported by others, (112) which could suggest high grafting density, high molecular weight, or both were obtained.
Analogously, SI-FRP can also be utilized without the prior immobilization of initiators on CNC surfaces at higher temperatures, however it is assumed that higher molecular weights are obtained, based on bulk FRP kinetics. In addition, the use of radical initiators attached to the surface of CNCs may be an alternative when high DP is desired despite the poor control over the molecular weight. (93) As many researchers have focused on using SI-FRP in order to produce nanocomposite hydrogels, (123,124,128,129,137,138) little is known about the molecular weight and dispersity of the grafted polymers.
SI-CRP approaches can be used to grow polymer brushes with controlled molecular weight and dispersity, however usually require an appropriate initiator or CTA to be attached to the CNC surface beforehand. An exception to this is an S-RAFT approach in which the polymerization is conducted in the presence of a free macro-RAFT agent and initiated by CAN. (96,180) Although initiator immobilization is a prerequisite to conduct Cu-mediated SI-CRP from CNCs, it has been shown that such initiators can be covalently attached in situ during the sulfuric acid hydrolysis process to produce functionalized CNCs. (95) In these Cu-mediated systems, the inherent surface chemistry of the CNCs, i.e. sulfate half-ester groups, also plays a role in the polymerization kinetics when performed in aqueous media, due to the formation of electrical double layers of Cu/ligand species. (173) Regarding SI-NMP, the synthetic strategy is simple in that, once the NMP initiator is grafted to the CNC surface, only monomer and heat are required to initiate polymerization. (143,181) Although information on the polymer graft characteristics is still lacking, free polymers synthesized in situ during SI-NMP had Mn values in excess of 100 kDa. (143) In most cases of SI-CRP, molecular weight >100 kDa may be obtained with the appropriate reaction conditions. In general, there are few reports on the analysis of polymers cleaved from the surface of CNCs in order to accurately determine their molecular weight and grafting density, (96,115,154,163,176) suggesting that there is much to be understood regarding polymer brush growth kinetics from CNCs compared to analogous solution polymerizations.
As the kinetics of surface-initiated polymerizations from other types of planar and curved interfaces are very often reported to differ significantly from solution polymerizations, (28) this also likely applies to polymer brush growth from CNCs and will depend highly on the polymerization technique, monomer, solvent, reactions conditions, etc. Utilizing cleavable functional groups for covalent attachment of initiator species (28,115) and applying new techniques to determine polymer brush grafting density (211) are promising approaches to address this challenge.
In addition to the development of appropriate strategies for the chemical synthesis of polymer-modified CNCs and fundamental investigations of polymer brush growth kinetics, a majority of these research efforts have focused on exploiting the unique thermomechanical properties of CNCs as a reinforcing filler in hydrophobic polymer matrices through surface functionalization. Such nanocomposites have been primarily processed by solution casting and various forms of melt processing.
A few works have sought to fabricate nanocomposites that do not suffer some of the processing limitations associated with blending hydrophilic CNCs with hydrophobic polymers by removing the polymer matrix component altogether; so-called OCNs. (93,112) These systems are advantageous in that they are less affected by phase separation of the components and can offer superior mechanical properties provided the molecular weight of polymer grafts is sufficient to allow chain entanglements between neighboring hybrid nanoparticles. One of the challenges in this regard is obtaining such high molecular weight polymer grafts without compromising the structural integrity of the CNCs. Due to the limited molecular weight that is often obtained using SI-ROP, (107) it may not be ideal for this type of application. In comparison to SI-CRP, it is already clear that this goal is probably best realized through SI-FRP techniques, since good control over polymer graft dispersity may not be as critical. However, there are many open research questions; what is the relationship between internanoparticle chain entanglements, anisotropy, molecular weight and grafting density? How does the dispersity of grafted polymers affect the thermomechanical properties of OCNs of rod-shaped nano-objects? Furthermore, one can imagine OCNs composed of block or statistical copolymer grafts with dissimilar, e.g., hard and soft phases, which might show interesting phase separation behaviors.
From the standpoint of LC research, anisotropic colloidal hybrid materials are useful models for elucidating the relationship between building block properties and self-assembly. Among this class, CNCs are unique in that they retain their chiral LC assemblies in the solid-state upon evaporation-induced self-assembly (EISA), which has opened new doors for the preparation of mesoporous inorganic solids with long-range chiral order. (191) Such chiral materials have great implications in enantioselective separations, asymmetric catalysis and chiral optics. Thus, far, such templating processes have been mostly limited to aqueous systems, and this puts a number of restraints on the types of chiral mesoporous solids one can fabricate. Moreover, the pitch of chiral nematic phases of CNCs is sensitive to the presence of additives, pH, ionic strength, external fields, and evaporation rate, due to the complex interplay between surface charge density, CNC chirality, aspect ratio, and dispersity. (31,212−214) Such factors have often been employed as a means to control the optical properties of CNC LC phases both in solution and in free-standing films. However, these means are not typically reversible processes, which are better realized by incorporating stimuli-responsive polymer systems.
In this regard, surface chemical modification with tethered polymers has not been widely explored. This is probably due to the disturbance that the macromolecular layers could cause on the self-assembly process. Nevertheless, simulations have shown that this effect is dependent on the packing density of the polymer tethers on the rods. (215) Therefore, selective modification at the reducing ends of CNCs could allow sufficient packing density and the formation of chiral nematic phases. Even though there is no experimental data for the formation of chiral nematic phases with such polymer end-group modified CNCs, Horsch et al. reported theoretical descriptions on chiral nematic phases of achiral end-tethered nanorods. (215−217) Furthermore, as described in section 3.4, Zoppe et al. proposed the starting point for symmetric (perhaps using CNCs from the cellulose II polymorph) and asymmetric CNCs (cellulose I polymorph) as new colloidal building blocks for self-assembled structures. (172) Moreover, much work remains in addressing some of the research challenges ranging from the synthesis of such polymer-modified CNCs to their self-assembly and applications.
In addition, CNC aspect ratio and polymer:tether length ratio will likely play a critical role in their self-assembly. Other questions include; what is the most promising approach to reduce the size distribution of CNCs? How does cellulose polymorph structure affect their native chiral morphology and assembly? Will end-tethered polymers allow polymer-modified CNCs to form chiral LC phases without sulfate half-ester groups? Will they phase separate into defined morphologies? These challenges will be investigated in our future research and we encourage others in the field to join us on this invigorating research path. Overall, our hope is that such end group-modified polymer-modified CNCs will open new avenues toward the fundamental understanding of the processes that govern the self-assembly of anisotropic colloids and a wide range of new nanocomposites and functional materials with programmed chirality.
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00733.
Overview of all polymers grafted from CNCs cited in this Perspective summarized in a table by the polymerization technique employed, including substrate, monomer(s), reaction conditions, resulting properties, and applications (PDF)
Terms & Conditions
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Author Information
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- Justin O. Zoppe - Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, Switzerland;
http://orcid.org/0000-0002-3599-9227;
- Wim Thielemans - Renewable Materials and Nanotechnology Research Group, Chemical Engineering, KU Leuven, Campus Kulak Kortrijk, Etienne Sabbelaan 53, 8500 Kortrijk, Belgium;http://orcid.org/0000-0003-4451-1964
- Christoph Weder - Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, Switzerland;http://orcid.org/0000-0001-7183-1790
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Biographies
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Sandra Wohlhauser
Originally from Venezuela, Sandra Wohlhauser received her Master’s degree in chemical engineering from the University of Carabobo, Venezuela, in 2005. After a few years of professional experience, she returned to academia and completed a Master’s degree in chemistry from the University of Fribourg, Switzerland, in 2015 while studying the potential mechano-responsive behavior of poly(sulfonate ester)s. She then joined the group of Prof. C. Weder in the Adolphe Merkle Institute at the University of Fribourg as a Ph.D. student, where she is currently working on the surface modification of CNCs with polymer chains to develop unique one-component nanocomposites in the solid state, which combine high strength, stiffness, and toughness.
Gwendoline Delepierre
Gwendoline Delepierre obtained her Bachelor’s degree at KU Leuven, where she worked in The Renewable Materials and Nanotechnology Research Group under the supervision of Prof. Wim Thielemans for various research projects involving cellulose nanocrystals. During her Master’s studies, she did an internship at Cambridge University in the research group of Dr. Silvia Vignolini. She worked on a project in collaboration with Dr. Wadood Hamad from FP Innovations on size sorting of cellulose nanocrystals. She went to Uppsala University for her Master’s thesis to work on a collaboration between Dr. Lukazs Pilarski’s group in Biomedicinskt centrum (BMC) and that of Dr. Anders Thapper at Uppsala’s Ångström laboratory. She finished her Master’s program in KU Leuven and started her Ph.D. work in the Adolphe Merkle Institute in October 2017 under the supervision of Dr. Justin Zoppe and Prof. Christoph Weder.
Marianne Labet
Marianne Labet is a research associate working at KU Leuven (Belgium). She received her MEng in paper science, printing, and converting from the French Graduate School of Engineering in Paper, Print Media, and Biomaterials and her MRes in materials science and engineering from Grenoble INP in 2006. She obtained her Ph.D. in chemistry and chemical engineering from the University of Nottingham (U.K.) working on the SI-ROP of ε-caprolactone from polysaccharide nanocrystals under the supervision of Wim Thielemans. She then worked as a postdoctoral researcher with Chris Breen of the Materials and Engineering Research Institute at Sheffield Hallam University (U.K.) and with Xavier Coqueret of the Institut de Chimie Moléculaire de Reims at the University of Reims (France). Her research interests include cellulose based composites, the recycling of polymer-based materials and the development of materials for packaging applications.
Gaëlle Morandi
Gaëlle Morandi was born in Le Mans, France, and received her Ph.D. in polymer chemistry and physical chemistry in 2007 from the Université du Maine under the supervision of Prof. L. Fontaine. She then joined W. Thielemans’ group in Nottingham, U.K., as a postdoctoral fellow, to develop cellulose nanocrystals grafted with polymeric chains through a SI-ATRP process. Since 2009, she is an assistant professor in INSA Rouen Normandie. Her research interests include the controlled synthesis of stimuli-sensitive amphiphilic copolymers and their association into mixed micelles of tunable stability.
Wim Thielemans
Wim Thielemans is Professor, Odysseus Fellow, and Province Chair of Advanced Materials at KU Leuven, Belgium where he leads the Renewable Materials and Nanotechnology research group. Wim received his Master in Chemical Engineering from KU Leuven (BE) in 1999 and his Ph.D. in Chemical Engineering from the University of Delaware (USA) in 2004. After postdoctoral work as a Marie Curie research fellow at the School of Engineering in Paper, Print Media, and Biomaterials of the National Polytechnic Institute of Grenoble (France), he became a lecturer, then associate professor at the University of Nottingham (U.K.) in 2006. In 2013, Wim completed the circle by moving back to KU Leuven with an Odysseus fellowship from Research Foundation Flanders as associate professor and was promoted to professor in 2017. Wim is currently also an honorary professor with the University of Nottingham and an affiliate professor with the University of Auburn. His research group focusses on the development of renewable (nano)materials.
Christoph Weder
Christoph Weder is Professor for Polymer Chemistry and Materials and Director of the Adolphe Merkle Institute (AMI) at the University of Fribourg (Switzerland). He also directs the Swiss National Center of Competence in Research Bio-Inspired Materials. Chris studied chemistry at ETH Zurich where he also earned his doctoral degree. After a postdoctoral stay at MIT, he held positions at ETH Zurich and Case Western Reserve University, before joining the AMI in 2009. Chris’ current research efforts are focused on stimuli-responsive polymers and nanomaterials. He coauthored some 250 research papers and book chapters and is coinventor of 20 patents. Chris has as mentored about 70 Ph.D. students and postdoctoral researchers. He serves as an Associate Editor of ACS Macro Letters and as codirector of the Swiss Chemical Society’s Polymer and Colloid Division and is a member of the Swiss Academy of Technical Sciences and a Fellow of the American Chemical Society’s Division of Polymer Chemistry.
Justin O. Zoppe
Justin O. Zoppe was born in Flint, Michigan (USA) in 1983 and graduated from the University of North Carolina at Wilmington with a Bachelor of Science degree in chemistry and a minor in mathematics. He was awarded a Ph.D. in 2011 from the Department of Forest Biomaterials at North Carolina State University under the supervision of Prof. Orlando Rojas. He then joined the Polymer Technology group of Prof. Jukka Seppälä at Aalto University as a postdoc before working as a Research Scientist at Clariant Specialty Chemicals in Frankfurt, Germany, in 2013. From 2014-2017, Dr. Zoppe was an EPFL Fellow cofunded by Marie Curie in the Polymers Laboratory of Prof. Harm-Anton Klok at the Swiss Federal Institute of Technology in Lausanne. Dr. Zoppe moved to the Polymer Chemistry & Materials Laboratory of Prof. Christoph Weder at the Adolphe Merkle Institute, University of Fribourg, in 2017 as a Group Leader and Ambizione Fellow funded by the Swiss National Science Foundation. In addition to surface modification of cellulose nanomaterials, he is interested in the fundamentals of transition metal-mediated SI-CRP carried out in polar media, surface forces, and colloidal liquid crystal assembly.
Acknowledgments
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Financial support from the Swiss National Science Foundation (SNSF) (Ambizione Grant No. PZ00P2_167900; J.O.Z. and G.D.) is gratefully acknowledged. S.W. and C.W. acknowledge financial support through the Army Research Office under Grant No. W911NF-15-1-0190. M.L. and W.T. acknowledge funding from Flanders Innovation & Entrepreneurship through the Plast-i-Com project (IWT 135100). W.T. also thanks the Provincie West-Vlaanderen for his Chair in Advanced Materials and Research Foundation Flanders for his Odysseus Fellowship (Grant G.0C60.13N).
| 13C | carbon isotope 13 |
| 1H NMR | proton nuclear magnetic resonance |
| 4-SS | sodium 4-vinylbenzenesulfonate |
| A(R)GET | activators (re)generated by electron transfer |
| AA | acrylic acid |
| Ac | sodium triacetoxyborohydride |
| ACVA | 4,4′-azobis(4-cyanovaleric acid) |
| AEM | 2-aminoethyl methacrylate |
| AEMA | 2-aminoethyl methacrylamide |
| AgNPs | silver nanoparticles |
| AGU | anhydroglucopyranose unit |
| AIBN | 2,2′-azobis(2-methylpropionitrile) |
| AM | acrylamide |
| (AMIM)Cl | 1-allyl-3-methylimidazolium chloride |
| APS | ammonium persulfate |
| ATRP | atom transfer radical polymerization |
| AuNPs | gold nanoparticles |
| BA | butyl acrylate |
| BIBA | α-bromoisobutyric acid |
| bis-TAD | 4,4′-(1,4-phenylene)bis(1,2,4-triazoline-3,5-dione) |
| BlocBuilder | 4-(diethoxyphosphinyl)-2,2,5,5-tetramethyl-3-azahexane-N-oxyl |
| Br | bromine |
| BiBB | 2-bromoisobutyryl bromide |
| CAN | ceric(IV) ammonium nitrate |
| CB[8] | cucurbit[8]uril |
| CDCl3 | deuterated chloroform |
| Ce4+ | ceric ion |
| Cl | chlorine |
| Cn | sodium cyanoborohydride |
| CNC | cellulose nanocrystal |
| CNC-1 | cellulose nanocrystals with high surface density of sulfate half-ester groups |
| CNC-2 | cellulose nanocrystals with low surface density of sulfate half-ester groups |
| CNC–COOH | carboxylate functionalized cellulose nanocrystals |
| CNC-SH | thiolated cellulose nanocrystals |
| CNF | cellulose nanofibrils |
| CO2 | carbon dioxide |
| CPADB | 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid |
| CP-MAS | cross-polarization magic angle spinning |
| CPT | cloud-point temperature |
| CSC | cellulose synthase complex |
| CTA | chain transfer agent |
| Cu | copper |
| DCC | N,N′-dicyclohexylcarbodiimide |
| DDMAT | 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid |
| DEAEMA | 2-(diethylamino)ethyl methacrylate |
| DEPN | N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide |
| d-LA | d-lactide |
| DLS | dynamic light scattering |
| DMAEMA | 2-(dimethylamino)ethyl methacrylate |
| DMAM | N,N-dimethylacrylamide |
| DMAP | 4-dimethylaminopyridine |
| DMAPMAM | N-[3-(dimethylamino)propyl]methacrylamide |
| DMF | N,N-dimethylformamide |
| DMSO | dimethyl sulfoxide |
| DMSO-d6 | deuterated dimethyl sulfoxide |
| DP | degree of polymerization |
| DSC | differential scanning calorimetry |
| E | elastic modulus |
| E′ | storage modulus |
| EANI | 4-ethoxy-9-allyl-1,8-naphthalimide |
| EBiB | ethyl α-bromoisobutyrate |
| EDA | ethylenediamine |
| EDC | N-(3-(dimethylamino)propyl)-N-ethylcarbodiimide hydrochloride |
| EGDMA | 2-(2-methyl-acryloyloxy)ethyl 2-methyl-acrylate |
| EISA | evaporation-induced self-assembly |
| Fe3O4 | magnetite |
| FRP | free radical polymerization |
| FTIR | Fourier transformed infrared |
| G′ | shear storage modulus |
| gg | gauche–gauche |
| gt | gauche–trans |
| HBr | hydrobromic acid |
| HCl | hydrochloric acid |
| KPS | potassium persulfate |
| LC | liquid crystal |
| LCEs | liquid crystal elastomers |
| LCST | lower critical solution temperature |
| l-LA | l-lactide |
| MA | methyl acrylate |
| MADIX | macromolecular design via the interchange of xanthates |
| METAC | [2-(methacryloyloxy)ethyl]trimethylammonium chloride |
| MgH2 | magnesium hydride |
| MMA | methyl methacrylate |
| MMAZO | 6-[4-(4-methoxyphenylazo)phenoxy] hexyl methacrylate |
| Mn | number-average molecular weight |
| D̵ | dispersity |
| Mo(CO)6 | molybdenumhexacarbonyl |
| MPC | 2-methacryloyloxyethyl phosphorylcholine |
| Mw | weight-average molecular weight |
| N2 | nitrogen |
| NaCl | sodium chloride |
| NaClO2 | sodium chlorite |
| NH2(CH2)6SH·HCl | 6-amino-1-hexanethiol hydrochloride |
| NHS | N-hydroxysuccinimide |
| NHS-BiB | 2-bromoisobutanoic acid N-hydroxysuccinimide ester |
| NIPAM | N-isopropylacrylamide |
| NMBA | N,N′-methylenebis(acrylamide) |
| NMP | nitroxide-mediated polymerization |
| NMR | nuclear magnetic resonance |
| NPs | nanoparticles |
| NpMA | naphtyl methacrylate |
| NVC | N-vinylcaprolactam |
| OCNs | one-component nanocomposites |
| OEGMA | oligoethylene glycol methacrylate |
| OEGMEMA | oligoethylene glycol methyl ether acrylate |
| –OH | hydroxyl |
| P(MAA-co-VSA) | poly(methacrylic acid-co-vinylsulfonic acid) |
| P4VP | poly(4-vinylpyridine) |
| PAA | poly(acrylic acid) |
| PAM | polyacrylamide |
| PBSA | poly(butylene succinate-co-adipate) |
| Pc | 2-picoline–borane complex |
| PCL | poly(ε-caprolactone) |
| PDMAEMA | poly[(dimethylamino)ethyl methacrylate] |
| PGMA | poly(glycidyl methacrylate) |
| pH | potential of hydrogen |
| PHB | poly(β-hydroxybutyrate) |
| PHEMA | poly(2-hydroxyethyl methacrylate) |
| PHMA | poly(hexyl methacrylate) |
| pKa | acid dissociation constant |
| PLA | poly(lactic acid) |
| PMMA | poly(methyl methacrylate) |
| PMAA | poly(methacrylic acid) |
| PNVC | N-vinylcaprolactam |
| POEGMA | poly(oligoethylene glycol) methacrylate |
| POEGMEMA | poly[(oligoethylene glycol) methyl ether acrylate] |
| POM | polarized optical microscopy |
| PPEGEEMA | poly[poly(ethylene glycol)ethyl ether methacrylate] |
| PSBAM | poly(soybean amide methacrylate) |
| PSty | polystyrene |
| PtBA | poly(tert-butyl acrylate) |
| PU | polyurethane |
| PVA | poly(vinyl alcohol) |
| PVAc | poly(vinyl acetate) |
| RAFT | reversible addition–fragmentation chain transfer |
| ROP | ring-opening polymerization |
| S | sulfur |
| SARA ATRP | supplemental activator and reducing agent atom transfer radical polymerization |
| SBAM | soybean amide methacrylate |
| SBS | sodium bisulfite |
| SEC | size-exclusion chromatography |
| SEM | scanning electron microscopy |
| SET LRP | single-electron transfer living radical polymerization |
| SI-AGET-ATRP | surface-initiated activators generated by electron transfer atom transfer radical polymerization |
| SI-ATRP | surface-initiated atom transfer radical polymerization |
| SI-CRP | surface-initiated controlled radical polymerization |
| SI-FRP | surface-initiated free radical polymerization |
| SI-NMP | surface-initiated nitroxide-mediated polymerization |
| SI-ROP | surface-initiated ring-opening polymerization |
| SMA | stearyl methacrylate |
| SMPs | shape-memory polymers |
| Sn(Oct)2 | tin(II) 2-ethylhexanoate |
| S-RAFT | surface reversible addition–fragmentation chain transfer |
| Sty | styrene |
| tBA | tert-butyl acrylate |
| Td | degradation temperature |
| TEM | transmission electron microscopy |
| TEMPO | (2,2,6,6-tetramethylpiperidin-1-yl)oxy |
| Tg | glass transition temperature |
| tg | trans–gauche |
| TGA | thermogravimetric analysis |
| THF | tetrahydrofuran |
| TMEDA | N,N,N′,N′-tetramethylethane-1,2-diamine |
| UCST | upper critical solution temperature |
| UPyMA | 2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate |
| UV | ultraviolet |
| VAc | vinyl acetate |
| wt % | weight percent |
| XPS | X-ray photoelectron spectroscopy |
| ΔCp | heat capacity variation |
| ε-CL | ε-caprolactone |
| σb | tensile at break |
References
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- 1Sanchez, C.; Belleville, P.; Popall, M.; Nicole, L. Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market. Chem. Soc. Rev. 2011, 40 (2), 696– 753, DOI: 10.1039/c0cs00136hGoogle Scholar1https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXptFSkug%253D%253D&md5=44c4c4201307a136e102df8f96865a2eApplications of advanced hybrid organic-inorganic nanomaterials: from laboratory to marketSanchez, Clement; Belleville, Philippe; Popall, Michael; Nicole, LionelChemical Society Reviews (2011), 40 (2), 696-753CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. Today cross-cutting approaches, where mol. engineering and clever processing are synergistically coupled, allow the chemist to tailor complex hybrid systems of various shapes with perfect mastery at different size scales, compn., functionality, and morphol. Hybrid materials with org.-inorg. or bio-inorg. character represent not only a new field of basic research but also, via their remarkable new properties and multifunctional nature, hybrids offer prospects for many new applications in extremely diverse fields. The description and discussion of the major applications of hybrid inorg.-org. (or biol.) materials are the major topic of this crit. review. Indeed, today the very large set of accessible hybrid materials span a wide spectrum of properties which yield the emergence of innovative industrial applications in various domains such as optics, micro-electronics, transportation, health, energy, housing, and the environment among others (526 refs.).
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- 4Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T. Review: current international research into cellulose nanofibres and nanocomposites. J. Mater. Sci. 2010, 45 (1), 1– 33, DOI: 10.1007/s10853-009-3874-0Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFKrur7L&md5=dc18078ac2514563b9702f24298252f3Review: current international research into cellulose nanofibres and nanocompositesEichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Toman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T.Journal of Materials Science (2010), 45 (1), 1-33CODEN: JMTSAS; ISSN:0022-2461. (Springer)A review. This paper provides an overview of recent progress made in the area of cellulose nanofiber-based nanocomposites. An introduction into the methods used to isolate cellulose nanofibers (nanowhiskers, nanofibrils) is given, with details of their structure. Following this, the article is split into sections dealing with processing and characterization of cellulose nanocomposites and new developments in the area, with particular emphasis on applications. The types of cellulose nanofibers covered are those extd. from plants by acid hydrolysis (nanowhiskers), mech. treatment and those that occur naturally (tunicate nanowhiskers) or under culturing conditions (bacterial cellulose nanofibrils). Research highlighted in the article are the use of cellulose nanowhiskers for shape memory nanocomposites, anal. of the interfacial properties of cellulose nanowhisker and nanofibril-based composites using Raman spectroscopy, switchable interfaces that mimic sea cucumbers, polymn. from the surface of cellulose nanowhiskers by atom transfer radical polymn. and ring opening polymn.
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- 6Klemm, D.; Kramer, F.; Moritz, S.; Lindstroem, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of Nature-Based Materials. Angew. Chem., Int. Ed. 2011, 50 (24), 5438– 5466, DOI: 10.1002/anie.201001273Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXmsValtLw%253D&md5=87920bb7ada80297e9837bcff7f7e175Nanocelluloses: A New Family of Nature-Based MaterialsKlemm, Dieter; Kramer, Friederike; Moritz, Sebastian; Lindstroem, Tom; Ankerfors, Mikael; Gray, Derek; Dorris, AnnieAngewandte Chemie, International Edition (2011), 50 (24), 5438-5466CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. Cellulose fibrils with widths in the nanometer range are nature-based materials with unique and potentially useful features. Most importantly, these novel nanocelluloses open up the strongly expanding fields of sustainable materials and nanocomposites, as well as medical and life-science devices, to the natural polymer cellulose. The nanodimensions of the structural elements result in a high surface area and hence the powerful interaction of these celluloses with surrounding species. This Review assembles the current knowledge on the isolation of microfibrillated cellulose from wood and its application in nanocomposites; the prepn. of nanocryst. cellulose and its use as a reinforcing agent; and the biofabrication of bacterial nanocellulose, as well as its evaluation as a biomaterial for medical implants.
- 7Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nanocomposites. Chem. Soc. Rev. 2011, 40 (7), 3941– 3994, DOI: 10.1039/c0cs00108bGoogle Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXns12ntLY%253D&md5=46105175ec4c68467ada5432cdffb09eCellulose nanomaterials review: structure, properties and nanocompositesMoon, Robert J.; Martini, Ashlie; Nairn, John; Simonsen, John; Youngblood, JeffChemical Society Reviews (2011), 40 (7), 3941-3994CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)This crit. review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphol., crystal structure, and properties. Also described are the self-assembly and rheol. properties of cellulose nanoparticle suspensions. The methodol. of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Addnl., advances in predictive modeling from mol. dynamic simulations of cryst. cellulose to the continuum modeling of composites made with such particles are reviewed (392 refs.).
- 8Kontturi, E.; Laaksonen, P.; Linder, M. B.; Nonappa; Gröschel, A. H.; Rojas, O. J.; Ikkala, O. Advanced Materials through Assembly of Nanocelluloses. Adv. Mater. 2018, 30, 1703779, DOI: 10.1002/adma.201703779Google ScholarThere is no corresponding record for this reference.
- 9Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A. Cellulose: Fascinating biopolymer and sustainable raw material. Angew. Chem., Int. Ed. 2005, 44 (22), 3358– 3393, DOI: 10.1002/anie.200460587Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXlsV2jtbY%253D&md5=804d758e637b3111b640b7272bea4de1Cellulose: Fascinating biopolymer and sustainable raw materialKlemm, Dieter; Heublein, Brigitte; Fink, Hans-Peter; Bohn, AndreasAngewandte Chemie, International Edition (2005), 44 (22), 3358-3393CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. As the most important skeletal component in plants, the polysaccharide cellulose is an almost inexhaustible polymeric raw material with fascinating structure and properties. Formed by the repeated connection of D-glucose building blocks, the highly functionalized, linear stiff-chain homopolymer is characterized by its hydrophilicity, chirality, biodegradability, broad chem. modifying capacity, and its formation of versatile semicryst. fiber morphologies. In view of the considerable increase in interdisciplinary cellulose research and product development over the past decade worldwide, this paper assembles the current knowledge in the structure and chem. of cellulose, and in the development of innovative cellulose esters and ethers for coatings, films, membranes, building materials, drilling techniques, pharmaceuticals, and foodstuffs. New frontiers, including environmentally friendly cellulose fiber technologies, bacterial cellulose biomaterials, and in-vitro syntheses of cellulose are highlighted together with future aims, strategies, and perspectives of cellulose research and its applications.
- 10Sacui, I. A.; Nieuwendaal, R. C.; Burnett, D. J.; Stranick, S. J.; Jorfi, M.; Weder, C.; Foster, E. J.; Olsson, R. T.; Gilman, J. W. Comparison of the Properties of Cellulose Nanocrystals and Cellulose Nanofibrils Isolated from Bacteria, Tunicate, and Wood Processed Using Acid, Enzymatic, Mechanical, and Oxidative Methods. ACS Appl. Mater. Interfaces 2014, 6 (9), 6127– 6138, DOI: 10.1021/am500359fGoogle Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXmsVGkt70%253D&md5=a722345b83852b456e73d1b3d0146647Comparison of the Properties of Cellulose Nanocrystals and Cellulose Nanofibrils Isolated from Bacteria, Tunicate, and Wood Processed Using Acid, Enzymatic, Mechanical, and Oxidative MethodsSacui, Iulia A.; Nieuwendaal, Ryan C.; Burnett, Daniel J.; Stranick, Stephan J.; Jorfi, Mehdi; Weder, Christoph; Foster, E. Johan; Olsson, Richard T.; Gilman, Jeffery W.ACS Applied Materials & Interfaces (2014), 6 (9), 6127-6138CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)This work describes the measurement and comparison of several important properties of native cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs), such as crystallinity, morphol., aspect ratio, and surface chem. Measurement of the fundamental properties of seven different CNCs/CNFs, from raw material sources (bacterial, tunicate, and wood) using typical hydrolysis conditions (acid, enzymic, mech., and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidn.), was accomplished using a variety of measurement methods. Atomic force microscopy (AFM), TEM, and 13C cross-polarization magic angle spinning (CPMAS) NMR spectroscopy were used to conclude that CNCs, which are rodlike in appearance, have a higher crystallinity than CNFs, which are fibrillar in appearance. CNC aspect ratio distributions were measured and ranged from 148±147 for tunicate-CNCs to 23±12 for wood-CNCs. Hydrophobic interactions, measured using inverse gas chromatog. (IGC), were found to be an important contribution to the total surface energy of both types of cellulose. In all cases, a trace amt. of naturally occurring fluorescent compds. was obsd. after hydrolysis. Confocal and Raman microscopy were used to confirm that the fluorescent species were unique for each cellulose source, and demonstrated that such methods can be useful for monitoring purity during CNC/CNF processing. This study reveals the broad, tunable, multi-dimensional material space in which CNCs and CNFs exist.
- 11Salas, C.; Nypelö, T.; Rodriguez-Abreu, C.; Carrillo, C.; Rojas, O. J. Nanocellulose properties and applications in colloids and interfaces. Curr. Opin. Colloid Interface Sci. 2014, 19 (5), 383– 396, DOI: 10.1016/j.cocis.2014.10.003Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvVOksLzF&md5=369bd595d6ea2b197a0f58d7e796cdcdNanocellulose properties and applications in colloids and interfacesSalas, Carlos; Nypelo, Tiina; Rodriguez-Abreu, Carlos; Carrillo, Carlos; Rojas, Orlando J.Current Opinion in Colloid & Interface Science (2014), 19 (5), 383-396CODEN: COCSFL; ISSN:1359-0294. (Elsevier Ltd.)In this review we introduce recent advances in the development of cellulose nanomaterials and the construction of high order structures by applying some principles of colloid and interface science. These efforts take advantage of natural assemblies in the form of fibers that nature constructs by a biogenetic bottom-up process that results in hierarchical systems encompassing a wide range of characteristic sizes. Following the reverse process, a top-down deconstruction, cellulose materials can be cleaved from fiber cell walls. The resulting nanocelluloses, mainly cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC, i.e., defect-free, rod-like cryst. residues after acid hydrolysis of fibers), have been the subject of recent interest. This originates from the appealing intrinsic properties of nanocelluloses: nanoscale dimensions, high surface area, morphol., low d., chirality and thermo-mech. performance. Directing their assembly into multiphase structures is a quest that can yield useful outcomes in many revolutionary applications. As such, we discuss the use of non-specific forces to create thin films of nanocellulose at the air-solid interface for applications in nano-coatings, sensors, etc. Assemblies at the liq.-liq. and air-liq. interfaces will be highlighted as means to produce Pickering emulsions, foams and aerogels. Finally, the prospects of a wide range of hybrid materials and other systems that can be manufd. via self and directed assembly will be introduced in light of the unique properties of nanocelluloses.
- 12Grishkewich, N.; Mohammed, N.; Tang, J.; Tam, K. C. Recent advances in the application of cellulose nanocrystals. Curr. Opin. Colloid Interface Sci. 2017, 29, 32– 45, DOI: 10.1016/j.cocis.2017.01.005Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXjt1KjsLo%253D&md5=91a60ff8f861a4142d2233ade42c338bRecent advances in the application of cellulose nanocrystalsGrishkewich, Nathan; Mohammed, Nishil; Tang, Juntao; Tam, Kam ChiuCurrent Opinion in Colloid & Interface Science (2017), 29 (), 32-45CODEN: COCSFL; ISSN:1359-0294. (Elsevier Ltd.)A review. This review describes recent advances on the application of cellulose nanocrystals (CNCs) in selected applications. CNCs are produced via acid hydrolysis of cellulosic materials, such as wood, cotton, tunicate, or other biomass. It possesses many desirable properties, such as large surface area, high tensile strength and stiffness, excellent colloidal stability, and potential for modification due to the abundance of surface hydroxyl groups. By modifying its surface with small mols., polymers, and nanoparticles, they can be utilized as a zero-dimension nanostructure for drug delivery, spun into 1-dimension fibers for enhanced strength, cast into 2-dimension films for flexibility, or molded into 3-dimension hydrogels and aerogels for compressibility or porous materials. The use and impact of CNCs in three industrial sectors: biomedical, wastewater treatment, energy and electronics are described and discussed, and we will offer our perspective on the future and new applications of this sustainable nanomaterial.
- 13Herbert, K. M.; Schrettl, S.; Rowan, S. J.; Weder, C. 50th Anniversary Perspective: Solid-State Multistimuli, Multiresponsive Polymeric Materials. Macromolecules 2017, 50 (22), 8845– 8870, DOI: 10.1021/acs.macromol.7b01607Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhslCmu7fF&md5=06424301852657f933adc74fa28f4adeAnniversary Perspective: Solid-State Multistimuli, Multiresponsive Polymeric MaterialsHerbert, Katie M.; Schrettl, Stephen; Rowan, Stuart J.; Weder, ChristophMacromolecules (Washington, DC, United States) (2017), 50 (22), 8845-8870CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)A review. The field of stimuli-responsive polymers has grown in the past three decades from a few obscure examples to one of the most vibrant domains of modern macromol. science. Indeed, the research, development, and implementation of tailored materials that can respond in predefined ways to specific stimuli now cut across most areas in which "traditional" polymers play a role, and an ever-growing range of com. products benefits from this class of materials. This Perspective is devoted to multistimuli, multiresponsive (MSMR) polymers, which are those materials that are able to respond to multiple, different stimuli with multiple, distinct responses. Somewhat akin to living systems, which have evolved to adapt and respond in complex ways to (combinations of) different environmental cues, MSMR polymers can offer a broad range of complex properties and functions. While much of the work on MSMR polymers has been devoted to the investigation of solns. or gels, this Perspective concs. on solid materials, as this is the state of matter in which the vast majority of polymers are currently employed. In a tutorial fashion, an outline of some of the most common mechanisms used to implement stimuli-responsive behavior in polymer solids is provided. To convey a glimpse of the potential and the challenges of these general design principles, select examples of materials that display multistimuli, single-response as well as single-stimulus multiresponse behavior are presented, before polymeric materials that are truly multistimuli, multiresponsive are discussed, and an outlook on possible directions that future work in the field may take is presented.
- 14Montero de Espinosa, L.; Meesorn, W.; Moatsou, D.; Weder, C. Bioinspired Polymer Systems with Stimuli-Responsive Mechanical Properties. Chem. Rev. 2017, 117 (20), 12851– 12892, DOI: 10.1021/acs.chemrev.7b00168Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1emsL3P&md5=bd46006db505c0118894512cbe6b2876Bioinspired Polymer Systems with Stimuli-Responsive Mechanical PropertiesMontero de Espinosa, Lucas; Meesorn, Worarin; Moatsou, Dafni; Weder, ChristophChemical Reviews (Washington, DC, United States) (2017), 117 (20), 12851-12892CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. Materials with switchable mech. properties are widespread in living organisms and endow many species with traits that are essential for their survival. Many of the mech. morphing materials systems found in nature are based on hierarchical structures, which are the basis for mech. robustness and often also the key to responsive behavior. Many "operating principles" involve cascades of events that translate cues from the environment into changes of the overall structure and/or the connectivity of the constituting building blocks at various levels. These concepts permit dramatic property variations without significant compositional changes. Inspired by the function and the growing understanding of the operating principles at play in biol. materials with the capability to change their mech. properties, significant efforts have been made toward mimicking such architectures and functions in artificial materials. Research in this domain has rapidly grown in the last two decades and afforded many examples of bioinspired materials that are able to reversibly alter their stiffness, shape, porosity, d., or hardness upon remote stimulation. This review summarizes the state of research in this field.
- 15Endes, C.; Mueller, S.; Kinnear, C.; Vanhecke, D.; Foster, E. J.; Petri-Fink, A.; Weder, C.; Clift, M. J. D.; Rothen-Rutishauser, B. Fate of Cellulose Nanocrystal Aerosols Deposited on the Lung Cell Surface In Vitro. Biomacromolecules 2015, 16 (4), 1267– 1275, DOI: 10.1021/acs.biomac.5b00055Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXks1OksrY%253D&md5=238e503a629fd1e826b9598e989cc8edFate of Cellulose Nanocrystal Aerosols Deposited on the Lung Cell Surface In VitroEndes, Carola; Mueller, Silvana; Kinnear, Calum; Vanhecke, Dimitri; Foster, E. Johan; Petri-Fink, Alke; Weder, Christoph; Clift, Martin J. D.; Rothen-Rutishauser, BarbaraBiomacromolecules (2015), 16 (4), 1267-1275CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)When considering the inhalation of high-aspect ratio nanoparticles (HARN), the characterization of their specific interaction with lung cells is of fundamental importance to help categorize their potential hazard. The aim of the present study was to assess the interaction of cellulose nanocrystals (CNCs) with a multicellular in vitro model of the epithelial airway barrier following realistic aerosol exposure. Rhodamine-labeled CNCs isolated from cotton (c-CNCs, 237 ± 118 × 29 ± 13 nm) and tunicate (t-CNCs, 2244 ± 1687 × 30 ± 8 nm) were found to display different uptake behaviors due to their length, although also dependent upon the applied concn., when visualized by laser scanning microscopy. Interestingly, the longer t-CNCs were found to exhibit a lower clearance by the lung cell model compared to the shorter c-CNCs. This difference can be attributed to stronger fiber-fiber interactions between the t-CNCs. In conclusion, nanofiber length and concn. has a significant influence on their interaction with lung cells in vitro.
- 16Endes, C.; Camarero-Espinosa, S.; Mueller, S.; Foster, E. J.; Petri-Fink, A.; Rothen-Rutishauser, B.; Weder, C.; Clift, M. J. D. A critical review of the current knowledge regarding the biological impact of nanocellulose. J. Nanobiotechnol. 2016, 14 (1), 78, DOI: 10.1186/s12951-016-0230-9Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVyltrk%253D&md5=f1b74cb149effd69e53d095e58321a84A critical review of the current knowledge regarding the biological impact of nanocelluloseEndes, C.; Camarero-Espinosa, S.; Mueller, S.; Foster, E. J.; Petri-Fink, A.; Rothen-Rutishauser, B.; Weder, C.; Clift, M. J. D.Journal of Nanobiotechnology (2016), 14 (), 78/1-78/14CODEN: JNOAAO; ISSN:1477-3155. (BioMed Central Ltd.)Several forms of nanocellulose, notably cellulose nanocrystals and nanofibrillated cellulose, exhibit attractive property matrixes and are potentially useful for a large no. of industrial applications. These include the paper and cardboard industry, use as reinforcing filler in polymer composites, basis for low-d. foams, additive in adhesives and paints, as well as a wide variety of food, hygiene, cosmetic, and medical products. Although the com. exploitation of nanocellulose has already commenced, little is known as to the potential biol. impact of nanocellulose, particularly in its raw form. This review provides a comprehensive and crit. review of the current state of knowledge of nanocellulose in this format. Overall, the data seems to suggest that when investigated under realistic doses and exposure scenarios, nanocellulose has a limited assocd. toxic potential, albeit certain forms of nanocellulose can be assocd. with more hazardous biol. behavior due to their specific phys. characteristics.
- 17Camarero-Espinosa, S.; Endes, C.; Mueller, S.; Petri-Fink, A.; Rothen-Rutishauser, B.; Weder, C.; Clift, M.; Foster, E. J. Elucidating the Potential Biological Impact of Cellulose Nanocrystals. Fibers 2016, 4 (3), 21, DOI: 10.3390/fib4030021Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXms1yhs7o%253D&md5=781c39a0db91d57701a371c602e2b449Elucidating the potential biological impact of cellulose nanocrystalsCamarero-Espinosa, Sandra; Endes, Carola; Mueller, Silvana; Petri-Fink, Alke; Rothen-Rutishauser, Barbara; Weder, Christoph; Clift, Martin James David; Foster, E. JohanFibers (2016), 4 (3), 21/1-21/15CODEN: FIBECU; ISSN:2079-6439. (MDPI AG)Cellulose nanocrystals exhibit an interesting combination of mech. properties and phys. characteristics, which make them potentially useful for a wide range of consumer applications. However, as the usage of these bio-based nanofibers increases, a greater understanding of human exposure addressing their potential health issues should be gained. The aim of this perspective is to highlight how knowledge obtained from studying the biol. impact of other nanomaterials can provide a basis for future research strategies to deduce the possible human health risks posed by cellulose nanocrystals.
- 18Majoinen, J.; Haataja, J. S.; Appelhans, D.; Lederer, A.; Olszewska, A.; Seitsonen, J.; Aseyev, V.; Kontturi, E.; Rosilo, H.; Osterberg, M.; Houbenov, N.; Ikkala, O. Supracolloidal Multivalent Interactions and Wrapping of Dendronized Glycopolymers on Native Cellulose Nanocrystals. J. Am. Chem. Soc. 2014, 136 (3), 866– 869, DOI: 10.1021/ja411401rGoogle Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXksVKm&md5=397df3f2966d5ecfa2168ed4fb54938fSupracolloidal Multivalent Interactions and Wrapping of Dendronized Glycopolymers on Native Cellulose NanocrystalsMajoinen, Johanna; Haataja, Johannes S.; Appelhans, Dietmar; Lederer, Albena; Olszewska, Anna; Seitsonen, Jani; Aseyev, Vladimir; Kontturi, Eero; Rosilo, Henna; Osterberg, Monika; Houbenov, Nikolay; Ikkala, OlliJournal of the American Chemical Society (2014), 136 (3), 866-869CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Cellulose nanocrystals (CNCs) are high aspect ratio colloidal rods with nanoscale dimensions, attracting considerable interest recently due to their high mech. properties, chirality, sustainability, and availability. In order to exploit them for advanced functions in new materials, novel supracolloidal concepts are needed to manipulate their self-assemblies. We report on exploring multivalent interactions to CNC surface and show that dendronized polymers (DenPols) with maltose-based sugar groups on the periphery of lysine dendrons and poly-(ethylene-alt-maleimide) polymer backbone interact with CNCs. The interactions can be manipulated by the dendron generation suggesting multivalent interactions. The complexation of the third generation DenPol (G3) with CNCs allows aq. colloidal stability and shows wrapping around CNCs, as directly visualized by cryo high-resoln. TEM and electron tomog. More generally, as the dimensions of G3 are in the colloidal range due to their ∼6 nm lateral size and mesoscale length, the concept also suggests supracolloidal multivalent interactions between other colloidal objects mediated by sugar-functionalized dendrons giving rise to novel colloidal level assemblies.
- 19Majoinen, J.; Hassinen, J.; Haataja, J. S.; Rekola, H. T.; Kontturi, E.; Kostiainen, M. A.; Ras, R. H. A.; Torma, P.; Ikkala, O. Chiral Plasmonics Using Twisting along Cellulose Nanocrystals as a Template for Gold Nanoparticles. Adv. Mater. 2016, 28 (26), 5262, DOI: 10.1002/adma.201600940Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XnsFWmurc%253D&md5=eebc099c55f054d3675e1a3ac79f007fChiral Plasmonics Using Twisting along Cellulose Nanocrystals as a Template for Gold NanoparticlesMajoinen, Johanna; Hassinen, Jukka; Haataja, Johannes S.; Rekola, Heikki T.; Kontturi, Eero; Kostiainen, Mauri A.; Ras, Robin H. A.; Toermae, Paeivi; Ikkala, OlliAdvanced Materials (Weinheim, Germany) (2016), 28 (26), 5262-5267CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)A simple route for chiral plasmonics is shown by binding cationic 8.5 nm gold nanoparticles on neg. charged cellulose nanocrystals (CNCs) having a lateral dimension of ≈ 7 nm and chiral twist along their rod axis dispersed in dil. aq. medium. This leads to a bisignate CD signal indicating a right-handed chiral plasmonic response. The electrostatic self-assembly leads to nanoscale fibrillar superstructures having lateral dimensions of 30-60 nm and length of 200-500 nm. The authors confirm their exptl. optical results by simulating the CD signals using exptl. detd. AuNP 3D coordinates from electron tomograms. The mutual sizes of CNCs and AuNPs have to match: too large AuNPs do not effectively bind on CNCs, and too small AuNPs do not provide strong enough plasmonic signal. The present concept is fundamentally different from the previously obsd. left-handed chiral plasmonic responses resulting from nanoparticles embedded within chiral liq. cryst. template by CNCs.
- 20Usov, I.; Nystrom, G.; Adamcik, J.; Handschin, S.; Schutz, C.; Fall, A.; Bergstrom, L.; Mezzenga, R. Understanding nanocellulose chirality and structure-properties relationship at the single fibril level. Nat. Commun. 2015, 6 (1), 7564, DOI: 10.1038/ncomms8564Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtF2ktrjP&md5=ab7b10221f75183c950de09c1db9d554Understanding nanocellulose chirality and structure-properties relationship at the single fibril levelUsov, Ivan; Nystrom, Gustav; Adamcik, Jozef; Handschin, Stephan; Schutz, Christina; Fall, Andreas; Bergstrom, Lennart; Mezzenga, RaffaeleNature Communications (2015), 6 (), 7564CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)Nanocellulose fibrils are ubiquitous in nature and nanotechnologies but their mesoscopic structural assembly is not yet fully understood. Here we study the structural features of rod-like cellulose nanoparticles on a single particle level, by applying statistical polymer physics concepts on electron and at. force microscopy images, and we assess their phys. properties via quant. nanomech. mapping. We show evidence of right-handed chirality, obsd. on both bundles and on single fibrils. Statistical anal. of contours from microscopy images shows a non-Gaussian kink angle distribution. This is inconsistent with a structure consisting of alternating amorphous and cryst. domains along the contour and supports process-induced kink formation. The intrinsic mech. properties of nanocellulose are extd. from nanoindentation and persistence length method for transversal and longitudinal directions, resp. The structural anal. is pushed to the level of single cellulose polymer chains, and their smallest assocd. unit with a proposed 2 x 2 chain-packing arrangement.
- 21Camarero-Espinosa, S.; Kuhnt, T.; Foster, E. J.; Weder, C. Isolation of Thermally Stable Cellulose Nanocrystals by Phosphoric Acid Hydrolysis. Biomacromolecules 2013, 14 (4), 1223– 1230, DOI: 10.1021/bm400219uGoogle ScholarThere is no corresponding record for this reference.
- 22Nicharat, A.; Sapkota, J.; Weder, C.; Foster, E. J. Melt processing of polyamide 12 and cellulose nanocrystals nanocomposites. J. Appl. Polym. Sci. 2015, 132 (45), 42752, DOI: 10.1002/app.42752Google ScholarThere is no corresponding record for this reference.
- 23Trache, D.; Hussin, M. H.; Haafiz, M. K. M.; Thakur, V. K. Recent progress in cellulose nanocrystals: sources and production. Nanoscale 2017, 9 (5), 1763– 1786, DOI: 10.1039/C6NR09494EGoogle Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhsVertLY%253D&md5=68ae9fc346fde2d8b251b037c076d654Recent progress in cellulose nanocrystals: sources and productionTrache, Djalal; Hussin, M. Hazwan; Haafiz, M. K. Mohamad; Thakur, Vijay KumarNanoscale (2017), 9 (5), 1763-1786CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)Cellulose nanocrystals, a class of fascinating bio-based nanoscale materials, have received a tremendous amt. of interest both in industry and academia owing to its unique structural features and impressive physicochem. properties such as biocompatibility, biodegradability, renewability, low d., adaptable surface chem., optical transparency, and improved mech. properties. This nanomaterial is a promising candidate for applications in fields such as biomedical, pharmaceuticals, electronics, barrier films, nanocomposites, membranes, supercapacitors, etc. New resources, new extn. procedures, and new treatments are currently under development to satisfy the increasing demand of manufg. new types of cellulose nanocrystals-based materials on an industrial scale. Therefore, this review addresses the recent progress in the prodn. methodologies of cellulose nanocrystals, covering principal cellulose resources and the main processes used for its isolation. A crit. and anal. examn. of the shortcomings of various approaches employed so far is made. Addnl., structural organization of cellulose and nomenclature of cellulose nanomaterials have also been discussed for beginners in this field.
- 24Eyley, S.; Thielemans, W. Surface modification of cellulose nanocrystals. Nanoscale 2014, 6 (14), 7764– 7779, DOI: 10.1039/C4NR01756KGoogle Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVKjtrnM&md5=e8f00868e0a641cf9c7a0c36d1e587d1Surface modification of cellulose nanocrystalsEyley, Samuel; Thielemans, WimNanoscale (2014), 6 (14), 7764-7779CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)A review. Chem. modification of cellulose nanocrystals is an increasingly popular topic in the literature. This review analyses the type of cellulose nanocrystal modification reactions that have been published in the literature thus far and looks at the steps that have been taken towards analyzing the products of the nanocrystal modifications. The main categories of reactions carried out on cellulose nanocrystals were oxidns., esterifications, amidations, carbamations and etherifications. More recently nucleophilic substitutions had been used to introduce more complex functionality to cellulose nanocrystals. Multi-step modifications were also considered. This review emphasizes quantification of modification at the nanocrystal surface in terms of degree of substitution and the validity of conclusions drawn from different anal. techniques in this area. The mechanisms of the modification reactions were presented and considered with respect to the effect on the outcome of the reactions. While great strides had been made in the quality of anal. data published in the field of cellulose nanocrystal modification, there was still vast scope for improvement, both in data quality and the quality of anal. of data. Given the difficulty of surface anal., cross-checking of results from different anal. techniques was fundamental for the development of reliable cellulose nanocrystal modification techniques.
- 25Habibi, Y. Key advances in the chemical modification of nanocelluloses. Chem. Soc. Rev. 2014, 43 (5), 1519– 1542, DOI: 10.1039/C3CS60204DGoogle Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitFWhtro%253D&md5=3c2f90ef998c2b65cd4fb106dd7dc73fKey advances in the chemical modification of nanocellulosesHabibi, YoussefChemical Society Reviews (2014), 43 (5), 1519-1542CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. Nanocelluloses, including nanocryst. cellulose, nanofibrillated cellulose and bacterial cellulose nanofibers, have become fascinating building blocks for the design of new biomaterials. Derived from the must abundant and renewable biopolymer, they are drawing a tremendous level of attention, which certainly will continue to grow in the future driven by the sustainability trend. This growing interest is related to their unsurpassed quintessential phys. and chem. properties. Yet, owing to their hydrophilic nature, their utilization is restricted to applications involving hydrophilic or polar media, which limits their exploitation. With the presence of a large no. of chem. functionalities within their structure, these building blocks provide a unique platform for significant surface modification through various chemistries. These chem. modifications are prerequisite, sometimes unavoidable, to adapt the interfacial properties of nanocellulose substrates or adjust their hydrophilic-hydrophobic balance. Therefore, various chemistries have been developed aiming to surface-modify these nano-sized substrates in order to confer to them specific properties, extending therefore their use to highly sophisticated applications. This review collocates current knowledge in the research and development of nanocelluloses and emphasizes more particularly on the chem. modification routes developed so far for their functionalization.
- 26Natterodt, J. C.; Petri-Fink, A.; Weder, C.; Zoppe, J. O. Cellulose Nanocrystals: Surface Modification, Applications and Opportunities at Interfaces. Chimia 2017, 71 (6), 376– 383, DOI: 10.2533/chimia.2017.376Google ScholarThere is no corresponding record for this reference.
- 27Roy, D.; Semsarilar, M.; Guthrie, J. T.; Perrier, S. Cellulose modification by polymer grafting: a review. Chem. Soc. Rev. 2009, 38 (7), 2046– 2064, DOI: 10.1039/b808639gGoogle Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXns1Slsbo%253D&md5=90367f757ab9435890764de2a4e85412Cellulose modification by polymer grafting: A reviewRoy, Debashish; Semsarilar, Mona; Guthrie, James T.; Perrier, SebastienChemical Society Reviews (2009), 38 (7), 2046-2064CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review on graft polymn. techniques involving cellulose and its derivs. Features of cellulose structure and cellulose reactivity were discussed. In addn. to the traditional grafting techniques, we highlight the recent developments in polymer synthesis that allow increased control over the grafting process and permit the prodn. of functional celluloses that possess improved phys. properties and chem. properties.
- 28Zoppe, J. O.; Ataman, N. C.; Mocny, P.; Wang, J.; Moraes, J.; Klok, H. A. Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes. Chem. Rev. 2017, 117 (3), 1105– 1318, DOI: 10.1021/acs.chemrev.6b00314Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhsl2qtrw%253D&md5=233affa8d1bfeb42e450f19fc052a5a9Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer BrushesZoppe, Justin O.; Ataman, Nariye Cavusoglu; Mocny, Piotr; Wang, Jian; Moraes, John; Klok, Harm-AntonChemical Reviews (Washington, DC, United States) (2017), 117 (3), 1105-1318CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. The generation of polymer brushes by surface-initiated controlled radical polymn. (SI-CRP) techniques has become a powerful approach to tailor the chem. and phys. properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Addnl., polymer brushes prepd. via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chem. and phys. characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.
- 29Barbey, R.; Lavanant, L.; Paripovic, D.; Schuwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Polymer Brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and Applications. Chem. Rev. 2009, 109 (11), 5437– 5527, DOI: 10.1021/cr900045aGoogle Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXht12iurbK&md5=57e160f93c817d4c6bf320bb3883ed5fPolymer Brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and ApplicationsBarbey, Raphael; Lavanant, Laurent; Paripovic, Dusko; Schuwer, Nicolas; Sugnaux, Caroline; Tugulu, Stefano; Klok, Harm-AntonChemical Reviews (Washington, DC, United States) (2009), 109 (11), 5437-5527CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. Prepn., architecture control, substrates, properties and applications of polymer brushes are reviewed.
- 30Habibi, Y.; Goffin, A.-L.; Schiltz, N.; Duquesne, E.; Dubois, P.; Dufresne, A. Bionanocomposites based on poly(ε-caprolactone)-grafted cellulose nanocrystals by ring-opening polymerization. J. Mater. Chem. 2008, 18 (41), 5002– 5010, DOI: 10.1039/b809212eGoogle Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXht1CqsLzE&md5=6ba5d3304361266254ca8b9bf6e3f989Bionanocomposites based on poly(ε-caprolactone)-grafted cellulose nanocrystals by ring-opening polymerizationHabibi, Youssef; Goffin, Anne-Lise; Schiltz, Nancy; Duquesne, Emmanuel; Dubois, Philippe; Dufresne, AlainJournal of Materials Chemistry (2008), 18 (41), 5002-5010CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)A 'grafting from' approach was used to graft poly(.vepsiln.-caprolactone) (PCL) polymers to cellulose nanocrystals by Sn(Oct)2-catalyzed ring-opening polymn. (ROP). The grafting efficiency was evidenced by the long-term stability of suspension of PCL-grafted cellulose nanocrystals in toluene. These observations were confirmed by Fourier Transform IR Spectroscopy (FT-IR) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Extd. nanohybrids were characterized by Differential Scanning Calorimetry (DSC), XPS, and contact angle measurements. The morphol. and cryst. structure of the PCL-grafted cellulose nanocrystals was examd. by transmission electron microscopy (TEM) and X-Ray diffraction, resp. Results showed that cellulose nanocrystals kept their initial morphol. integrity and their native crystallinity. Nanocomposites with high content of cellulose nanocrystals were prepd. using either neat cellulose nanocrystals or PCL-grafted cellulose nanocrystals and high mol. wt. PCL as matrix using a casting/evapn. technique. Thermo-mech. properties of processed nanocomposites were studied by DSC, dynamical mech. analyses (DMA) and tensile tests. A significant improvement in terms of Young's modulus and storage modulus was obtained.
- 31Parker, R. M.; Guidetti, G.; Williams, C. A.; Zhao, T.; Narkevicius, A.; Vignolini, S.; Frka-Petesic, B. The Self-Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance. Adv. Mater. 2018, 30, 1704477, DOI: 10.1002/adma.201704477Google ScholarThere is no corresponding record for this reference.
- 32Kulasinski, K.; Guyer, R.; Derome, D.; Carmeliet, J. Water Adsorption in Wood Microfibril-Hemicellulose System: Role of the Crystalline–Amorphous Interface. Biomacromolecules 2015, 16 (9), 2972– 2978, DOI: 10.1021/acs.biomac.5b00878Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsVSntr%252FM&md5=92f864c7bce953b6fafbf31eff3a2a30Water Adsorption in Wood Microfibril-Hemicellulose System: Role of the Crystalline-Amorphous InterfaceKulasinski, Karol; Guyer, Robert; Derome, Dominique; Carmeliet, JanBiomacromolecules (2015), 16 (9), 2972-2978CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)A two-phase model of a wood microfibril consisting of cryst. cellulose and amorphous hemicellulose is investigated with mol. dynamics in full range of sorption to understand the mol. origin of swelling and weakening of wood. Water is adsorbed in hemicellulose, and an excess of sorption is found at the interface, while no sorption occurs within cellulose. Water mols. adsorbed on the interface push away polymer chains, forcing the two phases to sep. and causing breaking of h-bonds, particularly pronounced on the interface. Existence of two different regions in moisture response is demonstrated. At low moisture content, water is uniformly adsorbed within hemicellulose, breaking a small amt. of hydrogen bonds. Microfibril does not swell, and the porosity does not change. As moisture content increases, water is adsorbed preferentially at the interface, which leads to addnl. swelling and porosity increase at the interface. Young's and shear moduli decrease importantly due to breaking of h-bonds and screening of the long-range interactions.
- 33French, A. D. Glucose, not cellobiose, is the repeating unit of cellulose and why that is important. Cellulose 2017, 24 (11), 4605– 4609, DOI: 10.1007/s10570-017-1450-3Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtlygurfP&md5=5c7cdd31bffa74244905e043b644eb02Glucose, not cellobiose, is the repeating unit of cellulose and why that is importantFrench, Alfred D.Cellulose (Dordrecht, Netherlands) (2017), 24 (11), 4605-4609CODEN: CELLE8; ISSN:0969-0239. (Springer)A review. Despite nomenclature conventions of the International Union of Pure and Applied Chem. and the International Union of Biochem. and Mol. Biol., the repeating unit of cellulose is often said to be cellobiose instead of glucose. This review covers arguments regarding the repeating unit in cellulose mols. and crystals based on biosynthesis, shape, crystallog. symmetry, and linkage position. It is concluded that there is no good reason to disagree with the official nomenclature. Statements that cellobiose is the repeating unit add confusion and limit thinking on the range of possible shapes of cellulose. Other frequent flaws in drawings with cellobiose as the repeating unit include incorporation of O-1 as the linkage oxygen atom instead of O-4 (the O-1 hydroxyl is the leaving group in glycoside synthesis). Also, n often erroneously represents the no. of cellobiose units when n should denote the d.p. i.e., the no. of glucose residues in the polysaccharide.
- 34Wertz, J.-L.; Mercier, J.-P.; Bédué, O. Cellulose Science and Technology, 1st ed.; EPFL Press: Lausanne, Switzerland, 2010.Google ScholarThere is no corresponding record for this reference.
- 35Hardy, B. J.; Sarko, A. Molecular dynamics simulations and diffraction-based analysis of the native cellulose fibre: Structural modelling of the I-alpha and I-beta phases and their interconversion. Polymer 1996, 37 (10), 1833– 1839, DOI: 10.1016/0032-3861(96)87299-5Google Scholar35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XjtlGmu7g%253D&md5=e8959e36b2ac4388663c9712f3da38aaMolecular dynamics simulations and diffraction-based analysis of the native cellulose fiber: structural modeling of the I-α and I-β phases and their interconversionHardy, Barry J.; Sarko, AnatolePolymer (1996), 37 (10), 1833-1839CODEN: POLMAG; ISSN:0032-3861. (Elsevier)The building of diffraction-based models of the 2 phases of cellulose I, which subsequently are subjected to mol. dynamics simulation, is described.. The models showed an interesting variety of behavior, including glycosidic and exocyclic torsional motion and isomerization, hydrogen-bond cleavage and formation, individual and collective chain motion, and sheet deformation in the non-hydrogen-bonding direction. The I-α phase exhibited a greater dynamic range of behavior than the I-b phase, including considerable movement of glycosidic torsions away from initial diffraction-based positions and considerable relative motion of the chains. Based on motions obsd. in the simulations, a break-slip model was suggested for the I-α → I-β phase transition, which proposed that the transition is initiated by heating-induced hydroxymethyl and OH side-group torsional rotations accompanied by hydrogen-bond cleavage. Hence, chains of the I-α phase are freed for rotation and sliding into the more stable I-β morphol. This model was tested with mol. mechanics refinement of likely intermediate structures. The results suggested that a facile transformation path is available via such a mechanism.
- 36Perez, S.; Samain, D. Structure and Engineering of Celluloses. Adv. Carbohydr. Chem. Biochem. 2010, 64, 25– 116, DOI: 10.1016/S0065-2318(10)64003-6Google ScholarThere is no corresponding record for this reference.
- 37Vietor, R. J.; Newman, R. H.; Ha, M.-A.; Apperley, D. C.; Jarvis, M. C. Conformational features of crystal-surface cellulose from higher plants. Plant J. 2002, 30 (6), 721– 731, DOI: 10.1046/j.1365-313X.2002.01327.xGoogle Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38Xls1Chtb8%253D&md5=661043c03a80aa5d84211d3b81297bbdConformational features of crystal-surface cellulose from higher plantsVietor, Remco J.; Newman, Roger H.; Ha, Marie-Ann; Apperley, David C.; Jarvis, Michael C.Plant Journal (2002), 30 (6), 721-731CODEN: PLJUED; ISSN:0960-7412. (Blackwell Science Ltd.)Native cellulose in higher plants forms cryst. fibrils a few nm across, with a substantial fraction of their glucan chains at the surface. The accepted crystal structures feature a flat-ribbon 21 helical chain conformation with every glucose residue locked to the next by hydrogen bonds from O-3' to O-5 and from O-2 to O-6'. Using solid-state NMR spectroscopy we show that the surface chains have a different C-6 conformation so that O-6 is not in the correct position for the hydrogen bond from O-2. We also present evidence consistent with a model in which alternate glucosyl residues are transiently or permanently twisted away from the flat-ribbon conformation of the chain, weakening the O-3' - 0-5 hydrogen bond. Previous mol. modeling and the modeling studies reported here indicate that this "translational" chain conformation is energetically feasible and does not preclude binding of the surface chains to the interior chains, because the surface chains share the axial repeat distance of the 21 helix. Reduced intramol. hydrogen bonding allows the surface chains to form more hydrogen bonds to external mols. in textiles, wood, paper and the living plant.
- 38Newman, R. H.; Davidson, T. C. Molecular conformations at the cellulose-water interface. Cellulose 2004, 11 (1), 23– 32, DOI: 10.1023/B:CELL.0000014778.49291.c6Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXptF2htg%253D%253D&md5=4047a61dc04d2b182066d268484328aaMolecular conformations at the cellulose-water interfaceNewman, Roger H.; Davidson, Tony C.Cellulose (Dordrecht, Netherlands) (2004), 11 (1), 23-32CODEN: CELLE8; ISSN:0969-0239. (Kluwer Academic Publishers)13C-NMR chem. shifts were measured for C-4 and C-6 in a collection of eight cryst. glucoses and glucosides. The influence of the hydroxymethyl conformation was greater at C-4 than at C-6, with mean chem. shifts for gauche-trans mols. displaced 3.1 ppm (C-4) and 2.5 ppm (C-6) relative to gauche-gauche mols. This information was used to interpret 13C-NMR spectra of cryst. celluloses. Chem. shifts for C-4 in the crystallite cores of celluloses I and II differed by just 0.2 ppm, but the corresponding chem. shifts for well-ordered crystallite surfaces differed by 3.0 ppm. The sepn. between crystallite-surface signals was attributed to different hydroxymethyl conformations at the cellulose-water interface, i.e., gauche-gauche and gauche-trans on crystallites of cellulose I and cellulose II, resp. A broad C-4 signal in the spectrum of cellulose II indicated gauche-gauche conformations in disordered cellulose. Chem. shifts for C-6 were consistent with these conformations.
- 39Nishiyama, Y.; Langan, P.; Chanzy, H. Crystal structure and hydrogen-bonding system in cellulose 1 beta from synchrotron X-ray and neutron fiber diffraction. J. Am. Chem. Soc. 2002, 124 (31), 9074– 9082, DOI: 10.1021/ja0257319Google ScholarThere is no corresponding record for this reference.
- 40Sugiyama, J.; Vuong, R.; Chanzy, H. Electron-Diffraction Study on the 2 Crystalline Phases Occurring in Native Cellulose from an Algal Cell-Wall. Macromolecules 1991, 24 (14), 4168– 4175, DOI: 10.1021/ma00014a033Google ScholarThere is no corresponding record for this reference.
- 41Wada, M.; Okano, T.; Sugiyama, J. Synchrotron-radiated X-ray and neutron diffraction study of native cellulose. Cellulose 1997, 4 (3), 221– 232, DOI: 10.1023/A:1018435806488Google Scholar41https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXmtlansL4%253D&md5=c0a4913ff273be0baeab731baa522255Synchrotron-irradiated x-ray and neutron diffraction study of native celluloseWada, Masahisa; Okano, Takeshi; Sugiyama, JunjiCellulose (London) (1997), 4 (3), 221-232CODEN: CELLE8; ISSN:0969-0239. (Chapman & Hall)Precise detn. of d-spacings and compositional ratio of cellulose Iα and Iβ in various native cellulose samples was successfully carried out by synchrotron-irradiated X-ray diffraction and time-of-flight (TOF) neutron diffraction from quasi-powder specimens. X-ray diffraction peaks were sepd. by the deconvolution method using 6 types of profile function, i.e., Gaussian, Lorentzian, intermediate Lorentzian, modified Lorentzian, pseudo-Voigt, and Pearson VII. In terms of R-factors, the pseudo-Voigt function gave the best fit with the observation, and was used for detn. of d-spacings. The numerical results for Valonia cellulose were: dIα (100)=0.613 nm; dIβ (11-0)=0.603 nm; dIβ (110)=0.535 nm; dIα (010)=0.529 nm; Iα content=0.65. The differences detd. between dIα (100) and dIβ (11-0) and between dIβ (110) and dIα (010) were similar to those previously reported. Comparison between unresolved peaks for the 2 types of cellulose samples revealed a small but definite difference between dIα (110) and dIβ (200). The TOF neutron diffractometry using deuterated samples confirmed this difference.
- 42Wang, T.; Phyo, P.; Hong, M. Multidimensional solid-state NMR spectroscopy of plant cell walls. Solid State Nucl. Magn. Reson. 2016, 78, 56– 63, DOI: 10.1016/j.ssnmr.2016.08.001Google Scholar42https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsVSrsbbN&md5=b76f0babb754b30a28c90e6c57409958Multidimensional solid-state NMR spectroscopy of plant cell wallsWang, Tuo; Phyo, Pyae; Hong, MeiSolid State Nuclear Magnetic Resonance (2016), 78 (), 56-63CODEN: SSNRE4; ISSN:0926-2040. (Elsevier)Plant biomass has become an important source of bio-renewable energy in modern society. The mol. structure of plant cell walls is difficult to characterize by most at.-resoln. techniques due to the insol. and disordered nature of the cell wall. Solid-state NMR (SSNMR) spectroscopy is uniquely suited for studying native hydrated plant cell walls at the mol. level with chem. resoln. Significant progress has been made in the last five years to elucidate the mol. structures and interactions of cellulose and matrix polysaccharides in plant cell walls. These studies have focused on primary cell walls of growing plants in both the dicotyledonous and grass families, as represented by the model plants Arabidopsis thaliana, Brachypodium distachyon, and Zea mays. To date, these SSNMR results have shown that (1) cellulose, hemicellulose, and pectins form a single network in the primary cell wall; (2) in dicot cell walls, the protein expansin targets the hemicellulose-enriched region of the cellulose microfibril for its wall-loosening function; and (3) primary wall cellulose has polymorphic structures that are distinct from the microbial cellulose structures. This article summarizes these key findings, and points out future directions of investigation to advance our fundamental understanding of plant cell wall structure and function.
- 43Wang, T.; Yang, H.; Kubicki, J. D.; Hong, M. Cellulose Structural Polymorphism in Plant Primary Cell Walls Investigated by High-Field 2D Solid-State NMR Spectroscopy and Density Functional Theory Calculations. Biomacromolecules 2016, 17 (6), 2210– 22, DOI: 10.1021/acs.biomac.6b00441Google Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XotVSksbs%253D&md5=c47b79b538faeda48e452e4140a2e7cdCellulose Structural Polymorphism in Plant Primary Cell Walls Investigated by High-Field 2D Solid-State NMR Spectroscopy and Density Functional Theory CalculationsWang, Tuo; Yang, Hui; Kubicki, James D.; Hong, MeiBiomacromolecules (2016), 17 (6), 2210-2222CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)The native cellulose of bacterial, algal, and animal origins was well studied structurally using x-ray and neutron diffraction and solid-state NMR spectroscopy, and is known to consist of varying proportions of two allomorphs, Iα and Iβ, which differ in hydrogen bonding, chain packing, and local conformation. In comparison, cellulose structure in plant primary cell walls is much less understood because plant cellulose has lower crystallinity and extensive interactions with matrix polysaccharides. Here we have combined two-dimensional magic-angle-spinning (MAS) solid-state NMR (solid-state NMR) spectroscopy at high magnetic fields with d. functional theory (DFT) calcns. to obtain detailed information about the structural polymorphism and spatial distributions of plant primary-wall cellulose. 2D 13C-13C correlation spectra of uniformly 13C-labeled cell walls of several model plants resolved seven sets of cellulose chem. shifts. Among these, five sets (denoted a-e) belong to cellulose in the interior of the microfibril while two sets (f and g) can be assigned to surface cellulose. Importantly, most of the interior cellulose 13C chem. shifts differ significantly from the 13C chem. shifts of the Iα and Iβ allomorphs, indicating that plant primary-wall cellulose has different conformations, packing, and hydrogen bonding from celluloses of other organisms. 2D 13C-13C correlation expts. with long mixing times and with water polarization transfer revealed the spatial distributions and matrix-polysaccharide interactions of these cellulose structures. Celluloses f and g are well mixed chains on the microfibril surface, celluloses a and b are interior chains that are in mol. contact with the surface chains, while cellulose c resides in the core of the microfibril, outside spin diffusion contact with the surface. Interestingly, cellulose d, whose chem. shifts differ most significantly from those of bacterial, algal, and animal cellulose, interacts with hemicellulose, is poorly hydrated, and is targeted by the protein expansin during wall loosening. To obtain information about the C6 hydroxymethyl conformation of these plant celluloses, we carried out DFT calcns. of 13C chem. shifts, using the Iα and Iβ crystal structures as templates and varying the C5-C6 torsion angle. Comparison with the exptl. chem. shifts suggests that all interior cellulose favor the tg conformation, but cellulose d also has a similar propensity to adopt the gt conformation. Cellulose in plant primary cell walls, due to their interactions with matrix polysaccharides, and has polymorphic structures that are not a simple superposition of the Iα and Iβ allomorphs, thus distinguishing them from bacterial and animal celluloses.
- 44Debzi, E. M.; Chanzy, H.; Sugiyama, J.; Tekely, P.; Excoffier, G. The I-Alpha-]I-Beta Transformation of Highly Crystalline Cellulose by Annealing in Various Media. Macromolecules 1991, 24 (26), 6816– 6822, DOI: 10.1021/ma00026a002Google ScholarThere is no corresponding record for this reference.
- 45Wada, M.; Kondo, T.; Okano, T. Thermally induced crystal transformation from cellulose I-alpha to I-beta. Polym. J. (Tokyo, Jpn.) 2003, 35 (2), 155– 159, DOI: 10.1295/polymj.35.155Google Scholar45https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXitlCms70%253D&md5=d52d61e2e797b33ba725f7f0c1e569d5Thermally induced crystal transformation from cellulose Iα to IβWada, Masahisa; Kondo, Tetsuo; Okano, TakeshiPolymer Journal (Tokyo, Japan) (2003), 35 (2), 155-159CODEN: POLJB8; ISSN:0032-3896. (Society of Polymer Science, Japan)It is known that cellulose Iα is metastable and mostly converted into Iβ phase by a heat treatment of 280°C in an inert gas, helium. To elucidate the mechanism of this heat-induced crystal transformation from cellulose Iα to Iβ, we measured X-Ray diffraction of the highly cryst. cellulose (Iα rich type) samples as a function of temps. In the heating process, d-spacings of equatorial reflections increased in line with the temp., and furthermore the rate of change of the d-spacings increased considerably above 200°C. This result indicated that 200°C was the crit. temp. for breaking intermol. hydrogen bonds. Above that temp., cellulose mol. chains became more flexible, inducing a thermal expansion of the crystal lattice, and it formed a transformation intermediate, a "high-temp." structure. In the cooling process, new types of hydrogen bonds may form at 200°C as a result of the transformation from Iα to Iβ phase. This indicates that Iα transformed into Iβ via the above expanded intermediate. The heat-induced expansion of the crystal lattice may be a trigger for the rearrangement of the hydrogen bonds, which may enhance the transformation from Iα to Iβ.
- 46Yamamoto, H.; Horii, F. CP/MAS 13C NMR Analysis of the Crystal Transformation Induced for Valonia Cellulose by Annealing at HighTemperatures. Macromolecules 1993, 26 (6), 1313– 1317, DOI: 10.1021/ma00058a020Google ScholarThere is no corresponding record for this reference.
- 47French, A. D.; Johnson, G. P. Cellulose and the twofold screw axis: modeling and experimental arguments. Cellulose 2009, 16 (6), 959– 973, DOI: 10.1007/s10570-009-9347-4Google Scholar47https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFaqsb3F&md5=0beaeb2a70cd9462b8b0e9a35d4365d8Cellulose and the twofold screw axis: modeling and experimental argumentsFrench, Alfred D.; Johnson, Glenn P.Cellulose (Dordrecht, Netherlands) (2009), 16 (6), 959-973CODEN: CELLE8; ISSN:0969-0239. (Springer)Crystallog. indicates that mols. in cryst. cellulose either have twofold screw-axis (21) symmetry or closely approx. it, leading to short distances between H4 and H1' across the glycosidic linkage. Therefore, modeling studies of cellobiose often show elevated energies for 21 structures, and exptl. observations are often interpreted in terms of intramol. strain. Also, some computer models of cellulose crystallites have an overall twist as well as twisted individual chains, again violating 21 symmetry. To gain insight on the question of inherent strain in 21 structures, modeling was employed and crystal structures of small mols. were surveyed. Residues in a disaccharide cannot be related by 21 symmetry because they are not identical but if their linkage geometry would lead to 21 symmetry for an infinite cellulose chain, the disaccharide would have 21 pseudo symmetry. Several initial structures in quantum mechanics (QM) studies of cellobiose minimized to structures having 21 pseudo symmetry. Similarly, a no. of relevant small mols. in exptl. crystal structures have pseudo symmetry. While the QM models of cellobiose with 21 pseudo symmetry had inter-residue hydrogen bonding, the exptl. studies included cellotriose undecaacetate, a mol. that cannot form conventional hydrogen bonds. Limitations in characterizing symmetry based on the linkage torsion angles .vphi. and ψ were also explored. It is concluded that 21 structures have little intrinsic strain, despite indications from empirical models.
- 48Huber, K. C.; McDonald, A.; BeMiller, J. N. Carbohydrate Chemistry. Handbook of Food Science, Technology, and Engineering 2006, 1, 1– 23, DOI: 10.1201/b15995-3Google ScholarThere is no corresponding record for this reference.
- 49Kovalenko, V. I. Crystalline Cellulose: Structure and Hydrogen Bonds. Russ. Chem. Rev. 2010, 79 (3), 231, DOI: 10.1070/RC2010v079n03ABEH004065Google Scholar49https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXovFejtbw%253D&md5=d9c70ae376df3dfa368fe3844e3ea5e1Crystalline cellulose: structure and hydrogen bondsKovalenko, V. I.Russian Chemical Reviews (2010), 79 (3), 231-241CODEN: RCRVAB; ISSN:0036-021X. (Turpion Ltd.)A review. The state-of-the-art studies of the cellulose structure, major cellulose polymorphs, the crystal packing, conformers and hydrogen bond systems are analyzed based on publications of the last decade. The bibliog. includes 50 refs.
- 50Tsuji, M.; Manley, R. S. Image-Analysis in the Electron-Microscopy of Cellulose Protofibrils. Colloid Polym. Sci. 1984, 262 (3), 236– 244, DOI: 10.1007/BF01458967Google Scholar50https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2cXhsFygu7o%253D&md5=855183d14f66cf7a5c6cfd880427ef8fImage analysis in the electron microscopy of cellulose protofibrilsTsuji, M.; Manley, R. S. J.Colloid and Polymer Science (1984), 262 (3), 236-44CODEN: CPMSB6; ISSN:0303-402X.The optical diffraction pattern (ODP) of approx. defocused electron microscope images of ramie cellulose [9004-34-6] protofibrils (Bohemia nivea) showed discrete max. in the resoln. range of 2-5 nm and was dominated by strong equatorial reflections arising from the lateral spacing of the protofibrils which is ∼5 nm. From the obsd. reciprocal lattice net, it was concluded that the protofibrils are characterized by some form of axial texture that repeats with a periodicity of ∼6 nm. It was emphasized that the ODP derives from gross structural features related to the morphol. of protofibrils and not from their internal structure.
- 51Brown, R. M.; Montezinos, D. Cellulose Microfibrils - Visualization of Biosynthetic and Orienting Complexes in Association with Plasma-Membrane. Proc. Natl. Acad. Sci. U. S. A. 1976, 73 (1), 143– 147, DOI: 10.1073/pnas.73.1.143Google ScholarThere is no corresponding record for this reference.
- 52Kimura, S.; Laosinchai, W.; Itoh, T.; Cui, X.; Linder, C. R.; Brown, R. M. Immunogold labeling of rosette terminal cellulose-synthesizing complexes in the vascular plant Vigna angularis. Plant Cell 1999, 11 (11), 2075– 2085, DOI: 10.2307/3871010Google ScholarThere is no corresponding record for this reference.
- 53Brown, R. M. The biosynthesis of cellulose. J. Macromol. Sci., Part A: Pure Appl.Chem. 1996, A33 (10), 1345– 1373, DOI: 10.1080/10601329608014912Google Scholar53https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XmtFyrtLg%253D&md5=615a57c711d8d72b67d156134bcc3bdfThe biosynthesis of celluloseBrown, R. Malcolm, Jr.Journal of Macromolecular Science, Pure and Applied Chemistry (1996), A33 (10), 1345-1373CODEN: JSPCE6; ISSN:1060-1325. (Dekker)A review, with 68 refs. Cellulose is one of the major com. products of Sweden and constitutes the most abundant of the natural polymer systems. Thus, it is of interest to review the mol. design and architecture of cellulose with particular ref. to the controls of its biosynthesis. The bioassembly process is highly ordered and structured, reflecting the intricate series of events which must occur to generate a thermodn. metastable cryst. submicroscopic, ribbonlike structure. The plant cell wall is an extremely complex composite of many different polymers. Cellulose is the "reinforcing rod" component of the wall. True architectural design demands a polymer which can withstand great flexing and torsional strain. Using comparative Hydrophobic Cluster Anal. of a bacterial cellulose synthase and other glycosyl transferases, the multidomain architecture of glycosyl transferases has been analyzed. All polymn. reactions which are processive require at least three catalytic sites located on two different domains. In contrast, retaining reactions with glycosyl transferases require only a single domain and two sites. Cellulose synthase appears to have evolved a mechanism to simultaneously bind at least three UDP-glucoses and to polymerize, by double addn., two UDP-glucoses in such a manner that the 2-fold screw axis of the β-1,4-glucan chain is maintained. Thus, no primer is required as the glucose monomers are added two-by-two to the growing chain. At the next higher level of assembly, the catalytic sites simultaneously polymerize parallel glucan chain polymers in close proximity so that they will favorably assoc. to crystallize into the metastable cellulose I allomorph. Recent energy anal. suggests that the first stage of this assocn. is the formation of a minisheet through van der Waals forces, followed by layering of these minisheets to form the cryst. microfibril. In native cellulose biogenesis, the microfibril shape and size appear to be detd. by a multimeric enzyme complex (TC) which resides in the plasma membrane. This complex, known as a terminal complex, was discovered through electron microscopy of freeze fracture replicas. The entire complex moves in the plane of the fluid plasma membrane as the result of polymn./crystn. reactions The assembly stages for native cellulose I are coordinated on a spatial/temporal scale, and they are under the genetic control of the organism. This might lead one to conclude that cellulose I could only be assembled with Nature's indigenous machinery; however, this is not the case. Recently, in collaboration with Professor Kobayashi and his colleagues in Sendai and Tokyo, we have synthesized cellulose I abiotically under conditions very different from those in the living cell or from isolated cell components. Purifn. of an endoglucanase from Trichoderma which serves as the catalyst and the addn. of β-cellobiosyl fluoride as the substrate in acetonitrile/acetate buffer has led to the assembly of synthetic cellulose I. Although natural and synthetic assembly pathways are very different, there are similar, underlying fundamental mechanisms common to both. These mechanisms will be discussed in relation to the more thermodn. stable allomorph of cellulose (cellulose II) first demonstrated by Professor Ranby in 1952. The evolution of cellulose biosynthesis will be summarized in terms of the demands for maintaining optimal cellular environments to generate the complex macromol. assemblies for cell wall biogenesis. Nature provides an exceptional model for cellulose biosynthesis that will lead us toward the biotechnol. prodn. of improved natural cellulose as well as synthetic cellulose and its derivs.
- 54Mueller, S. C.; Brown, R. M. Evidence for an Intramembrane Component Associated with a Cellulose Microfibril-Synthesizing Complex in Higher-Plants. J. Cell Biol. 1980, 84 (2), 315– 326, DOI: 10.1083/jcb.84.2.315Google ScholarThere is no corresponding record for this reference.
- 55Fernandes, A. N.; Thomas, L. H.; Altaner, C. M.; Callow, P.; Forsyth, V. T.; Apperley, D. C.; Kennedy, C. J.; Jarvis, M. C. Nanostructure of cellulose microfibrils in spruce wood. Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (47), E1195– E1203, DOI: 10.1073/pnas.1108942108Google ScholarThere is no corresponding record for this reference.
- 56Nishiyama, Y. Structure and properties of the cellulose microfibril. J. Wood Sci. 2009, 55 (4), 241– 249, DOI: 10.1007/s10086-009-1029-1Google Scholar56https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtVSlt7%252FN&md5=09379ba73c9da64fac02811ff2a0b302Structure and properties of the cellulose microfibrilNishiyama, YoshiharuJournal of Wood Science (2009), 55 (4), 241-249CODEN: JWSCFG; ISSN:1435-0211. (Springer Japan)A review. The current structural models of the cellulose microfibril as well as its mech. and thermal properties are reviewed. The cellulose microfibril can be considered as a single thin and long cryst. entity with highly anisotropic phys. properties. The contribution and limit of different methods employed such as electron microscopy, IR spectroscopy, X-ray scattering and diffraction, solid state NMR spectroscopy, and mol. modeling are also discussed.
- 57Nishiyama, Y.; Sugiyama, J.; Chanzy, H.; Langan, P. Crystal structure and hydrogen bonding system in cellulose 1(alpha), from synchrotron X-ray and neutron fiber diffraction. J. Am. Chem. Soc. 2003, 125 (47), 14300– 14306, DOI: 10.1021/ja037055wGoogle Scholar57https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXoslWhsr4%253D&md5=82a43945bad4c540b8692078a45d1555Crystal Structure and Hydrogen Bonding System in Cellulose Iα from Synchrotron X-ray and Neutron Fiber DiffractionNishiyama, Yoshiharu; Sugiyama, Junji; Chanzy, Henri; Langan, PaulJournal of the American Chemical Society (2003), 125 (47), 14300-14306CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The crystal and mol. structure, together with the hydrogen-bonding system in cellulose Iα, has been detd. using at.-resoln. synchrotron and neutron diffraction data recorded from oriented fibrous samples prepd. by aligning cellulose microcrystals from the cell wall of the freshwater alga Glaucocystis nostochinearum. The X-ray data were used to det. the C and O atom positions. The resulting structure is a one-chain triclinic unit cell with all glucosyl linkages and hydroxymethyl groups (tg) identical. However, adjacent sugar rings alternate in conformation giving the chain a cellobiosyl repeat. The chains organize in sheets packed in a "parallel-up" fashion. The positions of hydrogen atoms involved in hydrogen-bonding were detd. from a Fourier-difference anal. using neutron diffraction data collected from hydrogenated and deuterated samples. The differences between the structure and hydrogen-bonding reported here for cellulose Iα and previously for cellulose Iβ provide potential explanations for the solid-state conversion of Iα → Iβ and for the occurrence of two crystal phases in naturally occurring cellulose.
- 58Cosgrove, D. J. Re-constructing our models of cellulose and primary cell wall assembly. Curr. Opin. Plant Biol. 2014, 22, 122– 131, DOI: 10.1016/j.pbi.2014.11.001Google Scholar58https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvFSgs7fO&md5=457d7684de3d47233c899161367d5f63Re-constructing our models of cellulose and primary cell wall assemblyCosgrove, Daniel J.Current Opinion in Plant Biology (2014), 22 (), 122-131CODEN: COPBFZ; ISSN:1369-5266. (Elsevier Ltd.)A review. The cellulose microfibril has more subtlety than is commonly recognized. Details of its structure may influence how matrix polysaccharides interact with its distinctive hydrophobic and hydrophilic surfaces to form a strong yet extensible structure. Recent advances in this field include the first structures of bacterial and plant cellulose synthases and revised ests. of microfibril structure, reduced from 36 to 18 chains. New results also indicate that cellulose interactions with xyloglucan are more limited than commonly believed, whereas pectin-cellulose interactions are more prevalent. Computational results indicate that xyloglucan binds tightest to the hydrophobic surface of cellulose microfibrils. Wall extensibility may be controlled at limited regions ('biomech. hotspots') where cellulose-cellulose contacts are made, potentially mediated by trace amts. of xyloglucan.
- 59Wang, T.; Hong, M. Solid-state NMR investigations of cellulose structure and interactions with matrix polysaccharides in plant primary cell walls. J. Exp. Bot. 2016, 67 (2), 503– 14, DOI: 10.1093/jxb/erv416Google Scholar59https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xht1Squ7fP&md5=1121a4a85a9ecebe1b8744aef47ce3fdSolid-state NMR investigations of cellulose structure and interactions with matrix polysaccharides in plant primary cell wallsWang, Tuo; Hong, MeiJournal of Experimental Botany (2016), 67 (2), 503-514CODEN: JEBOA6; ISSN:0022-0957. (Oxford University Press)Until recently, the 3D architecture of plant cell walls was poorly understood due to the lack of high-resoln. techniques for characterizing the mol. structure, dynamics, and intermol. interactions of the wall polysaccharides in these insol. biomol. mixts. We introduced multidimensional solid-state NMR (SSNMR) spectroscopy, coupled with 13C labeling of whole plants, to det. the spatial arrangements of macromols. in near-native plant cell walls. Here we review key evidence from 2D and 3D correlation NMR spectra that show relatively few cellulose-hemicellulose cross peaks but many cellulose-pectin cross peaks, indicating that cellulose microfibrils are not extensively coated by hemicellulose and all three major polysaccharides exist in a single network rather than two sep. networks as previously proposed. The no. of glucan chains in the primary-wall cellulose microfibrils has been under active debate recently. We show detailed anal. of quant. 13C SSNMR spectra of cellulose in various wild-type (WT) and mutant Arabidopsis and Brachypodium primary cell walls, which consistently indicate that primary-wall cellulose microfibrils contain at least 24 glucan chains.
- 60Kono, H.; Yunoki, S.; Shikano, T.; Fujiwara, M.; Erata, T.; Takai, M. CP/MAS 13C NMR study of cellulose and cellulose derivatives. 1. Complete assignment of the CP/MAS 13C NMR spectrum of the native cellulose. J. Am. Chem. Soc. 2002, 124 (25), 7506– 7511, DOI: 10.1021/ja010704oGoogle Scholar60https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XktVKjsbo%253D&md5=c43b3db13fe24b90b0843e0af4b2e11bCP/MAS 13C NMR study of cellulose and cellulose derivatives. 1. Complete assignment of the CP/MAS 13C-NMR spectrum of the native celluloseKono, Hiroyuki; Yunoki, Shunji; Shikano, Tamio; Fujiwara, Masashi; Erata, Tomoki; Takai, MitsuoJournal of the American Chemical Society (2002), 124 (25), 7506-7511CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The precise assignments of cross polarization/magic angle spinning (CP/MAS) 13C-NMR spectra of cellulose Iα and Iβ were performed by using 13C-labeled cellulose biosynthesized by Acetobacter xylinum (A. xylinum) ATCC10245 strain from culture medium contg. D-[1,3-13C]glycerol or D-[2-13C]glucose as a C source. On the CP/MAS 13C-NMR spectrum of cellulose from D-[1,3-13C]glycerol, the introduced 13C labeling were obsd. at C1, C3, C4, and C6 of the biosynthesized cellulose. In the case of cellulose biosynthesized from D-[2-13C]glucose, the transitions of 13C labeling to C1, C3, and C5 from C2 were obsd. With the quant. anal. of the 13C transition ratio and comparing the CP/MAS 13C-NMR spectrum of the Cladophora cellulose with those of the 13C-labeled celluloses, the assignments of the cluster of resonances which belong to C2, C3, and C5 of cellulose, which have not been assigned before, were performed. As a result, all carbons of cellulose Iα and Iβ except for C1 and C6 of cellulose Iα and C2 of cellulose Iβ were shown in equal intensity of doublet in the CP/MAS spectrum of the native cellulose, which suggests that 2 inequiv. glucopyranose residues were contained in the unit cells of both cellulose Iα and Iβ allomorphs.
- 61Rowland, S. P.; Howley, P. S. Hydrogen-Bonding on Accessible Surfaces of Cellulose from Various Sources and Relationship to Order within Crystalline Regions. J. Polym. Sci., Part A: Polym. Chem. 1988, 26 (7), 1769– 1778, DOI: 10.1002/pola.1988.080260708Google Scholar61https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL1cXltFahtLc%253D&md5=743b16bb3f872684abc9b868b2c24521Hydrogen bonding on accessible surfaces of cellulose from various sources and relationship to order within crystalline regionsRowland, Stanley P.; Howley, Phyllis S.Journal of Polymer Science, Part A: Polymer Chemistry (1988), 26 (7), 1769-78CODEN: JPACEC; ISSN:0887-624X.Celluloses from a variety of common sources were analyzed for availabilities of O(2)H, O(3)H, and O(6)H in order to est. the extent of hydrogen bonding on accessible fibrillar surfaces. Celluloses from flax, ramie, sisal, and wood (both cellulose I and II from wood) together with liq. NH3-swollen cotton and NaOH-swollen cotton (cellulose II) had relative availabilities similar to those of native cotton. Celluloses from Valonia ventricosa and in rayon samples stood apart from each other and from the "cotton family.". The difference between Valonia and cotton celluloses results in addn. to the accepted smaller, less perfect crystallites in cotton, from an O(2)H hydrogen bond which is likely the intramol. bond between O(2)H and O(6')H that is present in Valonia and absent in cotton. Rayon samples also showed evidence of similar bonds involving O(2)H on accessible surfaces. Since the regenerated rayons had relative availabilities different from those of mercerized cotton and wood cellulose samples, it is proposed that chain packing arrangements are not the same in these 2 types of cellulose II.
- 62Eyley, S.; Shariki, S.; Dale, S. E. C.; Bending, S.; Marken, F.; Thielemans, W. Ferrocene-Decorated Nanocrystalline Cellulose with Charge Carrier Mobility. Langmuir 2012, 28 (16), 6514– 6519, DOI: 10.1021/la3001224Google Scholar62https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XltlahtLk%253D&md5=eca716b0196b4bf10c24e3c9d11c7ce0Ferrocene-Decorated Nanocrystalline Cellulose with Charge Carrier MobilityEyley, Samuel; Shariki, Sara; Dale, Sara E. C.; Bending, Simon; Marken, Frank; Thielemans, WimLangmuir (2012), 28 (16), 6514-6519CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Ferrocene-decorated cellulose nanowhiskers were prepd. by the grafting of ethynylferrocene onto azide functionalized cotton-derived cellulose nanowhiskers using azide-alkyne cycloaddn. Successful surface modification and retention of the cryst. morphol. of the nanocrystals was confirmed by elemental anal., inductively coupled plasma-at. emission spectroscopy, XPS, and x-ray diffraction. The coverage with ferrocenyl is high (∼1.14 × 10-3 mol g-1 or 4.6 × 1013 mol cm-2 corresponding to a specific area of 61 Å2 per ferrocene). Cyclic voltammetry measurements of films formed by deposition of ferrocene-decorated nanowhiskers showed that this small spacing of redox centers along the nanowhisker surface allowed conduction hopping of electrons. The apparent diffusion coeff. for electron (or hole) hopping via Fe(III/II) surface sites is estd. as Dapp = 10-19 m2s-1 via impedance methods, a value significantly less than nonsolvated ferrocene polymers, which would be expected as the 1,2,3-triazole ring forms a rigid linker tethering the ferrocene to the nanowhisker surface. In part, this is believed to be also due to bottleneck diffusion of charges across contact points where individual cellulose nanowhiskers contact each other. However, the charge-communication across the nanocrystal surface opens up the potential for use of cellulose nanocrystals as a charge percolation template for the prepn. of conducting films via covalent surface modification (with applications similar to those using adsorbed conducting polymers), for use in bioelectrochem. devices to gently transfer and remove electrons without the need for a soln.-sol. redox mediator, or for the fabrication of three-dimensional self-assembled conducting networks.
- 63Wakelyn, P. J.; Bertoniere, N. R.; French, A. D.; Thibodeaux, D. P.; Triplett, B. A.; Rousselle, M.-A.; Goynes, W. R.; Edwards, J. V.; Hunter, L.; McAlister, D. D.; Gamble, G. R. Cotton Fibers: Chemical properties of cotton. Handbook of Fiber Chemistry 2007, 20061511, 584– 628, DOI: 10.1201/9781420015270.ch9Google ScholarThere is no corresponding record for this reference.
- 64Atkins, P.; de Paula, J. Physical Chemistry, 8th ed.; W. H. Freeman and Company: New York, 2006.Google ScholarThere is no corresponding record for this reference.
- 65Beck, S.; Méthot, M.; Bouchard, J. General procedure for determining cellulose nanocrystal sulfate half-ester content by conductometric titration. Cellulose 2015, 22 (1), 101– 116, DOI: 10.1007/s10570-014-0513-yGoogle Scholar65https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitVenu7rN&md5=576425bc987a747ffa6f13bdfe5c27e8General procedure for determining cellulose nanocrystal sulfate half-ester content by conductometric titrationBeck, Stephanie; Methot, Myriam; Bouchard, JeanCellulose (Dordrecht, Netherlands) (2015), 22 (1), 101-116CODEN: CELLE8; ISSN:0969-0239. (Springer)Charged groups on the surface of cellulose nanocrystals (CNCs) control the colloidal stability and electrostatic and rheol. properties of aq. CNC suspensions, as well as their ability to self-assemble into liq. cryst. structures with unique optical properties. CNCs extd. from wood pulp by sulfuric acid hydrolysis typically contain 200-300 mmol/kg of anionic sulfate half-esters introduced at some of the hydroxyl sites. Two anal. methods to det. CNC surface charge are presented in the published literature: total sulfur content detn. by elemental anal. and protonated sulfate half-ester group detn. by conductometric titrn. with sodium hydroxide. The main drawbacks to elemental anal. are the expensive and complicated instrumentation and sample prepn. procedures it requires. Conductometric titrn. is a much simpler method, but requires complete protonation of the CNC samples in order to obtain an accurate value. Unfortunately, significant discrepancies between the sulfate half-ester contents measured with the two techniques are often obsd. in the literature, particularly with neutralized Na-CNCs. There are specific assumptions and pitfalls inherent to both anal. methods which must be taken into account and reconciled if comparable results are to be achieved by titrn. and elemental anal. In particular, sample prepn. is crucial to obtaining accurate detns. of sulfate half-ester content by conductometric titrn.; however, methods differ widely among labs. and are often not specified in detail, rendering published results meaningless. We have developed a rapid sample prepn. protocol which allows quant. and accurate detn. of the sulfate half-ester content of CNCs from various cellulosic feedstocks (for H- and Na-CNCs, in both never-dried and redispersed dried forms) by conductometric titrn., yielding results in good agreement with the total sulfur content detd. by elemental anal.
- 66Jiang, F.; Esker, A. R.; Roman, M. Acid-Catalyzed and Solvolytic Desulfation of H2SO4-Hydrolyzed Cellulose Nanocrystals. Langmuir 2010, 26 (23), 17919– 17925, DOI: 10.1021/la1028405Google Scholar66https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsVais7nJ&md5=46cefd47b697df351eb18bfb357157e3Acid-Catalyzed and Solvolytic Desulfation of H2SO4-Hydrolyzed Cellulose NanocrystalsJiang, Feng; Esker, Alan R.; Roman, MarenLangmuir (2010), 26 (23), 17919-17925CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Cellulose nanocrystals (CNCs) prepd. by H2SO4 hydrolysis have sulfate groups on their surface, which have neg. implications for some CNC applications. In this study, two desulfation methods were evaluated, and the properties of desulfated CNCs were compared to those of unsulfated CNCs, prepd. by HCl hydrolysis. H2SO4-hydrolyzed CNCs from softwood sulfite pulp were subjected to either a mild acid hydrolytic desulfation or a solvolytic desulfation in DMSO via the pyridinium salt. Removal of the sulfate groups was confirmed by conductometric titrn. and XPS. The effect of the desulfation procedure on the lateral crystallite dimensions was analyzed by X-ray diffraction. The extent of particle aggregation in the samples was assessed by at. force microscopy and dynamic light scattering. The acid hydrolytic method achieved only partial desulfation and produced gradually decreasing sulfate contents upon successive repetition. The solvolytic method achieved nearly complete desulfation in a single step. The desulfated CNCs showed similar particle aggregation as the HCl-hydrolyzed CNCs, but the extent of aggregation was slightly less.
- 67Kloser, E.; Gray, D. G. Surface Grafting of Cellulose Nanocrystals with Poly(ethylene oxide) in Aqueous Media. Langmuir 2010, 26 (16), 13450– 13456, DOI: 10.1021/la101795sGoogle Scholar67https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXptVSlt7k%253D&md5=bdd84434cc28a7bf97a595c0dfc8dbf8Surface Grafting of Cellulose Nanocrystals with Poly(ethylene oxide) in Aqueous MediaKloser, Elisabeth; Gray, Derek G.Langmuir (2010), 26 (16), 13450-13456CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Aq. suspensions of poly(ethylene oxide)-grafted nanocryst. cellulose (PEO-grafted NCC) were prepd. in order to achieve steric instead of electrostatic stabilization. A two-step process was employed: in the first step NCC suspensions prepd. by sulfuric acid hydrolysis were desulfated with sodium hydroxide, and in the second step the surfaces of the crystals were functionalized with epoxy-terminated poly(ethylene oxide) (PEO epoxide) under alk. conditions. The PEO-grafted samples were analyzed by conductometric titrn., ATR-IR, solid-state NMR, MALDI-TOF MS, SEC MALLS, and AFM. The covalent nature of the linkage was confirmed by wt. increase and MALDI-TOF anal. The PEO-grafted cellulose nanocrystals (CNCs) formed a stable colloidal suspension that remained well dispersed, while the desulfated nanoparticles aggregated and pptd. Upon concn. of the PEO-grafted aq. NCC suspension, a chiral nematic phase was obsd.
- 68Habibi, Y.; Chanzy, H.; Vignon, M. R. TEMPO-mediated surface oxidation of cellulose whiskers. Cellulose 2006, 13 (6), 679– 687, DOI: 10.1007/s10570-006-9075-yGoogle Scholar68https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XhtVWltbbO&md5=1cf31ded35a5ff53be949e819c8e373dTEMPO-mediated surface oxidation of cellulose whiskersHabibi, Youssef; Chanzy, Henri; Vignon, Michel R.Cellulose (Dordrecht, Netherlands) (2006), 13 (6), 679-687CODEN: CELLE8; ISSN:0969-0239. (Springer)Cellulose whiskers resulting from HCl acid hydrolysis of the animal cellulose tunicin (extd. from the mantle of Halocynthia roretzi) were subjected to TEMPO-mediated oxidn. under various conditions and the extent of the resulting oxidn. was characterized by Fourier-transform IR spectroscopy (FT-IR), conductometry, X-ray diffraction anal. and transmission electron microscopy (TEM). With degree of oxidn. of up to 0.1 the samples kept their initial morphol. integrity and native crystallinity, but at their surface the hydroxymethyl groups were selectively converted to carboxylic groups, thus imparting a neg. surface charge to the whiskers. When dispersed in water these oxidized whiskers did not flocculate and their suspensions appeared birefringent when viewed between cross polarizers, thus indicating a liq. cryst. behavior.
- 69Lee, S.-Y.; Mohan, D. J.; Kang, I.-A.; Doh, G.-H.; Lee, S.; Han, S. O. Nanocellulose reinforced PVA composite films: Effects of acid treatment and filler loading. Fibers Polym. 2009, 10 (1), 77– 82, DOI: 10.1007/s12221-009-0077-xGoogle Scholar69https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXislOhs7Y%253D&md5=4fac10d81f6d860e29a32e527c9451bbNanocellulose reinforced PVA composite films: Effects of acid treatment and filler loadingLee, Sun-Young; Mohan, D. Jagan; Kang, In-Aeh; Doh, Geum-Hyun; Lee, Soo; Han, Seong OkFibers and Polymers (2009), 10 (1), 77-82CODEN: FPIOA6; ISSN:1229-9197. (Korean Fiber Society)Nanocellulose was prepd. by acid hydrolysis of microcryst. cellulose (MCC) at different hydrobromic acid (HBr) concns. Polyvinyl alc. (PVA) composite films were prepd. by the reinforcement of nanocellulose into a PVA matrix at different filler loading levels and subsequent film casting. Chem. characterization of nanocelluloses was performed for the anal. of crystallinity (Xc), d.p. (DP), and mol. wt. (Mw). The mech. and thermal properties of the nanocellulose reinforced PVA films were also measured for tensile strength and thermogravimetric anal. (TGA). The acid hydrolysis decreased steadily the DP and Mw of MCC. The crystallinity of MCC with 1.5 M and 2.5 M HBr showed a significant increase due to the degrdn. of amorphous domains in cellulose. Higher cryst. cellulose showed the higher thermal stability than MCC. From X-ray diffraction (XRD) anal., nanocellulose samples showed the higher peak intensity than MCC cases. Redn. of MCC particle by acid hydrolysis was clearly obsd. from scanning electron microscope (SEM) images. The tensile and thermal properties of PVA composite films were significantly improved with the increase of the nanocellulose loading.
- 70Filson, P. B.; Dawson-Andoh, B. E.; Schwegler-Berry, D. Enzymatic-mediated production of cellulose nanocrystals from recycled pulp. Green Chem. 2009, 11 (11), 1808– 1814, DOI: 10.1039/b915746hGoogle ScholarThere is no corresponding record for this reference.
- 71Lu, Q.; Cai, Z.; Lin, F.; Tang, L.; Wang, S.; Huang, B. Extraction of Cellulose Nanocrystals with a High Yield of 88% by Simultaneous Mechanochemical Activation and Phosphotungstic Acid Hydrolysis. ACS Sustainable Chem. Eng. 2016, 4 (4), 2165– 2172, DOI: 10.1021/acssuschemeng.5b01620Google Scholar71https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XksVSktb4%253D&md5=7d1f3c1c5525f733f188b1898ad61753Extraction of Cellulose Nanocrystals with a High Yield of 88% by Simultaneous Mechanochemical Activation and Phosphotungstic Acid HydrolysisLu, Qilin; Cai, Zhenghan; Lin, Fengcai; Tang, Lirong; Wang, Siqun; Huang, BiaoACS Sustainable Chemistry & Engineering (2016), 4 (4), 2165-2172CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)An efficient approach for extg. cellulose nanocrystals (CNCs) was put forward through phosphotungstic acid (PTA) hydrolysis of cellulose raw materials under mechanochem. activation. Response surface methodol. was employed for exptl. design to det. the optimum conditions of CNCs prepn. with software Design Expert. The results showed that quadratic polynomial model was qualified to represent the relationship between the response and independent variables and the regression model defined well the true behavior of the system. Under the optimal conditions, a high yield of up to 88.4%, crystallinity index of 79.6%, and a higher thermal stability can be achieved by combining mechanochem. activation and phosphotungstic acid hydrolysis. This process can improve effectively the hydrolysis efficiency, avoid a lengthy sepn. process, and reduce the prepn. time. Meanwhile, compared to other techniques, mechanochem. activation is an energy-intensive method, and the process is environment-friendly. Phosphotungstic acid hydrolysis is easier to handle than liq. acids; meanwhile, the catalyst causes fewer corrosion hazards and can readily be recycled. Thus, an efficient green high-yield approach for the prepn. of CNCs was achieved in the study.
- 72Leung, A. C. W.; Hrapovic, S.; Lam, E.; Liu, Y.; Male, K. B.; Mahmoud, K. A.; Luong, J. H. T. Characteristics and Properties of Carboxylated Cellulose Nanocrystals Prepared from a Novel One-Step Procedure. Small 2011, 7 (3), 302– 305, DOI: 10.1002/smll.201001715Google Scholar72https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsFCrsb0%253D&md5=e610178c87a9abce4351b5eb8c00ed8bCharacteristics and Properties of Carboxylated Cellulose Nanocrystals Prepared from a Novel One-Step ProcedureLeung, Alfred C. W.; Hrapovic, Sabahudin; Lam, Edmond; Liu, Yali; Male, Keith B.; Mahmoud, Khaled A.; Luong, John H. T.Small (2011), 7 (3), 302-305CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)We have described a one-step procedure for producing carboxylated cellulose (CNCs) from cellulosic biomass that would carry significant benefits in terms of sustainability and low costs. Our approach could be considered as a chem. nanoscissor effective in processing raw cellulosics, in contrast to acid hydrolysis which requires pretreatment steps for the isolation of cellulose. Our facile procedure is amenable to the large-scale prodn. of CNCs with enhanced uniformity and crystallinity. Tunable hydropilicity or hydrophobicity can be easily attained by conjugation of the carboxyl group with pertinent mols.
- 73Zhou, Y.; Saito, T.; Bergström, L.; Isogai, A. Acid-Free Preparation of Cellulose Nanocrystals by TEMPO Oxidation and Subsequent Cavitation. Biomacromolecules 2018, 19, 633– 639, DOI: 10.1021/acs.biomac.7b01730Google Scholar73https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXitVCmsrbK&md5=9deec7639784749415c70a95480fa4aaAcid-Free Preparation of Cellulose Nanocrystals by TEMPO Oxidation and Subsequent CavitationZhou, Yaxin; Saito, Tsuguyuki; Bergstrom, Lennart; Isogai, AkiraBiomacromolecules (2018), 19 (2), 633-639CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)Softwood bleached kraft pulp (SBKP) and microcryst. cellulose (MCC) were oxidized using a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated system. The TEMPO-oxidized SBKP prepd. with 10 mmol/g NaClO (SBKP-10) had a higher carboxylate mass recovery ratio and higher content than the other prepd. celluloses including the TEMPO-oxidized MCCs. The SBKP-10 was then exposed to cavitation-induced forces through sonication in water for 10-120 min to prep. aq. dispersions of needle-like TEMPO-oxidized cellulose nanocrystals (TEMPO-CNCs) with homogeneous width of 3.5 to 3.6 nm and av. lengths of ∼200 nm. The av. chain lengths of the cellulose mols. that make up the TEMPO-CNCs were less than half the av. lengths of the TEMPO-CNCs. Compared with conventional CNCs prepd. by acid hydrolysis, the TEMPO-CNCs prepd. by the acid-free and dialysis-free process exhibited higher mass recovery ratios, significantly higher amts. of surface anionic groups, and smaller and more homogeneous widths.
- 74Van den Berg, O.; Capadona, J. R.; Weder, C. Preparation of Homogeneous Dispersions of Tunicate Cellulose Whiskers in Organic Solvents. Biomacromolecules 2007, 8 (4), 1353– 1357, DOI: 10.1021/bm061104qGoogle Scholar74https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXisV2qur4%253D&md5=26e7dfda710f212c676fb9b870662c10Preparation of Homogeneous Dispersions of Tunicate Cellulose Whiskers in Organic Solventsvan den Berg, Otto; Capadona, Jeffrey R.; Weder, ChristophBiomacromolecules (2007), 8 (4), 1353-1357CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)The presence of neg. charged sulfate ester moieties on the tunicate whisker surface is the detg. factor in the dispersability in polar aprotic solvents. Without surface charges, unmodified tunicate whiskers (e.g., HC1-TW) do not disperse in polar aprotic solvents. Protic solvents such as formic acid and m-cresol were shown to effectively disrupt the hydrogen bonds in aggregated whiskers, dispersing even the non-charged HC1-TW. To the best of our knowledge, this is the first time birefringent dispersions of unmodified tunicate whiskers were successfully produced. Tunicate whisker dispersions in org. solvents are valuable intermediates for the prepn. of polymer/tunicate whisker nanocomposites. The prepn. of such composites was previously stifled, due to the lack of a common solvent/dispersant for the polymer and the whiskers.
- 75Beck, S.; Bouchard, J.; Berry, R. Dispersibility in Water of Dried Nanocrystalline Cellulose. Biomacromolecules 2012, 13 (5), 1486– 1494, DOI: 10.1021/bm300191kGoogle Scholar75https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XltVOrsLc%253D&md5=f7a48d9f39e5ae4b448629c174a75973Dispersibility in Water of Dried Nanocrystalline CelluloseBeck, Stephanie; Bouchard, Jean; Berry, RichardBiomacromolecules (2012), 13 (5), 1486-1494CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)Dispersibility is important for nanocryst. cellulose (NCC) because recovering the unique suspension and particle properties is essential after the product has been dried for storage or transport. It is our goal to produce dried NCC that redisperses in water to yield colloidal suspensions without the use of additives or a large energy input. In contrast with the as-prepd. acidic form of NCC (H-NCC), suspensions of neutral sodium-form NCC (Na-NCC) dried by evapn., lyophilization, or spray-drying are readily dispersible in water. Suspension properties and NCC particle size detd. by light scattering were used as indicators of dispersion quality. The neutral counterion content, drying technique, freezing action, drying and redispersion concns., and moisture content in the dried NCC were all found to influence dispersibility. When a min. of 94% of the H+ counterion is exchanged for Na+, the neutral salt form is fully dispersible in water even when fully dried. Mild sonication is generally sufficient to recover measured particle sizes identical to those in the never-dried Na-NCC sample. A threshold moisture content of 4 wt % was found, above which dried H-NCC is fully dispersible in water.
- 76Foster, E. J.; Moon, R. J.; Agarwal, U. P.; Bortner, M. J.; Bras, J.; Camarero-Espinosa, S.; Chan, K. J.; Clift, M. J. D.; Cranston, E. D.; Eichhorn, S. J.; Fox, D. M.; Hamad, W. Y.; Heux, L.; Jean, B.; Korey, M.; Nieh, W.; Ong, K. J.; Reid, M. S.; Renneckar, S.; Roberts, R.; Shatkin, J. A.; Simonsen, J.; Stinson-Bagby, K.; Wanasekara, N.; Youngblood, J. Current characterization methods for cellulose nanomaterials. Chem. Soc. Rev. 2018, 47 (8), 2609– 2679, DOI: 10.1039/C6CS00895JGoogle Scholar76https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXnslKrt78%253D&md5=fe4f557d7974b93b74015ba665ebef21Current characterization methods for cellulose nanomaterialsFoster, E. Johan; Moon, Robert J.; Agarwal, Umesh P.; Bortner, Michael J.; Bras, Julien; Camarero-Espinosa, Sandra; Chan, Kathleen J.; Clift, Martin J. D.; Cranston, Emily D.; Eichhorn, Stephen J.; Fox, Douglas M.; Hamad, Wadood Y.; Heux, Laurent; Jean, Bruno; Korey, Matthew; Nieh, World; Ong, Kimberly J.; Reid, Michael S.; Renneckar, Scott; Roberts, Rose; Shatkin, Jo Anne; Simonsen, John; Stinson-Bagby, Kelly; Wanasekara, Nandula; Youngblood, JeffChemical Society Reviews (2018), 47 (8), 2609-2679CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A new family of materials comprised of cellulose, cellulose nanomaterials (CNMs), having properties and functionalities distinct from mol. cellulose and wood pulp, is being developed for applications that were once thought impossible for cellulosic materials. Commercialization, paralleled by research in this field, is fueled by the unique combination of characteristics, such as high on-axis stiffness, sustainability, scalability, and mech. reinforcement of a wide variety of materials, leading to their utility across a broad spectrum of high-performance material applications. However, with this exponential growth in interest/activity, the development of measurement protocols necessary for consistent, reliable and accurate materials characterization has been outpaced. These protocols, developed in the broader research community, are crit. for the advancement in understanding, process optimization, and utilization of CNMs in materials development. This review establishes detailed best practices, methods and techniques for characterizing CNM particle morphol., surface chem., surface charge, purity, crystallinity, rheol. properties, mech. properties, and toxicity for two distinct forms of CNMs: cellulose nanocrystals and cellulose nanofibrils.
- 77Carlsson, L.; Ingverud, T.; Blomberg, H.; Carlmark, A.; Larsson, P. T.; Malmström, E. Surface characteristics of cellulose nanoparticles grafted by surface-initiated ring-opening polymerization of ε-caprolactone. Cellulose 2015, 22 (2), 1063– 1074, DOI: 10.1007/s10570-014-0510-1Google Scholar77https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsFalsL0%253D&md5=ba3c7181021e92193ac9be969448a4b1Surface characteristics of cellulose nanoparticles grafted by surface-initiated ring-opening polymerization of ε-caprolactoneCarlsson, Linn; Ingverud, Tobias; Blomberg, Hanna; Carlmark, Anna; Larsson, Per Tomas; Malmstroem, EvaCellulose (Dordrecht, Netherlands) (2015), 22 (2), 1063-1074CODEN: CELLE8; ISSN:0969-0239. (Springer)In this study, surface-initiated ring-opening polymn. has been employed for the grafting of ε-caprolactone from cellulose nanoparticles, made by partial hydrolysis of cellulose cotton linters. A sacrificial initiator was employed during the grafting reactions, to form free polymer in parallel to the grafting reaction. The d.p. of the polymer grafts, and of the free polymer, was varied by varying the reaction time. The aim of this study was to est. the cellulose nanoparticle degree of surface substitution at different reaction times. This was accomplished by combining measurement results from spectroscopy and chromatog. The prepd. cellulose nanoparticles were shown to have 3.1 (±0.3) % of the total anhydroglucose unit content present at the cellulose nanoparticle surfaces. This effectively limits the amt. of cellulose that can be targeted by the SI-ROP reactions. For a certain SI-ROP reaction time, it was assumed that the resulting d.p. (DP) of the grafts and the DP of the free polymer were equal. Based on this assumption it was shown that the cellulose nanoparticle surface degree of substitution remained approx. const. (3-7 %) and seemingly independent of SI-ROP reaction time. We believe this work to be an important step towards a deeper understanding of the processes and properties controlling SI-ROP reactions occurring at cellulose surfaces.
- 78Labet, M.; Thielemans, W. Improving the reproducibility of chemical reactions on the surface of cellulose nanocrystals: ROP of ε-caprolactone as a case study. Cellulose 2011, 18 (3), 607– 617, DOI: 10.1007/s10570-011-9527-xGoogle Scholar78https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXlsFKiu7g%253D&md5=ad3f0491e3985322113c0d823794aad7Improving the reproducibility of chemical reactions on the surface of cellulose nanocrystals: ROP of ε-caprolactone as a case studyLabet, Marianne; Thielemans, WimCellulose (Dordrecht, Netherlands) (2011), 18 (3), 607-617CODEN: CELLE8; ISSN:0969-0239. (Springer)In our group, we work on the surface modification of cellulose nanocrystals. During this work, we have encountered reproducibility issues when the same reactions were performed on nanocrystals from different hydrolysis batches, indicating a variable surface compn. Given the inherent purity of the nanoparticles themselves, this issue was believed to be due to the presence of adsorbed species at the surface of the nanocrystals blocking reactive sites. To investigate this in detail, nanocrystals from several batches were extd. with different solvents. The effect of these extns. on the surface compn. of the nanowhiskers was investigated, followed by its effect on the Surface-Initiated Ring-Opening Polymn. (SI-ROP) of ε-caprolactone. The extd. impurities were analyzed by NMR (1H and 13C) and MS, showing a variety of adsorbed species which can be removed by solvent extn. A Soxhlet extn. using ethanol before the reaction was shown to be the most effective in removing adsorbed low mol. wt. org. compds. produced during the hydrolysis, resulting in improved reproducibility between reactions using nanocrystals from different batches, as confirmed by FTIR, elemental anal. and XPS. Extn. with ethanol should thus be performed before all reaction as these adsorbed species can be expected to interfere with all surface modification reactions.
- 79Labet, M.; Thielemans, W. Citric acid as a benign alternative to metal catalysts for the production of cellulose-grafted-polycaprolactone copolymers. Polym. Chem. 2012, 3 (3), 679– 684, DOI: 10.1039/c2py00493cGoogle Scholar79https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvFalu7k%253D&md5=5b28626abaca91f9a0826c8cc98786deCitric acid as a benign alternative to metal catalysts for the production of cellulose-grafted-polycaprolactone copolymersLabet, Marianne; Thielemans, WimPolymer Chemistry (2012), 3 (3), 679-684CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)Monocryst. cellulose nanocrystals (nanowhiskers), prepd. by acid hydrolysis of cotton wool followed by Soxhlet extn. in ethanol to remove adsorbed impurities, were modified with poly(.vepsiln.-caprolactone) using a "grafting from" approach, with citric acid, a benign naturally available org. acid, as the catalyst. The influence of catalyst concn., monomer concn., reaction time, and reaction temp. was studied to det. the optimal conditions for the ROP, and to enable us to control the grafted polymer content. The resulting materials were analyzed by FTIR, elemental anal., XPS and contact angle. In addn., homopolymer byproducts were analyzed by GPC. Modified nanoparticles with a PCL content (PCL shell around the cryst. cellulose core) of up to 58% were obtained, about 4 times higher than earlier reported grafted nanoparticles using the common tin(ii) ethylhexanoate catalyst. Nanoparticles with a controlled PCL content could be prepd. and max. PCL content was obtained for [.vepsiln.-CL] : [CA] : [OH]surf = 660 : 10 : 1 at 150° for 2 h under inert atm. Increasing the reaction time did not result in a statistically significant increase in the PCL content. As it is virtually impossible to remove all catalyst after polymn. reactions, the use of a benign, naturally available catalyst in the prodn. of materials aimed at increasing global sustainability is an important step forward.
- 80Bellani, C. F.; Pollet, E.; Hebraud, A.; Pereira, F. V.; Schlatter, G.; Averous, L.; Bretas, R. E. S.; Branciforti, M. C. Morphological, thermal, and mechanical properties of poly(ε-caprolactone)/poly(ε-caprolactone)-grafted-cellulose nanocrystals mats produced by electrospinning. J. Appl. Polym. Sci. 2016, 133 (21), 43445, DOI: 10.1002/app.43445Google ScholarThere is no corresponding record for this reference.
- 81Lin, N.; Chen, G.; Huang, J.; Dufresne, A.; Chang, P. R. Effects of polymer-grafted natural nanocrystals on the structure and mechanical properties of poly(lactic acid): A case of cellulose whisker-graft-polycaprolactone. J. Appl. Polym. Sci. 2009, 113 (5), 3417– 3425, DOI: 10.1002/app.30308Google Scholar81https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXntVGgsbc%253D&md5=dfc65afe364c442224dfb099ebd5e3a3Effects of polymer-grafted natural nanocrystals on the structure and mechanical properties of poly(lactic acid): A case of cellulose whisker-graft-polycaprolactoneLin, Ning; Chen, Guangjun; Huang, Jin; Dufresne, Alain; Chang, Peter R.Journal of Applied Polymer Science (2009), 113 (5), 3417-3425CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)In this work, polysaccharide nanocrystals-rodlike cellulose whiskers (CWs)-were surface-grafted with polycaprolactone (PCL) via microwave-assisted ring-opening polymn., and filaceous cellulose whisker-graft-polycaprolactone (CW-g-PCL) nanoparticles were produced. Moreover, the resultant nanoparticles were incorporated into poly(lactic acid) (PLA) as a matrix, and they showed superior function for enhancing the mech. performance of PLA-based materials in comparison with platelet-like nanoparticles of starch nanocrystal-graft-PCL. The optimal loading level of CW-g-PCL was 8 wt %, and this resulted in simultaneous enhancements of the strength and elongation of approx. 1.9- and 10.7-fold, resp., over those of the neat PLA material. In this case, the rigid CW nanoparticles contributed to the endurance of higher stress, whereas the grafted PCL chains improved the assocn. between the PLA matrix and the CW-g-PCL filler and hence facilitated the transfer of stress to the rigid CW nanoparticles. Furthermore, such a fully biodegradable PLA-based nanocomposite shows great potential for environmentally friendly materials because of its high mech. performance. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009.
- 82Simao, J. A.; Bellani, C. F.; Branciforti, M. C. Thermal properties and crystallinity of PCL/PBSA/cellulose nanocrystals grafted with PCL chains. J. Appl. Polym. Sci. 2017, 134 (8), 44493, DOI: 10.1002/app.44493Google ScholarThere is no corresponding record for this reference.
- 83Tian, C.; Fu, S.; Chen, J.; Meng, Q.; Lucia, L. A. Graft polymerization of ε-caprolactone to cellulose nanocrystals and optimization of grafting conditions utilizing a response surface methodology. Nord. Pulp Pap. Res. J. 2014, 29 (01), 058– 068, DOI: 10.3183/NPPRJ-2014-29-01-p058-068Google Scholar83https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXntVeit78%253D&md5=b656792643149b78711375a420acdd42Graft polymerization of ε-caprolactone to cellulose nanocrystals and optimization of grafting conditions utilizing a response surface methodologyTian, Chen; Fu, Shiyu; Chen, Jianhao; Meng, Qijun; Lucia, Lucian A.Nordic Pulp & Paper Research Journal (2014), 29 (1), 58-68CODEN: NPPJEG; ISSN:0283-2631. (Mid Sweden University)The objective of this study was the grafting, statistical polymn. optimization, and subsequent characterization of poly(ε-caprolactone)-grafted cellulose nanocrystals (CNCs) obtained by ring opening polymn. The most significant variables on the grafting ratio of poly(ε-caprolactone) as obtained from the response surface methodol. (Box-Behnken design) were monomer content, polymn. temp., and polymn. time. It was shown that an exptl. grafting ratio of 134.23% closely matched the predicted rate of 138.57%, within an R2 = 99.79% under the proposed optimized conditions. The optimum operating conditions obtained from the response surface methodol. were the following: ε-caprolactone: CNCs mass ratio (monomer content) of 14:1, polymn. temp. of 130°C and polymn. time of 26.5 h. It was shown that poly(ε-caprolactone) successfully grafted onto the cellulose nanocrystals while cellulose nanocrystals maintained their original morphol. and their native crystallinity. Although, the surface coverage was relatively low, the poly(ε-caprolactone)-grafted nanocellulose was shown to be much more hydrophobic compared to unmodified cellulose nanocrystals.
- 84Braun, B.; Dorgan, J. R.; Hollingsworth, L. O. Supra-Molecular EcoBioNanocomposites Based on Polylactide and Cellulosic Nanowhiskers: Synthesis and Properties. Biomacromolecules 2012, 13 (7), 2013– 2019, DOI: 10.1021/bm300149wGoogle ScholarThere is no corresponding record for this reference.
- 85Peltzer, M.; Pei, A.; Zhou, Q.; Berglund, L.; Jimenez, A. Surface modification of cellulose nanocrystals by grafting with poly(lactic acid). Polym. Int. 2014, 63 (6), 1056– 1062, DOI: 10.1002/pi.4610Google Scholar85https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsFGqtrnI&md5=193fdeb3996fb638c8624a8cb55a4be3Surface modification of cellulose nanocrystals by grafting with poly(lactic acid)Peltzer, Mercedes; Pei, Aihua; Zhou, Qi; Berglund, Lars; Jimenez, AlfonsoPolymer International (2014), 63 (6), 1056-1062CODEN: PLYIEI; ISSN:0959-8103. (John Wiley & Sons Ltd.)The use of biopolymers obtained from renewable resources is currently growing and they have found unique applications as matrixes and/or nanofillers in 'green' nanocomposites. Grafting of polymer chains to the surface of cellulose nanofillers was also studied to promote the dispersion of cellulose nanocrystals in hydrophobic polymer matrixes. The aim of this study was to modify the surface of cellulose nanocrystals by grafting from L-lactide by ring-opening polymn. in order to improve the compatibility of nanocrystals and hydrophobic polymer matrixes. The effectiveness of the grafting was evidenced by the long-term stability of a suspension of poly(lactic acid)-grafted cellulose nanocrystals in chloroform, by the presence of the carbonyl peak in modified samples detd. by Fourier transform IR spectroscopy and by the modification in C1s contributions obsd. by XPS. No modification in nanocrystal shape was obsd. in birefringence studies and transmission electron microscopy. © 2013 Society of Chem. Industry.
- 86Tian, C.; Fu, S.; Habibi, Y.; Lucia, L. A. Polymerization Topochemistry of Cellulose Nanocrystals: A Function of Surface Dehydration Control. Langmuir 2014, 30 (48), 14670– 14679, DOI: 10.1021/la503990uGoogle Scholar86https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvFSqurrJ&md5=4445ac732a645d5dde41269ff0146e7ePolymerization Topochemistry of Cellulose Nanocrystals: A Function of Surface Dehydration ControlTian, Chen; Fu, Shiyu; Habibi, Youssef; Lucia, Lucian A.Langmuir (2014), 30 (48), 14670-14679CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)The activation (dehydration) of cellulose nanocrystals (CNCs) toward surface "brush" polymn. is accomplished either by freeze drying or solvent exchange. However, the question of which one of these protocols to choose over the other is generally open-ended. The current study attempts to shed light on this question by installing a std. polymer, polycaprolactone (PCL), onto the surface of both freeze-dried and solvent-exchanged CNCs by ring-opening polymn. (ROP) and examg. the differences in polymn. and final product properties. The work is the first to demonstrate that the efficiency of surface polymn. and final product properties are in fact influenced by the protocols. The differences between the two sample PCL-grafted CNCs were investigated by XPS, elemental anal., gel permeation chromatog. (GPC), and contact-angle measurements. The freeze-dried samples had a significantly reduced PCL surface d. The crystallinity of the solvent-exchanged PCL-grafted CNCs (SECNC-g-PCL), however, was lower than that of either pure CNCs or freeze-dried PCL-grafted CNCs (FDCNC-g-PCL). It was detd. that solvent exchange sufficiently modified the CNC surface to provide enhanced reactivity, an effect that was not as apparent for FDCNC-g-PCL. The solvent-exchanged CNCs tended to have more porous, nanotextured surfaces that were tended to be more responsive toward brush polymn. In addn. to the phys. dissimilarities in surface morphol. and surface accessibility contributing to topochem. differences between the two species, it was also found that the dispersibility, aggregation, and thermal stability were different.
- 87Bitinis, N.; Verdejo, R.; Bras, J.; Fortunati, E.; Kenny, J. M.; Torre, L.; López-Manchado, M. A. Poly(lactic acid)/natural rubber/cellulose nanocrystal bionanocomposites Part I. Processing and morphology. Carbohydr. Polym. 2013, 96 (2), 611– 620, DOI: 10.1016/j.carbpol.2013.02.068Google Scholar87https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXksFejsb4%253D&md5=7cc8ebda83b4cfa3dc539e7665df2ac4Poly(lactic acid)/natural rubber/cellulose nanocrystal bionanocomposites Part I. Processing and morphologyBitinis, Natacha; Verdejo, Raquel; Bras, Julien; Fortunati, Elena; Kenny, Jose Maria; Torre, Luigi; Lopez-Manchado, Miguel AngelCarbohydrate Polymers (2013), 96 (2), 611-620CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)PLA/NR/cellulose nanowhisker composites were prepd. using three types of cellulose nanocrystals (CNC), i.e. unmodified CNC obtained from acid hydrolysis of microcryst. cellulose and two surface modified CNC. The two modification reactions, consisting on the grafting of long alkyl chains and of PLA chains onto the cellulose nanocrystals were carried out in order to facilitate the incorporation of the nanocrystals in the PLA/NR blend. A novel processing method was optimized combining solvent casting and extrusion in order to obtain a homogeneous dispersion of the nanofillers in the blend. The CNC modifications detd. their location in the PLA/NR blend and influenced its morphol.
- 88Goffin, A.-L.; Raquez, J.-M.; Duquesne, E.; Siqueira, G.; Habibi, Y.; Dufresne, A.; Dubois, P. From Interfacial Ring-Opening Polymerization to Melt Processing of Cellulose Nanowhisker-Filled Polylactide-Based Nanocomposites. Biomacromolecules 2011, 12 (7), 2456– 2465, DOI: 10.1021/bm200581hGoogle Scholar88https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXnt1Gnsro%253D&md5=2d703293a515c6c1a494236aa74521e2From Interfacial Ring-Opening Polymerization to Melt Processing of Cellulose Nanowhisker-Filled Polylactide-Based NanocompositesGoffin, Anne-Lise; Raquez, Jean-Marie; Duquesne, Emmanuel; Siqueira, Gilberto; Habibi, Youssef; Dufresne, Alain; Dubois, PhilippeBiomacromolecules (2011), 12 (7), 2456-2465CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)In the present work, cellulose nanowhiskers (CNWs), extd. from ramie fibers, were incorporated in polylactide (PLA)-based composites. Prior to the blending, PLA chains were chem. grafted on the surface of CNW to enhance the compatibilization between CNW and the hydrophobic polyester matrix. Ring-opening polymn. of L-lactide was initiated from the hydroxyl groups available at the CNW surface to yield CNW-g-PLA nanohybrids. PLA-based nanocomposites were prepd. by melt blending to ensure a green concept of the study thereby limiting the use of org. solvents. The influence of PLA-grafted cellulose nanoparticles on the mech. and thermal properties of the ensuing nanocomposites was deeply investigated. The thermal behavior and mech. properties of the nanocomposites were detd. using differential scanning calorimetry and dynamical mech. and thermal anal., resp. It was clearly evidenced that the chem. grafting of CNW enhances their compatibility with the polymeric matrix and thus improves the final properties of the nanocomposites. Large modification of the cryst. properties such as the crystn. half-time was evidenced according to the nature of the PLA matrix and the content of nanofillers.
- 89Wu, H.; Nagarajan, S.; Zhou, L.; Duan, Y.; Zhang, J. Synthesis and characterization of cellulose nanocrystal-graft-poly(d-lactide) and its nanocomposite with poly(l-lactide). Polymer 2016, 103, 365– 375, DOI: 10.1016/j.polymer.2016.09.070Google Scholar89https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhs1ejs7vI&md5=81df46b96d0d90b45972cf6cc67708cdSynthesis and characterization of cellulose nanocrystal-graft-poly(D-lactide) and its nanocomposite with poly(L-lactide)Wu, Hao; Nagarajan, S.; Zhou, Lijuan; Duan, Yongxin; Zhang, JianmingPolymer (2016), 103 (), 365-375CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)As a popular bioplastic, poly (L-lactic acid) (PLLA) still faces some drawbacks mainly related to its low crystn. rate and heat distortion resistance. Herein, we demonstrate a facile and promising route to solve these problems by compositing PLLA with poly(D-lactide) grafted cellulose nanocrystal (CNC-g-PDLA) which can be synthesized via ring-opening polymn. of D-lactide on the CNC surface. The grafting d. of PDLA on the CNC surface was quantified by newly developed spectral method using FTIR technique. Moreover, the non-isothermal and isothermal crystn. behavior of PLLA/CNC-PDLA composites was studied by DSC, POM and WXRD. Our results show that the addn. of small amt. of CNC-g-PDLA could enhance the crystn. rate of PLLA significantly due to the heterogeneous nucleation effect of CNC and the presence of stereocomplex crystals. In addn., the specific stereocomplex interaction in the interphase of PLLA matrix and modified CNC particles results in great improvement of both storage modulus and the heat distortion resistance of the PLLA/CNC-g-PDLA nanocomposites. This study provides a way to design advanced PLLA-based nanocomposites with fast matrix crystn. ability and excellent heat distortion resistance.
- 90Lizundia, E.; Vilas, J. L.; Leon, L. M. Crystallization, structural relaxation and thermal degradation in Poly(l-lactide)/cellulose nanocrystal renewable nanocomposites. Carbohydr. Polym. 2015, 123, 256– 265, DOI: 10.1016/j.carbpol.2015.01.054Google Scholar90https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXisVOkt7c%253D&md5=d2492c7fd15fca5b0d0abe768df77582Crystallization, structural relaxation and thermal degradation in Poly(L-lactide)/cellulose nanocrystal renewable nanocompositesLizundia, E.; Vilas, J. L.; Leon, L. M.Carbohydrate Polymers (2015), 123 (), 256-265CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)In this work, crystn., structural relaxation and thermal degrdn. kinetics of neat Poly(L-lactide) (PLLA) and its nanocomposites with cellulose nanocrystals (CNC) and CNC-grafted-PLLA (CNC-g-PLLA) have been studied. Although crystallinity degree of nanocomposites remains similar to that of neat homopolymer, results reveal an increase on the crystn. rate by 1.7-5 times boosted by CNC, which act as nucleating agents during the crystn. process. In addn., structural relaxation kinetics of PLLA chains has been drastically reduced by 53% and 27% with the addn. of neat and grafted CNC, resp. The thermal degrdn. activation energy (E) has been detd. from thermogravimetric anal. in the light of Kissinger's and Ozawa-Flynn-Wall theor. models. Results reveal a redn. on the thermal stability when in presence of CNC-g-PLLA, while raw CNC slightly increases the thermal stability of PLLA. Fourier transform IR spectroscopy and energy dispersive X-ray spectroscopy results confirm that the presence of residual catalyst in CNC-g-PLLA plays a pivotal role in the thermal degrdn. behavior of nanocomposites.
- 91Lizundia, E.; Fortunati, E.; Dominici, F.; Vilas, J. L.; Leon, L. M.; Armentano, I.; Torre, L.; Kenny, J. M. PLLA-grafted cellulose nanocrystals: Role of the CNC content and grafting on the PLA bionanocomposite film properties. Carbohydr. Polym. 2016, 142, 105– 113, DOI: 10.1016/j.carbpol.2016.01.041Google Scholar91https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhtl2jt70%253D&md5=98ff8772fbca360428f1da9a88d85eb4PLLA-grafted cellulose nanocrystals: Role of the CNC content and grafting on the PLA bionanocomposite film propertiesLizundia, Erlantz; Fortunati, Elena; Dominici, Franco; Vilas, Jose Luis; Leon, Luis Manuel; Armentano, Ilaria; Torre, Luigi; Kenny, Jose M.Carbohydrate Polymers (2016), 142 (), 105-113CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)Cellulose nanocrystals (CNC), extd. from microcryst. cellulose by acid hydrolysis, were grafted by ring opening polymn. of L-Lactide initiated from the hydroxyl groups available at their surface and two different CNC:L-lactide ratios (20:80 and 5:95) were obtained. The resulting CNC-g-PLLA nanohybrids were incorporated in poly(lactic acid) (PLA) matrix by an optimized extrusion process at two different content (1 wt.% and 3 wt.%) and obtained bionanocomposite films were characterized by thermal, mech., optical and morphol. properties. Thermal anal. showed CNC grafted with the higher ratio of lactide play a significant role as a nucleating agent. Moreover, they contribute to a significant increase in the crystn. rate of PLA, and the best efficiency was revealed with 3 wt.% of CNC-g-PLLA. This effect was confirmed by the increased in Young's modulus, suggesting the CNC graft ratio and content contribute significantly to the good dispersion in the matrix, pos. affecting the final bionanocomposite properties.
- 92Tang, J.; Sisler, J.; Grishkewich, N.; Tam, K. C. Functionalization of cellulose nanocrystals for advanced applications. J. Colloid Interface Sci. 2017, 494, 397– 409, DOI: 10.1016/j.jcis.2017.01.077Google Scholar92https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXitlOrsbg%253D&md5=dc0a305675e1768dc9e9b75ac44a7c50Functionalization of cellulose nanocrystals for advanced applicationsTang, Juntao; Sisler, Jared; Grishkewich, Nathan; Tam, Kam ChiuJournal of Colloid and Interface Science (2017), 494 (), 397-409CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)A review concerning functionalized cellulose nanocrystals as green, sustainable material for advanced applications, particularly as pollutant adsorbents, is given. Topics discussed include: introduction; cellulose nanocrystals (phys. properties, surface chem. properties); advanced applications (emulsion stabilizer, functional material templates, nanocrystal-inorg. hybrids, nanocrystal-org. hybrids); and conclusions and perspectives.
- 93Wohlhauser, S.; Meesorn, W.; Montero de Espinosa, L.; Weder, C. One-component nanocomposites based on homopolymer-grafted cellulose nanocrystals. Manuscript in preparation, 2018.Google ScholarThere is no corresponding record for this reference.
- 94Espino-Perez, E.; Gilbert, R. G.; Domenek, S.; Brochier-Salon, M. C.; Belgacem, M. N.; Bras, J. Nanocomposites with functionalised polysaccharide nanocrystals through aqueous free radical polymerisation promoted by ozonolysis. Carbohydr. Polym. 2016, 135, 256– 266, DOI: 10.1016/j.carbpol.2015.09.005Google ScholarThere is no corresponding record for this reference.
- 95Boujemaoui, A.; Mongkhontreerat, S.; Malmström, E.; Carlmark, A. Preparation and characterization of functionalized cellulose nanocrystals. Carbohydr. Polym. 2015, 115 (Supplement C), 457– 464, DOI: 10.1016/j.carbpol.2014.08.110Google ScholarThere is no corresponding record for this reference.
- 96Liu, T.; Xue, F.; Ding, E. Cellulose nanocrystals grafted with polyacrylamide assisted by macromolecular RAFT agents. Cellulose 2016, 23 (6), 3717– 3735, DOI: 10.1007/s10570-016-1083-yGoogle Scholar96https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhs1eltbfP&md5=0903c425c20dfa09314edf1bb1bc9a04Cellulose nanocrystals grafted with polyacrylamide assisted by macromolecular RAFT agentsLiu, Ting; Xue, Feng; Ding, EnyongCellulose (Dordrecht, Netherlands) (2016), 23 (6), 3717-3735CODEN: CELLE8; ISSN:0969-0239. (Springer)Graft modification is an excellent way to prep. cellulose nanocrystals (CNCs) with various functions. In this work, different water-sol. macro reversible addn. fragmentation chain transfer (macroRAFT) agents were used to mediate grafting of polyacrylamide with cellulose nanocrystals. Fourier-transform IR spectroscopy, 13C solid-state NMR(NMR), XPS, and elemental anal. were used to characterize the presence of polyacrylamide and quantify its content on the modified cellulose nanocrystals. The results show that two amphiphilic macroRAFTs, namely St5AA14RAFT and St7AA14RAFT, increased the wt. ratio of polyacrylamide on the modified cellulose nanocrystals, whereas a corresponding hydrophilic macroRAFT (AA14RAFT) impeded growth of the polymer chains on the cellulose nanocrystals. Examn. of the structure of the graft products and the cleaved polyacrylamide chains revealed that the amphiphilic macroRAFTs were involved in the graft reaction through effective chain transfer, thereby increasing the wt. ratio of polyacrylamide on the modified CNCs.
- 97Coulembier, O.; Dubois, P. Polyesters from beta-lactones. Handbook of Ring-Opening Polymerization 2009, 227– 254, DOI: 10.1002/9783527628407.ch9Google Scholar97https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtFegu7vJ&md5=2bd24e3f3bec5651f7d73e8f43399bcePolyester from β-lactonesCoulembier, Olivier; Dubois, PhilippeHandbook of Ring-Opening Polymerization (2009), (), 227-254CODEN: 69LWJV ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)A review on ring-opening polymn. of lactones. The basic principles of polyester synthesis produced from 2-oxetanone-type monomers, including details of pertinent mechanistic aspects and the general properties of the resultant polymeric materials are reviewed. First a brief overview of the prepn. of β-lactone key derivs. is given.
- 98Albertsson, A.-C.; Varma, I. K.; Srivastava, R. K. Polyesters from large lactones. Handbook of Ring-Opening Polymerization 2009, 287– 306, DOI: 10.1002/9783527628407.ch11Google Scholar98https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtFegu7vE&md5=06336af8ba1fa1a084a3b8d8fe3aee24Polyester from large lactonesAlbertsson, Ann-Christine; Varma, Indra K.; Srivastava, Rajiv K.Handbook of Ring-Opening Polymerization (2009), (), 287-306CODEN: 69LWJV ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)This review discusses the ring-opening polymn. and copolymn. og medium and large-ring sized lactones using nonenzymic (anionic, cationic, and organometallic initiators) as well as enzymic methods. The prepn. of polyesters with pendant functionality or end functionality is also described and the phys. properties and potential applications of such polyesters are briefly mentioned.
- 99Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Polyesters from dilactones. Handbook of Ring-Opening Polymerization 2009, 255– 286, DOI: 10.1002/9783527628407.ch10Google Scholar99https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtFegu7vK&md5=34eedd0f428758e36687eaa7f26b8a26Polyester from dilactonesDechy-Cabaret, Odile; Martin-Vaca, Blanca; Bourissou, DidierHandbook of Ring-Opening Polymerization (2009), (), 255-286CODEN: 69LWJV ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. The first section ficuses on coordination polymn. with metal complexes, classified by the nature of their ancillary ligands. The spectacular achievements reported recently in organo-catalyzed and stereocontrolled ROP are then presented. The third section concerns the macromol. engineering of poly(α-hydroxyacids) by varying both their substitution pattern and their architecture (block, star, dendritic). The well established and rapidly emerging applications of these synthetic polyesters are discussed briefly.
- 100Roda, J. Polyamides. Handbook of Ring-Opening Polymerization 2009, 165– 195, DOI: 10.1002/9783527628407.ch7Google Scholar100https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtFegu7vO&md5=b3aef27aef63d91a31e1d9611613da21PolyamidesRoda, JanHandbook of Ring-Opening Polymerization (2009), (), 165-195CODEN: 69LWJV ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)This review is based largely on the anionic ring-opening polymn. of lactams, using information gathered from a range of excellent review. It also incorporates the details of some interesting findings made over the past two decades. Mechanism of anionic ROP of lactams, initiators, activators, nonactivated polymn., cyclic oligomers of ε-caprolactam, copolymns., nanocomposites, ROP of pyrrolidone, and other aspects are described.
- 101Keul, H. Polycarbonates. Handbook of Ring-Opening Polymerization 2009, 307– 327, DOI: 10.1002/9783527628407.ch12Google Scholar101https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtFegu7vF&md5=15ce9195f26caf65b44b266b54627bcdPolycarbonatesKeul, HelmutHandbook of Ring-Opening Polymerization (2009), (), 307-327CODEN: 69LWJV ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)A review on ring-opening polymn. of cyclic carbonates. This method discloses an alternative route to polycarbonates, which follows a chain-growth mechanism comprising initiation and propagation with termination-, transfer-, and transesterification being absent in the ideal case. Under suitable conditions the ROP of cyclic carbonates fulfils the prerequisites for a controlled polymer synthesis.
- 102Hoogenboom, R. Polyethers and polyoxazolines. Handbook of Ring-Opening Polymerization 2009, 141– 164, DOI: 10.1002/9783527628407.ch6Google Scholar102https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtFegu7vM&md5=36d707f49db5995b303e30864d1eec0cPolyethers and polyoxazolinesHoogenboom, RichardHandbook of Ring-Opening Polymerization (2009), (), 141-164CODEN: 69LWJV ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)A review on the prepn. of polyethers (e.g. polyoxiranes, polyoxetanes, polyTHF) and polyoxazolines by (cationic) ring-opening polymn.
- 103Goffin, A.-L.; Raquez, J.-M.; Duquesne, E.; Siqueira, G.; Habibi, Y.; Dufresne, A.; Dubois, P. Poly(ϵ-caprolactone) based nanocomposites reinforced by surface-grafted cellulose nanowhiskers via extrusion processing: Morphology, rheology, and thermo-mechanical properties. Polymer 2011, 52 (7), 1532– 1538, DOI: 10.1016/j.polymer.2011.02.004Google Scholar103https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXjtFGgu74%253D&md5=30764f3b1d848d1244439a912b95bdacPoly(.vepsiln.-caprolactone) based nanocomposites reinforced by surface-grafted cellulose nanowhiskers via extrusion processing: Morphology, rheology, and thermo-mechanical propertiesGoffin, A.-L.; Raquez, J.-M.; Duquesne, E.; Siqueira, G.; Habibi, Y.; Dufresne, A.; Dubois, Ph.Polymer (2011), 52 (7), 1532-1538CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)Poly(ε-caprolactone)-grafted cellulose nanowhiskers (extd. from ramie: CNWr) synthesized by ring-opening polymn. of the corresponding lactone were studied as "masterbatches" by melt-blending within its com. poly(ε-caprolactone) matrix (PCL). For sake of comparison, unmodified CNWr were also dispersed in PCL. The goal of this study consists to evidence the impact of the covalent grafting of CNWr surface on thermo-mech. properties of the com. matrixes. Atomic force microscopy (AFM) attests of the excellent dispersion of the cellulose nanowhiskers within PCL matrix. As a result of the excellent interfacial compatibility between the nanofiller and the matrix, the thermo-mech. and rheol. performances were largely enhanced.
- 104Goffin, A.-L.; Habibi, Y.; Raquez, J.-M.; Dubois, P. Polyester-Grafted Cellulose Nanowhiskers: A New Approach for Tuning the Microstructure of Immiscible Polyester Blends. ACS Appl. Mater. Interfaces 2012, 4 (7), 3364– 3371, DOI: 10.1021/am3008196Google Scholar104https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xpt1Cgurk%253D&md5=aa2d7b02a470de7d6b48c09b5e63c9ebPolyester-Grafted Cellulose Nanowhiskers: A New Approach for Tuning the Microstructure of Immiscible Polyester BlendsGoffin, Anne-Lise; Habibi, Youssef; Raquez, Jean-Marie; Dubois, PhilippeACS Applied Materials & Interfaces (2012), 4 (7), 3364-3371CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)Cellulose nanowhiskers (CNW), extd. from ramie fibers by sulfuric acid hydrolysis, were used as substrates to compatibilize binary polyester blends contg. 50/50 (wt./wt.) polycaprolactone (PCL) and polylactide (PLA). To tailor their interfacial energy and fine-tune their adhesion with the components of the blend, CNW were subjected to different surface polyester grafting by the means of ring-opening polymn. PCL and PLA homopolyesters as well as P(CL-b-LA) diblock copolymers were successfully grafted on the surface of CNW and the resulting substrates were loaded into the PCL/PLA blend by melt-blending. Morphol. and rheol. analyses were conducted in order to evaluate the ability of these nanoparticles to enhance the compatibility of PCL/PLA blends. Our results showed that unmodified CNW as well as (co)polyester-grafted CNW improved, at different levels, the compatibility of PCL/PLA blends by preventing from coalescence the dispersed domains. (Co)polyester-grafted CNW also enhance the mech. properties of the blend, which can be explained by the formation of cocontinuous phase morphol. at the interface. Our findings suggest that (co)polyester-grafted CNW, esp. CNW-g-P(CL-b-LA) diblock copolymers, can serve as a suitable nanofiller to tune the compatibility of PCL/PLA blends and their related microstructures.
- 105Muiruri, J. K.; Liu, S.; Teo, W. S.; Kong, J.; He, C. Highly Biodegradable and Tough Polylactic Acid-Cellulose Nanocrystal Composite. ACS Sustainable Chem. Eng. 2017, 5 (5), 3929– 3937, DOI: 10.1021/acssuschemeng.6b03123Google Scholar105https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXks12iu7o%253D&md5=5580882869b078f12b8728cd936e984aHighly Biodegradable and Tough Polylactic Acid-Cellulose Nanocrystal CompositeMuiruri, Joseph K.; Liu, Songlin; Teo, Wern Sze; Kong, Junhua; He, ChaobinACS Sustainable Chemistry & Engineering (2017), 5 (5), 3929-3937CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)Poly(L-lactide) cellulose nanocrystals-filled nanocomposites were fabricated by blending of cellulose nanocrystals-g-rubber-g-poly(D-lactide) (CNC-rD-PDLA) and com. PLLA, in which CNC-g-rubber was synthesized by ring opening polymn. (ROP) of D-lactide and a ε-caprolactone mixt. to obtain CNC-P(CL-DLA), followed by further polymn. of D-lactide to obtain CNC-rD-PDLA. X-ray diffraction (XRD), NMR (NMR), and soly. tests confirmed successful grafting of the rubber segment and the PDLA segment onto CNC. Stereocomplexation between CNC-rD-PDLA nanofillers and PLLA matrix was confirmed by FT-IR, XRD, and differential scanning calorimetry (DSC) characterization. The PLLA/CNC-rD-PDLA nanocomposites exhibited greatly improved tensile toughness. With 2.5% CNC-rD-PDLA loading, strain at break of PLLA/CNC-rD-PDLA was increased 20-fold, and the composite shows potential to replace poly(ethylene terephthalate). SEM and small-angle X-ray scattering (SAXS) investigations revealed that fibrillation and crazing during deformation of PLLA/CNC-rD-PDLA nanocomposites were the major toughening mechanisms in this system. The highly biodegradable and tough cellulose nanocrystals-filled PLLA nanocomposites could tremendously widen the range of industrial applications of PLA.
- 106Ma, P.; Jiang, L.; Xu, P.; Dong, W.; Chen, M.; Lemstra, P. J. Rapid Stereocomplexation between Enantiomeric Comb-Shaped Cellulose-g-poly(l-lactide) Nanohybrids and Poly(d-lactide) from the Melt. Biomacromolecules 2015, 16 (11), 3723– 3729, DOI: 10.1021/acs.biomac.5b01135Google Scholar106https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslGiu7fI&md5=8cb4b9f375c99c568658826da28bf6faRapid Stereocomplexation between Enantiomeric Comb-Shaped Cellulose-g-poly(L-lactide) Nanohybrids and Poly(D-lactide) from the MeltMa, Piming; Jiang, Long; Xu, Pengwu; Dong, Weifu; Chen, Mingqing; Lemstra, Piet J.Biomacromolecules (2015), 16 (11), 3723-3729CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)In this work we report the in situ prepn. of fully biobased stereocomplex poly(lactide) (SC-PLA) nanocomposites grafted onto nanocryst. cellulose (NCC). The stereocomplexation rate by compounding high-molar-mass poly(D-lactide) (PDLA) with comb-like NCC grafted poly(L-lactide) is rather high in comparison with mixts. of PDLA and PLLA. The rapid stereocomplexation was evidenced by a high stereocomplexation temp. (Tc-sc = 145 °C) and a high SC crystallinity (Xc-sc = 38%) upon fast cooling (50 °C/min) from the melt (250 °C for 2 min), which are higher than currently reported values. Moreover, the half-life crystn. time (175-190 °C) of the SC-PLA was shortened by 84-92% in comparison with the PDLA/PLLA blends. The high(er) stereocomplexation rate and the melt stability of the SC in the nanocomposites were ascribed to the nucleation effect of the chem. bonded NCC and the "memory effect" of mol. pairs in the stereocomplex melt because of the confined freedom of the grafted PLLA chains.
- 107De Paula, E. L.; Roig, F.; Mas, A.; Habas, J.-P.; Mano, V.; Pereira, F. V.; Robin, J.-J. Effect of surface-grafted cellulose nanocrystals on the thermal and mechanical properties of PLLA based nanocomposites. Eur. Polym. J. 2016, 84, 173– 187, DOI: 10.1016/j.eurpolymj.2016.09.019Google Scholar107https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsFCmsbfF&md5=06c6e3a2acf5aed6b6d54bf089e28486Effect of surface-grafted cellulose nanocrystals on the thermal and mechanical properties of PLLA based nanocompositesde Paula, Everton Luiz; Roig, Frederic; Mas, Andre; Habas, Jean-Pierre; Mano, Valdir; Pereira, Fabiano Vargas; Robin, Jean-JacquesEuropean Polymer Journal (2016), 84 (), 173-187CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)The effects of polylactide-graft-cellulose nanocrystals on the thermal and mech. properties of poly(L-lactide) matrixes were investigated. Cellulose nanocrystals (CNCs) were grafted with polylactide chains via a solvent-free process by ring-opening polymn. of l-lactide using magnesium hydride as a catalyst. The efficiency of grafting was detd. by infra-red, XPS and NMR analyses. X-ray diffraction analyses showed that the cryst. nature of the CNCs was preserved. Nanocomposites based on poly(L-lactide) matrix contg. ungrafted nanocrystals (PLLA/CNCs) and grafted nanocrystals (PLLA/PLLA-g-CNCs) were investigated. DSC revealed that the grafted nanocrystals exhibited a strong influence on the crystallinity of the nanocomposites, inducing a significant enhancement of the mech. properties of PLLA/PLLA-g-CNCs compared with PLLA/CNCs material. The role played by the polylactide grafted layer on the interaction between the CNCs and PLLA matrix was revealed by mech. analyses in the solid and molten states.
- 108Miao, C.; Hamad, W. Y. In-situ polymerized cellulose nanocrystals (CNC)-poly(L-lactide) (PLLA) nanomaterials and applications in nanocomposite processing. Carbohydr. Polym. 2016, 153, 549– 558, DOI: 10.1016/j.carbpol.2016.08.012Google Scholar108https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhtlalu77J&md5=ac2375a7a08ed5e60bdd468f00865635In-situ polymerized cellulose nanocrystals (CNC)-poly(L-lactide) (PLLA) nanomaterials and applications in nanocomposite processingMiao, Chuanwei; Hamad, Wadood Y.Carbohydrate Polymers (2016), 153 (), 549-558CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)CNC-PLLA nanomaterials were synthesized via in-situ ring-opening polymn. of L-lactide in the presence of CNC, resulting in hydrophobic, homogeneous mixt. of PLLA-grafted-CNC and free PLLA homopolymer. The free PLLA serves two useful functions: as barrier to further prevent PLLA-g-CNC from forming aggregates, and in creating improved interfacial properties when these nanomaterials are blended with other polymers, hence enhancing their performance. CNC-PLLA nanomaterials can be used for medical or engineering applications as-they-are or by compounding with suitable biopolymers using versatile techniques, such as soln. casting, co-extrusion or injection molding, to form hybrid nanocomposites of tunable mech. properties. When compounded with com.-grade PLA, the resulting CNC-PLA nanocomposites appear transparent and have tailored (dynamic and static) mech. and barrier properties, approaching those of poly(ethylene terephthalate), PET. The effect of reaction conditions on the properties of CNC-PLLA nanomaterials have been carefully studied and detailed throughout the paper.
- 109Habibi, Y.; Aouadi, S.; Raquez, J.-M.; Dubois, P. Effects of interfacial stereocomplexation in cellulose nanocrystal-filled polylactide nanocomposites. Cellulose 2013, 20 (6), 2877– 2885, DOI: 10.1007/s10570-013-0058-5Google Scholar109https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvVarsLzP&md5=656e140bf8da9397d66dbe107fdd8249Effects of interfacial stereocomplexation in cellulose nanocrystal-filled polylactide nanocompositesHabibi, Youssef; Aouadi, Sabrina; Raquez, Jean-Marie; Dubois, PhilippeCellulose (Dordrecht, Netherlands) (2013), 20 (6), 2877-2885CODEN: CELLE8; ISSN:0969-0239. (Springer)Cellulose nanocrystals (CNC), extd. from cellulose fibers by the mean of acid hydrolysis, were subjected to ring opening polymn. of d-Lactide initiated from the hydroxyl groups available at their surface. The resulting CNC-g-PDLA nanohybrids were incorporated in poly(l-lactide) (PLLA) matrix through melt-blending and the obtained nanocomposites were characterized in terms of their thermo-mech. properties. Surface-induced polylactide (PLA) stereocomplexation resulting from the co-crystn. of grafted PDLA chains and unbounded PLLA ones from the matrix was evidenced. Acquired results and a thorough comparison with their counterparts grafted with PLLA chains reveal the effects of the stereocomplexed PLA structure surrounding CNC. Important alterations of both cryst. structure and its behavior of the final materials were obsd. originated from an efficient nucleation effect induced by the addn. of these nanohybrids. Significant enhancement of the mech. performances was also obtained most probably brought by the stereocomplexation that stiffens and stabilize further the percolation network.
- 110Ma, P.; Shen, T.; Lin, L.; Dong, W.; Chen, M. Cellulose-g-poly(D-lactide) nanohybrids induced significant low melt viscosity and fast crystallization of fully bio-based nanocomposites. Carbohydr. Polym. 2017, 155, 498– 506, DOI: 10.1016/j.carbpol.2016.09.003Google Scholar110https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsFWisrfJ&md5=06f9a57291a80968ed565cccc8056762Cellulose-g-poly(D-lactide) nanohybrids induced significant low melt viscosity and fast crystallization of fully bio-based nanocompositesMa, Piming; Shen, Tianfeng; Lin, Long; Dong, Weifu; Chen, MingqingCarbohydrate Polymers (2017), 155 (), 498-506CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)Comb-like nanocrystal cellulose graft poly (D-lactide) (PDLA), i.e., NCC-g-PDLA nanohybrids were synthesized and compounded with poly (L-lactide) (PLLA) and poly (hydroxyalkanoate)s (PHA) to make fully biobased nanocomposites. Surprisingly, the complex viscosity of the PLLA/PHA melts was reduced by more than one order of magnitudes, viz. from 4000 to 100 Pa s by incorporation of 2-4 wt% of the NCC-g-PDLA nanohybrids. Meanwhile, the crystn. of the PLLA component was accelerated by the NCC-g-PDLA nanohybrids due to the strong interaction between PDLA and PLLA macromols. The significant redn. in melt viscosity assocg. with unique core-shell-like microstructures due to the synergetic effect of the NCC-g-PDLA nanohybrids and the PHA would facilitate the prepn. of complex-shaped biomass articles and fibers under low(er) pressure and temps., which is beyond pure academic interest.
- 111Carlmark, A.; Larsson, E.; Malmstroem, E. Grafting of cellulose by ring-opening polymerization - A review. Eur. Polym. J. 2012, 48 (10), 1646– 1659, DOI: 10.1016/j.eurpolymj.2012.06.013Google Scholar111https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVOlt7nO&md5=e371da3e079147dc0bf053da97566accGrafting of cellulose by ring-opening polymerization - A reviewCarlmark, Anna; Larsson, Emma; Malmstroem, EvaEuropean Polymer Journal (2012), 48 (10), 1646-1659CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)A review. Homogeneous and heterogeneous grafting from cellulose and cellulose derivs. by ring-opening polymn. (ROP) are reported. Cellulose is biorenewable and biodegradable as well as a stiff material with a relatively low sp. wt., foreseen to be an excellent replacement for synthetic materials. By utilizing ROP of monomers such as ε-caprolactone or l-lactide from cellulose, composite materials with new and/or improved properties can be obtained. Grafting of solid cellulose substrates, such as cotton, microfibrillated cellulose (MFC) or cellulose nanocrystals, renders cellulose that can easily be dispersed into polymer matrixes and may be used as reinforcing elements to improve mech. and/or barrier properties of biocomposites. A surface grafted polymer can also tailor the interfacial properties between a matrix and the fibrillar structure of cellulose. When derivs. of cellulose are grafted with polymers in homogeneous media, amphiphilic materials with interesting properties can be achieved, anticipated to be utilized for applications such as encapsulation and release.
- 112Chen, G.; Dufresne, A.; Huang, J.; Chang, P. R. A Novel Thermoformable Bionanocomposite Based on Cellulose Nanocrystal-graft-Poly(ε-caprolactone). Macromol. Mater. Eng. 2009, 294 (1), 59– 67, DOI: 10.1002/mame.200800261Google Scholar112https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhs1OisrY%253D&md5=a6f3eef61bc2d5291e69eec1d8e3a404A novel thermoformable bionanocomposite based on cellulose nanocrystal-graft-poly(ε-caprolactone)Chen, Guangjun; Dufresne, Alain; Huang, Jin; Chang, Peter R.Macromolecular Materials and Engineering (2009), 294 (1), 59-67CODEN: MMENFA; ISSN:1438-7492. (Wiley-VCH Verlag GmbH & Co. KGaA)This is the first report on a thermoformable bionanocomposite based on a natural nanocrystal and formed by grafting long polymer chains onto the surface of microcryst. cellulose. For the cellulose nanocrystal-graft-poly(ε-caprolactone), the "graft from" strategy contributed to long and dense "plasticizing" PCL tails onto the CN surface as the key of thermoforming. The grafted PCL chains shielded the hydrophilic surface of CN and, hence, showed high water-resistance. Moreover, a strategy for developing new bionanocomposite materials based on natural nanocrystals was presented.
- 113Chen, J.; Wu, D.; Tam, K. C.; Pan, K.; Zheng, Z. Effect of surface modification of cellulose nanocrystal on nonisothermal crystallization of poly(β-hydroxybutyrate) composites. Carbohydr. Polym. 2017, 157, 1821– 1829, DOI: 10.1016/j.carbpol.2016.11.071Google ScholarThere is no corresponding record for this reference.
- 114Hansson, S.; Antoni, P.; Bergenudd, H.; Malmstrom, E. Selective cleavage of polymer grafts from solid surfaces: assessment of initiator content and polymer characteristics. Polym. Chem. 2011, 2 (3), 556– 558, DOI: 10.1039/C0PY00388CGoogle Scholar114https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXjvFWgurg%253D&md5=3707031791577450daadcea2cb7c4213Selective cleavage of polymer grafts from solid surfaces: assessment of initiator content and polymer characteristicsHansson, Susanne; Antoni, Per; Bergenudd, Helena; Malmstroem, EvaPolymer Chemistry (2011), 2 (3), 556-558CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)A novel initiator for atom transfer radical polymn., also allowing for selective cleavage of polymer grafts, was designed and immobilized on a solid substrate. After cleavage, the initiator content was detd. by utilizing Ellman's reagent and the cleaved polymer grafts were isolated and characterized by size exclusion chromatog.
- 115Morandi, G.; Thielemans, W. Synthesis of cellulose nanocrystals bearing photocleavable grafts by ATRP. Polym. Chem. 2012, 3 (6), 1402– 1407, DOI: 10.1039/c2py20069dGoogle Scholar115https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xms1aqt7w%253D&md5=0c261033361c7ace6efb874e4d04cf9fSynthesis of cellulose nanocrystals bearing photocleavable grafts by ATRPMorandi, G.; Thielemans, W.Polymer Chemistry (2012), 3 (6), 1402-1407CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)Cellulose nanocrystals (CNC) were grafted with photocleavable polymeric chains using Atom Transfer Radical Polymn. (ATRP). The nanoparticle synthesis relies on two main steps: first the grafting of a photosensitive moiety bearing an ATRP initiating site onto the surface of CNC, followed by surface initiated ATRP from the modified surface outwards. An original UV-sensitive linker bearing one ATRP initiating site and one anchoring hydroxyl functionality, which was grafted to the CNC surface using a diisocyanate linker, was designed to achieve this goal. Polystyrene (PS) was then polymd. from the modified CNC, resulting in CNC-g-photolinker-PS nanoparticles. The possibility to degraft the PS chains using UV-irradn. was further studied, and PS grafts with a narrow polydispersity index (1.08) and mol. wts. close to those of formed homopolymers through sacrificial initiator addn. were obtained. This is the first example where good control of SI-ATRP from CNC can be achieved. It also confirms that homopolymers initiated by sacrificial initiator in the polymn. medium can be an indication of the grafted polymer brush length. These materials further offer potential as smart delivery vehicles and smart materials.
- 116Klok, H.-A.; Genzer, J. Expanding the Polymer Mechanochemistry Toolbox through Surface-Initiated Polymerization. ACS Macro Lett. 2015, 4 (6), 636– 639, DOI: 10.1021/acsmacrolett.5b00295Google Scholar116https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXptF2lsL4%253D&md5=a31ffd1c4d173bb7ed5a46adf47067eeExpanding the Polymer Mechanochemistry Toolbox through Surface-Initiated PolymerizationKlok, Harm-Anton; Genzer, JanACS Macro Letters (2015), 4 (6), 636-639CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)Surface-initiated polymns. represent a versatile toolbox to generate densely grafted assemblies of chain end-tethered polymers. At sufficiently short interchain distances, surface-grafted polymers are forced into an extended chain conformation, which forms the basis of several unique properties, including their ability to withstand efficiently biofouling or to act as low friction coatings. While the effect on materials properties is well-established, only relatively recently first reports have appeared describing that chain stretching in surface-grafted polymer films also impacts chem. stability/reactivity. This Viewpoint presents surface-initiated polymn. as an alternative polymer mechanochem. tool. The absence of an external force field to induce chain elongation and the possibility to modulate chain stretching by varying brush mol. wt. and grafting d., in conjunction with electrostatic interactions and nanoinclusions that may be present inside the polymeric grafts, make surface-initiated polymn. an attractive tool to both study and understand the effects of polymer chain conformation on the stability/reactivity of surface-grafted polymers.
- 117Sheiko, S. S.; Panyukov, S.; Rubinstein, M. Bond Tension in Tethered Macromolecules. Macromolecules 2011, 44 (11), 4520– 4529, DOI: 10.1021/ma200328hGoogle Scholar117https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXlvFSrtr8%253D&md5=58b9f381b26085ad41a56f905ec3cdf0Bond Tension in Tethered MacromoleculesSheiko, Sergei S.; Panyukov, Sergey; Rubinstein, MichaelMacromolecules (Washington, DC, United States) (2011), 44 (11), 4520-4529CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)The paper presents scaling anal. of mech. tension generated in densely branched macromols. tethered to a solid substrate with a short linker. Steric repulsion between branches results in z-fold amplification of tension in the linker, where z is the no. of chain-like arms. At large z ∼ 100-1000, the generated tension may exceed the strength of covalent bonds and sever the linker. Two types of mol. architectures were considered: polymer stars and polymer "bottlebrushes" tethered to a solid substrate. Depending on the grafting d., one distinguishes the so-called mushroom, loose grafting, and dense grafting regimes. In isolated (mushroom) and loosely tethered bottlebrushes, the linker tension is by a factor of z smaller than the tension in a tethered star with the same no. of arms z. In densely tethered stars, the effect of interchain distance (d) and no. of arms (z) on the magnitude of linker tension is given by f ≃ f0z3/2(b/d) for stars in a solvent environment and f ≃ f0z2(b/d)2 for dry stars, where b is the Kuhn length and f0 ≃ kBT/b is intrinsic bond tension. These relations are also valid for tethered bottlebrushes with long side chains. However, unlike mol. stars, bottlebrushes demonstrate variation of tension along the backbone f ≃ f0s z1/2/d as a function of distance s from the free end of the backbone. In dense brushes (d ≃ bz) with z ≃ 1000, the backbone tension increases from f ≃ f0 ≃ 1 pN at the free end of the backbone (s ≃ b) to its max. f ≃ zf0 ≃ 1 nN at the linker to the substrate (s ≃ zb).
- 118Anzlovar, A.; Huskic, M.; Zagar, E. Modification of nanocrystalline cellulose for application as a reinforcing nanofiller in PMMA composites. Cellulose 2016, 23 (1), 505– 518, DOI: 10.1007/s10570-015-0786-9Google Scholar118https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslGltL7E&md5=526a7aa652d59ceede93f755e3bb0da3Modification of nanocrystalline cellulose for application as a reinforcing nanofiller in PMMA compositesAnzlovar, Alojz; Huskic, Miro; Zagar, EmaCellulose (Dordrecht, Netherlands) (2016), 23 (1), 505-518CODEN: CELLE8; ISSN:0969-0239. (Springer)Nanocryst. cellulose (NCC) with particle length of 100-500 nm was prepd. from microcryst. cellulose and modified with a reversible addn.-fragmentation chain transfer (RAFT) agent, i.e., 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPADB). From the surface of CPADB-modified NCC, Me methacrylate (MMA) was grafted to produce poly(Me methacrylate) (PMMA)-modified NCC. Unmodified NCC, CPADB-modified NCC, and PMMA-modified NCC were further used as reinforcing nanofillers in PMMA composite plates prepd. by bulk polymn. of MMA. Regardless of NCC type, the PMMA plates were transparent to visible light at low NCC concns. but became translucent with increasing NCC concn. At given NCC concn., the visible-light transparency of the PMMA/NCC composites decreased in the order PMMA-modified NCC, CPADB-modified NCC, unmodified NCC. PMMA-modified NCC and CPADB-modified NCC showed moderate potential for reinforcing the PMMA matrix, while unmodified NCC detrimentally affected the mech. properties of the PMMA composites due to more pronounced particle aggregation.
- 119Casas, J.; Persson, P. V.; Iversen, T.; Córdova, A. Direct Organocatalytic Ring-Opening Polymerizations of Lactones. Adv. Synth. Catal. 2004, 346 (9–10), 1087– 1089, DOI: 10.1002/adsc.200404082Google ScholarThere is no corresponding record for this reference.
- 120Hafrén, J.; Córdova, A. Direct Organocatalytic Polymerization from Cellulose Fibers. Macromol. Rapid Commun. 2005, 26 (2), 82– 86, DOI: 10.1002/marc.200400470Google Scholar120https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhtFektbo%253D&md5=2d2fe36acde649855799644f55a635c7Direct organocatalytic polymerization from cellulose fibersHafren, Jonas; Cordova, ArmandoMacromolecular Rapid Communications (2005), 26 (2), 82-86CODEN: MRCOE3; ISSN:1022-1336. (Wiley-VCH Verlag GmbH & Co. KGaA)We have developed the first direct, organocatalytic, bulk ring-opening polymn. (ROP) of ε-caprolactone (ε-CL) with solid cotton and paper cellulose as the initiators. The mild ROPs were performed without solvent, and are operationally simple, inexpensive and environmentally benign. Org.-acid-catalyzed heterogeneous derivatization of cellulose provides a novel route to valuable cellulose-based biocompatible nanomaterials. The furnished polymn. products were characterized by FTIR, 1H and 13C NMR spectroscopy, MALDI-TOF mass spectrometry and electron microscopy.
- 121Dadkhah Tehrani, A.; Neysi, E. Surface modification of cellulose nanowhisker throughout graft polymerization of 2-ethyl-2-oxazoline. Carbohydr. Polym. 2013, 97 (1), 98– 104, DOI: 10.1016/j.carbpol.2013.04.082Google ScholarThere is no corresponding record for this reference.
- 122Odian, G. Principles of Polymerization, 4th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2004.Google ScholarThere is no corresponding record for this reference.
- 123Liu, S.; Jin, M.; Chen, Y.; Gao, H.; Shi, X.; Cheng, W.; Ren, L.; Wang, Y. High internal phase emulsions stabilized by supramolecular cellulose nanocrystals and their application as cell-adhesive macroporous hydrogel monoliths. J. Mater. Chem. B 2017, 5 (14), 2671– 2678, DOI: 10.1039/C7TB00145BGoogle Scholar123https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXksVCrur4%253D&md5=2269faca3e956fa54ef3c0de5293d875High internal phase emulsions stabilized by supramolecular cellulose nanocrystals and their application as cell-adhesive macroporous hydrogel monolithsLiu, Sa; Jin, Min; Chen, Yunhua; Gao, Huichang; Shi, Xuetao; Cheng, Wenhua; Ren, Li; Wang, YingjunJournal of Materials Chemistry B: Materials for Biology and Medicine (2017), 5 (14), 2671-2678CODEN: JMCBDV; ISSN:2050-7518. (Royal Society of Chemistry)Nanosized celluloses are attractive building blocks to generate hierarchically advanced materials and have been gradually explored in emulsion applications. Here we report a high internal phase emulsion (HIPE) prepd. by using supramol. cellulose nanocrystals (CNCs) as Pickering stabilizers via one-step emulsification, and interconnected macroporous hybrid hydrogels were produced by utilizing this HIPE as a template. A quadruple hydrogen bonding moiety 2-ureido-4[1H]-pyrimidone (UPy) was firstly grafted onto the surface of cellulose nanocrystals through simple free radical polymn. The polymer grafting was confirmed by elemental anal. and thermogravimetry. The UPy modified CNCs (CNC-UPy) exhibited superior emulsion stabilizing ability compared to the pristine CNCs, and the oil-in-water emulsions with an internal phase vol. ratio of 80% showed good long-term stability. The properties of resulting macroporous polyHIPE hydrogels, such as swelling behaviors, porous structures and mech. strength, were investigated on the dependence of CNC-UPy concns. In addn., the macroporous hybrid hydrogel exhibits excellent cytocompatibility and cell adhesion as demonstrated by mouse bone mesenchymal stem cell (mBMSC) culture. With these promising properties, the developed hydrogels demonstrate great potential as active biol. scaffolds for tissue engineering.
- 124Wu, Y.; Wang, L.; Qing, Y.; Yan, N.; Tian, C.; Huang, Y. A green route to prepare fluorescent and absorbent nano-hybrid hydrogel for water detection. Sci. Rep. 2017, 7 (1), 4380, DOI: 10.1038/s41598-017-04542-7Google Scholar124https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC1cjjtFKisg%253D%253D&md5=153a2f24eacecac6dd51718e85ed21c0A green route to prepare fluorescent and absorbent nano-hybrid hydrogel for water detectionWu Yiqiang; Wang Lijun; Qing Yan; Yan Ning; Tian Cuihua; Huang Yuanxin; Wu Yiqiang; Qing Yan; Yan NingScientific reports (2017), 7 (1), 4380 ISSN:.An environment-friendly fluorescent nano-hybrid hydrogel has been synthesized successfully, from cellulose nanocrystal (CNC), acrylic acid (AA) and phosphorescent Eu(2+)/Dy(3+) doped SrAl2O4 via free radical polymerization. The hydrogel network matrix fixed Eu(2+)/Dy(3+) doped SrAl2O4 nanoparticles by polymer chains with coordinate bonds that prevented particles from aggregating and quenching in water. The fluorescent nano-hybrid hydrogel exhibited extremely high water absorption of which the swelling ratio in distilled water and NaCl salt solution were respectively of 323.35 g/g and 32.65 g/g. Furthermore, the hydrogel displayed excellent water retention property that can keep half of the moisture even exposed to 80 °C for 210 min. Besides, the hydrogel had bright green fluorescence under the sunlight or ultraviolet excitation, and the fluorescence intensity was up to 125477 after swelling 50 times in water. The time-resolved photoluminescence (TRPL) afterglow decay examination showed that the fluorescent emission persisted for 4 h after hydrogels excited at 368 nm wavelength UV-light for 10 min. The fluorescence intensity behaved significant linear relationship with the swelling ratio. As a result, these hydrogels were considered as promising candidates for the preparation of stable and sensitive sensor materials in green water detection.
- 125Anirudhan, T. S.; Rejeena, S. R.; Binusree, J. Adsorptive Separation of Myoglobin from Aqueous Solutions Using Iron Oxide Magnetic Nanoparticles Modified with Functionalized Nanocrystalline Cellulose. J. Chem. Eng. Data 2013, 58 (5), 1329– 1339, DOI: 10.1021/je400088gGoogle ScholarThere is no corresponding record for this reference.
- 126Tang, J.; Lee, M. F. X.; Zhang, W.; Zhao, B.; Berry, R. M.; Tam, K. C. Dual Responsive Pickering Emulsion Stabilized by Poly[2-(dimethylamino)ethyl methacrylate] Grafted Cellulose Nanocrystals. Biomacromolecules 2014, 15 (8), 3052– 3060, DOI: 10.1021/bm500663wGoogle Scholar126https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVyit7nF&md5=9ded3f44fc325dc0e0174fc02b9fd05dDual Responsive Pickering Emulsion Stabilized by Poly[2-(dimethylamino)ethyl methacrylate] Grafted Cellulose NanocrystalsTang, Juntao; Lee, Micky Fu Xiang; Zhang, Wei; Zhao, Boxin; Berry, Richard M.; Tam, Kam C.Biomacromolecules (2014), 15 (8), 3052-3060CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)A weak polyelectrolyte, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), was grafted onto the surface of cellulose nanocrystals via free radical polymn. The resultant suspension of PDMAEMA-grafted-cellulose nanocrystals (PDMAEMA-g-CNC) possessed pH-responsive properties. The grafting was confirmed by FTIR, potentiometric titrn., elementary anal., and thermogravimetric anal. (TGA); the surface and interfacial properties of the modified particles were characterized by surface tensiometer. Compared to pristine cellulose nanocrystals, modified CNC significantly reduced the surface and interfacial tensions. Stable heptane-in-water and toluene-in-water emulsions were prepd. with PDMAEMA-g-CNC. Various factors, such as polarity of solvents, concn. of particles, electrolytes, and pH, on the properties of the emulsions were investigated. Using Nile Red as a florescence probe, the stability of the emulsions as a function of pH and temp. was elucidated. It was deduced that PDMAEMA chains promoted the stability of emulsion droplets and their chain conformation varied with pH and temp. to trigger the emulsification and demulsification of oil droplets. Interestingly, for heptane system, the macroscopic colors varied depending on the pH condition, while the color of the toluene system remained the same. Reversible emulsion systems that responded to pH were obsd. and a thermoresponsive Pickering emulsion system was demonstrated.
- 127Sarac, A. S. Redox polymerization. Prog. Polym. Sci. 1999, 24 (8), 1149– 1204, DOI: 10.1016/S0079-6700(99)00026-XGoogle Scholar127https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXktFKhsQ%253D%253D&md5=004c1136271de24ab4e91cb493512593Redox polymerizationSarac, A. S.Progress in Polymer Science (1999), 24 (8), 1149-1204CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Science Ltd.)A review with refs.; virtually all free-radical chain reactions require a sep. initiation step in which a radical species is generated in the reaction mixt. Some types of chain reactions are initiated by adding a stable free radical, one that shows little or no tendency for self-combination, directly to the reactants, but a sep. initiation step is still involved because these stable radicals are most often inorg. ions or metals. A very effective method of generating free radicals under mild conditions is by one-electron transfer reactions, the most effective of which is redox initiation. This method has found wide application for initiating polymn. reactions and has industrial importance, e.g. in low-temp. emulsion polymns. In this review, in addn. to the classical examples of redox pairs, recently employed metal-ion-org.-compd. redox systems, electrochem. regeneration of reduced metal ions, redox initiation in nonaq. media and transition metal org. halide initiators and metal chelate initiators are all reviewed.
- 128Zhou, C.; Wu, Q.; Yue, Y.; Zhang, Q. Application of rod-shaped cellulose nanocrystals in polyacrylamide hydrogels. J. Colloid Interface Sci. 2011, 353 (1), 116– 123, DOI: 10.1016/j.jcis.2010.09.035Google Scholar128https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3cbivFSktw%253D%253D&md5=cd7921e8f92d08e300827413924c0a0eApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsZhou Chengjun; Wu Qinglin; Yue Yiying; Zhang QuanguoJournal of colloid and interface science (2011), 353 (1), 116-23 ISSN:.Rod-shaped cellulose nanocrystals (CNCs) were manufactured and used to reinforce polyacrylamide (PAM) hydrogels through in situ free-radical polymerization. The gelation process of the nanocomposite hydrogels was monitored on a rheometer using oscillatory shear. The chemical structure, morphology, swelling property, and compression strength of the formed gels were investigated. A possible mechanism for forming hydrogels was proposed. The results showed that CNCs accelerated the formation of hydrogels and increased the effective crosslink density of hydrogels. Thus CNCs were not only a reinforcing agent for hydrogel, but also acted as a multifunctional cross-linker for gelation. The shear storage modulus, compression strength and elastic modulus of the nanocomposite hydrogels were significantly improved because of good dispersion of CNCs in PAM as well as enhanced interfacial interaction between these two components. Among the CNC contents used, a loading of 6.7 w/w% led to the maximum mechanical properties for nanocomposite hydrogels.
- 129Zubik, K.; Singhsa, P.; Wang, Y.; Manuspiya, H.; Narain, R. Thermo-Responsive Poly(N-Isopropylacrylamide)-Cellulose Nanocrystals Hybrid Hydrogels for Wound Dressing. Polymers 2017, 9 (4), 119, DOI: 10.3390/polym9040119Google ScholarThere is no corresponding record for this reference.
- 130Feng, X. D.; Guo, X. Q.; Qiu, K. Y. Study of the initiation mechanism of the vinyl polymerization with the system persulfate/N, N,N′,N′-tetramethylethylenediamine. Makromol. Chem. 1988, 189 (1), 77– 83, DOI: 10.1002/macp.1988.021890108Google Scholar130https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL1cXpsFSjtg%253D%253D&md5=cf019d1138ecd8d1ba11b52c79590bc6Study of the initiation mechanism of the vinyl polymerization with the system persulfate/N,N,N',N'-tetramethylethylenediamineFeng, Xinde; Guo, Xinqiu; Qiu, KunyuanMakromolekulare Chemie (1988), 189 (1), 77-83CODEN: MACEAK; ISSN:0025-116X.The reaction mechanism of the redox initiation system persulfate-Me2NCH2CH2NMe2 (I) was studied by spin trapping technique and ESR spectra. The free radical Me2NCH2CH2NMeCH2· (II) was one of the initial free radicals responsible for the initiation of vinyl polymn. in addn. to the sulfate free radical (HO3SO·). Amino end group anal. confirmed the fact that II initiated vinyl polymn. and became the end group of the resulting polymers. The effect of I concn. on the final percentage of conversion of acrylamide polymn. and on the mol. wts. of the resulting polymer were also studied. An initiation mechanism was proposed.
- 131Feng, X. D. The role of amine in vinyl radical polymerization. Makromol. Chem., Macromol. Symp. 1992, 63 (1), 1– 18, DOI: 10.1002/masy.19920630105Google Scholar131https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3sXitlCmsrw%253D&md5=d639183f7ac7015abe0e25b09d0e5f30The role of amine in vinyl radical polymerizationFeng, XindeMakromolekulare Chemie, Macromolecular Symposia (1992), 63 (Int. Symp. Olefin Vinyl Polym. Funct.: React., Mech. Ind. Appl., 1991), 1-18CODEN: MCMSES; ISSN:0258-0322.Amine-peroxide (diacyl peroxides, org. hydroperoxides, or persulfates) radical initiation systems for vinyl monomer polymn. as well as photoinduced charge transfer initiation in the presence of amines are reviewed with 36 refs. The role of amine in the form of aminium radical is discussed.
- 132Kan, K. H. M.; Li, J.; Wijesekera, K.; Cranston, E. D. Polymer-Grafted Cellulose Nanocrystals as pH-Responsive Reversible Flocculants. Biomacromolecules 2013, 14 (9), 3130– 3139, DOI: 10.1021/bm400752kGoogle Scholar132https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtFamsbjO&md5=cab48d44c9e75b09a0c5200fe570d3c4Polymer-Grafted Cellulose Nanocrystals as pH-Responsive Reversible FlocculantsKan, Kevin H. M.; Li, Jian; Wijesekera, Kushlani; Cranston, Emily D.Biomacromolecules (2013), 14 (9), 3130-3139CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)Cellulose nanocrystals (CNCs) are a sustainable nanomaterial with applications spanning composites, coatings, gels, and foams. Surface modification routes to optimize CNC interfacial compatibility and functionality are required to exploit the full potential of this material in the design of new products. In this work, CNCs have been rendered pH-responsive by surface-initiated graft polymn. of 4-vinylpyridine with the initiator ceric(IV) ammonium nitrate. The polymn. is a one-pot, water-based synthesis carried out under sonication, which ensures even dispersion of the cellulose nanocrystals during the reaction. The resultant suspensions of poly(4-vinylpyridine)-grafted cellulose nanocrystals (P4VP-g-CNCs) show reversible flocculation and sedimentation with changes in pH; the loss of colloidal stability is visible by eye even at concns. as low as 0.004 wt %. The presence of grafted polymer and the ability to tune the hydrophilic/hydrophobic properties of P4VP-g-CNCs were characterized by Fourier transform IR spectroscopy, elemental anal., electrophoretic mobility, mass spectrometry, transmittance spectroscopy, contact-angle measurements, thermal anal., and various microscopies. Atomic force microscopy showed no observable changes in the CNC dimensions or degree of aggregation after polymer grafting, and a liq. cryst. nematic phase of the modified CNCs was detected by polarized light microscopy. Controlled stability and wettability of P4VP-g-CNCs is advantageous both in composite design, where cellulose nanocrystals generally have limited dispersibility in nonpolar matrixes, and as biodegradable flocculants. The responsive nature of these novel nanoparticles may offer new applications for CNCs in biomedical devices, as clarifying agents, and in industrial sepn. processes.
- 133Pracella, M.; Haque, M. M.-U.; Puglia, D. Morphology and properties tuning of PLA/cellulose nanocrystals bio-nanocomposites by means of reactive functionalization and blending with PVAc. Polymer 2014, 55 (16), 3720– 3728, DOI: 10.1016/j.polymer.2014.06.071Google Scholar133https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtFCgsb3N&md5=8754336616e39141756745df90c67014Morphology and properties tuning of PLA/cellulose nanocrystals bio-nanocomposites by means of reactive functionalization and blending with PVAcPracella, Mariano; Haque, Md. Minhaz-Ul; Puglia, DeboraPolymer (2014), 55 (16), 3720-3728CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)A novel method for the prepn. of PLA bio-nanocomposites contg. cellulose nanocrystals (CNCs) is reported. In order to enhance interfacial adhesion and dispersion of nanocrystals into PLA matrix, functionalization of PLA and CNCs by radical grafting of glycidyl methacrylate (GMA) and pre-dispersion of CNCs in poly (vinyl acetate) (PVAc) emulsion were applied. Morphologies, thermal and mech. properties of nanocomposites for CNCs content of 1-6 wt.% were examd. Addn. of functionalized components (PLA-GMA, CNC-GMA) and/or PVAc dispersed CNCs both improved the phase distribution of nanofiller and tensile properties, compared to the binary PLA/CNC nanocomposites. Thermal analyses demonstrated that glass transition, melting temp. and crystallinity of PLA were affected by the PVAc amt. Nanocomposites with PVAc dispersed CNCs exhibited higher thermal resistance than other composites. The filler effectiveness (CFE) was evaluated for all samples on the basis of storage modulus values: CNC-GMA and PVAc dispersed CNCs (3 wt. %) resulted the most effective fillers.
- 134Kedzior, S. A.; Graham, L.; Moorlag, C.; Dooley, B. M.; Cranston, E. D. Poly(methyl methacrylate)-grafted cellulose nanocrystals: One-step synthesis, nanocomposite preparation, and characterization. Can. J. Chem. Eng. 2016, 94 (5), 811– 822, DOI: 10.1002/cjce.22456Google Scholar134https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XkvV2iu7c%253D&md5=d3636b0452001b1990bbffc48524f049Poly(methyl methacrylate)-grafted cellulose nanocrystals: One-step synthesis, nanocomposite preparation, and characterizationKedzior, Stephanie A.; Graham, Lexa; Moorlag, Carolyn; Dooley, Brynn M.; Cranston, Emily D.Canadian Journal of Chemical Engineering (2016), 94 (5), 811-822CODEN: CJCEA7; ISSN:0008-4034. (John Wiley & Sons, Inc.)Cellulose nanocrystals (CNCs) are ideal reinforcing agents for polymer nanocomposites because they are lightwt. and nano-sized with a large aspect ratio and high elastic modulus. To overcome the poor compatibility of hydrophilic CNCs in non-polar composite matrixes, we grafted poly(Me methacrylate) (PMMA) from the surface of CNCs using an aq., one-pot, free radical polymn. method with ceric ammonium nitrate as the initiator. The hybrid nanoparticles were characterized by CP/MAS NMR, XPS, IR spectroscopy, contact angle, thermogravimetric anal., X-ray diffraction, and at. force microscopy. Spectroscopy demonstrates that 0.11 g/g (11 wt%) PMMA is grafted from the CNC surface, giving PMMA-g-CNCs, which are similar in size and crystallinity to unmodified CNCs but have an onset of thermal degrdn. 45 °C lower. Nanocomposites were prepd. by compounding unmodified CNCs and PMMA-g-CNCs (0.0025-0.02 g/g (0.25-2 wt%) loading) with PMMA using melt mixing and wet ball milling. CNCs improved the performance of melt-mixed nanocomposites at 0.02 g/g (2 wt%) loading compared to the PMMA control, while lower loadings of CNCs and all loadings of PMMA-g-CNCs did not. The difference in Young's modulus between unmodified CNC and polymer-grafted CNC composites was generally insignificant. Overall, ball-milled composites had inferior mech. and rheol. properties compared to melt-mixed composites. SEM showed aggregation in the samples with CNCs, but more pronounced aggregation with PMMA-g-CNCs. Despite improving interfacial compatibility between the nanoparticles and the matrix, the effect of PMMA-g-CNC aggregation and decreased thermal stability dominated the composite performance.
- 135Tang, J.; Berry, R. M.; Tam, K. C. Stimuli-Responsive Cellulose Nanocrystals for Surfactant-Free Oil Harvesting. Biomacromolecules 2016, 17 (5), 1748– 1756, DOI: 10.1021/acs.biomac.6b00144Google Scholar135https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xlslejsr0%253D&md5=6351dd1840d62d221160869d0791af1cStimuli-Responsive Cellulose Nanocrystals for Surfactant-Free Oil HarvestingTang, Juntao; Berry, Richard M.; Tam, Kam C.Biomacromolecules (2016), 17 (5), 1748-1756CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)Cellulose nanocrystals with grafted binary polymer brushes (CNC-BPB), poly(oligoethylene glycol) methacrylate (POEGMA) and poly(methacrylic acid) (PMAA), were prepd. by cerium-mediated polymn. in aq. soln. The phys. properties of CNC-BPB can be controlled by external triggers, such as temp. and pH, which can be used to stabilize and destabilize oil-water emulsions. By virtue of the modifications, these bi-functionalized CNCs diffused to the oil-water interface and stabilized the oil droplets at high pHs. When the pH was lowered to 2, strong hydrogen bonding between POEGMA and PMAA chains grafted on the CNC induced the coalescence of the emulsion droplets, resulting in the phase sepn. of oil and water. For emulsions stabilized by CNC-POEGMA and free PMAA mixts., instantaneous coalescence was not obsd. at low pHs. Successive stabilization-destabilization over 5 cycles was demonstrated by modulating the pH with the addn. of acid or base without any loss in efficiency. Functional sustainable nanomaterials can be used for small scale oil-water sepns., particularly for oil droplet transportation and harvesting of lipophilic compds.
- 136Arthur, J. C.; Baugh, P. J.; Hinojosa, O. ESR study of reactions of cellulose initiated by the ceric ion method. J. Appl. Polym. Sci. 1966, 10 (10), 1591– 1606, DOI: 10.1002/app.1966.070101015Google Scholar136https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaF2sXjvFKj&md5=59f8e02f29c1266468aed9270f1b05fbElectron spin resonance study of reactions of cellulose initiated by the ceric ion methodArthur, Jett C., Jr.; Baugh, Peter J.; Hinojosa, OscarJournal of Applied Polymer Science (1966), 10 (10), 1591-1606CODEN: JAPNAB; ISSN:0021-8995.An E.S.R. spectroscopic study was made to det. the effects of ceric ion concn., atm., temp., and graft polymerization with acrylonitrile on the formation and decay rates of free radicals in microcryst. cellulose (I) and purified cotton cellulose (II) under both static and dynamic conditions. Under static conditions, the free radical concns. were detd. at -100 or -160°. Under dynamic conditions, I and II were treated with the ceric ion at 25°. The decay rate of the free radicals was decreased in the presence of O. The radical concn. increased and then decreased on the initiation of copolymerization with acrylonitrile. During graft polymerization, the radical site initially on the cellulose mol. (III) was retained on the end of the growing polymer chain (IV). Then addnl. ceric ion coordinated with the hydroxyl groups of I and II, leading to the formation of addnl. radical sites. An Arrhenius interpretation of the effect of temp. on the formation of these addnl. radical sites gave apparent activation energies for radical formation as 29 kcal./mole on I and 34 kcal./mole on II.
- 137Yang, J.; Han, C.-R.; Duan, J.-F.; Ma, M.-G.; Zhang, X.-M.; Xu, F.; Sun, R.-C.; Xie, X.-M. Studies on the properties and formation mechanism of flexible nanocomposite hydrogels from cellulose nanocrystals and poly(acrylic acid). J. Mater. Chem. 2012, 22 (42), 22467– 22480, DOI: 10.1039/c2jm35498eGoogle Scholar137https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVOmsrrP&md5=5cdfaba11ac3b2fdab1997be09730c95Studies on the properties and formation mechanism of flexible nanocomposite hydrogels from cellulose nanocrystals and poly(acrylic acid)Yang, Jun; Han, Chun-Rui; Duan, Jiu-Fang; Ma, Ming-Guo; Zhang, Xue-Ming; Xu, Feng; Sun, Run-Cang; Xie, Xu-MingJournal of Materials Chemistry (2012), 22 (42), 22467-22480CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)A novel series of nanocomposite hydrogels based on cellulose nanocrystals (CNCs) and poly(acrylic acid) (PAA) have been synthesized by in situ free radical polymn. within an aq. medium. Rheol. measurements were applied to monitor the gelation process and results indicated that the gelation took place as monomers (acrylic acid, AA) grafted from the CNC surface and PAA chains entangled to produce flexible CNC-PAA gels. By tailoring the concn. of CNC (CCNC) over a wide range of 0.02-1 wt%, two crit. CCNC, C* and C**, were found which corresponded to polymer chains that occurred in overlapping entanglements and promoted conformational rearrangements on the basis of earlier gel precursors, resp. The formation mechanism of CNC based nanocomposite hydrogels, in which the nanoparticles transformed from the isolated state below C* to the spatially continuous percolation structure above C**, was proposed. The CNC-PAA gels exhibited excellent, compn.-dependent mech. properties, such as a large elongation ratio (>1100%) and high tensile strength (>350 kPa). Transmission electron microscopy (TEM) revealed that the CNCs were surrounded by grafted chains and formed inter-connected network structures, where the CNCs acted as multifunctional crosslinks with an av. effective functionality of 75. The mech. measurements indicated that the increase of CCNC led to an increase in the hydrogel's viscous characteristics and contributed to the energy dissipating mechanism, which was responsible for CNC-PAA gels excellent flexibility. The swelling and partial dissoln. behaviors of the hydrogels were examd., focusing on the effect of CCNC on the gels characteristic partial deswelling and gel-to-sol transition. Some new chain entanglements were formed under concd. conditions after drying treatment above the glass transition temp. (Tg) which was verified by observation of the greater tensile strength and modulus. All the results corresponded to the self-consistent network structure model for CNC-PAA gels.
- 138Yang, J.; Han, C.-R.; Duan, J.-F.; Ma, M.-G.; Zhang, X.-M.; Xu, F.; Sun, R.-C. Synthesis and characterization of mechanically flexible and tough cellulose nanocrystals–polyacrylamide nanocomposite hydrogels. Cellulose 2013, 20 (1), 227– 237, DOI: 10.1007/s10570-012-9841-yGoogle Scholar138https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpt12nug%253D%253D&md5=0d45998bc81d999638fe20afa3315a15Synthesis and characterization of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsYang, Jun; Han, Chun-Rui; Duan, Jiu-Fang; Ma, Ming-Guo; Zhang, Xue-Ming; Xu, Feng; Sun, Run-CangCellulose (Dordrecht, Netherlands) (2013), 20 (1), 227-237CODEN: CELLE8; ISSN:0969-0239. (Springer)The unique combinations of hard and soft components with core/shell structures were proposed to synthesize high strength nanocomposite hydrogels. The elastomeric hydrogels contg. rod-like cellulose nanocrystals (CNCs) core and polyacrylamide shell were made from aq. solns. via free radical polymn. in the absence of chem. cross-links. The obtained hydrogels possessed greater tensile strength and elongation ratio when compared with chem. cross-linked counterparts. Oscillatory shear expts. indicated that CNCs interacted with polymer matrix via both chem. and phys. interactions and contributed to the rubbery elasticity of the hydrogels. The nanocomposite hydrogels were more viscous than the chem. hydrogels, suggesting the addn. of CNC led to the increase of energy dissipating and viscoelastic properties. The network structure model was proposed and it suggested that the high extensibilities and fracture stresses were related to the well-defined network structures with low crosslinking d. and lack of noncovalent interactions among polymer chains, which may promote the rearrangements of network structure at high deformations.
- 139Lee, Y. R.; Park, D.; Choi, S. K.; Kim, M.; Baek, H. S.; Nam, J.; Chung, C. B.; Osuji, C. O.; Kim, J. W. Smart Cellulose Nanofluids Produced by Tunable Hydrophobic Association of Polymer-Grafted Cellulose Nanocrystals. ACS Appl. Mater. Interfaces 2017, 9, 31095, DOI: 10.1021/acsami.7b08783Google Scholar139https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtlCnsrzM&md5=ee1c6d657e023fee41d832ef6e0e5de4Smart Cellulose Nanofluids Produced by Tunable Hydrophobic Association of Polymer-Grafted Cellulose NanocrystalsLee, Yea Ram; Park, Daehwan; Choi, Sang Koo; Kim, Miju; Baek, Heung Soo; Nam, Jin; Chung, Chan Bok; Osuji, Chinedum O.; Kim, Jin WoongACS Applied Materials & Interfaces (2017), 9 (36), 31095-31101CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)Cellulose fibrils, unique plant-derived semicryst. nanomaterials with exceptional mech. properties, have significant potential for rheol. modification of complex fluids due to their ability to form a phys. assocd. semiflexible fibrillary network. Here, we report new associative cellulose nanocrystals (ACNCs) with stress-responsive rheol. behaviors in an aq. soln. The surface-mediated living radical polymn. was employed to graft poly(stearyl methacrylate-co-2-methacryloxyethyl phosphorylcholine) brushes onto the nanofibrils, and then 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidn. was conducted to produce nanoscale ACNCs in the aq. soln. The ACNCs displayed interfibril assocn. driven by the hydrophobic interaction that resulted in the formation of a nanofibrillar cryst. gel phase. We obsd. that the viscosity of the ACNC fluid showed reversible shear thinning and temp.-induced thickening in response to applied shear stress and thermal shock. Moreover, thanks to generation of a mech. robust nanofibrillar cryst. gel network, the ACNC suspension showed extraordinary stability to changes in salinity and pH. These results highlighted that the interfibril hydrophobic assocn. of ACNCs was vital and played an essential role in regulation of stimuli-responsive sol-gel transitions.
- 140Dorman, G.; Prestwich, G. D. Benzophenone Photophores in Biochemistry. Biochemistry 1994, 33 (19), 5661– 5673, DOI: 10.1021/bi00185a001Google Scholar140https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXivFehsrw%253D&md5=496856015156004dfd870857a2c060efBenzophenone Photophores in BiochemistryDorman, Gyorgy; Prestwich, Glenn D.Biochemistry (1994), 33 (19), 5661-73CODEN: BICHAW; ISSN:0006-2960.A review, with ∼85 refs. The photoactivatable aryl ketone derivs. have been rediscovered as biochem. probes in the last 5 yr. The expanding use of benzophenone (BP) photoprobes can be attributed to three distinct chem. and biochem. advantages. First, BPs are chem. more stable than diazo esters, aryl azides, and diazirines. Second, BPs can be manipulated in ambient light and can be activated at 350-360 nm, avoiding protein-damaging wavelengths. Third, BPs react preferentially with unreactive C-H bonds, even in the presence of solvent water and bulk nucleophiles. These three properties combine to produce highly efficient covalent modifications of macromols., frequently with remarkable site specificity. This perspective includes a brief review of BP photochem. and a selection of specific applications of these photoprobes, which address questions in protein, nucleic acid, and lipid biochem.
- 141Yi, J.; Xu, Q.; Zhang, X.; Zhang, H. Chiral-nematic self-ordering of rodlike cellulose nanocrystals grafted with poly(styrene) in both thermotropic and lyotropic states. Polymer 2008, 49 (20), 4406– 4412, DOI: 10.1016/j.polymer.2008.08.008Google Scholar141https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtFegtLnL&md5=d0c6798ebc586ca4d5bee1db48eb01eeChiral-nematic self-ordering of rodlike cellulose nanocrystals grafted with poly(styrene) in both thermotropic and lyotropic statesYi, Jie; Xu, Qunxing; Zhang, Xuefei; Zhang, HailiangPolymer (2008), 49 (20), 4406-4412CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)Graft copolymers of rodlike cellulose nanocrystals (CNC) with poly(styrene) (PSt) were synthesized through atom transfer radical polymn. (ATRP). The hydroxyl groups on CNC were esterified with 2-bromoisobutyrylbromide to yield 2-bromoisobutyryloxy groups, which were used to initiate the polymn. of poly(styrene). The graft copolymers were characterized by thermogravimetric anal. (TGA), fourier transform IR spectroscopy (FT-IR) and gel permeation chromatog. (GPC). The size of the original CNC is 10-40 nm in width and 100-400 nm in length, which was characterized by at. force microscopy (AFM). The thermal and liq. cryst. properties of the graft copolymers were investigated by differential scanning calorimeter (DSC) and polarizing optical microscope (POM). The graft copolymers exhibit fingerprint texture in both thermotropic and lyotropic states. In thermotropic state, the PSt-grafted CNC orient spontaneously in isotropic melt (PSt side chains acting as a solvent). The thermotropic liq. crystal phase behavior is similar to the lyotropic phase behavior.
- 142Zeinali, E.; Haddadi-Asl, V.; Roghani-Mamaqani, H. Nanocrystalline cellulose grafted random copolymers of N-isopropylacrylamide and acrylic acid synthesized by RAFT polymerization: effect of different acrylic acid contents on LCST behavior. RSC Adv. 2014, 4 (59), 31428– 31442, DOI: 10.1039/C4RA05442CGoogle Scholar142https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtFSgsb7I&md5=49a8b3bc35c1e5696c76dfd3525a370eNanocrystalline cellulose grafted random copolymers of N-isopropylacrylamide and acrylic acid synthesized by RAFT polymerization: effect of different acrylic acid contents on LCST behaviorZeinali, Elnaz; Haddadi-Asl, Vahid; Roghani-Mamaqani, HosseinRSC Advances (2014), 4 (59), 31428-31442CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)Free and nanocryst. cellulose (NCC) attached homopolymers of N-isopropylacrylamide and acrylic acid (AA) as temp.- and pH-sensitive materials and also their dual-sensitive copolymers with different contents of AA were synthesized by RAFT polymn. NCCs were obtained from microcryst. cellulose by an acid hydrolysis process. Then, the surface of NCC was chem. modified by 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) as chain transfer agent. In situ synthesis of polymers in the presence of 1 wt% of NCC was carried out at 70 °C via the R-group approach. Successful attachment of DDMAT and polymer chains on the backbone of NCC was studied by X-ray photoelectron (XPS), Fourier transform IR, proton NMR, and Raman spectroscopies. Elemental anal., thermogravimetric anal., and XPS were also used to evaluate the grafting of DDMAT and polymers. Thermal behavior of the NCC-attached polymers was also studied by differential scanning calorimetry. The lower crit. soln. temps. (LCST) of polymers as phase sepn. temps. were measured by the cloud point method using dynamic light scattering. Addn. of NCC, AA content, and pH at higher pH values results in increase of LCST. At low pH values, increase of LCST occurred by an increase of NCC and also pH value. The morphol. and cryst. structure of polymer-grafted NCCs were examd. by transmission electron microscopy and X-ray diffraction resp.
- 143Roeder, R. D.; Garcia-Valdez, O.; Whitney, R. A.; Champagne, P.; Cunningham, M. F. Graft modification of cellulose nanocrystals via nitroxide-mediated polymerisation. Polym. Chem. 2016, 7 (41), 6383– 6390, DOI: 10.1039/C6PY01515HGoogle Scholar143https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhs1Sjsr%252FM&md5=65ff0e3f4e5d2acaea0b591c7f195295Graft modification of cellulose nanocrystals via nitroxide-mediated polymerisationRoeder, Ryan D.; Garcia-Valdez, Omar; Whitney, Ralph A.; Champagne, Pascale; Cunningham, Michael F.Polymer Chemistry (2016), 7 (41), 6383-6390CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)Cellulose nanocrystals (CNC) have become the subject of increasing research interest because of their unique phys., chem. and mech. properties, including being a renewable material. While CNC shows promise as a reinforcing material in polymer-based composites, the hydrophilic surface of CNC makes dispersibility in most hydrophobic polymers very difficult which limits potential applications. In this study, we report the first graft modification of CNC using nitroxide-mediated radical polymn. The CNC surface was first functionalised with the nitroxide SG1 (4-(diethoxyphosphinyl)-2,2,5,5-tetramethyl-3-azahexane-N-oxyl), yielding a CNC-macroalkoxyamine. Poly(Me acrylate) and poly(Me methacrylate) chains were then grafted from the CNC-macroalkoxyamine surface to yield polymer graft modified CNC.
- 144Matyjaszewski, K. Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives. Macromolecules 2012, 45 (10), 4015– 4039, DOI: 10.1021/ma3001719Google Scholar144https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XlsVaqs7w%253D&md5=350b580bd1bb46c21ba5dbfd65a6811dAtom Transfer Radical Polymerization (ATRP): Current Status and Future PerspectivesMatyjaszewski, KrzysztofMacromolecules (Washington, DC, United States) (2012), 45 (10), 4015-4039CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)A review. Current status and future perspectives in atom transfer radical polymn. (ATRP) are presented. Special emphasis is placed on mechanistic understanding of ATRP, recent synthetic and process development, and new controlled polymer architectures enabled by ATRP. New hybrid materials based on org./inorg. systems and natural/synthetic polymers are presented. Some current and forthcoming applications are described.
- 145Jakubowski, W.; Min, K.; Matyjaszewski, K. Activators Regenerated by Electron Transfer for Atom Transfer Radical Polymerization of Styrene. Macromolecules 2006, 39 (1), 39– 45, DOI: 10.1021/ma0522716Google Scholar145https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXht1Kkt7rI&md5=1b7fc67b0e0c071e441b7b762d50060fActivators Regenerated by Electron Transfer for Atom Transfer Radical Polymerization of StyreneJakubowski, Wojciech; Min, Ke; Matyjaszewski, KrzysztofMacromolecules (2006), 39 (1), 39-45CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)The amt. of Cu-based catalysts in atom transfer radical polymn. (ATRP) of styrene has been reduced to a few ppm in the presence of the appropriate reducing agents such as FDA approved tin(II) 2-ethylhexanoate (Sn(EH)2) or glucose. The reducing agents constantly regenerate ATRP activator, the Cu(I) species, from the Cu(II) species, formed during termination process, without directly or indirectly producing initiating species that generate new chains. Moreover, the reducing agents allow starting an ATRP with the oxidatively stable Cu(II) species. The reducing/reactivating cycle may also eliminate air or some other radical traps in the system. This new catalytic system is based on regeneration of the activators for an ATRP by electron transfer and therefore was named activators regenerated by electron transfer (ARGET) ATRP. The optimum amt. of reducing agent and minimal amt. of ATRP Cu catalyst depend on the particular system. For example, styrene was polymd. with 10 ppm of CuCl2/Me6TREN and 100 ppm of Sn(EH)2 resulting in a polystyrene with Mn = 63 000 (Mn,th = 64 000) and Mw/Mn = 1.17.
- 146Min, K.; Gao, H.; Matyjaszewski, K. Use of Ascorbic Acid as Reducing Agent for Synthesis of Well-Defined Polymers by ARGET ATRP. Macromolecules 2007, 40 (6), 1789– 1791, DOI: 10.1021/ma0702041Google Scholar146https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhvFagur0%253D&md5=874c1acec61ae8518938c7f4da04185bUse of Ascorbic Acid as Reducing Agent for Synthesis of Well-Defined Polymers by ARGET ATRPMin, Ke; Gao, Haifeng; Matyjaszewski, KrzysztofMacromolecules (Washington, DC, United States) (2007), 40 (6), 1789-1791CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)A heterogeneous redox reaction between ascorbic acid, sparingly sol. in anisole, and Cu(II) species is sufficiently slow to maintain a slow but efficient regeneration of Cu(I) species. The results in well-controlled activators regenerated by electron transfer ATRP of Me acrylate, Bu acrylate, Me methacrylate, and styrene.
- 147Bai, L.; Zhang, L.; Cheng, Z.; Zhu, X. Activators generated by electron transfer for atom transfer radical polymerization: recent advances in catalyst and polymer chemistry. Polym. Chem. 2012, 3 (10), 2685– 2697, DOI: 10.1039/c2py20286gGoogle Scholar147https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xht1yksrfI&md5=811202e4002bc2cfa87dbfa09c914907Activators generated by electron transfer for atom transfer radical polymerization: recent advances in catalyst and polymer chemistryBai, Liangjiu; Zhang, Lifen; Cheng, Zhenping; Zhu, XiulinPolymer Chemistry (2012), 3 (10), 2685-2697CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)A review. Activators generated by electron transfer for atom transfer radical polymn. (AGET ATRP) was introduced by Matyjaszewski and coworkers in 2005. The development of AGET ATRP has profound industrial implications because it lowers the amt. of necessary catalyst, while still allowing excellent control over mol. wt. and mol. wt. distribution. Herein, the authors highlight recent works on the mechanistic understanding of AGET ATRP towards the advance of catalysis and the design and synthesis of functional polymers, with a particular emphasis on: (a) mechanistic understanding of AGET ATRP; (b) redn. of catalyst concn.; (c) aq.-phase systems by AGET ATRP; (d) iron-mediated AGET ATRP; and (e) functional polymers designed by AGET ATRP. AGET ATRP is a robust tool due to its simplicity, broad applicability, and its ability to prep. previously inaccessible well-defined polymeric materials.
- 148Rosen, B. M.; Percec, V. Single-Electron Transfer and Single-Electron Transfer Degenerative Chain Transfer Living Radical Polymerization. Chem. Rev. 2009, 109 (11), 5069– 5119, DOI: 10.1021/cr900024jGoogle Scholar148https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXht1Cjs7%252FJ&md5=68e02c6974a936bf46513f1a0a933d05Single-Electron Transfer and Single-Electron Transfer Degenerative Chain Transfer Living Radical PolymerizationRosen, Brad M.; Percec, VirgilChemical Reviews (Washington, DC, United States) (2009), 109 (11), 5069-5119CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. Recent progress in two related polymn. methodologies that rely on single-electron transfer (SET): single-electron transfer degenerative chain transfer living radical polymn. (SET-DTLRP) and single-electron transfer living radical polymn. (SET-LRP) are discussed. SET-DTLRP proceeds via SET initiation and competition of SET activation/deactivation and degenerative transfer (DT). SET-LRP proceeds exclusively through a SET initiation, activation, and deactivation. Catalyst systems, e.g., Cu halides and sulfonyl halides; solvent systems; mechanisms of polymn.; and applications of SET-LRP are discussed.
- 149Konkolewicz, D.; Wang, Y.; Zhong, M.; Krys, P.; Isse, A. A.; Gennaro, A.; Matyjaszewski, K. Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. A Critical Assessment of the SARA ATRP and SET-LRP Mechanisms. Macromolecules 2013, 46 (22), 8749– 8772, DOI: 10.1021/ma401243kGoogle Scholar149https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvVWms7%252FP&md5=84dbd44edb4c0d59b856363770012d35Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. A Critical Assessment of the SARA ATRP and SET-LRP MechanismsKonkolewicz, Dominik; Wang, Yu; Zhong, Mingjiang; Krys, Pawel; Isse, Abdirisak A.; Gennaro, Armando; Matyjaszewski, KrzysztofMacromolecules (Washington, DC, United States) (2013), 46 (22), 8749-8772CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)A review. Reversible-deactivation radical polymn. (RDRP) in the presence of Cu0 is a versatile technique that can be used to create well-controlled polymers with complex architectures. Despite the facile nature of the technique, there has been a vigorous debate in the literature as to the mechanism of the reaction. One proposed mechanism, named supplemental activator and reducing agent atom transfer radical polymn. (SARA ATRP), has CuI as the major activator of alkyl halides, Cu0 acting as a supplemental activator, an inner-sphere electron transfer occurring during the activation step, and relatively slow comproportionation and disproportionation. In SARA ATRP slow activation of alkyl halides by Cu0 and comproportionation of CuII with Cu0 compensates for the small no. of radicals lost to termination reactions. Alternatively, a mechanism named single electron transfer living radical polymn. (SET-LRP) assumes that the CuI species do not activate alkyl halides, but undergo instantaneous disproportionation, and that the relatively rapid polymn. is due to a fast reaction between alkyl halides and "nascent" Cu0 through an outer-sphere electron transfer. In this article a crit. assessment of the exptl. data are presented on the polymn. of Me acrylate in DMSO with Me6TREN as the ligand in the presence of Cu0, in order to discriminate between these two mechanisms. The exptl. data agree with the SARA ATRP mechanism, since the activation of alkyl halides by CuI species is significantly faster than Cu0, the activation step involves inner-sphere electron transfer rather than an outer-sphere electron transfer, and in DMSO comproportionation is slow but occurs faster than disproportionation, and activation by CuI species is much faster than disproportionation. The rate of deactivation by CuII is essentially the same as the rate of activation by CuI, and the system is under ATRP equil. The role of Cu0 in this system is to slowly and continuously supply CuI activating species and radicals, by supplemental activation and comproportionation, to compensate for CuI lost due to the unavoidable radical termination reactions. With the mechanistic understanding gained by analyzing the exptl. data in the literature, the reaction conditions in SARA ATRP can be tailored toward efficient synthesis of a new generation of complex architectures and functional materials.
- 150Lligadas, G.; Grama, S.; Percec, V. Single-Electron Transfer Living Radical Polymerization Platform to Practice, Develop, and Invent. Biomacromolecules 2017, 18 (10), 2981– 3008, DOI: 10.1021/acs.biomac.7b01131Google Scholar150https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtl2ls7bL&md5=dee4a019f59437284824461298bccd0bSingle-Electron Transfer Living Radical Polymerization Platform to Practice, Develop, and InventLligadas, Gerard; Grama, Silvia; Percec, VirgilBiomacromolecules (2017), 18 (10), 2981-3008CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)The most fundamental aspects of single-electron transfer (SET) principles are presented. They are discussed according to different definitions used by expert practitioners and are applied to SET living radical polymn. (SET-LRP) according to the definition of the division of org. chem. of IUPAC that relies on principles elaborated by Taube, Eberson, Chanon, and Kochi. Addnl. definitions are also discussed to help clarify for the nonexpert contradictory literature reports. Subsequently, the principles and evolution of SET-LRP together with the methodologies currently available to practice it are discussed. It is expected that this Perspective will be able to help experts and nonexperts practice, develop, and invent new concepts and methodologies for SET-LRP to advance its status and the status of other living radical polymn. methods to the level of the most precise living polymn. methods.
- 151Xu, Q.; Yi, J.; Zhang, X.; Zhang, H. A novel amphotropic polymer based on cellulose nanocrystals grafted with azo polymers. Eur. Polym. J. 2008, 44 (9), 2830– 2837, DOI: 10.1016/j.eurpolymj.2008.06.010Google Scholar151https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtFentrfE&md5=27fd48d1cb5e523ff449c46db5ad5151A novel amphotropic polymer based on cellulose nanocrystals grafted with azo polymersXu, Qunxing; Yi, Jie; Zhang, Xuefei; Zhang, HailiangEuropean Polymer Journal (2008), 44 (9), 2830-2837CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)A novel amphotropic polymer which could exhibit liq.-cryst. behavior both in the solvent and in the heating process was synthesized through azo polymers grafting from cellulose nanocrystals (CNCs). The CNCs, prepd. by acid hydrolysis of filter paper, were characterized by At. Force Microscopy (AFM). Poly{6-[4-(4-methoxyphenylazo)phenoxy] hexyl methacrylate} (PMMAZO), which was a liq.-cryst. polymers (LCP), was successfully to graft from CNC via Atom transfer radical polymn. (ATRP). The structure and thermal properties of the PMMAZO-grafted CNC were investigated using FT-IR and thermogravimetric analyses (TGA). Its phase structures and transitions were studied by DSC and polarized optical microscopy (POM). The exptl. results showed that the PMMAZO-grafted CNC exhibited both types of liq. crystal formation, thermotropic and lyotropic.
- 152Yi, J.; Xu, Q.; Zhang, X.; Zhang, H. Temperature-induced chiral nematic phase changes of suspensions of poly(N,N-dimethylaminoethyl methacrylate)-grafted cellulose nanocrystals. Cellulose 2009, 16 (6), 989– 997, DOI: 10.1007/s10570-009-9350-9Google Scholar152https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFaqsbrP&md5=bf631b60ff18808f978b901e26e60d04Temperature-induced chiral nematic phase changes of suspensions of poly(N,N-dimethylaminoethyl methacrylate)-grafted cellulose nanocrystalsYi, Jie; Xu, Qunxing; Zhang, Xuefei; Zhang, HailiangCellulose (Dordrecht, Netherlands) (2009), 16 (6), 989-997CODEN: CELLE8; ISSN:0969-0239. (Springer)Temp.-induced copolymers of poly(N,N-dimethylaminoethyl methacrylate)-grafted cellulose nanocrystals (PDMAEMA-grafted CNC) were synthesized by surface-initiated atom transfer radical polymn. (ATRP). The graft copolymers were characterized by thermogravimetric anal. (TGA), Fourier-transform IR spectroscopy (FT-IR), and gel permeation chromatog. (GPC). The size of the original CNC was 10-40 nm in width and 100-400 nm in length, as characterized by at. force microscopy (AFM). The liq.-cryst. properties of the graft copolymers were studied by using polarizing optical microscopy (POM). The graft copolymers exhibited fingerprint texture in lyotropic state. The temp.-induced fingerprint texture changes of PDMAEMA-grafted CNC aq. suspensions were studied at various temps. With increasing temp., the spacing of the fingerprint lines decreases. Temp.-induced changes of PDMAEMA polymer chains result in changes of fingerprint texture.
- 153Hemraz, U. D.; Lu, A.; Sunasee, R.; Boluk, Y. Structure of poly(N-isopropylacrylamide) brushes and steric stability of their grafted cellulose nanocrystal dispersions. J. Colloid Interface Sci. 2014, 430, 157– 165, DOI: 10.1016/j.jcis.2014.05.011Google Scholar153https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtV2qt7zN&md5=d4f149bd04f3aebb4f9a645ae787e963Structure of poly(N-isopropylacrylamide) brushes and steric stability of their grafted cellulose nanocrystal dispersionsHemraz, Usha D.; Lu, Ang; Sunasee, Rajesh; Boluk, YamanJournal of Colloid and Interface Science (2014), 430 (), 157-165CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)Thermo-responsive poly(N-isopropylacrylamide) (poly(NIPAAm)) brushes were grafted from the surface of cellulose nanocrystals (CNC) via living radical polymn. (LRP) using different initiator and monomer concns. The dry film thickness of the poly(NIPAAm) layer around CNC was calcd. based on SEM and dynamic light scattering (DLS) measurements. The wet film thicknesses of grafted poly(NIPAAm) brushes in water were calcd. to be 15 and 9 nm for NIPAAm-CNC-1 and NIPAAm-CNC-2, resp. Grafted chain densities and wet film thicknesses at below and above the crit. temp. (T = 34°C) of polyNIPAAm were calcd. by applying mean-field anal. theory. The non-ionic poly(NIPAAm) brushes screened the surface charges of CNC particles, leading to a significant decrease in the abs. zeta potential values for the poly(NIPAAm) grafted CNCs compared to the unmodified and initiator modified CNC samples. Nevertheless, the colloidal stability of poly(NIPAAm) grafted CNC particles were still maintained by steric stabilization below the crit. temp. On the other side, hydrophobic attractions among poly(NIPAAm) grafted CNC rods above 34 °C lead to coagulation and phase sepn. While both poly(NIPAAm) grafted CNC samples showed thermo-responsive behavior, the reversibility of this temp. triggered property was dependent on grafting d.
- 154Wu, W.; Huang, F.; Pan, S.; Mu, W.; Meng, X.; Yang, H.; Xu, Z.; Ragauskas, A. J.; Deng, Y. Thermo-responsive and fluorescent cellulose nanocrystals grafted with polymer brushes. J. Mater. Chem. A 2015, 3 (5), 1995– 2005, DOI: 10.1039/C4TA04761CGoogle Scholar154https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvF2kur7O&md5=054d5346ae3ebdb1524e8b000e0324a0Thermo-responsive and fluorescent cellulose nanocrystals grafted with polymer brushesWu, Weibing; Huang, Fang; Pan, Shaobo; Mu, Wei; Meng, Xianzhi; Yang, Haitao; Xu, Zhaoyang; Ragauskas, Arthur J.; Deng, YulinJournal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (5), 1995-2005CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)Cellulose nanocrystals (CNCs) grafted with fluorescent and thermo-responsive poly(N-isopropylacrylamide) (PNIPAAM) brushes were prepd. via surface-initiated activators generated by electron transfer for atom transfer radical polymn. (SI-AGET-ATRP) in the CH3OH-H2O mixing solvent with different vol. ratios. The successful grafting was supported by Fourier transform IR (FTIR) and NMR measurements. Gravimetric anal. plus 1H NMR and gel permeation chromatog. (GPC) measurements showed that there was an increase in the monomer conversion and mol. wt. of polymer brushes with increasing H2O proportion of the solvent system. The variation trend of graft length was further evidenced by the gradual change of decompn. and glass transition temps. of the surface-grafted CNCs. A large scale of chain transfer occurred on the surface of CNCs in view of the minute quantity of free polymers generated by a sacrificial initiator. Free polymers cannot be used as a substitute to characterize grafted polymers in terms of the big difference between their mol. wts. The obtained surface-grafted CNCs showed thermo-enhanced fluorescence owing to the thermal-driven chain dehydration of the grafted PNIPAAM brushes.
- 155Hemraz, U. D.; Campbell, K. A.; Burdick, J. S.; Ckless, K.; Boluk, Y.; Sunasee, R. Cationic Poly(2-aminoethylmethacrylate) and Poly(N-(2-aminoethylmethacrylamide)) Modified Cellulose Nanocrystals: Synthesis, Characterization, and Cytotoxicity. Biomacromolecules 2015, 16 (1), 319– 325, DOI: 10.1021/bm501516rGoogle ScholarThere is no corresponding record for this reference.
- 156Wang, Z.; Zhang, Y.; Yuan, L.; Hayat, J.; Trenor, N. M.; Lamm, M. E.; Vlaminck, L.; Billiet, S.; Du Prez, F. E.; Wang, Z.; Tang, C. Biomass Approach toward Robust, Sustainable, Multiple-Shape-Memory Materials. ACS Macro Lett. 2016, 5 (5), 602– 606, DOI: 10.1021/acsmacrolett.6b00243Google Scholar156https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XmslWgsbk%253D&md5=389b505faaaa02cdd997ff70cfa285b6Biomass Approach toward Robust, Sustainable, Multiple-Shape-Memory MaterialsWang, Zhongkai; Zhang, Yaqiong; Yuan, Liang; Hayat, Jeffery; Trenor, Nathan M.; Lamm, Meghan E.; Vlaminck, Laetitia; Billiet, Stijn; Du Prez, Filip E.; Wang, Zhigang; Tang, ChuanbingACS Macro Letters (2016), 5 (5), 602-606CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)We report biomass-derived, shape-memory materials prepd. via simple reactions, including "grafting from" ATRP and TAD click chem. Although the biomass, including plant oils and cellulose nanocrystals, has heterogeneous chem. structures in nature, these materials exhibit excellent multiple shape-memory properties toward temp., water, and org. solvents, which are comparable to petroleum counterparts. The work presented herein provides burgeoning opportunities to design the next-generation, low-cost, biomass-prevalent, green materials for niche applications.
- 157Yin, Y.; Tian, X.; Jiang, X.; Wang, H.; Gao, W. Modification of cellulose nanocrystal via SI-ATRP of styrene and the mechanism of its reinforcement of polymethylmethacrylate. Carbohydr. Polym. 2016, 142, 206– 212, DOI: 10.1016/j.carbpol.2016.01.014Google Scholar157https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xot1KjtQ%253D%253D&md5=fc5518f8f77f16119012d8badabed4deModification of cellulose nanocrystal via SI-ATRP of styrene and the mechanism of its reinforcement of polymethylmethacrylateYin, Y.; Tian, X.; Jiang, X.; Wang, H.; Gao, W.Carbohydrate Polymers (2016), 142 (), 206-212CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)Cellulose nanocrystal (CNC) is a promising strengthener but is used limitedly since its poor compatibility with org. materials. The graft polymn. of styrene via surface-initiated atom transfer radical polymn. (SI-ATRP) of cellulose nanocrystal is adopted to modify its thermo-stability and compatibility. The modified crystals have been dosed into polymethylmethacrylate (PMMA) nanocomposites by the soln. casting. The polymeric layer on the surface of CNCs should improve the thermal stability of CNCs, and provide significant dispersibility and compatibilization for the nanocomposites. Thermogravimetry anal. proved that the initial degrade temp. of CNC was increased 50 °C with the modification. The scanning electronic microscope showed that the modified CNCs homogeneously dispersed in PMMA matrix. Breaking strength and elongation at break of the composites were improved, which was attributed to the reinforcement of CNCs modified with styrene. Transmittance of nanocomposite films measurement showed that the transmittance of PMMA/1%CNC was almost close to that of pure PMMA.
- 158Hatton, F. L.; Kedzior, S. A.; Cranston, E. D.; Carlmark, A. Grafting-from cellulose nanocrystals via photoinduced Cu-mediated reversible-deactivation radical polymerization. Carbohydr. Polym. 2017, 157, 1033– 1040, DOI: 10.1016/j.carbpol.2016.10.064Google Scholar158https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVahsrfN&md5=a05f20387301933d1b84364da9b0e8fbGrafting-from cellulose nanocrystals via photoinduced Cu-mediated reversible-deactivation radical polymerizationHatton, Fiona L.; Kedzior, Stephanie A.; Cranston, Emily D.; Carlmark, AnnaCarbohydrate Polymers (2017), 157 (), 1033-1040CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)In this work we describe the grafting of cellulose nanocrystals (CNCs) by surface-initiated photoinduced Cu-mediated reversible-deactivation radical polymn. (RDRP). Initially, CNCs obtained through sulfuric acid hydrolysis were functionalized with a tertiary bromo-ester moiety as an initiating group for the subsequent RDRP of Me acrylate, targeting three different ds.p. for the polymer grafts: 50, 300 and 600. The polymns. proceeded in DMSO in the presence of CuBr2 and Me6TREN as the catalytic system utilizing a UV source (λmax ≈ 360 nm). The technique proved highly versatile for the modification of CNCs with poly(Me acrylate), where considerably high grafting was achieved in short reaction times (90 min), with simple purifn. steps. CNC morphol. was maintained and polymer grafts were evident through FT-IR spectroscopy, thermal anal., contact angle measurements, X-ray photoelectron microscopy and x-ray diffraction.
- 159Grishkewich, N.; Akhlaghi, S. P.; Zhaoling, Y.; Berry, R.; Tam, K. C. Cellulose nanocrystal-poly(oligo(ethylene glycol) methacrylate) brushes with tunable LCSTs. Carbohydr. Polym. 2016, 144, 215– 222, DOI: 10.1016/j.carbpol.2016.02.044Google Scholar159https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XivVCisbk%253D&md5=001e5e700ffdcf0e2f03d37ee5c489b0Cellulose nanocrystal-poly(oligo(ethylene glycol) methacrylate) brushes with tunable LCSTsGrishkewich, Nathan; Akhlaghi, Seyedeh Parinaz; Zhaoling, Yao; Berry, Richard; Tam, Kam C.Carbohydrate Polymers (2016), 144 (), 215-222CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)This paper reports on the synthesis of poly(oligoethylene glycol) Me ether acrylate (POEGMA) grafted cellulose nanocrystals (CNCs) via surface initiated atom transfer radical polymn. (ATRP). An ATRP initiator (α-Bromoisobutyryl bromide) was covalently bonded to the surface of CNCs, followed by copolymg. di(ethylene glycol) Me ether methacrylate (MEO2MA) and oligoethylene glycol Me ether methacrylate (OEGMA300) monomers from the surface using Cu(I)Br/2,2-dipyridal. Multiple POEGMA-g-CNC systems with varying MEO2MA/OEGMA300 content were synthesized, and they displayed a range of lower crit. soln. temps. (LCSTs) in aq. medium. μDSC endotherms and microstructural anal. indicated the collapse of POEGMA chains, followed by the aggregation of nanoparticles above their LCSTs. Cloud point measurements demonstrated a hysteresis in the heating and cooling of the POEGMA-g-CNC systems. It was found that the LCST of the nanoparticles could be tuned to between 23.8 to 63.8° by adjusting the OEGMA300 content of the POEGMA brushes.
- 160Zhang, X.; Zhang, J.; Dong, L.; Ren, S.; Wu, Q.; Lei, T. Thermoresponsive poly(poly(ethylene glycol) methylacrylate)s grafted cellulose nanocrystals through SI-ATRP polymerization. Cellulose 2017, 24, 4189, DOI: 10.1007/s10570-017-1414-7Google Scholar160https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1OmtLzL&md5=4251cd327b5770bc0a56b3077a9a7915Thermoresponsive poly(poly(ethylene glycol) methylacrylate)s grafted cellulose nanocrystals through SI-ATRP polymerizationZhang, Xiuqiang; Zhang, Jinlong; Dong, Lili; Ren, Suxia; Wu, Qinglin; Lei, TingzhouCellulose (Dordrecht, Netherlands) (2017), 24 (10), 4189-4203CODEN: CELLE8; ISSN:0969-0239. (Springer)We report a series of thermoresponsive cellulose nanocrystals (CNCs) decorated with poly(poly(ethylene glycol) methylacrylate) copolymers (poly(PEGMA)-g-CNCs) synthesized by surface initiated-atom transfer radical polymn. (SI-ATRP). The chem. structures and surface morphologies were subsequently confirmed by FT-IR, XPS, and AFM measurements. With regard to thermally responsive behavior, poly(PEGMA)-g-CNCs show tunable lower crit. soln. temp. (LCST) values in the range of 34-66 °C by varying the feeding ratios of comonomers. The reversible morphol. transformation from individual nano-rod structures to larger globule aggregates was further verified by AFM during the LCST transition. These functionalized CNCs have potential as smart film filters and biosensors.
- 161Zhang, J.; Wu, Q.; Li, M.-C.; Song, K.; Sun, X.; Lee, S.-Y.; Lei, T. Thermoresponsive Copolymer Poly(N-Vinylcaprolactam) Grafted Cellulose Nanocrystals: Synthesis, Structure, and Properties. ACS Sustainable Chem. Eng. 2017, 5 (8), 7439– 7447, DOI: 10.1021/acssuschemeng.7b02033Google Scholar161https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtVKgsb7J&md5=3c8710f20a53969aa51474635a62bfc1Thermoresponsive Copolymer Poly(N-Vinylcaprolactam) Grafted Cellulose Nanocrystals: Synthesis, Structure, and PropertiesZhang, Jinlong; Wu, Qinglin; Li, Mei-Chun; Song, Kunlin; Sun, Xiuxuan; Lee, Sun-Young; Lei, TingzhouACS Sustainable Chemistry & Engineering (2017), 5 (8), 7439-7447CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)A novel thermoresponsive copolymer poly(N-vinylcaprolactam) grafted cellulose nanocrystals (PVCL-g-CNCs) was synthesized using surface-initiated atom transfer radical polymn., and its temp. responsive behavior was studied in this work. The chem. structure characterization by Fourier transform IR spectroscopy, solid-state CP/MAS 13C NMR spectroscopy, and XPS confirmed the presence of covalently grafting PVCL chains on the CNC surface. The cryst. structure and nanorod-shaped morphol. of CNCs were well preserved after polymn. Transmission electron microscope results indicated that the surface of CNCs was covered with grafted PVCL brushes. The viscoelastic properties of PVCL-g-CNC aq. suspensions (1.0 wt %) by dynamic rheol. measurements confirmed the thermally induced phase transition behavior. The work presented herein paves the way to design CNC-based advanced functional materials benefiting both from the intrinsic characteristics of CNCs and the new properties imparted by the temp. sensitive grafted polymer chains.
- 162Morandi, G.; Heath, L.; Thielemans, W. Cellulose Nanocrystals Grafted with Polystyrene Chains through Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP). Langmuir 2009, 25 (14), 8280– 8286, DOI: 10.1021/la900452aGoogle Scholar162https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXktVOks7Y%253D&md5=891a5a951763f3117dd199bbf9b43652Cellulose Nanocrystals Grafted with Polystyrene Chains through Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP)Morandi, Gaelle; Heath, Lindy; Thielemans, WimLangmuir (2009), 25 (14), 8280-8286CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)This paper reports the synthesis of cellulose nanocrystals grafted by polystyrene chains via surface-initiated ATRP. Naturally occurring cellulose was first hydrolyzed to obtain cellulose nanocrystals. Their surface was then chem. modified using 2-bromoisobutyryl bromide to introduce initiating sites for ATRP. A varying extent of surface modification was achieved by changing reaction conditions. Further initiation of styrene polymn. from these modified nanocrystals with a CuBr/PMDETA (N,N,N',N',N''-pentamethyldiethylenetriamine) catalytic system and in the presence of a sacrificial initiator produced polysaccharide nanocrystals grafted by polystyrene chains. A range of nanocrystals-g-polystyrene with different graft lengths (theor. polymn. degree = 27-171) was synthesized through this method and characterized by elemental anal., XPS, FT-IR, TEM, and contact angle measurements. We are thus able to produce cellulose nanoparticles with varying grafting densities (by altering extent of initiator surface modification) and varying polymer brush length (through polymn. control). The nanocrystals-g-polystyrene (NC-g-PS) particles were tested for their capacity to absorb 1,2,4-trichlorobenzene from water. The results obtained show that they can absorb the equiv. of 50% of their wt. in pollutant compared to 30 wt % adsorption for nonmodified nanocrystals, while also displaying faster absorption kinetics.
- 163Zoppe, J. O.; Habibi, Y.; Rojas, O. J.; Venditti, R. A.; Johansson, L.-S.; Efimenko, K.; Osterberg, M.; Laine, J. Poly(N-isopropylacrylamide) Brushes Grafted from Cellulose Nanocrystals via Surface-Initiated Single-Electron Transfer Living Radical Polymerization. Biomacromolecules 2010, 11 (10), 2683– 2691, DOI: 10.1021/bm100719dGoogle Scholar163https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtFGru7nF&md5=9d4f387d7033e40ab6936748feb5489cPoly(N-isopropylacrylamide) Brushes Grafted from Cellulose Nanocrystals via Surface-Initiated Single-Electron Transfer Living Radical PolymerizationZoppe, Justin O.; Habibi, Youssef; Rojas, Orlando J.; Venditti, Richard A.; Johansson, Leena-Sisko; Efimenko, Kirill; Osterberg, Monika; Laine, JanneBiomacromolecules (2010), 11 (10), 2683-2691CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)Cellulose nanocrystals (CNCs) or nanowhiskers produced from sulfuric acid hydrolysis of ramie fibers were used as substrates for surface chem. functionalization with thermoresponsive macromols. The CNCs were grafted with poly(N-isopropylacrylamide) brushes via surface-initiated single-electron transfer living radical polymn. (SI-SET-LRP) under various conditions at room temp. The grafting process was confirmed via Fourier transform IR spectroscopy and XPS and the different mol. masses of the grafts were quantified and found to depend on the initiator and monomer concns. used. No observable damage occurred to the CNCs after grafting, as detd. by X-ray diffraction. Size exclusion chromatog. analyses of polymer chains cleaved from the cellulose nanocrystals indicated that a higher d.p. was achieved by increasing initiator or monomer loading, most likely caused by local heterogeneities yielding higher rates of polymn. It is expected that suspension stability, interfacial interactions, friction, and other properties of grafted CNCs can be controlled by changes in temp. and provide a unique platform for further development of stimuli-responsive nanomaterials.
- 164Chen, X.; Huang, L.; Sun, H.-J.; Cheng, S. Z. D.; Zhu, M.; Yang, G. Stimuli-Responsive Nanocomposite: Potential Injectable Embolization Agent. Macromol. Rapid Commun. 2014, 35 (5), 579– 584, DOI: 10.1002/marc.201300720Google ScholarThere is no corresponding record for this reference.
- 165Majoinen, J.; Walther, A.; McKee, J. R.; Kontturi, E.; Aseyev, V.; Malho, J. M.; Ruokolainen, J.; Ikkala, O. Polyelectrolyte Brushes Grafted from Cellulose Nanocrystals Using Cu-Mediated Surface-Initiated Controlled Radical Polymerization. Biomacromolecules 2011, 12 (8), 2997– 3006, DOI: 10.1021/bm200613yGoogle ScholarThere is no corresponding record for this reference.
- 166McKee, J. R.; Appel, E. A.; Seitsonen, J.; Kontturi, E.; Scherman, O. A.; Ikkala, O. Healable, Stable and Stiff Hydrogels: Combining Conflicting Properties Using Dynamic and Selective Three-Component Recognition with Reinforcing Cellulose Nanorods. Adv. Funct. Mater. 2014, 24 (18), 2706– 2713, DOI: 10.1002/adfm.201303699Google Scholar166https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXjsVChtL8%253D&md5=7751d3e86afafd787a34f4d8dd4a2e39Healable, Stable and Stiff Hydrogels: Combining Conflicting Properties Using Dynamic and Selective Three-Component Recognition with Reinforcing Cellulose NanorodsMcKee, Jason R.; Appel, Eric A.; Seitsonen, Jani; Kontturi, Eero; Scherman, Oren A.; Ikkala, OlliAdvanced Functional Materials (2014), 24 (18), 2706-2713CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)Nanocomposite hydrogels are prepd. combining polymer brush-modified 'hard' cellulose nanocrystals (CNC) and 'soft' polymeric domains, and bound together by cucurbit[8]uril (CB[8]) supramol. crosslinks, which allow dynamic host-guest interactions as well as selective and simultaneous binding of two guests, i.e., Me viologen (the first guest) and naphthyl units (the second guest). CNCs are mech. strong colloidal rods with nanometer-scale lateral dimensions, which are functionalized by surface-initiated atom transfer radical polymn. to yield a dense set of methacrylate polymer brushes bearing naphthyl units. They can then be non-covalently crosslinked through simple addn. of poly(vinyl alc.) polymers contg. pendant viologen units as well as CB[8]s in aq. media. The resulting supramol. nanocomposite hydrogels combine three important criteria: high storage modulus (G' > 10 kPa), rapid sol-gel transition (<6 s), and rapid self-healing even upon aging for several months, as driven by balanced colloidal reinforcement as well as the selectivity and dynamics of the CB[8] three-component supramol. interactions. Such a new combination of properties for stiff and self-healing hydrogel materials suggests new approaches for advanced dynamic materials from renewable sources.
- 167Rosilo, H.; McKee, J. R.; Kontturi, E.; Koho, T.; Hytonen, V. P.; Ikkala, O.; Kostiainen, M. A. Cationic polymer brush-modified cellulose nanocrystals for high-affinity virus binding. Nanoscale 2014, 6 (20), 11871– 11881, DOI: 10.1039/C4NR03584DGoogle Scholar167https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht1Kkt7vM&md5=1a07c5831a910254512b1d39a45489d9Cationic polymer brush-modified cellulose nanocrystals for high-affinity virus bindingRosilo, Henna; McKee, Jason R.; Kontturi, Eero; Koho, Tiia; Hytonen, Vesa P.; Ikkala, Olli; Kostiainen, Mauri A.Nanoscale (2014), 6 (20), 11871-11881CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)Surfaces capable of high-affinity binding of biomols. are required in several biotechnol. applications, such as purifn., transfection, and sensing. Therein, the rod-shaped, colloidal cellulose nanocrystals (CNCs) are appealing due to their large surface area available for functionalization. In order to exploit electrostatic binding, their intrinsically anionic surfaces have to be cationized as biol. supramols. are predominantly anionic. Here we present a facile way to prep. cationic CNCs by surface-initiated atom-transfer radical polymn. of poly(N,N-dimethylaminoethyl methacrylate) and subsequent quaternization of the polymer pendant amino groups. The cationic polymer brush-modified CNCs maintained excellent dispersibility and colloidal stability in water and showed a ζ-potential of +38 mV. Dynamic light scattering and electron microscopy showed that the modified CNCs electrostatically bind cowpea chlorotic mottle virus and norovirus-like particles with high affinity. Addn. of only a few wt. percent of the modified CNCs in water dispersions sufficed to fully bind the virus capsids to form micrometer-sized assemblies. This enabled the concn. and extn. of the virus particles from soln. by low-speed centrifugation. These results show the feasibility of the modified CNCs in virus binding and concg., and pave the way for their use as transduction enhancers for viral delivery applications.
- 168Malho, J.-M.; Morits, M.; Lobling, T. I.; Nonappa; Majoinen, J.; Schacher, F. H.; Ikkala, O.; Groschel, A. H. Rod-Like Nanoparticles with Striped and Helical Topography. ACS Macro Lett. 2016, 5 (10), 1185– 1190, DOI: 10.1021/acsmacrolett.6b00645Google Scholar168https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhs1aju7fK&md5=fc66c61a5fe04c3ae98aaf1ad3a7b366Rod-Like Nanoparticles with Striped and Helical TopographyMalho, Jani-Markus; Morits, Maria; Lobling, Tina I.; Nonappa; Majoinen, Johanna; Schacher, Felix H.; Ikkala, Olli; Groschel, Andre H.ACS Macro Letters (2016), 5 (10), 1185-1190CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)The behavior of nanoparticles in soln. is largely dominated by their shape and interaction potential. Despite considerable progress in the prepn. of patchy and compartmentalized particles, access to nanoparticles with complex surface patterns and topogs. remains limited. Here, we show that polyanionic brushes tethered to rod-like cellulose nanocrystals (CNCs) spontaneously develop a striped or helical topog. through interpolyelectrolyte complexation with polycationic diblock copolymers. Using cryogenic transmission electron microscopy (cryo-TEM) and tomog. (cryo-ET), we follow the complexation process and analyze the delicate 3D topog. on the CNC surface. The described approach is facile and modular and can be extended to other block chemistries, nanoparticles, and surfaces, thereby providing a versatile platform toward surface-patterned particles with complex topogs. and spatially arranged functional groups.
- 169Yu, J.; Wang, C.; Wang, J.; Chu, F. In situ development of self-reinforced cellulose nanocrystals based thermoplastic elastomers by atom transfer radical polymerization. Carbohydr. Polym. 2016, 141, 143– 150, DOI: 10.1016/j.carbpol.2016.01.006Google Scholar169https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XktlCjtw%253D%253D&md5=9e0276618941606bebcf6ae0639543baIn situ development of self-reinforced cellulose nanocrystals based thermoplastic elastomers by atom transfer radical polymerizationYu, Juan; Wang, Chunpeng; Wang, Jifu; Chu, FuxiangCarbohydrate Polymers (2016), 141 (), 143-150CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)Recently, the utilization of cellulose nanocrystals (CNCs) as a reinforcing material has received a great attention due to its high elastic modulus. In this article, a novel strategy for the synthesis of self-reinforced CNCs based thermoplastic elastomers (CTPEs) is presented. CNCs were first surface functionalized with an initiator for surface-initiated atom transfer radical polymn. (SI-ATRP). Subsequently, SI-ATRP of Me methacrylate (MMA) and Bu acrylate (BA) was carried out in the presence of sacrificial initiator to form CTPEs in situ. The CTPEs together with the simple blends of CNCs and linear poly(MMA-co-BA) copolymer (P(MMA-co-BA)) were characterized for comparative study. The results indicated that P(MMA-co-BA) was successfully grafted onto the surface of CNCs and the compatibility between CNCs and the polymer matrix in CTPEs was greatly enhanced. Specially, the CTPEs contg. 2.15 wt% CNCs increased Tg by 19.2 °C and tensile strength by 100% as compared to the linear P(MMA-co-BA).
- 170Wang, H.-D.; Roeder, R. D.; Whitney, R. A.; Champagne, P.; Cunningham, M. F. Graft modification of crystalline nanocellulose by Cu(0)-mediated SET living radical polymerization. J. Polym. Sci., Part A: Polym. Chem. 2015, 53 (24), 2800– 2808, DOI: 10.1002/pola.27754Google Scholar170https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXht1entbrJ&md5=dbdfe6633ebeabc74cfbf16f4ed9d49aGraft modification of crystalline nanocellulose by Cu(0)-mediated SET living radical polymerizationWang, Hai-Dong; Roeder, Ryan D.; Whitney, Ralph A.; Champagne, Pascale; Cunningham, Michael F.Journal of Polymer Science, Part A: Polymer Chemistry (2015), 53 (24), 2800-2808CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)Cryst. nanocellulose (CNC) was grafted with poly(Me acrylate) (PMA) to yield modified CNC that is readily dispersed in a range of org. solvents [including THF, chloroform, DMF, and DMSO (DMSO)], in contrast to native CNC which is dispersible primarily in aq. solns. First, a CNC macroinitiator with high bromine initiator d. was prepd. through a 1,1'-carbonyldiimidazole-mediated esterification reaction in DMSO-based dispersant. MA was then grafted from the CNC macroinitiator through SET living radical polymn. (LRP) at room temp. using Cu(0) (copper wire) as the catalyst. The LRP grafting proceeded rapidly, with ∼30% monomer conversion achieved within 30 min, yielding approx. six times the mass of PMA with respect to CNC macroinitiator. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015.
- 171Arredondo, J.; Jessop, P. G.; Champagne, P.; Bouchard, J.; Cunningham, M. F. Synthesis of CO2-responsive cellulose nanocrystals by surface-initiated Cu(0)-mediated polymerisation. Green Chem. 2017, 19, 4141, DOI: 10.1039/C7GC01798GGoogle Scholar171https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1Grsb3J&md5=e8f9a16457a7652b0ad504b734b5dcb1Synthesis of CO2-responsive cellulose nanocrystals by surface-initiated Cu(0)-mediated polymerizationArredondo, Joaquin; Jessop, Philip G.; Champagne, Pascale; Bouchard, Jean; Cunningham, Michael F.Green Chemistry (2017), 19 (17), 4141-4152CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)Cellulose nanocrystals (CNC) were converted into a CO2-responsive composite nanomaterial by grafting poly(dimethylaminoethy methacrylate) (PDMAEMA) and poly(diethylaminoethyl methacrylate) (PDEAEMA) on its surface using a grafting-from approach via surface-initiated copper(0)-mediated polymn. (SI-Cu(0)-ATRP). The Cu(0)-ATRP homopolymn. kinetics of these monomers were first studied to det. suitable conditions to perform the grafting step. Reasonable mol. wt. control and livingness were obsd. during the polymns. Following functionalization of the CNC with ATRP initiating groups, a grafting-from approach was employed to graft PDMAEMA and PDEAEMA onto the CNC surface. The surface charge of the graft-modified CNC could be reversibly switched by protonation/deprotonation of the tertiary amine groups simply by sparging with CO2 and N2, resp.
- 172Zoppe, J. O.; Dupire, A. V. M.; Lachat, T. G. G.; Lemal, P.; Rodriguez-Lorenzo, L.; Petri-Fink, A.; Weder, C.; Klok, H.-A. Cellulose Nanocrystals with Tethered Polymer Chains: Chemically Patchy versus Uniform Decoration. ACS Macro Lett. 2017, 6, 892– 897, DOI: 10.1021/acsmacrolett.7b00383Google Scholar172https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1OrsbzK&md5=e4915a58ef772f0b6ed4c8622cdaaf74Cellulose Nanocrystals with Tethered Polymer Chains: Chemically Patchy versus Uniform DecorationZoppe, Justin O.; Dupire, Alix Vaimiti Marie; Lachat, Theo Gaston Gerard; Lemal, Philipp; Rodriguez-Lorenzo, Laura; Petri-Fink, Alke; Weder, Christoph; Klok, Harm-AntonACS Macro Letters (2017), 6 (9), 892-897CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)The site-sp. surface modification of colloidal substrates, yielding "patchy" nanoparticles, is a rapidly expanding area of research as a result of the new complex structural hierarchies that are becoming accessible to chemists and materials scientists through colloidal self-assembly. The inherent directionality of cellulose chains, which feature a nonreducing and a reducing end, within individual cellulose nanocrystals (CNCs) renders them an interesting exptl. platform for the synthesis of asym. nanorods with end-tethered polymer chains. Here, we present water-tolerant reaction pathways toward patchy and uniformly modified CNC hybrids based on atom transfer radical polymn. (ATRP) and initiators that were linked to the CNCs with carbodiimide-mediated coupling and Fischer esterification, resp. Various monomers, including N-isopropylacrylamide (NIPAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC), and sodium 4-vinylbenzenesulfonate (4-SS), were polymd. from both types of initiator-modified CNCs, yielding chem. patchy and uniform CNC hybrids, via surface-initiated ATRP (SI-ATRP). Interestingly, the stereochem. of tethered PNIPAM was affected by the precise location of ATRP initiating sites, as evidenced by 1H NMR and CD (CD) spectroscopy. This effect may be related to the inherent right-handed chirality of CNCs. CNC/PMETAC hybrids were labeled with gold nanoparticles (AuNPs) to visualize the precise location of polymer tethers via cryo-electron microscopy. In some instances, the AuNPs were indeed concd. at the end groups of the patchy CNC hybrids.
- 173Zoppe, J. O.; Xu, X.; Kanel, C.; Orsolini, P.; Siqueira, G.; Tingaut, P.; Zimmermann, T.; Klok, H.-A. Effect of Surface Charge on Surface-Initiated Atom Transfer Radical Polymerization from Cellulose Nanocrystals in Aqueous Media. Biomacromolecules 2016, 17 (4), 1404– 1413, DOI: 10.1021/acs.biomac.6b00011Google ScholarThere is no corresponding record for this reference.
- 174Turgman-Cohen, S.; Genzer, d. J. Simultaneous Bulk- and Surface-Initiated Controlled Radical Polymerization from Planar Substrates. J. Am. Chem. Soc. 2011, 133 (44), 17567– 17569, DOI: 10.1021/ja2081636Google Scholar174https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXht1Onsb%252FI&md5=1436a01dc79bb96f097441c58d1f17ceSimultaneous Bulk- and Surface-Initiated Controlled Radical Polymerization from Planar SubstratesTurgman-Cohen, Salomon; Genzer, JanJournal of the American Chemical Society (2011), 133 (44), 17567-17569CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)We employ Monte Carlo computer simulations to study the simultaneous controlled radical polymn. in soln. and from a flat surface. The bulk polymers grow at faster rates and possess narrower mol. wt. distribution than polymers initiated from flat, impenetrable surfaces. The rate of surface-initiated polymn. depends on the d. of initiator sites. Our results provide evidence that the assumption that the mol. wt. of surface-initiated polymers is equal to that of polymers grown in bulk, invoked often in detg. the grafting d. of surface-bound polymers, is generally invalid.
- 175Tang, W.; Kwak, Y.; Braunecker, W.; Tsarevsky, N. V.; Coote, M. L.; Matyjaszewski, K. Understanding atom transfer radical polymerization: Effect of ligand and initiator structures on the equilibrium constants. J. Am. Chem. Soc. 2008, 130 (32), 10702– 10713, DOI: 10.1021/ja802290aGoogle Scholar175https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXos12qs7o%253D&md5=b8bd60b588b23b623dc60a95dfdaa210Understanding Atom Transfer Radical Polymerization: Effect of Ligand and Initiator Structures on the Equilibrium ConstantsTang, Wei; Kwak, Yungwan; Braunecker, Wade; Tsarevsky, Nicolay V.; Coote, Michelle L.; Matyjaszewski, KrzysztofJournal of the American Chemical Society (2008), 130 (32), 10702-10713CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Equil. consts. in Cu-based atom transfer radical polymn. (ATRP) were detd. for a wide range of ligands and initiators in acetonitrile at 22°. The ATRP equil. consts. obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the CuI/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equil. consts. are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equil. consts. are largest for the nitrile derivs., followed by those for the benzyl derivs. and the corresponding esters. Other equil. consts. that are not readily measurable were extrapolated from the values for the ref. ligands and initiators. Excellent correlations of the equil. consts. with the CuII/I redox potentials and the carbon-halogen bond dissocn. energies were obsd.
- 176Hu, H.; Hou, X.-J.; Wang, X.-C.; Nie, J.-J.; Cai, Q.; Xu, F.-J. Gold nanoparticle-conjugated heterogeneous polymer brush-wrapped cellulose nanocrystals prepared by combining different controllable polymerization techniques for theranostic applications. Polym. Chem. 2016, 7 (18), 3107– 3116, DOI: 10.1039/C6PY00251JGoogle Scholar176Gold nanoparticle-conjugated heterogeneous polymer brush-wr