Electrostatically Anchored Branched Brush Layers
- Xiaoyan Liu ,
- Andra Dedinaite ,
- Mark Rutland ,
- Esben Thormann ,
- Ceslav Visnevskij ,
- Ricardas Makuska , and
- Per M. Claesson
Abstract

A novel type of block copolymer has been synthesized. It consists of a linear cationic block and an uncharged bottle-brush block. The nonionic bottle-brush block contains 45 units long poly(ethylene oxide) side chains. This polymer was synthesized with the intention of creating branched brush layers firmly physisorbed to negatively charged surfaces via the cationic block, mimicking the architecture (but not the chemistry) of bottle-brush molecules suggested to be present on the cartilage surface, and contributing to the efficient lubrication of synovial joints. The adsorption properties of the diblock copolymer as well as of the two blocks separately were studied on silica surfaces using quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry. The adsorption kinetics data highlight that the diblock copolymers initially adsorb preferentially parallel to the surface with both the cationic block and the uncharged bottle-brush block in contact with the surface. However, as the adsorption proceeds, a structural change occurs within the layer, and the PEO bottle-brush block extends toward solution, forming a surface-anchored branched brush layer. As the adsorption plateau is reached, the diblock copolymer layer is 46–48 nm thick, and the water content in the layer is above 90 wt %. The combination of strong electrostatic anchoring and highly hydrated branched brush structures provide strong steric repulsion, low friction forces, and high load bearing capacity. The strong electrostatic anchoring also provides high stability of preadsorbed layers under different ionic strength conditions.
1 Introduction
| (i) | The polymer should adsorb strongly to negatively charged surfaces, and for this reason a cationic anchor block was used. | ||||
| (ii) | The side chains should be able to generate strongly repulsive forces, and 45 unit long PEO chains have previously been shown to achieve this. (10, 15) Thus, such side chains were used also in this case. | ||||
| (iii) | Theoretical modeling has predicted that cationic diblock bottle-brush structures adsorb to larger amounts than cationic random bottle-brush structures. (20, 21) | ||||
| (iv) | It has been suggested that biological bottle-brush molecules are oriented preferentially perpendicular to the cartilage surface and whereby aid lubrication. (9) Modeling suggests (20, 21) that such a structure can be achieved more efficiently by using diblock bottle-brushes, rather than the random bottle-brushes used so far. Indeed, the lowest friction coefficients reported for random bottle-brush polymers were obtained for adsorbed layers where the bottle-brush structure was partly oriented away from the surface. (10) | ||||
| (v) | The bottle-brush polymers found in nature are typically highly polydisperse. (6) Thus, a significant polydispersity of the bottle-brush block in the cationic diblock bottle-brush structure is not regarded as a disadvantage for the lubrication properties. | ||||
2 Materials and Methods
2.1 Materials
2.2 Synthesized Polymers
| polymer | Mn·10–3 | Mw·10–3 | Mw/Mn | DP | Dapp (m2/s) | RH (nm) |
|---|---|---|---|---|---|---|
| (METAC)m | 18.7 | 23.4 | 1.25 | 90 | 6.2 × 10–11 | 4 |
| (PEO45MEMA)n | 236 | 871 | 3.69 | 113 | 1.2 × 10–11 | 21 |
| (METAC)m-b-(PEO45MEMA)n | 235 | 698 | 3.01 | 90 + 102 | 1.1 × 10–11 | 22 |
2.3 Methods
2.3.1 Optical Reflectometry
(1)2.3.2 QCM-D
(2)
(3)where ω is the angular frequency, Q is the quality factor, ED is the energy dissipated, and ES is the energy stored in one oscillation, respectively. The frequency change (Δf) and the dissipation change (ΔD) were recorded using the Q-tools program (Q-sense, Gothenburg). In order to convert the measured quantities to, e.g., mass, a model has to be invoked, and several different ones have been suggested in the literature. (28-30) In this work we employed the Sauerbrey model when rigid layers were formed, and the Voigt model for viscoelastic layers.2.3.2.1 The Sauerbrey Model
(4)where n is the overtone number, Δf is the frequency change, and C is a constant that is a characteristic for a specific type of QCM-D crystal. In our case, the value of C is 0.177 mg m–2 Hz–1. The above equation is accurate for rigid films, i.e., when the change in dissipation is small, and this model was employed when evaluating the data obtained for the cationic block. This model is not appropriate for more viscoelastic layers, and for that reason we employed the Voigt model when analyzing QCM-D data obtained for the nonionic bottle-brush block and for the diblock copolymer.2.3.2.2 The Voigt Model
(5)
(6)where the β-parameter introduced in this model is analogous to the complex impedance (Z*) used in the Johannsmann model. (29) The β-parameter contains information on the viscoelasticity of the adsorbed film.
(7)
(8)where μf is the elastic shear modulus, ηf is the shear viscosity, and τ = ηf/μf is the characteristic relaxation time of the adsorbed film.
(9)
(10)where all symbols have the same meaning as before. The subscript “b” denotes the bulk liquid, and δ is the viscous penetration depth defined as
(11)In the analysis of the data, the frequency and dissipation changes are fitted by eqs 9 and 10, respectively.
(12)The effective hydrodynamic thickness of the adsorbed layer was calculated according to eq 13:
(13)where deff is the effective hydrodynamic thickness of the adsorbed layer, and ρpolymer is the bulk density of the polymer. (31) The use of eqs 12 and 13 relies on the assumption of insignificant volume change of mixing. A value for the layer thickness, dVoigt, was also obtained directly from the Voigt model.2.3.3 AFM
3 Results
3.1 Kinetic Aspects of Layer Formation
Figure 1

Figure 1. (a) Adsorbed mass and (b) sensed mass, calculated from overtone 3, as a function of time during adsorption of (METAC)m-b-(PEO45MEMA)n on silica from water (pH ∼ 6). The diblock copolymer at a concentration of 50 ppm was injected at time = 0. The arrows mark the start of the rinsing process with Milli-Q water. The insets focus on the initial adsorption, and the dashed lines in the insets are provided to emphasize the increase in adsorption rate that occurs after a few hundred seconds.
Figure 2

Figure 2. Dissipation change (ΔD) as a function of frequency change (−Δf) during adsorption of (METAC)m-b-(PEO45MEMA)n (upper curve) and (PEO45MEMA)n (lower curve) on silica from a solution with a polymer concentration of 50 ppm in pure water. The inset shows the data in the range of −Δf up to 40 Hz in more detail.
3.2 Equilibrium Layer Properties
| (METAC)m | (PEO45MEMA)n | (METAC)m-b-(PEO45MEMA)n | |
|---|---|---|---|
| Γrefl(mg/m2) | 0.17 ± 0.01 | 0.94 ± 0.05 | 2.75 ± 0.2 |
| ΓQCM-D (mg/m2) | 0.4 ± 0.02 | 2.15 ± 0.2 | 48.6 ± 0.3 |
| water content (wt%) | 57.5 ± 0.5 | 56 ± 0.5 | 94.5 ± 0.2 |
| μf (Pa) | (2.5 ± 0.1) × 105 | (2.5 ± 0.2) × 105 | |
| ηf(Pa·s) | (2.2 ± 0.3) × 10–3 | (4.1 ± 0.3) × 10–3 | |
| deff (nm) | 0.4 ± 0.03 | 2.1 ± 0.3 | 48 ± 1 |
| dVoigt (nm) | 2.8 ± 0.1 | 46 ± 2 |
3.3 Effect of Ionic Strength on Adsorption
Figure 3

Figure 3. Adsorbed amount (Γrefl(■)) and sensed mass (ΓQCM-D(●)) of (METAC)m-b-(PEO45MEMA)n on silica as a function of NaCl concentration. The points marked with “*” are obtained with no added salt. The concentration of the diblock copolymer was 50 ppm, pH ∼ 6, and the temperature 22 °C. The error bars correspond to standard deviations from three measurements.
Figure 4

Figure 4. Water content of (METAC)m-b-(PEO45MEMA)n layers formed by adsorption from water (∗) and NaCl solutions of different concentrations (□). Thickness of the layer calculated by eq 13 (●) and from the Vogit model (▲), the thickness of the layer after rinse with water (∗).
3.4 Desorption of Preadsorbed Layers
Figure 5

Figure 5. Adsorbed amount and sensed mass of (METAC)m-b-(PEO45MEMA)n layers after rinsing with NaCl solutions of different concentrations. The polymer was initially adsorbed from a 50 ppm aqueous solution at pH ∼ 6 (∗), and then the layer was subsequently rinsed with polymer-free solutions of increasing NaCl concentration (■).The rinse time at each NaCl concentration was about 30 min.
3.5 Surface Forces and Friction
Figure 6

Figure 6. (a) Force normalized by radius between silica surfaces coated with an adsorbed (METAC)m-b-(PEO45MEMA)n layer across a 50 ppm solution of the diblock copolymer as a function of separation. Filled and unfilled symbols represent data obtained on approach and retraction, respectively. (b) Friction force versus load for two uncoated silica surfaces across water (triangles), and for silica coated with an adsorbed (METAC)m-b-(PEO45MEMA)n layer across a 50 ppm solution of the diblock copolymer (circles). Filled and unfilled symbols represent data obtained on loading and unloading, respectively.
4 Discussion
4.1 Build-Up of the Layer Formed by the (METAC)m-b-(PEO45MEMA)n Diblock Copolymer
Figure 7

Figure 7. Schematic illustration of the time evolution of the structure of (METAC)m-b-(PEO45MEMA)n during the adsorption process. We note that the Rg of the cross-section of the bottle-brush block in solutions is about 3 nm, (40, 41) and thus the side chains are more coiled than illustrated in this simple sketch.
4.2 Effect of Ionic Strength on the Adsorption of (METAC)m-b-(PEO45MEMA)n
4.3 Effect of Ionic Strength on Preadsorbed Layers of (METAC)m-b-(PEO45MEMA)n
4.4 Comparison with Random Bottle-Brush Polymers
Figure 8

Figure 8. (a) Adsorbed amount, (b) layer thickness, and (c) number of PEO45 side chains m–2 for PEO45MEMA:METAC-X (symbols) compared to that obtained for (METAC)m-b-(PEO45MEMA)n (horizontal lines) as a function of mol % charged segments in the random copolymers. Data for the random copolymers are from refs 36 and 39.
5 Conclusions
Supporting Information
Additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
Terms & Conditions
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Acknowledgment
X.L. acknowledges a stipend from the Chinese Scholarship Council (CSC), P.C. and M.R. acknowledge financial support from VR. The project was carried out within the framework of the SSF program “Microstructure, Corrosion and Friction Control”. C.V. and R.M. gratefully acknowledge financial support from the Research Council of Lithuania under the project MIP-50/2010. Lubica Macakova at the Institute for Surface Chemistry, YKI, is thanked for valuable discussions.
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- 31Naderi, A.; Iruthayaraj, J.; Vareikis, A.; Maku ka, R.; Claesson, P. M. Surface Properties of Bottle-Brush Polyelectrolytes on Mica: Effects of Side Chain and Charge Densities Langmuir 2007, 23, 12222– 12232[ACS Full Text
], [CAS], Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXht1Sgtb3L&md5=34659ab5e726995ee3a3e8e328fe088aSurface Properties of Bottle-Brush Polyelectrolytes on Mica: Effects of Side Chain and Charge DensitiesNaderi, Ali; Iruthayaraj, Joseph; Vareikis, Ausvydas; Makuska, Ricardas; Claesson, Per M.Langmuir (2007), 23 (24), 12222-12232CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Surface properties of a series of cationic bottle-brush polyelectrolytes, poly(ethylene glycol)-Me ether methacrylate (PEO-MEMA) with 45-unit-long PEO side chains and methacryloxyethyltrimethylammonium chloride (METAC) were studied by phase modulated ellipsometry and surface force measurements. The evaluation of the adsorbed mass of polymer on mica by ellipsometry is complex due to the transparency of mica and its birefringence and low dielec. const.; a new method was developed to overcome these difficulties. The charge and the poly(ethylene oxide) side chain d. of the bottle-brush polymers were varied from zero charge d. and one side chain per segment to one charge per segment and no side chains, thus spanning the realm from a neutral bottle-brush polymer, via a partly charged brush polyelectrolyte, to a linear fully charged polyelectrolyte. The adsorption properties depend crucially on the polymer architecture. A min. charge d. of the polymer is required to facilitate adsorption to the oppositely charged surface. The max. adsorbed amt. and the max. side chain d. at the surface are obtained for the polymer with 50% charged segments and the remaining 50% of the segments carrying poly(ethylene oxide) side chains. Brush-like layers are formed when 25-50% of the segments carry poly(ethylene oxide) side chains. The repulsion between the side chains results in an adsorbed layer that is non-homogeneous on the mol. level. As a result, not all side chains will contribute equally to the steric repulsion but some will be stretched along the surface rather than perpendicular to it. By comparison with linear polyelectrolytes, the presence of the side chains counteracts adsorption. This is due to the entropic penalty of confining the side chains to the surface region. - 32Green, C. P.; Lioe, H.; Cleveland, J. P.; Proksch, R.; Mulvaney, P.; Sader, J. E. Normal and Torsional Spring Constants of Atomic Force Microscope Cantilevers Rev. Sci. Instrum. 2004, 75, 1988– 1996[Crossref], [CAS], Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXks1els7g%253D&md5=b471951beab1e5b10eaa71e2ff34e4edNormal and torsional spring constants of atomic force microscope cantileversGreen, Christopher P.; Lioe, Hadi; Cleveland, Jason P.; Proksch, Roger; Mulvaney, Paul; Sader, John E.Review of Scientific Instruments (2004), 75 (6), 1988-1996CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)Two methods commonly used to measure the normal spring consts. of at. force microscope cantilevers are the added mass method of Cleveland et al. [J. P. Cleveland et al., Rev. Sci. Instrum. 64, 403 (1993)], and the unloaded resonance technique of Sader et al. [J. E. Sader, J. W. M. Chon, and P. Mulvaney, Rev. Sci. Instrum. 70, 3967 (1999)]. The added mass method involves measuring the change in resonant frequency of the fundamental mode of vibration upon the addn. of known masses to the free end of the cantilever. In contrast, the unloaded resonance technique requires measurement of the unloaded resonant frequency and quality factor of the fundamental mode of vibration, as well as knowledge of the plan view dimensions of the cantilever and properties of the fluid. In many applications, such as frictional force microscopy, the torsional spring const. is often required. Consequently, in this article, we extend both of these techniques to allow simultaneous calibration of both the normal and torsional spring consts. We also investigate the validity and applicability of the unloaded resonance method when a mass is attached to the free end of the cantilever due to its importance in practice.
- 33Sader, J. E.; Chon, J. W. M.; Mulvaney, P. Calibration of Rectangular Atomic Force Microscope Cantilevers Rev. Sci. Instrum. 1999, 70, 3967– 3969[Crossref], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXmt1yhtr4%253D&md5=ad9d2b3c05ccc21e2e21ed66a797bae8Calibration of rectangular atomic force microscope cantileversSader, John E.; Chon, James W. M.; Mulvaney, PaulReview of Scientific Instruments (1999), 70 (10), 3967-3969CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)A method to det. the spring const. of a rectangular at. force microscope cantilever is proposed that relies solely on the measurement of the resonant frequency and quality factor of the cantilever in fluid (typically air), and knowledge of its plan view dimensions. This method gives very good accuracy and improves upon the previous formulation by Sader et al. [Rev. Sci. Instrum. 66, 3789 (1995)] which, unlike the present method, requires knowledge of both the cantilever d. and thickness.
- 34Pettersson, T.; Nordgren, N.; Rutland, M. W.; Feiler, A. Comparison of Different Methods to Calibrate Torsional Spring Constant and Photodetector for Atomic Force Microscopy Friction Measurements in Air and Liquid Rev. Sci. Instrum. 2007, 78, 093702– 8[Crossref], [PubMed], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhtFKgtbzM&md5=b63f76941ed98a6ee727df6c1346f1acComparison of different methods to calibrate torsional spring constant and photodetector for atomic force microscopy friction measurements in air and liquidPettersson, Torbjoern; Nordgren, Niklas; Rutland, Mark W.; Feiler, AdamReview of Scientific Instruments (2007), 78 (9), 093702/1-093702/8CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)A no. of at. force microscopy cantilevers were exhaustively calibrated by a no. of techniques to obtain both normal and frictional force consts. to evaluate the relative accuracy of the different methods. These were of either direct or indirect character-the latter relies on cantilever resonant frequencies. The so-called Sader [Rev. Sci. Instrum. 70, 3967(1999)] and Cleveland [Rev. Sci. Instrum. 64, 403(1993)] techniques are compared for the normal force const. calibration and while agreement was good, a systematic difference was obsd. For the torsional force consts., all the techniques displayed a certain scatter but the agreement was highly encouraging. By far the simplest technique is that of Sader, and it is suggested in view of this validation that this method should be generally adopted. The issue of the photodetector calibration is also addressed since this is necessary to obtain the cantilever twist from which the torsional force is calcd. Here a technique of obtaining the torsional photodetector sensitivity by combining the direct and indirect methods is proposed. Direct calibration measurements were conducted in liq. as well as air, and a conversion factor was obtained showing that quant. friction measurements in liq. are equally feasible provided the correct calibration was performed.
- 35Bijelic, G.; Shovsky, A.; Varga, I.; Makuska, R.; Claesson, P. M. Adsorption Characteristics of Brush Polyelectrolytes on Silicon Oxynitride Revealed by Dual Polarization Interferometry J. Colloid Interface Sci. 2010, 348, 189– 197Google ScholarThere is no corresponding record for this reference.
- 36Iruthayaraj, J.; Olanya, G.; Claesson, P. M. Viscoelastic Properties of Adsorbed Bottle-Brush Polymer Layers Studied by Quartz Crystal Microbalance Dissipation Measurements J. Chem. Phys. C 2008, 112, 15028– 15036[ACS Full Text
], [CAS], Google Scholar36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtVeitLbI&md5=a00546c0f934211677a143a91f0615deViscoelastic Properties of Adsorbed Bottle-brush Polymer Layers Studied by Quartz Crystal Microbalance - Dissipation MeasurementsIruthayaraj, Joseph; Olanya, Geoffrey; Claesson, Per M.Journal of Physical Chemistry C (2008), 112 (38), 15028-15036CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)Adsorbed layers of a series of bottle-brush polyelectrolytes with 45 units long poly(ethylene oxide) [PEO], side chains were investigated by the quartz crystal microbalance technique with dissipation monitoring. The data have been evaluated with three different models, the Sauerbrey model, the Johannsmann model, and the Voigt model. It is found that all three models predict the same trend in the variations of sensed mass and hydrodynamic layer thickness with polymer architecture, i.e., with the backbone charge to side chain d. ratio. However, the simple Sauerbrey model underestimates the sensed mass by a factor of 1.15-1.45 compared to the more accurate Voigt model. By following the evolution of the layer dissipation, elasticity, and viscosity with increasing surface coverage it was concluded that the layers formed by brush polymers with intermediate charge densities undergo a structural change as the coverage is increased. Initially, the polymers are anchored to the surface via the PEO side chains. However, as the adsorption proceeds a structural change that brings the backbone to the surface and forces the side chains to extend from it is obsd. The layer elasticity and viscosity as a function of surface coverage go through a max. in this transition region. - 37Linse, P. Adsorption of Weakly Charged Polyelectrolytes at Oppositely Charged Surfaces Macromolecules 1996, 29, 326– 336[ACS Full Text
], [CAS], Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXpvVaqur8%253D&md5=b77ca99a234f848d06309a9eab0cb292Adsorption of Weakly Charged Polyelectrolytes at Oppositely Charged SurfacesLinse, PerMacromolecules (1996), 29 (1), 326-36CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)The adsorption of weakly charged polyelectrolytes at planar and oppositely charged surfaces was modeled by using a mean-field lattice theory for flexible polyelectrolytes in soln. The nature of the adsorption was described in terms of vol. fraction profiles, adsorbed amt., thickness of adsorbed layer, and conformational characteristics at different salt concns. The effect on the adsorption of (i) polyelectrolyte length, (ii) linear charge d. of the polyelectrolyte, (iii) vol. fraction of the polyelectrolyte, (i.v.) nonelectrostatic polyelectrolyte-surface interaction, and (v) surface potential or surface charge d., all at different salt concns. and different surface conditions, was investigated. In most cases, as the salt concn. is increased, the adsorbed amt. is reduced but the thickness of the adsorbed layer is increased. At low salt concn. and at const. surface charge d., the amt. adsorbed is governed by the surface charge d. through a polyelectrolyte-surface charge matching. At const. surface potential, a more diverse picture has emerged. A less regular distribution of the charges along the polyelectrolyte chain facilitates the adsorption. - 38Lundin, M.; Macakova, L.; Dedinaite, A.; Claesson, P. Interactions between Chitosan and SDS at a Low-Charged Silica Substrate Compared to Interactions in the Bulk – The Effect of Ionic Strength Langmuir 2008, 24, 3814– 3827
- 39Olanya, G.; Iruthayaraj, J.; Poptoshev, E.; Makuska, R.; Vareikis, A.; Claesson, P. M. Adsorption Characteristics of Bottle-Brush Polymers on Silica: Effect of Side Chain and Charge Density Langmuir 2008, 24, 5341– 5349
- 40Bastardo, L. A.; Iruthayaraj, J.; Lundin, M.; Dedinaite, A.; Vareikis, A.; Makuška, R.; van der Wal, A.; Furó, I.; Garamus, V. M.; Claesson, P. M. Soluble Complexes in Aqueous Mixtures of Low Charge Density Comb Polyelectrolyte and Oppositely Charged Surfactant Probed by Scattering and NMR J. Colloid Interface Sci. 2007, 312, 21– 33Google ScholarThere is no corresponding record for this reference.
- 41Dedinaite, A.; Bastardo, L. A.; Oliveira, C. L. P.; Pedersen, J. S.; Claesson, P. M.; Vareikis, A.; Makuška, R. Solution Properties of Bottle-Brush PolyelectrolytesIn Proceedings of the Baltic Polymer Symposium, Druskininkai, Lithuania, September 19–21, 2007.Google ScholarThere is no corresponding record for this reference.
- 42Van de Steeg, H. G. M.; Cohen Stuart, M. A.; De Keizer, A.; Bijsterbosch, B. H. Polyelectrolyte Adsorption: A Subtle Balance of Forces Langmuir 1992, 8, 2538– 2546
- 43Bailey, F. E.; Callard, R. W. Some Properties of Poly(ethylene oxide)1 in Aqueous Solution J. Appl. Polym. Sci. 1959, 1, 56– 62[Crossref], [CAS], Google Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaF3cXhsVSguw%253D%253D&md5=b541ee6dbbb69771f345e8e505d29c63Properties of poly(ethylene oxide) in aqueous solnBailey, F. E., Jr.; Callard, R. W.Journal of Applied Polymer Science (1959), 1 (), 56-62CODEN: JAPNAB; ISSN:0021-8995.Poly(ethylene oxide) (I) is sol. in water owing to an apparently unique hydration of the ether O. It is disrupted by thermal and salting out effects; the latter are smaller than those observed on polyelectrolytes. A high degree of shear dependence of high-mol.-wt. I is due in part to the essential linearity of the structure together with the enormous degrees of polymerization possible. In aq. solns. the ether linkages are apparently stiffened by hydration so that the polymer coil extension approaches that found in hydrocarbons in which hindered rotation leads to extended configurations. The high degree of entanglement of polymer chains in more concd. solns. (several %) seems to result in a network structure that is distended by relatively low shearing forces. The degree of distortion of the polymer configurations in soln. tends to reach a limiting value at very high rates of shear. The linear, relatively nonpolar structure of I displays a high degree of polymer solvent interaction in aq. soln. which is observed in the development of an unusual degree of structural viscosity.
- 44Nilsson, P. G.; Wennerstroem, H.; Lindman, B. Structure of Micellar Solutions of Nonionic Surfactants. Nuclear Magnetic Resonance Self-Diffusion and Proton Relaxation Studies of Poly(ethylene oxide) Alkyl Ethers J. Phys. Chem. 1983, 87, 1377– 1385[ACS Full Text
], [CAS], Google Scholar44https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3sXhsV2qurY%253D&md5=7695fe18882896bf7cc5432b1f2a7641Structure of micellar solutions of nonionic surfactants. Nuclear magnetic resonance self-diffusion and proton relaxation studies of poly(ethylene oxide) alkyl ethersNilsson, Per Gunnar; Wennerstroem, Haekan; Lindman, BjoernJournal of Physical Chemistry (1983), 87 (8), 1377-85CODEN: JPCHAX; ISSN:0022-3654.Binary systems of water and nonionic surfactants of the dodecyl oxyethylene monoether of the type C12H25(OCH2CH2)nOH were studied by 1H NMR techniques. For n = 5, C12E5, and n = 8, C12E8, the self-diffusion coeffs. of the surfactants in the isotropic, L1, phase were measured by using the pulsed field gradient technique. In addn., the line widths of the different proton signals were monitored and some samples of liq. cryst. character were also studied. Dramatic broadening of the methylene signals of the C12 chain is obsd. as the hexagonal liq. cryst. phase is approached in the C12E5-water system, while only a small broadening is obsd. in the C12E8-water system, showing that at low temps. there is a growth of C12E5 micelles to rods with increasing concns., while the C12E8 micelles at low temps. remain small in the whole concn. range. The self-diffusion coeffs. of the surfactants decrease rapidly with increasing concn. until a min. is reached after which there is a slow increase. The location of the min. occurs at lower surfactant concns. the closer the temp. is to the cloud point, where the system separates into 2 isotropic phases. In the line width studies, broadening is found at a certain temp. interval when the temp. is increased in the C12E5 system. These results taken together indicate that, in the C12E5 system, the surfactant aggregates grow in size as the cloud point is approached. The aggregates are flexible and probably not of a definite shape close to the cloud point. In the C12E8 system, the micelles are much less affected by an increase in temp. and micellar growth cannot be unambiguously established. The methylene signals of the ethylene oxide moieties consistently show narrower 1H signals, showing that in the aggregates they are less ordered than the chain methylenes. The various changes in aggregate size and shape are correlated with the stability ranges of the isotropic and liq. cryst. phases according to phase diagrams from the literature. Both aggregate size and phase structures are in qual. agreement with considerations based on the effective (as a result of coiling, hydration, and head group size) shape of the mols. at different temps. and concns. - 45Nilsson, P. G.; Lindman, B. Water Self-Diffusion in Nonionic Surfactant Solutions. Hydration and Obstruction Effects J. Phys. Chem. 1983, 87, 4756– 4761[ACS Full Text
], [CAS], Google Scholar45https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3sXlvVWrtLs%253D&md5=4792a22df4b4daeb165774f56c60591bWater self-diffusion in nonionic surfactant solutions. Hydration and obstruction effectsNilsson, Per Gunnar; Lindman, BjoernJournal of Physical Chemistry (1983), 87 (23), 4756-61CODEN: JPCHAX; ISSN:0022-3654.The self-diffusion coeff. was detd. of D2O in aq. solns. of nonionic surfactants of different concns. and at different temps. by using the NMR spin-echo pulsed-field-gradient method. The nonionic surfactants are pentaethylene glycol dodecyl ether (C12E5) and octaethylene glycol dodecyl ether (C12E8). The self-diffusion coeff. of D2O is lower than that of neat D2O because the surfactant aggregates obstruct the paths of the D2O mols. and because a fraction of it is bound to the surfactant (hydration) and hence has a lower translational mobility. The hydration is ∼4-6 H2O mols. per ethylene oxide group for a 10% surfactant soln. and decreases with increasing surfactant concn. The nonionic surfactants were compared with a poly(ethylene glycol) (PEG). The hydration of the ethylene oxide groups seems to be independent of the particular system and mainly detd. by the temp. and the compn. of the system. The consequences of this for the understanding of the clouding phenomenon are discussed. - 46Reimhult, E.; Larsson, C.; Kasemo, B.; Höök, F. Simultaneous Surface Plasmon Resonance and Quartz Crystal Microbalance with Dissipation Monitoring Measurements of Biomolecular Adsorption Events Involving Structural Transformations and Variations in Coupled Water Anal. Chem. 2004, 76, 7211– 7220
- 47Iruthayaraj, J.; Poptoshev, E.; Vareikis, A.; Makuška, R. a.; van der Wal, A.; Claesson, P. M. Adsorption of Low Charge Density Polyelectrolyte Containing Poly(ethylene oxide) Side Chains on Silica: Effects of Ionic Strength and pH Macromolecules 2005, 38, 6152– 6160
- 48Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J.; Böhmer, M. R. Formation and Stability of Multilayers of Polyelectrolytes Langmuir 1996, 12, 3675– 3681[ACS Full Text
], [CAS], Google Scholar48https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XjvFWksLw%253D&md5=03926b7941609ba1f1c3e8fab5cc3e5dFormation and Stability of Multilayers of PolyelectrolytesHoogeveen, Nynke G.; Cohen Stuart, Martien A.; Fleer, Gerard J.; Boehmer, Marcel R.Langmuir (1996), 12 (15), 3675-3681CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Sequential addn. of anionic and cationic polyelectrolytes may lead to the formation of multilayers at a solid surface. The buildup of such multilayers is characterized by a stepwise increase of the adsorbed amt. and layer thickness and by alternating highly pos. and highly neg. values for the ζ-potential. The prime variables which det. the stability of these structures are the polymer charge and the ionic strength. Very stable multilayers are formed when both polymers are highly charged and when the ionic strength is low. For weakly stable multilayers complexation at the surface may first occur, followed by desorption of the complexes. For strongly charged polyelectrolytes the charge stoichiometry, which is not always 1:1, seems to be unique for each pair of polyelectrolytes; no influence of the substrate, of the pH, or of the ionic strength could be obsd. - 49Dedinaite, A.; Thormann, E.; Olanya, G.; Claesson, P. M.; Nystrom, B.; Kjoniksen, A.-L.; Zhu, K. Friction in Aqueous Media Tuned by Temperature-Responsive Polymer Layers Soft Matter 2010, 6, 2489– 2498[Crossref], [CAS], Google Scholar49https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXmsFequr8%253D&md5=fd0b73cdad7396404f593ade1041ee3cFriction in aqueous media tuned by temperature-responsive polymer layersDedinaite, Andra; Thormann, Esben; Olanya, Geoffrey; Claesson, Per M.; Nystroem, Bo; Kjoniksen, Anna-Lena; Zhu, KaizhengSoft Matter (2010), 6 (11), 2489-2498CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)An at. force microscope colloidal probe technique has been employed to probe normal and friction forces between silica surfaces coated with adsorbed layers of a N-isopropylacrylamide48-(3-acrylamidopropyl)trimethylammonium chloride20 diblock copolymer, abbreviated PNIPAAM48-b-PAMPTMA(+)20. The interactions between the PNIPAAM48-b-PAMPTMA(+)20-coated surfaces across a 0.1 mM NaCl (pH 6) soln. at 25 °C are purely repulsive, due to a combination of steric and electrostatic double-layer forces. However, when the temp. is increased to 35 °C, and subsequently to 45 °C, an attractive force develops at short sepns. due to the unfavorable PNIPAAM-water interaction at these temps. The temp.-dependent polymer-water interaction has implications for the friction force between the layers. At 25 °C a frictional force that increases linearly with increasing load is obsd. once the surfaces are brought into close contact. At higher temps. significantly higher friction forces appear as a consequence of attractive segment-segment interactions. Further, a clearly expressed hysteresis between friction forces encountered on loading and unloading is detected. Our results demonstrate that both normal and friction forces between surfaces can be controlled by temp. changes when temp.-responsive polymers are employed, and friction forces can be adjusted as required from low to high.
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Abstract

Figure 1

Figure 1. (a) Adsorbed mass and (b) sensed mass, calculated from overtone 3, as a function of time during adsorption of (METAC)m-b-(PEO45MEMA)n on silica from water (pH ∼ 6). The diblock copolymer at a concentration of 50 ppm was injected at time = 0. The arrows mark the start of the rinsing process with Milli-Q water. The insets focus on the initial adsorption, and the dashed lines in the insets are provided to emphasize the increase in adsorption rate that occurs after a few hundred seconds.
Figure 2

Figure 2. Dissipation change (ΔD) as a function of frequency change (−Δf) during adsorption of (METAC)m-b-(PEO45MEMA)n (upper curve) and (PEO45MEMA)n (lower curve) on silica from a solution with a polymer concentration of 50 ppm in pure water. The inset shows the data in the range of −Δf up to 40 Hz in more detail.
Figure 3

Figure 3. Adsorbed amount (Γrefl(■)) and sensed mass (ΓQCM-D(●)) of (METAC)m-b-(PEO45MEMA)n on silica as a function of NaCl concentration. The points marked with “*” are obtained with no added salt. The concentration of the diblock copolymer was 50 ppm, pH ∼ 6, and the temperature 22 °C. The error bars correspond to standard deviations from three measurements.
Figure 4

Figure 4. Water content of (METAC)m-b-(PEO45MEMA)n layers formed by adsorption from water (∗) and NaCl solutions of different concentrations (□). Thickness of the layer calculated by eq 13 (●) and from the Vogit model (▲), the thickness of the layer after rinse with water (∗).
Figure 5

Figure 5. Adsorbed amount and sensed mass of (METAC)m-b-(PEO45MEMA)n layers after rinsing with NaCl solutions of different concentrations. The polymer was initially adsorbed from a 50 ppm aqueous solution at pH ∼ 6 (∗), and then the layer was subsequently rinsed with polymer-free solutions of increasing NaCl concentration (■).The rinse time at each NaCl concentration was about 30 min.
Figure 6

Figure 6. (a) Force normalized by radius between silica surfaces coated with an adsorbed (METAC)m-b-(PEO45MEMA)n layer across a 50 ppm solution of the diblock copolymer as a function of separation. Filled and unfilled symbols represent data obtained on approach and retraction, respectively. (b) Friction force versus load for two uncoated silica surfaces across water (triangles), and for silica coated with an adsorbed (METAC)m-b-(PEO45MEMA)n layer across a 50 ppm solution of the diblock copolymer (circles). Filled and unfilled symbols represent data obtained on loading and unloading, respectively.
Figure 7

Figure 7. Schematic illustration of the time evolution of the structure of (METAC)m-b-(PEO45MEMA)n during the adsorption process. We note that the Rg of the cross-section of the bottle-brush block in solutions is about 3 nm, (40, 41) and thus the side chains are more coiled than illustrated in this simple sketch.
Figure 8

Figure 8. (a) Adsorbed amount, (b) layer thickness, and (c) number of PEO45 side chains m–2 for PEO45MEMA:METAC-X (symbols) compared to that obtained for (METAC)m-b-(PEO45MEMA)n (horizontal lines) as a function of mol % charged segments in the random copolymers. Data for the random copolymers are from refs 36 and 39.
References
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- 28Sauerbrey, G. Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung Z. Phys. A: Hadrons Nucl. 1959, 155, 206– 222[Crossref], [CAS], Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaG1MXoslSjtQ%253D%253D&md5=ff97a9cc63637eb9823aebfeb57a367bThe use of quartz oscillators for weighing thin layers and for microweighingSauerbrey, GuntherZeitschrift fuer Physik (1959), 155 (), 206-22CODEN: ZEPYAA; ISSN:0044-3328.The frequency of a quartz plate is altered if a layer of foreign material is deposited on the quartz, e.g. by evapn. Since changes in frequency can be measured very accurately, the phenomenon can be used for weighing thin layers. The change in frequency is proportional to the d. of the foreign layer and the proportionality const. can be calcd. from the characteristic frequency of the quartz plate. The accuracy of the method is limited by the temp. variation of the characteristic frequency of the quartz oscillator. For a temp. fluctuation of 1° the accuracy is ±4.10-9 g./cm. This corresponds to a mean thickness of the layer of 0.4 A. at a d. of 1 g./cc. The method can also be used for direct microweighing, e.g. by evapg. a drop of a soln. on the quartz plate.
- 29Johannsmann, D.; Mathauer, K.; Wegner, G.; Knoll, W. Viscoelastic Properties of Thin Films Probed with a Quartz-Crystal Resonator Phys. Rev. B 1992, 46, 7808Google ScholarThere is no corresponding record for this reference.
- 30Rodahl, M.; Kasemo, B. On the Measurement of Thin Liquid Overlayers with the Quartz-Crystal Microbalance Sens. Actuators, A 1996, 54, 448– 456[Crossref], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XntFGhtL4%253D&md5=2676cb392cdf0d96de40dc2e47554480On the measurement of thin liquid overlayers with the quartz-crystal microbalanceRodahl, Michael; Kasemo, BengtSensors and Actuators, A: Physical (1996), 54 (1-3, Transducers '95, Proceedings of the 8th International Conference on Solid-State Sensors and Actuators Eurosensors IX), 448-456CODEN: SAAPEB; ISSN:0924-4247. (Elsevier)We present a simple model that predicts the changes in resonance frequency and dissipation factor for a quartz-crystal microbalance (QCM) when it is coated with a viscous film that may or may not slip on the crystal. In this context, the validity of the Sauerbrey equation is discussed. The Sauerbrey equation gives an accurate est. of the film thickness, tf, only if (i) the film is thin compared to the shear-wave penetration depth, .vdelta., into the liq., i.e., tf«.vdelta., and (ii) the film does not slide on the QCM electrode(s). We have shown that by measuring both the QCM resonance frequency and the dissipation factor simultaneously, the thickness range over which tf can be measured accurately can be extended to about 2.vdelta. for non-slipping films. If the film slips, which we have only obsd. for molecularly thin films, changes in dissipation factor can be used to calc. the coeff. of friction between the film and the substrate. We also show the usefulness of measuring the dissipation factor of the QCM when studying solid to liq. phase transitions.
- 31Naderi, A.; Iruthayaraj, J.; Vareikis, A.; Maku ka, R.; Claesson, P. M. Surface Properties of Bottle-Brush Polyelectrolytes on Mica: Effects of Side Chain and Charge Densities Langmuir 2007, 23, 12222– 12232[ACS Full Text
], [CAS], Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXht1Sgtb3L&md5=34659ab5e726995ee3a3e8e328fe088aSurface Properties of Bottle-Brush Polyelectrolytes on Mica: Effects of Side Chain and Charge DensitiesNaderi, Ali; Iruthayaraj, Joseph; Vareikis, Ausvydas; Makuska, Ricardas; Claesson, Per M.Langmuir (2007), 23 (24), 12222-12232CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Surface properties of a series of cationic bottle-brush polyelectrolytes, poly(ethylene glycol)-Me ether methacrylate (PEO-MEMA) with 45-unit-long PEO side chains and methacryloxyethyltrimethylammonium chloride (METAC) were studied by phase modulated ellipsometry and surface force measurements. The evaluation of the adsorbed mass of polymer on mica by ellipsometry is complex due to the transparency of mica and its birefringence and low dielec. const.; a new method was developed to overcome these difficulties. The charge and the poly(ethylene oxide) side chain d. of the bottle-brush polymers were varied from zero charge d. and one side chain per segment to one charge per segment and no side chains, thus spanning the realm from a neutral bottle-brush polymer, via a partly charged brush polyelectrolyte, to a linear fully charged polyelectrolyte. The adsorption properties depend crucially on the polymer architecture. A min. charge d. of the polymer is required to facilitate adsorption to the oppositely charged surface. The max. adsorbed amt. and the max. side chain d. at the surface are obtained for the polymer with 50% charged segments and the remaining 50% of the segments carrying poly(ethylene oxide) side chains. Brush-like layers are formed when 25-50% of the segments carry poly(ethylene oxide) side chains. The repulsion between the side chains results in an adsorbed layer that is non-homogeneous on the mol. level. As a result, not all side chains will contribute equally to the steric repulsion but some will be stretched along the surface rather than perpendicular to it. By comparison with linear polyelectrolytes, the presence of the side chains counteracts adsorption. This is due to the entropic penalty of confining the side chains to the surface region. - 32Green, C. P.; Lioe, H.; Cleveland, J. P.; Proksch, R.; Mulvaney, P.; Sader, J. E. Normal and Torsional Spring Constants of Atomic Force Microscope Cantilevers Rev. Sci. Instrum. 2004, 75, 1988– 1996[Crossref], [CAS], Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXks1els7g%253D&md5=b471951beab1e5b10eaa71e2ff34e4edNormal and torsional spring constants of atomic force microscope cantileversGreen, Christopher P.; Lioe, Hadi; Cleveland, Jason P.; Proksch, Roger; Mulvaney, Paul; Sader, John E.Review of Scientific Instruments (2004), 75 (6), 1988-1996CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)Two methods commonly used to measure the normal spring consts. of at. force microscope cantilevers are the added mass method of Cleveland et al. [J. P. Cleveland et al., Rev. Sci. Instrum. 64, 403 (1993)], and the unloaded resonance technique of Sader et al. [J. E. Sader, J. W. M. Chon, and P. Mulvaney, Rev. Sci. Instrum. 70, 3967 (1999)]. The added mass method involves measuring the change in resonant frequency of the fundamental mode of vibration upon the addn. of known masses to the free end of the cantilever. In contrast, the unloaded resonance technique requires measurement of the unloaded resonant frequency and quality factor of the fundamental mode of vibration, as well as knowledge of the plan view dimensions of the cantilever and properties of the fluid. In many applications, such as frictional force microscopy, the torsional spring const. is often required. Consequently, in this article, we extend both of these techniques to allow simultaneous calibration of both the normal and torsional spring consts. We also investigate the validity and applicability of the unloaded resonance method when a mass is attached to the free end of the cantilever due to its importance in practice.
- 33Sader, J. E.; Chon, J. W. M.; Mulvaney, P. Calibration of Rectangular Atomic Force Microscope Cantilevers Rev. Sci. Instrum. 1999, 70, 3967– 3969[Crossref], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXmt1yhtr4%253D&md5=ad9d2b3c05ccc21e2e21ed66a797bae8Calibration of rectangular atomic force microscope cantileversSader, John E.; Chon, James W. M.; Mulvaney, PaulReview of Scientific Instruments (1999), 70 (10), 3967-3969CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)A method to det. the spring const. of a rectangular at. force microscope cantilever is proposed that relies solely on the measurement of the resonant frequency and quality factor of the cantilever in fluid (typically air), and knowledge of its plan view dimensions. This method gives very good accuracy and improves upon the previous formulation by Sader et al. [Rev. Sci. Instrum. 66, 3789 (1995)] which, unlike the present method, requires knowledge of both the cantilever d. and thickness.
- 34Pettersson, T.; Nordgren, N.; Rutland, M. W.; Feiler, A. Comparison of Different Methods to Calibrate Torsional Spring Constant and Photodetector for Atomic Force Microscopy Friction Measurements in Air and Liquid Rev. Sci. Instrum. 2007, 78, 093702– 8[Crossref], [PubMed], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhtFKgtbzM&md5=b63f76941ed98a6ee727df6c1346f1acComparison of different methods to calibrate torsional spring constant and photodetector for atomic force microscopy friction measurements in air and liquidPettersson, Torbjoern; Nordgren, Niklas; Rutland, Mark W.; Feiler, AdamReview of Scientific Instruments (2007), 78 (9), 093702/1-093702/8CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)A no. of at. force microscopy cantilevers were exhaustively calibrated by a no. of techniques to obtain both normal and frictional force consts. to evaluate the relative accuracy of the different methods. These were of either direct or indirect character-the latter relies on cantilever resonant frequencies. The so-called Sader [Rev. Sci. Instrum. 70, 3967(1999)] and Cleveland [Rev. Sci. Instrum. 64, 403(1993)] techniques are compared for the normal force const. calibration and while agreement was good, a systematic difference was obsd. For the torsional force consts., all the techniques displayed a certain scatter but the agreement was highly encouraging. By far the simplest technique is that of Sader, and it is suggested in view of this validation that this method should be generally adopted. The issue of the photodetector calibration is also addressed since this is necessary to obtain the cantilever twist from which the torsional force is calcd. Here a technique of obtaining the torsional photodetector sensitivity by combining the direct and indirect methods is proposed. Direct calibration measurements were conducted in liq. as well as air, and a conversion factor was obtained showing that quant. friction measurements in liq. are equally feasible provided the correct calibration was performed.
- 35Bijelic, G.; Shovsky, A.; Varga, I.; Makuska, R.; Claesson, P. M. Adsorption Characteristics of Brush Polyelectrolytes on Silicon Oxynitride Revealed by Dual Polarization Interferometry J. Colloid Interface Sci. 2010, 348, 189– 197Google ScholarThere is no corresponding record for this reference.
- 36Iruthayaraj, J.; Olanya, G.; Claesson, P. M. Viscoelastic Properties of Adsorbed Bottle-Brush Polymer Layers Studied by Quartz Crystal Microbalance Dissipation Measurements J. Chem. Phys. C 2008, 112, 15028– 15036[ACS Full Text
], [CAS], Google Scholar36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtVeitLbI&md5=a00546c0f934211677a143a91f0615deViscoelastic Properties of Adsorbed Bottle-brush Polymer Layers Studied by Quartz Crystal Microbalance - Dissipation MeasurementsIruthayaraj, Joseph; Olanya, Geoffrey; Claesson, Per M.Journal of Physical Chemistry C (2008), 112 (38), 15028-15036CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)Adsorbed layers of a series of bottle-brush polyelectrolytes with 45 units long poly(ethylene oxide) [PEO], side chains were investigated by the quartz crystal microbalance technique with dissipation monitoring. The data have been evaluated with three different models, the Sauerbrey model, the Johannsmann model, and the Voigt model. It is found that all three models predict the same trend in the variations of sensed mass and hydrodynamic layer thickness with polymer architecture, i.e., with the backbone charge to side chain d. ratio. However, the simple Sauerbrey model underestimates the sensed mass by a factor of 1.15-1.45 compared to the more accurate Voigt model. By following the evolution of the layer dissipation, elasticity, and viscosity with increasing surface coverage it was concluded that the layers formed by brush polymers with intermediate charge densities undergo a structural change as the coverage is increased. Initially, the polymers are anchored to the surface via the PEO side chains. However, as the adsorption proceeds a structural change that brings the backbone to the surface and forces the side chains to extend from it is obsd. The layer elasticity and viscosity as a function of surface coverage go through a max. in this transition region. - 37Linse, P. Adsorption of Weakly Charged Polyelectrolytes at Oppositely Charged Surfaces Macromolecules 1996, 29, 326– 336[ACS Full Text
], [CAS], Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXpvVaqur8%253D&md5=b77ca99a234f848d06309a9eab0cb292Adsorption of Weakly Charged Polyelectrolytes at Oppositely Charged SurfacesLinse, PerMacromolecules (1996), 29 (1), 326-36CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)The adsorption of weakly charged polyelectrolytes at planar and oppositely charged surfaces was modeled by using a mean-field lattice theory for flexible polyelectrolytes in soln. The nature of the adsorption was described in terms of vol. fraction profiles, adsorbed amt., thickness of adsorbed layer, and conformational characteristics at different salt concns. The effect on the adsorption of (i) polyelectrolyte length, (ii) linear charge d. of the polyelectrolyte, (iii) vol. fraction of the polyelectrolyte, (i.v.) nonelectrostatic polyelectrolyte-surface interaction, and (v) surface potential or surface charge d., all at different salt concns. and different surface conditions, was investigated. In most cases, as the salt concn. is increased, the adsorbed amt. is reduced but the thickness of the adsorbed layer is increased. At low salt concn. and at const. surface charge d., the amt. adsorbed is governed by the surface charge d. through a polyelectrolyte-surface charge matching. At const. surface potential, a more diverse picture has emerged. A less regular distribution of the charges along the polyelectrolyte chain facilitates the adsorption. - 38Lundin, M.; Macakova, L.; Dedinaite, A.; Claesson, P. Interactions between Chitosan and SDS at a Low-Charged Silica Substrate Compared to Interactions in the Bulk – The Effect of Ionic Strength Langmuir 2008, 24, 3814– 3827
- 39Olanya, G.; Iruthayaraj, J.; Poptoshev, E.; Makuska, R.; Vareikis, A.; Claesson, P. M. Adsorption Characteristics of Bottle-Brush Polymers on Silica: Effect of Side Chain and Charge Density Langmuir 2008, 24, 5341– 5349
- 40Bastardo, L. A.; Iruthayaraj, J.; Lundin, M.; Dedinaite, A.; Vareikis, A.; Makuška, R.; van der Wal, A.; Furó, I.; Garamus, V. M.; Claesson, P. M. Soluble Complexes in Aqueous Mixtures of Low Charge Density Comb Polyelectrolyte and Oppositely Charged Surfactant Probed by Scattering and NMR J. Colloid Interface Sci. 2007, 312, 21– 33Google ScholarThere is no corresponding record for this reference.
- 41Dedinaite, A.; Bastardo, L. A.; Oliveira, C. L. P.; Pedersen, J. S.; Claesson, P. M.; Vareikis, A.; Makuška, R. Solution Properties of Bottle-Brush PolyelectrolytesIn Proceedings of the Baltic Polymer Symposium, Druskininkai, Lithuania, September 19–21, 2007.Google ScholarThere is no corresponding record for this reference.
- 42Van de Steeg, H. G. M.; Cohen Stuart, M. A.; De Keizer, A.; Bijsterbosch, B. H. Polyelectrolyte Adsorption: A Subtle Balance of Forces Langmuir 1992, 8, 2538– 2546
- 43Bailey, F. E.; Callard, R. W. Some Properties of Poly(ethylene oxide)1 in Aqueous Solution J. Appl. Polym. Sci. 1959, 1, 56– 62[Crossref], [CAS], Google Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaF3cXhsVSguw%253D%253D&md5=b541ee6dbbb69771f345e8e505d29c63Properties of poly(ethylene oxide) in aqueous solnBailey, F. E., Jr.; Callard, R. W.Journal of Applied Polymer Science (1959), 1 (), 56-62CODEN: JAPNAB; ISSN:0021-8995.Poly(ethylene oxide) (I) is sol. in water owing to an apparently unique hydration of the ether O. It is disrupted by thermal and salting out effects; the latter are smaller than those observed on polyelectrolytes. A high degree of shear dependence of high-mol.-wt. I is due in part to the essential linearity of the structure together with the enormous degrees of polymerization possible. In aq. solns. the ether linkages are apparently stiffened by hydration so that the polymer coil extension approaches that found in hydrocarbons in which hindered rotation leads to extended configurations. The high degree of entanglement of polymer chains in more concd. solns. (several %) seems to result in a network structure that is distended by relatively low shearing forces. The degree of distortion of the polymer configurations in soln. tends to reach a limiting value at very high rates of shear. The linear, relatively nonpolar structure of I displays a high degree of polymer solvent interaction in aq. soln. which is observed in the development of an unusual degree of structural viscosity.
- 44Nilsson, P. G.; Wennerstroem, H.; Lindman, B. Structure of Micellar Solutions of Nonionic Surfactants. Nuclear Magnetic Resonance Self-Diffusion and Proton Relaxation Studies of Poly(ethylene oxide) Alkyl Ethers J. Phys. Chem. 1983, 87, 1377– 1385[ACS Full Text
], [CAS], Google Scholar44https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3sXhsV2qurY%253D&md5=7695fe18882896bf7cc5432b1f2a7641Structure of micellar solutions of nonionic surfactants. Nuclear magnetic resonance self-diffusion and proton relaxation studies of poly(ethylene oxide) alkyl ethersNilsson, Per Gunnar; Wennerstroem, Haekan; Lindman, BjoernJournal of Physical Chemistry (1983), 87 (8), 1377-85CODEN: JPCHAX; ISSN:0022-3654.Binary systems of water and nonionic surfactants of the dodecyl oxyethylene monoether of the type C12H25(OCH2CH2)nOH were studied by 1H NMR techniques. For n = 5, C12E5, and n = 8, C12E8, the self-diffusion coeffs. of the surfactants in the isotropic, L1, phase were measured by using the pulsed field gradient technique. In addn., the line widths of the different proton signals were monitored and some samples of liq. cryst. character were also studied. Dramatic broadening of the methylene signals of the C12 chain is obsd. as the hexagonal liq. cryst. phase is approached in the C12E5-water system, while only a small broadening is obsd. in the C12E8-water system, showing that at low temps. there is a growth of C12E5 micelles to rods with increasing concns., while the C12E8 micelles at low temps. remain small in the whole concn. range. The self-diffusion coeffs. of the surfactants decrease rapidly with increasing concn. until a min. is reached after which there is a slow increase. The location of the min. occurs at lower surfactant concns. the closer the temp. is to the cloud point, where the system separates into 2 isotropic phases. In the line width studies, broadening is found at a certain temp. interval when the temp. is increased in the C12E5 system. These results taken together indicate that, in the C12E5 system, the surfactant aggregates grow in size as the cloud point is approached. The aggregates are flexible and probably not of a definite shape close to the cloud point. In the C12E8 system, the micelles are much less affected by an increase in temp. and micellar growth cannot be unambiguously established. The methylene signals of the ethylene oxide moieties consistently show narrower 1H signals, showing that in the aggregates they are less ordered than the chain methylenes. The various changes in aggregate size and shape are correlated with the stability ranges of the isotropic and liq. cryst. phases according to phase diagrams from the literature. Both aggregate size and phase structures are in qual. agreement with considerations based on the effective (as a result of coiling, hydration, and head group size) shape of the mols. at different temps. and concns. - 45Nilsson, P. G.; Lindman, B. Water Self-Diffusion in Nonionic Surfactant Solutions. Hydration and Obstruction Effects J. Phys. Chem. 1983, 87, 4756– 4761[ACS Full Text
], [CAS], Google Scholar45https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3sXlvVWrtLs%253D&md5=4792a22df4b4daeb165774f56c60591bWater self-diffusion in nonionic surfactant solutions. Hydration and obstruction effectsNilsson, Per Gunnar; Lindman, BjoernJournal of Physical Chemistry (1983), 87 (23), 4756-61CODEN: JPCHAX; ISSN:0022-3654.The self-diffusion coeff. was detd. of D2O in aq. solns. of nonionic surfactants of different concns. and at different temps. by using the NMR spin-echo pulsed-field-gradient method. The nonionic surfactants are pentaethylene glycol dodecyl ether (C12E5) and octaethylene glycol dodecyl ether (C12E8). The self-diffusion coeff. of D2O is lower than that of neat D2O because the surfactant aggregates obstruct the paths of the D2O mols. and because a fraction of it is bound to the surfactant (hydration) and hence has a lower translational mobility. The hydration is ∼4-6 H2O mols. per ethylene oxide group for a 10% surfactant soln. and decreases with increasing surfactant concn. The nonionic surfactants were compared with a poly(ethylene glycol) (PEG). The hydration of the ethylene oxide groups seems to be independent of the particular system and mainly detd. by the temp. and the compn. of the system. The consequences of this for the understanding of the clouding phenomenon are discussed. - 46Reimhult, E.; Larsson, C.; Kasemo, B.; Höök, F. Simultaneous Surface Plasmon Resonance and Quartz Crystal Microbalance with Dissipation Monitoring Measurements of Biomolecular Adsorption Events Involving Structural Transformations and Variations in Coupled Water Anal. Chem. 2004, 76, 7211– 7220
- 47Iruthayaraj, J.; Poptoshev, E.; Vareikis, A.; Makuška, R. a.; van der Wal, A.; Claesson, P. M. Adsorption of Low Charge Density Polyelectrolyte Containing Poly(ethylene oxide) Side Chains on Silica: Effects of Ionic Strength and pH Macromolecules 2005, 38, 6152– 6160
- 48Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J.; Böhmer, M. R. Formation and Stability of Multilayers of Polyelectrolytes Langmuir 1996, 12, 3675– 3681[ACS Full Text
], [CAS], Google Scholar48https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XjvFWksLw%253D&md5=03926b7941609ba1f1c3e8fab5cc3e5dFormation and Stability of Multilayers of PolyelectrolytesHoogeveen, Nynke G.; Cohen Stuart, Martien A.; Fleer, Gerard J.; Boehmer, Marcel R.Langmuir (1996), 12 (15), 3675-3681CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Sequential addn. of anionic and cationic polyelectrolytes may lead to the formation of multilayers at a solid surface. The buildup of such multilayers is characterized by a stepwise increase of the adsorbed amt. and layer thickness and by alternating highly pos. and highly neg. values for the ζ-potential. The prime variables which det. the stability of these structures are the polymer charge and the ionic strength. Very stable multilayers are formed when both polymers are highly charged and when the ionic strength is low. For weakly stable multilayers complexation at the surface may first occur, followed by desorption of the complexes. For strongly charged polyelectrolytes the charge stoichiometry, which is not always 1:1, seems to be unique for each pair of polyelectrolytes; no influence of the substrate, of the pH, or of the ionic strength could be obsd. - 49Dedinaite, A.; Thormann, E.; Olanya, G.; Claesson, P. M.; Nystrom, B.; Kjoniksen, A.-L.; Zhu, K. Friction in Aqueous Media Tuned by Temperature-Responsive Polymer Layers Soft Matter 2010, 6, 2489– 2498[Crossref], [CAS], Google Scholar49https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXmsFequr8%253D&md5=fd0b73cdad7396404f593ade1041ee3cFriction in aqueous media tuned by temperature-responsive polymer layersDedinaite, Andra; Thormann, Esben; Olanya, Geoffrey; Claesson, Per M.; Nystroem, Bo; Kjoniksen, Anna-Lena; Zhu, KaizhengSoft Matter (2010), 6 (11), 2489-2498CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)An at. force microscope colloidal probe technique has been employed to probe normal and friction forces between silica surfaces coated with adsorbed layers of a N-isopropylacrylamide48-(3-acrylamidopropyl)trimethylammonium chloride20 diblock copolymer, abbreviated PNIPAAM48-b-PAMPTMA(+)20. The interactions between the PNIPAAM48-b-PAMPTMA(+)20-coated surfaces across a 0.1 mM NaCl (pH 6) soln. at 25 °C are purely repulsive, due to a combination of steric and electrostatic double-layer forces. However, when the temp. is increased to 35 °C, and subsequently to 45 °C, an attractive force develops at short sepns. due to the unfavorable PNIPAAM-water interaction at these temps. The temp.-dependent polymer-water interaction has implications for the friction force between the layers. At 25 °C a frictional force that increases linearly with increasing load is obsd. once the surfaces are brought into close contact. At higher temps. significantly higher friction forces appear as a consequence of attractive segment-segment interactions. Further, a clearly expressed hysteresis between friction forces encountered on loading and unloading is detected. Our results demonstrate that both normal and friction forces between surfaces can be controlled by temp. changes when temp.-responsive polymers are employed, and friction forces can be adjusted as required from low to high.
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