Surface Grafted Chitosan Gels. Part I. Molecular Insight into the Formation of Chitosan and Poly(acrylic acid) Multilayers
Abstract

Composite polyelectrolyte multilayers of chitosan and low molecular weight poly(acrylic acid) (PAA) have been assembled by sequential adsorption as a first step toward building a surface anchored chitosan gel. Silane chemistry was used to graft the first chitosan layer to prevent film detachment and decomposition. The assembly process is characterized by nonlinear growth behavior, with different adsorption kinetics for chitosan and PAA. In situ analysis of the multilayer by means of surface sensitive total internal reflection Raman (TIRR) spectroscopy, combined with target factor analysis of the spectra, provided information regarding composition, including water content, and ionization state of weak acidic and basic groups present in the thin composite film. Low molecular weight PAA, mainly in its protonated form, diffuses into and out of the composite film during adsorption and rinsing steps. The higher molecular weight chitosan shows a similar behavior, although to a much lower extent. Our data demonstrate that the charged monomeric units of chitosan are mainly compensated by carboxylate ions from PAA. Furthermore, the morphology and mechanical properties of the multilayers were investigated in situ using atomic force microscopy operating in PeakForce tapping mode. The multilayer consists of islands that grow in lateral dimension and height during the build-up process, leading to close to exponentially increasing roughness with deposition number. Both diffusion in and out of at least one of the two components (PAA) and the island-like morphology contribute to the nonlinear growth of chitosan/PAA multilayers.
1 Introduction
2 Materials and Methods
2.1 Polyelectrolytes and Reagents
2.2 Experimental Procedures
2.2.1 Silanation of Silica Substrates
2.2.2 Solutions and Grafting of Chitosan
2.2.3 Sequential Polyelectrolyte Adsorption
2.3 Techniques
2.3.1 Quartz Crystal Microbalance with Dissipation, QCM-D
2.3.2 Total Internal Reflection Raman Spectroscopy
(1)
(2)where z is the distance from the interface of the exponentially decaying field. (32)E0t at the interface is related to the incident electric field (E0i) by the corresponding Fresnel factors, which for an S polarized beam can be written as
(3)
(4)Note that the transmitted angles are complex in TIR conditions. Equations 1–4 indicate that upon changes in the refractive index of the solution, both the penetration depth (d) and the transmitted electric field at the interface are expected to vary, which will have direct consequences in the absolute intensities measured by TIRR.2.3.3 Calculating Adsorbed Amounts of Each Polyelectrolyte from the TIRR Spectra
2.3.4 Atomic Force Microscopy
3 Results and Discussion
3.1 Grafting of First Chitosan Layer
Figure 1

Figure 1. TIRR spectrum of the CH stretching region after silanation of the silica substrate with (3-glycidyloxypropyl)trimethoxysilane. The spectrum was collected in air using an angle of incidence of 51°.
3.2 Dynamic Events during Sequential Adsorption
Figure 2

Figure 2. Frequency and dissipation changes as a function of time during (a) sequential adsorption of PAA and chitosan (CHI), (b) details observed during formation of a layer (7th) of CHI and (c) a layer (8th) of PAA (d) a layer (11th) of CHI. The measurement started by injection of PAA on a silica surface carrying a pregrafted layer of CHI. The black arrows indicate the injections of polyelectrolytes (CHI:100 ppm, PAA:200 ppm) or a rinsing solution. The frequency, dissipation, and fitting curves from the Voigt model are represented by black, blue, and red curves, respectively. The data shown are from the 5th overtone.
Figure 3

Figure 3. (a) The change in frequency (circles) and dissipation (triangles) after each adsorption and rinsing step evaluated from the QCM-D fifth overtone. Solid and open symbols represent the addition of PAA and chitosan, respectively. Error bars correspond to the standard deviation calculated from three different measurements. (b) Voigt thickness and Voigt mass estimated using the Voigt model for viscoelastic films. Data from overtones 3, 5, and 7 were used.
3.3 Molecular Information Obtained from TIRR
Figure 4

Figure 4. Selected TIRR spectra collected during the sequential adsorption process in the (a) fingerprint/double bond and (b) CH/OH stretching spectral regions. Odd and even numbered layers, correspond to the addition of chitosan and poly(acrylic acid), respectively. The spectra in panel a have been subtracted by the spectrum of Layer 1, while the raw data is presented in panel b. All spectra were collected under the Sy polarization combination.
| chitosan (48-50) | polyacrylic acid (PAA) (51, 52) | ||
|---|---|---|---|
| wavenumber (cm–1) | assignments | wavenumber (cm–1) | assignments |
| 1326 | τ(CH2) | 1324 | τ(CH2) |
| 1380 | HCC and HCO bending* | 1355 | ω(CH2) |
| 1418 | HCC and HCO bending* | 1420 | νs(COO–) |
| 1460 | δ(CH2) | 1455 | δ(CH2) |
| 1656 | ν(C═O) amide | 1566 | νas(COO–) |
| 2905 | ν(CH) | 1710 | ν(C═O) |
| 2945 | νs(CH2) or ν(CH)* | 2878 | ν(CH) |
| 3315 | νs(NH2) | 2938 | νs(CH2) |
| 3370 | νas(NH2) | 2989 | νas (CH2) |
ν, δ, ω, and τ stand for stretching, bending (scissoring), wagging, and twisting vibrational modes, respectively. The subscripts “s” and “as”, denote symmetric and asymmetric modes. Asterisks “*” denote tentative assignments.
Figure 5

Figure 5. (a) Target factor analysis weight component for chitosan (CHI) and PAA as a function of deposition number evaluated from peaks in the CH/OH stretching region. The corresponding weight component for water is shown in the inset. (b) Calculated adsorbed amounts for CHI and PAA obtained from the TIRR data. The refractive index of the rarer medium estimated from a two layer model is also shown in the inset. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
3.4 Adsorbed Amounts from TIRR
3.5 Layer Hydration
Figure 6

Figure 6. Water weight percentage in the adsorbed film as a function of the number of depositions, calculated by combining QCM-D and TIRR results. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
3.6 Polyelectrolyte Monomeric Unit Mole Fractions
Figure 7

Figure 7. Monomeric unit mole fraction (including charged and uncharged fractions) as a function of deposition number obtained from the TIRR data. The dashed line at 0.5 is a guide to the eyes. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
3.7 Overcompensation and Complexation
Figure 8

Figure 8. Calculated adsorbed amounts for protonated and deprotonated monomeric units of PAA as a function of number of deposited layers. Inset: percentage of protonated PAA monomeric units in the adsorbed layer along the build-up process. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
3.8 Structural Information Obtained with AFM
Figure 9

Figure 9. AFM topography images of some specific deposited layers (deposition numbers provided in the images) in 30 mM NaCl at pH 5.7. Note the change in the height bar scales along the build-up process. Image size: 10 × 10 μm2. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
Figure 10

Figure 10. Root mean square roughness (a) and roughness factor (b) as a function of deposition number obtained from the AFM images, size: 10 × 10 μm2. The line in panel a following an exponential pattern is just a guide to the eye. Odd and even numbered layers correspond to deposition of chitosan and PAA, respectively.
Summary and Concluding Remarks
Supporting Information
Voigt modeling of the QCM-D data, description of steps followed in Target Factor Analysis to determine adsorbed polyelectrolyte masses, QCM-D curves at different overtones including parameters used in the Voigt modeling of film mass and thickness, bulk Raman spectra from protonated and deprotonated PAA, estimation of the refractive index in the two layer model for determining the absolute mass from the TIR Raman data, mole fraction of charged monomeric units (overcompensation) as a function of deposited layer obtained by TIR Raman, and Surface Material properties of the multilayer obtained by AFM. This material is available free of charge via the Internet at http://pubs.acs.org/.
Terms & Conditions
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Acknowledgment
All authors acknowledge individual financial support from the Swedish Research Council, VR. E.Th. and E.Ty. also acknowledge support from the Swedish Foundation for Strategic Research (SSF) through the programs “Microstructure, Corrosion and Friction Control” and “Future Research Leaders-5”, respectively.
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Abstract

Figure 1

Figure 1. TIRR spectrum of the CH stretching region after silanation of the silica substrate with (3-glycidyloxypropyl)trimethoxysilane. The spectrum was collected in air using an angle of incidence of 51°.
Figure 2

Figure 2. Frequency and dissipation changes as a function of time during (a) sequential adsorption of PAA and chitosan (CHI), (b) details observed during formation of a layer (7th) of CHI and (c) a layer (8th) of PAA (d) a layer (11th) of CHI. The measurement started by injection of PAA on a silica surface carrying a pregrafted layer of CHI. The black arrows indicate the injections of polyelectrolytes (CHI:100 ppm, PAA:200 ppm) or a rinsing solution. The frequency, dissipation, and fitting curves from the Voigt model are represented by black, blue, and red curves, respectively. The data shown are from the 5th overtone.
Figure 3

Figure 3. (a) The change in frequency (circles) and dissipation (triangles) after each adsorption and rinsing step evaluated from the QCM-D fifth overtone. Solid and open symbols represent the addition of PAA and chitosan, respectively. Error bars correspond to the standard deviation calculated from three different measurements. (b) Voigt thickness and Voigt mass estimated using the Voigt model for viscoelastic films. Data from overtones 3, 5, and 7 were used.
Figure 4

Figure 4. Selected TIRR spectra collected during the sequential adsorption process in the (a) fingerprint/double bond and (b) CH/OH stretching spectral regions. Odd and even numbered layers, correspond to the addition of chitosan and poly(acrylic acid), respectively. The spectra in panel a have been subtracted by the spectrum of Layer 1, while the raw data is presented in panel b. All spectra were collected under the Sy polarization combination.
Figure 5

Figure 5. (a) Target factor analysis weight component for chitosan (CHI) and PAA as a function of deposition number evaluated from peaks in the CH/OH stretching region. The corresponding weight component for water is shown in the inset. (b) Calculated adsorbed amounts for CHI and PAA obtained from the TIRR data. The refractive index of the rarer medium estimated from a two layer model is also shown in the inset. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
Figure 6

Figure 6. Water weight percentage in the adsorbed film as a function of the number of depositions, calculated by combining QCM-D and TIRR results. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
Figure 7

Figure 7. Monomeric unit mole fraction (including charged and uncharged fractions) as a function of deposition number obtained from the TIRR data. The dashed line at 0.5 is a guide to the eyes. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
Figure 8

Figure 8. Calculated adsorbed amounts for protonated and deprotonated monomeric units of PAA as a function of number of deposited layers. Inset: percentage of protonated PAA monomeric units in the adsorbed layer along the build-up process. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
Figure 9

Figure 9. AFM topography images of some specific deposited layers (deposition numbers provided in the images) in 30 mM NaCl at pH 5.7. Note the change in the height bar scales along the build-up process. Image size: 10 × 10 μm2. Odd and even numbered layers correspond to the deposition of chitosan and PAA, respectively.
Figure 10

Figure 10. Root mean square roughness (a) and roughness factor (b) as a function of deposition number obtained from the AFM images, size: 10 × 10 μm2. The line in panel a following an exponential pattern is just a guide to the eye. Odd and even numbered layers correspond to deposition of chitosan and PAA, respectively.
References
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Supporting Information
Supporting Information
ARTICLE SECTIONSVoigt modeling of the QCM-D data, description of steps followed in Target Factor Analysis to determine adsorbed polyelectrolyte masses, QCM-D curves at different overtones including parameters used in the Voigt modeling of film mass and thickness, bulk Raman spectra from protonated and deprotonated PAA, estimation of the refractive index in the two layer model for determining the absolute mass from the TIR Raman data, mole fraction of charged monomeric units (overcompensation) as a function of deposited layer obtained by TIR Raman, and Surface Material properties of the multilayer obtained by AFM. This material is available free of charge via the Internet at http://pubs.acs.org/.
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