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

    Year: 2014

    Journal: Langmuir, 2014, 30 (29), pp 8866–8877, 20141007

    Authors: Chao Liu †, Esben Thormann ‡, Per M. Claesson †§, and Eric Tyrode †

    Last authors: Eric Tyrode

    Organizations: † School of Chemical Science and Engineering, Department of Chemistry, Surface and Corrosion Science, KTH Royal Institute of Technology, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden ‡ Department of Chemistry, Technical University of Denmark, Kemitorvet 207, DK-2800 Kongens Lyngby,Denmark § Chemistry, Materials, and Surfaces, SP Technical Research Institute of Sweden, P.O. Box 5607, SE-114 86 Stockholm Sweden

    Country: Sweden, Sverige, Denmark

    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.