Robust "one-component" chitosan-based ultrathin films fabricated using layer-by-layer technique

"One-component" chitosan-based multilayer films have been formed and characterized for the first time. Two derivatives of chitosan containing oppositely charged strong ionic groups were synthesized for that purpose. Low molecular weight chitosan was modified by glycidyltrimethylammonium chloride to obtain the cationic form (CCh) of the polymer. The anionic form of chitosan (ACh) was obtained by sulfonation of carboxymethylchitosan with trimethylamine-sulfur trioxide. The two derivatives of chitosan were used to prepare stable multilayer polyelectrolyte films through a layer-by-layer (LbL) deposition technique. The films revealed smooth surfaces and linear growth of the thickness during LbL adsorption as measured by atomic force microscope (AFM). Contact angle measurements revealed a very hydrophilic nature of the formed films. In contrast, the hybrid films, made of one of the chitosan derivatives and the synthetic oppositely charged polyelectrolyte, demonstrated non-linear growths and much higher surface roughnesses. So far, the multilayer films built of natural polymers like chitosan and alginate as well as the mentioned hybrid films have shown difficulties in regular formation related to the differences in polymer backbones and/or the charge distribution along them. The approach proposed here omits the inherent entropic barrier and leads to the controlled formation of robust "one-component" multilayer films. The entropy-driven formation of such films was also supported by the calorimetric studies on the model polyelectrolyte complexes in solution. Thanks to biocompatibility and bacteriostatic properties of chitosan and its derivatives, CCh/ACh multilayer films may be potentially useful for many biomedical and environmental protection applications.