Sorption-active transparent films based on chitosan.

In this paper we describe the preparation of alkanoic acid-based aqueous chitosan solutions, which show no sign of acid-catalysed depolymerisation over time - something commonly accompanying other preparation methods. Longer chitosan chains have previously been shown to exhibit more advantageous biological activities, and constant viscosities are essential for consistent quality in biomedical applications. Avoiding acid-catalysed depolymerisation requires careful control of the stoichiometry between the free amino groups of chitosan and the appropriate solubilising acid. Acetic and butyric acid are both suitable solubilising agents, but chitosan butyrate exhibits lower solution viscosities due to a combined electric and steric shielding of the chains. These solutions dry into clear transparent films that remain fully water soluble and absorb up to 70 wt% of water from 90%-RH vapour phase at 25 °C. The absorption follows simple first-order kinetics and the rate constants increase with increasing humidity up to approx. 71%-RH, where a metastable chitosan trihydrate salt appears to be formed. Desorption is slightly faster, but more complex, as it exhibits two distinct first-order processes. In addition, films prepared in this way are thermally more stable than the usual chitosan hydrochloride.