Formation and characterisation of a physical chitin gel

The formation of N-acetyl chitosan gels in an acetic acid-water-propanediol solution was studied. Side reactions arising from the esterification of 1,2-propanediol by acetic anhydride, and hydrolysis of acetic anhydride were studied, as well as their possible role on the gel formation. The gels were studied by FTIR and H-1 NMR spectroscopy. Independently of the initial acetylation degree of chitosan, a minimal acetylation degree is required for gelation (80%). The final acetylation degree of the gels increases with the molar ratio acetic anhydride/amine, but does not reach 100%. An increase in temperature favours the molecular mobility, inter-and intra-molecular hydrogen bondings and hydrophobic interactions, responsible for gelation which appears to depend on two parameters: a critical acetylation degree and a critical molecular weight allowing the aggregation of the polymer chains. The nature of the solvent used to avoid chitosan side reactions during gelation (ethanol or propanediol) does not influence the chemical structure of the gel but only the kinetics of gelation. (C) 1997 Elsevier Science Ltd.