Dependence on the Preparation Procedure of the Polymorphism and Crystallinity of Chitosan Membranes

Differences in the polymorphism and crystallinity of chitosan were found in membranes prepared by different procedures when examined by X-ray diffraction measurements for four samples of chitosan differing in the degree of polymerization. When an acetic acid solution of chitosan was dried in air and then soaked in an alkaline solution (method A), both hydrated and anhydrous polymorphs of chitosan were present in the resulting membranes; the latter polymorph made chitosan insoluble in common solvents of chitosan, and its crystallinity increased with decreasing chitosan molecular weight. When a highly concentrated chitosan solution in aqueous acetic acid was neutralized with an alkaline solution (method B), no anhydrous polymorphs were detected in the membrane because of incomplete drying. When aqueous formic acid was used as the solvent, behavior basically similar to that in aqueous acetic acid was observed. In contrast, even with method A, aqueous hydrochloric acid gave a chitosan membrane having very little anhydrous crystallinity. The crystalline polymorph called "1-2", which has been proposed to be one of four chitosan polymorphs, is considered to be a mixture of hydrated and anhydrous crystals.