Crystallinity of Partially N-Acetylated Chitosans

In order to search for a chitosan with low crystallinity, partially N-deacetylated chitins (PDC) and partially N-acetylated chitosans (PAC) with a low degree of N-acetylation (DAc) were examined by X-ray powder diffraction measurements. Three PDC samples, having less than 30% DAc and prepared by solid-state deacetylation, gave X-ray powder patterns showing the presence of α-chitin, a hydrated crystal of chitosan, or their mixture, respectively. When these PDC samples were treated with an acid-alkali, however, reduced crystallinity was observed. By annealing in water at 160 or 200°C, the latter PDC having DAc less than approx. 22% gave powder patterns indicating the presence of an anhydrous crystal which may spoil the chitosan’s functionality. In contrast, PAC prepared by N-acetylating pure chitosan (DAc=0%) in a swollen state, which can be expected to have random copolymers in the chain, was always less crystallized than PDC, this crystallinity depending on the molecular weight. In the case of high-molecular-weight PAC samples, whose DAc was in the range of 5–21%, the effect of high molecular weight on reducing crystallinity was larger than that of the degree of N-acetylation.