Optical activity of solutions and films of chitosan acetate

Abstract

The optical activity of solutions, oligomers, and films of chitosan acetate in a molecular weight (М) range of 660—640 000 g mol–1 was studied by the optical rotation dispersion method. Monotonic dispersion curves lying in the range of negative values of specific optical rotation [α] are observed for all samples in a wavelength range of 300—700 nm. The absolute value of [α] decreases as the degree of polymerization decreases for solutions of chitosan acetate. Three regions are observed on the dependence [α] = f(logМ): constant [α] values for solutions of highmolecular-weight chitosan samples, their relatively weak change for low-molecular-weight samples, and a sharp change for oligomers. The transition to the low-molecular-weight analog of the monomeric chitosan unit, d-glucosamine, is characterized by the inversion of the rotation sign as high positive values of [α] are attained. The nontrivial for high-molecular-weight compounds dependence [α] = f(logМ) was established for chitosan acetate films: the module of [α] of the films increases with a decrease in the degree of polymerization. It is shown that the module of optical activity of the studied samples is determined by the mobility of structural units of oligomer, polymer, or film. It is found by X-ray diffractometry that differences in optical activity of the solutions and films are due to restriction of the mobility of macromolecular segments during the formation of high-molecular-weight chitosan films and incomplete relaxation processes of film systems to the state of energetically favorable conformations.