Molecular motions of chitosan in the solid state

Abstract

The relaxation behavior of chitosan has been investigated by dynamic mechanical and dielectric spectroscopy over a wide temperature and frequency range. Two dispersion regions are found in the spectrum of the dry polysaccharide: a relaxation (γ) at low temperature (− 102° at 3 Hz; °H=47 kJ/mol) and another relaxation process above room temperature (130° at 3 Hz; °H ∼ 100 kJ/mol). Absorbed water strongly modifies the relaxation spectrum of chitosan below room temperature, giving rise to a new relaxation (βd) at temperatures higher than the γ peak. With increasing water content, the γ peak is progressively depressed, while the βd relaxation intensifies and moves to lower temperatures. In room-stored samples (∼10% absorbed water) the βd peak is the only observable relaxation in the viscoelastic spectrum of chitosan below room temperature. The γ and βd loss processes are attributed to local motions of the polysaccharide backbone and of complex polymer-water units, respectively. The characteristic parameters of the high-temperature peak indicate that the relaxation originates from short-range rather than cooperative molecular motions.