Unique gelation of chitosan in an alkali/urea aqueous solution

Abstract

The gelation behavior of chitosan in an alkali/urea aqueous solution was studied by means of rheology, X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The sol-gel transition temperature of the chitosan alkaline solution strongly depends on chitosan concentration and heating rate. The equilibrium sol-gel transition temperature, Tc, which was obtained by an extrapolation method based on multiwave oscillation, decreases with increasing chitosan concentration, and could be fitted to a parabolic equation, Tc = −4.1 + 48.0w - 11.5w2, where w is the weight fraction of chitosan in the alkaline aqueous solution. At the sol-gel transition point, the relaxation behavior shows a strong dependence on kinetics and the equilibrium critical relaxation exponent ne decreases as chitosan concentration increases. The FESEM images verified that the chitosan gel is formed as a fibrillar network and the fibrillar network is denser at a higher polymer concentration or a lower heating rate, leading to an irreversibility of the sol-gel transition of chitosan in the alkaline aqueous solution. A schematic diagram has been proposed to describe the change of gel network structure with chitosan concentration.