Thermal gelation of chitosan in an aqueous alkali–urea solution

Abstract

Chitosan can readily dissolve in a precooled aqueous alkali–urea solution, a solvent that has previously been developed to dissolve cellulose. Upon heating, the resulting solutions quickly become a gel. The thermal gelling of the chitosan solutions was studied by rheology. Initially, a temperature ramp test was used to determine the gelation temperatures (T_gel). It was found that T_gel does not significantly change with chitosan concentration. The in situ formed gels liquefy on cooling, but the liquefication temperature (T_liq) is considerably lower than T_gel, indicating a large hysteresis in the cooling process. In addition, T_liq decreases with increasing polymer concentration. The kinetics of thermal gelation was then studied by isothermal curing. The solution gels were cured not only at temperatures above the T_gel, which was determined in the temperature ramp test, but also at temperatures far below the T_gel, provided that the solution is cured at the temperature for a long enough time. The solutions become gel faster when cured at higher temperatures. When cured at the same temperature, higher concentration solutions become gel faster. The apparent activation energy for the thermal gelation of the chitosan solutions was determined to be ∼200 kJ mol⁻¹. Physical gels of pure chitosan were obtained by repeated soaking the in situ formed gels in water. Preliminary test shows that new gels are highly biocompatible.