Chitosan and glycerophosphate concentration dependence of solution behaviour and gel point using small amplitude oscillatory rheometry

In this study, linear viscoelastic properties of a chitosan system were investigated in the sol and gel states in terms of glycerophosphate (GP) (CGP) and polymer (CC) concentrations. The chitosan solutions showed a liquid-like behaviour at low β-GP and chitosan content, but deviated from the sol state at higher concentrations. The complex viscosity at 0.1 rad/s (η0.1rad/s*) presented a strong power-law relationship with chitosan concentration, i.e. η0.1rad/s*~CC5.0. The flow activation energy (Eaf) calculated from the master curves of the linear viscoelastic data increased with decreasing CGP and increasing CC. The opposite behaviour was explained in terms of enhanced hydrophobic interactions at high glycerophosphate concentration and high temperature, confirmed by the calculated ionic strength. Monitoring of the heat-induced gelation showed that the data obtained at different β-GP concentrations could be superposed in a single master curve in the gel state, indicating that the gelation process and kinetics were identical regardless of β-GP content. However, measurements obtained at different chitosan concentrations could not be shifted onto a single curve. Higher CC resulted in lower gelation temperature but slower kinetics due to the high viscosity and slower molecular diffusion. The 3D sol–gel phase diagram was derived from the rheological measurements during the heat-induced gelation. A synergistic effect at high β-GP and polymer concentrations resulted in a gelation on the edge of concentration-induced and heat-induced gelation.