Chitosan as a Yield Stress Fluid: Concentration Dependent Rheology and Microdynamics

Chitosan and its derivatives are of interest due to their versatility and potential both as a biomaterial and additive for rheological control. For chitosan concentrations ranging from the dilute to concentrated regime, we report two regions of shear thinning at high and low shear rates. The shear thinning at low shear rates, although reported for chitosan derivatives, has not been investigated in aqueous chitosan and is consistent with the existence of an apparent yield-stress. We probe the physicochemical and microdynamical origins of this behavior by the addition of urea – a hydrogen bond and hydrophobic interaction disrupter – and by dynamic light scattering (DLS). Our findings suggest that urea is not able to fully disrupt chitosan associations and only marginally lowers the yield stress parameter and viscosity. The microstructure of highly concentrated chitosan solutions, as revealed by DLS, is dominated by extremely slow microdynamics, suggesting a network of large, associating aggregates. At the highest concentration analyzed by DLS, the dynamics are consistent with weak gel behavior.