Rheology of Rigid Macromolecules in the Semi-Dilute Region

Abstract

Various rheological properties were measured of a solution containing a rigid macromolecule, Xanthan Gum, including: steady shear viscosity, stress jump and stress relaxation after steady shear and exponential shear. The steady shear viscosity was found to shear thin for all the solutions tested. The stress jump on cessation of steady shear was found to increase with increasing shear rate and depend on concentration. When the stress jump was made dimensionless with the stress at steady state prior to cessation the stress jump was found to be independent of concentration at higher shear rates and solvent viscosity and increase with increasing shear rate. This is consistent with predictions of dilute solution rigid molecule theories. However, the relaxation after the jump showed that the stress decayed with a multitude of relaxation times not a single relaxation time as predicted by dilute solution theories. The instantaneous viscosity in exponential shear exhibited a large overshoot and the stress jump and subsequent relaxation after cessation depended on the time of shear.