RELAXATIONAL PHENOMENA AND ANISOTROPY IN LYOTROPIC POLYMERIC LIQUID-CRYSTALS

The transient rheological behaviour of a lyotropic polymeric liquid crystalline material has been investigated systematically. The sample consisted of a 12% by weight solution of poly(γ-benzyl-l-glutamate) in m-cresol. The rheological experiments included stepwise decrease and increase in shear rate, as well as flow reversal. Both the effects of shear rate and temperature have been investigated to detect correlations between the underlying structural mechanisms. The experiments yielded complex transients which are shown to be governed by at least two different physical mechanisms. The first part of the stress relaxation curves scales with viscosity, whereas the final part is inversely proportional to the previous shear rate. In stepwise decrease in shear rate the stress response consists of an initial fast decrease followed by a damped oscillation. The initial fast response is identical with the one obtained in stress relaxation. The subsequent damped oscillation scales with the strain the sample experienced at the final shear rate, similar to the stress response resulting from a stepwise increase in shear rate. Comparing the latter with flow reversal proves the rheological anisotropy of the sample under consideration. © 1990.