TRANSIENT SHEAR RESPONSE AND FLOW-INDUCED MICROSTRUCTURE OF ISOTROPIC AND NEMATIC RIGID-ROD POLY(P-PHENYLENEBENZOBISTHIAZOLE) SOLUTIONS

Poly(p-phenylenebenzobisthiazole) (PBZT), a highly rigid-rod-like polymer that forms a lyotropic solution in strong acids such as methanesulfonic acid, was used as a model system to examine the transient flow properties and shear-induced microstructure near the isotropic-nematic phase transition. Upon flow reversal, two types of responses were observed in the nematic phase depending on the rod concentration and the applied shear rate. At low concentrations and low shear rates, the shear stress versus strain exhibits no overshoot, and the first normal stress transient is always positive. Such a response resembles those of the isotropic solutions. At higher concentrations and higher shear rates, damped oscillations in the shear stress and a negative transient followed by oscillatory behavior in the first normal stress were recorded. The material texture during flow and following flow cessation was examined using optical microscopy, and the macroscopic order parameter was measured using UV dichroism. The results indicate that the threshold shear rate dividing the flow response corresponds to the onset of shear-induced changes in the polydomain texture. This division is in agreement with the qualitative classification of regions I and III proposed by Onogi and Asada1 for the flow behavior of liquid-crystalline polymers. The transient flow behavior of anisotropic PBZT solutions also depends on the solvent. In poly(phosphoric acid), the oscillations in shear stress upon flow reversal and step shear persist for a long time with very little damping in amplitude. Flow instability in a rotational viscometer at high shear rates was observed in both solvent systems.