STUDIES ON THE THERMODYNAMIC, RHEOLOGICAL, AND RHEO-OPTICAL PROPERTIES OF A RIGID-ROD POLYMER IN SOLUTION

The equilibrium, steady-state and time-dependent properties of a rod-like polymer, polybisbenzothiazole (PBT), in methane sulfonic acid (MSA) have been studied. The effects of temperature and concentration on the transition to a liquid-crystalline solution were explored for polymers of differing intrinsic viscosities. The phase diagrams were found to be very similar for all the polymers. The critical concentration for the conversion to a nematic state was temperature dependent; the coexistence region was found to be very narrow. Disparity between the observed and predicted phase behavior was examined on the basis of intermolecular association effects.; Steady-state rheological and rheo-optical properties were studied for concentrated and semi-dilute solutions. The shear rate dependence of the steady-state viscosity and recoverable compliance was similar to that found for solutions of flexible polymers, but the concentration dependence of these properties (at vanishing shear rate) was different from that observed for flexible chain polymers. The zero shear viscosity, (eta)(,0), in particular, was found to increase extremely rapidly near the critical concentration for all the polymers. The phase transition appeared to affect the behavior of the zero shear compliance also. The variation of (eta)(,0) with concentration was found to agree qualitatively with the predictions of Doi and Edwards.; The gross features of the transient responses (creep, stress growth and relaxation) were found to be very similar to the behavior of flexible polymers, but the magnitudes of the parameters in the transient behavior were different.; Stress-optical coefficients were derived from measurements of flow birefringence in the 1-2 plane for dilute solutions; comparisons between these and measurements in the 1-3 plane on concentrated solutions were made using a modified stress-optical coefficient for the latter. The parameter (tau)(,c) was found to successfully reduce viscosity, recoverable compliance and flow birefringence data at various temperatures.; The effect of post-polymerization processing methods on the phase behavior, and on the rheological and rheo-optical properties, was explored for the polymer PBT-72. Substantial differences were found in the magnitudes of the viscosity, recoverable compliance and flow birefringence between the polymers obtained by precipitation from polyphosphoric acid and by precipitation from MSA. This was attributed to different degrees of association.; Finally, comparisons between the relaxation of flow birefringence and of the shear stress following steady shear flow showed that the shear stress relaxed faster than the birefringence.