BIREFRINGENCE, X-RAY-SCATTERING, AND NEUTRON-SCATTERING MEASUREMENTS OF MOLECULAR-ORIENTATION IN SHEARED LIQUID-CRYSTAL POLYMER-SOLUTIONS

Recent studies of molecular orientation in sheared liquid crystalline polymers have often yielded contradictory results. To check the self-c onsistency of methods for quantitative measurements of molecular orien tation, liquid crystalline solutions of (hydroxypropyl)cellulose [HPC] and poly(benzyl glutamate) [PEG] have been studied using flow birefri ngence, X-ray scattering, and neutron scattering. HPC X-ray scattering patterns show an arclike pattern with a distinct peak as a function o f scattering vector, while PEG patterns show a more diffuse equitorial streak. These differences are attributed to more strongly correlated lateral packing in HPC solutions due to their higher concentration. Me asurements of orientation in steady shear flow agree well among the th ree techniques. Lyotropic HPC and PEG solutions differ in orientation at low shear rates. KPC solutions exhibit near zero orientation at low rates, while X-ray and neutron scattering measurements confirm previo us birefringence data showing a low shear rate plateau of moderate ori entation in PEG. Differences with recent neutron scattering measuremen ts on PEG solutions that show low orientation at low shear rate are at tributed to choice of solvent, rather than choice of technique. X-ray and optical data are consistent in showing decreasing orientation in H PC solutions during relaxation, but discrepancies are found in relaxat ion of PEG solutions. Large increases in flow birefringence suggest su bstantial orientation enhancement. X-ray data on one PEG solution conf irm increasing orientation, but X-ray and neutron scattering data on a more concentrated solution show only modest changes in orientation. I t is suggested that flow birefringence fails in this case due to textu re coarsening to the point where there is no longer effective averagin g over the distribution of director orientations along the light path.