MOLECULAR-ORIENTATION AND RHEOLOGY IN SHEARED LYOTROPIC LIQUID-CRYSTALLINE POLYMERS

This article summarizes recent experiments relating measurements of mo lecular orientation to bulk rheological behavior in liquid crystalline polymers (LCPs) under shear. The principal experimental techniques ar e flow birefringence and x-ray scattering. Since LCPs usually exhibit a ''polydomain'' texture, measurements of flow-induced orientation ref lect both the local distribution of molecular orientation around the d irector and the heterogeneous distribution of director orientations in the sheared LCP. In model lyotropic solutions of poly(benzyl glutamat e) (PBG) and hydroxypropylcellulose (HPC), there are clear structural signatures of a transition from director tumbling dynamics at low Debo rah number to flow alignment at high Deborah number. Rheo-optical meas urements of the full refractive index tensor in PEG allow the orientat ion predictions of microstructural theories for LCP rheology to be qua ntitatively tested. At low shear rates the two model materials differ: PEG solutions exhibit significant orientation, while HPC solutions sh ow little orientation. This is correlated with the presence of so-call ed ''Region I'' shear thinning in HPC solutions. Conversely, in PEG so lutions of high concentration, x-ray scattering measurements demonstra te that Region I arises from the presence of a hexagonal phase. The mo del systems are further differentiated in relaxation. Molecular orient ation increases in PEG solutions, but decreases in HPC solutions upon flow cessation; these differences are manifested in the evolution of d ynamic properties. Finally, structural investigations of a PEG solutio n and a nematic surfactant solution during step changes in shear condi tions are used to interrogate tumbling dynamics at low shear rates and test microstructural tumbling models.