Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers

This article summarizes recent experiments relating measurements of molecular orientation to bulk rheological behavior in liquid crystalline polymers (LCPs) under shear. The principal experimental techniques are flow birefringence and x-ray scattering. Since LCPs usually exhibit a "polydomain" texture, measurements of flow-induced orientation reflect both the local distribution of molecular orientation around the director and the heterogeneous distribution of director orientations in the sheared LCP. In model lyotropic solutions of poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC), there are clear structural signatures of a transition from director tumbling dynamics at low Deborah number to flow alignment at high Deborah number. Rheo-optical measurements of the full refractive index tensor in PEG allow the orientation predictions of microstructural theories for LCP rheology to be quantitatively tested. At low shear rates the two model materials differ: PEG solutions exhibit significant orientation, while HPC solutions show little orientation. This is correlated with the presence of so-called "Region I" shear thinning in HPC solutions. Conversely, in PEG solutions of high concentration, x-ray scattering measurements demonstrate that Region I arises from the presence of a hexagonal phase. The model systems are further differentiated in relaxation. Molecular orientation increases in PEG solutions, but decreases in HPC solutions upon flow cessation; these differences are manifested in the evolution of dynamic properties. Finally, structural investigations of a PEG solution and a nematic surfactant solution during step changes in shear conditions are used to interrogate tumbling dynamics at low shear rates and test microstructural tumbling models.