Molecular Alignment of Polymer Liquid Crystals in Shear Flows. 1. Spectrographic Birefringence Technique, Steady-State Orientation, and Normal Stress Behavior in Poly(benzyl glutamate) Solutions

We describe results of an experimental investigation into the orientation state of liquid crystalline solutions of poly(benzyl glutamate) under shear flow and how the microscopic structure relates to the macroscopic mechanical rheological behavior. The technique of flow birefringence was used to study the degree of molecular orientation. A spectrographic flow birefringence apparatus is described that eliminates ambiguities associated with multiple orders of retardation in birefringence measurements. The birefringence observed in textured solutions under shear flow is always less than that measured in quiescent, defect-free monodomains of the solutions. At low shear rates, the birefringence is roughly constant and in the range of 53–63% of that observed in a monodomain; there is no evidence of a low-orientation, “piled polydomain” structure. At high shear rates, the birefringence is again roughly constant and around 90% of the monodomain value. The transition between low- and high-orientation states as a function of shear rate is closely correlated with changes in sign of the first normal stress difference of these solutions, leading us to identify it as a manifestation of a transition between regimes of director tumbling at low shear rates and flow alignment at high shear rates. These observations are compared qualitatively and qualitatively with predictions of the nonlinear Doi molecular model for textureless samples [Larson, R. G. Macromolecules 1990, 23, 3983] and the linear Larson and Doi tumbling polydomain model for textured samples [Larson, R. G.; Doi, M. J. Rheol. 1991, 35, 539]. An accompanying paper considers transient flow phenomena at low shear rates.