Measurement of the Full Refractive Index Tensor in Sheared Liquid Crystalline Polymer Solutions

The full refractive index tensor of sheared liquid crystalline solutions of poly (γ-benzyl glutamate) in m-cresol has been measured using a combination of optical techniques. At all shear rates studied, the average molecular orientation direction is within 2 deg of the flow direction. In the low shear rate tumbling regime, the orientation angle is positive, in agreement with the Larson-Doi polydomain model [Larson, R. G.; Doi, M. J. Rheol. 1991,35,539]. As shear rate is increased, the orientation angle changes sign, consistent with a transition from tumbling to flow alignment, as predicted by the Doi molecular model [Marrucci, G.; Maffettone, P. L. Macromolecules 1989, 22, 4076. Larson, R. G. Macromolecules 1990, 23, 3983]. This transition appears to be shifted to higher shear rates at higher concentrations. The low shear rate regime is characterized by a biaxial average refractive index tensor, with enhanced molecular orientation along the vorticity direction relative to the shear gradient direction. This is in qualitative agreement with the Larson-Doi model, which however underpredicts the misalignment of domains, both within and out of the flowvorticity plane. At high shear rates, the average refractive index tensor is approximately uniaxial. Long after cessation of shear flow at a low rate, the resulting high orientation state is uniaxial but is characterized by a negative orientation angle when the previous shear rate is in the low shear rate, tumbling regime. This indicates that the average orientation direction changes sign during relaxation under these conditions.