Flow birefringence of poly(n-hexyl isocyanate) solutions

Abstract

A correction for optical quantities measured by a slit rheometer is proposed in order to obtain the corresponding optical properties at wall. This correction together with the Rabinowitch correction enables studies of the relation between the optical and the mechanical properties. A simple optical flow cell of a slit geometry was designed for the simultaneous measurements of the mechanical and the optical properties of flowing birefringent materials. The setup and the correction were applied to investigate the flow birefringence of the isotropic solutions of a liquid crystalline polymer, poly(n-hexyl isocyanate). The apparent birefringence defined as the refractive index difference between the ordinary light and the extraordinary light was found to increase with the increasing shear rates and was higher for solutions of high concentrations at a given shear rate. The stress optical law was checked. The ratio of the apparent birefringence to the shear stress was found to be independent of the shear rate at concentration of 10 wt%. For solutions of 15 and 20 wt%, the ratios increase with the shear rate when the shear rate is small, and they reach a maximum then decrease with increasing shear rate. The optical-stress behavior of PHIC solutions qualitatively follows the theoretical prediction of the Doi theory.