RHEOLOGY OF SIDE-GROUP LIQUID-CRYSTALLINE POLYMERS - EFFECT OF ISOTROPIC NEMATIC TRANSITION AND EVIDENCE OF FLOW-ALIGNMENT

The dynamics of a nematic side-group liquid-crystalline polymer (SG-LC P) melt are investigated using transient stress and birefringence meas urements. This SG-LCP shows a pronounced drop in the dynamic moduli at sufficiently low frequency upon passing through the transition from t he isotropic to the nematic state. In contrast to previous studies, th is suggests that liquid-crystalline order does affect the relaxation d ynamics of SG-LCP melts. In the isotropic phase we simultaneously meas ure dynamic birefringence and stress to determine the stress-optic rat io (SOR). Results in the isotropic state near T(ni) show the anomalous ly large SOR characteristic of pretransitional effects in liquid-cryst alline systems in general. In the nematic state, we find that prolonge d, large-amplitude oscillatory shear dramatically reduces the turbidit y and increases the birefringence of the sample, suggesting that shear ing induces a preferred alignment in this SG-LCP melt. This is accompa nied by a decrease in the effective dynamic moduli of the nematic.