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-LCP) melt are investigated using transient stress and birefringence measurements. This SG-LCP shows a pronounced drop in the dynamic moduli at sufficiently low frequency upon passing through the transition from the isotropic to the nematic state. In contrast to previous studies, this suggests that liquid-crystalline order does affect the relaxation dynamics of SG-LCP melts. In the isotropic phase we simultaneously measure dynamic birefringence and stress to determine the stress-optic ratio (SOR). Results in the isotropic state near T_ni show the anomalously large SOR characteristic of pretransitional effects in liquid-crystalline systems in general. In the nematic state, we find that prolonged, large-amplitude oscillatory shear dramatically reduces the turbidity and increases the birefringence of the sample, suggesting that shearing induces a preferred alignment in this SG-LCP melt. This is accompanied by a decrease in the effective dynamic moduli of the nematic.