In situ X-ray scattering study of a model thermotropic copolyester under shear: Evidence and consequences of flow-aligning behavior

While director tumbling is responsible for much of the unique rheological b ehavior of lyotropic liquid-crystalline polymers (LCPs), it is not clear wh ether tumbling routinely occurs in main-chain thermotropes. We present expe riments on a model thermotropic LCP under shear, using in-situ X-ray scatte ring as a probe of molecular orientation. This model LCP, PSHQ6-12, was syn thesized by Chang and Han [Macromolecules 1996, 29, 2383] and consists of r igid mesogens randomly copolymerized with flexible spacer chains of two dif ferent lengths. This architecture suppresses crystallization and lowers the nematic-isotropic transition temperature to an accessible level, allowing a well-defined thermal history to be established prior to now. We present m easurements of molecular orientation in steady shear flow, during relaxatio n, and during shear flow start-up. The evolution of molecular orientation o bserved during now inception and the observation of shear-induced transpare ncy in the material lead to the hypothesis that PSHQ6-12 is shear aligning rather than tumbling. Consequences of this hypothesis are explored by polyd omain simulations based on Ericksen's transversely isotropic fluid model, w hich are capable of capturing many aspects of the experimentally observed r heology and shear-induced structure.