A rheo-optical and dynamic X-ray-scattering study of flow-induced textures in main-chain thermotropic liquid-crystalline polymers. The influemce of molecular weight

Abstract

In situ optical microscopy, small–angle light–scattering and X–ray–scattering techniques have been applied to study flow–induced textures in main–chain thermotropic liquid–crystalline polymers (LCPs). In order to gain more insight into the underlying mechanism leading to the formation of microstructure, the effect of molecular weight has been investigated. The well–known ‘banded’ texture, commonly observed in fibres and sheared thin films of LCPs, is observed to develop after cessation of shear. Furthermore, the banded–texture formation is molecular–weight dependent, in that it occurs only above a certain ‘critical’ molecular weight, M c w . For polymers with M w > M w c the banded texture forms within a few seconds of cessation of shear, relaxing to a disordered ‘tight’ texture. On the other hand, for polymers with M w M w c , a ‘striped’ texture, with stripes parallel to the flow direction, develops after cessation of shear, relaxing to a ‘domain’ texture, with no evidence of banded–texture formation. The ‘striped’ texture corresponds simply to the elongation in the shear direction of the original microstructure of disclinations. The critical molecular weight for banded–texture formation corresponds approximately to the molecular weight threshold for the transition from log rolling to flow aligning previously observed for these materials by Romo–Uribe and Windle. Rheological measurements (stress relaxation and recoverable strain) along with microscopic observations suggest that a retraction process along the nematic director at the cessation of shear provides a mechanism for crumpling the microstructure and thus for banded–texture formation.