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

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) over bar(W)(c). For polymers with (M) over bar(W) > (M) over bar(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) over bar(W) < (M) over bar(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.