Long‐range orientation correlations and molecular alignment in sheared thermotropic copolyester. In situ light and X‐ray scattering

Orientation correlations induced by shear flow and their relaxation were investigated using in situ small‐angle light scattering (SALS) in the thermotropic random copolyester of 60 mol% hydroxybenzoic acid (B) and 40 mol% ethylene terephthalate (ET). B‐ET displays a nematic polydomain texture, the SALS and wide‐angle X‐ray scattering (WAXS) patterns are amorphous and isotropic. Shear flow produced optical defect multiplication with the consequent reduction of the micro–domains size. However, SALS detected long‐range spatial correlations within the optically chaotic texture, the SALS patterns showed bimodal orientation of defects. After cessation of shear the orientation correlation rapidly relaxed back to a polydomain and the SALS pattern became again isotropic. Above a threshold shear rate of about the SALS pattern showed unimodal orientation arising from line defects oriented nearly orthogonal to the velocity axis. Strikingly, the texture relaxation now showed the well known “banded texture”. The threshold shear rate coincided with a significant increase in the degree of molecular alignment as determined from in situ X‐ray scattering. This technique also showed that shear flow always oriented the molecular chains along the flow direction regardless of the shear rate. Copyright © 2007 John Wiley & Sons, Ltd.