Shear-induced long-range spatial correlation and banded texture in thermotropic copolyester. In situ light and X-ray scattering

In situ small-angle light scattering (SALS) has enabled to elucidate shear-induced orientation correlations and monitor their relaxation in the thermotropic copolyester of 60 mol% hydroxybenzoic acid (B) and 40 mol% ethylene terephthalate (ET). At 280 °C B-ET displays a nematic polydomain texture, the SALS and WAXS patterns are amorphous and isotropic. Applying steady shear, optical defect multiplication occurred and the microdomain sizes were reduced. However, the SALS pattern now showed anisotropy, the SALS pattern transitioned from a unimodal to a bimodal orientation. After cessation of shear, the orientation correlation rapidly relaxed to a polydomain and the SALS pattern became again isotropic. Above a threshold shear rate of about _c ∼ 2 s⁻¹ shear now induced line defects oriented nearly orthogonal to the velocity axis. The texture relaxation above _c was also distinctly different, the well-known "banded texture" was formed upon cessation of shear. In situ X-ray scattering showed that the molecular chains always aligned along the flow direction regardless of the shear rate. However, the degree of macromolecular alignment improved significantly above _c and this is a condition to obtain the banded texture.