Smectic-like order in the log-rolling flow of thermotropic random copolymers. A time-resolved wide-angle X-ray scattering study

Under a suitable combination of molecular weight, temperature and shear rat e thermotropic random copolymers display macromolecular orientation orthogo nal to tile flow direction and along the vorticity axis; Romo-Uribe & Windl e have called this a 'log-rolling' flow. Time-resolved wide-angle X-ray sca ttering has enabled tile detection of a 002 meridional reflection as the or thogonal orientation develops from tile non-oriented molten phase. Therefor e, similar sequences of tire random copolymer self-assemble during the log- rolling how, giving rise to a smectic-like phase. After cessation of shear, neither the molecular sequence assemblage is lost nor is macromolecular or ientation relaxation observed. Furthermore, applying shear to macromolecule s that are already orthogonal to the velocity axis disrupts the chain regis ter due to the macromolecular disorientation prior to the transition to til e flow-aligning regime. Strikingly, after cessation of shear, the macromole cules recoil towards the orthogonal orientation and the molecular sequence assembly Is re-established. The smectic-like order is therefore the mechani sm that stabilizes tile log-rolling flow.