LOG-ROLLING ALIGNMENT IN MAIN-CHAIN THERMOTROPIC LIQUID-CRYSTALLINE POLYMER MELTS UNDER SHEAR - AN IN-SITU WAXS STUDY

This paper sets down in detail the observation, first announced as a n ote (Romo-Uribe, A.; Windle, A. H. Macromolecules 1993, 26, 7100), of a strident and reversible orientation transition in a specially synthe sized series of thermotropic random copolyester melts of the Vectra A9 00 type. The transition is between the flow-aligning regime normally s een in nematics and alignment of the global director at right angles t o flow-alignment and parallel to the vorticity axis, which we refer to as the ''log-rolling'' regime. The orientation transition is observed in a comparatively narrow band of temperature above the crystal melti ng point, the width of this band decreasing with increasing strain rat e and increasing molecular weight. We believe that observation of log- rolling in the main-chain thermotropic random copolymers provides conf irmatory evidence for the occurrence of a smectic phase in isolated re gions which are the precursors of the non periodic layer (npl) crystal s which form on further cooling. The fact that the fog-rolling regime is not observed at high molecular weights and higher strain rates sugg ests that the low density of entanglements, which will occur in liquid crystalline polymers with less than completely rigid chains, will nev ertheless compromise the log-rolling regime. Slow strain rates and low er molecular weights would each tend to enhance the relaxation of enta nglements during shear and thus reduce the deleterious effect which en tanglements would have on the log-rolling regime. Recent rheological m easurements on the same set of polymers add support to this hypothesis .