X-ray scattering studies of orientation in channel flows of a thermotropic liquid-crystalline polymer

Abstract

In situ wide-angle X-ray scattering (WAXS) orientation measurements are reported for a commercial thermotropic copolyester melt in channel flows. This investigation includes extensive examination of the orientation effects of mixed shear-extensional kinematics produced by channel flows with varying cross sections. Extensional gradients in a slit-contraction flow produce large increases in orientation, similar to those seen in lyotropes. In sharp slit-expansion flow, a bimodal orientation state is observed in which one population is aligned in the downstream direction while the other lies perpendicular to the expected local streamlines. This is attributed to stretching transverse to the flow direction in expansion regions. Weaker extensional gradients in a gradual expansion lead to a smaller effect. Flow along the centerline upstream and downstream of an obstruction leads to behavior similar to that observed in expansions and contractions, while flow around a sharp 90° bend produces orientation that tracks the expected streamlines, without significant change in the degree of orientation. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3411–3428, 1999