Phase studies of binary mesogenic systems

Abstract

Although several theories exist for rod-like nematogens, our understanding of thermotropic mesomorphism in semiflexible chain polymers is less well developed. Flory observed that the nematic behaviour of Kuhn chain polymers is governed by the axial ratio of the Kuhn link and is nearly independent of the contour length of the chain. Following this lead, we attempted to fit binary phase diagrams of two polymer + diluent systems using the Warner–Flory treatment of rod-like nematogens. The only modification involved replacement of the axial ratio of the rod by that of the Kuhn link. This theory assumes the nematic phase is stabilized by orientation-dependent interactions and that the axial ratio of the rod is independent of temperature. Poor agreement with the experimental phase diagrams suggests some other factor is important. We then took into account the temperature dependence of the unperturbed dimensions through appropriate modification of Flory's treatment of Kuhn chain polymers. This predicts a thermotropic nematic–isotropic transition, even in the absence of orientation-dependent interactions, at the temperature which reduces the axial ratio of the Kuhn link to the critical value. Persistence lengths measured for two polymers at different temperatures, when extrapolated to their isotropization temperatures, produce axial ratios very near the critical value. This treatment predicts that all semiflexible polymers that behave as Kuhn chains should be in corresponding states at their isotropization temperatures. The application of this concept is illustrated by examining the thermal behaviour of a number of substituted poly(p-phenylene terephthalates).