A THERMODYNAMIC INTERPRETATION OF POLYMER MOLECULAR-WEIGHT EFFECT ON THE PHASE-TRANSITIONS OF MAIN-CHAIN AND SIDE-CHAIN LIQUID-CRYSTAL POLYMERS

In this paper we follow up the dependence of the mesomorphic temperature range as a function of the degree of polymerization through simple thermodynamic considerations. In particular, we consider the dependence between the free energies of the crystalline (Gk), liquid crystalline (G1c), and isotropic (Gi) phases on the temperature for different polymerization degrees and three different thermodynamic situations. These dependences were converted into phase-transition temperature-degree of polymerization relationships. The slope of the isotropization temperature (Ti)-polymerization degree dependence is steeper than that of the corresponding dependence of the melting temperature (Tm). It is shown that this has the consequence of promoting or enhancing the mesophase, the exact nature of the effect being determined by the relative positions of the two curves, namely Ti versus molecular weight (M) and Tm versus M. In particular, it explains the enlargement of an enantiotropic mesophase of a monomeric structural unit, and the transformation of a virtual or monotropic monomeric unit mesophase into a monotropic or enantiotropic mesophase upon increasing its degree of polymerization. It shows further why and how the virtual mesophase of the monomeric structural unit is narrowed upon increasing its degree of polymerization. All these predictions are supported by experimental data available in the literature on the phase-transition temperature-molecular weight dependence of both main-chain and side-chain liquid-crystal polymers. © 1990, American Chemical Society. All rights reserved.