NEMATIC TO ISOTROPIC TRANSITION IN CHEMICALLY DISORDERED OR MULTICOMPONENT LIQUID-CRYSTALLINE POLYMERS

A Landau theory is presented for the nematic to isotropic (N-I) transition of main-chain liquid-crystalline polymers that are chemically disordered, i.e., statistical copolymers. The two constituent monomers are assumed to differ in flexibility but could be otherwise identical. For such a system, the chemical disorder contributes a random component to the local bending elastic constant of each chain. We find a coupling between the conventional nematic (orientational) order parameter and a compositional order parameter that expresses nonuniformity in the density of flexible monomers. This coupling leads to a biphasic region at the N-I transition in accordance with the recent experiments of Stupp et al. (Macromolecules 1988,21,1217-1234). In the limit of high molecular weight, the width of the biphasic window is proportional to the difference between the elastic constants of the two monomers and is inversely proportional to the square root of the molecular weight. Segregation by flexibility at the N-I transition is also analyzed for binary blends of thermotropic homopolymers. © 1990, American Chemical Society. All rights reserved.