Phase Equilibria in Mixtures of Thermotropic Small Molecular Liquid Crystals and Flexible Polymers

Abstract

Lebwohl-Lasher nematogenic model and Flory-Huggins theory of mixtures have been combined to describe the phase equilibria in mixtures of thermotropic small molecular liquid crystals and flexible polymers. Monte Carlo simulations on the same model have also been carried out in order to confirm the theoretical predictions. A comparison between theory and Monte Carlo simulation gives very good agreement. The findings include a widening of the nematic (N)/isotropic (I) two phase region with increasing polymer chain length and the isotropic force between polymer subunit and nematogen, exclusion of the polymer coils from the nematic phase and the appearance of an I/I biphasic region when the isotropic interaction exceeds the critical Flory–Huggins interaction parameter χc. A comparison with a real experimental system is also made and the agreement is quite good considering the simplicity of the theoretical model. A discrepancy between theory and experiment is imputed to the existence of an anisotropic interaction between polymer subunit and nematogen according to the existing 2H NMR results, which has not been taken into account in the present stage of theoretical approach. All of these findings are in accord with the present model and lend support to the general validity of Flory–Huggins treatment of mixtures and the Lebwohl–Lasher model for the thermotropic nematic behavior.