Phase Behavior of Mixtures of Low Molecular Weight Nematic Liquid Crystals and Photochemically Crosslinked Polyacrylates

Abstract

The present work deals with theoretical and experimental studies to explore some physical properties of composite materials made of crosslinked poly(n-butylacrylate) networks and low molecular weight nematic liquid crystals (LCs). The chemically crosslinked polymers were obtained by exposure to UV radiation of initial solutions composed of a reactive monomer, n-butylacrylate, a small amount of a crosslinking agent, hexanedioldiacrylate, and a photoinitiator. To obtain different polymer network densities, the ratio of n-butylacrylate to hexanedioldiacrylate was varied prior to exposure to UV irradiation. Immersion in excess LC solvent of the obtained polymer networks allowed for the measurement of the solvent uptake by determining the size increase as function of temperature by polarized optical microscopy. A strong change of the swelling ratio was found around the nematic-isotropic transition temperatures of the two LCs involved, i.e., 4-cyano-4′-n-pentylbiphenyl known as 5CB, and the eutectic commercial mixture of cyanoparaphenylene derivatives (E7). The obtained swelling data were used to establish phase diagrams in terms of composition and temperature for three different network densities and two LCs. The phase diagrams exhibit upper criticial solution temperatures and isotropic network + nematic solvent (I + N), isotropic network + isotropic solvent (I + I), and isotropic swollen network (I) coexistence regions. The results were successfully analyzed within a theoretical model using a combination of the Flory-Rehner theory of isotropic mixing and the Maier-Saupe theory of nematic ordering.