Effect of molar mass of an epoxy oligomer on the phase separation in epoxy based polymer dispersed liquid crystals

Abstract

Polymer dispersed liquid crystals based on epoxy-amine crosslinked matrices and a nematic liquid crystal, E7, have been studied over the course of polymerisation, i.e. as a function of the polymerisation conversion. The influence of epoxy oligomer molar mass on the initial temperature-concentration and the temperature-conversion phase diagrams has been investigated.An increase of the epoxy oligomer molar mass greatly reduces the initial liquid crystal solubility and brings the cloud points to earlier polymerisation conversions, which have been quantified. Thus the phase separation is markedly enhanced.The temperature-conversion phase diagrams have been characterised at two isothermal polymerisation temperatures for one liquid crystal composition (50wt.). These diagrams (isotropic-nematic and nematic-isotropic transition temperatures) are shown to obey master curves when the epoxy molar mass is varied.Finally, the size of the liquid crystal droplets is shown to decrease when the epoxy molar mass increases. This effect is mainly due to the viscosity increase resulting from the oligomer mass increase. Viscosity measurements were made at intervals during polymerisation.