Polymer Dispersed Liquid Crystals Based on Epoxy Networks: Transformations and Phase Separation Phenomena During Polycondensation

Abstract

A diepoxy-diamine/liquid crystal mixture (50/50 wt%) is studied in the course of the crosslinking step-polymerization at different isothermal reaction temperatures or with a two temperature cure cycle. Depending on the reaction temperature, the systems undergoe phase separation either isothermally, when the reaction temperature is far below the nematic-isotropic (N-I) temperature of the neat LC, or on cooling partially reacted systems when the reaction temperature is above the N-I temperature of the neat LC. Phase separation is characterized simultaneously by two glass transition temperatures (Tg ), one nematic-isotropic transition (T N-1) and the appearance of nematic droplets as determined by differential scanning calorimetry (DSC) or polarized optical microscopy (POM). Thus, temperature-polymerization conversion phase diagrams are presented in the form of the evolution of Tg 's and of T N-1 of the LC-rich phase. Furthermore, we attempted to use the calorimetric data to calculate the mass fraction of the LC-rich phase and the mass fraction of separated LC. The gel point conversions of the epoxy-amine network are determined. For example, T N-1 of the LC phase markedly increases near the gel point during an isothermal crosslinking at 30°C. The observation of the PDLC samples confined between two glass plates showed that final morphology (at full conversion) depends strongly on the thermal cycles of crosslinking. Finally, for a cure at 100°C, in bulk samples, evidence is given for a macrosyneresis of the LC in the fully crosslinked network. However, this phenomenon occurs very slowly with time. This means that depending on the cure cycles, on the LC content and on the confinement of the LC, the resulting PDLC may be prepared in off equilibrium conditions.