Polymer-dispersed liquid crystals based on polystyrene and EBBA: Analysis of phase diagrams and morphologies generated

Abstract

As an example of thermoplastic/liquid crystal blends that do not exhibit a liquid-liquid immiscibility region in their phase diagrams, a polystyrene (PS)/N-4-ethoxybenzylidene-4′-butylaniline (EBBA) blend was analyzed. The complete phase diagram was built up using thermal transitions determined by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The boundary of the nematic + isotropic region was fitted with the Flory-Huggins-Maier-Saupe model, extended to consider the polydispersity of PS. Main factors controlling the morphologies generated by thermal-induced phase separation (TIPS) were the initial EBBA concentration in the blend and the cooling rate. Cooling at a fast rate led to small nematic domains with a narrow size distribution. Slow cooling rates led to the coexistence of very large and small dispersed domains. This was because of the large extent of coalescence of the droplets first generated associated with the continuation of the nucleation/growth process in a medium of increasing viscosity. The use of fast cooling rates might be important for the generation of a narrow size-distribution of nematic droplets by TIPS in polymer-dispersed liquid crystals (PDLC) used in electrooptical devices.