Thermodynamics, transition dynamics, and texturing in polymer-dispersed liquid crystals with mesogens exhibiting a direct isotropic/smectic-A transition

Experimental studies of phase equilibrium and growth morphologies of novel polymer-dispersed liquid crystal (PDLC) mixtures of PS (polystyrene) and liquid crystals that exhibit a direct isotropic/smecticA (lamellar) mesophase transition were performed for PS/10CB (decyl-cyanobiphenyl) and PS/12CB (dodecyl-cyanobiphenyl). The studies were performed using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). Partial phase diagrams were determined for different compositions of both materials, determining both phase separation (liquid/liquid demixing) and phase ordering (isotropic/smectic-A transition) temperatures. For moderate to high concentrations of liquid crystal, isotropic/isotropic phase separation (TII) and isotropic/smectic-A phase ordering (TAI) were observed separately. PDLCs with low concentrations of liquid crystal exhibited simultaneous phase separation and ordering (TI/AI) at temperatures below the pure liquid crystal phase transition. The Flory-Huggins theory of isotropic mixing and Maier-Saupe-McMillan theory for smectic-A liquid crystalline ordering were used to computationally determine phase diagrams for both systems, in strong agreement with the experimentally determined results. In addition to thermodynamic observations, growth morphology relations were determined depending on phase transition sequence, quench rate, and material composition. Three stages of liquid crystal-rich domain growth morphology were observed: spherical macroscale domain growth (“stage I”), highly anisotropic domain growth (“stage II”), and sub-micron spheroid domain growth (“stage III”). Morphologies observed during stage II growth are typical of direct isotropic/smectic-A phase transitions, such as highly anisotropic ”batonnets” and filaments. These morphologies, unique to smectic PDLCs, could provide new functionality and applications for these functional materials. ∗Institute of Materials Science and Technology (INTEMA), University of Mar del Plata and National Research Council (CONICET), J.B. Justo 4302, 7600 Mar del Plata, Argentina †Department of Chemical Engineering, McGill University, Montréal, Québec H3A 2B2, Canada