Polymerization-Induced Phase Separation. 2. Morphology of Polymer-Dispersed Liquid Crystal Thin Films

Abstract

Phase separation in a polymerizing diacrylate/LC mixture is shown to be driven by liquid−gel demixing rather than by liquid−liquid demixing. The structure of the gel strongly influences the initial morphology: “early” phase separation (at low conversion) produces spherical domains, whereas “late” phase separation (at high conversion) produces nonspherical domains. The higher the conversion at phase separation, the smaller the domains. A new method, simultaneous photo DSC/turbidity measurement, provides the conversion at the appearance of a nematic phase, and optical microscopy shows the development of morphology. A nonspherical droplet shape reflects the inhomogeneous structure of the polymer network. The dependence of the initial morphology on the LC content, temperature of the reaction, and cross-linker content can be explained using conversion−phase diagrams obtained from the Flory−Huggins−Dušek theory.1 The observable part of the demixing process in a model system composed of 4-n-pentyl-4‘-cyanobiphenyl (K15) and tetraethylene glycol diacrylate (TEGDA) probably proceeds through nucleation and growth rather than through spinodal decomposition. The phase diagram of the unpolymerized monomer/LC mixture is also reported.