Polymerization-induced phase separation .2. Morphology of polymer-dispersed liquid crystal thin films

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. Anew 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-Dusek 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.