Theoretical simulation of holographic polymer-dispersed liquid-crystal films via pattern photopolymerization-induced phase separation

A theoretical simulation has been performed to elucidate the emergence of nematic domains during pattern photopolymerization-induced phase separation in holographic polymer-dispersed liquid crystals. We consider a reference system consisting of a single-component nematic, namely, 4-n-heptyl-4(')-cyanobiphenyl (T(NI)=42 degrees C), and a polymer network made from multifunctional monomers. To mimic pattern photopolymerization, the reaction rate was varied periodically in space through wave mixing. In the theoretical development, the photopolymerization kinetics was coupled with the time-dependent Ginzburg-Landau model C equations by incorporating the local free energy densities pertaining to isotropic liquid-liquid mixing, nematic ordering, and network elasticity. The simulated morphological patterns in the concentration and orientation order parameter fields show discrete layers of liquid-crystal droplets alternating periodically with polymer-network-rich layers. The Fourier transforms of these patterns show sharp diffraction spots arising from the periodic layers. As the layer thickness is reduced, the liquid-crystal molecules are confined in the narrow stripes. The liquid-crystal domains appear uniform along the stripes, which in turn gives rise to sharper diffraction spots in Fourier space. Of particular interest is that our simulated stratified patterns are in qualitative agreement with reported experimental observations.