Temperature dependence of the dynamical behavior of holographic gratings formed in polymer dispersed liquid crystal films

Using films made of liquid crystal (LC), pre-polymer and photoinitiator dye mixtures: we fabricated holographic gratings under various curing temperatures. Mixtures having LC contents of 20 wt% and 40 wt% were prepared. Qualitative studies on phase separation using these mixtures were first conducted using a light scattering method. We then investigated the dynamical behavior of the gratings at various ambient temperatures. The grating morphologies were finally investigated using scanning electron microscopy (SEM). The results showed that no macroscopic phase separation was observed in the samples which have a LC content of 20wt%. The thermal grating was found to dominate in the initial period during the formation of gratings on these films. Later, it was offset by the photopolymerization effect which eventually dominated and determined the characteristics of the final grating. For samples having a LC content of 40wt%, the thermal grating effect was negligible, due to the molecular diffusions in these samples. The diffraction efficiencies of the final gratings were analyzed, and were found to have a good correlation with their SEM images.