Electro-optics and structural peculiarities of liquid crystal-nanoparticle-polymer composites

The structural peculiarities and electro-optic performance of liquid crystal (LC)-colloidal nanoparticle (NP)-polymer (P) composites formed by photoinduced phase separation are considered. We classify these materials under two groups according to two limiting cases of polymer morphology. The first group corresponding to small polymer concentration comprises LCs filled with NPs that are stabilized with a polymer network. It is found that, in addition to the light scattering caused by the LC orientational defects, the refractive index mismatch between LC and NP aggregates may significantly affect the electro-optic contrast and its angular characteristics. The second group is represented by polymer dispersed liquid crystals (PDLCs) filled with NPs. It is established that, in the process of photoinduced phase separation of the LC-NP-prepolymer mixture, the nanoparticles are mainly involved with the polymer, serving as building blocks for the polymer matrix. When the aggregation rate of the NPs is high or their size is large, the NPs enhance light scattering in the polymer. For low aggregation rate, NPs modify the effective refractive index and/or the absorption coefficient of the polymer phase without producing any noticeable optical inhomogeneity. Additionally, we found that TiO2 NPs may cause a photochromic effect, which manifests itself in color changes in the course of the photoinduced phase separation. For PDLCs with optically transparent polymer matrices modified by NPs, it is shown that doping with NPs can be used to control the refractive index ratio of the LC and polymer. In this way one can modify the contrast and substantially reduce the off-axis haze of the PDLC. The observed effects show LC-NP-P composites as materials of considerable promise for LCD and other electro-optic applications.