Diffraction properties of photopolymerized liquid-crystal composites

Abstract

Experimental samples of polymer-dispersed liquid crystals (PDLC) of uniform and periodic spatial structure have been obtained and studied. The characteristic sizes of PDLC droplets and the refraction-index modulation range have been estimated by analyzing the texture of these samples and the diffracted-light intensity. IntroductionThe polymer-dispersed liquid crystals (PDLC) with field-controlled light scattering are widely used in reflection and projection displays and in working elements of switchable windows [1,2]. The structure of these materials usually appears as a polymer film with finely dispersed LCs of micrometer and subniicrometer sizes embedded into it. Without taking special precautions, the director field of an encapsulated nematic liquid crystal (NLC) under tangential conditions at the NLC-polymer interface usually takes on a bipolar configuration. The droplets with such a directorfield configuration are randomly oriented in the polymeric-film bulk and scatter an incident light, because of the mismatch between the refraction indices of the polymeric matrix and the NLC. With a sufficiently intense electric field applied, the NLC molecules align (if the NLCs have a positive dielectric anisotropy A o Q ) , and, if the refraction index np of the polymeric matrix is matched to the ordinary refraction index no of the NLC along the direction of the incident light, the film appears transparent, PDLCs are typically formed through phase-separation and emulsification methods. Among the various phase-separation methods, photopolymerixation-induced LC-polymer separation initiated by a spatially uniform illumination has found wide application. Simultaneously, by exposing an initial pre-polymer NLC mixture to a spatially periodic intensity field obtained with the help of an amplitude grating or two interfering plane waves, one may prepare a composite material in the film bulk of which PDLC stripes alternate with polymeric stripes, with a spatial period determined by the interference pattern. Since the early 90-ths, the interference-assisted PDLC production method, also known as the holographic technique, has been progressing rapidly in