Electro-Optic Response of Polymer-Dispersed Liquid Crystals.

The electro-optic properties of polymer-dispersed liquid crystals (PDLC) are studied for two specific cases: (1) when the elongated molecules are anchored perpendicular to the droplet surface, resulting in either a radial or axial director configuration; (2) when the elongated molecules are tangentially anchored, resulting in a bipolar director configuration. The results of electro-optic measurements are compared to theoretical models developed for each case. Nematic director configurations in large droplets (>=q 2mum diameter) are identified by comparing their optical textures under a polarizing microscope with computer simulated pictures of nematic droplets, calculated assuming a ray picture to be valid and only phase shifts and polarization rotations to contribute in the supramicron limit. The simulated pictures compare favorably to photographs of nematic droplets in PDLCs, particularly in the case of droplets with the bipolar configuration. Impedance measurements, combined with a simple circuit model, determine whether a sample operates in the resistive or dielectric regime. The first measurement of the molecular anchoring strength is reported for the case where liquid crystals are confined to spherical cavities. This measurement results from electro-optic studies of PDLCs with radially configured =droplets when perpendicular surface anchoring conditions exist. The anchoring strength values result from a comparison of theoretical models by Zumer with studies of the radial -to-axial configuration transition in PDLCs. Field-induced configuration transitions provide values for the reduced local electric fields inside droplets. Light transmission experiments investigate the effects of film thickness, sample impedance, and droplet size, shape, and density on response times and switching voltages for PDLCs with bipolar droplets. Shearing and compressing PDLCs results in samples with nonspherical droplets which have much shorter response times than similar samples with spherical droplets. Angel-dependent light transmission measurements of sheared PDLCs are used to study reorientation of confined liquid crystals when the bipolar configuration exists in droplets with tangential surface anchoring. The theory of the movement of a droplet director between equilibrium ON and OFF orientations, calculated from torque-balance equations for ellipsoidal droplets, is verified by these experiments. The work in this dissertation provides a qualitative description of the electro-optic principles governing PDLCs with the two most common droplet configurations. These investigations yield possible ways of optimizing PDLC materials for display applications.