Investigation into the phase separation of polymer dispersed liquid crystals

Nicole Leigh Gill

Abstract

Polymer dispersed liquid crystals (PDLCs), micron-sized liquid crystal droplets dispersed within a solid polymer matrix, may be electrically switched between opaque and transparent states without the use of polarizers. PDLCs have potential applications as flat panel displays, privacy windows, Bragg gratings, and light shutters. The electro-optical properties of PDLCs depend on the phase-separated morphology, which is controlled by the coupled phase separation and polymerization processes. The phase separation process of PDLCs was examined using a model system composed of the liquid crystal mixture E7 and its component mixtures in the thiol-ene based pre-polymers NOA65 and UV1. E7 is composed of K15 (4-pentyl-4 ' cyanobiphenyl), K21 (4-heptyl-4' cyanobiphenyl), M24 (4-octyloxy-4 ' cyanobiphenyl), and T15 (4-pentyl-4' cyanoterphenyl). NOA65 is an industrially made thiol-ene pre-polymer with benzophenone as the photo-initiator. UV1, the equivalent to NOA65, was prepared at The University of Southern Mississippi. To fully investigate the phase-separation process for this model system, the research was composed of four major stages: the pure liquid crystal component mixtures, the liquid crystal component mixtures in the pre-polymer, the liquid crystal with increasing percent conversion of the polymer, and the liquid crystal in the polymer network. The first stage of the research examined the thermal behavior and the phase sequence of the liquid crystal of the single components (K15, K21, M24, and T15) and the binary component mixtures (K15-K21, K15-M24, K15-T15, M24-T15, K21-M24, and K21-T15), and the melting behavior was numerically-fitted using the le-Chatelier-Schröder-Van Laar (CSL) equations. Using the CSL equations, a new approach for determining the latent heats of melting was identified. The second stage examined the liquid crystal single components, binary component mixtures, ternary component mixtures, and E7 in the thiol-ene pre-polymers where the phase sequences and phase separation temperatures were characterized. Stage two was the foundation for the liquid crystal mixtures in the polymer in outlining the experimental limits for the different liquid crystal components where phase separation occurs. In the third stage, the phase separation temperatures for liquid crystal/UV1 mixtures were examined as the percent conversion of UV1 increased. In the last stage of the project, PDLC samples were polymerized at varying increments above the liquid crystal/pre-polymer phase-separation temperatures to determine how the morphology changes with the cure temperature, the liquid crystal component, and liquid crystal percentage. The PDLCs were prepared with the following liquid crystal components and liquid crystal mixtures: K15, K21, M24, K21-M24, K15-M24, and K15-K21. The characterization techniques included polarized optical microscopy, scanning electron microscopy, laser-light transmission, differential scanning calorimetry, and differential photo-calorimetry.