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Date of Award
Physics and Astronomy
Paul H. Keyes
The dielectric properties of nematic liquid crystals were studied in both the achiral and chiral limits. For achiral nematics, the literature documents that pretransitional curvature occurs for polar molecules on both sides of the nematic and isotropic phase transition. This curvature is due to anti-parallel dimer formation. However, past models have failed to quantitatively describe pretransitional curvature. Through a generalization of the order parameter, a macroscopic model has been developed to mathematically describe the pretransitional curvature on the isotropic side of the transition. The new model was fitted to dielectric data from the literature. Meaningful parameter estimates were extracted.
The dielectric response of chiral nematic systems has not been well studied in the literature. A system with tunable chirality was dielectrically studied by mixing two highly chiral liquid crystals: cholesteryl oleyl carbonate (left handed) and cholesteryl chloride (right handed). An apparatus was designed and built to systematically measure the dielectric response of the mixtures. Optical cross-polarized microscopy was used to identify the transition temperatures of each phase. The transition temperatures were then correlated with the dielectric response.
The initial intentions of studying chiral systems was two-fold: to see how chirality played a role in the pretransitional curvature, and to see if the blue phases were dielectrically distinguishable. While the initial intentions were null and indecisive, respectively, interesting results were obtained. First, the phase transitions were monotropic for the highest chirality mixtures. Second, the estimated discontinuity at the isotropic to mesogenic transition followed theoretical predictions that had not been tested for dielectric measurements.
Godfrey, Garrett Justin, "Dielectric Anomalies Of Both Chiral And Achiral Nematogens Near The Isotropic To Mesogenic Phase Transition" (2016). Wayne State University Dissertations. 1533.