Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Noel A. Clark

Second Advisor

Joseph E. Maclennan

Third Advisor

Matthew A. Glaser

Fourth Advisor

Charles T. Rogers

Fifth Advisor

David M. Walba

Abstract

In chiral smectic A liquid crystals, an applied electric field induces a tilt of the optic axis from the layer normal. When these materials are of the de Vries-type, the electroclinic tilt susceptibility is unusually large, with the field-induced director reorientation accompanied by a substantial increase in optical birefringence, essentially no change in the smectic layer spacing, and a polarization current response that has double peaks. In order to account for this behavior, we assume that the molecular orientation distribution in the smectic A has two degrees of freedom, azimuthal orientation and tilt of the molecular long axis from the layer normal, with the tilt confined to a narrow range of angles. We present a generalized Langevin-Debye model of the response of this distribution to applied field, using an effective mean field obtained from a 3D-XY model of interacting hard rods confined to reorient on a cone. The resultant evolution of this distribution with increasing applied field yields field-induced optic axis tilt, birefringence, and polarization dependence that agree well with experimental measurements, including reproducing the observed double-peaked response of the polarization current.

The family of single tail bent-core molecules with a polar termination on one end and a siloxane terminated tail exhibit a ferroelectric orthorhombic fluid smectic liquid crystal phase, the SmAPF. These SmAPF materials exhibit large magnitude of spontaneous polarization, leading to several novel electro-optic effects. Fluid ferroelectrics with large macroscopic polarization exhibit block polarization reorientation, in which the polarization is rendered uniform by space charge, and responds to static applied voltage by reorienting such that the field in the LC is reduced to E = 0. In the SmAPF this behavior enables a simple mode of analog control of birefringence. With time-dependent applied voltage the E field appears in the LC to generate reorientation of P. We have investigated theoretically the dynamic behavior of block polarization in the SmAPF phase, finding that a reorienting LC polarization block behaves electrically as a resistor, with a minimum resistance of R ∝ γ/P2, where γ is the LC orientational viscosity, which decreases with increasing P. Experimental studies of W623 confirm this behavior, revealing the low resistance of the block reorienting LC.

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