Optical and Resonant X-Ray Diffraction Studies of Nano-Scale Orientation Order in Liquid Crystals

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

This project focuses on (1) understanding the nature of relevant phase transitions, (2) determining the underlying intermolecular ordering and interactions in the liquid crystal phases, and (3) studying thin free-standing films to observe two-dimensional and surface phenomena. Experimental results shed new light into the fundamentals of molecular packing in various liquid crystal phases and provide theorists with important physical parameters for theoretical advances. Chiral smectic liquid-crystals are an important class of liquid crystals. Thus far resonant x-ray diffraction (RXRD) has been the most powerful experimental probe to obtain the nano-scale molecular orientational order. Expanding the use of this probe, RXRD is being applied to binary mixtures (AxB1-x) with x = 0.10. Here the compound A has a heavy element (e.g., sulfur) in the core part of the molecule, while the compound B does not. An important breakthrough, the use of binary mixtures has greatly enhanced experimental capabilities. Table-top optical probes are irreplaceable in-house experimental tools to acquire essential information before and after RXRD runs. With the need to manufacture high speed electro-optical switching devices and with a new interest in special materials to be employed in optic fiber networks, the characterization of exotic liquid crystal mesophases capable of such applications is critically important. Liquid crystal mesophases show both liquid- and crystal-like order. Some compounds display a series of phase transitions (five or more mesophases) in a relatively small temperature window (~30o F). They provide a novel system to explore the richness of structures and molecular packing of many organic molecules. This project will provide unique opportunities to train both undergraduate and graduate students in designing and constructing state-of-the-art optical and x-ray diffraction systems as well as acquiring, analyzing and understanding high quality experimental data.