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

本项目的重点是(1)了解相关相变的本质，(2)确定液晶相中潜在的分子间有序和相互作用，以及(3)研究自支撑薄膜以观察二维和表面现象。实验结果揭示了各种液晶相中分子堆积的基本原理，并为理论发展提供了重要的物理参数。手性近晶液晶是一类重要的液晶材料。到目前为止，共振X射线衍射仪是获得纳米级分子取向有序的最有力的实验探针。将这一探头的应用范围扩大到x=0.10的二元混合物(AxB1-x)。在这里，化合物A在分子的核心部分有重元素(例如硫)，而化合物B没有。一项重要突破是，二元混合物的使用大大增强了实验能力。桌面光学探头是不可替代的内部实验工具，用于在RXRD运行前后获取必要的信息。随着制造高速电光开关器件的需要和对光纤网络中使用的特殊材料的新的兴趣，能够用于这种应用的奇异液晶中间相的表征变得至关重要。液晶中间相既有液晶有序，又有类晶体有序。一些化合物在相对较小的温度窗口(~30oF)内表现出一系列的相变(五个或更多个中间相)。它们提供了一种新的系统来探索许多有机分子的结构和分子堆积的丰富性。该项目将提供独特的机会，培训本科生和研究生设计和建造最先进的光学和X射线衍射系统，以及获取、分析和理解高质量的实验数据。

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.