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)研究独立薄膜以观察二维和表面现象。实验结果揭示了不同液晶相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。迄今为止，共振x射线衍射（RXRD）是获得纳米级分子取向秩序的最有力的实验探针。RXRD扩展了该探针的用途，用于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一个重元素（如硫），而化合物B则没有。一个重要的突破，二元混合物的使用大大提高了实验能力。台式光学探头是不可替代的内部实验工具，可以在RXRD运行前后获取重要信息。随着制造高速电光开关器件的需要和对用于光纤网络的特殊材料的新兴趣，能够用于此类应用的奇异液晶中间相的表征至关重要。液晶中间相既显示液态有序，也显示晶体有序。一些化合物在相对较小的温度窗（~30o F）内显示一系列相变（5个或更多中间相）。它们为探索许多有机分子的丰富结构和分子包装提供了一个新的系统。该项目将为本科生和研究生提供独特的机会，培养他们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.