Structure at Homeotropic Liquid Crystal Surfaces

向列相液晶由没有规则重复位置的细长分子组成。它们确实有取向顺序，因为分子的长轴倾向于平行排列。这种对齐方向称为导向器。还有其他的液晶相，例如近晶A相，它可以在低于向列相的温度下形成。在近晶A相中，分子呈层状排列，形成规则的堆积，长轴垂直于层。指向器的表面取向对许多向列型液晶器件的工作至关重要。无处不在的扭曲向列相装置使导向器平行于表面。更新的设备，如电视显示器中使用的那些，使用导向器垂直于表面的同向性对准。向列型液晶在同向同性表面的结构是这一建议的主要主题。向列型液晶的结构在靠近表面的区域被改变，因为分子有形成近晶层的趋势。这是用掠入射X射线反射在空气/向列相界面上观察到的。然而，由于X射线方法的局限性，衬底与向列相的相互作用的重要影响还没有被探索。这项建议是使用更适合于固体衬底的中子反射。中子反射将确定类近晶有序，并给出表面近晶有序的定量测量。椭偏测量法将量化表面附近向列相或各向同性相的取向顺序的任何增强。因此，将获得近地表结构的完整图像。通过使用不同的取向层来研究与衬底的相互作用的影响。液晶材料的成层倾向也将通过使用具有不同相行为的分子来改变，以验证理论预测。此外，还将研究混合物中某一组分形成异常第一层和优先层的可能性，以探索如何控制和调整同向排列。同样的实验方法将被用来阐明电子寻址命令表面的性质。在这些表面上，分子在一层中的取向是电性切换的，并控制着相邻的向列相大块薄膜中的取向。

A nematic liquid crystal consists of elongated molecules which have no regularly repeated positions. They do have orientational order in that the long axes of the molecules tend to align parallel. This direction of alignment is called the director. There are other liquid crystal phases such as the smectic A phase which can form at temperatures below the nematic. In the smectic A phase the molecules are arranged in layers which form a regular stack with the long axes perpendicular to the layers. The alignment of the director by surfaces is crucial to the operation of many nematic liquid crystal devices. The ubiquitous twisted nematic device has the director lying parallel to the surface. More recent devices such as those used in TV displays use homeotropic alignment where the director is perpendicular to the surface. The structure of nematic liquid crystals at homeotropic surfaces is the principal topic of this proposalThe structure of a nematic liquid crystal is modified in the region close to a surface because there is a tendency for the molecules to form into smectic layers. This has been observed using grazing incidence X-ray reflection at the air/nematic interface. However the important influence of the substrate interaction with the nematic has not been explored because of limitations to the X-ray method. This proposal is to use neutron reflection which is better suited to solid substrates. Neutron reflection will determine the smectic-like order and give a quantitative measure of the smectic order at the surface. Ellipsometry will quantify any enhancement of the orientational order of the nematic or isotropic phase near the surface. Thus a complete picture of the near surface structure will be obtained. The influence of the interaction with the substrate will be studied by using different alignment layers. The layer forming tendency of the liquid crystal material will also be varied by using molecules with different phase behaviour in order to test theoretical predictions. In addition, the possibility of an anomalous first layer and preferential layer formation by one component in a mixture will be studied to explore how homeotropic alignment may be controlled and adjusted. The same experimental methodology will be used to elucidate the nature of electrically addressed command surfaces. These are surfaces where the orientation of the molecules in one layer is switched electrically and controls the orientation in an adjacent bulk film of nematic.