Liquid crystal alignment by rubbed polymer surfaces: a microscopic bond orientation model

We discuss the microscopic origin of a previously poorly understood phenome non, the alignment of a nematic liquid crystal (LC), consisting of rod-like molecular units, when placed on a rubbed polymer surface. After giving a b rief review of the phenomenon and its technological utilization in flat pan el displays we discuss the use of surface sensitive, polarization dependent near edge X-ray absorption spectroscopy for the study of rubbed polymer su rfaces. These measurements are shown to provide a microscopic picture for t he origin of the alignment process. It is shown that the LC orientation dir ection is set by an asymmetry in the molecular bonds, i.e. of the charge, a t the rubbed polymer surface. The experimental results are explained by a g eneral theory, based on tensor order parameters, which states that the mini mum energy state of the interaction between the LC and oriented polymer sur face corresponds to maximum directional overlap of the respective anisotrop ic charge distributions. (C) 1998 Elsevier Science B.V. All rights reserved .