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

We discuss the microscopic origin of a previously poorly understood phenomenon, the alignment of a nematic liquid crystal (LC), consisting of rod-like molecular units, when placed on a rubbed polymer surface. After giving a brief review of the phenomenon and its technological utilization in flat panel displays we discuss the use of surface sensitive, polarization dependent near edge X-ray absorption spectroscopy for the study of rubbed polymer surfaces. These measurements are shown to provide a microscopic picture for the origin of the alignment process. It is shown that the LC orientation direction is set by an asymmetry in the molecular bonds, i.e. of the charge, at the rubbed polymer surface. The experimental results are explained by a general theory, based on tensor order parameters, which states that the minimum energy state of the interaction between the LC and oriented polymer surface corresponds to maximum directional overlap of the respective anisotropic charge distributions. (C) 1998 Elsevier Science B.V. All rights reserved.