Molecular Orientation of Polyimide Films for Liquid Crystal Alignment Studied by Infrared Dichroism

Abstract

An orientation structure of polyimide film, which is introduced by rubbing in liquid crystal device (LCD) fabrication, is clarified by polarized infrared spectroscopy. Samples rotated in-plane around the polarized incident beam reveal that the orientation direction of the rubbed polymer chain coincides with the rubbing direction. A sample-tilt measurement, in which the sample is tilted around the vertical axis, reveals that the polymer chains are tilted at an angle to the substrate surface. A simple model is proposed in order to estimate the thickness of the oriented layer of the rubbed film. In the model, the degree of dichroism of parallel and perpendicular polarized IR spectra is calculated as a function of polymer film thickness. Comparing the model with experimental data, it is determined that the thickness of the oriented layer is less than 14 nm.