Characterization of polymeric alignment layers and alignment processes for liquid crystal displays

Abstract

Alignment processes and characterization of alignment layers were discussed. In this thesis the alignment layers were confined to polymeric materials. Polyimides and photo-curing polymers were introduced as the alignment layers, briefly in Chapter I, and applied to orient, especially, nematic liquid crystal molecules. In chapter II, three alignment processes were discussed; conventional rubbing, wet rubbing, and photo-alignment techniques. The wet rubbing process has been proposed to overcome the drawbacks of the conventional rubbing. It utilizes suspensions as the buffing medium under uniform flow. The process conditions in the wet rubbing were controlled accurately with ease by varying physical properties related to the suspensions and the flow field. Contrast to the difficulties of defining process conditions in the dry rubbing, the wet rubbing process can give us reproducible operation, even in weak rubbing strength. In the wet rubbing process, it was found that the threshold stress of 400 Pa is required to align LC molecules on PI layers. Smaller and harder particles turned out to be more effective in view of stabilizing the flow field. A phase modulation technique is applied to measure the induced retardation of the alignment layers generated by various alignment processes. Such efforts become effective especially in cases that alignment layer has weak anchoring energy, and that orientation direction is unknown after LC injection. By measuring the induced retardation, it was possible to correlate the ordering of the polymer chains with the alignment of the liquid crystal molecules. Further, inclination in the polymeric layer was directly correlated with the macroscopic alignment properties, such as pretilt angle. After a brief introduction of photo-alignment processes in chapter II, detailed mechanism of LC alignment on the photo-polymer surface was discussed in chapter III. The surface hydrogenation reaction was performed in order to clarify the propo...