Influence of Chain Stiffness and Preparation Methods on the Efficiency of Polyimide Alignment Layers for Liquid Crystal Orientation

Abstract

In this work the influences of chain stiffness, solution concentration, and preparation methods on the formation and efficiency of highly oriented polyimide alignment layers are investigated. To quantify the performance of the alignment layers in liquid-crystalline (LC) cells, the orientation of a calamitic bis-azo dye in a low-molecular-weight nematic LC guest–host mixture was employed. The degree of orientation was quantified by measuring the dichroic ratio (DRUV) of the dye with polarized UV spectroscopy. Alignment layers were prepared from precursors of polyimide polymers with different chain stiffnesses. Additionally, the effect of polymer solution concentration, denoted as low (approx. 2 wt%) and high (50 wt%), and orientation techniques (casting and shearing) were systematically investigated. Cells assembled with buffed polyimide alignment layers originating from 1.5 wt% solutions in N-methyl-2-pyrrolidone (NMP) of a rigid para-linked poly(amic ethyl ester) precursor polymer resulted in a very high DRUV of 15. This value can be compared to values obtained with commercially available flexible polyimide layers, which had a DRUV of only 8. Pretilt angles of selected cells were also measured and were found to be in the range of 0.7 to 5.1°.