Liquid Crystal Photoalignment Layer Using Coumarin-Containing Block Copolymers

Abstract

The intrinsic combination of anisotropy and fluidity of liquid crystals enables a wide range of their applications such as displays and optical and photonic devices, for which an initial orientation of liquid crystal molecules at the macroscopic scale is essential. The use of an optically treated surface, termed the photoalignment layer, to induce this initial liquid crystal orientation is an attractive alternative to the use of a mechanically rubbed surface because, among other advantages, using light makes it possible to spatially pattern the photoalignment layer to organize the liquid crystal orientation. Herein, we introduce coumarin-containing block copolymers as a material system for the surface photoalignment layer. A diblock copolymer comprising a non-photosensitive polyurethane and a photoactive polymethacrylate bearing coumarin side groups was synthesized in different compositions and was investigated for the use of a photoalignment layer for a nematic liquid crystal. The microphase separation in the block copolymer results in distinct self-assembled surface morphologies during the casting and processing of the alignment layers, which influence the induced liquid crystal orientation. Interestingly, even with the coumarin polymer forming nanodomains dispersed in the non-photosensitive polymer matrix, the photoalignment layer can effectively align liquid crystal molecules, demonstrating a dominant cooperative effect in propagating liquid crystal orientation in all directions. Given the numerous possibilities for designing photosensitive block copolymers, the concept of exploring photoalignment layers featuring not only surface anisotropy but also microphase separation of the blocks on the surface offers perspectives for controlling liquid crystal orientation crucial for applications.