A versatile photochemical procedure to introduce a photoreactive molecular layer onto a polyimide film for liquid crystal alignment

A polyimide film containing a cholesteryl moiety spin-coated on a solid substrate is treated with vacuum ultraviolet light from a Xe-2(*) excimer lamp at 20 Pa. This procedure generates oxidized chemically functional groups, such as hydroxyl groups, selectively on the top-most surface without changing the nature of the bulk polymer. Onto the chemically functionalized surface, photofunctional units such as azobenzene and cinnamate units are attached at a monolayer or sub-monolayer level. When nematic liquid crystal (LC) cells are fabricated, such photoresponsive surfaces work as effective 'command surfaces' for photoswitching the zenithal LC orientation and photoaligment layers to achieve the homogeneous azimuthal orientation applicable for vertical alignment mode LC devices. The proposed photochemical treatment provides a simple but versatile alternative process for the preparation of surface molecular films comparable to self-assembled monolayers or Langmuir-Blodgett films. Polymer Journal (2012) 44, 639-645; doi:10.1038/pj.2012.45; published online 28 March 2012