Enhanced, parallel liquid crystal alignment based on polystyrene substituted with phthalimidoyl groups

A series of polystyrene (PS) polymers substituted with phthalimide (PPH#), where # is the degree of substitution, were synthesized via a polymer analogous reaction to investigate liquid crystal (LC) alignment behaviors of LC cells fabricated using PPH# films. The PPH# films exhibited higher optical transparency in the visible light region compared to a commercial polyimide (PI) film. The transmittance of each PPH# film ranged from 91 to 93%, whereas that of PI was 80.5% at 550 nm. The LC cell fabricated with a pristine PS film showed planar LC alignment perpendicular with respect to the rubbing direction. The introduction of the phthalimide group to the side chain of PS diverted the LC alignment direction from perpendicular to parallel with respect to the rubbing direction. The LC cells fabricated with PPH# films exhibited planar LC alignment parallel with respect to the rubbing direction when the degree of substitution of phthalimidoyl moieties was >20 mol%. In addition, the long-term stability of the LC cell composed of pristine PS was very low. However, the long-term stability of the LC cells fabricated with the PPH# series was remarkably enhanced in addition to their ultraviolet stability. Therefore, this study contributes to the development of planar and parallel LC alignments on PS derivatives and can be used in the next-generation display industry for the production of flexible displays.