Optical retardation and FT-IR characteristics of rubbed polyimide langmuir-blodgett alignment layers of liquid crystals

Optical retardation and FT-IR characteristics of polyimide alignment layer of liquid crystals were investigated with Langmuir-Blodgett (LB) films rubbed perpendicularly to the dipping direction. While the retardations of LB films remained unchanged as the number of transfers increased, those of rubbed films increased up to 7 layers, but after a maximum retardation, they decreased steadily as the number of transfers increased. The retardations of films thicker than about 7–9 layers shifted negatively, indicating that the optical axis completely switched to the direction of rubbing. Further, difference of retardation between unrubbed and rubbed films indicates that there existed a limit in the number of layers, i.e., about 30 layers, to be influenced by the surface forces. It is an interesting observation that the substrate surface freezes the polymer chains up to 7–9 layers, persistent to the rubbing strength, while the surface rubbing force penetrates into about 30 layers. The IR dichroic ratio (A¦/A⊥) of 1,500 cm-1 and of 1,240 cm-1 decreases by rubbing, while little change at 1,720 cm-1, indicating that benzene groups are reoriented to the direction of rubbing, while imide groups are not reoriented.