Polarized Infrared Spectroscopic Studies of Polarized UV-exposed Polyimide Films for Liquid Crystal Alignment

Polarized Fourier transform infrared (FTIR) and ultraviolet-visible (UV-VIS) spectroscopy were used to investigate the cause of liquid crystal alignment on polarized UV (PUV) irradiated polyimide (PI) films. PI films strongly absorb below 350 nm. PUV irradiation of the PI film caused a decrease in the intensity of the original IR absorbance peaks of the PI, due to degradation of the PI molecules, along with the appearance of a new peak at 3261 cm(-1). Because the PI molecules oriented parallel to the direction of UV polarization preferentially degrade, the predominant orientation of the remaining PI molecules after PUV irradiation is perpendicular to the PUV polarization direction. Liquid crystal (LC) molecules align parallel to the predominant orientation of the PI and perpendicular to the PUV polarization direction.