Improvement of liquid crystal alignment using photo-fragment reactions

Abstract

Two photo-activators, AAP and CAP, were synthesized with cyclohexanone oxime with methyl and cronic anhydrides. They were used to improve the liquid crystal (LC) alignment properties by increasing the surface azimuthal anchoring energy, that has been an image-sticking problems. These photo-activators were respectively mixed with twisted nematic (TN) planar alignment agent (TN-PAA) or photo-polyamic acid (P-PAA), respectively. It is remarkable that a photo-activator using crotonic anhydride (CAP) what has allyl double bond showed very high anchoring energy, 6.92 x 10(-5) J/m(2), which is comparable or better than that obtained with rubbing, 1.65 x 10(-5) J/m(2). The anchoring and the alignment properties were observed with the P-PAA were the lower than the TN-PAA. The photochemical reactions and the surface properties of the photo-polyimide (P-PI) were more complex due to more than double photochemical reactions. The fourier transform infrared spectroscopy (FT-IR), contact angle, atomic force microscope (AFM) and differential thermal analysis and thermogravimetic analysis (DTA-TGA) measurements indicated that the surface of polyimide (PI) became more hydrophobic due to the photoreactions of alkyl radicals. These radicals were generated by the photo-irradiation and the reduction of hydrophilic functional groups. The high anchoring energy was assisted by the rough and robust surface induced by carbon dioxide generation. These properties contribute to prevent the image-sticking and shorten the irradiation time with energy.