Generation of nematic liquid crystal alignment with polyimides exposed to linearly polarized light of long wavelength

The alignment behavior of nematic liquid crystal (LC) on the surface of four types of polyimide (PI) films, three aromatic PIs and a PI with an alicyclic dianhydride, exposed to linearly polarized light of long wavelength (366 nm) was investigated by polarizing microscopy. It was found that the alignment of the LC was induced by all of the exposed PIs and the alignment direction of the LC was perpendicular to the polarization direction of the linearly polarized light. The degree of uniformity of LC alignment increased with increasing exposure energy incident on PI films. To produce the uniform alignment state, the PI with an alicyclic dianhydride had to be exposed to higher energy than aromatic PIs. Ultraviolet (UV) -visible spectroscopy indicated that the absorbance changes of the aromatic PIs were much smaller than that of the PI with an alicyclic dianhydride when uniform alignment of the LC was caused. Furthermore, both UV-visible and infrared spectroscopy showed the change in the absorbance of the aromatic PI with a diphenyl ether diamine was still very small even though it was exposed to linearly polarized light with a large dose. These results demonstrate that the aromatic PIs are more favorable as a photosensitive alignment layer and the uniform alignment of the LC can be induced without significant destruction of the chemical structure of PI with a diphenyl ether diamine.