The electro-optical switching behaviors of ferroelectric liquid crystal cell with alignment layers of a photo-degradable polyimide, CBDA-BAPP, having cyclobutane rings in the backbone structure prepared by the polarized UV light (PUVL) irradiation was investigated.
The chemical changes of PI surfaces by the PUVL irradiation were monitored with a UV-visible spectroscope, and an FT-IR analyzer. The absorption intensities of UV and IR decreased as the PUVL irradiation time increased. The dichroic absorption spectra of polarized UV and IR were measured as an indicator of the anisotropic interaction between liquid crystal molecules and alignment layer. The intensities of UV and IR absorption parallel to the polarization direction of the irradiated PUVL were much smaller than those perpendicular to the polarization direction. However, no dichroic UV absorption spectra were observed in the region of above 290nm that might arise from the by-products of photo-degradation. Therefore, it is our observation that the anisotropy of the PI films was induced by the preferential cleavages of the PI chains oriented parallel along the polarization direction of the irradiated PUVL and by the subsequent random reorientation of the cleaved groups.
Textures of FLC cells prepared with various rubbing strengths and PUVL irradiation time were observed with a polarized optical microscope. While textures with a lot of zigzag defects were observed in the rubbed cell irrespective of the rubbing strength, the well-aligned FLC textures without zigzag defects were obtained in the slowly cooled cell prepared by the PUVL irradiation at normal incidence. The formation of zigzag defects in surface-stabilized ferroelectric liquid crystal is strongly affected by the surface roughness and pretilt angles. The smooth surface and small pretilt angle after the PUVL irradiation enable FLC to be aligned without zigzag defects. The contrast ratio of the FLC cell was much higher when it was prepared by photo-induced alignment method than by rubbing method, and the highest value of the contrast ratio achieved in the photo-aligned cells was 23:1. The FLC alignment was also strongly dependent on the cooling rate. The more slowly the cell cools, the better the alignment is.
The switching behaviors of FLC including spontaneous polarization, response time, and decay time were measured. As the PUVL irradiation time and the rubbing strength increased, the spontaneous polarization was increased. When it was irradiated for more than 40min, the spontaneous polarization of FLC was higher than that in the rubbed cells. Also, the slowly cooled cells showed higher spontaneous polarization, and the faster response time with the order of microsecond that makes it possible to realize high quality moving images. On the other hand, the slower decay time, indicating the longer memory time, was measured in the cells with alignment layers irradiated by the PUVL for the shorter period that induces the lower anchoring energy.
