Alignment of ferroelectric liquid crystal by photodimerization of benzylideneacetophenone (chalcone) based side chain polymer

Abstract

The alignment and optical properties of ferroelectric liquid crystal cells with alignment films of benzylideneacetophenone (chalcone) side chain polymer treated by a linearly polarized UV light irradiation were investigated. The polymer films were exposed to linearly polarized UV light and characterized by using UV and FT-IR spectroscopies. The absorption spectra of UV have a maximum point at 290nm and two isosbestic points at 238nm and 258nm. The gradual decrease at long absorption band of UV and the changes of IR spectra as the irradiation times increased resulted from cycloaddition and isomerization reaction of the chalcone based side chains. The UV dichroism $(A_para-A_perp)$ was measured to confirm the anisotropic changes of alignment films and observed a maximum value at low exposure energy (0.5 J/㎠). It implied that anisotropic photoisomerization of alignment films have occurred in the early stage of irradiation because of low energy requirement. But the large UV dichroism did not lead to always the good alignment condition of LCs because anisotropy by the isomerization without dimerization was feeble for heat and light energy. The direction of LC alignment and order parameter on the alignment films were determined by monitoring the dichroism of dichroic dye (disperse blue 14, Aldrich Co.) dissolved in nematic LC (E7) because the orientation of dye molecules dissolved in LC followed LC molecules. The texture of FLC cells with a linearly polarized UV irradiation times were observed with a cross-polarizing optical microscope. When a linearly PUVL have irradiated for more than 5min, well-aligned defect-free cells were observed and obtained high contrast ratios. These microscopic results and high contrast ratios confirmed that the defect-free state was achieved. Pretilt angle of alignment films and layer spacing of FLCs in the SSFLC cells was measured by crystal rotation method and X-ray diffraction method respectively. We observed that the photo-alignment m...