Characteristics of polyimide alignment layers with different surface treatments

Abstract

Two alignment methods — rubbing and Langmuir-Blodgett (LB) techniques — were investigated with respect to textures and spontaneous polarization of ferroelectric liquid crystals by varying the rubbing strength and the deposition pressure, respectively. After the cells were rubbed, and LB layers were assembled, the alignment was achieved by varying the cooling rates and the AC field. The texture under the optical microscope was observed to be improved as the rubbing strength increased. When the cell was rapidly cooled, the smectic alignment was not complete but the slowly cooled cell showed a uniform alignment with some zig-zag defects. Under the AC field, all defects were reduced in most cases. LB films resulted in a better alignment in the range between 25 mN/m and collapse pressure. The optical texture and spontaneous polarization of liquid crystals in the cell indicated that good alignment could be achieved at a moderate rubbing strength and at field-on slow cooling. The spontaneous polarizations of LB cells were slightly less than those of the rubbed cells, but those of field-on slowly cooled cells were identical with those of rubbed cells.