Phase transitions and orientational behavior of liquid crystalline polyurethane mixtures with polar compounds

Liquid crystalline polyurethanes (LCPUs), having azobenzene mesogenic groups, formed a nematic phase on heating and cooling processes. Liquid crystalline binary mixtures were obtained by mixing LCPUs and strongly polar compounds with mesogenic groups. The liquid crystalline binary mixtures exhibited an induced smectic A phase on heating and cooling processes. LCPUs exhibited a schlieren texture in the nematic phase. In the smectic A phase of the liquid crystalline binary mixtures, however, a fan texture was formed. The X-ray diffraction patterns of the liquid crystalline binary mixtures were composed of the sharp inner and the broad outer reflections. The X-ray inner reflections correspond to the layer spacing of the smectic A phase. On the other hand, the X-ray broad outer reflections reveal the existence in a short-range order within the layer. Mixing of weakly polar compounds with LCPUs was also examined. The weakly polar compounds did not mix well with LCPUs, and their liquid crystalline binary mixtures did not induce a smectic A phase. The interaction between the azobenzene mesogenic group and the strongly polar compound can lead to the control of oriented structures of LCPUs.