Polymer liquid crystals. Contribution of the links between mesogenic units and flexible spacers to the structure-properties relationship in poly(ether ester) liquid crystals

The effect of ether and ester bonds linking an aromatic mesogenic unit and a flexible spacer in poly(ether ester) liquid crystals is presented. Two groups of polymers, each of them possessing the same aromatic ester mesogen and the same eight-membered oligooxyethylene spacer, but different linkages between mesogen and spacer (ester and ether bond), have been studied by DSC, polarizing microscopy, X-ray, and IR spectroscopy. The structure analysis reveals the formation of the smectic phase when the spacer is attached to a rigid mesogenic unit by an ester bond whereas, in the case of ether bond, the nematic phase can be observed. IR spectra of polymers in the solid, liquid-crystalline, and isotropic phases suggest the conformational changes in the polymer chain during the transition to mesomorphic state. The formation of a smectic phase is accompanied by an increase of the number of gauche conformers. The lack of essential conformational changes in the case of nematic polymers with ether bridges can be explained by a low-energy barrier to rotation characteristic of ether bonds. Spectra. 21 ref.--AA