Low-Melting Nematic Quasi-Rigid Rod-Like Polyesters

Thermotropic quasi‐rigid rod‐like polyesters with broad nematic phases in temperature ranges that allow melt processing in the mesophase have been synthesized. The desired mesomorphic temperature range can be realized by adjusting the chain flexibility and packing density as well as the axial ratio of the mesogenic segments. The chain flexibility was slightly increased without giving the molecule much conformational freedom by introducing ethylene or dioxyethylene linkages between the phenyl groups of the aromatic polyester (preferably in the acid part of the molecule). The sensitive balance between the axial ratio of the mesogenic segments and the chain flexibility was preserved to retain the mesogeneity of the molecule. Replacing some of the planar aromatic phenyl groups by staggered trans‐cyclohexyl moieties effectively depresses Tm without affecting Ti. The effect of symmetrical and asymmetrical lateral substitution with alkyl groups of different van der Waals volume on Tm and Ti was studied. Asymmetric lateral substitution is quite effective in depressing Tm with little effect on Ti. Combinations of these structural modifications are useful tools to adjust the mesomorphic properties of liquid crystalline polyesters.