SYNTHESIS AND CHARACTERIZATION OF A NEW SERIES OF SEMIFLEXIBLE LIQUID-CRYSTALLINE POLYESTERS WITH AROMATIC TRIAD MESOGENS

A new series of semiflexible liquid-crystalline polyesters 1-n or 2-n based on a terephthalate-methylhydroquinone-terephthalate triad ester mesogen and a flexible spacer segment of variable number n of methylene units (n = 4-12) was prepared and studied. Two synthetic procedures were used involving a diacid/diphenol direct polycondensation in the presence of a condensing agent (polyesters 1-n), or a diacyl chloride/diphenolate polycondensation under phase transfer conditions (polyesters 2-n). By the former synthetic method, high molecular weight polymers 1-n were obtained, that were characterized by a greater thermodynamic stability and a wider persistence range of the nematic phase with respect to the corresponding low molecular weight analogues 2-n. The mesophase behavior depended on the length and parity of the spacer segment, and the nematic-isotropic transition temperature exhibited a strong even-odd oscillation. For polyesters 1-10 and 1-12, the nematic phase was quenched to room temperature and in the latter sample a cybotactic nematic structure was frozen-in.