Synthesis and liquid-crystalline properties of thermotropic poly(ester-urethane)s

Seven new mesogenic diol monomers CmCn, consisting of two bis(p-oxybenzoyl) units interconnected by a polymethylene segment containing n methylene groups and flanked by two linear ω-hydroxyalkyl segments containing m methylene groups were reacted with 1,6-hexamethylenediisocyanate (HDI) to give a new series of liquid-crystalline poly(ester-urethane)s HDI-CmCn. All of the precursors CmCn exhibited a nematic phase, and in two of them a smectic mesophase also occurred. Poly(ester-urethane)s HDI-CmCn were semicrystalline and formed a monotropic mesophase. The mesomorphic properties of the polymers were studied as a function of their chemical structure by changing alternatively m or n. The effect of the molecular weight of the polymers on the stability of the mesophase was also investigated for various samples of poly(ester-urethane) HDI-C6C6 (M_n = 3000-22,000). It was found that a competition existed between the tendencies of the polymer to crystallinity and liquidcrystallinity, and the molecular weight dependence of the crystallization temperature determined the onset of a monotropic mesophase.