Semiflexible liquid-crystalline polyesters based on twin bis(p-oxybenzoyl) units. Part 2.—Effect of molar mass on mesomorphic behaviour

The effect of the molar mass (Mn) and molar mass distribution (Mw/Mn) on liquid-crystalline phase-transition behaviour was investigated for four polymers of the general class CmCn, which were based on two mesogenic bis(p-oxybenzoyl) units and two linear alkylene spacer segments containing m(m= 5 or 7) and n(n= 5, 7 or 10) methylene groups, respectively. Each polymer exhibited smectic and nematic mesophases. The polymers were fractionated into various samples having different Mn and Mw/Mn values. The liquid-crystalline phase-transition parameters of samples C7C7 and C7C10 increased with Mn until a limiting value of Mn≈6000–8000 g mol–1, and then remained constant. In complete contrast, for samples C5C5 and C5C7 the stability of the smectic mesophase increased, whereas that of the nematic mesophase decreased with increasing Mn. In particular, for the former samples the nematic mesophase turned from enantiotropic to monotropic with increasing Mn, and then disappeared in correspondence to a critical value of Mn≈ 7000 g mol–1. In addition, wide Mw/Mn values were found to increase the stability of the nematic mesophase.