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 (M_n) and molar mass distribution (M_w/M_n) on liquid-crystalline phase-transition behaviour was investigated for four polymers of the general class C_mC_m 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 M_n and M_w/M_π values. The liquid-crystalline phase-transition parameters of samples C₇C₇ and C₇C₁₀ increased with M_n until a limiting value of M_n≈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 M_n. In particular, for the former samples the nematic mesophase turned from enantiotropic to monotropic with increasing M_n, and then disappeared in correspondence to a critical value of M_n≈7000 g mol^-1. In addition, wide M_w/M_n values were found to increase the stability of the nematic mesophase.