Semiflexible Liquid-crystalline Polyesters Based on Twin Di(p-oxybenzoyl) Units: Effect of Molar Mass on mesomorphic behaviour

Four new series of thermotropic liquid-crystalline polyesters have been prepared, comprising two mesogenic bis(p- oxybenzoyl) units and two linear alkylene spacer segments containing m (m = 7, 8, 9, 11) and n (n = 5-12) methylene groups. Polymers C₈C_n exhibited a nematic mesophase, whereas a nematic and a smectic mesophase were shown by polymers C₇C_n, C₉C_n and C₁₁C_n. Within each series, the nematic-isotropic transition temperatures and entropies as well as the smectic-nematic transition temperatures showed typical odd-even alternations in which homologues with even number n exhibited higher values than adjacent homologues. In contrast, a reversed odd-even alternation was observed for the smectic-nematic transition entropy, in which members possessing an odd number n of methylene groups presented the higher values. The effect of the parity and length of the poly(methylene) spacers on the incidence of mesophases and on their thermodynamic parameters is discussed.