SEMIFLEXIBLE LIQUID-CRYSTALLINE POLYESTERS BASED ON TWIN BIS(P-OXYBENZOYL) UNITS .1. EFFECT OF SPACER LENGTH ON MESOMORPHIC BEHAVIOR

Four new series of thermotropic liquid-crystalline polyesters have bee n prepared, comprising two mesogenic bis(p-oxybenzoyl) units and two l inear alkylene spacer segments containing m (m = 7, 8, 9, 11) and n (n = 5-12) methylene groups. Polymers C8Cn exhibited a nematic mesophase , whereas a nematic and a smectic mesophase were shown by polymers C7C n, C9Cn and C11Cn. Within each series, the nematic-isotropic transitio n 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 smec tic-nematic transition entropy, in which members possessing an odd num ber n of methylene groups presented the higher values. The effect of t he parity and length of the poly(methylene) spacers on the incidence o f mesophases and on their thermodynamic parameters is discussed.