Semiflexible liquid-crystalline polyesters based on twin bis(p-oxybenzoyl) units. Part 1.—Effect of spacer length 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 C8Cn exhibited a nematic mesophase, whereas a nematic and a smectic mesophase were shown by polymers C7Cn, C9Cn and C11Cn. 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.