Effect of isotropization on the thermal behavior of two thermotropic copolyesters containing spirobicromane moieties

Two thermotropic copolyesters with inherent rigid spirobicromane (SPI) moieties were prepared to evaluate their registry effects of neighboring chains in the mesomorphic state. Due to this chain registry effect, copolyester I-19, synthesized from polycondensation of terephthaloyl bis(4-oxybenzoyl chloride) (TOBC), heptanediol (HD, 90 mol%), and SPI (10 mol%) exhibited different thermal behavior between samples cooled from the mesomorphic and the isotropic liquid states. Chain arrangements in copolyester I-19, once randomized by isotropization, can not be readily improved by a further annealing process. In contrast, crystallinity of copolyester II-19 prepared from reaction of TOBC, HD (90 mol%), and bis(6-hydroxyhexanoxy)spirobicromane, 10 mol%) (BHS) can be significantly enhanced by annealing even when previous thermal treatment in the isotropic liquid state was performed. Suggestively, the minor component (i.e. SPI or BHS) in the corresponding copolyesters plays a decisive role in their responses toward annealing. The rigid, bent SPI moieties in copolyester I-19 efficiently retarded the crystallization process. In contrast, the flexible 1,6-dioxyhexamethyl spacers introduced in the BHS moieties greatly reduce the detrimental effect of the bent SPI moieties on the crystallization behavior of copolyester II-19.