Chain folding in the smectic phase of main-chain polyesters

Morphology of the two types of main-chain smectic liquid crystal polyesters, BB-n and PB-n, was studied by small angle X-ray scattering (SAXS). The SAXS for the crystalline BB-6 specimens prepared by cooling the smectic A melt shows well-defined reflection maxima which are attributable to the stacked lamellar structure. The lamellar spacings are distributed around 250 Angstrom so that an appreciable number of chain foldings are included in a chain. The lamellar spacing is increased by annealing the crystal. In contrast, it is not essentially altered by annealing the smectic A phase. The increment of the lamellar spacing was caused by a decrease in the crystallization temperature. This trend is contrary to that observed in conventional polymers which crystallize from the isotropic melt. The results show that the chain foldings are at a thermodynamic equilibrium by an entropy effect in the smectic A phase. The well-defined reflection maxima were observed for the smectic H phase of PB-14 polyester. The lamellar size increased from 300 Angstrom to 500 Angstrom with an increase in the liquid crystallization temperature of 190 degrees C to 220 degrees C. The relationship between the lamellar thickness and isotropization temperature is well described by the Thomson-Gibbs equation. When the sample was annealed at a smectic ii temperature of 215 degrees C, the isotropization temperature and enthalpy of the smectic H phase increased. The overall results thus indicate that the smectic H liquid crystallization at a certain temperature takes place imperfectly in a finite period due to the chain folding and that the succeeding annealing causes the alternation of the chain conformation from a folded form to an extended one as observed in the crystallization of conventional polymers.