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 SA XS for the crystalline BB-6 specimens prepared by cooling the smectic A mel t shows well-defined reflection maxima which are attributable to the stacke d lamellar structure. The lamellar spacings are distributed around 250 Angs trom so that an appreciable number of chain foldings are included in a chai n. The lamellar spacing is increased by annealing the crystal. In contrast, it is not essentially altered by annealing the smectic A phase. The increm ent of the lamellar spacing was caused by a decrease in the crystallization temperature. This trend is contrary to that observed in conventional polym ers which crystallize from the isotropic melt. The results show that the ch ain 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 An gstrom to 500 Angstrom with an increase in the liquid crystallization tempe rature of 190 degrees C to 220 degrees C. The relationship between the lame llar thickness and isotropization temperature is well described by the Thom son-Gibbs equation. When the sample was annealed at a smectic ii temperatur e of 215 degrees C, the isotropization temperature and enthalpy of the smec tic 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.