Chain-Folded Lamellar Structure in the Smectic H Phase of a Main-Chain Polyester

Smectic state morphology of the main-chain liquid crystal PB-14 polyester was studied by small-angle X-ray scattering (SAXS), differential scanning calorimetry, and transmission electron microscope methods. The PB-14 polyester forms isotropic liquid (I), smectic H (S H ), and crystalline phases in order of decreasing temperature. The transition temperature between the I and S H phases was 226 °C on heating and 210 °C on cooling when it was measured at a rate of 10 °C min -1 . SAXS for the S H phase formed from the isotropic melt showed well-defined reflection maxima which are attributed to the chain-folded lamellar structure. The lamellar size increased from 300 Å to 500 Å with an increase of the liquid crystallization temperature of 190 °C to 220 °C. The relationship between the lamellar thickness and isotropization temperature is well described by the Thomson−Gibbs equation. According to this relationship, the surface free energy of the lamellae and the equilibrium isotropization temperature of S H were elucidated as 72 erg cm -2 and 263 °C, respectively. When the sample was annealed at a S H temperature of 215 °C, the isotropization temperature of the S H phase increased from 228 °C to 257 °C, and simultaneously the enthalpy change increased from 4.9 kcal mol -1 to 6.0 kcal mol -1 . These trends can be explained by the lamellar thickening. The overall results thus indicate that the S 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 alteration of the chain conformation from a folded form to an extended one as observed in the crystallization of conventional polymers.