Chain Folding of Main-Chain Polyesters in the Smectic A and CA Phases with a Liquid-Like Association of Biphenyl Mesogens within a Layer.

Abstract

The solid state morphology of poly (methylene p,p-bibenzoate) (BB-n) was studied by the small-angle X-ray scattering (SAXS) method. The BB-n polyesters with n of 6 and 8 form isotropic, smectic A, and crystal phases in the order of decreasing temperature; thereby their crystallization taking place from the smectic A phase. The SAXS for the crystalline specimens shows the well-defined reflection maxima which are attributable to the stacked lamellar structure. The lamellar spacings are distributed around 250-300 A so that an appreciable number of chain foldings are included in a chain. The lamellar size is increased by annealing the crystal like in other crystalline polymers. In contrast, it is not essentially altered by annealing the preceding smectic A phase. Most interesting is that the lamellar spacing increases with a decrease in the crystallization temperature. The trend is contrary to that observed in conventional polymers as well as in BB-10 and BB-12 which crystallize from the isotropic melt and can be explained if in the preceding smectic A phase the chain foldings exist at a thermodynamic equilibrium as an entropy effect. Such a chain folding was also predicted from anomalous orientation of molecules in the fibers drawn from the smectic melt. In these fibers, the polymer chains align in a direction perpendicular to the fiber axis. This orientation is completely retained even if the fiber is further elongated up to the strain of 100%. These results suggest that the chain foldings take place at a limited space to form the lamellar smectic domain.