Molecular packing and crystalline morphologies of biodegradable poly(alkylene dicarboxylate)s derived from 1,6-hexanediol

The crystalline structures of two aliphatic polyesters derived from 1,6-hexanediol and suberic or dodecanodioic acid are studied by means of X-ray and electron diffraction and real space electron microscopy. Chain-folded lamellar crystals have been obtained by isothermal crystallization of dilute n-hexanol or n-heptanol solutions and their crystalline habit have been studied. A regular folding habit is deduced from polyethylene decoration techniques. An orthorhombic unit cell containing four chain segments with a planar zig-zag conformation is determined for the two studied polyesters. A P21ab space group symmetry can be assumed taking into account the detected systematic absences and the symmetry of diffraction patterns. In addition, evidence of a second non-orthorhombic structure is found for the suberic acid derivative. The orthorhombic crystal packing of these polymers is characterized by a setting angle close to 45°, showing a similarity with polyethylene. Refinement of the X-ray fiber diffraction pattern of polyester 6-12 allows to determine the c-chain axis shift between the molecular chains, being observed an alignment of the CH2–O methylene groups with the CH2–O oxygen atoms of neighbouring chains. Crystallization from the melt renders spherulites with a negative birefringence and a fibrillar texture, whereas banded morphologies constituted by multilayered single crystals can be obtained by evaporation of xylene solutions. Electron diffraction patterns of ultrathin samples allow the spherulitic radial growth direction to be correlated with the crystalline structure.