Structure and Morphology of the Aliphatic Polyester Poly-<i>β</i>-propiolactone in Solution-Grown Chain-Folded Lamellar Crystals

Abstract

Solution-grown, chain-folded lamellar crystals of poly-β-propiolactone (PPL) have been crystallized isothermally from cyclohexanone at 55 °C. The lathlike crystals have been studied by transmission electron microscopy (imaging and diffraction), atomic force microscopy, and X-ray diffraction. A new crystal structure, the γ-structure, has been discovered. Using the experimental diffraction data and computerized modeling the structure has been refined, including the detailed geometry of the adjacent re-entry folds. The chain is in an all-trans or 1-fold helix conformation and the structure consists of a two-chain, C-faced, orthorhombic unit cell with the following parameters: a = 0.700 ± 0.002 nm, b = 0.490 ± 0.002 nm, and c (chain axis) = 0.493 ± 0.002 nm. The setting angles, with respect to the a axis, are ± 51.5° for the corner and center chains, respectively. The lamellae are 5 nm in thickness and the chains run orthogonal to the lamellar surface, although there is evidence for slight c-axis shearing of the chain-folded sheets. The general fold direction is along the a axis (long axis of the crystal) and the chain folds successively in the (110) and (11̄0) planes. Comparisons are made with previous structures for PPL fibers and related chain-folded polymers.