Asymmetric Co-unit Inclusion in Statistical Copolyesters

Abstract

The biodegradation of isodimorphic random butanediol-succinate/adipate copolymers (PBSAs) can be affected by a change in the semicrystalline morphology. A crucial point for the final semicrystalline structure is the incorporation of monomers of the minority component into the crystals of the majority component. We use proton time-domain NMR and wide-angle X-ray scattering to characterize the copolymers across their complete composition range and analyze the possible inclusion of minority comonomers in the PBA-rich 20/80 and the PBS-rich 60/40 composition by applying different solid-state 13C NMR experiments. For the PBA-rich copolymers, we find a nearly constant volumetric crystallinity of 50% up to the 50/50 composition combined with an increase of the crystalline lamella thickness. NMR has detected the BS units incorporated into the BA crystalline structure, where on average every tenth monomer is a BS comonomer. In PBS-rich crystals, the crystallinity and crystallite thickness decrease from neat PBS to PBSA 60/40 by 37% and 20%, respectively. We also find evidence that the BA units are excluded from the bulk crystal but accumulate at the crystallite surface. The amount of BA units in these pocket-like structures can be estimated to be 7-9% of the BA comonomer content.