Interfacial and Crystalline Gauche OCH₂-CH₂O Layers in Biodegradable and Recyclable Polyethylene-like Polyesters...

Polyethylene-like aliphatic polyesters are promising biodegradable polymers; however, their conformational and supramolecular structures are not well understood. Here, we used solid-state nuclear magnetic resonance (NMR) to investigate three synthetically accessible polyesters made from doubly ¹³C-labeled ethylene diol units and unlabeled dicarboxylic acids of 12-, 18-, and 48-carbon length (PE-2,12 to PE-2,48). Signals of abundant gauche OCH₂-CH₂O conformers observed in all samples are spectrally resolved from the sharp peak of crystalline anti OCH₂-CH₂O segments in PE-2,12 and PE-2,18. Layers of disordered and immobilized gauche OCH₂-CH₂O units at the crystal-amorphous interfaces are present in all samples. PE-2,12 and PE-2,18 additionally contain mobile amorphous and crystalline gauche OCH₂-CH₂O units. The unexpected crystalline gauche conformation deduced from the chemical shift and slow ¹³C spin-lattice relaxation was proved by fast decay in centerband-only detection of exchange (CODEX) NMR. The location of these gauche OCH₂ groups deep inside the crystallites was confirmed by ¹H spin diffusion from the amorphous layers. Crystalline gauche moieties, observed in three different samples of PE-2,12, account for about 1/3 of its crystalline OCH₂ groups. Based on quantitative NMR and spin diffusion, specific models of the layered supramolecular structures were developed, with gauche OCH₂ in interfacial layers at the crystal surfaces. While PE-2,18 and PE-2,12 contain two or three anti diol/diester layers within each crystallite, most OCH₂ groups in PE-2,48 are immobilized at the interfaces, and mobile gauche or crystalline anti OCH₂ units are insignificant. Thus, PE-2,48 contains all-polyethylene crystalline lamellae capped by diol/diester interfacial layers, indicating chemical control of the crystallite thickness.