Abstract
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 (13)C-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 OCH(2)-CH(2)O conformers observed in all samples are spectrally resolved from the sharp peak of crystalline anti OCH(2)-CH(2)O segments in PE-2,12 and PE-2,18. Layers of disordered and immobilized gauche OCH(2)-CH(2)O 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 OCH(2)-CH(2)O units. The unexpected crystalline gauche conformation deduced from the chemical shift and slow (13)C spin-lattice relaxation was proved by fast decay in centerband-only detection of exchange (CODEX) NMR. The location of these gauche OCH(2) groups deep inside the crystallites was confirmed by (1)H 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 OCH(2) groups. Based on quantitative NMR and spin diffusion, specific models of the layered supramolecular structures were developed, with gauche OCH(2) 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 OCH(2) groups in PE-2,48 are immobilized at the interfaces, and mobile gauche or crystalline anti OCH(2) 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.