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
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
OCH
-CH
O conformers observed in all samples are spectrally resolved from the sharp peak of crystalline
OCH
-CH
O segments in PE-2,12 and PE-2,18. Layers of disordered and immobilized
OCH
-CH
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
OCH
-CH
O units. The unexpected crystalline
conformation deduced from the chemical shift and slow
C spin-lattice relaxation was proved by fast decay in centerband-only detection of exchange (CODEX) NMR. The location of these
OCH
groups deep inside the crystallites was confirmed by
H spin diffusion from the amorphous layers. Crystalline
moieties, observed in three different samples of PE-2,12, account for about 1/3 of its crystalline OCH
groups. Based on quantitative NMR and spin diffusion, specific models of the layered supramolecular structures were developed, with
OCH
in interfacial layers at the crystal surfaces. While PE-2,18 and PE-2,12 contain two or three
diol/diester layers within each crystallite, most OCH
groups in PE-2,48 are immobilized at the interfaces, and mobile
or crystalline
OCH
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.