Elucidation of the Chain Conformation in a Glassy Polyester, PET, by Two-Dimensional NMR

Abstract

The chain conformation of glassy poly(ethylene terephthalate) (PET) was characterized by two-dimensional double-quantum nuclear magnetic resonance (NMR). In amorphous carbon-13–labeled PET, the statistics of the O–¹³CH₂–¹³CH₂–O torsion angle were determined, on the basis of the distinct shapes of the two-dimensional NMR patterns of trans and gauche conformations. In crystalline PET, the trans content is 100 percent, but in the amorphous PET it is only 14 percent (±5 percent). An average gauche torsion angle of 70 degrees (±9 degrees) was obtained. Implications for materials properties of polyesters are discussed.