Structural analysis of uniaxially oriented C-13-labelled poly(ethylene terephthalate) films studied with solid-state C-13 nuclear magnetic resonance spectroscopy

The structure of uniaxially oriented poly(ethylene terephthalate) (PET) with different draw ratios was studied in the solid state using C-13 nuclear magnetic resonance (n.m.r.) spectroscopy. All of the carbonyl carbons of the samples were isotopically labelled with C-13, and therefore a detailed C-13 n.m.r. analysis, including the determination of the chemical-shift tensor for the carbonyl carbons, was possible. The chemical-shift tensor direction relative to the molecular symmetry axis was determined with the aid of semiempirical quantum-chemical calculations by using the FPT-INDO (finite perturbation theory/intermediate neglect of differential overlap) method. The spectra of uniaxially oriented PET films observed by changing the angle between the draw direction and the magnetic field were analysed. Two components, one oriented and the other unoriented, were detected in the spectra of samples with draw ratios of 3 and 4. A highly oriented component was observed in addition to these two for samples having draw ratios of 5 (before and after heat treatment) and 5.66. The fraction of each component was determined by computer simulation of spectra that had been obtained as a function of sample orientation angle in the magnetic field. The angle theta between the phenylene para C-C axis and the chain axis was 16 degrees +/- 10 degrees and 29 degrees +/- 5 degrees for the oriented components, which is in agreement (within experimental errors) with the values reported previously by others. The angle-dependent spectra simulated with small p (characterizing the distribution of the fibre axis) confirm the presence of a highly oriented structure in the samples. Copyright (C) 1996 Elsevier Science Ltd.