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K. Schmidt-Rohr,* W. Hu, N. Zumbulyadis
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-^sup 13^CH^sub 2^-^sup 13^CH-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.
PET (Fig. 1) and related aromatic polyesters find widespread uses in tough, transparent packaging materials with good barrier properties, in polyester fibers, and in thin films for photographic or magnetic-tape applications, which represent multibillion dollar industries ( 1 ). The degree of crystallinity can vary from 0 to 50%, and noncrystalline glassy polyesters can be obtained by quenching from the melt, which produces transparent films. To understand details of the materials properties of amorphous and semicrystalline polyesters, such as glasstransition and melting temperatures, crystallization rates, maximum crystallinities, or gas-barrier properties, knowledge of their microscopic structure is required. Although the chemical and the crystal structures of PET are well known (Fig. 1) (2-4), the chain conformations in the amorphous state are poorly characterized. The torsion around the OC-CO single bond (Fig. 1) is the major degree of freedom; bond lengths and angles vary only slightly, and the sp^sup 2^hybridized units are expected to be mostly coplanar. In the probability distribution of the OC-CO torsion angle psi, maxima are expected near 180 deg, the trans conformation (5), and near +/-60 deg, the gauche conformation (2, 3, 5).
Many groups have attempted to estimate the positions and populations of the trans and gauche conformations for PET (2, 3, 6-9), which are important parameters in rotational-isomeric-state (RIS) models (2, 3). Calculations unconstrained by experimental data tend to produce relatively high trans populations (6, 7), which predict a larger extension of PET chains in solution than is observed experimentally (2, 3). Therefore, the choice of RIS parameters is usually guided by dipole moments or J couplings of low-molar-mass model compounds (2, 3, 6-10). Nevertheless, these procedures have failed to produce a...