Selective Observation and Quantification of Amorphous Trans Conformers in Doubly 13C-Labeled Poly(ethylene terephthalate), PET, by Zero-Quantum Magic-Angle-Spinning Solid-State NMR

Abstract

A new method for identifying and quantifying conformations of 13 C-labeled polymer segments in magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra is introduced. The observed magnetization is dephased by the difference of the recoupled anisotropic chemical shifts, reflecting relative orientation, of two sites with a significant 13 C- 13 C dipolar interaction. Signals of trans conformations decay more slowly than those near gauche or cis if the chemical-shift tensors in the trans conformation are related by inversion symmetry. This permits an estimate of the trans:gauche ratio. The experiments, which are based on zero-quantum (ZQ) or double-quantum/single-quantum (DQ-2SQ) dephasing under the recoupled chemical-shift anisotropy (CSA), yield selective MAS spectra of methylene groups in trans or gauche conformations. For the O-CH 2 -CH 2 -O moieties in amorphous and semicrystalline poly(ethylene terephthalate) (PET), the ZQ-CSA-dephasing and (DQ-2SQ)-CSA-dephasing methods enable selective observation of the amorphous trans components, which cannot be detected by traditional CP/ MAS experiments due to complete overlap with dominant gauche and crystalline trans signals. The observed amorphous trans chemical shift is close to that in PET crystallites; it cannot be rationalized based on the empirical γ-gauche effect. These experiments also provide signal decay curves from which torsion angles can be determined, with high angular resolution around the trans state. The data yield 12 ± 3% trans in amorphous PET, with a approximate 16° standard deviation of the torsion angles from the ideal trans state. In semicrystalline PET, the total trans fraction is 43 ± 3% and the best estimate of the crystalline-trans fraction, which corresponds to the degree of crystallinity, is 23 ± 5%; the amorphous trans fraction is 20 ± 5%. The trans fraction in the amorphous regions of semicrystalline PET, 26 ± 5%, is significantly higher than the 12 ± 3% in amorphous PET. The experiment is easily combined with double-quantum spectroscopy of isotropic chemical shifts, which provides further information on the isotropic chemical shifts of specific conformers. In amorphous PET, the two CH 2 groups in trans conformers are found to experience similar local fields.