Conformation and dynamics of atactic poly(acrylonitrile).: 1.: Trans/gauche ratio from double-quantum solid-state 13C NMR of the methylene groups

The trans/gauche ratio of backbone bonds in atactic poly(acrylonitrile) (aPAN) was characterized by several two-dimensional solid-state NMR experiments, using 15% (CH2)-C-13-carbon labeled aPAN. An estimate of the conformational ratio was obtained from a double-quantum filtered NMR experiment, which correlates the C-13-C-13 internuclear vector of two adjacent (CH2)-C-13 groups with their C-13 chemical shift anisotropies. The orientation of the chemical shift tensor was determined in auxiliary experiments. The backbone conformation of two successive bonds was characterized by double-quantum NMR spectroscopy, which correlates the orientation-dependent C-13 chemical shifts of two adjacent (CH2)-C-13 segments. The experiments were performed with and without C-13-C-13 dipolar decoupling. From the three two-dimensional NMR spectra that were obtained, the trans/gauche ratio of aPAN was determined as 90%:10% (+/-10%). This value is compatible with the previously reported experimental density and wide-angle X-ray diffraction patterns of aPAN. The high trans content is also consistent with the single CH2 resonance line observed under magic-angle spinning.