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% 13 CH 2 -carbon labeled aPAN. An estimate of the conformational ratio was obtained from a double-quantum filtered NMR experiment, which correlates the 13 C− 13 C internuclear vector of two adjacent 13 CH 2 groups with their 13 C 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 13 C chemical shifts of two adjacent 13 CH 2 segments. The experiments were performed with and without 13 C− 13 C 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 CH 2 resonance line observed under magic-angle spinning.