Rotational motions in atactic poly(acrylonitrile) studied by one- and two-dimensional 15N solid-state NMR and dielectric measurements

The dynamics of atactic poly(acrylonitrile) (aPAN) have been investigated by variable-temperature N-15 chemical shift anisotropy (CSA), two-dimensional 2D N-15 CSA exchange, and 2D N-15 pure-exchange NMR experiments on an aPAN sample with 100% N-15-labeled nitrile side groups and by dielectric measurements. In contrast to the dynamics below 110 degreesC, where only small-angle motions are observed with an activation energy E-a = 68 kJ mol(-1), it is found that large-amplitude motions with Ea = 180 kJ mol(-1) set in above 110 degreesC. From the detailed analysis of the NMR spectra, it is found that aPAN undergoes restricted uniaxial rotational diffusion motion around the chain axis above 110 degreesC. The 2D spectra prove that there is a distribution of amplitudes of these restricted motions. The standard deviation of the amplitude distribution increases from 30degrees to 100degrees in the temperature range 120-140 degreesC. At the highest temperature, there is strong evidence of dynamic averaging within the distribution of angles between the CdropN bond and the fiber axis. The experimental findings indicate that the motion occurs in the disordered pseudohexagonal lattice and thus favor a single-phase model for aPAN. An alternative explanation by crankshaft motion can be ruled out mainly by the small gauche content of 10%, which is confirmed by variable-temperature C-13 CP/MAS experiments. The solid-state NMR results are in excellent agreement with those in dielectric relaxation experiments, where a relaxation peak of growing amplitude and frequency position is observed.