Chemical Reactions and Their Kinetics of atactic-Polyacrylonitrile As Revealed by Solid-State 13C NMR

Inter- and intramolecular chemical reactions and their kinetics for C-13-labeled atactic-polyacrylonitrile (aPAN) powder heat-treated at 220-290 degrees C under air and vacuum were investigated by various solid-state nuclear magnetic resonance (ssNMR) techniques. By applying C-13 direct polarization magic angle spinning (DPMAS) as well as through-bond and through space double quantum/single quantum ssNMR techniques, it was concluded that aPAN heat-treated under air at 290 degrees C for 300 min adopted the ladder formation, namely, conjugated six-membered aromatic rings with partially cross-linked and oxidized rings and polyene components. In contrast, aPAN heat-treated under vacuum at the same condition thermally decomposed into oligomeric chains that were mainly composed of isolated aromatic rings connected by alkyl segments. Furthermore, early stages of the chemical reactions were investigated by C-13 cross-polarization (CP) and DPMAS spectra. The latter provided quantitative information regarding the kinetics of the chemical reactions. As a result, it was shown that chemical reactions under oxygen occurred homogeneously with a higher activation energy (E-a) of 122 +/- 3 kJ/mol compared to that of vacuum at 47 +/- 2 kJ/mol. By comparing both chemical structures and kinetics under two different conditions, the chemical reaction mechanisms of aPAN will be discussed in detail.