Selective Observation of Chemical Structures at Surface and Core Regions of Heat-treated Poly(Acrylonitrile) Films by Solid-State NMR Spectroscopy

Detailed chemical structures and kinetics of chemical reactions at the surface and core regions of 13C labeled atactic-poly(acrylonitrile) (aPAN) films heat-treated under air are for the first time investigated by solid-state (ss) nuclear magnetic resonance (NMR). The chemical structures of aPAN powders heat-treated under air and vacuum are also studied as reference data by ssNMR and electron spin resonance (ESR). The unpaired electron spin density is determined to be 1.43 × 1015 and 3.30 × 1014 spins/mg in the heat-treated powder under air and vacuum, respectively, by ESR. 1H spin–lattice relaxation time in the laboratory frame (T1H) is 167 and 572 ms for the former and latter, respectively. A large difference in the T1H values is attributed to differences of the unpaired electron densities. It is demonstrated that both 13C direct polarization magic-angle spinning (DPMAS) NMR spectral patterns and T1H values highly depend on the film thickness. Thickness dependence of apparently single T1H values can be ...