Structural Identification of Polyacrylonitrile during Thermal Treatment by Selective 13C Labeling and Solid-State 13C NMR Spectroscopy

Four kinds of C-13-labeled polyacrylonitrile (PAN) samples were prepared respectively by solution polymerization of acrylonitrile (AN) with selective C-13 labeling of different molecular sites. The composition and structure of the residues from the thermal treatment of PAN in argon at 250 and 350 degrees C were quantitatively analyzed in detail by one-and two-dimensional solid-state C-13 nuclear magnetic resonance (ssNMR) experiments. Compared with the NMR spectrum of each labeled carbon in AN monomer unit, nine chemical structures created during the heat treatment process have been identified accurately. On this basis, four reaction routes were proposed. It is noted that the main chemical change for PAN started from a cyclization reaction at a relatively low temperature, then experienced an aromazation reaction to form a molecular chain basically composed of isolated pyridine units, instead of the commonly reported ladder structure. This work also shows that the combination of selectively C-13-labeled technique and a high spinning speed of 20 kHz in magic-angle spinning (MAS) NMR experiment could improve the detection sensitivity to nearly 2 orders of magnitude, and provide a clear ssNMR spectra with little peak overlaps, which will be helpful to discover the complex reaction mechanism in the manufacture of carbon fibers with high performance.