Investigation of the chemical changes during thermal treatment of polyacrylonitrile and 15N-labelled polyacrylonitrile by means of in-situ FTIR and 15N NMR spectroscopy

Abstract

Fourier transform infrared (FTIR) spectroscopy is used to analyze in situ the temperature-dependent changes in homopolymer polyacrylonitrile (PAN) and 15N-labelled PAN during the thermal treatment between room temperature (RT) and 420 °C under a dinitrogen atmosphere. It is found that the nitrile peak at 2244 cm−1 (C≡N0) for the starting PAN disappears in favor of a new nitrile peak at 2199 cm−1 (C≡N1) which finally at higher temperatures is transformed into a yet unrecognized third nitrile peak at 2221 cm−1 (C≡N2). Additionally to the C≡N (nitrile) range, the C=C/C=N range and the C–H/N–H-range of the IR spectra are analyzed in detail, taking in particular into account for the first time the differences between 14N- and 15N-PAN and comparing the results for PAN with those of well-defined model compounds. Additionally, for the first time 15N NMR spectroscopy is performed on 15N-labelled model compounds as well as on 15N-labelled PAN and thermally treated 15N-labelled PAN. DFT calculations on relevant moieties are presented. Also, an investigation of the volatiles during the thermal treatment by Simultaneous Thermal Analysis (STA) is done providing for the first time quantitative data for the amounts of evolved ammonia and HCN. On the basis of these findings new aspects about the mechanism of the chemical transformations are explored.