Chemical reactions in poly(acrylonitrile-co-itaconic acid) during stabilization as revealed by solid-state NMR spectroscopy and 13C isotope labeling

Abstract

Understanding and controlling chemical reactions of atactic-Poly(acrylonitrile) (PAN) and its copolymer during thermal heat-treatment under air are critical subjects to produce PAN-based carbon-fibers. Copolymerization of PAN with itaconic acid (IA) monomers effectively lowers initial chemical reaction temperature and thus promote chemical reactions during stabilization. In this work, detailed chemical structure of PAN and poly(acrylonitrile-co-itaconic acid) (PAI) stabilized under air atmosphere were studied by combined use of solid-state (ss) NMR spectroscopy and 13C isotope labeling. By applying 13C–13C INADEQUATE and fully relaxed 13C DPMAS NMR spectra, it was elucidated that 2% cyclized structure can be initially formed in PAI during stabilization. Moreover, five elemental reactions including cyclization, aromatization, oxidization, dehydrogenation, and intermolecular cross-linking were identified and corresponding structures were quantitatively analyzed in both PAN and PAI as a function of stabilization temperature. It was revealed that all the reactions concertedly occur in PAN in the narrow temperature range (290–340 °C). For PAI, it was demonstrated that cyclization and aromatization are dominant in the low temperature range (180–270 °C, regime I), dehydrogenation and intermolecular cross linking are promoted in the high temperature range (270–340 °C, regime II), and oxidization occurs across two regimes. The sensitivity enhanced ssNMR study not only provides fundamental understanding of the chemical reactions in PAN and PAI during stabilization but also guide ones to potentially control the stabilized structures from one dimensional structure to two dimensional one for PAI by choosing a proper temperature and time.