The stabilization process of Poly(acrylonitrile) (PAN) is one of the important steps to make industrially important carbon-fiber materials. The stabilization process takes several hours and thus consumes a lot of energy. It is known that various copolymers such as itaconic acid, acrylamide, and methacrylic acid lower down stabilization temperature and time of PAN and thus are major precursors in industrial application. However, detailed reactions mechanisms in the early stage are not well studied due to the sensitivity problem. In this study, advanced solid-state nuclear magnetic resonance (ssNMR) spectroscopy combined with 13C selective isotope labeling method is employed for the quantitative tracking of the structural evolution of PAN and poly(acrylonitrile-itaconic acid) (PAI) with 3% itaconic acid (IA) during the thermal-oxidative stabilization. Two-dimensional (2D) refocused 13C-13C INADEQUATE combined with 30 % 13C three-sites (C3) labeled PAN and PAI is used to successfully separate complex chemical reactions at different spectral positions. Consequently, it is found that cyclization and aromatization initially occur in PAI and is followed by oxidization and dehydrogenization while all reactions simultaneously occur in PAN. Moreover, single-site 13C CN (C-1) and CH2 (C-3) carbon labeled PAI and PAN could allow us to separately observe temperature effects on cyclization and stabilization process, respectively. It was demonstrated PAI sample gives significant increase in cyclization (CI) and stabilization indexes (SI) at the low temperatures compared to those in PAN. Understanding detailed chemical reactions will be useful for the study of the chemical reactions of industrial carbon fiber precursors.

Sequence order of complex chemical reactions in the early state of stabilization of poly(acrylonitrile) and its copolymer as studied by solid-state NMR spectroscopy

ACS Fall 2020 Virtual Meeting

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