Kinetics of cyclization reaction in poly(acrylonitrile/methyl acrylate/dimethyl itaconate) copolymer determined by a thermal analysis

The PAN/MA/DMI (poly(acrylonitrile/methyl acrylate/dimethyl itaconate)) copolymer was synthesized for being used as a carbon fiber precursor. Its high melt-spinnability at 175-210 °C has been demonstrated elsewhere but the cyclization kinetics was thoroughly investigated by DSC analysis herein. The isothermal analysis based on the Kamal equation demonstrates that at a given temperature, the cyclization rate constant k of PAN/MA/DMI system is five to seven times that of a regular PAN/MA system. The activation energy Ea of the PAN/MA/DMI system, owing to the acid-catalysis by the DMI compound, is approximately 30 kJ/mol lower than that of a regular PAN/MA system. An isothermal Avrami analysis confirmed the above finding. Furthermore, based on the obtained nucleation index n, DMI may induce intermolecular cyclization which can rapidly stabilize the fiber shape of PAN/MA/DMI precursor. A nonisothermal analysis, obtained from the Ozawa's method, revealed the same tendency as did the isothermal results, but the difference between the average activation energies Ea obtained with and without DMI was reduced from 30 to 25 kJ/mol. Finally, a cyclization mechanism that involves the formation of a cyclic anhydride was proposed.