A high molecular weight acrylonitrile copolymer prepared by mixed solvent polymerization: I. effect of monomer feed ratios on polymerization and stabilization

A bifunctional comonomer β-methylhydrogen itaconate (MHI) was employed to prepare high molecular weight poly[acrylonitrile-co-(β-methylhydrogen itaconate)] [P(AN-co-MHI)] copolymers by radical polymerization in a mixed solvent of dimethyl sulfoxide/deionized water = 60/40 (wt/wt), which was used as a carbon fiber precursor instead of acrylonitrile terpolymers for improving the stabilization and spinnability simultaneously. The structure and stabilization of P(AN-co-MHI) copolymers with different monomer feed ratios were characterized by elemental analysis (EA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that both the polymerization conversion and molecular weight reduce with the increase of MHI content in the feed due to the larger molecular volume of MHI. The monomer reactivity ratios were calculated by Fineman–Ross and Kelen–Tüdõs methods and the reactivity of MHI is higher than that of AN. Two parameters Es = A1618 cm−1/A2244 cm−1 and SI = (I0 − Is)/I0 were defined to evaluate the extent of stabilization and the activation energy (Ea) of the cyclization. The FTIR, XRD and DSC results show that P(AN-co-MHI) copolymers exhibit significantly improved stabilization characteristics than the PAN homopolymer and poly(acrlonitrile–methyl acrylate–acrylic acid) terpolymer, such as a larger extent of stabilization, lower initiation temperature and smaller Ea of cyclization, which is attributed to the ionic initiation of the MHI comonomer and is beneficial for preparing high performance carbon fiber.