Molecular design and pre-oxidation mechanism of acrylonitrile copolymer used as carbon fiber precursor

In order to improve the pre-oxidation and spinnability of polyacrylonitrile (PAN) at the same time, a bifunctional comonomer 3-ammoniumcarboxylate-butenoic acid-methyl ester (ACBM) was designed and synthesized to prepare poly(acrylonitrile-co-3-ammoniumcarboxylate-butenoic acid-methyl ester) [P(AN-co-ACBM)] copolymers used as carbon fiber precursor instead of acrylonitrile terpolymers. The P(AN-co-ACBM) copolymers with different compositions were characterized by elemental analysis, Fourier transform infrared spectroscopy, X-ray diffraction and differential scanning calorimetry. Both the conversion of polymerization and molecular weight decease with the increasing of ACBM content in P(AN-co-ACBM) due to the larger molecular volume of ACBM than AN, and the monomer reactivity ratios study shows that ACBM possesses higher reactivity than AN. Two parameters \( {E}_s={A}_{1619c{m}^{-1}}/{A}_{2244c{m}^{-1}} \) and SI = (I ₀ − I _S )/I ₀ were defined to evaluate the extent of pre-oxidation, and the activation energy (E _a) of the cyclization was calculated by Kissinger and Ozawa methods. The results show that P(AN-co-ACBM) copolymers exhibit significantly improved pre-oxidation characteristics than PAN homopolymer, such as lower initiation temperature, larger extent of pre-oxidation and smaller E _a of cyclization, which is attributed to the initiation of ACBM through ionic mechanism. In addition, the rheological analysis shows that the spinnability of P(AN-co-ACBM) copolymer is better that that of PAN, which is beneficial to preparing high performance carbon fiber.