Radiation assisted pre-oxidation of polyacrylonitrile fiber: Graphite formation and lower crystal size revealed by 2D WAXD at a synchrotron facility

Abstract

To accurately understand how irradiation treatment influences the crystal structure variation of polyacrylonitrile (PAN) fibers during the pre-oxidation process, the crystal structure evolution of the irradiated PAN fibers after pre-oxidation was investigated by synchrotron wide-angle X-ray diffraction (WAXD) at beamline BL16B1 in transmission geometry, using a three-slit system at the Shanghai Synchrotron Radiation Facility. By employing highly sensitive 2D WAXD analysis, we observed the deformation of the orientation structure in the irradiated PAN fibers. It was found that irradiation treatment can reduce the quasicrystal structure formed in the early stage of the pre-oxidation process (200–220 °C). Irradiation further enhanced the decrease in crystal size, crystallinity, and preferred orientation during the pre-oxidation. The diffraction peaks for PAN fibers at q = 12 nm−1 and 20.7 nm−1 almost disappeared at 260 °C; meanwhile, a new peak appeared for the irradiated samples at q = 18.2 nm−1, indicating the formation of a graphite structure, which confirmed by Raman spectra. The results showed that irradiation treatment can accelerate the pre-oxidation reaction of PAN fiber, reduce the time required for pre-oxidation, and improve the properties of carbon fiber by reducing the crystal size and crystallinity.