Synchrotron Microfocus Wide-Angle X-ray Scattering Reveals the Evolution of Radial Microstructural Heterogeneity in γ-Irradiated/Heated Polyacrylonitrile Fibers

In order to accurately understand the radial structural evolution of polyacrylonitrile (PAN) fibers in irradiation and heat treatment, trapezoidal scanning was performed on the fibers with synchrotron microfocus wide-angle X-ray scattering (μ-WAXS), and radial scanning with confocal Raman was used on the cross section of the fibers. The results indicate that PAN precursors and γ-irradiated/heated PAN fibers present a heterogeneous structure and share the same trend in terms of microstructure changes at different radial positions. The crystal size, crystallinity, and orientation of the (100) crystal plane in the skin layer of fibers are larger than that those in the core layer, and the d spacing of the (100) crystal plane in the skin layer of fibers is smaller than that present in the core layer. The difference is that γ-irradiated PAN fibers have no secondary crystallization in the core layer during heat treatment; this is because the strong penetration of γ irradiation can act on the core of the fibers. In addition, some tests including WAXS were employed to characterize the evolution of the crystalline structure, thermal properties, and chemical structure of the PAN fiber as a whole after irradiation and heat treatment. Combined with the analysis of regional structure evolution, the above results show that the irradiation treatment advances the inflection points of the crystal structures of PAN fibers in the skin and core layers during heat treatment, and the strong penetration of γ irradiation caused both to advance by a similar amount of time. The coupling of irradiation and heat treatment resulted in smaller differences between the skin and core layers of PAN preoxidized fibers in terms of d spacing, orientation, and graphitized structure.