The Crosslinking Structures Of PAN Fibers During Stabilization

The stabilization of polyacrylonitrile(PAN) fibers is a crux to produce carbon fibers, during which the linear molecules gradually transformed into trapezoidal and crosslinking structures through thermal oxidation or other means. Gamma irradiation is a supplementary method, which not only cyclizes the nitrile groups, but also crosslinks the molecules. However, the properties and forming mechanism of the crosslinking structures and whether side reactions like molecular scission took place during irradiation are still not clear.In this paper, the PAN fibers, films and dilute solution were irradiated. Crosslinking structures of PAN fibers after gamma ray irradiation were verified by gel fractions(the insoluble substance of PAN fibers in 75 wt.% H2SO4/H2O). The structures of that were conjectured through Fourier Transform Infrared Spectroscopy(FTIR) and Ultraviolet /visible spectra. Besides, the thermal reaction properties were studied by Differential Scanning Calorimetry(DSC), in the last, the origin of crosslinking structures and thermal reactions of irradiated PAN fibers were deduced via. Electron Paramagnetic Resonance(EPR) spectra.During irradiation, partial chemical bonds in PAN molecules broke and generated many radicals. These radicals combined with each other and formed crosslinked bonds, at the same time, some backbones broke which resulted in molecular scission. If the samples exposed in oxygen containing atmosphere there radicals would combine with oxygen and form ether and carbonyl groups. The crosslinking structures resulted from irradiation in air conclude the crosslinking bonds formed by combination of nitrile groups and oxygen bridges which came from oxidative condensation. In the same time, oxygen inhibited the crosslinking of nitrile groups. It is surprising that the nitrile crosslinking bonds were reversible, in the further stabilization these crosslinking bonds would break and generate polyimine radicals, then initiated cyclization. This surprising result makes us wonder whether it would form nitrile crosslinking bonds during thermal treatment. Therefore, in this paper, the PAN fibers heat treated in vacuum was investigated. It is found that nitrile crosslinking bonds generated at 170 ℃, increased and peaked 190 ℃, nearly 92%. At 230 ℃, the fibers could be dissolved again, which indicated that the crosslinking bonds disappeared. In oxygen containing atmosphere, nitrile crosslinking was inhibited, but more stable crosslinking structures were formed at higher temperature. The initial temperature of cyclization of PAN fibers heat treated in vacuum decreased, and exothermic peaks broadened, which indicated that the nitrile crosslinking bonds would break into polyimine radicals and initiated cyclization. Thus, these two kinds of crosslinking bonds in PAN fibers are similar, deriving from heat treatment and gamma irradiation.Understanding the structures of the crosslinking bonds of PAN fibers and atmosphere affecting on that would do good to optimize process conditions of stabilization, and improve orientation of fibers and minimize flaws. Thereby, the mechanical properties of carbon fibers would be improved.