Free radical evolution and decay of PAN nano-fibers formed by irradiation and thermal stabilization

•Free radicals of PAN nano-fibers were stable in vacuum but decayed fast in air.•The polyimine radicals show higher stability at room and elevated temperatures.•Radiation oxidation occurred throughout PAN nano-fibers due to smaller diameter.•Radiation oxidation facilitated dehydrogenation to drive the stabilization process. Radiation technique was found to accelerate the stabilization process of PAN nano-fibers in the production of carbon nano-fibers. The evolution and decay of the free radicals of electrospun PAN nano-fibers induced by gamma-rays irradiation were investigated using electron spin resonance. Mainly alkyl and polyimine radicals were formed and the total free radical concentration reached saturation of 1.3×1019spins/g at the dose of 200 kGy in vacuum. The free radicals were very stable with a half-life period longer than 40 days in vacuum but rapidly decayed in air. PAN nano-fibers were subject to radiation oxidation due to their small diameter when irradiated in air. The polyimine radicals were found to be more stable than alkyl radicals at room and elevated temperatures. Mainly polyene radicals were formed resulting from dehydrogenation during the stabilization process and the irradiated sample always contained higher radical concentration. The radiation oxidation facilitated the dehydrogenation to drive the thermal stabilization process.