Higher dose rate effect of 500-keV EB irradiation favoring free radical annealing and pre-oxidation of polyacrylonitrile fibers

Abstract

In this study, free radicals in polyacrylonitrile (PAN) fibers induced by irradiation of 500-keV electron beam (EB) accelerator were investigated via electron spin resonance (ESR). Alkyl radicals and polyimine radicals were formed in PAN fibers after irradiation, and the relative content of the two free radicals changed with respect to the absorbed dose and dose rate. The effect of temperature on the relative content of the two radicals was investigated via analyzing the ESR spectra of alkyl radicals and polyimine radicals, and two parameters (PHR and δPHR) were set for evaluation. The value of δPHR was related to the temperature. The instantaneous temperature change in the PAN fibers during the irradiation process was inferred via analyzing the change in δPHR. Changes in the free radicals in irradiated PAN fibers during the stabilization process were also understood by ESR. Irradiation induced cross-linking and decreased the temperature of pre-oxidation for the PAN fibers. Increases in the dose rate of EB irradiation efficiently induced cross-linking and also led to pre-oxidation of the PAN fiber due to heat accumulation by EB irradiation. Radiation crosslinking significantly reduced the temperature required for peroxidation, and the trend was more evident at higher dose rates. For example, the PAN fiber irradiated to 400 kGy at a dose rate of 2 kGy/s was pre-oxidized at approximately 150 °C while the PAN fiber irradiated to 400 kGy at a dose rate of 13 kGy/s exhibited a darker color and was pre-oxidized at approximately 130 °C.