Defect evolution and molecular transformation in the crystalline region of polyacrylonitrile fibers during irradiation-heat pre-oxidation: integrated simulation and experimental study

Defects generated during the pre-oxidation of polyacrylonitrile (PAN)-based carbon fibers can significantly affect the ultimate quality of the resultant carbon fibers. Introducing irradiation in the pre-oxidation process has emerged as a potential approach to optimize the defect structure of polyacrylonitrile fibers. However, the evolution of defects and molecular transformation under irradiation-heat pre-oxidation remains unclear at the molecular-scale. This study employs molecular dynamics simulations alongside experimental characterization of polyacrylonitrile fibers to investigate the formation and evolution of defects in a crystalline region of a polyacrylonitrile model subjected to irradiation followed by heat treatment. Under constant dose rate conditions, the simulation results revealed that increasing irradiation energy induces the transition from vacancy defects to void defects in polyacrylonitrile crystalline domains. Defects caused by free-volume fluctuations are positively correlated with irradiation energy. The molecular species increased with rising irradiation energy, resulting in more severe morphological damage to the polyacrylonitrile model. Irradiation at 7.5 keV induced maximum chain scission in the crystalline regions of polyacrylonitrile, exerting a detrimental effect on heat treatment outcomes. Conversely, 10 keV irradiation generated the most extensive area of chain cross-linking. The study proposes that irradiation energy around 7.5 keV serves as a critical threshold for inducing maximum defect formation in PAN crystalline molecular models.