Molecular dynamics simulation of the effect of irradiation and heat treatment time-sequence on the molecular structure of pre-oxidized polyacrylonitrile fibres

BackgroundPre-oxidation is a critical step in the fabrication of polyacrylonitrile-based (PAN) carbon fibres, during which the fibre undergoes cyclization, oxidation, dehydrogenation, and cross-linking reactions to form a thermally stable ladder structure. γ-irradiation has been demonstrated to promote pre-oxidation by generating free radicals; however, the mechanism by which the time-sequence of γ-irradiation and heat treatment affects this process remains unclear.PurposeThis study aims to elucidate the molecular-level mechanisms by which the time-sequence of γ-irradiation and heat treatment influences the pre-oxidation behavior of PAN fibres.MethodsFirstly, based on a ternary copolymer system, a PAN molecular bundle model (density 1.2 g·cm-3, 21 vol% O2) was constructed and subjected to energy minimization and relaxation under the number-volume-temperature (NVT) ensemble. Then, two pre-oxidation sequences were simulated using ReaxFF reactive molecular dynamics: γ-irradiation followed by heating (γ-T) and heating followed by γ-irradiation (T-γ), with irradiation modelled by primary knock-on atom (PKA) bombardment at 2.5 keV (equivalent to 100 kGy) and heat treatment applied via gradient heating from 443 K to 603 K. Finally, molecular chain morphology, species distribution, and reaction pathways were analyzed using LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) and OVITO (Open Visualization Tool), and the simulation results were validated by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR).ResultsThe validation results show that the γ-T sequence generates substantially more free radicals at an earlier stage (27 H atoms released during irradiation vs. only 1 in the T-γ heating stage), promoting cross-linking and oxidation; at the end of pre-oxidation, the γ-T system retains only 36 residual O2 molecules (vs. 71 in T-γ) and produces fewer small-chain fragments (C: 57 vs. 161), while generating more cross-linked products (C300‒313O>8 and C>313: 6 vs. 0). In contrast, insufficient free radical sites in the T-γ heating stage cause the PAN chains to fracture under subsequent irradiation, yielding a more disordered and complex product distribution.ConclusionsThe γ-T pre-oxidation sequence achieves a higher degree of cross-linking, oxidation, and structural stability than the T-γ sequence, providing theoretical guidance for optimising the irradiation–heat treatment time-sequence in industrial PAN fibre pre-oxidation to produce high-performance carbon fibres.