Evolution of molecular structure within radial partitions of polyacrylonitrile fibres in irradiation-heat pre-oxidation: a molecular dynamics study

Pre-oxidation is a crucial step in the preparation of polyacrylonitrile (PAN)-based carbon fibres, but the process suffers from severe fibre radial structural heterogeneity, which adversely affects the overall performance of the subsequently generated carbon fibres. However, research on the mechanisms that govern the molecular-scale evolution of the radially heterogeneous structure of PAN fibres during pre-oxidation remains unreported. Therefore, in this paper, a molecular dynamics approach is used to simulate the radial distribution of PAN fibres and to validate it with known experimental results to explore the effect of irradiation and heat treatment on the structural evolution of different regions of the PAN fibres radial direction during the pre-oxidation process. The results indicated that irradiation led to the generation of a significant number of free radicals within the PAN molecular chain, thereby facilitating the cyclisation and cross-linking of the PAN fibre polymer. The mechanism of oxidative cyclisation varied across different regions of oxygen content during the heating phase, and itaconic acid and methyl acrylate were effective in mitigating the structural heterogeneity resulting from the uneven radial distribution of oxygen. The ReacNetGenerator tool was employed for the first time to analyse the atomic migration paths during the pre-oxidation of PAN fibres. The results indicated that the pre-oxidation process involves a series of reactions, including cyclisation, oxidation, cross-linking, and pyrolytic chain-breaking. Furthermore, the frequency of these reactions is influenced by the radial oxygen concentration within the PAN fibres, which is an important reason for the heterogeneous structure of PAN fibres. This study demonstrates possible processes in the pre-oxidation stage that should be taken into account when changing the conditions for the preparation of carbon fibres.