Exploring the Atomistic-Scale Formation and Evolution of Rings in PAN-Based Carbon Fibers Using a Reparameterized ReaxFF Approach

Carbon fiber (CF) is an ideal lightweight material to replace traditional engineering materials. For the high performance of CF, there is an urgent need to elucidate all reactions involved in carbon fiber from polymerization and preoxidation to carbonization. Alternatively, exploration of molecular dynamics (MD) simulations can bridge the gap by providing intuitive insights; however, it faces a lack of well-developed and validated reactive force field (ReaxFF) tailored for these reactions. Herein, an expanded CHONSi-2024 ReaxFF potential suitable for all reactions for carbon fiber preparation is first developed by training the charge distributions, bond formations, angle parameters, molecular structures, and energy landscapes. Its accuracy and reliability are further validated through comprehensive MD simulations of the CF formation processes. Several significant reactions are observed and analyzed in detail. During polymerization in the PAN-based models, the homopolymerization of acrylonitrile (AN) and copolymerization of AN with itaconic acid (IA) occur, in the way of head–head, head–tail, and head–tail connections involving cyano nitrogen atoms, in agreement with experimental observations. In the preoxidation stage, the reaction between polyacrylonitrile and oxygen is observed through a series of primary reactions, including continuous dehydrogenation of PAN backbone, the cleavage of cyanide groups, the formation of nitrogen-containing conjugated rings, oxidation of β-carbon atoms by oxygen, and partial carbon cyclization, ultimately leading to the formation of stable ladder structures. In the higher temperature carbonization of the PAN-based models, the elimination of various small molecular species and the nitrogen transformation mechanism─critical for promoting carbon cluster growth─are explored. This process drives the formation of the final turbostratic carbon structure. Ultimately, a carbon fiber structure characterized by five-, six-, and seven-membered carbon rings is obtained and characterized. The strong agreement between these simulation results and established experimental reaction pathways demonstrates that this newly developed force field enables rapid progress in studying the intricate chemistry of carbon fiber preparation and paves the way for the optimization of high-performance carbon fiber synthesis.