Open Access Open Access  Restricted Access Subscription or Fee Access

MD-CF: A Molecular Model to Predict the Structure and Properties of PAN Based Carbon Fibers



Carbon fibers (CFs) are increasingly the reinforcement of choice for many highperformance composites, due to their high stiffness and strength, combined with their low density. However, while commercial fibers can approach the modulus of ideal graphite, high-strength fibers achieve no more than 10% of the ideal strength. Strength is limited by the presence of defects in the CF and thus, the development of highstrength fibers requires establishing relationships between processing, microstructure and properties. To contribute to the development of this knowledge base, we developed a molecular model, MD-CF, that uses a combination of kinetic Monte Carlo (kMC) and molecular dynamics (MD) to simulate the processes of carbonization and graphitization of PAN-based CFs; importantly, the molecular-level simulation predicts the development of microstructure during processing. The MD-CF method has been used previously to capture the cross-sectional microstructure and stiffness of carbon fibers and was shown to generate structures with experimentally observed features such as hairpins and graphitic sheets. In this paper, we extend this model to three dimensions to capture the fiber microstructure along the fiber axis, a key step towards the prediction of longitudinal modulus and strength. The model starts with ladder-like molecular chains that are broadly accepted to be the result of the stabilization of PAN precursors and uses kMC to describe the chemical reactions leading to the formation of graphitic sheets, and MD to describe the relaxation of the system during the process of cure. We find that the density of simulated structures ranges from 1.55 to 1.70 g/cc and is in excellent agreement with commercial PAN-based fibers. In addition, the predicted longitudinal moduli are in reasonable agreement with recent experiments. However, the transverse compressive moduli are an order of magnitude higher than experimental values, as a result of crosslinking in the transverse direction, preventing sliding between graphitic sheets.


Full Text: