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

摘要

Carbon fibers (CFs) are increasingly the reinforcement of choice for many highperformancecomposites, due to their high stiffness and strength, combined with theirlow density. However, while commercial fibers can approach the modulus of idealgraphite, high-strength fibers achieve no more than 10% of the ideal strength. Strengthis limited by the presence of defects in the CF and thus, the development of highstrengthfibers requires establishing relationships between processing, microstructureand properties. To contribute to the development of this knowledge base, wedeveloped a molecular model, MD-CF, that uses a combination of kinetic MonteCarlo (kMC) and molecular dynamics (MD) to simulate the processes of carbonizationand graphitization of PAN-based CFs; importantly, the molecular-level simulationpredicts the development of microstructure during processing.The MD-CF method has been used previously to capture the cross-sectionalmicrostructure and stiffness of carbon fibers and was shown to generate structureswith experimentally observed features such as hairpins and graphitic sheets. In thispaper, we extend this model to three dimensions to capture the fiber microstructurealong the fiber axis, a key step towards the prediction of longitudinal modulus andstrength. The model starts with ladder-like molecular chains that are broadly acceptedto be the result of the stabilization of PAN precursors and uses kMC to describe thechemical reactions leading to the formation of graphitic sheets, and MD to describethe relaxation of the system during the process of cure. We find that the density ofsimulated structures ranges from 1.55 to 1.70 g/cc and is in excellent agreement withcommercial PAN-based fibers. In addition, the predicted longitudinal moduli are inreasonable agreement with recent experiments. However, the transverse compressivemoduli are an order of magnitude higher than experimental values, as a result ofcrosslinking in the transverse direction, preventing sliding between graphitic sheets.