Molecular Modeling of Stabilization during Processing of Polyacrylonitrile-Based Carbon Fibers

The chemical process of stabilization is a critical step in the fabrication of polyacrylonitrile (PAN)-based carbon fibers; it transforms the spun polymer precursor into a thermally stable ladder compound capable of undergoing the processes of carbonization and graphitization. The molecular structure of the stabilized polymer strongly influences the microstructure and, consequently, the properties of the resulting fiber. However, molecular models of the process of stabilization are lacking, and so is an understanding of the role of the molecular structure of the spun fiber. We developed a model that combines stochastic chemical reactions with molecular dynamics (MD) to simulate the process of stabilization of PAN in atomic detail; we describe dehydrogenation, activation, and cyclization. The rates of the various reactions are adjustable parameters and depend on the local environment of the active sites. We compared the stabilization of unstretched amorphous PAN and a sample that had undergone stretching and, thus, includes molecularly ordered and disordered regions. We find that the molecular alignment accomplished via spinning does not increase the amount of cyclization but favors intramolecular over intermolecular reactions and the number of contiguous rings.