Microscopic pyrolysis mechanism on the octyphenylsiloxane flame retarded polycarbonate by reactive molecular dynamics

Abstract Pyrolysis mechanisms and flame retardant properties of polycarbonate (PC) and PC/ octyphenylsiloxane (OPS) composites at different temperatures (2000–4000 K with an interval of 500 K) were studied using reactive force field (ReaxFF) molecular dynamics simulation. The initial decomposition mechanism, products distribution, and main products generation and consumption paths of PC and PC/OPS composites were analyzed. Results showed that decomposition of PC molecular chain was caused by the rupture of the C O bond, and the decomposition of OPS molecule was caused by the rupture of the C-Si bond. The breaking of partial C Si bond made OPS from cage structure into a lamellar-like or reticular structure, which has been proven this structure was a key factor to improve the flame retardance of PC by suppressing the transfer of heat, flammable volatile gaseous products. From the potential energy and decomposition time of PC and PC/OPS composites, the addition of OPS slowed down the decomposition rate of the PC molecular chain and improved the thermal stability of the composites. These have overall good agreement with the reported results. The amounts of CO2, CO, C6H5OH and their derivatives in PC/OPS composites were lower than those in PC at the later stage of reaction, which was because unsaturated Si atoms in OPS adsorbed small oxygen-containing molecules to suppress these harmful materials. Moreover, OPS promoted the combination of molecular products into carbon clusters and large thermally stable organic structures. Thus, the ReaxFF method could be effectively used to study the pyrolysis mechanism of PC/OPS composites, and provide a new method for the prediction and analysis of flame retardance.