(451e) Polymer Decomposition Mechanisms for Polyethylene Determined From Reactive Molecular Dynamics

Understanding molecular-level information is key to understanding the underlying chemistry and physics of condensed-phase pyrolysis. However, current experimental techniques can only evaluate macroscopic properties.  The insight gained from individual rates constants including the effects of degree of polymerization is important for use in population balance and kinetic models that extend the predictions to full-scale product distributions.  Reactive Molecular Dynamics (RMD) is a tool that probes individual kinetic events through the natural evolution of temporal dynamics.  The simulations were conducted using RxnMD.  The reactions are represented using RMDff, which is a valence-bond description which uses switching functions to smoothly represent the atomistic changes that occur during a chemical reaction.  Simulations were conducted using linear polyethylene to determine the effects of coupled backbone scission and beta-scission reactions on polymer decomposition.  The effects of beta-scission reactions are considered with and without energy redistribution after the initial scission event.  The polymer decomposition as determined from RMD simulations reveal that there is competition between scission and beta-scission reactions.  Using the individual rate constants determined from RMD, comparisons are made between the experimental mass loss data and that obtained using a kinetic Monte Carlo analysis with the RMD kinetics.  While the calculations do not exactly match the data, the calculations reveal that the individual rates are a reasonable approximation.