(77b) Understanding Polymer Pyrolysis through Advances in Reactive Molecular Simulation

We have used our new reactive forcefield, RMDff, and Reactive Molecular Dynamics program, RxnMD, to simulate the backbone scission reactions in the decomposition of high-density polyethylene of various lengths. The results reveal the effect of chain size and entanglement on the backbone scission rate of high-density polyethylene. Current experimental techniques are not amenable to the investigation of polymer decomposition from a mechanistic or microscopic perspective. However, such detail is beneficial for the further advancement and development of fire-resistant polymers. Our code, RxnMD, couples a standard molecular dynamics routine that solves Newton’s classical equations of motion while having a reactive forcefield that can accurately describe chemical transitions along the reactive pathway. We have developed our RMDff forcefield to permit smooth transitions from reactant to product atom descriptions through smoothly transitioning switching functions. The simulations demonstrate the locations of polyethylene backbone scissions to be random, as seen in experiments. We have found that once the polymer becomes long enough, the activation energy for scission decreases with increasing polymer length. We believe that increased lateral interactions of the longer chains decrease chain mobility and allow the chain to break with lower activation energy.