A comparison of molecular dynamics chemical reaction heuristics applied to crosslink formation

Atomistic models of crosslinked polymers provide invaluable insights into the nanoscale properties otherwise inaccessible to experiments. These amorphous, highly interconnected structures can only be produced in simulations using artificially accelerated crosslinking reactions, completing within the nanosecond timescale available. Atomistic reaction modeling techniques density functional theory (DFT) and ReaxFF are often too expensive for the large systems required for polymer modeling, and cure reactions too slow for simulation timescales. Heuristics have been designed to identify reactive functionalities and instantaneously perform crosslinking reactions during classical molecular dynamics simulations. We compare the networks produced by two major approaches used in literature for a model epoxy-amine polymer of diglycidyl ether of bisphenol A (DGEBA) and m-xylylenediamine (MXDA). In one method, the reactants are modified into ‘activated’ intermediate forms, while the other performs all bond formations and cleavages of a reaction simultaneously. We find minor differences in kinetics during the cure, but fully crosslinked bulk polymers are indistinguishable. However, the presence of a solid surface during the cure can result in significant differences in interphase structures due to the activation of reactants. We conclude that while both methods may be used equally in producing bulk networks, activated reactants may not produce accurate structures at interfaces.