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

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