Molecular dynamics studying on welding behavior in thermosetting polymers due to bond exchange reactions

Covalent adaptable network (or dynamic covalent network) polymers can alternate their network topology through bond exchange reactions, whereby an active atom replaces an atom in an existing bond, resulting in a new bond and a new active atom. This process repeats itself numerous times in the network, bestowing interesting material behaviors, such as stress relaxation, surface welding, self-healing and recycling, upon the network. In this paper, molecular dynamics simulations are used to investigate surface welding due to bond exchange reactions in an epoxy system. Two epoxy networks are constructed and brought into contact, and bond exchange reactions are then activated. The trajectory of active atoms is tracked, which shows how the active atoms cross the interface. Based on simulation results, this work analyzes the influence of welding conditions, such as welding time, welding temperature, degree of polymerization and crosslinking density of original networks, on the mechanical properties of welded materials. These parameters determine the number of connected bonds on the interface, which consequently affects the welding performance. Eventually, with a sufficiently long welding time, the system can fully recover the same modulus and yielding stress as those of a fresh network. Finally, the active atoms' penetration depth, obtained from MD simulation, shows good agreement with the predictions of some existing theories.