The Effect of Mixed-Mechanism Bond Exchange on Dynamic Network Rearrangement

Covalent adaptive networks (CAN) are characterized by their thermally responsive topologies and self-healing properties due to the presence of dynamic bonds that serve as crosslinks. CANs can be classified into dissociative or associative networks, depending on how the dynamic bonding mechanism leads to topological rearrangement; dissociative crosslinks break and re-form, and associative crosslinks undergo bond exchange. The latter leads to more robust materials, but the former is easier for reprocessing. Mixed-mechanism networks have the potential to possess both characteristics, in particular, in the situation that dissociated bonds generate free reactive sites that can also participate associative reactions. In this study, we use coarse-grained simulation to understand the relationship between bond energies and binding barriers for mixed associative-dissociative CANs. The presence of free reactive sites governs the bond lifetime of dynamic covalent bonds, indicating that the relaxation dynamics of CANs can be tailored through precise molecular design. By analyzing the diffusion coefficient of free reactive sites, we can gain deeper insight into the relationship between the number of free reactive sites and the facilitation of bond exchange. Furthermore, the mechanical properties of mixed associative-dissociative CANs can be elucidated by examining their stress relaxation behavior, which reveals how the dynamic exchange of bonds contributes to overall material performance. This study aims to provide molecular-level insights into the dynamic bond exchange and cross-link rearrangement by examining the bond lifetime and the dynamics of free reactive sites, enabling the rational design of CANs with tunable properties for future applications.