Topology annealing effects: Meso-scale analysis of bond exchangeable network

   Incorporating dynamic covalent bonds into cross-linked network polymers has attracted significant attention as an effective approach to impart sustainable functionalities. These materials are referred to as covalent adaptable networks (CANs) or vitrimers, depending on the bond exchange mechanism. Rapid advances have expanded the chemical library of such bond exchangeable materials and enabled new functionalities.

   As these functional materials move toward practical applications, several issues require assessment. One is the potential alteration of the network’s topological structure during bond exchange. In most cross-linked materials, the initial network forms under kinetic control, which often results in network inhomogeneity. Bond exchange could induce changes in topological features, leading to altered physical properties—thermal, mechanical, and relaxation behaviors. Topological changes following bond exchange have received limited attention in the past.

   In this study, we thus attempt to extract effects of “network topological annealing” for bond exchangeable materials. Specifically, curing is performed via UV-triggered reactions at low temperature, during which the sample undergoes vitrification. This approach yields an inhomogeneous network with respect to cross-link-density distribution. During post-curing thermal treatment at the bond exchange state, the initial inhomogeneous network evolves toward a more uniform state, modifying rheological properties and stress-relaxation behavior. Importantly, topological alteration is evidenced by small-angle neutron scattering analysis. The resulting reduction in network inhomogeneity could enable the development of tougher cross-linked resins, with potential advantages for structural materials, coatings, and adhesives, thereby broadening the applications of the bond exchange concept.