Network Topology Alternation for Bond Exchangeable Cross-Linked Polymers: A Pathway for Solving Inhomogeneous Network Formation

For cross-linked polymers, network inhomogeneity has long been a persistent issue. In an inhomogeneous network, local stress concentrations under deformation can trigger macroscopic fracture. In this study, for bond exchangeable cross-linked polymers within the framework of covalent adaptable networks (CANs) and vitrimers, we demonstrate that bond exchange induced by high-temperature treatment enables a transition from an initially inhomogeneous network to a more homogeneous network, in terms of the spatial distribution of cross-link points and the presence of dense cross-link clusters. Furthermore, in the latter part of the study, we show that performing the curing reaction above the bond exchange activation temperature generates a network with suppressed inhomogeneity at the initial state. These findings are experimentally validated by cyclic temperature-sweep rheology, cyclic stress relaxation, and small-angle neutron scattering (SANS) measurements for network topology assessment, where the enhancement of mechanical toughness due to topology alteration is also demonstrated. Although bond exchangeable cross-linked polymers have largely been explored for sustainability, here we unveil an unexplored facet of the bond exchange concept.