Reaction/Diffusion Competition Controls Anomalous Vitrimer Relaxation Processes

Crosslinked polymer networks with dynamic covalent bonds, vitrimers, can rearrange and relax without significant change in crosslink density. At high T, vitrimers exchange bonds to relax the network without disintegration. This could combine the recyclability of glasses and physical networks with the mechanical stability of permanently crosslinked networks. Vitrimer relaxation challenges our understanding of dynamic covalent networks: unlike classical physical networks, vitrimer relaxation rates are decoupled from segmental relaxation. By combining simulations, theory, and experiments we probe vitrimer dynamics and uncover two distinct regimes of behavior. At high T, crosslink reaction rates control relaxation while at low T, segmental diffusion rates dominate relaxation. This reaction/diffusion competition creates a crossover between Arrhenius and super-Arrhenius behavior in systems where bond exchange occurs via sub-α rattling motions. We define two types of transient networks based on the ratio of the atomic displacements required to exchange bonds to the atomic displacement required to relax a segment. These findings resolve the longstanding questions around vitrimer relaxation providing a foundation for rational vitrimer design.