Reaction/Diffusion Competition Drives Anomalous Relaxation of Vitrimers

Abstract: Vitrimers are an emerging class of crosslinked polymer networks in which dynamic covalent bonds can exchange and rearrange, without any significant change in population of bonded states.1 Because these bonds can rearrange at high temperatures without disintegration of the network, vitrimers could combine the mechanical stability of traditional permanently crosslinked networks with the recyclability of glasses and physical networks. However, unlike classical transient/physical networks, vitrimers can relax at a rate that is decoupled from the mobility of their polymer segments. This challenges our fundamental understanding of how dynamic covalent networks relax and deform. Here we combine simulations, theory, and experiments to show that vitrimer dynamics exhibit two very different regimes: (i) a high temperature regime controlled by the local bond-exchange reaction rate and (ii) a low temperature regime controlled by segmental diffusion. This fundamental difference from traditional physical networks explains why vitrimers can exhibit Arrhenius dynamics, providing a foundation for their rational design.