Reaction/Diffusion Competition Drives Anomalous Relaxation of Vitrimers

Since their discovery in 2011, vitrimers - covalent associative network polymers - have challenged the traditional understanding of soft matter relaxation dynamics: unlike in typical glass-forming liquids, vitrimers' viscous relaxation can be entirely decoupled from their underlying structural (segmental) dynamics. Beyond this fundamental mystery, the origin of vitrimers' Arrhenius viscosity in the presence of super-Arrhenius structural relaxation behavior has been of high interest due to vitrimers' potential to provide readily reprocessable high-performance plastics. Here, we combine simulations, theory, and experiments to establish a foundational understanding of vitrimer relaxation dynamics. We identify two types of transient networks based on the ratio of atomic displacement scales required for bond exchange to those required to relax a segment. In systems where bond exchange only requires sub-segmental motion, we show that network relaxation is governed by a competition between chemical exchange reactions and segmental diffusion. This competition produces vitrimers' signature network/segment decoupling, while also driving a crossover between Arrhenius and super-Arrhenius behavior that is observed for many vitrimers. This work provides an explanation for longstanding puzzling features of vitrimer dynamics and establishes a foundation for rational vitrimer design.