Structural Relaxation, Bond Exchange Dynamics, and Glass Transition in Vitrimers

Vitrimers are a class of polymer networks featuring dynamic crosslinks that can undergo bond exchange. There has been recent interest in these materials due to their promise as recyclable thermosets or self-healing polymers due to the ability of vitrimer networks to rearrange at the molecular level and undergo macroscopic flow. However, the practical use of these materials often occurs in the supercooled regime or glassy state, where the implications of dynamic bonds are complicated by the interplay between slow activated segmental dynamics, crosslink (i.e. bond-exchange) kinetics, and ultimately material properties. We combine coarse-grained molecular dynamics simulation with microscopic statistical mechanical theory to understand how crosslinking kinetics affects material dynamics, and how this couples to segmental relaxation of the polymeric network strands across a spectrum of length and time scales. We characterize the Kuhn segmental alpha relaxation time and bond exchange time for vitrimer systems across various crosslink densities, temperatures, and bond exchange rates. Simulation and theory both exhibit a bending-up behavior for bond exchange time upon cooling, suggesting a coupling between bond exchange dynamics and segmental relaxation. This effect is most prominent when the bond exchange time is similar to the Kuhn segment alpha relaxation time, and the resulting coupling of these two relaxation processes is tied to both the probability of a free end to find a bonded pair and the timescale of the constraints imposed by the dynamic crosslinks.