Revealing the bond exchange and network rearrangement mechanism in vitrimers.

Vitrimers combine thermosets stability with thermoplastic reprocessability, yet their rational design is hindered by an insufficient understanding of their viscoelasticity. We investigated the fundamental relationships among segmental dynamics, bond exchange kinetics, and network rearrangements in model poly(propylene glycol)-based vitrimers. Our results show that the bond rearrangement time (τr) and its temperature dependence remain nearly unchanged at high temperatures, where their segmental relaxation time (τα) differs by several orders. This indicates that rate-limited bond exchange kinetics controls high-temperature bond rearrangements largely independent of segmental dynamics. We further demonstrated that τr can exhibit either a stronger or weaker temperature dependence than τα, depending on the observed temperature regime, addressing the long-standing puzzling of the relationship between these two processes. The observed results support the recently proposed framework where τr can be described as a two-step process, one controlled by bond exchange and another by segmental relaxation. However, simple additive description of characteristic timescales fails to provide consistent quantitative description.