Combinatorial Entropy Controls Novel Viscoelasticity in Model Vitrimers

Vitrimers are associative covalent adaptive networks whose viscoelastic behavior is characterized by two transitions: a glass transition and a topological transition. Models elucidate the theoretical basis using coarse-grained simulations with approximate bond-swap reaction kinetics, and calculations of mobility, viscoelasticity, and creep. We numerically investigate the vitrimer's transition of time-temperature dependence with different reactive monomer distributions and different crosslink densities. Results show that the high sensitivity of the topology-like transition stems primarily from the combinatorial entropy caused by crosslink density and crosslink asymmetry rather than defects induced by intra-molecular crosslinks. Further, the bond swap reaction barrier and this entropic effect, which control the reaction rate together, drive the network toward two opposite limiting states—permanent crosslinking and no crosslinking. Across a wide temperature range, we observe multiple crossover regimes, which further prompts us to consider the determination and definition of the topological transition temperature. To stay rigorous, we present time–temperature crossover landscapes instead of declaring one transition point. Our results highlight crosslink asymmetry as a crucial factor for reprocessable networks, offering new avenues to control viscoelastic properties in vitrimer-like materials.