The Crucial Role in Controlling the Dynamic Properties of Polyester-Based Epoxy Vitrimers: The Density of Exchangeable Ester Bonds (nu)

Vitrimer materials that obscure the line between "thermoset" and "thermoplastic" have already attracted a great deal of interest since they exhibit dynamic properties of stress relaxation or reprocessability without losing their permanently cross-linked networks. While much research has been aimed at developing new exchangeable dynamic bonds to broaden the scope, there is a dearth of effort in exploring the key factors that influence the dynamic properties of vitrimer materials. These explorations are not only useful in the exact design and synthesis of vitrimer materials but also quite important in their theory establishment and computer simulation. However, only a few ways including catalyst control have been confirmed to be the key factors in controlling the dynamic properties of classical vitrimer materials like the polyester-based epoxy vitrimer. Here, we proposed that the density of exchangeable ester bonds (nu) in networks also has a crucial role in adjusting the dynamic properties of epoxy vitrimers. Four polyester-based epoxy vitrimers were synthesized from the tricarboxylic acid curing agent and different epoxy monomers, resulting in various nu's. These epoxy vitrimers were named "E-202-vitrimer", "E-380-vitrimer", "E-500-vitrimer", and "E-640-vitrimer" according to the different molecular weights of the epoxy monomer. It was revealed that the dynamic properties of these vitrimer materials varied with the nu value. From the E-640-vitrimer to E-202-vitrimer, the stress relaxation behavior sped up with the rise in nu and the characterized relaxation times (tau's) obviously decreased. Meanwhile, the activation energy (E-a) and the Arrhenius prefactor (tau(0)), which are two important intrinsic parameters in evaluating the dynamic properties of vitrimers, exhibited heavy dependence on nu, while linear models were applied to describe their relationships. Moreover, the tau at various temperatures could be also predicted from a linear model that relied on the nu in polyester vitrimers. These results indicated that nu has a crucial role in controlling the dynamic properties of polyester-based epoxy vitrimers and the value of nu could be applied to calculate the tau , E-a, and tau(0). Besides, the influence of nu on the dynamic properties of polyester-based epoxy vitrimers and the resulted models might be workable in other vitrimer materials, which can be utilized to design desired vitrimer materials for various applications.