Rheology of the Critical Transition State of an Epoxy Vitrimer

Vitrimer network polymers are very special in that, above a characteristic temperature T v, their dynamic covalent network connectivity allows macroscopic flow while remaining cross-linked. The vitrimer is able to change from being a typical elastomer to a network polymer with thermoreversible bond-exchange reactions. This has aroused enthusiastic interest, especially for designing novel materials to meet the increasing requirements of environmental protection, self-healing, and sustainable development. In this work, the viscoelasticity of the curing and cured epoxy vitrimer was investigated. Time-resolved multifrequency rheometry during the early stages of curing disclosed network ripening due to transesterification. After being fully cured, one of the most defining properties of vitrimers is a reversible, temperature-induced transition at T v, where the vitrimer assumes a pronounced critical state. Surprisingly, this transition state was found to behave like a critical gel having a relaxation modulus, G ( t ) = St – n, over a fairly broad temperature interval of 20 K. Viscoelastic moduli G ′(ω, T ) and G ″(ω, T ) can be approximately shifted into a single set of master curves for the entire experimental temperature range. Large strain behavior above T v becomes nonlinear very quickly. No resemblance was found with a glass transition near T v .