Epoxy Vitrimers: The Effect of Transesterification Reactions on the Network Structure

TLDR

Vitrimers are covalently crosslinked polymers that behave as conventional thermosets below Tg but can flow above a temperature Tv > Tg through bond‑exchange reactions, with epoxy vitrimers relying on transesterification to enable flow, thermoforming, recycling, self‑healing, and stress relaxation, while their network structure can evolve during high‑temperature use. The authors employed a fragment‑based statistical model to analyze how transesterification, acting as a disproportionation that converts two monoester fragments into a glycol and a diester, increases crosslink and pendant chain concentrations while preserving a sol fraction. The model provides an analytical solution for a diepoxide–dicarboxylic acid formulation and a numerical solution for a diepoxide–tricarboxylic acid system, demonstrating its applicability to polyfunctional monomers.

Abstract

Vitrimers are covalently crosslinked polymers that behave as conventional thermosets below the glass transition temperature (Tg) but can flow above a particular temperature, Tv > Tg, by bond exchange reactions. In epoxy vitrimers, transesterification reactions are responsible for their behavior at T > Tv that enables flow, thermoforming, recycling, self-healing and stress relaxation. A statistical analysis based on the fragment approach was performed to analyze the evolution of the network structure of epoxy vitrimers during transesterification reactions. An analytical solution was obtained for a formulation based on a diepoxide and a dicarboxylic acid. A numerical solution was derived for the reaction of a diepoxide with a tricarboxylic acid, as an example of the way to apply the model to polyfunctional monomers. As transesterification acts as a disproportionation reaction that converts two linear fragments (monoesters) into a terminal fragment (glycol) and a branching fragment (diester), its effect on network structure is to increase the concentration of crosslinks and pendant chains while leaving a sol fraction. Changes in the network structure of the epoxy vitrimer can take place after their synthesis, during their use at high temperatures, a fact that has to be considered in their technological applications.