Correlating the thermomechanical properties of a novel bio-based epoxy vitrimer with its crosslink density

Vitrimers have covalent crosslink structures as normal thermoset polymers, but they possess unique properties of being able to go through plastic deformation due to their crosslinks being dynamically exchangeable above a particular temperature. This new type of polymers is getting increased attention owing to their reprocessing, remolding, and recycling properties, along with the thermoset characteristics. As a thermoset, the materials' degree of crosslinking, namely crosslink density, is essential to their physical and mechanical properties. Herein, a series of dynamic epoxy crosslinking networks with different crosslink densities were prepared by changing the ratio of two epoxies N,N-diglycidyl-4-glycidyloxyaniline, and butanediol diglycidyl ether but keeping the same stoichiometric ratios when cured with a hardener synthesized from bio-based lignin-derived vanillin. The reaction enthalpy (Delta H) and glass transition temperature (Tg) were shown to decrease with decreasing crosslink density. The dynamic behavior of the resins was attributed to the dynamically exchangeable imine bonds brought in by the curing agent. The dynamic properties under various temperatures were analyzed to determine the activation energy for bond exchange (Ea) and relaxation times (tau). The results revealed that the resin with the highest crosslink density showed the highest tau as well as the highest Ea. The resins with higher crosslink density showed higher mechanical and thermal properties but lower strength retention efficiency (SRE). This study provides important insights into understanding the properties of this bio-based epoxy vitrimer.