Chitosan-Enhanced Vitrimerization of Thermoset Epoxy Resins: Multiple-Dynamic Networks for Sustainable Approaches, Improved Mechanical Properties, and Rapid Stress Relaxation

This study explores the use of chitosan as a sustainable alternative in the vitrimerization of thermoset epoxy resins. Epoxy/chitosan vitrimer composites, with and without an external catalyst (zinc acetate), as well as epoxy/zinc acetate vitrimers as reference samples, were successfully fabricated through mechanochemical ball-milling, followed by hot-pressing. Comprehensive multiscale analyses were conducted, and the results were compared with the existing literature. Incorporating chitosan significantly enhanced the vitrimerization process, leading to remarkable improvements in mechanical performance, solvent resistance, and stress relaxation. Specifically, the inclusion of 15 wt % chitosan, without an external catalyst, increased the epoxy vitrimer's elastic modulus by 100% and tensile strength by 35% compared to virgin epoxy resin, while reducing the relaxation time to approximately 800 s, an order of magnitude lower than epoxy/cellulose vitrimers reported in the literature. Remarkably, in the presence of zinc acetate, the relaxation time of epoxy/chitosan vitrimer composites further decreased to 150 s, with modulus and tensile strength increasing by 210% and 80%, respectively, compared to those of virgin epoxy resin. The reduction in relaxation time was attributed to chitosan's primary amine and hydroxyl groups, which facilitated multiple exchange reactions (transesterification, ester-to-amide, and transamidation) and acted as self-catalysts for transesterification. Enhancements in mechanical properties were attributed to zinc-coordinated interactions within the polymer networks. This study also investigates the influence of cross-linking density and activation energy as competitive parameters on the relaxation behavior and glass transition temperature (Tg) of the vitrimers. The multiple-dynamic network and rapid stress relaxation characteristics of the epoxy/chitosan vitrimers make them promising candidates for a variety of applications, supporting a circular economy. This work represents the first use of chitosan in the vitrimerization of thermoset epoxy resins, advancing the development of sustainable and recyclable thermoset materials.