Polyphenols-derived epoxy vitrimers for smart applications: Electrical conductivity, Joule heating, and strain sensing

Epoxy resins play a crucial role in several industrial applications. However, their irreversible crosslinked structure and need for precursors from fossil-fuels provide sustainability issues. This study explores the synthesis of bio-based epoxy vitrimers using glycidylated derivatives of gallic acid (GGA) and tannic acid (GTA) as eco-friendly alternatives. ATR-FTIR and NMR spectroscopies confirmed the successful glycidylation reaction. The thermal curing of epoxy monomers with Vitrimax imine T130 was performed after thoughtful DSC and TGA analyses, achieving reprocessable and thus recyclable materials. Indeed, thanks to the covalent adaptable networks (CANs) based on imine bonds the reprocessing of polyphenols-based composites was possible by hot-pressing their powders after a grinding step. Carbon nanotubes (CNT) were introduced into natural polyphenol-based materials at 1 and 2 wt% contents to improve electrical conductivity and piezoresistive properties. Thermomechanical performance of the bio-based composites was assessed as a function of CNT content, measuring a glass transition temperature of approximately 60 °C. Electrical conductivity measurements revealed an outstanding capability of polyphenols-based composites to conduct electricity with a percolation threshold at 1 wt% of CNT, reaching a maximum of 0.1 S/m and 0.4 S/m for GGA and GTA, respectively. Moreover, unlike systems with 1 wt%, the composites with 2 wt% of CNT exhibited significant Joule heating capabilities reaching 60 °C by just applying about 50 V. Finally, strain-sensing tests demonstrated the electromechanical responsiveness of the composites, showing outstanding gauge factors of 89 and 17 with the GGA_1CNT and GTA_1CNT, respectively, highlighting their potential in structural health monitoring (SHM) applications. This work underscores the feasibility of electrically conductive natural polyphenol-based composites as sustainable, recyclable, and multifunctional alternatives to conventional epoxy systems.