Dual Functioning Deep Eutectic Solvents and Imidazolium-Based Ionic Liquids for Synergistic Toughening of Thermoset System

A greener strategy for toughening thermoset epoxy resin by the combination of imidazolium based ionic liquid (IL) and a newly synthesized salicylic acid (HBA) and triethylenetetramine (HBD) based deep eutectic solvent (DES) is demonstrated in this work. DES served as both a fracture-toughening agent and curing activator for IL(1-butyl-3-methylimidazolium chloride, bromide, and 1-ethyl-3-methylimidazolium acetate) cured epoxy systems, outperforming the traditional triethylenetetramine (TETA)-cured systems. Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) confirmed a single-phase curing mechanism, with a 30% increase in glass transition temperature (T_g), indicating effective curing. Mechanical properties showed marked improvement, with tensile strength increasing by up to 55%, impact strength by 60–235%, and fracture toughness by up to 301%, depending on the type of IL involved. Acetate-based IL and TETA systems also showed enhanced properties, albeit with slightly lower crosslinking density. Dynamic mechanical analysis (DMA) revealed lower tan delta values, indicative of a flexible network and scanning electron microscopy (SEM) confirmed the brittle-to-ductile transition on the fractured surfaces. The results demonstrate the potential of DES as a multifunctional additive, leveraging hydrogen bonding, etherification, and esterification to enhance epoxy performance.