Impact of the Ionic Liquids-Initiated Curing Mechanisms on Epoxy Polymeric Networks

Ionic liquids (ILs) have recently been used as curing agents for epoxy. They have significant benefits over traditional diamine-based curing agents because of their latent curing ability, low toxicity, and thermal stability. Specifically, ILs containing imidazolium and phosphonium cations have shown to be effective epoxy latent curing agents.[1] Multiple reaction pathways have been proposed, however, the exact mechnisms have not been proven.[2] To elucidate these mechanisms, we investigated 1-ethyl-3-methyl imidazolium (EMIM-) and trihexyltetradecylphosphonium (THTDP)-based ILs with various counter-anions using differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), thermogravimetric analysis–mass spectrometry (TGA–MS), and high-resolution mass spectrometry (HR–MS).

Our results show that both anion- and cation-routes can initiate epoxy curing, but the dominant pathway depends strongly on the cation-anion pair and curing conditions. Specifically, EMIM–acetate (Ac) exhibited exclusively anion-driven curing, while EMIM–dicyanamide (DCA) supported multiple reaction pathways. The complementary analyses techniques used in this study also provided the first experimental evidence of carbene-derived intermediates formed during the thermal activation of EMIM-based ILs. Comparison among IL systems demonstrated that EMIM–DCA generated a higher exothermic enthalpy (∆H) and glass transition temperature (T_g) than THTDP–DCA, indicating that the additional initiation routes enhance crosslinking density. Similarly, EMIM–DCA exhibited higher ∆H and T_g than EMIM–Ac, confirming that ion pairing significantly governs network formation and thermal behavior. Collectively, these findings reveal the mechanistic complexity of ionic liquid–initiated epoxy curing and underscore how molecular structure and ion pairing dictate crosslinking density and thermal properties in polymeric network systems.