Yu-Min Wang, Daniel V Krogstad
The use of ionic liquid (IL) latent curing agents for epoxy resins has grown significantly. However, the reaction pathways responsible for IL-initiated curing remain underexplored, particularly the effect of cation-induced pathways for imidazolium-based ILs. Here, the curing behavior of epoxy resins initiated by 1-ethyl-3-methylimidazolium (EMIM)- and trihexyltetradecylphosphonium (THTDP)-based ILs was investigated using nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS) and differential scanning calorimetry (DSC) to increase our understanding of the curing mechanisms and their effect on the glass transition temperature (Tg) of the resultant epoxy networks. DSC experiments showed that 1-ethyl-3-methylimidazolium dicyanamide (EMIM-DCA) exhibited bimodal curing behavior, whereas phosphonium-based ILs and EMIM acetate (EMIM-Ac) showed single exothermic peaks consistent with a predominantly anion-mediated curing route. Chemical analysis of the thermally activated EMIM-DCA supported the analysis that the dual curing was the result of both anion- and cation-mediated routes by providing evidence for the formation of imidazole-derived species and for the possible involvement of carbene-related pathways. Additionally, molar ratio-matched EMIM-DCA and THTDP-DCA exhibited comparable curing temperatures for the primary exothermic peak, indicating that it was DCA-mediated. Interestingly, isothermal curing of EMIM-DCA showed that curing the sample at the intermediate temperature of 120 °C exhibited the highest Tg (134 °C), potentially due to contributions from both the anion and cation-mediated curing mechanisms, while curing at 80 °C and 170 °C resulted in lower Tg of 92 °C and 114 °C, respectively, due to curing mechanisms dominated by a single mechanism (cation and anion, respectively). These results highlight the influence of curing conditions on network properties.