Curing mechanisms of imidazolium- and phosphonium-based ionic liquids in epoxy resins: linking initiation pathways to network glass transition temperature.
basic_science · Level V
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- Record sourced from PubMed, PMID 42725403.
- Also identified by DOI 10.1039/d6sm00667a.
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Abstract
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 (<i>T</i><sub>g</sub>) 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 <i>T</i><sub>g</sub> (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 <i>T</i><sub>g</sub> 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.