Cure monitoring and characterization of epoxy/amine networks, modified with 1-butyl-3-bethylimidazolium tetrafluoroborate ionic liquid

In the present work, electrochemical impedance spectroscopy (EIS) was employed for cure monitoring of bisphenol A/F based epoxy resin-polyoxypropylenediamine hardener systems modified with various concentrations of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) ionic liquid (IL). These systems exhibited phase separation induced by an increase in the molar mass through the curing of the resin. The progress of the curing process was accompanied by a steady increase of impedance for all types of samples, as expected, due to the increasing viscosity of the medium for increased degree of cure. The results were compared to those of unmodified resin-hardener systems. The sensitivity of EIS as a cure monitoring technique increased dramatically with the addition of the IL. The cure kinetics of the epoxy resin was characterized by differential scanning calorimetry and the data were used to create a model for comparison with the EIS results. The change of the ionic conductivity during curing as a function of IL concentration and the effect of IL loading on mechanical properties of the resin were also studied. We demonstrate that [BMIM]BF4 loading of 5wt.% increased the room temperature ionic conductivity of the fully cured product by an order of magnitude with only a 2.5% reduction in compression strength from that of pure epoxy.