Formation of Multicompartment Ion Gels by Stepwise Self-Assembly of a Thermoresponsive ABC Triblock Terpolymer in an Ionic Liquid

Ion gels are materials composed of an ionic liquid immobilized by a network-forming polymer, resulting in a soft solid with high ionic conductivity and tunable mechanical strength. Here, we demonstrate the thermoreversible gelation of a triblock terpolymer poly(ethylene-alt-propylene)-block-poly(ethylene oxide)-block-poly(N-isopropylacrylamide) (PON) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). At high temperature, the polymers self-assemble into micelles with P cores and ON coronas. Below the upper critical solution temperature of PNIPAm in this ionic liquid, the N blocks aggregate, resulting in macroscopic gelation. The ion gels were characterized at concentrations of 1, 2, 5, and 10 wt %. Rheological measurements demonstrated that the PON ion gels had a greater percentage of elastically effective strands, leading to significantly higher mechanical strength than for the corresponding NON ion gels. This is consistent with the expectation that an ABC terpolymer architecture can suppress looping of the middle blocks. A combination of small-angle X-ray scattering, rheology, and dynamic light scattering demonstrated that PON ion gels exhibited a broader sol-gel transition compared to previously studied PON hydrogels, a result which is attributed to the different thermoresponsive phase behavior of poly(N-isopropylacrylamide) in ionic liquids and water. Overall, this work demonstrates that the use of ABC triblock terpolymers allows for more efficient formation of strong ion gel networks at low polymer concentrations, which can be readily processed at moderate temperatures.