Synergistic Increase in Ionic Conductivity and Modulus of Triblock Copolymer Ion Gels

Abstract

Ion gels formed with ABA triblock polymers and ionic liquids (IL) have recently attracted significant attention. Because of their high ionic conductivity, high capacitance, and good mechanical integrity, ion gels prepared from triblock polymers of polystyrene-b-poly(methyl methacrylate)-b-polystyrene (SMS) and polystyrene-b-poly(ethylene oxide)-b-polystyrene (SOS) and an IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) have been successfully applied as the dielectric layer in thin film transistors. However, water absorption can negatively affect the stability of the dielectric layer and lead to electrical breakdown. Consequently, the preferred polymer of these two is SMS. However, the high glass transition temperature (Tg) of PMMA limits the usable SMS polymer concentration in order to ensure comparable ionic conductivity to that of SOS ion gel; this constraint limits the modulus of the gel to about 103 Pa. In this work, we developed a new ABA triblock ion gel system using poly(ethyl acrylate) (PEA) as a low Tg and hydrophobic midblock. The low Tg of the midblock ensures the ionic conductivity of the resulting ion gels is comparable to that of SOS ion gels at polymer concentrations up to 50 wt %, which is a significant improvement relative to the currently used SMS ion gels. Additionally, by decreasing the size of the midblock at constant polymer concentration, the modulus and ionic conductivity of the ion gels increase synergistically. This interesting and counterintuitive effect reflects the concurrent increase in the number density and chain stretching of midblocks, accompanied by a net reduction in midblock concentration within the conducting phase. We demonstrate that electrolyte gated transistors (EGTs) made with SEAS ion gels have improved stability under ambient humid conditions in comparison to those made with SOS ion gels.