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 (T g ) 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 10 3 Pa. In this work, we developed a new ABA triblock ion gel system using poly(ethyl acrylate) (PEA) as a low T g and hydrophobic midblock. The low T g 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.

Brush polymer ion gels

“…Conductivity generally decreases as polymer concentration increases due to an increase in T g and concomitant reduction in ion mobility, a trend also found with both chemically and physically cross‐linked linear polymer gels. The low glass transition temperature of PEO (≈−60 °C) results in similar curvature for each ion gel concentration versus temperature dataset.…”

“…): g PS 3 ‐ g PEO 85 ‐ g PS 3 > g PS 6 ‐ g PEO 87 ‐ g PS 5 > g PS 15 ‐ g PEO 119 ‐ g PS 15 . The result that lower ratios of N A : N B yield higher conductivities at equivalent temperatures and polymer concentrations seems intuitive but actually contrasts observations with linear polymer ion gels . Typically, larger N A at constant N B (e.g., 9‐20‐9 vs. 2‐20‐2) produces a higher conductivity, since the fraction of ionic liquid in the conducting B matrix at a constant polymer wt % loading increases.…”

“…Linear ABA triblock copolymers with ionophobic A blocks and ionophilic B blocks form reversible physically cross‐linked networks comprising A block micelles in a B + IL matrix . Zhang et al have reported the connection between ionic conductivity and rheological properties for polymers including poly(styrene‐ block ‐methyl methacrylate‐ block ‐styrene) (PS‐PMMA‐PS), poly(styrene‐ block ‐ethylene oxide‐ block ‐styrene) (PS‐PEO‐PS), poly(styrene‐ block ‐ethyl acrylate‐ block ‐styrene) (PS‐PEA‐PS), and PS‐PEO‐PS with chemically cross‐linked PS cores in 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). The potential industrial impact delivered by this class of soft materials is highlighted by their successful implementation in a variety of electronic devices …”

Ink‐Jet Printable, Self‐Assembled, and Chemically Crosslinked Ion‐Gel as Electrolyte for Thin Film, Printable Transistors

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voltages for operation [1][2][3][4][5][6][7] and remarkable progress has been made in the development of semiconducting channel materials [8][9][10][11] and gate insulators. [12][13][14] The EGTs are switched on/off by applying a gate potential, which leads to a migration and local redistribution of ions at the semiconductor/electrolyte interfaces. The formed electric double layer (EDL) generates high charge accumulation in the adjacent semiconducting channel, which, for instance, renders the channel conductive at a certain gate potential.

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Self-assembly and structure-property behaviour of block copolymer/ionic liquid composites

“…In order to overcome this limitation a new variant of triblock copolymer was investigated where the midblock was replaced with a structurally similar but lower T g block poly(ethyl acrylate). 144 The enabled the conductivity of the resulting gels to be comparable to that of PS-b-PEO-b-PS at polymer concentrations up to 50 wt %. Furthermore, the modulus and ionic conductivity of the gels could be increased synergistically by decreasing the size of the midblock at constant polymer concentration.…”