Ionic Conductivity, Capacitance, and Viscoelastic Properties of Block Copolymer-Based Ion Gels

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Abstract

The effects of composition, temperature, and polymer identity on the electrical and viscoelastic properties of block copolymer-based ion gels were investigated. Ion gels were prepared through the self-assembly of poly(styrene-b-ethylene oxide-b-styrene) (SOS) and poly(styrene-b-methyl methacrylate-b-styrene) (SMS) triblock copolymers in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([EMI][TFSI]). The styrene end-blocks associate into micelles, whereas the ethylene oxide and methyl methacrylate midblocks are well-solvated by this ionic liquid. The properties of the ion gels were examined over the composition range of 10−50 wt % polymer and temperature range of 25−160 and 25−200 °C for the SOS- and SMS-based gels, respectively. The response of the ion gels to ac electric fields below 1 MHz can be represented by a resistor and constant phase element (CPE) series circuit, with a characteristic time corresponding to the establishment of stable electrical double layers (EDLs) at the gel/electrode interfaces. The ionic conductivity and specific capacitance were found to range from 3 × 10−5 to 3 × 10−2 S/cm and 0.3 to 10 μF/cm2, respectively. For 1 mm thick gels, the corresponding RC time constants ranged from 2 × 10−5 to 5 × 10−3 s. Notably, at high polymer concentrations, the ionic conductivity is much higher in SOS than SMS due to the higher glass transition of the methyl methacrylate block. Two relaxation modes have been observed in the ion gels under oscillatory mechanical shear. The faster mode corresponds to the relaxation of the midblocks in the ionic liquid, while the slow mode reflects motion of the end-blocks within their micellar cores. The plateau modulus of the gels was found to vary from 0.5 to 100 kPa over the measured composition and temperature ranges. While the ionic conductivity generally decreases as the modulus increases, it is possible to achieve conductivities greater than 0.01 S/cm with moduli above 10 kPa in the SOS system.

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