Polymer Actuators Using Ion-Gel Electrolytes Prepared by Self-Assembly of ABA-Triblock Copolymers

Abstract

Well-defined ABA-triblock copolymers, polystyrene- block -poly(methyl methacrylate)- block- polystyrene (SMS), which have two different polystyrene (PSt) weight fractions ( f PSt ), were synthesized by successive atom-transfer radical polymerizations. Ion gels consisting of SMS and an ionic liquid, (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [C 2 mim][NTf 2 ]), were prepared using the cosolvent evaporation method with tetrahydrofuran. Atomic force microscope images of the ion gels indicated that PSt is phase-separated to form sphere domains that serve as physical cross-linking points because PSt is not compatible with [C 2 mim][NTf 2 ], while a continuous poly(methyl methacrylate) (PMMA) phase with dissolved [C 2 mim][NTf 2 ] is formed to serve as ion conduction paths. Accordingly, the ion gels are formed by the self-assembly of SMS and the preferential dissolution of [C 2 mim][NTf 2 ] into the PMMA phase. The viscoelastic properties of the gels can be easily controlled by changing f PSt in SMS and [C 2 mim][NTf 2 ] concentration in the ion gels. The ion gels that exhibit high ionic conductivities (>10 –3 S cm –1 ) at room temperature were used as an electrolyte of an ionic polymer actuator, which has a trilaminar structure consisting of the ion-gel electrolyte sandwiched between two composite carbon electrodes containing high-surface-area activated carbon powders. By applying low voltages (<3.0 V) to the electrodes, the actuator exhibited a soft bending motion toward the anodic side.