Electrical Impedance of Spin-Coatable Ion Gel Films

The electrical properties (capacitance, resistance, and conductivity) of ion gel films were examined as a function of film geometry and temperature by using electrical impedance spectroscopy. Ion gel films, which consist of a triblock copolymer, poly (styrene-b-methyl methacrylate-b-styrene) [SMS], and an ionic liquid, 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide, [EMI] [TFSI], were deposited by spin coating from ethyl acetate solution. The thickness (2.2-13.4 mu m) and the area (0.01-0.06 cm(2)) of the film sandwiched between two gold electrodes were varied systematically to investigate the relation between the electrical properties and the geometry of the film. The resistance (R) was directly proportional to the thickness and the reciprocal area, as expected, whereas the specific capacitance (C) was insensitive to the film geometry. Importantly, the gel polarization time constants (RC, where C = C' x area) were as small as 2.8 mu s for 2.2 mu m thick ion gel films. Conductivity and capacitance of the film both increase with increasing temperature, with conductivity following the Vogel-Fulcher-Tamman equation, indicating entropically activated behavior, and capacitance at 10 Hz showing Arrhenius-type activation.