Ionic Polymer-metal Composites (IPMCs) are soft actuators and sensors. They generally consist of a thin perfluorinated ionomer membrane, metal electrodes plated on both faces, and are neutralized with certain counter cations, balancing the charge of the anions covalently fixed to the membrane. Under a suddenly applied step function (1 to 3 V), the IPMC in alkali-metal cation forms exhibits a fast bending motion towards the anode, followed by a slow relaxation. For Nafion-based IPMCs, this slow relaxation is towards the cathode, whereas for Flemion-based IPMCs, the slow relaxation continues the initial fast motion towards the anode. IPMC samples in sulfonic forms having sodium as cations are prepared, their electromechanical properties are characterized, and their actuation responses to various electric stimuli are investigated. Results show that for Nafion-based IPMCs, initial motion towards the anode can be ultimately eliminated by applying a slowly increasing potential, due to very slow charge accumulation and extensive cation redistribution within a boundary layer near the cathode electrode.
An ionic polymer-metal composite (IPMC) consisting of a thin perfluorinated ionomer (usually, Nafion® or Flemion®) strip, platinum and/or gold plated on both faces, undergoes large bending motion when a small electric field is applied across its thickness. When the same membrane is suddenly bent, a small electric potential of the order of millivolts is produced across its surfaces. This actuation and sensing response depends on the structure of the ionomer, the morphology of the metal electrodes, the nature of cations, and the level of hydration. IPMCs in alkali-metal cation form under direct current (DC) show a fast motion towards the anode, followed by a slow relaxation. For Nafion-based IPMCs, this slow relaxation is towards the cathode, whereas for Flemion-based IPMCs, the slow relaxation continues the initial fast motion towards the anode. In contrast, the actuation of both Nafion- and Flemion-based IPMCs in tetrabutylammonium (TBA<sup>+</sup>) cation form consists of a continuous slow motion towards the anode. We have discovered that when an IPMC is neutralized by combined Na<sup>+</sup> and TBA<sup>+</sup> cations to produce a suitable Na-TBA-form membrane, different actuation behavior results. The proportion of the cations can be tailored to obtain a desired actuation response, e.g., to control the duration, speed, and the maximum amplitude of the initial motion towards the anode, or the magnitude and the speed of the subsequent relaxation. A series of cation combination tests on both Nafion- and Flemion-based IPMCs are carried out. Various essential physical properties of the IPMCs in various cation compositions are measured and compared. A summary of these results is presented.