| Ionic polymer-metal composites (IPMCs) consist of a perfluorinated ionomer membrane (usually NafionRTM or FlemionRTM) plated on both faces with a noble metal such as gold or platinum and neutralized with a certain amount of counterions that balance the electrical charge of anions covalently fixed to the backbone ionomer. IPMCs are electroactive materials with potential applications as actuators and sensors. Their electrical-chemical mechanical response is dependent on the cations used, the nature and the amount of solvent uptake, the morphology of the electrodes, the composition of the backbone ionomer, the geometry and boundary conditions of the composite element and the magnitude and spatial and time-variation of the applied potential. With water as the solvent, the applied electric potential must be limited to less than 1.3V at room temperature, to avoid electrolysis. Moreover, water evaporation in open air presents additional problems. These and related factors limit the application of IPMCs with water as the solvent. We present the results of a series of tests on both Nafion- and Flemion-based IPMCs with ethylene glycol, glycerol, and crown ethers as solvents. IPMCs with these solvents have greater solvent uptake, and can be subjected to relatively high voltages without electrolysis. They can be actuated in open air for rather long time periods, and at low temperatures. They may be good actuators when high-speed actuation is not necessary. In addition, their slow response in open air allows direct observation of the physical characteristics of the cathode and anode surfaces of a cantilever during actuations. This can provide additional clues for unraveling the underpinning micromechanisms of their actuation. We seek to model the IPMCs' actuation and compare results with the experimental data. The modeling rests on the observation that a sudden application of a step potential (DC) of several volts (1--3 V) alters the distribution of cations within the ionomer, forcing cations out of the clusters near the anode and additional cations into the clusters near the cathode. The clusters within a thin boundary layer near the anode are thus depleted of their cations while cations accumulate in the clusters near the cathode boundary layer. We first seek to determine the spatial and temporal variation of the cation distribution across the thickness of the IPMC for various cations and solvents, using implicit finite difference numerical solution of the basic field equations, and compare the results with those of approximate analytical estimates. Based on this information, we then calculate the changes in the osmotic, electrostatic, and elastic forces that tend to expand or contract the clusters in the anode and cathode boundary layers. Finally, we calculate the amount of solvent out of or into the clusters that produces the bending motion of the cantilever. |
