Effect of solvents on the chemical and physical properties of ionic polymer-metal composites

Abstract

Ionic polymer-metal composites ͑IPMCs͒ consist of a perfluorinated ionomer membrane ͑usually Nafion® or Flemion®͒ 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 1.3 V 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. Remarkably, solvents are found to have profound effects on the nature of the IPMCs' actuation. For example, Nafion-based IPMCs in Li + form show very small back relaxation when hydrated, but extensive back relaxation with all other solvents that we have considered. On the other hand, the same membrane in the K + form has extensive back relaxations when solvated with water, or ethylene glycol, or glycerol, but none with 18-Crown-6.

Modeling of electrochemomechanical response of ionic polymer-metal composites with various solvents

“…With crown ethers, on the other hand, we discovered experimentally 6 that there may be a remarkable qualitative change in the actuation, depending on the cation and the crown ether. For example, Nafion-based IPMCs in Li + -form show only a small relaxation towards the cathode with water, but with ethylene glycol, glycerol, or 12-Crown-4 they show extensive back relaxation towards the cathode after an initial small actuation towards the anode.…”

“…6 For solvents such as glycerol or crown ethers, the sensitivity of our measurement devices was insufficient to discern in certain cases the necessary signals from that of the background noise. So for the purpose of modeling, a value within the measured range is used for the effective capacitance.…”

“…For Nafion-based IPMCs that are neutralized with alkali metal cations and solvated by water, ethylene glycol, or glycerol, the strip then slowly relaxes back towards the cathode, while still under electric potential. 2,[4][5][6] The sample eventually reaches an equilibrium state ͑while the electric potential is still on͒, which is generally far from its initial equilibrium position. If the electric potential is removed as the two electrodes are shorted, the Nafion-based IPMC sample ͑under the above-mentioned conditions͒ displays a relatively fast bending deformation towards the cathode and then slowly relaxes back towards the anode, seldom attaining its initial state.…”

“…In the solution of 4.2 g of 18CR6 ͑m 18CR6 = 4.2 g͒ and 10 ml of glycerol ͑V G =10 ml͒ ͑see Nemat-Nasser et al 6 for details on preparation of the crown ether solutions͒, the moles of glycerol, x G , are given by…”

Transport modeling in ionomeric polymer transducers and its relationship to electromechanical coupling

“…Beyond that, though, there have been a number of models based on different physical phenomena that can closely match experimental data. The models that provide a direct comparison between experiment and theory can generally be separated into phenomenological models based on curvefits of experimental data [6][7][8][9] and physics-based models that attempt to predict the material response from governing equations.…”