Polypyrrole for Artificial Muscles: Ionic Mechanisms

Some conjugated polymers can convert electrical energy to mechanical energy (via chemical energy), thereby acting as electro-chemo-mechanical actuators or “artificial muscles”. The advantage of this type of actuator is that the process can be driven by the application of a small potential (1-5 V), opening the possibility of making control and measurement both safe and accurate. The actuation process is identical to charging and discharging an electrochemical cell during redox cycling of a rechargeable battery. It involves ions moving between the electrolyte and being inserted in, or expelled from the matrix of a polymer electrode – thereby causing volume expansion which can be converted into work. Solvent molecules are able to penetrate the polymer too. A precise description of the nature of these ionic and solvent movements is therefore important for understanding and improving the performance. This work examines the influence of solvent, ionic species and electrolyte concentration on the fundamental question about the ionic mechanism involved: Is the actuation process driven by anion motion, cation motion, or a mixture of the two? In addition: What is the extent of solvent motion? The discussion is centered on polypyrrole (PPy), which is the material most used and studied. The tetraethyl ammonium cation (TEA) is shown to be able to move in and out of PPy(DBS) polymer films, in contrast to expectations. There is a switching between ionic mechanisms during cycling in TEACl electrolyte.