Performance of Compliant Electrodes in Electro Active Polymer (EAP) Actuators

Abstract

Dielectric elastomer (DE) actuators, based on the field-induced deformation of elastomeric polymers with compliant electrodes, can produce a large strain response. To accommodate the high strain during actuation the electrodes around the DE should also deform without imposing any restraints while maintaining their conductivity. The electrodes have to be compatible with the DE in their mechanical properties. In addition to mechanical properties such as elastic moduli, hysterisis, etc., other properties such as conductivity, percolation, are also of importance. Therefore the compliant electrode is a key feature of the DE actuator technology.

Many types of compliant electrodes used in conjunction with DE actuators have been reported in the literature. Among them are particle included polymers, metals, and conductive polymers. Particle included polymer based electrodes generally consist of carbon or silver as particles and a polymer medium such as silicone. Conductive polymers such as polypyrrole can be used as compliant electrode as well.

In this work an effort has been made to characterize various compliant electrodes on dielectric elastomer EAPs under different process conditions. Characterization of the electrodes includes their response to applied voltage, their conductivity values under different test conditions and their topography.

Three different types of compliant electrode have been characterized. These were rubber electrodes, grease electrodes and polypyrrole electrodes. The results showed that the crack formation was related with the amount of polymer carrier for grease and rubber electrodes. Both rubber and grease electrodes, which were prepared with Nusil CF19-2186, showed the worst results in terms of uniformity of the electrodes and areal strain rates to the applied voltage. For rubber electrodes, electrodes, which were prepared with Sylgard 186 and Sylgard 184, showed similar results in terms of uniformity of the electrode. Generally Sylgard 184 rubber electrodes showed higher areal strain rates to the applied voltage than that of Sylgard 186 electrodes. Higher conductivity values were achieved with Sylgard 186 rubber electrodes comparing to Sylgard 184 rubber electrodes.

For grease electrodes, electrodes, which were prepared with Sylgard 186 and Sylgard 184 did not show similar results in terms of uniformity and areal strain. Higher conductivity values were observed with Sylgard 186 grease electrodes comparing to Sylgard 184 grease electrodes.

Both Sylgard 184 and Sylgard 186 rubber electrodes lost its conductivity at 100%-100% nominal strain rate.

It was observed that increasing the number of polymerizing process, thus number of rinsing, enable to remove black pyrrole particles more efficiently for polypyrrole electrodes. Relatively higher conductivity values and lower areal strain values were achieved with Ppy electrodes comparing to rubber and grease electrodes. Ppy electrodes lost its conductivity at 100%-100% nominal strain ratio.