Reliability of high strain ionomeric polymer transducers fabricated using the directassembly process

Abstract

Ionomeric polymer transducers have received considerable attention in the past severalyears. These actuators, sometimes referred to as artificial muscles, have the ability togenerate large bending strain and moderate stress at low applied voltages. As sensors, ionicpolymer transducers generate an electric response due to mechanical strain. Typically, ionicpolymer transducers are composed of Nafion-117 membranes with platinum electrodes andare saturated with water diluents. Recently the authors have developed a novel fabricationtechnique called the direct assembly process (DAP), which allowed improved control of theelectrode morphology and composition. The DAP consists of spraying two highsurface area metal–ionomer electrodes on a Nafion membrane. The benefits ofthe DAP process over previous methods is the ability to control the thickness ofthe electrode, the ability to control the composition of the electrode layer of thetransducer, and the ability for it to be used with a wide variety of diluents. In pastwork we have demonstrated that platinum, ruthenium dioxide, and single-walledcarbon nanotubes can be used as electrode material with diluents such as water,formamide, and ionic liquids. In this work we will present a reliability study oftransducers fabricated using the DAP. Water-hydrated transducers dehydrateand stop moving within 5 min while operating in air under the application of ± 2 V. Ionic liquid based transducers are demonstrated to operate in air for over 400 000 cycleswith little loss in performance, and are reliable up to 1 million cycles with a performanceloss of less than 43%. The main source of degradation is the adhesion of the conductivesurface to the high surface area electrode. This is enhanced in this study by using a PUUlinking polymer that has good adhesion properties to gold. Large voltage and large strainare proven to decrease the life of the transducer. Formamide based samples are stable for 3days under a 1 V actuation signal, while they are only reliable for 3–4 h under a2 V actuation signal. Solvent evaporation is the main reason for degradation informamide samples and it is increased at 2 V, indicating some electrochemicalactivity at such high voltages. Finally the initial drop in performance and thefluctuation in the generated strain are shown to be due to the loss of humidityabsorbed from ambient air and the fluctuation in this ambient humidity, respectively.