Dielectric elastomer actuators (DEAs) have gained an increasing importance in various research applications. In literature the most explored dielectric elastomers (DEs) are silicones and acrylics. As promising alternatives, polyurethanes have been identified due to their inherent higher dielectric constant, higher dielectric strength, and appropriate response behavior. However, polyurethane systems are known to have hygroscopic properties that can lead to changes in the dielectric properties when they are exposed to moisture. In this work, a circular DEA composed of a thermosetting polyurethane film and carbon grease electrodes is exposed to four stabilized levels of relative humidity (RH = 22 %, 36 %, 56 %, 70 %) at a constant temperature of 22 °C. The actuation strain is measured to study the effect of moisture absorption on the DEA performance. In addition, impedance spectroscopy is performed on the DEA to investigate the influence of moisture on its dielectric properties. Impedance, phase angle and dielectric constant of the DEA are evaluated at the respective RH level. The results show a rise in the dielectric constant of 15 % with increasing the humidity from 22 % to 70 %. However, increasing the humidity does not necessarily lead to an increase in actuation strain.
Electroactive polymers are a major component of dielectric elastomer actuators (DEA). The performance of DEAs depends on the Young’s modulus, dielectric constant and film thickness of the electroactive polymer as well as the compliance of the electrodes and the applied voltage. In literature mostly experiments on silicone- and acrylic-based DEAs are reported. However, better actuator performance can be expected with materials that have a higher dielectric constant. Therefore, unconventional electroactive polymers, such as polyurethane, chloroprene, or nitrile rubber, are currently attracting increasing interest for DEA applications. Besides their inherent dielectric properties, ferroelectric fillers embedded in the electroactive polymer can increase the dielectric constant even further. Provided that the filler concentration does not significantly increase the Young’s modulus, ferroelectrically filled polymers can be expected to have a better actuator performance than standard materials. In this work, barium titanate particles with different concentrations were embedded in crosslinked polyurethane elastomer films. The hyperelastic material behavior of the polyurethane elastomer is represented by a Mooney-Rivlin model. Impedance spectroscopy is used to determine the dielectric constant of the electroactive compounds. Planar DEAs are designed from both unfilled and barium titanate-filled polyurethane films and compared with respect to their mechanical, dielectric, and actuator properties. The electric field response of unfilled and barium titanate-filled polyurethane-based DEAs is investigated experimentally and compared to an analytical solution of the actuator deformation. Additionally, a comparison between experimental DEA operation and an FEA prediction is carried out and discussed.
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