Smart materials for artificial muscle and energy harvesting

Dielectric elastomer actuators (DEA) are a new type of electromechanical actuators based on the ability of soft materials to change dimensions when an electric field is applied. Their useful properties such as large strains and stresses, light-weight, ease of processability, simplicity, noiselessness, low cost, and high mechanical flexibility made them attractive for applications including: robotics, haptics, fluidics, optics, biomimetics, and muscle replacement. In addition DEA can also be used as sensors, energy harvesting devices, capacitors, or even as memories. However, a disadvantage of DEA is the large activation voltage required to induce their mechanical deformation. Furthermore, a linear relation between deformation and voltage is desirable, allowing fora wealth of possible applications in morphing and energy harvesting. To obtain this feature, DEAs would have to be made piezoelectric. It is the aim of this project to develop new reliable materials to be used in the construction of actuators which are used at low voltage for muscle replacement (artificial muscle) or for energy harvesting. In order to achieve these goals, the performance of the dielectric material must be improved. Polydimethylsiloxane elastomers (PDMS) available in the applicant`s lab will be blended with nanoparticles containing large permanent dipole moments. These nanoparticles will be prepared by the miniemulsion polymerization technique. Monomers are the target, which after polymerization provide a high Tg polymer. These monomers will be used either to encapsulate molecules having large dipole moments or to copolymerize with polar monomers. After thin films (or actuators) are formed by cross-linking the composite, the films (actuator) will be processed as described in the detailed research plan to make them piezoelectric. With these novel materials, actuators will be constructed by using doctor blading and printing techniques. The use of such actuators to generate electrical energy will be investigated in close collaboration with the Johannes-Kepler University (JKU). For the characterization, broadband dielectric spectroscopy, several techniques for measuring stress strain relationships of elastomers under various experimental conditions (changing temperatures, applied voltages), as well as poling techniques for rendering the elastomers piezoelectric is available.

Elastomers, in everyday language rubber, are soft materials useful in actuators and devices for the conversion of renewable mechanical energy. In these applications we use capacitors formed by the elastomer and conformable electrodes. An electric voltage is applied to the elastomer capacitor, which is then deformed by electrostatic forces. Currently one major obstacle in employing these actuators is the high voltage needed for driving the circuit, whereas withstanding high voltages is advantageous for dielectric elastomer generators. Material synthesis must account for these different requirements. Acrylic elastomers, natural and synthetic rubber and polydimethylsiloxane (PDMS) are potential candidates for the next generation of dielectric elastomer actuators and generators. In addition, dielectric elastomers may be promising materials for highly conformable piezoelectrics. In this project we first investigated the potential of charge controlled actuators, which should avoid the pull-in instability of voltage driven actuators. The pull-in instability causes an uncontrolled thickness reduction of the actuator, finally leading to catastrophic failure by electrical breakdown. Our investigations have shown that charge controlled actuation leads to a new instability, similar to the necking instability in mechanical testing. We also investigated temporal effects in minimum energy elastomer actuators, co-developed at the Johannes Kepler University, in order to derive guidelines for long term stable dielectric elastomer actuators. In the field of materials development, we showed the potential of natural rubber for dielectric elastomer generators. In the last year of the project and in concurrent work this year, we revealed, in close co-operation with the EMPA group, first piezoelectric signals in PDMS elastomers with strongly polar molecules. These results may pave a way towards highly conformable piezoelectric materials that can adjust to arbitrary three-d-forms.