Design of High Permittivity Polysiloxane Elastomers for Dielectric Elastomer Transducers

Dielectric elastomer transducers consist of a dielectric elastomer between two compliant electrodes. When the elastic capacitor is electrically charged, the dielectric elastomer elongates until a balance of elastic and electrostatic force is reached. Electrical energy is converted into mechanical work. Contrary, in the generator mode, the capacitor is stretched by an outside mechanical force. The mechanical work is converted into electrical energy due to the change of capacitance. The need of a high voltage power supply is the main obstacle for the elastomer transducer technology in various applications. Siloxane elastomers are the most frequently tested elastomers with excellent mechanical stability, processability and electrically insulating properties. It is well-known that an increase in permittivity of the silicone elastomer would allow a reduction of the driving voltage of the elastic transducer. Hence, manifold research activities have been initiated to enhance the permittivity of siloxane elastomers. In the present work, siloxane elastomers were developed that combined increased permittivity with high mechanical stability and cost-efficient processability of the siloxane materials. The synthesis of polar thiols as the first step of siloxane functionalization was studied. Serious challenges in the synthesis of polar groups that can be integrated by thiol-ene addition as side group of polysiloxanes were found. Therefore, commercially available alkylthiols were selected for the functionalization of siloxane due to their chemical stability. The alkylthiols were grafted onto the polysiloxane by thiol-ene addition and subsequently cross-linked in the well-established organotin-catalyzed condensation reaction. The impact of different types of cross-linker on the electro-mechanical properties of the polar siloxane elastomers was analyzed. A significant improvement in the performance of actuator test devices was demonstrated. In addition, the electromechanical stability of the polar siloxane elastomer was illustrated by 50.000 actuation cycles. However, solvent had to be used in the production process of the siloxane elastomers and the cross-linking reaction time was about two days. Apart from their application in thin film actuators, these polar siloxane elastomers were studied with respect to their performance in dielectric elastomer generators together with siloxane composites. Nanospring carbon nanotubes were blended as high permittivity fillers in siloxanes subsequent to cross-linking. Indeed, improved performances of the high permittivity silicone elastomers were found and a deeper understanding for the design of dielectric elastomers for generator applications was gained. The most significant result of this work was the development of polar siloxane elastomers that could be processed into thin film elastomers without any solvent within a few minutes. Two different synthetic approaches were developed based on low viscosity mixtures of cross-linkable oligo(ethylthioethyl)(methyl)siloxanes and multifunctional thiol cross-linkers. The cross-linking reaction was selectively initiated by UV-induced radical formation. These siloxane elastomers possessed an increased permittivity and showed a high electromechanical stability.