3D printing of dielectric elastomer transducers (DET) would significantly accelerate their application in soft robotics. Direct ink writing (DIW) of DET is limited by multiple factors, such as the need for a multi-material printing of dielectric and compliant electrodes and the relatively large thickness, high stiffness, and poor mechanical properties of elastomers. Increasing the permittivity of elastomers is the only tunable material parameter, which can reduce the actuation voltage, or increase the sensor signal, as the minimum thickness is fixed by the printing resolution. We present DIW printable high-permittivity polysiloxanes. Besides the high-permittivity further material parameters and interdependencies between ink requirements and final material performance are explored. The facile printing of these high-permittivity dielectrics with standard 3D printers is demonstrated. Lastly, the performance of various DIW printed DETs are presented.
Polar group-modified polysiloxanes obtained by anionic ring-opening polymerization possess high dielectric permittivity and are of great interest for application in dielectric elastomer actuators (DEAs). A self-healing elastomer can be obtained by in situ polymerization and cross-linking using a cyclic siloxane monomer with polar side groups and a cross-linker consisting of multiple connected siloxane rings. In previous works, a non-polar cross-linker has been used, which requires the addition of a solvent for compatibilization with the polar monomer. In polymerization reactions of siloxanes, the addition of solvent leads to a more pronounced formation of cyclic by-products. These cycles impair the mechanical properties of the elastomer and cannot be removed after the reaction, as the material is already cross-linked. Therefore, in this work, we use a polar cross-linker that can be mixed with the polar monomer without adding solvent. Nitrile groups have been studied extensively for increasing the permittivity of the polysiloxane backbone. For a functionalization of 100%, a dielectric permittivity of ~18 was reached. In most cases, the nitrile group was attached to the siloxane backbone in the form of cyanopropyl groups. Still, the influence of the alkyl spacer on the material's dielectric and mechanical properties has not been studied. In this work, we synthesize cyanoalkyl-functional cyclic siloxanes with different lengths of the alkyl spacer and polymerize them solvent-free to high-permittivity polysiloxanes.
The development of novel functional dielectric materials can open the doors to major technological innovations with societal impact. Stretchable capacitors transduce electrical into mechanical energy or vice-versa. Over the last 20 years, they have received significant interest from academia and industry. However, this technology still needs both improved dielectrics as well as conductive elastomers to achieve the desired low driving voltage and to realize devices with attractively high sensitivity. The currently most explored dielectric elastomers are polydimethylsiloxanes (PDMS). However, because of their low dielectric permittivity of only 3, the devices made of them require high voltages for operation. We synthesized polar polysiloxanes with different types and contents of polar groups, investigated their thermal and dielectric properties, and selected the most suitable groups to achieve the highest dielectric permittivity, yet sufficiently low glass transition temperature (Tg) to afford an excellent elastomer at room temperature after cross-linking. This research guided us to several promising polar polysiloxane elastomers modified with nitrile and nitroaniline groups, for which the properties were optimized. We reproducibly achieved dielectric elastomers with a dielectric permittivity of about 18. Some respond to a voltage as low as 200 V, while some give very large actuation and have a breakdown field reaching 100 V μm-1. By carefully selecting suitable synthetic chemistry, we could also achieve self-healable high permittivity elastomers. The materials can be processed into thin films by melt pressing. Stack actuators can be easily manufactured manually and give 5.4% actuation at an electric field as low as 3.2 V μm-1. Furthermore, the actuators can self-repair after a breakdown and be recycled after complete failure. A graphene nanoplatelets (GNPs) composite in PDMS as a conductive electrode was developed via in-situ polymerization. The synthesis and the processing by screen-printing were conducted solvent-free, making this composite the greenest electrode for this technology. This presentation gives an overview of recent research on improved materials for dielectric elastomer transducers (DETs) conducted at Empa. We are confident that our materials will impact fields including actuators, sensors, energy harvesting, artificial muscles, and soft robotics.
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