High dielectric permittivity elastomers: synthesis, processability, and device manufacturing

Abstract

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.