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Humidity, temperature and membrane prestretch influence the dielectric breakdown strength of elastomer membranes. These three factors thus also influence the maximum force and strain that dielectric elastomer actuators (DEAs) can generate, as the Maxwell pressure is proportional to the applied voltage squared, and limited by the breakdown field. We compared several commercial silicones, 10 to 25 µm thick under different equibiaxial prestretch conditions, for temperatures between 20 °C and 80 °C and relative humidity from 10% to 90%, measuring both breakdown fields and mechanical properties in order to compute two figures of merit for DEAs. The silicone films all have breakdown strengths of order 100 V µm−1. Higher humidity (90% RH) leads to decreased dielectric breakdown strengths compared to lower humidity (10% RH): up to 43% reduction for prestretches of 1.3 and 1.5, but only 2% to 10% reduction for prestretch of 1.1. Higher prestretch leads to up to 50% higher breakdown field, but also leads to higher effective Young’s modulus due to strain stiffening. Higher temperatures (80 °C) lead to up to 30% lower breakdown voltages compared to lower temperatures (20 °C). Higher prestretch generally enhances the maximum Maxwell stress because of higher dielectric breakdown strength, but reduces the strain figure of merit owing to increased Young’s moduli at high prestretch. For silicone elastomers, Sylgard 184 shows the highest stress figures of merit and LSR 4305 the highest strain figures of merit. Data for VHB is also presented for comparison. This work allows identifying elastomers better suited to harsh environments, and to selecting operating voltages that enable safe operation for a wide range of environmental conditions.

Dielectric elastomer actuators (DEAs) have gained growing interest during the last decade from both the scientific community and industry. However, the high operating voltages needed to drive these actuators reduce their application potential. To date, several reports have been published on high permittivity elastomers that can be operated at low electric fields; however, little attention has been given to their processability into thin films and reliability as dielectrics in actuators. This work presents the synthesis of low molecular weight polar polysiloxanes, which have low viscosity and can, therefore, be processed into thin films by the solvent-free doctor blade deposition technique. Additionally, the prepared polysiloxanes have vinyl end-groups which are subsequently used for cross-linking into thin films, where multifunctional thiol cross-linkers and a UV initiator are used. The prepared films are elastic and show a permittivity of about two times higher as compared to commercial polydimethylsiloxane elastomers. In addition, their elastic modulus is easily tuned between 1.2 and 0.4 MPa using a dithiol as a chain prolongation reagent, while their viscoelastic behavior is comparable to commercial silicone elastomers. When used as dielectrics in DEAs the developed elastomers allow reliable operation over 180 000 actuation cycles at reduced voltages.

Chemically cross-linked polydimethylsiloxane (PDMS) elastomers have gained importance as a dielectric in elastic capacitor actuators, which elongate when electrically charged. Common PDMS elastomers have a dielectric permittivity of only about 3 and thus elongations interesting for real applications occur only at high voltages. In this work, a new class of silicone-based elastomers with increased permittivity are synthesized starting from polymethylvinylsiloxane and thioacetic acid via a photo-induced thiol-ene reaction, whereby polar thioacetate groups are introduced at every siloxy unit. The silanol end-groups of the formed polymer are subsequently used for cross-linking into thin films using a condensation reaction with poly(methylhydrosiloxane-co-dimethylsiloxane). These films show good mechanical properties and have a low glass transition temperature (-58 °C), which allows for a wide range of use temperatures. Further they have an increased permittivity (ε’ = 4.7) and an increased sensitivity to electric field as compared to regular polydimethylsiloxane elastomers. Our new elastomers are easily accessible and can be processed into very thin films, which allows access to actuators operable way below 1000 V. An actuation strain of 12.8% at an electric field of 21.5 V/μm was measured. Keywords: Stimuli responsive polymers, Electroresponsive polymers, Dielectric elastomer actuators, Polar silicones, Dielectric permittivity, Thiolene addition