High dielectric permittivity sulfonyl-modified polysiloxanes as a dielectric for soft actuators

Abstract

Dielectric elastomer actuators (DEAs) are soft transducers with great potential in soft robotics applications. Reducing the driving voltage and increasing the reliability of DEAs remain major challenges that need to be addressed. This can be achieved using thin dielectric elastomer films with high dielectric permittivity and low Young's modulus. Herein, high dielectric permittivity polysiloxanes were synthesized by functionalizing a polymethyvinylsiloxane with varying ratios of polar 3-mercaptosulfolane and mercaptobutyl groups, which allows for fine-tuning the dielectric permittivity of the resulting polymers. Thin films were cast using the doctor blade process and subsequently cross-linked via a UV-induced thiol–ene addition reaction to yield elastomers with a maximum dielectric permittivity of 15 at 1 Hz caused by the permanent dipole polarization. Differential scanning calorimetry shows a glass transition temperature (T_g) below room temperature for all polymers. Dielectric impedance spectroscopy at different frequencies and temperatures revealed a secondary relaxation transition attributed to adsorbed water influencing the dielectric response of nearby polar groups. While such effects have been observed in other polymers, this is the first time they have been demonstrated in polar polysiloxanes. The polymer containing half of the repeat units modified by the sulfonyl group and half by butyl exhibited the most suitable dielectric and mechanical properties and, therefore, was further investigated as a dielectric in actuators. DEAs constructed from it can be operated over a wide voltage range and exhibit a lateral actuation strain of 7.2% at 14 V μm⁻¹. Stack actuators constructed from this material exhibited an in-thickness actuation strain of 2.5% at 13.8 V μm⁻¹ only, despite using a dielectric film with a thickness of 145 μm. Although our stack exhibits a lower dielectric breakdown field than stacks using acrylates or polydimethylsiloxane elastomers, it operates effectively at much lower electric fields. Since actuation scales with the inverse square of the dielectric thickness, reducing the thickness greatly enhances not only the actuation but also the breakdown field, supporting applications where low-voltage operation is critical.