Ionic liquids utilized in silicone elastomers for transducer applications

Dielectric elastomer transducers (DETs), which are lightweight and have high flexibility, considerable actuation strain, and fast response are of great interest in various electromechanical applications including sensors, actuators, and  generators. DETs consist of a flexible dielectric elastomer (DE) membrane sandwiched by two compliant electrodes, thus forming a variable capacitor capable of energy transduction. Silicone DEs are one of the most promising materials for DETs due to their high energy density, low viscoelasticity, and large Maxwell stress. However, due to the low relative permittivity and high elastic modulus of silicone dielectric elastomers, the driving voltages of the resulting DETs are usually high when working in actuator mode, and the sensitivity is relatively low when working in sensor mode. The low sensitivity and high driving voltage of silicone-based DETs limit their societal and commercial applications in artificial muscles, soft robotics, wearable electronics, and haptic devices.

The performance of DETs can be improved by optimizing the dielectric and mechanical properties of the silicone DEs and/or by designing suitable compliant electrodes. Ionic liquids with inherent high relative permittivity, conductivity, nonvolatility, and chemical and thermal stability, not only can be applied to optimize the relative permittivity and elastic modulus of silicone DEs, but also can be used as highly compliant and conductive electrodes of DETs. The objective of this project is therefore accomplished by chemically modifying silicone DEs with ionic liquids and designing a suitable ionic liquid-based electrode to improve the performance of DETs.

Firstly, a flexible, and highly sensitive pressure sensor is developed by using an elastomeric dielectric layer with particularly high permittivity and homogeneity due to the inclusion of synthesized ionic liquid-grafted silicone oil. The ionic liquid-grafted silicone oil not only possesses a very high relative permittivity (9.6×105 at 10^-1 Hz) but also has excellent compatibility with silicone elastomers due to the covalently connected structure of ionic liquid and chloropropyl silicone oil. An optimal silicone elastomer with 10 phr of the ionic liquid-grafted silicone oil presents a relative permittivity of 22 at 10^-1 Hz, Young’s modulus of 0.78 MPa, and excellent homogeneity. A high-performance pressure sensor is fabricated by using the optimized silicone elastomer. The sensitivity of the pressure sensor is 0.51 kPa^-1, which is 100 times higher than that of the pristine elastomer. In addition, high durability, and rapid response and recovery time of approximately 60 ms are achieved.

Secondly, a novel strategy is developed to prepare silicone elastomers via the crosslinking reaction between multifunctional imidazole-grafted PDMS with difunctional bis(1-ethylene-imidazole-3-ium) bromide ionic liquid (bis-IL). The prepared ionic liquid crosslinked elastomer entails uniformly dispersed ionic liquid and presents stable mechanical and dielectric properties due to the covalent nature of the crosslinking as opposed to previously reported physical mixing in of ionic liquids. The relative permittivity is improved up to 200 % by including the bis-IL in the elastomer and the Young’s modulus is around 0.04 MPa. As a result of the excellent combination of properties, the dielectric actuator developed exhibits an area strain of 20 % at 15 V/μm.

Lastly, a fibre actuator assembled from a silicone elastomer shaped as a hollow fibre and ionic liquid-based electrodes is developed. Due to the spinning process, the developed and optimized silicone hollow fibre with a small outer diameter of 463 μm and uniform wall thickness of 78 μm shows a ~5-fold increase in tensile strength (0.64 MPa) and ~7-fold increase in tensile strain (596 %) compared to these of the reference planar film. Due to the highly transparent PDMS elastomer layer and ionic electrodes, the fibre DE actuator presents transparency of ~91 % in the visible light spectrum. Interestingly, the fibre DE actuator exhibits a large linear strain of 9 %, and repeatable and stable linear actuation strain over 1000 cyclic actuation tests. Furthermore, the fibre DE actuator can be assembled into bundles for increased forces.