Silicone-based dielectric elastomers with high permittivity ionic liquid loading

Xue Liu

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Dielectric elastomers (DEs), which have high flexibility, low weight, high electrical and mechanical breakdown strength and elasticity, as well as great stability with respect to a large
range of chemicals, are well-known materials with a large variety of novel and advanced electromechanical applications, namely as actuators, sensors and generators. Silicone elastomers are one of the most promising materials for DEs, as they can be operated at a wide frequency range and offer high efficiency and fast response time. However, the most obvious challenge facing current silicone elastomers is that they possess low dielectric permittivity  and need high driving voltages to activate them, limiting the applications of silicone-based DEs. Therefore, new approaches must be undertaken in order to improving the performance of silicone-based DEs. The objective of this project is improving the performance of silicone-based DEs as actuators or sensors, particularly enhancing the dielectric properties, sensing properties, and biocompatibility. This ambitious goal is accomplished by incorporating liquid filler with high permittivity and a high biocompatible solid filler into silicone network, and each of the methods for improving the target property is demonstrated in the following chapters.
First, high-permittivity silicone-based DEs were prepared by synthesizing a silicone elastomer loaded with ionic liquids (ILs). The influence of the structure and the amounts of ILs on the material properties was discussed, and important properties for material applications as DEs, such as dielectric permittivity, gel fraction, and mechanical properties, were also investigated. It was found that 1-butyl-3-methylimidazolium hexafluoroantimonate (BmimSbF6) is the most suitable IL for the given system. The dielectric permittivity of the material with 90 parts per hundred rubber (phr) BmimSbF6 was 3.2 times higher than that of the pristine silicone elastomer. The Young’s modulus decreased in line with the increasing amount of BmimSbF6 in the elastomer, as expected. A simpler figure of merit (F*om) than the traditional figure of merit for actuators was used, and the F*om of the elastomer with a 90 phr IL loading was 10.4, thereby indicating that the material has a great advantage when used in actuators. 

Secondly, wool keratin, with high biodegradability, biocompatibility and stability, wasselected as a solid filler for increasing the mechanical properties and biocompatibility of the silicone-based DEs. Three keratin models and 621 ILs, including 27 cations and 23 anions,were used to evaluate keratin dissolution capability via a screening method based on COSMO-RS. From the prediction results of logarithmic activity coefficients (lnγ) for the three keratin models, it can be concluded that anions play a leading role in keratindissolution, while cations only have a moderate effect on the dissolution process. In addition, the experimental solubility of ten ILs was in accordance with that of the prediction lnγ,thereby indicating that the prediction value of lnγ via COSMO-RS can effectively reflect thekeratin dissolution capability of ILs.

Lastly, a novel pressure sensor with remarkably improved force sensing characteristics was obtained through combined usage of PDMS and IL. Keratin was dispersed homogeneously in the PDMS matrix to serve as a reinforcing filler. High conductivity IL was employed as sensitivity-enhancing constituent in the elastomer, and the effect of the amount of IL on elastomers’ pressure-sensing performance was investigated. The elastomer with 70 phr IL showed excellent pressure-sensing performance. This novel pressure sensor demonstrates high linear sensitivity (0.037 kPa-1) in a large pressure region of 0-10 kPa. Response and recovery times were 8 ms and 11 ms, respectively, which are much shorter than previously reported times. Moreover, the pressure sensor could distinguish different pressures via stable sensing signals in the pressure range of 0 to 50 kPa. The excellent performance of the novel pressure sensor has application potential in various fields, such as health monitoring and soft robotics.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages115
Publication statusPublished - 2020

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