Increased electromechanical sensitivity of polysiloxane elastomers by chemical modification with thioacetic groups

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 degrees C), which allows for a wide range of use temperatures. Further they have an increased permittivity (epsilon' = 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/mu m was measured. (c) 2019 The Authors. Published by Elsevier Ltd.

High Permittivity Silicones for Dielectric Elastomer Actuators

Simon Johannes Dünki

Many technical advances of modern society sprout from the creation of specially customized polymer materials exhibiting unique combinations of properties uncharted so far. Amid this plurality of highly versatile materials, the class of electroactive polymers (EAPs) bears the special ability to undergo large shape changes when exposed to electrical stimulation. Such materials offer new perspectives in automation, robotics and medicine. Especially the dielectric elastomer actuators (DEA) technology is considered as highly promising among EAPs due to the simple working principle and actuation characteristics similar to mammalian muscles. Despite great research efforts and remarkable progress in regard to materials and devices, the commercial breakthrough of the DEA technology is still outstanding. A major obstacle for their application is the high driving voltage needed to obtain a reasonable actuation. The development of tailored elastomers that overcome this deficiency is therefore a main research focus in academia and industry. The exceptional elastic and dielectric characteristics of silicones nominate them as an ideal vantage point to start this endeavor. Their low permittivity inherently decreases the actuation performance and should therefore be optimized with all other properties remaining unaffected. In the present work, dipoles were covalently attached to a siloxane backbone in order to increase the permittivity thereof and achieve a material with an enhanced actuation performance at low driving voltages for DEAs. The introduction of the polar groups to the polymer side chains was carried out via thiol-ene chemistry, which enabled a quantitative functionalization of the polymer backbone. Three different polar groups, nitrile, thioether and sulfones, were investigated and the corresponding functionalized polysiloxanes were prepared and characterized. Thorough dielectric, mechanical and electromechanical investigations were conducted on polysiloxane based elastomers with different content of nitrile groups. In addition, first results were obtained with thioethers and sulfones which are also promising candidates to enhance the permittivity of polysiloxanes. Most of the synthesized polysiloxanes show glass transition temperatures well below room temperature, an important prerequisite to ensure elastic properties. Polysiloxanes that carry a butylthioether moiety at every repeat unit doubled the permittivity of the elastomer. With the same amount of the nitrile or sulfone dipoles, a six respectively eight times higher permittivity as compared to regular siloxane is achieved. However, with enhanced dipole content also an increase of the electrical conductivity of the polymer was observed. A simple one-step process to prepare cross-linked polar silicone films was developed using UV light triggered thiol-ene addition. A strong correlation between the dipole content in the material and the resulting actuation strain was found. Finally, a polar silicone elastomer was prepared that showed up to 20.5% lateral strain under a field as low as 10.8 V/um. Compared to commercial silicone this corresponds to an effective voltage reduction by about 70%. Processing such materials in films the thickness of which is below 10 um would result in an actuator that can be driven below 100 V.

EPFL2017

Transient Elastomers with High Dielectric Permittivity for Actuators, Sensors, and Beyond

Holger Frauenrath, Yauhen Sheima

Dielectric elastomers (DEs) are key materials in actuators, sensors, energy harvesters, and stretchable electronics. These devices find applications in important emerging fields such as personalized medicine, renewable energy, and soft robotics. However, even after years of research, it is still a great challenge to achieve DEs with increased dielectric permittivity and fast recovery of initial shape when subjected to mechanical and electrical stress. Additionally, high dielectric permittivity elastomers that show reliable performance but disintegrate under normal environmental conditions are not known. Here, we show that polysiloxanes modified with amide groups give elastomers with a dielectric permittivity of 21, which is 7 times higher than regular silicone rubber, a strain at break that can reach 150%, and a mechanical loss factor tan delta below 0.05 at low frequencies. Actuators constructed from these elastomers respond to a low electric field of 6.2 V m(-1), giving reliable lateral actuation of 4% for more than 30 000 cycles at 5 Hz. One survived 450 000 cycles at 10 Hz and 3.6 V m(-1). The best actuator shows 10% lateral strain at 7.5 V m(-1). Capacitive sensors offer a more than a 6-fold increase in sensitivity compared to standard silicone elastomers. The disintegrated material can be re-cross-linked when heated to elevated temperatures. In the future, our material could be used as dielectric in transient actuators, sensors, security devices, and disposable electronic patches for health monitoring.

AMER CHEMICAL SOC2022

Infrared properties of the Si-SiO2 interface from first principles

Feliciano Giustino

We study the infrared properties of the Si-SiO2 interface within a first-principles approach. In order to provide an atomic-scale description of the dielectric permittivity (both high-frequency and static) and of the infrared absorption at the interface, we introduce two theoretical schemes of general validity. First, we develop a method for investigating atomic-scale dielectric permittivity profiles across interfaces between insulators. From the microscopic charge density induced by an applied electric field, we calculate a local permittivity which describes variations of the dielectric response over length scales of the order of interatomic distances. In order to establish a relation between the dielectric response and the underlying microscopic structure, the local permittivity is further analysed in terms of maximally localized Wannier functions. Second, we develop a method for calculating from first principles both the transverse-optical and longitudinal-optical infrared absorption spectra at surfaces and interfaces. We derive expressions for the total absorption spectra of the system under consideration, and then define a spatial decomposition which provides the evolution of the infrared activity across that system. Such a decomposition is particularly suited for associating specific spectral features to the underlying local bonding arrangements. By using the first method, we determine the profile of the local permittivity across several structural models of the Si-SiO2 interface. We are able to show that the dielectric transition from the silicon to the oxide occurs within a width of only a few angstroms, and that the interfacial layer carries an enhanced permittivity with respect to bulk vitreous silica. Correspondingly, the equivalent oxide thickness of the interfacial oxide is found to be smaller than the corresponding physical thickness by 0.2–0.3 nm, with beneficial consequences for the scaling of Si-based electronic devices. By using the second method, we solve a long-standing controversy related to the red shifts of the high-frequency peaks observed in the infrared spectra of ultrathin oxides on silicon with decreasing thickness. By calculating the transverseoptical and the longitudinal-optical absorption spectra across a realistic model of the Si(100)-SiO2 interface, we are able to assign the microscopic origin of these shifts to the lengthening of the Si-O bonds in the interfacial substoichiometric oxide.