Anne Ladegaard Skov

The Current State of Silicone‐Based Dielectric Elastomer Transducers

Silicone elastomers are promising materials for dielectric elastomer transducers (DETs) due to their superior properties such as high efficiency, reliability and fast response times. DETs consist of thin elastomer films sandwiched between compliant electrodes, and they constitute an interesting class of transducer due to their inherent lightweight and potentially large strains. For the field to progress towards industrial implementation, a leap in material development is required, specifically targeting longer lifetime and higher energy densities to provide more efficient transduction at lower driving voltages. In this review, the current state of silicone elastomers for DETs is summarised and critically discussed, including commercial elastomers, composites, polymer blends, grafted elastomers and complex network structures. For future developments in the field it is essential that all aspects of the elastomer are taken into account, namely dielectric losses, lifetime and the very often ignored polymer network integrity and stability.

A new soft dielectric silicone elastomer matrix with high mechanical integrity and low losses

Though dielectric elastomers (DEs) have many favourable properties, the issue of high driving voltages limits the commercial viability of the technology. Driving voltage can be lowered by decreasing the Youngs modulus and increasing the dielectric permittivity of silicone elastomers. A decrease in Youngs modulus, however, is often accompanied by the loss of mechanical stability and thereby the lifetime of the DE. A new soft elastomer matrix, with no loss of mechanical stability and high dielectric permittivity, was prepared through the use of alkyl chloride-functional siloxane copolymers. Furthermore, the increase in dielectric permittivity (43%) was obtained without compromising other important properties of DEs such as viscous and dielectric losses as well as electrical breakdown strength.

High Breakdown-Strength Composites from Liquid Silicone Rubbers

In this paper we investigate the performance of liquid silicone rubbers (LSRs) as dielectric elastomer transducers. Commonly used silicones in this application include room-temperature vulcanisable (RTV) silicone elastomers and composites thereof. Pure LSRs and their composites with commercially available fillers (an anatase TiO2, a core–shell TiO2-SiO2 and a CaCu3Ti4O12 filler) are evaluated with respect to dielectric permittivity, elasticity (Youngs modulus) and electrical breakdown strength. Film formation properties are also evaluated. The best-performing formulations are those with anatase TiO2 nanoparticles, where the highest relative dielectric permittivity of 5.6 is obtained, and with STX801, a core–shell morphology TiO2-SiO2 filler from Evonik, where the highest breakdown strength of 173 V μm−1 is obtained.

Dipolar cross-linkers for PDMS networks with enhanced dielectric permittivity and low dielectric loss

Dipole grafted cross-linkers were utilized to prepare polydimethylsiloxane (PDMS) elastomers with various chain lengths and with various concentrations of functional cross-linker. The grafted cross-linkers were prepared by reaction of two alkyne-functional dipoles, 1-ethynyl-4-nitrobenzene and 3-(4-((4-nitrophenyl)diazenyl)phenoxy)-prop-1-yn-1-ylium, with a synthesized silicone compatible azide-functional cross-linker by click chemistry. The thermal, mechanical and electromechanical properties were investigated for PDMS films with 0 to 3.6 wt% of dipole-cross-linker. The relative dielectric permittivity was found to increase by 20% at only 0.46 wt% of incorporated dipole without significant changes in the mechanical properties. Furthermore, the dielectric losses were proved to be remarkably low while the electrical breakdown strengths were high.

Visualisation and characterisation of heterogeneous bimodal PDMS networks

The existence of short-chain domains in heterogeneous bimodal PDMS networks has been confirmed visually, for the first time, through confocal fluorescence microscopy. The networks were prepared using a controlled reaction scheme where short PDMS chains were reacted below the gelation point into hyperbranched structures using a fluorescent silicone compatible cross-linker. The formation of the hyperbranched structures was confirmed by FTIR, 1H-NMR and size exclusion chromatography (SEC). The short-chain hyperbranched structures were thereafter mixed with long-chain hyperbranched structures to form bimodal networks with short-chain domains within a long-chain network. The average sizes of the short-chain domains were found to vary from 2.1 to 5.7 μm depending on the short-chain content. The visualised network structure could be correlated thereafter to the elastic properties, which were determined by rheology. All heterogeneous bimodal networks displayed significantly lower moduli than mono-modal PDMS elastomers prepared from the long polymer chains. Low-loss moduli as well as low-sol fractions indicate that low-elastic moduli can be obtained without compromising the networks structure.

Investigation of the properties of fully reacted unstoichiometric polydimethylsiloxane networks and their extracted network fractions

We investigated the linear dynamic response of a series of fully reacted unstoichiometric polydimethylsiloxane networks and of the two corresponding network fractions namely the sol and the washed network. The sol and the washed network were separated by a simple extraction process. This way, it was possible to obtain rheological data from the washed network without interference from the sol fraction and furthermore from the sol fraction without interference from the elastic washed network. When the stoichiometry increased towards perfectly reacted networks and beyond, we observed harder networks both qualitatively and by rheology, and the properties of the two fractions became more and more different. At the gel point, the sol fraction and the washed networks have more or less identical properties which our data also show. The storage and loss moduli, G′ and G″, were analysed with the gel equation as proposed by Winter and Chambon (J Rheol 30:367–382, 1986) and Chambon and Winther (J Rheol 31:683–697, 1987). We observed that one of the investigated samples which before the swelling experiment did not show any elastic response gave an elastic washed network after swelling; this was verified by analysis with the gel equation. We also calculated the weight fraction of the sol fraction by using the theory by Villar et al. (Macromolecules 29(11):4072–4080, 1996) and compared this with experimentally found values.

Synthesis of telechelic vinyl/allyl functional siloxane copolymers with structural control

Multifunctional siloxane copolymers with terminal vinyl or allyl functional groups are synthesised through the borane-catalysed polycondensation of hydrosilanes and alkoxysilanes. Copolymers of varying molecular weights (w = 13 200–70 300 g mol−1), spatially well-distributed functional groups and high end-group fidelity are obtained in a facile and robust synthetic scheme involving polycondensation, end-group transformation and different functionalisation reactions such as Cu(I)-mediated azide–alkyne cycloaddition. Pendant alkyl chloride, alkyl azide, bromoisobutyryl, 4-nitrobenzene and 1-ethyl-imidazolium chloride fragments with programmable spatial distributions are incorporated in the copolymer backbones. NMR and FTIR spectroscopy as well as size exclusion chromatography corroborate the efficacy and versatility of this modular approach.

Enhancement of dielectric permittivity by incorporating PDMS-PEG multiblock copolymers in silicone elastomers†

A silicone elastomer from PDMS-PEG multiblock copolymer has been prepared by use of silylation reactions for both copolymer preparation and crosslinking. The dielectric and mechanical properties of the silicone elastomers were carefully investigated, as well as the morphology of the elastomers was investigated by SEM. The developed silicone elastomers were too conductive to be utilized as dielectric elastomers but it was shown that when the above silicone elastomers were mixed with a commercial silicone elastomer, the resulting elastomer had very favourable properties for dielectric elastomers due to a significantly increased dielectric permittivity. The conductivity also remained low due to the resulting discontinuity in PEG within the silicone matrix.

The electrical breakdown strength of pre-stretched elastomers, with and without sample volume conservation

In practice, the electrical breakdown strength of dielectric electroactive polymers (DEAPs) determines the upper limit for transduction. During DEAP actuation, the thickness of the elastomer decreases, and thus the electrical field increases and the breakdown process is determined by a coupled electro-mechanical failure mechanism. A thorough understanding of the mechanisms behind the electro-mechanical breakdown process is required for developing reliable transducers. In this study, two experimental configurations were used to determine the stretch dependence of the electrical breakdown strength of polydimethylsiloxane (PDMS) elastomers. Breakdown strength was determined for samples with and without volume conservation and was found to depend strongly on the stretch ratio and the thickness of the samples. PDMS elastomers are shown to increase breakdown strength by a factor of ~3 when sample thickness decreases from 120 to 30 μm, while the biaxial pre-stretching (λ = 2) of samples leads similarly to an increase in breakdown strength by a factor of ~2.5.

Dielectric elastomers, with very high dielectric permittivity, based on silicone and ionic interpenetrating networks

Dielectric elastomers (DEs), which represent an emerging actuator and generator technology, admittedly have many favourable properties, but their high driving voltages are one of the main obstacles to commercialisation. One way to reduce driving voltage is by increasing the ratio between dielectric permittivity and the Youngs modulus of the elastomer. One system that potentially achieves this involves interpenetrating polymer networks (IPNs), based on commercial silicone elastomers and ionic networks from amino- and carboxylic acid-functional silicones. The applicability of these materials as DEs is demonstrated herein, and a number of many and important parameters, such as dielectric permittivity/loss, viscoelastic properties and dielectric breakdown strength, are investigated. Ionic and silicone elastomer IPNs are promising prospects for dielectric elastomer actuators, since very high permittivities are obtained while dielectric breakdown strength and Youngs modulus are not compromised. These good overall properties stem from the softening effect and very high permittivity of ionic networks – as high as e′ = 7500 at 0.1 Hz – while the silicone elastomer part of the IPN provides mechanical integrity as well as relatively high breakdown strength. All IPNs have higher dielectric losses than pure silicone elastomers, but when accounting for this factor, IPNs still exhibit satisfactory performance improvements.