Dielectric elastomer (DE) sensors have great potential for applications in soft robotics, wearable devices and medical diagnostic. A novel pressure sensor with remarkably improved force sensing characteristics was obtained through combined usage of polydimethylsiloxane (PDMS) and ionic liquid (IL). The regenerated keratin from wool was added and dispersed homogeneously in the PDMS matrix acting as reinforcing fillers. The influence of the amount of IL on the electro-mechanical properties of the composites was investigated. One obvious result was that the permittivity of the ILcontaining elastomers increased dramatically with the increased amount of IL loaded. Furthermore, the sensitivity of the composite elastomers as pressure sensors was investigated by recording the response of the voltage when a small force is applied to the top surface of the pressure sensor. The elastomers with IL loaded exhibit excellent response of the voltage and the maximum sensitivity of the composite elastomer is 2.64 mV/N.
Novel insights into the electromechanical failure of dielectric elastomers are presented. Measurements are conducted by coupling a high-speed camera to electrical breakdown strength equipment. It is shown that the breakdown behavior is far from simple since the thin elastomer film undergoes complex dimensional changes before the breakdown. Multiple geometries of the electrodes were investigated and different behaviors were observed. The breakdown patterns were categorized and the underlying theory behind this complex process will be presented.
Silicone elastomers have been heavily investigated as candidates for dielectric elastomers and are as such almost ideal candidates with their inherent softness and compliance but they suffer from low dielectric permittivity. This shortcoming has been sought optimized by many means during recent years. However, optimization with respect to the dielectric permittivity solely may lead to other problematic phenomena such as premature electrical breakdown. In this work, we investigate the electrical breakdown phenomena of various types of permittivity-enhanced silicone elastomers. Two types of silicone elastomers are investigated and different types of breakdown are discussed. Furthermore the use of voltage stabilizers in silicone-based dielectric elastomers is investigated and discussed.
High driving voltages currently limit the commercial potential of dielectric elastomers (DEs). One method used to lower driving voltage is to increase dielectric permittivity of the elastomer. A novel silicone elastomer system with high dielectric permittivity was prepared through the synthesis of siloxane copolymers, thereby allowing for the attachment of high dielectric permittivity molecules through copper-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC). The synthesized copolymers allow for a high degree of chemical freedom, as several parameters can be varied during the preparation phase. Thus, the space between the functional groups can be varied, by using different dimethylsiloxane spacer units between the dipolar molecules. Furthermore, the degree of functionalization can be varied accurately by changing the feed of dipolar molecules. As a result, a completely tunable elastomer system, with respect to functionalization, is achieved. It is investigated how the different functionalization variables affect essential DE properties, including dielectric permittivity, dielectric loss, elastic modulus and dielectric breakdown strength, and the optimal degree of chemical functionalization, where these important properties are not significantly compromised, is also determined. Thus, the best overall properties were obtained for a silicone elastomer prepared with 5.6 wt% of the dipolar molecule 1-ethynyl-4-nitrobenzene. Here, a high increase in dielectric permittivity (~70%) was obtained without compromising other vital DE properties such as elastic modulus, gel fraction, dielectric and viscous loss and electrical breakdown strength.
The energy density of dielectric elastomers (DEs) is sought increased for better exploitation of the DE technology since an increased energy density means that the driving voltage for a certain strain can be lowered in actuation mode or alternatively that more energy can be harvested in generator mode. One way to increase the energy density is to increase dielectric permittivity of the elastomer. A novel silicone elastomer system with high dielectric permittivity was prepared through the development of interpenetrating networks from ionically assembled silicone polymers and covalently crosslinked silicones. The system has many degrees of freedom since the ionic network is formed from two polymers (amine and carboxylic acid functional, respectively) of which the chain lengths can be varied, as well as the covalent silicone elastomer with many degrees of freedom arising from amongst many the varying content of silica particles. A parameter study is performed to elucidate which compositions are most favorable for the use as dielectric elastomers. The elastomers were furthermore shown to be self-repairing upon electrical breakdown.
Dielectric elastomers (DEs) have many favourable properties. The obstacle of high driving voltages, however, limits the commercial viability of the technology at present. Driving voltage can be lowered by decreasing the Young’s modulus and increasing the dielectric permittivity of silicone elastomers. A decrease in Young’s modulus, however, is often accompanied by the loss of mechanical stability and thereby the lifetime of the DE. New soft elastomer matrices with high dielectric permittivity and low Young’s modulus, with no loss of mechanical stability, were prepared by two different approaches using chloropropyl-functional silicone polymers. The first approach was based on synthesised chloropropyl-functional copolymers that were cross-linkable and thereby formed the basis of new silicone networks with high dielectric permittivity (e.g. a 43% increase). These networks were soft without compromising other important properties of DEs such as viscous and dielectric losses as well as electrical breakdown strength. The second approach was based on the addition of commercially available chloropropyl-functional silicone oil to commercial LSR silicone elastomer. Two-fold increase in permittivity was obtained by this method and the silicone oil decreased the Young’s modulus significantly. The viscous losses, however, also increased with increasing content of silicone oil. Cross-linkable chloropropyl-functional copolymers offer a new silicone elastomer matrix that could form the basis of dielectric elastomers of the future, whereas the chloropropyl silicone oil approach is an easy tool for improvement of the properties of existing commercial silicone elastomers.
Liquid silicone rubbers (LSRs) have been shown to possess very favorable properties as dielectric electroactive polymers
due to their very high breakdown strengths (up to 170 V/μm) combined with their fast response, relatively high tear
strength, acceptable Young’s modulus as well as they can be filled with permittivity enhancing fillers. However, LSRs
possess large viscosity, especially when additional fillers are added. Therefore both mixing and coating of the required
thin films become difficult. The solution so far has been to use solvent to dilute the reaction mixture in order both to
ensure better particle dispersion as well as allowing for film formation properties. We show that the mechanical
properties of the films as well as the electrical breakdown strength can be affected, and that the control of the amount of
solvent throughout the coating process is essential for solvent borne processes. Another problem encountered when
adding solvent to the highly filled reaction mixture is the loss of tension in the material upon large deformations. These
losses are shown to be irreversible and happen within the first large-strain cycle.
A new approach based on silicone interpenetrating networks with orthogonal chemistries has been investigated with
focus on developing soft and flexible elastomers with high energy densities and small viscous losses. The
interpenetrating networks are made as simple two pot mixtures as for the commercial available silylation based
elastomers such as Elastosil RT625. The resulting interpenetrating networks are formulated to be softer than RT625 to
increase the actuation caused when applying a voltage due to their softness combined with the significantly higher
permittivity than the pure silicone elastomers.
To our knowledge no known technologies or processes are commercially available for embossing microstructures and
sub-micron structures on elastomers like silicones in large scale production of films. The predominantly used
technologies to make micro-scale components for micro-fluidic devices and microstructures on PDMS elastomer are 1)
reaction injection molding 2) UV lithography and 3) photolithography, which all are time-consuming and not suitable for
large scale productions. A hot-embossing process to impart micro-scale corrugations on an addition curing vinyl
terminated PDMS (polydimethyl siloxane) film, which is thermosetting elastomer, was established based on the existing
and widely applied technology for thermoplasts. We focus on hot-embossing as it is one of the simplest, most costeffective
and time saving methods for replicating structures for thermoplasts. Addition curing silicones are shown to
possess the ability to capture and retain an imprint made on it 10-15 minutes after the gel-point at room temperature. This
property is exploited in the hot-embossing technology.