Actuator materials that can reproduce the multifunctionality of natural muscles have long been desired for the development of biologically inspired robots. Electroactive polymers (EAPs) have attracted increasing attention as potential candidates for artificial muscles. While several types of EAPs have been investigated, [1][2][3][4] electroelastomers, also known as dielectric elastomers, have been particularly attractive for largestrain and high-power applications. [5][6][7] Electroelastomers based on acrylic copolymer elastomers (e.g., 3M VHB 4910) and compliant electrodes have been shown to exhibit electromechanical strains of up to 380 % in terms of area expansion. Furthermore, the specific elastic energy density (3.4 J g -1), stress (up to 8 MPa), and electromechanical conversion efficiency (60-90 %) are all extraordinarily high. However, this outstanding performance is only observed when the acrylic films are highly prestrained. The reported specific elastic-energy density and stress are calculated from the weight or volume of the active acrylic elastomers. The performance of the packaged actuators is substantially lower.A number of actuator configurations, such as bow, bowtie, rigid-frame, diaphragm, and spring-roll actuators, have been designed to support the required high prestrain. [8,9] Each of these designs has its own unique advantages for certain applications, but without exception, the prestrain-supporting structures occupy significantly more space and weigh significantly more than the films themselves. The consequence of this is that the supporting structures cause a large performance gap between the active material and the packaged actuators. In addition, the lifetimes of the actuators are limited by the concentration of stress at the interfaces between the soft polymer film and the rigid supporting structure. The shock tolerance of the actuators is also reduced because of the introduction of rigid structural components. The prestrained films exhibit stress relaxation that affects the subsequent actuation.[10]Therefore, it would be highly desirable to eliminate mechanical prestraining while still retaining its performance benefits. We report here the development of new electroelastomers that exhibit high strain without requiring high prestrain. Electrically induced strain is proportional to the square of the applied electric field. High strain necessitates high breakdown strength. Prestrain enhances the dielectric-breakdown field of the elastomer films.[11] Mechanistic reasons for the enhancement of dielectric strength via mechanical prestraining are not well understood; we attribute it to the increased probability of hot electrons colliding with polymer chains realigned in parallel to the film surfaces. The prestrain also realigns defects, such as fibrous impurities, non-spherical voids, and gel particles, which may be responsible for premature dielectric breakdown. Here, we describe new interpenetrating elastomeric networks in which the benefits resulting from prestrain are obtained without exte...
Certain dielectric elastomers such as the 3M VHB 4910 acrylic adhesive films have exhibited electrically induced strains as high as 380 % in area expansion. The calculated maximum specific energy density of 3.4 J g -1 and maximum stress of 8 MPa are attractive for a wide range of applications including robotics, prosthetic devices, medical implants, pumps, and valves. [1][2][3][4][5][6] However, the performance of actuators based on the VHB films is substantially lower than the calculated values which reflect the maximum intrinsic material properties. Defects in the soft dielectric films, such as gel particles, uneven thickness, non-uniform crosslinking, and stress concentration, are possible causes for the reduced actuator performance. They also reduce the actuator reliability, hindering technology commercialization. [5,6] We introduce self-clearable compliant electrode materials that could enhance the fault-tolerance of the dielectric elastomers actuators. Thin metallic layers (10-100 nm) are being used for polypropylene thin-film capacitors for fault-tolerance. The electrode materials locally evaporate, or self-clear, around defects during highvoltage breakdown. [7][8][9] Metallic films are not sufficiently compliant for the dielectric elastomers. To enhance the compliancy of metallic electrodes, zig-zag shaped metallic lines were used to obtain 80 % area strains.[10] Uniform metallic coatings on corrugated silicone elastomer surfaces supported linear strains up to 33 %. [11] The commonly used compliant electrode materials for the VHB elastomers are powdered carbon graphite, carbon black, or carbon fibrils dispersed in grease or silicon oil, and electrolyte solutions. [12] No self-clearing has been reported for these electrode materials. Our experiments showed that single-walled carbon nanotubes (SWNTs) can overcome this morass. The SWNT electrodes have exhibited flexibility as transparent electrodes in lightemitting diodes, solar cells, and thin-film transistors. [13][14][15][16] With thin SWNT electrodes, the dielectric elastomer can not only be strained larger than 200 % in area expansion, but can achieve fault-tolerance through localized degradation of SWNTs. Figure 1 shows a 300 % biaxially prestrained VHB 4910 film (62 lm thick after prestrain) with the SWNT electrodes at rest (Fig. 1a) and during actuation at 5 kV (Fig. 1b). The calculated strain is 200 %. This value is comparable with the same film using conventional carbon grease electrodes. The observed strain for 300 % biaxially prestrained VHB 4905 films (31 lm thick after prestrain) was 190 % at 3.5 kV.The thickness of the SWNT electrodes greatly affects the obtained strain. Figure 2 shows the strain as a function of the applied voltage for different thicknesses of SWNT on a 300 % biaxially prestrained VHB 4905 film. For comparison, equal thicknesses of SWNT were applied on the two surfaces of each acrylic elastomers film. The surface resistances were 0.1, 0.2, 0.4, 4, and 20 kX square -1 , corresponding to calculated SWNT thicknesses of 250, 125,...
Mechanical prestrain is generally required for most electroelastomers to obtain high electromechanical strain and high elastic energy density. However, prestrain can cause several serious problems, including a large performance gap between the active materials and packaged actuators, instability at interfaces between the elastomer and prestrain-supporting structure, and stress relaxation. Difunctional and trifunctional liquid additives were introduced into 400% biaxially prestrained acrylic films and subsequently cured to form the second elastomeric network. The goal of this research was to determine the effect of different functional additives and concentrations on the microstructure, the mechanical properties, and the actuation of composite films. In the as-obtained interpenetrating polymer networks (IPNs), the additive network can effectively support the prestrain of the acrylic films and as a result, eliminate the external prestrain-supporting structure. However, the large amount of additive used to completely preserve prestrain was found to make the films too stiff, causing damage to IPN composite films. Furthermore, the interpenetrating network formed from a trifunctional monomer is more effective than that formed from a difunctional monomer in supporting the high tension of the VHB network. This high efficiency trifunctional additive leads to the enhancement of the breakdown field, due to less damage on the microstructure. The IPN composite films without external prestrain exhibit electrically-induced strains up to 300% in area, comparable to those of VHB 4910 films under high prestrain conditions.
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