Dielectric elastomer actuators (DEAs) can be optimized by modifying the dielectric or mechanical properties of the electroactive polymer. In this work both properties were improved simultaneously by a simple process, the one-step film formation for polyurethane and silicone films. The silicone network contains polydimethylsiloxane (PDMS) chains, as well as cross-linker and grafted molecular dipoles in varying amounts. The process leads to films, which are homogenous down to the molecular level and show higher permittivities as well as reduced stiffnesses. The dipole modification of a new silicone leads to 40 times higher sensitivities, compared to the unmodified films. This new material reaches the sensitivity of the widely used acrylate elatomer VHB4905. A similar silicone modification was obtained by using smart fillers consisting of organic dipoles and additional groups realizing a high compatibility to the silicon network. Polyurethanes are alternative elastomers for DEAs which are compared with the silicones in important properties. Polyurethanes have an intrinsically high dielectric constant (above 7), which is based on the polar nature of the polyurethane fragments. Polyurethanes can be made in roll-to-roll process giving constant mechanical and electrical properties on a high level.
Dielectric elastomer actuators (DEAs) can be optimized by modifying the dielectric or mechanical properties
of the electroactive polymer. In this work both properties were improved simultaneously by a simple process,
the one-step film formation. The silicone elastomer network contains polydimethylsiloxane (PDMS) chains, as
well as crosslinker and grafted molecular dipoles in varying amounts. This leads to films, which are homogenous
down to the molecular level. Films with higher permittivity and reduced stiffness were obtained. As matrix
two PDMS-materials with different molar masses, leading to other network chain lengths, were compared. This
directly influences the network density and thus the mechanical properties. A higher electrical field response for
long chain matrix materials was found. The actuation sensitivities for both materials were enhanced by 6.3 and
4.6 times for the short and long chain matrix material, respectively.
Silicone elastomers are highly suitable for application in the field of dielectric elastomer actuators (DEA) due
to their unique material properties (e.g. low glass temperature, thermal stability, large capability of chemical
tailoring). The elastomer forming Polydimethysiloxane (PDMS) employed for this study consists of chains
with vinyl termination and is cross linked via hydrosilylation to a cross linking molecule in the presence of
platinum catalyst. Here, dipole molecules (N-Allyl-N-methyl-4-nitroaniline) were specifically synthesized such
that they could chemically graft to the silicone network. The most prominent advantage of this approach is the
achievement of a homogeneous distribution of dipoles in the PDMS matrix and a suppression of phase separation
due to the grafting to the junction points of the rubber network. Several films with dipole contents ν ranging
from 0 %wt up to 10.9 %wt were prepared. The films were investigated to determine their mechanical (tensile
testing), dielectric (dielectric relaxation spectroscopy) and electrical (electrical breakdown) properties. This new
approach for composites on the molecular level leads to homogeneous films with enhanced material properties for
DEA applications. An increase in permittivity from 3.3 to 6.0, a decrease in electrical breakdown from 130 V/μm
to 50 V/μm and a lowering of the mechanical stiffness from 1700 kPa to 300 kPa was observed.
Dielectric elastomer actuators are soft electro-mechanical transducers with possible uses in robotic, orthopaedic and
automotive applications. The active material must be soft and have a high ability to store electrical energy. Hence, three
properties of the elastic medium in a dielectric elastomer actuator affect the actuation properties directly: dielectric
constant, electric breakdown strength, and mechanical stiffness. The dielectric constant of a given elastomer can be
improved by mixing it with other components with a higher dielectric constant, which can be classified as insulating or
conducting. In this paper, an overview of all approaches proposed so far for dielectric constant improvement in these soft
materials will be provided.
Insulating particles such as TiO2 nanoparticles can raise the dielectric constant, but may also lead to stiffening of the
composite, such that the overall actuation is lowered. It is shown here how a chemical coating of the TiO2 nanoparticles
leads to verifiable improvements. Conducting material can also lead to improvements, as has been shown in several
cases. Simple percolation, relying on the random distribution of conducting nanoparticles, commonly leads to drastic
lowering of the breakdown strength. On the other hand, conducting polymer can also be employed, as has been
demonstrated. We show here how an approach based on a specific chemical reaction between the conducting polymer
and the elastomer network molecules solves the problem of premature breakdown which is otherwise typically found.
We discuss various approaches to increasing the dielectric constant of elastomer materials, for use in dielectric elastomer
actuators. High permittivity metal-oxide nano-particles can show elevated impact compared to larger size particles, but
suffer from water uptake. Composites with conducting particles lead to extremely high permittivity caused by
percolation, but they often suffer early breakdown. We present experiments on approaches combining metal-oxides and
metal particles, which compensate for the drawbacks, and may lead to useful DEA materials in which all relevant
properties are technologically useful. The key seems to be to avoid percolation and achieve a constant nearest-neighbor
separation.
Dielectric elastomer actuators deform due to voltage-induced Maxwell-stress, which interacts with the mechanical
properties of the material. Such actuators are considered for many potential applications where high actuation strain and
moderate energy density comparable to biological muscle are required. However, the high voltage commonly required to
drive them is a limitation, especially for biomedical applications. The high driving voltage can be lowered by developing
materials with increased permittivity, while leaving the mechanical properties unaffected. Here, an approach to lowering
the driving voltage is presented, which relies on a grafted nano-composite, in which conducting nanoparticles are
integrated directly into a flexible matrix by chemical grafting. The conducting particles are π-conjugated soft
macromolecules, which are grafted chemically to a polymer matrix flexible backbone. Dielectric spectroscopy, tensile
mechanical analysis, and electrical breakdown strength tests were performed to fully characterize the electro-mechanical
properties. Planar actuators were prepared from the resulting composites and actuation properties were tested in two
different modes: constant force and constant strain. With this approach, it was found that the mechanical properties of the
composites were mostly unaffected by the amount of nanoparticles, while the permittivity was seen to increase from 2.0
to 15, before percolation made further concentration increases impossible. Hence, it could be demonstrated that the socalled
"optimum load" was independent from the permittivity (as expected), while the operating voltage could be
lowered, or higher strains could be observed at the same voltage.
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