Mechanical,dielectric, and electromechanical properties of silicone dielectric elastomer actuators

Silicone elastomer actuators were investigated to develop a simple and industrially scalable product with improved mechanical properties, such as a low modulus, high tearing strength, and good resilience, and enhanced electromechanical actuation properties. Silicone elastomers were fabricated via a hydrosilylation addition reaction with a vinyl‐end‐functionalized poly(dimethyl siloxane) (V), a multivinyl‐functionalized silicone resin, and a crosslinker in the presence of a platinum catalyst. For the larger electromechanical actuation response, the silicone dielectric elastomer actuator had to have a larger molecular weight of poly(dimethyl siloxane), a smaller hardener content, and a resin‐free composition. However, the silicone elastomer actuators needed to include a small amount of resin to improve the tearing strength. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40030.     

Preparation and electrodeformation of silicone dielectric elastomers containing poly(propylene glycol diacetate) with different molecular weights

In this study, poly(propylene glycol diacetate)s (PPGDAs) with different molecular weights were obtained by the esterification reaction of poly(propylene glycol) and acetic anhydride. We effectively reduced the residual moisture and hydrophilicity of PPGDA. Then, poly(dimethyl siloxane) (PDMS) was modified by the addition of only 5 wt % PPGDA, which possessed a high dielectric constant (k) and a large actuated strain at a low electric field. PPGDA was used to enhance the molecular polarity because of the more polar oxygen atoms and the greater number of ester groups. The great increase in k and the low elastic modulus of the PPGDA–PDMS composites lead to a great increase in the electromechanical sensitivity. When the molecular weight of PPGDA was about 4000, the PPGDA–PDMS composites had the largest actuated strain. As a result, compared to the pure silicone elastomer (8.94%), it exhibited a greater strain of 17.31% at a low electric field of 10.5 V/μm (an increase of ca. 1.94 times). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45329.     

Octakis(phenyl)-T8-silsesquioxane-filled silicone elastomers with enhanced electromechanical capability

The first dielectric elastomer actuators based on electroactive nanocomposites with octakis(phenyl)-T8-silsesquioxane (phenyl-T8), obtained ex-situ, used as voltage stabilizer filler for silicone elastomers are reported. The incorporation and homogeneous dispersion of crystalline phenyl-T8 in percentages of 2.5, 3.5, 5, and 10 into the amorphous matrix consisting in a polydimethylsiloxane-α,ω-diol with Mn = 240,000 g/mol was successfully achieved by solution mixing and crosslinking. For the sample with the best actuation performance (that containing 3.5 wt.% filler), an optimized filled elastomer was obtained by dispersing 3.6 wt. % phenyl-T8 in the matrix using a suitable surfactant (Pluronic L81), thus gaining an increased electrical breakdown of 30% compared with the pristine sample. Beside dielectric strength, the matured films were characterized in terms of morphology, mechanical, dielectric and actuation tests. In spite of structural incompatibility between the filler and the matrix, the obtained materials are soft elastomers showing high strain (~800%) and low Young's modulus of 50–100 kPa. The use of phenyl-T8 in a silicone matrix lead to electroactive films with slightly increased lateral actuation strain and electric breakdown strength.     

Improvement of actuation performance of dielectric elastomers by barium titanate and carbon black fillers

Dielectric elastomers are promising materials for actuators resembling human muscle. Among elastomers, acrylic rubbers (ACM) have shown good actuation performance but its use is limited by the high operating voltages required. The present work demonstrates that simultaneous incorporation of nanostructured carbon black and dielectric fillers offers an increase in a dielectric permittivity and a suitable modulus of the elastomers matrix, enabling an improved electro‐mechanical actuation performance at low voltages. By the use of reinforcing carbon black and barium titanate in an acrylic elastomer matrix a sixfold increase in the dielectric permittivity was realized. A fine tuning of the actuation stress and, consequently, actuation strain can be done by a judicial selection of the different filler concentrations in the soft rubber matrix. Finally, a synergistic effect of the fillers was observed in the improved actuation performance of the developed materials. This work may pave the way to design dielectric elastomers for actuator fabrication. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44116.     

Vegetable fillers for electric stimuli responsive elastomers

Dielectric elastomer actuators (DEAs) have been studied widely in recent years for artificial muscle applications, but their implementation into production is limited due to high operating voltages required. The actuation behavior of dielectric elastomer under an applied electric field is predicted by Maxwell's pressure and thickness strain equations. According to these equations, the best electromechanical response is achieved when the relative permittivity is high and elastic modulus is low. The potential source for additives increasing the relative permittivity of rubbers can be vegetable powders that have much higher dielectric constant than common elastomers. In the present research, the dielectric and actuation properties of polyacrylate rubber (ACM) were studied after the addition of different vegetable‐based fillers such as potato starch, corn starch, garlic, and paprika. The results were compared to ACM filled with barium titanate. The compounds containing vegetable fillers showed higher relative dielectric permittivity at 1 Hz frequency than the compounds containing barium titanate due to higher interfacial polarization. The actuation studies showed that lower electric fields are required to generate certain actuation forces when the starches and garlic are used in the rubber instead of barium titanate. Therefore, the vegetable‐based fillers can be used to improve actuation performance of DEAs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45081.     

Property reinforcement of silicone dielectric elastomers filled with self‐prepared calcium copper titanate particles

In this article, submicron and micron calcium copper titanate (CCTO) crystallites with different morphologies were successfully designed and prepared by directly thermal treatment method and molten salt method, respectively. Then, the silicone elastomer filled with self‐prepared CCTO particles had high dieletric constant, low dielectric loss, and actuated strain which was greatly improved at low electric field. The dieletric constant at 50 Hz obviously increased from 2.15 for pure silicone elastomer to 4.37 and 4.18 for the submicron and micron CCTO/poly (dimethyl siloxane) (PDMS) composites. The dielectric loss of the composites retained at a low value (less than 0.06). Meanwhile, the elastic modulus of CCTO/PDMS composites was increased slightly only with a good flexibility. Compared to pure silicone elastomer (2.25%), the submicron and micron CCTO/PDMS composites with 2 wt % content exhibited a greater strain of 7.69% and 9.83% at a low electric field of 5 V/μm. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42613.     

Plasticized thermoplastic polyurethanes for dielectric elastomers with improved electromechanical actuation

In this article, four different plasticizers are blended in thermoplastic polyurethane (PU) to improve its electromechanical actuation performance. The selected plasticizers include dibutyl phthalate, triphenyl phosphate, polyethylene glycol (PEG), and an unsaturated polyester PMG. The plasticization effect of various plasticizers on the mechanical properties, dielectric properties, and the electromechanical actuation of PU films is carefully characterized and compared. Results demonstrate that the actuated strain under low electric fields and the electromechanical coupling efficiency of PU can be substantially improved by blending with appropriate type and amount of plasticizers. The oligomer‐type plasticizers, PEG and PMG, act more efficiently in the improvement of actuation. An actuated strain in thickness of 1.54%, 140 times higher than that of pure PU, along with an electromechanical coupling efficiency of 0.60 under a low electric field of 5 V/μm was achieved for the PU plasticized with PMG suggesting an attractive approach toward advanced dielectric elastomer actuators. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45123.     

Electromechanical instability in silicone‐ and acrylate‐based dielectric elastomers

Electromechanical instability (EMI) is regarded as a significant factor in preventing dielectric elastomers (DEs) from achieving large voltage‐induced deformations. In this study, the strain‐stiffening effect was used to control the occurrence of EMI in DEs. The results show that the stretching ratio required to provide a feasible strain‐stiffening effect in silicone rubber (SR) was smaller than that needed for a commercial DE material, VHB 4910. The experimental data were compared with currently used models for the simulation of EMI in DEs. We found that EMI could be eliminated in the deformation of these elastomers when prestretching was used. Through the application of a prestretching ratio of above 2.0, EMI was suppressed in both the VHB 4910 and SR samples. The findings of this research are of great significance in the maximization of the electromechanical performance of DE materials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45733.     

Dielectric strength of Al2O3/silicone composites after high‐temperature aging

Silicones are widely used for electrical insulation owing to their high dielectric strength and thermal stability. However, recent studies revealed insufficient stability of silicone for high‐temperature applications. To study the effect of Al₂O₃ fiber on silicone stability, we measured the dielectric strength of unfilled silicone and Al₂O₃/silicone composites as a function of aging time at 250°C in air and analyzed data by Weibull probability distribution to determine characteristic dielectric strength (E₀) and shape parameter (β). Prior to aging, unfilled silicone and composites had similar behavior, with E₀ at about 20 kV/mm and β > 15. During aging, unfilled silicone developed both micro‐ and macrocracks, with β dropped below five in 240 h and E₀ decreased significantly. Composites developed microcracks, with β dropped below 5 in longer time and E₀ remained almost constant. Addition of Al₂O₃ slowed down crack growth in silicone matrix, resulting in longer lasting high‐temperature dielectric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41170.     

Fabrication and performance of a donut‐shaped generator based on dielectric elastomer

Dielectric elastomers (DEs) have been suggested as generators to harvest electrical energy from natural mechanical energy sources, such as human movements and ocean waves. In this study, a donut‐shaped DE generator (DEG) has been fabricated and its performance is characterized depending on the stretch deformation. A simple new stretchable electrode system using multi‐walled carbon nanotubes has been suggested. Measurements on the resistance, capacitance, and electrical power generation are made depending on the area expansion. The capacitance and harvested energy are parabolically increased with increased area expansions. The theoretical prediction of energy harvesting is in good agreement with measured values of capacitance changes with stretching. FE analysis is also applied for calculation of strains for the DEG to figure out the distribution of strains. It is suggested that the DEG has promising applications in the field of designing an energy harvesting device depending on the type of energy available. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40076.     

Enhancement of electromechanical properties of natural rubber by adding barium titanate filler: An electro-mechanical study

Natural rubber is one of the most potential electro-active polymers for sensors, actuators, and energy harvesting applications. Enhancing the characteristic properties of polymers by reinforcing with fillers that possess multifunctional attributes have attracted considerable attention. In the present study, barium titanate reinforced natural rubber composite is prepared by using two-roll mill mixing. Afterwards, mechanical, electrical, and electromechanical properties of the composites are extensively analyzed by reinforcing different amounts of barium titanate into the matrix of natural rubber. The fabricated dielectric composite shows excellent properties such as high dielectric constant, low dielectric losses, high dielectric breakdown strength, and extreme stretchability. It is observed that as the filler loading reaches the value of 11 parts per hundred rubber (phr), maximum agglomeration of the particles occurs. Maximum stretchability and highest ratio of dielectric constant to elastic modulus are obtained at 8 phr of barium titanate fillers and at the loading, a maximum actuation strain of 11.24% is achieved. This study provides a simple, economical, and effective method for preparing enhanced mechanical, electrical, and electromechanical properties of natural rubber composites, facilitating the wide applications of dielectric materials as actuators and generators.     

Cauchy stresses and vibration frequencies for the instability parameters of dielectric elastomer actuators

An analysis of the effect of Cauchy stresses, vibration frequency response, and instability on the transient dynamic response of step‐voltage‐driven dielectric elastomer actuators (DEAs) is presented in this paper. Material nonlinearities associated with the hyperelastic constitutive law are taken into account, and the membrane is assumed to be made of an isotropic, homogeneous, and incompressible material. The results for the neo‐Hookean material model are further extended to analyze relatively complex multiparameter hyperelastic models (Mooney–Rivlin and Ogden) that are often employed for investigating the behavior of DEAs. The dynamic instability parameters are predicted using energy‐based extraction of static instability and validated by the response of the material in the vicinity of the dynamic instability. The natural modes of the membrane are used to approximate the nonlinear deformation field using the Galerkin method. A detailed parametric analysis of the equations of motion for the prestretched membrane shows the natural frequencies and mode shapes of the membrane and the strong influence of the stretching ratios and material parameters on the linear and nonlinear oscillations of the membrane. The results of the present investigation show the electric field–frequency relations, resonance curves, and bifurcation diagrams using the nonlinear dynamics of DEAs subjected to electrical loads. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46215.     

Toward green three‐phase composites with enhanced dielectric permittivity

This work aims to investigate the dielectric potential of microcrystalline cellulose, a green biosourced material, as a third constituent in the three‐phase composites based on ethylene vinyl acetate‐vinyl ester of versatic acid (EVA‐VeoVa) terpolymer and BaTiO₃. For that, new green three‐phase composites were prepared using an economic and green process, with simple implementation at room temperature and using water as a solvent. Compared with the binary composite EVA‐VeoVa/BaTiO₃, the three‐phase composite EVA‐VeoVa/BaTiO₃/microcrystalline cellulose showed an improvement of the BaTiO₃ particles dispersion, enhanced relative permittivity, and reduced dielectric loss, which explains the significance of this study. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46147.     

Structural development in ion‐irradiated poly(ether ether ketone) as studied by dielectric relaxation spectroscopy

Structural alterations to amorphous poly(ether ether ketone) (PEEK) produced by ion irradiation (11.2 MeV H⁺ and 25.6 MeV He²⁺ ions) were investigated by dielectric relaxation spectroscopy. The analysis in terms of the Havriliak–Negami (HN) equation and the scaling model showed an increase in the intermolecular correlation with increasing irradiation dose. The dynamic fragility index (m) was estimated from Vogel–Fulcher–Tammann analysis. Ion irradiation not only elevated the glass‐transition temperature (T_g) but interestingly decreased m of the PEEK chains around T_g. This was due to increasing polar interaction and better packing efficiency of the irradiated samples compared with those of amorphous PEEK. The average size of the cooperative rearranging region decreased in line with decreasing m and indicated an increase in the rigid amorphous phase fraction after irradiation. The analysis of the direct‐current conductivity confirmed that there was a strong coupling between the macroscopic ion transport and concerted segmental motion. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39929.     

Polymeric piezoelectric fiber with metal core produced by electrowetting‐aided dry spinning method

An electrowetting‐aided dry spinning method is developed to produce morphologically stable polymeric piezoelectric fibers with a metal core covered by a beta‐phase poly(vinylidene fluoride) [or poly(vinylidene‐trifluoroethylene)] layer. Each fiber consists of a 100 μm copper core (enameled with 6 μm polyester‐imide), a 3–10 μm piezoelectric layer, and a sputtered 100 nm gold electrode. The morphological properties of the fibers are analyzed with scanning electron microscopy, X‐ray diffraction, and a step profiler. The piezoelectric properties are tested in a vibration‐detecting application. Both morphological observation and piezoelectric testing demonstrate that the electrowetting‐aided dry spinning helps in forming high‐quality polymeric piezoelectric fibers. Moreover, this method can also be applied in different fabrications, where adhesion between a liquid and solid surface needs to be enhanced. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43968.     

Outstanding performance of parylene polymers as electrets for energy harvesting and high-temperature applications

Parylene C is used in many applications due to its high properties but it remains a material with moderate performance as long as it is intended for use as an electret. Hence, the generally accepted idea, rightly so, in the scientific and industrial community not to necessarily select parylene (i.e., parylene C) for applications where the endurance of the electret is a strong criterion. Our study provided a new perspective on the performance of parylenes as electret. In this case, we will talk about fluorinated Parylenes of the VT-4 type and especially AF-4 variant. Their thermal stability is outstanding and a charge stability is almost total up to 100 °C. A 50% reduction in the charge is recorded at a temperature as high as of 220 °C (9 μm thick Parylene AF-4), making it one of the most efficient polymer electrets to date. Negatively and positively charged Parylene AF-4 electrets presented similar performance over long durations, which is out of ordinary for the commonly employed polymeric electrets. Finally, these fluorinated polymers are therefore particularly promising new candidates for applications in electret-based converters for energy harvesting. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48790.     

Voltage‐induced wrinkling behavior of dielectric elastomer

Wrinkles, with regular periodic patterns in soft dielectric membrane, are interesting, since they are induced electrically by applying a voltage. An experimental investigation is presented to study the wrinkling behavior of dielectric elastomer. Steady wrinkles, without the accompany of electrical breakdown were attained. According to the relationship between wrinkling and breakdown, the electromechanical behaviors of DE membrane can be divided into the following types: Type A: breakdown directly without wrinkles; Type B: wrinkle and immediate breakdown; Type C: form steady wrinkles within a voltage span. Three different electromechanical behaviors of DE membrane are classified in a phase chart. A theoretical analysis is presented and discussed, involving the effect of prestretch and configurations to predict the relationship between mechanical wrinkling and electrical breakdown. Wrinkles at on‐demand location can be triggered. The results agree with the experiments. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43258.     

Synthesis and characterization of side‐chain liquid‐crystalline elastomers containing cholesterol

A series of new smectic and cholesteric liquid‐crystalline elastomers were prepared by graft polymerization of mesogenic monomer with the chiral and nonmesogenic crosslinking agent using polymethylhydrosiloxane as backbone. The chemical structures of the monomers and polymers obtained were confirmed by Fourier transform infrared and proton nuclear magnetic resonance spectra. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. M₁ showed cholesteric phase during the heating and the cooling cycle. Polymer P₁, elastomers P₂ and P₃ exhibited smectic phase, elastomers P₄? P₆ showed chiral smectic C phase, P₇ showed cholesteric phase, and P₈ displayed stress‐induced birefringence. The elastomers containing less than 15 mol % M₂ displayed elasticity and reversible phase transition with wide mesophase temperature ranges. Experimental results demonstrated that the glass transition temperatures, the isotropization temperatures, and the mesophase temperature ranges decreased with increasing content of the crosslinking unit. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 383–390, 2005     

Lead magnesium niobate-filled silicone dielectric elastomer with large actuated strain

A silicone dielectric elastomer filled with lead magnesium niobate with a maximum actuated strain of 7.4% at 45 kV/mm was fabricated by optimizing the amount of dielectric filler, amount of plasticizing agent, and crosslink density of the elastomer. The actuated strain of dielectric elastomers (DEs) is determined by both the dielectric constant and the elastic modulus. Although the dielectric constant of the silicone elastomer increased with increasing loading amount of lead magnesium niobate, actuated strain did not increase as expected because the elastic modulus increased at the same time. The elastic modulus of silicone dielectric elastomer was decreased by reducing the crosslink density or adding plasticizing agent, leading to a visible increase in actuated strain. It was also revealed that actuated strain of silicone dielectric elastomer always goes up with increasing ratio of dielectric constant to elastic modulus. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Embedded macroporous elastomers by hydrostatic fracturing for flexible strain‐sensor applications

A method is proposed for fabricating flexible materials embedded with macroporous regions by inducing fractures under point loading. Possible use of these structures in strain sensing is demonstrated. Injecting air at high pressure through a needle‐tip generates 3‐dimensional fractures in homogeneously crosslinked polydimethylsiloxane (PDMS) media, whereas a 2‐dimensional planar fracture is generated in a sandwich‐like structure wherein a softer layer is bounded by two stiffer layers. Size‐dependence of 3‐dimensional fractures on stiffness of the media which is controlled by the crosslinker concentration shows a maximum, suggesting an optimal stiffness for generating largest fracture. The size of the 2‐dimensional fractures (～5 cm) generated inside the sandwiched layer is huge as compared to the 3‐dimensional fractures (～1 mm) under the similar conditions. Two dimensional fractured surfaces show ridges with feature length monotonically becoming smaller with stiffness. Embedded rough planar domains are created by introducing 2‐dimensional fractures at distances close enough to overlap. Using this method an embedded 2‐dimensional porous domain of polyaniline nanostructures is realized in flexible PDMS matrix. An Ohmic nature of these embedded polyaniline domains with an ability to change resistance under compression establishes their suitability for developing inexpensive and flexible strain sensors. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43681.