Electro-thermal and -mechanical model of thermal breakdown in multilayered dielectric elastomers

Multiple breakdown phenomena may take place when operating dielectric elastomers. Thermal breakdown, which occurs due to Joule heating, becomes of special importance when using multilayered stacks of dielectric elastomers, due to the large volume-to-surface-area-ratio. In this article, a 2D axisymmetric finite-element model of a multilayered stack of dielectric elastomers is set up in COMSOL Multiphysics®. Both the electro-thermal and electro-mechanical couplings are considered, allowing for determination of the onset of thermal breakdown. Simulation results show that an entrapped particle in the dielectric elastomer drastically reduces the possible number of layers in the stack. Furthermore, the possible number of layers is greatly affected by the ambient temperature and the applied voltage. The performance of three hyperelastic material models for modeling the elastomer deformation are compared, and it is established that the Gent model yields the most restrictive prediction of breakdown point, while the Ogden model yields the least restrictive estimation.     

Post Curing as an Effective Means of Ensuring the Long-term Reliability of PDMS Thin Films for Dielectric Elastomer Applications

Post curing can be used to facilitate volatile removal and thus produce polydimethylsiloxane (PDMS) films with stable elastic and electrical properties over time. In this study, the effect of post curing was investigated for commercial silicone elastomer thin films as a means of improving long-term elastomer film reliability. The Young’s moduli and electrical breakdown strengths of commercial (silica-reinforced) PDMS elastomer films, with and without additional 35 parts per hundred rubber titanium dioxide (TiO₂), were investigated after high-temperature (200°C) post curing for various time spans. The elastomers were found to contain less than 2% of volatiles (significantly higher for TiO₂-filled samples), but nevertheless a strong effect from post curing was observed. The young’s moduli as well as the strain-dependent behavior were found to change significantly upon post curing treatment, where Young’s moduli at 5% strain increase with post curing. Furthermore, the determined dielectric breakdown parameters from Weibull analyses showed that greater electrical stability and reliability could be achieved by post curing the PDMS films before usage, and this method therefore paves a way toward more reliable dielectric elastomers.     

Maximum actuation strain for dissipative dielectric elastomers with simultaneous effect of prestretch and temperature

A dielectric elastomer can generate giant deformation by the voltage actuation, but the deformation is often hindered by the electromechanical instability and “snap‐through deformation,” which may lead to electrical breakdown. In this study, for the first time, the mathematical model is established for dissipative dielectric elastomers in the dynamic model with simultaneous effect of prestretch and temperature in order to achieve maximum actuation strain. The deformation of the dissipative dielectric elastomer: VHB 4905/4910 is investigated for the two simple actuation methods: constant and ramping voltage actuation, respectively. The best combined conditions of voltage and prestretch to obtain a large deformation at different operating temperatures are studied in detail. Under the best combined conditions, the influences of three factors: voltage, prestretch, and temperature on the maximum actuation strain are analyzed. This study should offer a great help in the design of dielectric elastomer actuators, and give the guidance to the accomplishment of the large deformation of dissipative dielectric elastomer actuators. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45850.     

Electrical breakdown of an acrylic dielectric elastomer: effects of hemispherical probing electrode’s size and force

Dielectric elastomers are widely investigated as soft electromechanically active polymers (EAPs) for actuators, stretch/force sensors, and mechanical energy harvesters to generate electricity. Although the performance of such devices is limited by the dielectric strength of the constitutive material, the electrical breakdown of soft elastomers for electromechanical transduction is still scarcely studied. Here, we describe a custom-made setup to measure electrical breakdown of soft EAPs, and we present data for a widely studied acrylic elastomer (VHB 4905 from 3M). The elastomer was electrically stimulated via a planar and a hemispherical metal electrode. The breakdown was characterized under different conditions to investigate the effects of the radius of curvature and applied force of the hemispherical electrode. With a given radius of curvature, the breakdown field increased by about 50% for a nearly 10-fold increase of the applied mechanical stress, while with a given mechanical stress the breakdown field increased by about 20% for an approximately twofold increase of the radius of curvature. These results indicate that the breakdown field is highly dependent on the boundary conditions, suggesting the need for reporting breakdown data always in close association with the measurement conditions. These findings might help future investigations in elucidating the ultimate breakdown mechanism/s of soft elastomers.     

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.     

Silicone elastomers with high dielectric permittivity and high dielectric breakdown strength based on dipolar copolymers

Dielectric elastomers (DEs) are a promising new transducer technology, but high driving voltages limit their current commercial potential. 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-catalysed azide-alkyne 1,3-dipolar cycloaddition (CuAAC). The copolymers have a high degree of chemical freedom, as the dimethylsiloxane spacer units between the functional groups, as well as the degree of functionalisation, can be varied. Thus, the best overall properties were obtained for an elastomer prepared with a copolymer with a 1200 g mol⁻¹ dimethylsiloxane spacer unit and 5.6 wt% of the high dielectric permittivity molecule 1-ethynyl-4-nitrobenzene. Here, a high increase in dielectric permittivity (∼70%) was obtained without compromising other favourable DE properties such as elastic modulus, gel fraction, dielectric loss and electrical breakdown strength.     

Silicone rubbers for dielectric elastomers with improved dielectric and mechanical properties as a result of substituting silica with titanium dioxide

One prominent method of modifying the properties of dielectric elastomers (DEs) is by adding suitable metal oxide fillers. However, almost all commercially available silicone elastomers are already heavily filled with silica to reinforce the otherwise rather weak silicone network and the resulting metal oxide filled elastomer may contain too much filler. We therefore explore the replacement of silica with titanium dioxide to ensure a relatively low concentration of filler. Liquid silicone rubber (LSR) has relatively low viscosity, which is favorable for loading inorganic fillers. In the present study, four commercial LSRs with varying loadings of silica and one benchmark room-temperature vulcanizable rubber (RTV) were investigated. The resulting elastomers were evaluated with respect to their dielectric permittivity, tear and tensile strengths, electrical breakdown, thermal stability and dynamic viscosity. Filled silicone elastomers with high loadings of nano-sized titanium dioxide (TiO₂) particles were also studied. The best overall performing formulation had 35 wt.% TiO₂ nanoparticles in the POWERSIL® XLR LSR, where the excellent ensemble of relative dielectric permittivity of 4.9 at 0.1 Hz, breakdown strength of 160 V µm^?1, tear strength of 5.3 MPa, elongation at break of 190%, a Young’s modulus of 0.85 MPa and a 10% strain response (simple tension) in a 50 V μm^?1 electric field was obtained.     

热氧老化对聚酯薄膜力学和电性能的研究

通过对电机用聚酯薄膜在不同热氧老化温度下进行了加速热处理,讨论了热处理温度对材料的力学和电性能影响因素。结果表明,随着热氧老化温度提高,薄膜电气强度和局放电压逐渐增大,拉伸强度、断裂伸长率、表面电阻率、介质损耗因数逐渐下降;相对电容率、体积电阻率变化不大。经分析,这些性能的变化主要与聚对苯二甲酸乙二醇酯(PET)薄膜黏度下降和结晶度增加有非常大的关系,黏度下降影响占主导地位。     

PDA型PUR弹性体的制备与性能

以聚己二酸二乙二醇酯二醇(PDA)为软段,4,4′–二苯基甲烷二异氰酸酯(MDI)和1,4–丁二醇(BDO)为硬段,采用预聚体法制备一系列PDA型PUR弹性体。采用力学性能测试、广角X射线衍射(WAXD)、傅立叶变换红外光谱(FTIR)、差示扫描量热(DSC)、热重(TG)分析和维卡软化点温度测定等研究手段,研究硬段含量对其力学性能、微观形态和热性能的影响。结果表明,随着硬段含量的增加,PDA型PUR弹性体的硬度、拉伸强度、300%定伸应力、拉伸永久变形和撕裂强度都增大,当硬段含量为40.1%时,弹性体的综合力学性能最佳,硬度(邵A)为88,拉伸强度为33.9 MPa,300%定伸应力为12.5 MPa,拉伸永久变形为31%,撕裂强度为90.3 k N/m;WAXD分析表明,弹性体为无定型结构;FTIR分析表明,硬段含量的增加使弹性体总的氢键化程度增加,微相分离程度改善;DSC测试表明,硬段含量的增加使弹性体的微相分离程度提高;TG和维卡软化点温度测试表明,弹性体的热性能随着硬段含量的增加而提高,当硬段含量为40.1%时,弹性体的初始分解温度(失重5%的温度)和维卡软化点温度分别达到324.5℃和144.1℃,具有较好的热性能。     

Mechanical, electrical, and thermal properties of irradiated low-density polyethylene by electron beam

The effect of electron-beam (EB) irradiation on the mechanical, electrical, and thermal properties of low-density polyethylene (LDPE) was studied The LDPE was irradiated by using 3?MeV EB machine at doses ranging from 25 to 250?kGy in air at room temperature and analyzed for mechanical, thermal, and electrical properties. It was revealed by differential scanning calorimetry analysis that the crystallinity of the EB-radiated LDPE decreased slightly as verified by a marginal reduction in the densities, enthalpy, and melting points. Thermogravimetric analysis test showed that the thermal degradation of LDPE improved by increasing irradiation. The results obtained from both gel content and hot set tests, indicating whether the applicable LDPE has been properly cross-linked or not, showed that under the EB irradiation conditions employed, the cross-linking of the LDPE samples occur mainly in the amorphous region, and the cross-linking density at each irradiation dose depends almost on the amorphous portions of the LDPE. A significant improvement in the tensile strength of the neat LDPE samples was obtained upon EB up to 250?kGy with a concomitant decline in elongation at break. The results on the electrical properties revealed that the surface resistance, volume resistivity, and dielectric strength of the LDPE increase with irradiation dose and reaches a maximum at a 250?kGy irradiation dose. No considerable change of breakdown voltage, dielectric constant, and dielectric loss factor were observed with increasing irradiation dose. The enhancement in the heat deformation, hardness, and thermal aging properties of LDPE upon EB irradiation, suggests that irradiated LDPE is more thermally and mechanically stable than virgin LDPE.     

Deep eutectic solvent inclusions for high-k composite dielectric elastomers

Recent advances in novel electroactive devices have placed new requirements on material development. High-performance dielectric elastomers with good mechanical stretchability and high dielectric constant are under high demand. However, the current strategy for fabricating these materials suffers from high cost or low thermal stability, which greatly hinders large-scale industrial production. Herein, we have successfully developed a novel strategy for improving the dielectric constant of polymeric elastomers via deep eutectic solvent inclusion by taking advantage of the low cost, convenient and environmentally benign synthesis process and high ionic conductivity from deep eutectic solvents. The as-prepared composite elastomers showed good stretchability and a greatly enhanced dielectric constant with a negligible increase in dielectric dissipation. Moreover, we have proven the universality of our strategy by using different types of deep eutectic solvents. It is believed that low-cost, easy-synthesis and environmentally friendly deep eutectic solvents including composite elastomers are highly suitable for large-scale industrial production and can greatly broaden the application fields of dielectric elastomers.     

Dynamically vulcanized nitrile rubber/polyoxymethylene thermoplastic elastomers

A new type of oil‐resistant thermoplastic elastomer of nitrile rubber (NBR) and polyoxymethylene (POM) prepared by dynamic vulcanization were studied. The effects of the curing systems for NBR, acrylonitrile content in NBR, POM content, thermal aging, and reprocessing have on the mechanical properties of NBR/POM thermoplastic elastomers were investigated in detail. NBR/POM thermoplastic elastomers have high tensile strength and excellent oil resistance at elevated temperature. NBR/POM thermoplastic elastomers can be produced in a wide range of hardness. NBRs with higher acrylonitrile content are more suitable for preparing NBR/POM thermoplastic elastomer with high tensile strength and good oil resistance. The NBR/POM thermoplastic elastomer has significantly improved electrical insulation properties compared with NBR (Gessler, M. U.S. Pat. 3,037,954, 1962). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2641–2645, 2000     

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.     

Synthesis and characterization of the properties of thermosensitive elastomers with thermoplastic and magnetic particles for application in soft robotics

In the currently rapidly developing field of soft robots, smart materials with controllable properties play the central role. Thermosensitive elastomers are soft, smart materials whose material properties can be controlled by changing their temperature. The aim of this work is to investigate the mechanical properties, to analyze the surface, the inner structure, and the heat transfer within the thermosensitive elastomer materials. This should provide a knowledge base for new combinations, such as a combination of thermosensitive and the well-known magneto sensitive elastomers, in order to realize new applications. Thermoplastic polycaprolactone particles were incorporated into a flexible polydimethylsiloxane matrix to produce thermosensitive elastomer samples. With a low melting point in the range of 58–60°C, polycaprolactone offers good application potential compared to other thermoplastic materials such as polymethamethylacrylate with a melting point above 160°C. Test samples of different material compositions and geometries were made to examine temperature-depending material properties. Two useful effects were identified: temperature-dependent change in stiffness and the shape memory effect. In certain examinations, carbonyl iron particles were also included to find out if the two particle systems are compatible with each other and can be combined in the polydimethylsiloxane matrix without disadvantages. Changes in shore hardness before and after the influence of temperature were investigated. Micro computed tomography images and scanning electron microscopy images of the respective samples were also obtained in order to detect the temperature influence on the material internally as well as on the surface of the thermosensitive elastomers in combination with carbonyl iron particles. In order to investigate the heat transfer within the samples, heating tests were carried out and the influence of different particle concentrations of the thermosensitive elastomers with and without carbonyl iron particles was determined. Further work will focus on comprehensive investigations of thermo-magneto-sensitive elastomers, as this will enable the functional integration in the material to be implemented with increased efficiency. By means of the different investigations, the authors see future applications for this class of materials in adaptive sensor and gripper elements in soft robotics.     

Self-Strengthening Dielectric Elastomer of Triblock Copolymer with Significantly Improved Electromechanical Performance under Low Voltage

Dielectric elastomer actuators (DEAs) are promising soft electromechanical transducers for soft robotics. Fabricating a high-performance DEA actuated by sub-kV voltage remains challenging. Here, a facile method not only to fabricate ultrathin dielectric elastomer films of triblock copolymers but also to enhance the dielectric breakdown strength and thus enhance the electromechanical performance is reported. A thick thermoplastic elastomer film of poly(styrene-b-butyl acrylate-b-styrene) from solution blading is symmetrically pre-stretched and relaxed at 120 °C to fabricate a freestanding ultrathin DE film. Compared with the pristine DE film of the same thickness (12 µm), the thermally-relaxed DE film with equally biaxial pre-stretch ratio 3.5 × 3.5 exhibits increased electrical breakdown strength by a factor of 1.9 (from 43 to 82 V µm⁻¹), maximum actuation area strain by a factor of 1.9 (from 11.7% to 22.4%), and highest energy density by a factor of 5.7 (from 4.5 to 25.8 kJ m⁻³). The enhancement may be ascribed to the self-reinforcement of the dielectric breakdown strength due to the morphology change of polystyrene nanodomains from spheres to oblate spheroids. Thanks to the ultra-thinness, the high electromechanical performance is achieved within sub-kV driving voltage in all cases.     

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     

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.     

Viscoelastic properties and heat generation in urethane elastomers

Static and dynamic properties were studied in a series of polyurethane elastomers as a function of selected compositional variables such as curative system, curative level, catalyst level, and curing temperature. A number of physical properties including swelling ratio, density, glass transition temperature, stress–strain behavior, and thermal conductivity were also measured on these elastomers. The selected variables affect dynamic mechanical properties as well as heat buildup. A good correlation was noted between the loss modulus and the heat generation. The loss modulus and the heat generation decrease with decreasing curative level. The elastomers cured with a mixture of triol and diamine give lower loss modulus and heat buildup than those cured with diamine alone. These responses are believed due to the increase in covalent crosslinks. The observed low heat generation of the elastomer cured with 0.2 phr azelaic acid as a catalyst level was also attributed to the high crosslink density. The curing temperature, in the range investigated, appears to have very little effect on the properties. Thus, the choice of formulation variables, especially the use of diamine–triol blends, provides an effective means of minimizing heat generation in dynamic applications of polyurethane elastomers.     

Morphology,mechanical, and dielectric breakdown properties of PBT/PET/TPE,PBT/PET/PA66, PBT/PET/LMPE,and PBT/PET/TiO2 blends

The morphology and thermal behavior of tertiary blends of poly(butylene terephthalate) [PBT]/poly(ethylene terephthalate) [PET] alloy resin (Alloy)/thermoplastic elastomer (TPE), Alloy/nylon 66 (PA66), Alloy/low molecular weight polyester (LMPE), and Alloy/titanium dioxide (TiO₂) have been investigated by scanning electron microscopy, differential scanning calorimetry, and thermogravimetry (TG). In case of TPE and LMPE containing blends did not show any morphological change compared with Alloy whereas for PA66 containing blends, the size of the dispersed phase increased with the increase the PA66 contents. The crystallization peak of Alloy/TPE blends became undetectable with an increasing TPE content. In contrast, the crystallization temperature (T_c1) of Alloy/PA66, Alloy/LMPE, Alloy/TiO₂ blends increased significantly as the PA66, LMPE, and TiO₂ content increases. TG traces show a significant shift of the weight loss towards higher temperature for Alloy/TiO₂ blend whereas Alloy/TPE, Alloy/PA66 and Alloy/LMPE blends slightly decreases in thermal stability than Alloy. To investigate the applicability for insulation material, the prepared blend samples were extruded an electric wire and the breakdown voltage (BDV) of wires was investigated. The melt flow rate of Alloy increased as the amount of TPE increased, and a contrary trend was found when the increase of the amount of TiO₂. The dielectric breakdown test applies a voltage that is greater than the product's rated operating voltage for a specific period of time during which dielectric breakdown must not occur. The BDV of Alloy/TPE and Alloy/PA66 blends is higher than other blends before and after thermal aging 225°C for 30 min. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers