polydimethylsiloxane composites with high dielectric constant, low dielectric loss and improved actuated strain

Composites Science and Technology 99 (2014) 37–44

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Thermally expanded graphene nanoplates/polydimethylsiloxane composites with high dielectric constant, low dielectric loss and improved actuated strain Ming Tian a,b, Zhaoyang Wei b, Xiaoqing Zan b, Liqun Zhang a,b, Jing Zhang a,b,c, Qin Ma a,b,c, Nanying Ning a,b,⇑, Toshio Nishi c a

State Key Lab of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China c Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan b

a r t i c l e

i n f o

Article history: Received 13 July 2013 Received in revised form 21 April 2014 Accepted 5 May 2014 Available online 13 May 2014 Keywords: Nanocomposites Polymer–matrix composites (PMCs) Dielectric properties Thermally expanded graphene nanoplates (TGNPs)

a b s t r a c t Thermally expanded graphene nanoplates (TGNPs) were introduced into polydimethylsiloxane (PDMS) matrix by using solution mixing method to obtain TGNPs/PDMS dielectric composites with high dielectric constant (k), low dielectric loss and large actuated strain. The results indicated that the k at 103 Hz was obviously increased from 3.1 for pure PDMS to 18.3 and 89.5 for the composite with 1.6 wt% and 2.0 wt% TGNPs, respectively. Meanwhile, the volume resistivity of all the composites was larger than 109 X cm, indicating a low direct current conductance. As a result, the dielectric loss at 103 Hz retained a low value for all the composites. In addition, a significant increase in actuated strain was obtained from 1.4% for pure PDMS to 3.6% with the addition of 2.0 wt% TGNPs under a low electric field (15 V/lm). The mechanism for the largely improved dielectric properties was carefully discussed based on the uniform dispersion of TGNPs in PDMS matrix, the gradual formation of many parallel or serial micro-capacitor structures (low direct current conductance) with the content of TGNPs increasing, and the remained oxygenic groups coated on TGNPs after high temperature thermal exfoliation. Our work provided a simple, lowcost and effective method to prepare high performance dielectric elastomer (DE), facilitating the wide application of DE composites, especially in the biological and medical fields such as biosensors, artificial muscles, and prosthetics. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Dielectric elastomers (DEs) are one of the most promising classes of actuator materials that exhibit excellent performance such as large active strain, high energy density, fast response, high electromechanical coupling efficiency, reliability, durability, as well as easiness of processing [1,2]. They have been widely used in various devices, such as eyeball actuators, tunable gratins [3], tactile displays, and inchworm robots [4]. Therefore, DEs have attracted more and more attention during the past decades [1,5,6]. By spraying electrodes on both sides of dielectric elastomer film, a delicately produced dielectric elastomer actuators (DEA) is obtained [7]. However, a key limitation for the practical application of DEAs ⇑ Corresponding author at: State Key Lab of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China. Tel.: +86 10 6443 4860; fax: +86 10 6443 3964. E-mail address: [email protected] (N. Ning). http://dx.doi.org/10.1016/j.compscitech.2014.05.004 0266-3538/Ó 2014 Elsevier Ltd. All rights reserved.

is the high electric field (>100 kV/mm) to drive them [8,9]. Such a high electric field is very dangerous for humans and equipment, particularly in biological and medical fields, which largely limits its wide application [8–10]. Therefore, the preparation of DEAs with high actuated strain at a low electric field is the biggest challenge for DEAs. According to the well-known Maxwell equation as previously reported, to obtain a DE with high actuated strain at a low electric field, a high electromechanical sensitivity (b) is required, which is defined as the ratio of the dielectric constant (k) to the elastic modulus (Y) (b = k/Y) [2]. Therefore, an effective solution is to largely increase the k and largely decrease Y of DE [11]. Most DEs, such as silicone elastomer and acrylic rubber, have quite a low k [12]. One common method to obtain a DE with a high k is to introduce high-k ceramics into the elastomer matrix [13]. This strategy usually requires high loading fractions to effectively improve the k, and thus produces the large increase in elastic modulus, resulting in a low actuated strain, largely limiting the wide application of

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DEs [14]. Another commonly used method is to add conductive fillers such as carbon nanotubes (CNTs) into the matrix to improve the k. Because of the curl and entanglement of CNTs with fibrous nanostructure, even quite a low content of CNTs could lead to the formation of CNTs network, and thus leading to the large increase in k [15]. However, in this case, a high dielectric loss was also obtained because of the high direct current (DC) conductance caused by the direct connection of CNTs [16]. In addition, the high cost of CNTs is not good for the industrial application of CNTs/ elastomer materials. As a naturally abundant, cheap and environmentally friendly conductive material, graphite or expanded graphite has unique layered structure, and thus has attracted considerable attention as dielectric filler to improve the k of elastomers [17]. For example, Skov et al. recently reported that a good dispersion of expanded graphite (EG) in silicone matrix was obtained by using speed mixing method. As a result, a 2.7-fold increase in the k of silicone were obtained by adding 4 wt% of EG, without increasing the Y significantly [6]. However, the increase in k by adding graphite or expanded graphite is still far less than expected because of the micron scale thickness of graphite. As a monolayer of sp2-bonded carbon atoms packed into a honeycomb crystal structure, graphene nanoplates (GNPs) have a nanoscale thickness and a larger aspect ratio, and thus it is more easy to form a large number of parallel micro-capacitors at a low filler content, facilitating the preparation of graphene/DE composites with high k [18]. On the other hand, the dielectric loss can be effectively decreased by preventing the direct connection of GNPs, which can be realized via the coating of functional groups (CAOAC, CAOH, and C@O) on GNPs through chemical modification of natural graphite (NP) during preparation [16,19]. However, the chemical modification could also lead to the severe disruption of graphite structure, which could lead to a low k. Therefore, the reduction of GNPs is required to obtain a DE composite with high k and low dielectric loss. Thermally expanding technique provides a simple and effective method to prepare GNPs via the simultaneous exfoliation and reduction of graphite oxide (GO) [17]. Therefore, introducing thermally expanded graphene nanoplates (TGNPs) into an elastomer, DE with high k and low dielectric loss is expected to be obtained because of the formation of a large number of micro-capacitors (high k) and the coating of the functional groups (low loss). Actually, it has been reported that the addition of these TGNPs can not only lead to an increase in k but also a decrease in dielectric loss [17]. As one of the most important DEs, poly(dimethyl)siloxane (PDMS) has a rather low modulus, a good thermal stability over a wide temperature range, a fast speed of response, high efficiency and excellent biocompatibility for artificial muscle [11]. However, it has a rather low k. Therefore, in this work, TGNPs was introduced into PDMS matrix to achieve a DE with high k, low dielectric loss, and large actuated strain at a low electric field. TGNPs was prepared via simultaneously exfoliation and reduction of GO by using high temperature thermally expanding method, because it is simple, low-cost and easy to realize the large scale preparation of TGNPs in industry. The solution mixing method was used to obtain a uniform dispersion of TGNPs in PDMS matrix. 2. Experimental 2.1. Materials A commercial grade of polydimethylsiloxane (PDMS) (SILASTIC 3481) and the curing agent (81-F) were purchased from Dow Corning Co., Ltd. (America). The viscosity and density of PDMS material used is 22.1 Pa s and 1.213 g/cm3, respectively. The natural graphite (NG) with average lateral size of 18 lm was purchased from Qingdao Huatai Lubricant Sealing S&T (China).

Concentrated sulfuric acid (98%), sodium nitrate, potassium permanganate, and hydrogen peroxide (30%) were purchased from Beijing agent company (China). 2.2. Preparation of thermally exfoliated TGNPs GO was firstly prepared from NG by using Hummers method [20]. First of all, 3 g NG was stirred in 98% H2SO4 (100 mL) for 10 min, and 3 g NaNO3 was gradually added to the dispersion, followed by the addition of 12 g KMnO4 while keeping at a low temperature. After two hours, the mixture was diluted with distilled water and heated at 98 °C for 20 min. The reaction was terminated by addition of distilled water (350 mL) and 30% H2O2 solution (70 mL). The mixture was washed by repeated vacuum filtration, first with 10% HCl aqueous solution, and then with methyl alcohol. The as-prepared GO was then thermally exfoliated in a muffle furnace at 1050 °C for 30 s [21]. Then the TGNPs were obtained by sonication technique for 2 h. 2.3. Preparation of TGNPs/PDMS composites TGNPs/PDMS composites were prepared by solution mixing followed by vulcanization at room temperature. A stable TGNPs/tetrahydrofuran (THF) suspension was achieved by sonication (1000W) for 2 h in THF solvent, during which a kind of surfactant named sodium dodecyl benzene sulfonate (SDBS) was added into the suspension. Subsequently, the well-dispersed TGNPs/SDBS/ THF suspension was mixed with the PDMS/THF solution under stirring for 2 h to obtain a stable TGNPs/PDMS suspension, followed by the addition of 5 wt% of curing agent. After mechanical stirring for 5 min, the suspension was poured into a mold and dried in a fume hood for curing at room temperature to form a thin film with a thickness of 0.5 mm. The composite films with different content of TGNPs (0.1 wt%, 0.8 wt%, 1.6 wt% and 2.0 wt%) were achieved by changing the content of TGNPs by solution casting method. For comparison purpose, pure PDMS films were also prepared by solution casting method. 2.4. Measurements A 2500VB2 + PCX-ray diffraction (XRD) purchased from RIGAKU Co. (Japan) was used to study the oxidation of NG and exfoliation of GO. An H-800 high-resolution transmission electron microscope (HRTEM) purchased from Hitachi Co. (Japan) was used to observe the morphology and thickness of TGNPs. An atomic force microscope (AFM) (NanoScope Analysis, Bruker, Germany) was used to observe the thickness of TGNPs. A ESCALAB250 X-ray photoelectron spectroscopy (XPS) purchased from Thermo Fisher Scientific Company (American) was used to study the chemical compositions of GO and TGNPs. Dielectric properties were measured by using a HP4294A impedance analyzer (Agilent, U.S.A) in the frequency range of 102–106 Hz at room temperature. The volume resistivity of the composites was tested by using a high resistance meter (EST 121, Beijing HuaJingHui Scientific and Technical Co., Ltd., China) at room temperature. The Y of pure PDMS and TGNPs/PDMS composites were obtained by calculating the slopes of the stress–strain curves at the strain of 5%, which were obtained by using a tensile apparatus (CMT4104, Shenzhen SANS Testing Machine Co., Ltd., China) at a strain rate of 50 mm/min. The actuated strain was measured by using a circular strain test and the diameter of the active area is 1 cm. Before test, two main surfaces of the films were sprayed with graphite suspension using an airbrush to fabricate compliant electrodes. The film was first fixed on two circle frames without a pre-strain, then electric field was applied on both sides of the film. The electric field needed was equipped with a high

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voltage direct current generator (DTZG-60, Wuhan Dotek Electric Co., Ltd.). The strain was calculated as the change of the pixel of the electrodes area divided by the original pixel. A video camera was fixed at the same focal length to capture the actuator plain before and after voltage exerting. Three actuators were studied three times for each composition, and the average values are reported. The thickness of pure PDMS and TGNPs/PDMS composites were 0.5 mm. The electric breakdown filed was obtained by testing the actuated stains of composites until electric breakdown occurred. 3. Results and discussion 3.1. Characterization of TGNPs

Intensity (a.u.)

Fig. 1 shows XRD patterns of NG, GO and TGNPs. For NG, a strong and sharp diffraction peak at 26.6° is clearly observed, which is ascribed to the highly ordered crystallized structure of NG. For GO, the characterization peak of NG at about 26.6° is absent, whereas a relatively weak diffraction peak at 12.5° is observed, indicating that the highly ordered crystallized structure of NG is disrupted after oxidation. This is due to the complete intercalation of graphite, facilitating the preparation of TGNPs by the subsequent thermal expansion procedure. For TGNPs, no obvious diffraction peak is observed, indicating the disruption of the periodic layered structure of GO caused by the successful thermal exfoliation of GO at high temperature (1050 °C). This facilitates the preparation of TGNPs/PDMS composites with TGNPs uniformly dispersed in PDMS matrix, as will be discussed later. These results agree well with that reported in many studies [22,23]. The SEM images of NG, GO and TGNPs and the HR-TEM images of TGNPs after sonication for 2 h are shown in Fig. 2. Both NG and GO exhibit block shape, as shown in Fig. 2(a and b). The diameter of NG is in the range of 10–30 lm, whereas that of GO is slightly decreased. After thermal exfoliation, the blocked shaped GO are exfoliated into thin sheet-like TGNPs, as shown in Fig. 2(c). The preparation of TGNPs using high temperature thermal expansion technique can also be verified by HR-TEM observation, as shown in Fig. 2(d and e). From the image with low magnification, as shown in Fig. 2(d), we can observe that GO has been efficiently exfoliated to ultrathin TGNPs with many ripples and corrugations, which facilitate the thermodynamic stability of the two dimensional graphene crystal that was thermodynamically unstable. Under high magnification, the paralleled structure of stacks of TGNPs can be observed, as shown in Fig. 2(e). From an edge-on

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view, the thickness of these TGNPs is about 2–2.5 nm. In addition, AFM was also used to determine the thickness of graphene, and many sheets were measured in order to get an average thickness. As shown in Fig. 3(a), TGNPs has an average thickness of approximately 2.8 nm with a lateral dimension of several micrometers. In addition, under high temperature thermal treatment, GO was not only thermal exfoliated into ultrathin sheets, but also underwent an in situ reduction, as demonstrated by the composition variations during the thermal treatment. Therefore, XPS was used to analyze the composition variations during the thermal exfoliation process, and the results are shown in Fig. 3(b and c). From the C1s XPS spectra of GO shown in Fig. 3(b), we can observe four characteristic peaks corresponding to carbon atoms in different functional groups: the non-oxygenated ring C (284.8 eV), the C in CAO bonds (286.6 eV), the carbonyl C (C@O, 287.7 eV) and the carboxylate C (OAC@O, 288.8 eV). The C1s XPS spectra of TGNPs are shown in Fig. 3(c). We can also observe the same characteristic peaks representing oxygen functionalities as GO, indicating that there are still some oxygenic groups on TGNPs. The coating of these remained oxygenic groups on TGNPs facilitates the preparation of TGNPs/PDMS composites with a low dielectric loss, as will be discussed in Section 3.4. However, the peak intensities of TGNPs are much weaker than those of GO, indicating that most oxygen functional groups have been removed during thermal exfoliation. This demonstrates that GO has been successfully reduced during exfoliation under 1050 °C. 3.2. Microstructure of PDMS/TGNPs composites The microstructure of pure PDMS and TGNPs/PDMS composites were observed by using SEM, and the results are shown in Fig. 4. We can observe that the fractured surface of pure PDMS is smooth, as shown in Fig. 4(a). The fractured surface of all the composites with 0.8 wt%, 1.6 wt% and 2.0 wt% of TGNPs exhibits many ripples and corrugations, representing the presence of TGNPs, as shown in Fig. 4(b–d). Obviously, no TGNPs aggregates are observed in all the composites, indicating the uniform dispersion of TGNPs in PDMS matrix for all the composites by using the solution casting method. On the other hand, TGNPs filler network structure is not formed in the composite with 0.8 wt% of TGNPs. The filler network structure with TGNPs connect with one another is gradually formed with the increase in the content of TGNPs to 1.6 wt%, and the filler network structure becomes stronger with the further increase in the content of TGNPs to 2.0 wt%. The gradual formation of TGNPs filler network structure can also be demonstrated by the change in volume resistivity of the composites with different content of TGNPs, as shown in Fig. 5. We can observe that the volume resistivity decreases slightly with the addition of 0.1 wt% and 0.8 wt% of TGNPs, again indicating that the TGNPs filler network structure is not formed in the composites with 0.1 wt% and 0.8 wt% of TGNPs. The volume resistivity starts to decrease obviously with the addition of 1.6 wt% of TGNPs (percolation threshold), indicating the formation of TGNPs filler network structure. The volume resistivity decreases sharply with the further increase in the content of TGNPs to 2.0 wt%, indicating that TGNPs filler network structure becomes stronger. These results are consistent well with the SEM results. The TGNPs filler network structure plays an important role in the formation of micro-capacitor network, and thus the increase in the dielectric properties of TGNPs/PDMS composites. 3.3. Electromechanical properties

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The dielectric properties of pure PDMS and PDMS/TGNPs composites with different content of TGNPs are shown in Fig. 6. Fig. 6(a and b) show the frequency dependency of k and dielectric loss, respectively. We can observe that both k and dielectric loss

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Fig. 2. The SEM images of (a) natural graphite (NG), (b) graphite oxide (GO), (c) thermally exfoliated grapheme nanoplates (TGNPs) and high-resolution TEM (HRTEM) images of TGNPs (d) in low magnification, and (e) in high magnification.

behave like a constant for the composites with TGNPs content lower than the percolation threshold (1.6 wt%), indicating a frequency independence of dielectric properties at a low TGNPs content. This is mainly attributed to the formation of separated microcapacitors, as will be discussed in detail in Section 3.4. However, both k and dielectric loss are remarkably decreased with increasing frequency at the content of TGNPs above the percolation threshold (2.0 wt%). The detailed values of the dielectric properties at 103 Hz are summarized in Fig. 6(c). We can see that k obviously increases from 3.1 for pure PDMS to 18.3 for the composite with 1.6 wt% TGNPs, and it sharply increases to 89.5 as the content of TGNPs further increasing to 2.0 wt%. This indicates that TGNPs can effectively improve the k of PDMS. Here, it should be noted that the k at 103 Hz of the composites with 2.0 wt% TGNPs is 5 times higher than the composites with the same content of TGNPs prepared by melt mixing method, as reported by Romasanta et al. [17]. This could be ascribed to the more uniform dispersion of TGNPs in PDMS matrix for the composites prepared by using solution casting method, comparing with the melt mixing method. In addition, the dielectric loss of the composites at a low content of TGNPs is at a low level, slightly higher than that of pure PDMS (0.02 at

103 Hz). Even for the composite with 1.6 wt% TGNPs, the dielectric loss is only 0.16 at 103 Hz. As the content of TGNPs reaching 2.0 wt%, the dielectric loss is further increased to 1.5, which retains a low value. These results indicate that a DE with a high k and low dielectric loss has been successfully prepared by using TGNPs as the dielectric filler. The Y of pure PDMS and TGNPs/PDMS composites are shown in Fig. 7(a). We can observe that the Y of the composites with 0.1 wt% and 0.8 wt% of TGNPs is almost the same as that of pure PDMS (0.5 MPa), and it obviously increases to 1.2 MPa and 1.9 MPa for the composites with 1.6 wt% and 2.0 wt% of TGNPs, respectively. Despite of the 1.4-fold and 2.8-fold increase in the Y for the composites with 1.6 wt% and 2.0 wt% of TGNPs, a 5-fold and 29-fold increase of the k of the composite was obtained, compared with neat PDMS. As a result, the b increases from 6 at 103 Hz for pure PDMS to 15 and 47.3 at 103 Hz for the composite with 1.6 wt% and 2 wt% of TGNPs, respectively, as shown in Fig. 7(a). Comparing with pure PDMS, b of the composites increases by 150% and 690% by adding 1.6 wt% and 2 wt% of TGNPs, respectively. The large increase in b leads to the large increase in actuated strain of TGNPs/PDMS composites at a low electric field (see below).

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Fig. 3. (a) Atomic force microscope height image of TGNPs, The C1s X-ray photoelectron spectroscopy of (b) GO and (c) TGNPs.

Fig. 4. The SEM images of TGNPs/PDMS composites with different TGNPs content: (a) 0 wt%, (b) 0.8 wt%, (c) 1.6 wt% and (d) 2.0 wt%.

The actuated strain of pure PDMS and TGNPs/PDMS composites at a low electric field (0–15 V/lm) are shown in Fig. 7(b). The actuated strain of all the samples shows almost a linear increase with the increase in electric field, because it has a quadratic relationship with the applied electric field [24]. More importantly, under the same electric field, the actuated strain of PDMS is significantly increased with the content of TGNPs increasing. For example, the actuated strain at 15 V/lm is obviously increased from 1.4% for pure PDMS to 3.6% for the composite with 2.0 wt% of TGNPs. This indicates that the actuated strain of PDMS can be largely increased

with the addition of TGNPs under a low electric field (0–15 V/lm), facilitating the wide application of DE in the biological and medical fields. Here, it should be noted that the actuated strain of pure PDMS reaches 10% under a high electric field (50 V/lm), as reported in our previous work [25]. However, such a high electric field is dangerous especially for human beings, and thus limits its application in biological and medical fields [9–11]. The breakdown strength is very important for a dielectric elastomer, thus has also been investigated in this study, and the results are shown in Fig. 7(c). We can observe that the electric breakdown

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properties relationship was carefully studied in this work. With regard to dielectric composites, the increase in k could be attributed to two mechanisms: the micro-capacitance-structure model and the interfacial polarization effect (also named Maxwell–Wagner–Sillars (MWS) effect) [27]. The microcapacitance-structure model postulates that many parallel or serial micro-capacitors connected with one another are formed. MWS effect is ascribed to the accumulation of many charge carriers at the internal interfaces between TGNPs and PDMS. For both mechanisms, the dispersion and spatial distribution of TGNPs in the matrix are the key factors to affect the dielectric performance. Thus, based on the layered structure of TGNPs, which can easily form micro-capacitors, the micro-capacitance-structure model is used to analyze the mechanism for the increase in k, as reported in previous studies [27]. The evolution process of the dielectric properties in TGNPs/PDMS composites with the change of TGNPs content can be divided into two stages, which can be demonstrated by the change in microstructure (see Fig. 4) and volume resistivity of these composites (see Fig. 5). A schematic illustration of the different micro-structures of TGNPs/PDMS composites is shown in Fig. 8. The first stage is shown in Fig. 8(a), in which the microcapacitors are separated from one another. Initially, some microcapacitor structures are formed because of the addition of a small amount of TGNPs (0.1 wt%), resulting in a slight increase in k over that of pure PDMS. As the content of TGNPs increases to 0.8 wt%, k is further increased owing to the formation of more microcapacitors. In these cases, the separation of micro-capacitors leads to a low direct current (DC) conductance, and thus a low dielectric loss (see Section 3.3). In addition, the separation of microcapacitors leads to frequency independence of dielectric properties for these composites at a low content of TGNPs. The second stage is

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strength of the composites at a low content of TGNPs (0.1–1.6 wt%) only slightly decreases, whereas the breakdown strength of the composite with 2.0 wt% of TGNPs obviously decreases because of the increase in DC conductance and defects of the sample at a high content of TGNPs. However, the dielectric composite with 2 wt% of TGNPs is safe when the operated electric voltage is lower than the breakdown voltage, as reported in the reference [26]. 3.4. Mechanism for the largely improved dielectric properties The dielectric properties depend strongly on the microstructure of the dielectric composites. Thus, the microstructure–dielectric

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Fig. 7. (a) Young’s modulus and b values of TGNPs/PDMS composites with different content of TGNPs, (b) actuated strains of TGNPs/PDMS composites under different voltages, (c) breakdown strength of TGNPs/PDMS composites with different content of TGNPs.

Fig. 8. Schematic representation of different micro-structures of TGNPs/PDMS dielectric composites (a) with low content of TGNPs (0.1–0.8 wt%) and (b) with high content of TGNPs (1.6–2.0 wt%).

shown in Fig. 8(b), in which a network with micro-capacitors connected with one another is formed as the content of TGNPs reaches the percolation threshold (1.6 wt%) or above the percolation threshold (2.0 wt%), leading to a significant increase in k. On the other hand, the DC conductance is also remarkably increased, resulting in an increase in dielectric loss. In this case, although the percolation threshold is reached, the volume resistivity of the composite is still larger than 109 X cm (see Fig. 5), indicating that the DC conductance of the composite is still at a low level. This could be due to the remained oxygenic groups coated on TGNPs after high temperature thermal exfoliation. As a result, a relatively low dielectric loss was obtained. On the other hand, at the content of TGNPs above the percolation threshold, the

interfacial polarization effect (also named as Maxwell–Wagner– Sillars (MWS) effect) caused by the accumulation of plentiful charges carriers at the internal interfaces between TGNPs and PDMS inside the composites, also plays an important role in dielectric properties, resulting in a relatively high frequency dependence of dielectric properties. 4. Conclusion TGNPs are prepared by using high temperature thermal expanding method, during which GO is exfoliated and reduced simultaneously. The as-prepared TGNPs are uniformly dispersed in PDMS matrix by using solution mixing method. Many parallel

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or serial micro-capacitor structures are gradually formed with the increase in the content of TGNPs. The volume resistivity of all the composites is larger than 109X cm, indicating a low direct current conductance because of the coating of the remained oxygenic groups on TGNPs after high temperature thermal exfoliation. As a result, a high k, a low dielectric loss and a significantly improved actuated strain under a low electric field are obtained. This work provides a simple, low-cost and effective method to prepare high performance dielectric elastomer (DE), and thus facilitating the wide application of dielectric materials, especially in the biological and medical fields.

Acknowledgments We would like to express our sincere thanks to the National Natural Science Foundation of China (Grant Nos. 51173007 and 51221002) for financial supports.

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