Tuning the surface chemistry of graphene oxide for enhanced

Subscriber access provided by University of Winnipeg Library

Article

Tuning the surface chemistry of graphene oxide for enhanced dielectric and actuated performance of silicone rubber composites Mahyar Panahi-Sarmad, Ehsan Chehrazi, Mina Noroozi, Mohammad Raef, Mehdi Razzaghi-Kashani, and Mohammad Ali Haghighat Baian ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00042 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Electronic Materials

Tuning the surface chemistry of graphene oxide for enhanced dielectric and actuated performance of silicone rubber composites Mahyar Panahi-Sarmad*a,b, Ehsan Chehrazic, Mina Noroozia, Mohammad Raefa, Mehdi RazzaghiKashani *a and Mohammad Ali Haghighat Baiand a Polymer

Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, P. O. Box: 14115-114, Tehran, I.R. Iran

b Young

Researchers and Elite Club, Science and Research Branch, Islamic Azad University, Tehran, Iran

c Department

of Polymer Reaction Engineering, Faculty of Chemical Engineering, Tarbiat Modares University, P. O. Box: 14115-114, Tehran, I.R. Iran

d Department

of Polymer Engineering and Color Technology, Amirkabir University of Technology, P.O. Box:15875-4413, Tehran, I.R. Iran

First

corresponding Author, Email: [email protected] ORCID ID: https://orcid.org/0000-0002-8918-4875

Second corresponding author, Email: [email protected] ORCID ID: https://orcid.org/0000-0002-4207-8573

1 ACS Paragon Plus Environment

ACS Applied Electronic Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The influence of reduction temperature on the electromechanical properties and actuation behavior of Polydimethylsiloxane (PDMS) dielectric elastomer containing the thermally reduced graphene oxide (rGO) with different surface chemistry has been systematically investigated. A set of rGO nanosheets were prepared by thermal reduction of graphene oxide (GO) at four temperatures (150, 200, 300 and 400 °C). The dielectric permittivity, dielectric loss and elastic modulus of PDMS composites were increased, while the electrical breakdown strength of composites was decreased with increasing the reduction temperature of GO. A thermodynamic model applied for studying the electromechanical deformation and stability of PDMS/GO(rGOx) dielectric elastomer composites showed that the optimum value of break point was observed in PDMS/rGO-300. It is shown for the first time that variation of electromechanical instability and recovery behavior are related to the surface chemistry of rGOs. A critical reduction temperature was observed at 300°C which can be considered as desired rGO nanosheets for electromechanical applications. By employing an equivalent circuit on impedance spectroscopy, the interfacial polarization is recognized as the dominant mechanism rather than the intrinsic polarization of the matrix and nanosheets. Noteworthy, PDMS composites containing rGO, reduced at higher temperatures, have more interfacial polarized charges at the interface.


Page 2 of 32

Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Keywords: Dielectric Elastomer, Polymer Composite, Graphene Oxide (GO), Thermally Reduced Graphene Oxide (rGO), Electromechanical Properties, Electromechanical Instability, Actuation Behavior 1.Introduction Nowadays, the hydraulic, pneumatic and electromechanical systems are receiving significant attention due to their applications in actuator devices.1 However, the performance of actuator devices is limited by the challenges associated with the material behavior such as low dielectric permittivity, high dielectric loss and high elastic modulus.2,3 The actuator devices used in engineering machines are mostly rigid materials consisted of the many moving parts which can be damaged in harsh conditions. On the other hand, soft materials such as dielectric elastomer actuators (DEA) with optimal design capable of withstanding in such conditions have intrinsic flexibility, low weight and energy efficiency.3–5 Therefore, dielectric elastomers have great potentials to become a suitable alternative to the traditional actuator technologies for electromechanical transduction applications.6–8 A dielectric elastomer actuator is consisted of an elastomeric film sandwiched between two very thin and flexible electrodes in a capacitor-like structure.9 When the voltage is applied, the electrostatic stress, known as Maxwell stress, exerted on the two surfaces of the elastomeric film leads to contraction in thickness direction of the film and expansion in the planar direction.10,11 According to the Maxwell strain, a reasonable way to increase the actuation strain is the increase of the dielectric permittivity to the elastic modulus ratio (εr/Y), which is defined as electromechanical sensitivity (β). 12,13 Dielectric elastomer actuators display highly nonlinear electromechanical behavior. This kind of actuator can form wrinkles under voltage, endure snap-through instability, and suffer electrical breakdown.14,15 However, the performance of dielectric elastomers is limited by



electromechanical instability (EMI). Stark and Garton applied an electrical field to irradiated polyethylene under temperature variation and observed that EMI is responsible for electrical failure of this deformable polymer.16 To address this problem, incorporation of conductive nanoparticles into the polymer matrix has been recognized as a promising method for increasing the dielectric permittivity (εr) of composite actuators.17 However, the percolation of nanoparticles on one hand, increases the dielectric permittivity and on the other hand, increases the elastic modulus and the dielectric loss of composites.18,19 Therefore, conductive nanoparticles can largely affect the actuation performance of DEAs by influencing the different mechanical and electrical properties such as dielectric permittivity, dielectric loss, electrical breakdown strength and elastic modulus.20–22 Recently, graphene and graphene oxide (GO) have attracted considerable interest in the field of dielectric elastomers actuators.20,23 Graphene, a single-atom-thick honeycomb planar structure of SP2 carbon atoms, is one of the most attractive and preferential nanoparticles due to the its prominent conductivity, electron mobility, and large specific surface area.24,25 The prevalent method for production of graphene is the initial oxidation of graphite to GO, followed by the thermal or chemical reduction process.26,27 Therefore, a large number of oxygene-containing groups on surface of GO can affect on SP2 structure and polarity of graphene.28 Thermal reduction method with manipulated level of reduction gives the reduced graphene oxide (rGO) or graphene with high specific surface area, high electrical conductivity 29,30 and also can maintain layers at a low number.31 The reduction and oxidation processes govern the overall oxygen contents on the surface of GO, dispersion of rGO within the polymer matrix as well as the dielectric and the mechanical properties, curing condition and permeability of resultant composite actuators.32–35 Although the incorporation of GO into the polymer matrix increases the


Page 4 of 32

Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dielectric permittivity and improves the actuation strain of elastomeric composites, but the elastic modulus and the dielectric loss are inevitably increased.13,36 In addition, the interfacial interactions between the polymer matrix and GO nanosheets have crucial roles in determining the GO dispersion characteristics, electrical properties and energy dissipation effects. The surface chemistry of GO nanosheets can be manipulated by functionalization to minimize the dissipation effects and to create strong interactions between polymer matrix and GO nanosheets.37–41 However, the functionalization impairs the conductivity of GO nanosheets to such an extent that the GO nanosheets are not suitable for a dielectric elastomer actuator. Therefore, a balance between the intrinsic conductivity of GO nanosheets and the GO/matrix interfacial characteristics should be achieved in order to optimize the final properties of the actuator composites. In this study, reduced graphene oxide (rGO) nanosheets with different surface chemistries are synthesized by controlling the reduction temperature. Dielectric elastomer actuators containing 0.5 vol.% of different reduced graphene oxide (rGO) nanosheets are fabricated to investigate the effect of reduction temperature on the electromechanical properties. Moreover, electrical data are quantitatively analyzed to separate the influence of PDMS/nanosheets interfacial polarization and intrinsic properties of nanosheets. In addition, the instability behavior of composites is evaluated by thermodynamic predictions. Finally, the actuation strain of the composites measured as a function of electric field, as well as time for elucidating recovery behavior. 2. Experimental Section 2.1. Materials and Preparation Methods Graphite, sulfuric acid (H2SO4), peroxide (H2O2), tetrahydrofuran (THF), hydrochloric acid (HCl) and potassium permanganate (KMnO4) were purchased from Merck, Germany, to



synthesis the graphene oxide. A filler-free PDMS (GELEST® OE 41.4, USA) was employed for fabricating the polymer composites. Details about the synthesis procedure of graphene oxide (GO) and reduced graphene oxide (rGO) using Hummers method can be found elsewhere 23. As a result, the different oxygencontaining groups such as hydroxyl and carboxyl groups can be formed on the GO surface. Then, the GO powders were thermally reduced at different temperatures of 150˚C, 200˚C, 300˚C, and 400˚C. The samples were labeled as the rGO-x, that x refers to the reduction temperature. The PDMS/GO and PDMS/rGO-x composites were prepared by the solution mixing method, which was followed by vulcanization process for 15 min at 120 °C. For this intention, a stable suspension of GO nanosheets in THF was prepared by ultrasonication (Hielscher 400 Watts) for 30 min. Subsequently, the well-dispersed suspension was added to a solution of PDMS in THF under stirring for 60 min to obtain a stable mixture. Then, a certain quantity of the curing agent was added to the mixture. The mixture was stirred for 5 min and degassed under vacuum at room temperature for 5 hr. Finally, the resultant mixture was poured into a Teflon mold and cured in an oven at 120 °C for 15 min to form a thin film of PDMS containing 0.5 vol.% of different GO or rGO-x nanosheets with a thickness of about 0.75 mm. 2.2. Testing Methods The crystalline structure of synthesized GO and rGO-x was studied by X-ray diffraction (XRD) using X’ Pert-Philips diffractometer (using Cobalt/kα radiation, λ=1.540 60 Å). X-ray photoelectron spectroscopy (XPS) measurements were utilized to characterize the chemical groups on the GO and rGO-x surfaces and to quantify the ratio of oxygen to carbon (O/C ratio) with monochromatic Al Ka (1486.71 eV), X-ray radiation (15 kV and 10 mA) and hemispherical electron energy analyzer (Thermo Scientific ESCALAB 250Xi). Thermal gravimetric analysis


Page 6 of 32

Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(TGA) was conducted using PerkinElmer Pyirs apparatus under nitrogen environment. The electrical properties and electrical breakdown strength of the composite samples were analyzed using LCR-meter from GW Instek and ASTM D149, respectively. Tensile tests of the composites were performed on Hiwa machine (Hiwa Eng Co.) following ASTM Standard D 412. To ensure data accuracy and repeatability, three specimens were tested for each composite and the average value was reported. The morphology of the cryogenically fractured surface of the pure PDMS and the PDMS/GO(rGO-x) composites was studied with a field emission scanning electron microscopy (FE-SEM). The thickness and diameter of disc-shaped sample for actuation test were 0.75 mm and 2.5 cm, respectively. The static contact angle method was employed for measuring surface energy of GO(rGO-x), according to our previous work.23 A mixture of graphite and grease was used as a compliant electrode and DC electrical field was applied in the shape of a step function (instantaneous increase in DC field at 0 time holding time of 8s) was applied to both sides of the samples. In order to obtain the planar strain, the relatively safe electric fields (less than 25 kV/mm) were used because a high electric field can be harmful to humans and damage equipment. A disc shape sample for the actuation test had a thickness of 0.7-0.8 mm and a diameter of 2.5 cm. Compliant electrodes, constitute of graphite in grease, were applied to both sides of the elastomeric disc. As it is difficult to accurately measure the change in thickness, the change in surface area was measured to evaluate the thickness actuation strain (Sz) or thickness stretch ratio (λz = 1+ Sz), considering incompressibility for the sample and no pre-stretch was applied to the sample as Equation (1). 𝜆𝑥𝜆𝑦𝜆𝑧 = 1, 𝑏𝑖𝑎𝑥𝑖𝑎𝑙→𝜆𝑥 = 𝜆𝑦 = 𝜆→ (1)

1 𝜆𝑧𝜆2 = 1= (1+ 𝑆𝑠)(1+ 𝑆𝑝)→𝑆𝑝 = ― 1 𝜆𝑧



Page 8 of 32

According to Equation (X), the planar strain (Sp) increases with increasing the thickness strain because the thickness stretch ratio is negative. During actuation, images of the surface area of samples were captured by a general camera (Nikon D810) fitted with a wide-angle lens. The relative change in planar area was determined by analyzing the captured images with Adobe Photoshop software. The planar strain was calculated by Equation (2) as follow: 𝐴 𝑆𝑝 = ( 𝐴 )― 1 0

(2)

where A is the actuated planar area and A0 is the original area. 3. Results and Discussions 3.1. Characterization of GO and rGO-x XRD analysis was used to investigate the effect of thermal reduction on the interlayer spacing between the graphitic layers and the crystalline structure of graphite and its derivatives. Figure 1a shows that the graphite has an interlayer distance (d-spacing) of approximately 0.35 nm, associated to the basal reflection (002) peak at 2θ = 26.2°. This crystalline peak is a characteristic of hexagonal pristine graphite.42 After oxidation process, this peak has disappeared in the XRD pattern of GO and a new diffraction peak (001) at 2θ = 11.5° has appeared corresponding to the d-spacing of 1 nm. The larger interlayer spacing of GO is due to the presence of oxygencontaining functional groups on the GO surface. Moreover, the distance between nanosheets (dspacing) is decreased with increasing the reduction temperature, because of the decrease in the oxygen-containing groups on the GO surface. In addition, with increasing the reduction temperature, the position of 2θ peak for rGO-x nanosheets is shifted to the higher wavelength. The shifted peaks are observed at 2θ = 17.3, 19.0, 22.1 and 25.4° for rGO-150, rGO-200, rGO300 and rGO-400, respectively.


Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The TGA plots of GO and rGO-x samples show the main weight loss in the range of 190 - 240 °C due to the elimination of oxygen-containing functional groups (Figure 1b). The main weight loss is decreased with increasing the reduction temperature in the order of GO > rGO-150 > rGO-200 > rGO-300 > rGO-400. This behavior can prove the existence of lower value of oxygenic groups on surface of rGO-400.

Figure 1- (a) XRD pattern and (b) the TGA plots of graphite, GO and rGO-x nanosheets

XPS measurements were carried out to quantitatively study the functional groups of the GO and rGO-x samples with different reduction levels (Figure 2). The GO sample shows a significant degree of oxidation with four types of carbon atoms including C=C (284.8 eV), C−O (286.6 eV), C=O (287.2 eV), and O−C=O (288.9 eV), (Figure 2a). Moreover, the ratio of area of the peaks related to oxygen-containing functional groups to the total area, O/C, for GO is 0.54 and decrease with thermal reduction in the order of rGO-150 > rGO-200 > rGO-300 > rGO-400 (Figures 2b, c, d and e). Furthermore, the C 1s XPS spectra exhibit the different amount of oxygen-containing functional groups on the surface of rGO-x nanosheets. Accordingly, the C=C peaks of the rGO-x are sharper than that of GO. However, the C=C fraction is insensitive to the reduction level, as evidenced by the slight variation in the C=C fraction from rGO-150 to rGO9 ACS Paragon Plus Environment


400.28,43 Howbeit, the C=C fraction is almost constant but the free electrons are increased in rGO-x with increasing the reduction level (Figure 2f) owing to decreasing the oxygenic groups and enhancing the number of SP2 bond in system.

Figure 2. XPS C1s core-level spectra for (a) GO and different rGO-x, (b) rGO-150, (c) rGO-200, (d) rGO-300, (f) rGO-400 and (e) oxygen to carbon ratio (O/C)

3.2. Mechanical behavior of PDMS Composites Tensile stress-strain curves for the cured PDMS composites containing 0.5 vol.% of GO and rGO-x are shown in Figure 3a. The stress and strain levels of the neat PDMS is boosted with embedding the nanosheets in the matrix. In addition, the nanosheets with lower O/C ratio 10 ACS Paragon Plus Environment

Page 10 of 32

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


demonstrate more pronounced reinforcing efficiencies for PDMS. The PDMS/rGO-400 composite shows the highest tensile stress at break-point (σbr) value as 1.61 MPa, which increased by 1.52 times, compared to the PDMS/GO composite. It is worth noting that the difference in σbr values of PDMS/rGO-x composites at the same content of nanosheets is ascribed to the difference in the amount of oxygen-containing groups on the surface of rGO-x. In addition, when the GO nanosheets are thermally reduced at higher temperatures, the tensile strain at break point values (εbr) are monotonically decreased, as specified in Table 1. These observations can be attributed to the improved dispersion of nanosheets within the PDMS matrix by decrease in the O/C ratio, due to the enhancement of interfacial interactions arising from their enhanced affinity between rGO-x surface and PDMS.28,44 In order to assess the dispersion of nanosheets within the PDMS matrix, the fractured surfaces of PDMS/GO(rGO-x) composites were studied by FE-SEM, as shown in Figure S1, in supporting information. The GO nanosheets were agglomerated in the PDMS/GO composite and led to the formation of large clusters of GO (Figure S1) due to the hydrophilic nature of GO and low polarity of PDMS. However, the thermal reduction of GO resulted in the improved dispersion of rGO-x and the breakdown of clusters into smaller ones. Furthermore, to systematically investigate the dispersion of rGO-x within the PDMS composite, the thermodynamic predictors on the basis of surface energy can be employed.28,45 In this regard, the adhesion energy of PDMS-GO(rGO-x) (WPF) can be estimated through dispersive (γd) and polar (γp) components of the surface energy for both constituents contact angle measurements through Equation (3): 𝑊𝑃𝐹 = 2( 𝛾𝑓𝑑 + 𝛾𝑑𝑝 + 𝛾𝑓𝑝 + 𝛾𝑝𝑝)

(3)



Page 12 of 32

where the subscript of F and P corresponding the GO or rGO-x and PDMS. Another thermodynamic parameter describing the driving force for the reaggregation of GO(rGO-x) after dispersion is as stated by Equation (4): 2

2

∆𝑊𝑎 = 2( 𝛾𝑝𝑓 ― 𝛾𝑝𝑝) + 2( 𝛾𝑓𝑑 ― 𝛾𝑝𝑑)

(4)

Other thermodynamic parameter is recognized as interfacial tension (𝛾𝑃𝐹), which describes the strength of interface and the quality of PDMS-GO(rGO-x) interaction that is defined as follows: 𝛾𝑃𝐹 = 𝛾𝑝𝑓 + 𝛾𝑓𝑑 + 𝛾𝑝𝑝 + 𝛾𝑑𝑝 ― 2( 𝛾𝑝𝑓𝛾𝑝𝑝 + 𝛾𝑓𝑑𝛾𝑝𝑑) (5) These parameters, which can be calculated by measuring the GO(rGO-x) and PDMS surface energies (the raw data is presented in Table S1 at supplementary information), are summarized in Figure 3b. These results can clarify that the degree of thermal reduction of GO is an important parameter to engineer the dispersion of nanosheets in PMDS matrix. According to Figure 3b, the 𝛾𝑃𝐹 and ∆𝑊𝑎 have decreased, and also the 𝑊𝑃𝐹 show higher values as the reduction temperature of GO increase. Therefore, the PDMS-GO(rGO-x) affinity, dispersion and interfacial interaction can improve by reducing the oxygen-containing groups on the surface of GO nanosheets during the reduction process. To gain deeper insights into the effects of thermal reduction of GO on the mechanical behavior the tensile data were recast in the form of the reduced stress, σ*, versus reciprocal extension ratio, 1/λ, known as Mooney-Rivlin plot, (Figure 3c). In order to calculate the reduced stress, the mechanical stress is divided by the strain function , λ-λ-2, proposed by the molecular theory of rubber elasticity 46,47: 𝜎∗ =

𝜎 ―2

𝜆― 𝜆

(6)

= 2𝐶2𝜆―1 + 2𝐶1

The λ is stretch ratio and defined as L/L0, which L0 and L are the initial and final lengths of sample, respectively. In addition, the terms C1 and C2 are constants that C1 is related to the nature of elastomer network. Table 1 shows the C1 and C2 values for the PDMS/GO and rGO-x composites. The C1 values are increased by increasing the reduction temperature of GO, which 12 ACS Paragon Plus Environment

Page 13 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


can be attributed to the non-Gaussian nature of the molecular network. However, the stronger interactions between PDMS and rGO-x than those between PDMS and GO ensures the efficient load transfer from the matrix to filler. Thus, these results can consider as the preconditions to study the influence of the surface chemistry of nanosheets on the dielectric properties of PDMS composites.

Figure 3. (a) Tensile stress-strain responses of PDMS composites containing 0.5 vol.% of GO, (b) rGO-x, the 𝛾𝑃𝐹, 𝑊𝑃𝐹 and ∆𝑊𝑎 as function of O/C ratio and (c) Mooney-Rivlin curves obtained from the tensile stress-strain data.



Page 14 of 32

Table 1. Young's modulus (Y), stress (σbr) and strain (εbr) at break-point as well as Mooney-Rivlin constants values (C1 and C2) of different PDMS/GO(rGO-x) composites. PDMS

Young's modulus (Y),

Stress at break

%Stain at break

Composite

MPa

(σbr), MPa

(εbr)

Neat PDMS

0.136

0.412

GO

0.212

rGO-150

2×C1

2×C2

304.7

0.0563

0.0956

1.06

460.5

0.1365

0.1301

0.238

1.21

425.4

0.2050

0.1622

rGO-200

0.247

1.30

396.9

0.2265

0.1926

rGO-300

0.264

1.48

364.1

0.2601

0.2524

rGO-400

0.281

1.61

345.3

0.2737

0.4223

3.3. Electrical Properties of PDMS Composites The change in the dielectric loss and permittivity by addition of nanosheets can be attributed to three mechanisms: intrinsic properties of the nanosheets, the micro-capacitance-structure model and the interfacial polarization effect (or Maxwell–Wagner–Sillars effect). The surface chemistry can dictate the intrinsic conductivity of GO and rGO-x owing to distortion and hybridization state (SP2) of its 2D planar lattice.25,48 The functionalization of nanosheets surface can influence the dispersion of nanosheets within the PDMS by changing the interaction of PDMS with GO and rGO-x. Maxwell–Wagner–Sillars (MWS) effect is ascribed to the accumulation of many charge carriers at the internal interfaces between rGOs and PDMS matrix. The micro-capacitance structure model assumes that a large number of parallel or series micro-capacitors connected with one another are formed. A micro-capacitance-structure model is consisted of a thin


Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


insulating polymer layers that trapped by two rGO-x nanosheets. A large number of microcapacitance-structure models in the composite can increase the intensity of local electric field. The dielectric permittivity, dielectric loss (at a frequency of 103 Hz) and electrical breakdown strength of PDMS/rGO-x composites are presented in Table 2 (the dielectric permittivity and dielectric loss versus frequency are presented in Fig. S2). With increasing the reduction temperature, more free electrons are trapped in PDMS/rGO interface. This leads to monotonically increase in the dielectric permittivity and dielectric loss owing to increase in the intrinsic conductivity of the nanosheets, number of micro-capacitances in system and interfacial polarization effect (Table 2). The dielectric loss of PDMS composites incorporated with conductive rGO-x is mainly produced by the leakage current. The higher free electrons on the rGO-x surface could construct more conductive pathways, and thus leads to more significant dielectric loss and leakage current. The dielectric permittivity and loss depend strongly on the dispersion of the filler and microstructure of the composites.49,50 The electrical breakdown strength (Ebr) values of the PDMS/rGO-x composites are lower than that of the PDMS/GO composite, as shown in Table 2. The Ebr values are decreased with further increases in reduction temperature of GO, because of the increased leakage current and DC conductance, conductive paths and dissipative mechanisms. Table 2. Dielectric permittivity, dielectric loss and electrical breakdown strength of PDMS/GO(rGO-x) composites. PDMS Composite

Dielectric Loss ()ε at′ 103

Electrical Breakdown

at 103 Hz

Hz

KV Strength ( mm)

Neat PDMS

2.2

0.01

122.2

GO

3.4

0.09

97.9

rGO-150

4.9

0.26

90.2

Dielectric Permittivity ()ε

′



Page 16 of 32

rGO-200

6.8

0.31

85.14

rGO-300

8.6

0.38

81.18

rGO-400

9.6

0.41

75.24

A equivalent circuit must be used to simulate the impedance spectra of composite for further explore of the mechanism behind the electrical property of PDMS/GO(rGO-x) composites. Figure 4 shows the Nyquist plots (impedance results) of PDMS/GO(rGO-x) composites in the frequency range of 10 Hz to 106 Hz. These plots represent the relationship between imaginary part of impedance (Z") and real part of impedance (Z'). When the polarization occurs in the composites, semicircle shape is obtained in the Nyquist plots. The diameter of semicircles corresponds to the bulk resistivity of the components in the composite.21 The impedance value of the PDMS/GO composite is the highest, followed by PDMS/rGO-150, PDMS/rGO-200, PDMS/rGO-300 and PDMS/rGO-400 composites. The impedance of composite is determined by both the intrinsic properties of components (matrix and nanosheets) and interfacial polarization.21,51–53 In this regard, the decrease in the impedance of the composite with increasing the reduction temperature of GO again specifies the enhancement of interfacial polarized charges in the composites with lower O/C ratio. Impedance results provide a unique opportunity to quantitatively separate the influence of the interfacial polarization from the influence of intrinsic properties of nanosheets and matrix on the electrical properties.51–53 For this purpose, an equivalent circuit is proposed by a resistance from the impedance measurements consisting of the resistors (R) and the constant phase (CP) elements. The CP is defined by two values, CP-T and CP-P, the latter is a constant ranging from 0 to 1 and CP-T is a constant phase element with respect to counter electrodes. If CP-P equals 1 then, the CP is identical to a capacitor. However, it is important to note that, if CP-P equals 0.5, 16 ACS Paragon Plus Environment

Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a 45-degree line is produced on the Complex-Plane graph and thus, the interfacial behavior of the composite can be investigated.54 When a CP is placed in parallel to a resistor, a Cole-Element (depressed semi-circle) is produced. As described in previous studies 21,51–53,55, the equivalent circuit can be presented as two parallel resistor-capacitor (RC) circuits in series (R1-CP1 and R2- CP2) PDMS/rGO-x composites, shown as the inset in Figure 4. By varying the values of capacitance, the electrochemical behavior of the system can be found. Here, CP1 in the circuit represents capacitance produced by the polarization of PDMS and nanosheets, whereas CP2 represents the interfacial polarization.56 In addition, R1 represents the composites resistance which consist of the PDMS and rGO-x resistances, while R2 represents the interfacial resistance.

Figure 4. Nyquist plots of composites containing GO and rGO-x. The inset is an equivalent circuit.

The impedance data of composites can be analyzed by Zview2 software and the parameters of the equivalent circuit are calculated according to the following equation 57:



𝑍∗ =

Page 18 of 32

𝑅 1+ 𝑗𝑤𝜏

(7)

where Z*, τ = R×C and ω are complex impedance, the time constant of the circuit and the angular frequency, respectively. The values of each element, achieved by fitting the impedance data to the equivalent circuit, were reported in Table 3. The conductivity of composites increases, while R1 decreases with increasing the reduction temperature of GO. with decreasing the O/C ratio of rGO-x nanosheets, both the CP1 and CP2 values for the PDMS/rGO-x composites are increased. The CP2 values are larger than the CP1 values at the same composite, indicating that the interfacial polarization is the main mechanism rather than the intrinsic polarization of the matrix and nanosheets in all the composites. Thus, the dominant mechanism in electrical properties of the PDMS/rGO-x composites is the interfacial polarization. By comparing the simulation results in Table 3, the PDMS composites containing nanosheets with the lower O/C ratio show higher CP2 value, indicating the stronger interfacial polarization. This phenomenon is exacerbated with increasing the reduction temperature of GO because the improved dispersion and distribution of nanosheets within the PDMS matrix. Thus, the equivalent circuit results reveal that the one important parameter in interfacial polarizability of PDMS/rGO-x composites can be the reduction temperature of GO.

Table 3. Simulated values of elements by fitting the impedance data and the equivalent circuit. Composites

R1

CP1

R2

CP2

PDMS/GO

1.42×107

3.15×10-10

1.56×107

4.41×10-9

PDMS/rGO-150

8.67×106

6.39×10-10

5.85×107

5.86×10-9

PDMS/rGO-200

2.54×106

8.21×10-9

9.77×106

7.48×10-8


Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PDMS/rGO-300

8.87×105

1.03×10-9

3.36×106

9.96×10-7

PDMS/rGO-400

1.32×105

8.62×10-8

7.95×105

1.68×10-7

3.4. Tracing instability of actuators According to the Helmholtz free energy concept, a thermodynamic model was derived for forecasting the actuator properties.58 In the ideal state of a dielectric elastomer, the electromechanical behavior at equi-biaxial mode is achieved by the following equation 59,60: ∂𝑊𝑠 𝜎𝑥.𝑦 ― 𝜎𝑧 = 𝜆𝑥.𝑦 ― 𝜀𝐸2 ∂𝜆𝑥.𝑦 𝑟

(8)

where 𝜎, 𝜆, 𝑊𝑠, 𝜀𝑟 and 𝐸 are stress, stretch ratio, free energy of elastomer, dielectric permittivity and electric field, respectively. The subscripts x, y and z refer to the directions. To estimate the free energy values resulting from the stretching of PDMS/GO(rGO-x) composites, Ws, we assumed that the rubber’s hyper-elasticity is not affected by the addition of small amounts of GO or rGO-x. To calculate the extension limit of a elastomer, the Gent model was used as 61: 𝑊𝑠 =

𝜆2𝑥 + 𝜆2𝑦 + 𝜆𝑧2 ― 3 ln (1 ― ) 𝐽𝑙𝑖𝑚 6

―𝑌𝐽𝑙𝑖𝑚

(9)

Where Y and Jlim are the effective elastic modulus of the PDMS/GO(rGO-x) composite and a constant describing the extension limit, respectively. The stretches are restricted as 0≤ 𝜆𝑥2 + 𝜆2𝑦 + 𝜆𝑧2 ― 3 𝐽𝑙𝑖𝑚

< 1. When

+ 𝜆𝑦2 + 𝜆𝑧2 ―3). When

𝜆2𝑥 + 𝜆𝑦2 + 𝜆2𝑧 ― 3

𝑌

→0, the Gent model retrieves the neo-Hookean model, 𝑊𝑠 = 6(𝜆𝑥2

𝐽𝑙𝑖𝑚 𝜆𝑥2 + 𝜆2𝑦 + 𝜆2𝑧 ― 3 𝐽𝑙𝑖𝑚

→1, the free energy characterized by equation (9), and the

elastomer styles the extension limit. The composites are considered as an incompressible material; thus, the Poisson ratio value is equal to 0.5 (𝜆𝑥𝜆𝑦𝜆𝑧 = 1). By employing the model to



Page 20 of 32

investigate a dielectric elastomer actuator composites subject to fixed equal-biaxial mode, 𝜆𝑥 = 𝜆𝑦 𝑌 𝐽𝑙𝑖𝑚( 3)

1

= 𝜆 and 𝜆𝑧 = 𝜆2, without pre-stretch condition (𝜎𝑧 = 0. 𝜎𝑥.𝑦 = (𝜆2 ― 𝜆―4)(𝐽

𝑙𝑖𝑚

)), the

― (2𝜆2 + 𝜆―4 ― 3)

equation (8) is converted to equation (10): 𝑌 𝐽𝑙𝑖𝑚( 3)

2 ―4 𝐸(𝜆)= 𝜆―2 (𝜆 ― 𝜆 )(𝐽 ― (2𝜆2 + 𝜆―4 ― 3)) 𝑙𝑖𝑚

(10)

𝜀0𝜀𝑟 According to the equation (5), the electromechanical performance of actuators can be traced with recognizing the material properties of composites such as dielectric permittivity (εr), elastic modulus (Y) and maximum strain extension (Jlim). The electrical breakdown is defined as the 𝜙

ratio of voltage to the thickness of specimens, 𝐸𝑏𝑟 = 𝑇0 . When a DEA is actuated, the thickness is 𝑇

decreased (𝑇0 = 𝜆𝑧 = 𝜆―2), therefore, the electrical breakdown of PDMS composites have drastically declined during the actuation process (by this equation 𝐸𝑏𝑟 =

𝜆―2𝜙 𝑇

).

Figure 5 shows the 𝐸(𝜆)―𝜆 and 𝐸𝑏𝑟(𝜆)―𝜆 curves for PDMS/GO(rGO-x) composite using the data reported in Tables 1 and 2. The unstable region is detected by gradient of 𝐸(𝜆)―𝜆 curve ( 𝑑𝐸(𝜆)

) and instability point stand at starting of unstable region (

𝑑𝜆

𝑑𝐸(𝜆) 𝑑𝜆

≤0

= 0). The PDMS/GO composite

shows the highest unstable region (Figure 5a). The break point of composites existed at encounter of 𝐸(𝜆)―𝜆 and 𝐸𝑏𝑟(𝜆)―𝜆 curve for each sample, and also the composites loses the potential of actuation at this point. The maximum extension values of PDMS/GO(rGO-x) composites were monotonically decreased with increasing the reduction temperature due to the decrease in oxygen content at nanosheets surface (Figures 5a, b, c, d and e). In this work and the previous study 15, it was clearly shown that the electromechanical instability of all composites 20 ACS Paragon Plus Environment

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


occurred at a constant stretch ratio (~λ = 1.15). As presented in Figures 5a to e, the unstable regions have been decreased because of the increase in the reduction temperature of GO. The values of break-point and unstable region of all composites are summarized in Figure 5f. The break point results show the optimum composite to be the PDMS/rGO-300 composite. Moreover, these observations demonstrate that the optimum reduction level for achieving the best performance of PDMS/GO(rGO-x) dielectric elastomer actuators is 300 °C.



Figure 5. The electric field–stretch dependence curves and electrical breakdown- stretch ratio of PDMS composites containing (a) GO, (b) rGO-150, (c) rGO-200, (d) rGO-300, (e) rGO-400, and (f) reports the range of unstable region and stretch value of break point for each PDMS composite.

3.5. Electromechanical Performance of actuators The electromechanical sensitivity (β = εr/Y) values of PDMS/GO(rGO-x) were calculated using the data in Tables 1 and 2 and shown in Figure 6a. The β values of PDMS composites containing 0.5 vol.% of rGO-150, rGO-200, rGO-300 and rGO-400 are obviously increased by 1.29, 1.72, 2.03 and 2.13 times, respectively, in comparison to the β value of PDMS/GO composite. The elastic modulus and the dielectric permittivity values were increased by adding 0.5 vol.% of rGO-x at different reduction temperatures to PDMS. However, the increasing rate of the dielectric permittivity is greater than the elastic modulus of composites with increasing the temperature of reduction. Besides, the elastic modulus and dielectric permittivity values of PDMS composites are employed for calculating Maxwellian actuation strain. The Maxwellian actuation strain is calculated at 20 kV/mm field and it noticeably enhances from 5.7% for GO/PDMS composite to 7.3%, 9.7%, 11.5% and 12.1% for PDMS composites containing rGO150, rGO-200, rGO-300 and rGO-400 respectively. Figure 6b shows the planar actuation strains of the PDMS/GO(rGO-x) composites as a function of applied electric field. The maximum planar actuation strain at the electric field of 20 KV/mm is increased from 5.07% for the PDMS/GO to 8.94%, 11.16%, 13.32% and 12.87% for the PDMS/rGO-150, PDMS/rGO-200, PDMS/rGO-300 and PDMS/rGO-400 composites, respectively. In addition, the planar actuation strain of the PDMS/rGO-300 at a low electric field (4 KV/mm) is lower than that of the PDMS/rGO-400, as shown in Figure 6b. The actuation strain value of PDMS/rGO-300 at large electric fields (>4 KV/mm) are higher than that of 22 ACS Paragon Plus Environment

Page 22 of 32

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PDMS/rGO-400. The reason for this observation is the increase in dissipation mechanisms (such as loss modulus and dielectric loss) in the PDMS/rGO-400 composite due to the higher conductivity and better dispersion of rGO-400 within the PDMS. The PDMS/rGO-300 composite shows the optimized reduction level. On the other hand, PDMS/rGO-400 composite shows the highest mechanical and electrical dissipation that leads to deteriorating performance of dielectric elastomer actuator.62 The planar actuation strains of all composites at the electric field of 20 KV/mm as a function of time is shown in Figure 6c. The planar actuation strain shows a sudden increase when the DC electric field was applied (in the beginning time), followed by a slow rate of increase with increasing the time. Although the large magnitude of electromechanical sensitivity, β, represents the high rate of actuation strain growth in the primitive moments, after some seconds the rate of actuation strains of composites decline due to the appearance of dissipation mechanisms. After removing the electric field, the planar actuation strains go to zero with delay. In the recovery behavior of PDMS composites, the dissipation mechanism is quite obvious due to the lack of presence of the Maxwell stress in samples.



Figure 6. a) Electro-mechanical sensitively (𝛽) and modulus, b) Planar actuation strain (%), and c) Electro-actuation profiles (under 20 KV/mm DC field) of composites containing GO and rGO-x

4. Conclusions In the present scenario, dielectric permittivity and electrical conductivity of PDMS composites containing 0.5 vol.% of graphene oxide (GO) and reduced graphene oxides (rGO) at different reduction temperatures ranging from 150 to 400˚C were investigated. By varying the reduction temperature, it was shown that the ratio of oxygen to carbon (O/C) of GO is controllable. The role of reduction temperature of GO in the interfacial polarization, mechanical properties and electrical properties of the composites was carefully studied. To separate the influence of the interfacial polarization from the influence of intrinsic properties of matrix and GO (rGO-x) nanosheets, an equivalent circuit was employed. The results prove that there are more interfacial 24 ACS Paragon Plus Environment

Page 24 of 32

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


polarized charges at the PDMS/rGO-x interface in the composites containing thermally reduced GO at higher temperatures. Many parallel or serial micro-capacitor structures are gradually formed because of improved dispersion of GO with decrease in the O/C ratio. As a result, dielectric breakdown strength of composites is monotonically decreased, while the dielectric permittivity, dielectric loss and elastic modulus of composites are increased with increasing the reduction temperature of GO. By removing the oxygen groups on the GO surface, the great increase in dielectric permittivity leads to the great increase in electromechanical sensitively, β. Therefore, the maximum planar actuated strains were increased (at 20 KV/mm) from 5.07% for the PDMS/GO to 8.94%, 11.16%, 13.32% and 12.87% for the composite with 0.5 vol.% of rGO150, rGO-200, rGO-300 and rGO-400, respectively. Another achievement of this research is investigating the influence of surface chemistry on the instability and break point of dielectric elastomers, and also engineering these parameters. This work provides a simple method to prepare high performance dielectric elastomer actuators without makes any hybrid filler and chemical functionalization. Supporting Information The SEM micrograph, the dielectric permittivity versus frequency and dielectric loss versus frequency of composites, as well as the preparation method and the raw surface energy data of GO(rGO-x) and PDMS are reported in Supporting Information. The Supporting Information are available free of charge. Acknowledgment The authors want to thank the families for their support during the period that they were working on the project. References



(1) (2) (3) (4)

(5)

(6)

(7)

(8) (9)

(10) (11)

(12)

(13)

Carpi, F. Electromechanically Active Polymers. Polym. Int. 2017, 59 (3), 277–278. https://doi.org/10.1002/pi.2790. Carpi, F.; De Rossi, D.; Kornbluh, R.; Pelrine, R.; Sommer-Larsen, P. Dielectric Elastomers as Electromechanical Transducers; Elsevier, 2008; Vol. 23. https://doi.org/10.1016/B978-0-08-047488-5.X0001-9. Romasanta, L. J.; Lopez-Manchado, M. A.; Verdejo, R. Increasing the Performance of Dielectric Elastomer Actuators: A Review from the Materials Perspective. Prog. Polym. Sci. 2015, 51, 188–211. https://doi.org/10.1016/j.progpolymsci.2015.08.002. Tugui, C.; Bele, A.; Tiron, V.; Hamciuc, E.; Varganici, C. D.; Cazacu, M. Dielectric Elastomers with Dual Piezo-Electrostatic Response Optimized through Chemical Design for Electromechanical Transducers. J. Mater. Chem. C 2017, 5 (4), 824–834. https://doi.org/10.1039/C6TC05193F. Chen, T.; Qiu, J.; Zhu, K.; Li, J.; Wang, J.; Li, S.; Wang, X. Achieving High Performance Electric Field Induced Strain: A Rational Design of Hyperbranched Aromatic Polyamide Functionalized Graphene-Polyurethane Dielectric Elastomer Composites. J. Phys. Chem. B 2015, 119 (12), 4521–4530. https://doi.org/10.1021/jp508864b. Chen, B.; Lu, J. J.; Yang, C. H.; Yang, J. H.; Zhou, J.; Chen, Y. M.; Suo, Z. Highly Stretchable and Transparent Ionogels as Nonvolatile Conductors for Dielectric Elastomer Transducers. ACS Appl. Mater. Interfaces 2014, 6 (10), 7840–7845. https://doi.org/10.1021/am501130t. Yin, G.; Yang, Y.; Song, F.; Renard, C.; Dang, Z.-M.; Shi, C.-Y.; Wang, D. Dielectric Elastomer Generator with Improved Energy Density and Conversion Efficiency Based on Polyurethane Composites. ACS Appl. Mater. Interfaces 2017, 9 (6), 5237–5243. https://doi.org/10.1021/acsami.6b13770. Madsen, F. B.; Yu, L.; Skov, A. L. Self-Healing, High-Permittivity Silicone Dielectric Elastomer. ACS Macro Lett. 2016, 5 (11), 1196–1200. https://doi.org/10.1021/acsmacrolett.6b00662. Wang, D.; Bao, Y.; Zha, J. W.; Zhao, J.; Dang, Z. M.; Hu, G. H. Improved Dielectric Properties of Nanocomposites Based on Poly(Vinylidene Fluoride) and Poly(Vinyl Alcohol)-Functionalized Graphene. ACS Appl. Mater. Interfaces 2012, 4 (11), 6273–6279. https://doi.org/10.1021/am3018652. Kollosche, M.; Kofod, G.; Suo, Z.; Zhu, J. Journal of the Mechanics and Physics of Solids Temporal Evolution and Instability in a Viscoelastic Dielectric Elastomer. J. Mech. Phys. Solids 2015, 76, 47–64. https://doi.org/10.1016/j.jmps.2014.11.013. Yang, D.; Ruan, M.; Huang, S.; Wu, Y.; Li, S.; Wang, H.; Shang, Y.; Li, B.; Guo, W.; Zhang, L. Improved Electromechanical Properties of NBR Dielectric Composites by Poly(Dopamine) and Silane Surface Functionalized TiO 2 Nanoparticles. J. Mater. Chem. C 2016, 4 (33), 7724–7734. https://doi.org/10.1039/C6TC01504B. Liu, S.; Sun, H.; Ning, N.; Zhang, L.; Tian, M.; Zhu, W.; Chan, T. W. Aligned Carbon Nanotubes Stabilized Liquid Phase Exfoliated Graphene Hybrid and Their Polyurethane Dielectric Elastomers. Compos. Sci. Technol. 2016, 125, 30–37. https://doi.org/10.1016/j.compscitech.2016.01.022. Liu, S.; Tian, M.; Yan, B.; Yao, Y.; Zhang, L.; Nishi, T.; Ning, N. High Performance Dielectric Elastomers by Partially Reduced Graphene Oxide and Disruption of Hydrogen Bonding of Polyurethanes. Polymer (Guildf). 2015, 56, 375–384. https://doi.org/10.1016/j.polymer.2014.11.012. 26 ACS Paragon Plus Environment

Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14)

(15) (16) (17)

(18)

(19)

(20)

(21)

(22)

(23) (24)

(25) (26)

Jiménez, S. M. A.; Mcmeeking, R. M. A Constitutive Law for Dielectric Elastomers Subject to High Levels of Stretch during Combined Electrostatic and Mechanical Loading: Elastomer Stiffening Crossmark and Deformation Dependent Dielectric Permittivity. Int. J. Non. Linear. Mech. 2016, 87 (October), 125–136. https://doi.org/10.1016/j.ijnonlinmec.2016.10.004. Li, B.; Chen, H.; Zhou, J. Electromechanical Stability of Dielectric Elastomer Composites with Enhanced Permittivity. Compos. Part A Appl. Sci. Manuf. 2013, 52, 55–61. https://doi.org/10.1016/j.compositesa.2012.11.013. Stark, K. H.; Garton, C. G. Electric Strength of Irradiated Polythene [12]. Nature. 1955, pp 1225–1226. https://doi.org/10.1038/1761225a0. Wang, D.; Zhang, X.; Zha, J. W.; Zhao, J.; Dang, Z. M.; Hu, G. H. Dielectric Properties of Reduced Graphene Oxide/Polypropylene Composites with Ultralow Percolation Threshold. Polymer (Guildf). 2013, 54 (7), 1916–1922. https://doi.org/10.1016/j.polymer.2013.02.012. Zhang, T.; Yang, J.; Zhang, N.; Huang, T.; Wang, Y. Achieving Large Dielectric Property Improvement in Poly(Ethylene Vinyl Acetate)/Thermoplastic Polyurethane/Multiwall Carbon Nanotube Nanocomposites by Tailoring Phase Morphology. Ind. Eng. Chem. Res. 2017, 56 (13), 3607–3617. https://doi.org/10.1021/acs.iecr.6b04763. Arjmand, M.; Mahmoodi, M.; Park, S.; Sundararaj, U. An Innovative Method to Reduce the Energy Loss of Conductive Filler / Polymer Composites for Charge Storage Applications. Compos. Sci. Technol. 2013, 78, 24–29. https://doi.org/10.1016/j.compscitech.2013.01.019. Zhang, F.; Li, T.; Luo, Y. A New Low Moduli Dielectric Elastomer Nano-Structured Composite with High Permittivity Exhibiting Large Actuation Strain Induced by Low Electric Field. Compos. Sci. Technol. 2018, 156, 151–157. https://doi.org/10.1016/j.compscitech.2017.12.016. Wu, S. Q.; Wang, J. W.; Shao, J.; Wei, L.; Yang, K.; Ren, H. Building a Novel Chemically Modified Polyaniline/Thermally Reduced Graphene Oxide Hybrid through ππ Interaction for Fabricating Acrylic Resin Elastomer-Based Composites with Enhanced Dielectric Property. ACS Appl. Mater. Interfaces 2017, 9 (34), 28887–28901. https://doi.org/10.1021/acsami.7b07785. Ning, N.; Ma, Q.; Liu, S.; Tian, M.; Zhang, L.; Nishi, T. Tailoring Dielectric and Actuated Properties of Elastomer Composites by Bioinspired Poly(Dopamine) Encapsulated Graphene Oxide. ACS Appl. Mater. Interfaces 2015, 7 (20), 10755–10762. https://doi.org/10.1021/acsami.5b00808. Panahi-Sarmad, M.; Razzaghi-Kashani, M. Actuation Behavior of PDMS Dielectric Elastomer Composites Containing Optimized Graphene Oxide. Smart Mater. Struct. 2018, 27 (8), 085021. https://doi.org/10.1088/1361-665X/aacfe6. Kuang, B.; Song, W.; Ning, M.; Li, J.; Zhao, Z.; Guo, D.; Cao, M.; Jin, H. Chemical Reduction Dependent Dielectric Properties and Dielectric Loss Mechanism of Reduced Graphene Oxide. Carbon N. Y. 2018, 127, 209–217. https://doi.org/10.1016/j.carbon.2017.10.092. Dang, Z.; Zheng, M.; Zha, J. 1D/2D Carbon Nanomaterial-Polymer Dielectric Composites with High Permittivity for Power Energy Storage Applications. Small 2016, 12 (13), 1688–1701. https://doi.org/10.1002/smll.201503193. Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 27 ACS Paragon Plus Environment


(27) (28) (29) (30)

(31)

(32)

(33)

(34) (35)

(36)

(37)

(38)

80 (6), 1339. https://doi.org/10.1021/ja01539a017. Lahaye, R. J. W. E.; Jeong, H. K.; Park, C. Y.; Lee, Y. H. Density Functional Theory Study of Graphite Oxide for Different Oxidation Levels. Phys. Rev. B - Condens. Matter Mater. Phys. 2009, 79 (12). https://doi.org/10.1103/PhysRevB.79.125435. Tang, Z.; Zhang, L.; Feng, W.; Guo, B.; Liu, F.; Jia, D. Rational Design of Graphene Surface Chemistry for High-Performance Rubber/Graphene Composites. Macromolecules 2014, 47 (24), 8663–8673. https://doi.org/10.1021/ma502201e. Zhao, B.; Liu, P.; Jiang, Y.; Pan, D.; Tao, H.; Song, J.; Fang, T.; Xu, W. Supercapacitor Performances of Thermally Reduced Graphene Oxide. J. Power Sources 2012, 198, 423– 427. https://doi.org/10.1016/j.jpowsour.2011.09.074. Acik, M.; Lee, G.; Mattevi, C.; Pirkle, A.; Wallace, R. M.; Chhowalla, M.; Cho, K.; Chabal, Y. The Role of Oxygen during Thermal Reduction of Graphene Oxide Studied by Infrared Absorption Spectroscopy. J. Phys. Chem. C 2011, 115 (40), 19761–19781. https://doi.org/10.1021/jp2052618. Dolbin, A. V.; Khlistyuck, M. V.; Esel’son, V. B.; Gavrilko, V. G.; Vinnikov, N. A.; Basnukaeva, R. M.; Maluenda, I.; Maser, W. K.; Benito, A. M. The Effect of the Thermal Reduction Temperature on the Structure and Sorption Capacity of Reduced Graphene Oxide Materials. Appl. Surf. Sci. 2016, 361, 213–220. https://doi.org/10.1016/j.apsusc.2015.11.167. Liu, H.; Xu, P.; Yao, H.; Chen, W.; Zhao, J.; Kang, C.; Bian, Z.; Gao, L.; Guo, H. Controllable Reduction of Graphene Oxide and Its Application during the Fabrication of High Dielectric Constant Composites. Appl. Surf. Sci. 2017, 420, 390–398. https://doi.org/10.1016/j.apsusc.2017.05.181. Aradhana, R.; Mohanty, S.; Nayak, S. K. Comparison of Mechanical, Electrical and Thermal Properties in Graphene Oxide and Reduced Graphene Oxide Filled Epoxy Nanocomposite Adhesives. Polym. (United Kingdom) 2018, 141, 109–123. https://doi.org/10.1016/j.polymer.2018.03.005. Wei, Y.; Hu, X.; Jiang, Q.; Sun, Z.; Wang, P.; Qiu, Y.; Liu, W. Influence of Graphene Oxide with Different Oxidation Levels on the Properties of Epoxy Composites. Compos. Sci. Technol. 2018, 161, 74–84. https://doi.org/10.1016/j.compscitech.2018.04.007. Yang, E.; Ham, M. H.; Park, H. B.; Kim, C. M.; Song, J. ho; Kim, I. S. Tunable SemiPermeability of Graphene-Based Membranes by Adjusting Reduction Degree of Laminar Graphene Oxide Layer. J. Memb. Sci. 2018, 547 (April 2018), 73–79. https://doi.org/10.1016/j.memsci.2017.10.039. Tian, M.; Wei, Z.; Zan, X.; Zhang, L.; Zhang, J.; Ma, Q.; Ning, N.; Nishi, T. Thermally Expanded Graphene Nanoplates/Polydimethylsiloxane Composites with High Dielectric Constant, Low Dielectric Loss and Improved Actuated Strain. Compos. Sci. Technol. 2014, 99, 37–44. https://doi.org/10.1016/j.compscitech.2014.05.004. Chen, T.; Pan, L.; Lin, M.; Wang, B.; Liu, L.; Li, Y.; Qiu, J.; Zhu, K. Dielectric, Mechanical and Electro-Stimulus Response Properties Studies of Polyurethane Dielectric Elastomer Modified by Carbon Nanotube-Graphene Nanosheet Hybrid Fillers. Polym. Test. 2015, 47, 4–11. https://doi.org/10.1016/j.polymertesting.2015.08.001. Liu, S.; Sun, H.; Ning, N.; Zhang, L.; Tian, M.; Zhu, W.; Chan, T. W. Aligned Carbon Nanotubes Stabilized Liquid Phase Exfoliated Graphene Hybrid and Their Polyurethane Dielectric Elastomers. Compos. Sci. Technol. 2016, 125, 30–37. https://doi.org/10.1016/j.compscitech.2016.01.022. 28 ACS Paragon Plus Environment

Page 28 of 32

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) (40)

(41)

(42) (43)

(44)

(45) (46) (47) (48) (49)

(50)

(51)

Park, S.; He, S.; Wang, J.; Stein, A.; Macosko, C. W. Graphene-Polyethylene Nanocomposites: Effect of Graphene Functionalization. Polymer (Guildf). 2016, 104, 1–9. https://doi.org/10.1016/j.polymer.2016.09.058. Chen, T.; Qiu, J.; Zhu, K.; Li, J. Electro-Mechanical Performance of Polyurethane Dielectric Elastomer Flexible Micro-Actuator Composite Modified with Titanium Dioxide-Graphene Hybrid Fillers. Mater. Des. 2016, 90, 1069–1076. https://doi.org/10.1016/j.matdes.2015.11.068. Tian, M.; Ma, Q.; Li, X.; Zhang, L.; Nishi, T.; Ning, N. High Performance Dielectric Composites by Latex Compounding of Graphene Oxide-Encapsulated Carbon Nanosphere Hybrids with XNBR. J. Mater. Chem. A 2014, 2 (29), 11144. https://doi.org/10.1039/c4ta01600a. Chen, J.; Chi, F.; Huang, L.; Zhang, M.; Yao, B.; Li, Y.; Li, C.; Shi, G. Synthesis of Graphene Oxide Sheets with Controlled Sizes from Sieved Graphite Flakes. Carbon N. Y. 2016, 110, 34–40. https://doi.org/10.1016/j.carbon.2016.08.096. Xiao, F.; Yang, S.; Zhang, Z.; Liu, H.; Xiao, J.; Wan, L.; Luo, J.; Wang, S.; Liu, Y. Scalable Synthesis of Freestanding Sandwich-Structured Graphene/Polyaniline/Graphene Nanocomposite Paper for Flexible All-Solid-State Supercapacitor. Sci. Rep. 2015, 5 (1), 9359. https://doi.org/10.1038/srep09359. Yang, Z.; Liu, J.; Liao, R.; Yang, G.; Wu, X.; Tang, Z.; Guo, B.; Zhang, L.; Ma, Y.; Nie, Q.; Wang, F. Rational Design of Covalent Interfaces for Graphene/Elastomer Nanocomposites. Compos. Sci. Technol. 2016, 132, 68–75. https://doi.org/10.1016/j.compscitech.2016.06.015. Natarajan, B.; Li, Y.; Deng, H.; Brinson, L.; Schadler, L. S. Effect of Interfacial Energetics on Dispersion and Glass Transition Temperature in Polymer Nanocomposites. Macromolecules 2013. Rivlin, R. S.; Gee, G. A Uniqueness Theorem in the Theory of Highly-Elastic Materials. Math. Proc. Cambridge Philos. Soc. 1948, 44 (04), 595. https://doi.org/10.1017/S0305004100024610. Rivlin, R. Further Remarks on the Stress-Deformation Relations for Isotropic Materials. Indiana Univ. Math. J. 1955, 4 (5), 681–702. https://doi.org/10.1512/iumj.1955.4.54025. Sreeprasad, T. S.; Berry, V. How Do the Electrical Properties of Graphene Change with Its Functionalization? Small 2013, 9 (3), 341–350. https://doi.org/10.1002/smll.201202196. Kummali, M. M.; Miccio, L. A.; Schwartz, G. A.; Alegría, A.; Colmenero, J.; Otegui, J.; Petzold, A.; Westermann, S. Local Mechanical and Dielectric Behavior of the Interacting Polymer Layer in Silica Nano-Particles Filled SBR by Means of AFM-Based Methods. Polymer (Guildf). 2013, 54 (18), 4980–4986. https://doi.org/10.1016/j.polymer.2013.07.032. Xie, Z.-T.; Fu, X.; Wei, L.-Y.; Luo, M.-C.; Liu, Y.-H.; Ling, F.-W.; Huang, C.; Huang, G.; Wu, J. New Evidence Disclosed for the Engineered Strong Interfacial Interaction of Graphene/Rubber Nanocomposites. Polymer (Guildf). 2017, 118, 30–39. https://doi.org/10.1016/j.polymer.2017.04.056. Sun, H.; Zhang, H.; Liu, S.; Ning, N.; Zhang, L.; Tian, M.; Wang, Y. Interfacial Polarization and Dielectric Properties of Aligned Carbon Nanotubes/Polymer Composites: The Role of Molecular Polarity. Compos. Sci. Technol. 2018, 154, 145–153. https://doi.org/10.1016/j.compscitech.2017.11.008. 29 ACS Paragon Plus Environment


(52) (53)

(54)

(55)

(56) (57) (58) (59)

(60) (61) (62)

Jiang, M. J.; Dang, Z. M.; Bozlar, M.; Miomandre, F.; Bai, J. Broad-Frequency Dielectric Behaviors in Multiwalled Carbon Nanotube/Rubber Nanocomposites. J. Appl. Phys. 2009, 106 (8), 1–6. https://doi.org/10.1063/1.3238306. Dang, Z. M.; Yao, S. H.; Yuan, J. K.; Bai, J. Tailored Dielectric Properties Based on Microstructure Change in Batio 3-Carbon Nanotube/Polyvinylidene Fluoride Three-Phase Nanocomposites. J. Phys. Chem. C 2010, 114 (31), 13204–13209. https://doi.org/10.1021/jp103411c. Hirschorn, B.; Orazem, M. E.; Tribollet, B.; Vivier, V.; Frateur, I.; Musiani, M. Electrochimica Acta Determination of Effective Capacitance and Film Thickness from Constant-Phase-Element Parameters. Electrochim. Acta 2010, 55 (21), 6218–6227. https://doi.org/10.1016/j.electacta.2009.10.065. Yao, S.; Yuan, J.; Zhou, T.; Dang, Z.; Bai, J. Stretch-Modulated Carbon Nanotube Alignment in Ferroelectric Polymer Composites: Characterization of the Orientation State and Its Influence on the Dielectric Properties. J. Phys. Chem. C 2011, 115 (40), 20011– 20017. https://doi.org/10.1021/jp205444x. Dang, Z. M.; Yuan, J. K.; Zha, J. W.; Zhou, T.; Li, S. T.; Hu, G. H. Fundamentals, Processes and Applications of High-Permittivity Polymer-Matrix Composites. Prog. Mater. Sci. 2012, 57 (4), 660–723. https://doi.org/10.1016/j.pmatsci.2011.08.001. Wu, S.; Wang, J.; Shao, J.; Wei, L.; Ge, R.; Ren, H. An Approach to Developing Enhanced Dielectric Property Nanocomposites Based on Acrylate Elastomer. Mater. Des. 2018, 146, 208–218. https://doi.org/10.1016/j.matdes.2018.03.023. Lefèvre, V.; Lopez-Pamies, O. Nonlinear Electroelastic Deformations of Dielectric Elastomer Composites: I—Ideal Elastic Dielectrics. J. Mech. Phys. Solids 2017, 99, 409– 437. https://doi.org/10.1016/j.jmps.2016.07.004. Lu, T.; Huang, J.; Jordi, C.; Kovacs, G.; Huang, R.; Clarke, D. R.; Suo, Z. Dielectric Elastomer Actuators under Equal-Biaxial Forces, Uniaxial Forces, and Uniaxial Constraint of Stiff Fibers. Soft Matter 2012, 8 (22), 6167. https://doi.org/10.1039/c2sm25692d. Suo, Z. Theory of Dielectric Elastomers. Acta Mech. Solida Sin. 2010, 23 (6), 549–578. https://doi.org/10.1016/S0894-9166(11)60004-9. Gent, A. N. A New Constitutive Relation for Rubber. Rubber Chem. Technol. 1996, 69 (1), 59–61. https://doi.org/10.5254/1.3538357. Yang, D.; Huang, S.; Ruan, M.; Li, S.; Wu, Y.; Guo, W.; Zhang, L. Improved Electromechanical Properties of Silicone Dielectric Elastomer Composites by Tuning Molecular Flexibility. Compos. Sci. Technol. 2018, 155, 160–168. https://doi.org/doi.org/10.1016/j.compscitech.2017.12.010.

Graphical Abstract:


Page 30 of 32

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




Graphical Abstract 197x79mm (600 x 600 DPI)

ACS Paragon Plus Environment

Page 32 of 32

Tuning the surface chemistry of graphene oxide for enhanced

Recommend Documents