Article Actuation Behavior of Multilayer Nanosheets/ Composite Films

Chunmei Zhang 1 , Tianliang Zhai 2,* , Chao Zhan 2, Qiuping Fu 1 and Chao Ma 1

1 College of Chemistry and Materials Engineering, Guiyang University, Guiyang 550005, China; [email protected] (C.Z.); [email protected] (Q.F.); [email protected] (C.M.) 2 Guizhou Building Material Quality Supervision Testing Center, Guiyang 550000, China; [email protected] * Correspondence: [email protected]; Tel: +86-851-84619925

 Received: 28 October 2018; Accepted: 7 November 2018; Published: 9 November 2018 

Abstract: The graphene nanosheets (GNS)/polydimethylsiloxane (PDMS) composite films with out-of-plane dielectric actuation behavior were prepared through a layer-by-layer spin coating process. The GNS-PDMS/PDMS composite films with 1~3 layers of GNS-PDMS films were spin coated on top of the PDMS film. The dielectric, mechanical, and electromechanical actuation properties of the composite films were investigated. The dielectric constant of the GNS-PDMS3/PDMS composite film at 1 kHz is 5.52, which is 1.7 times that of the GNS-PDMS1/PDMS composite film. The actuated displacement of the GNS-PDMS/PDMS composite films is greatly enhanced by increasing the number of GNS-PDMS layers. This study provides a novel alternative approach for fabricating high-performance actuators with out-of-plane actuation behavior.

Keywords: graphene nanosheets; polydimethylsiloxane; dielectric constant; electromechanical actuation

1. Introduction Recently, much attention has been paid to dielectric elastomers because they are lightweight, have mechanical resiliency, low cost, facile processability, direct voltage control, noiseless performance, scalability and shock tolerance [1–3]. Dielectric elastomers can directly convert electrical energy into mechanical work or vice versa, which made them promising materials for advanced electromechanical applications in actuators, generators and sensors [4–6]. Generally, dielectric elastomer actuators consist of an elastomer film sandwiched between two thin and compliant electrodes, thereby forming a capacitor capable of energy transduction. When an external voltage is applied to the electrodes, electrostatic forces are created. Electrostatic pressure acting on the film squeezes the elastomer in thickness, and as the elastomer is incompressible it consequently expands in the in-plane direction. The driving force for the actuation process is the resulting reduction in charge density when the electrode area is enlarged. When the external voltage is switched off, the elastomer film returns to its original shape. For dielectric elastomer actuators, the elastomer matrix is very important because it governs dielectric constant, dielectric breakdown strength and obtainable strain. Silicone elastomers, including the polydimethylsiloxane (PDMS), are one of the most frequently used materials for dielectric elastomer actuators due to their good elasticity, reliability and fast response speed [7–11]. They produce repeatable, reproducible actuation upon activation, they show little tendency for Mullins (stress softening) and ageing effects and they have been shown to actuate (at low strains) to more than 4 million cycles, in some instances more than 400 million cycles, without failure [12,13]. They also possess significantly lower viscous losses than acrylics, indicating that they can be operated at higher

Polymers 2018, 10, 1243; doi:10.3390/polym10111243 www.mdpi.com/journal/polymers Polymers 2018, 10, 1243 2 of 10 frequencies with lower losses and less heat generation. Furthermore, silicone elastomers can be operated at a broader temperature range and possess inherent softness and stability. However, the silicone elastomers are non-polar, which demonstrate limited deformation under electrical field due to their inherent low dielectric constant [14]. This means that silicone elastomers, despite possessing superior mechanical properties compared to other dielectric elastomers, do not reach their full potential when used as actuators. To enhance the actuation efficiency of the silicone elastomer actuators, the dielectric constant of the system needs to be increased. The addition of a filler with high constant to the elastomer system is one of the most investigated solutions to this challenge. Inorganic particles (TiO2 and titanates) [7,15], metals (iron, cobalt and titanium complexes) [16], organo-clays (montmorillonite) [17], as well as conductive fillers (carbon black, carbon nanotubes, and exfoliated graphite) [18,19] have been added in silicone matrix to investigate the actuator application. For the optimal system, a balance will have to be found between reinforcement and constant increase while not compromising the elastic modulus. Compared with the traditional method of adding a mass of high-dielectric-constant ceramics, the way of loading a much reduced quantity of conductive fillers can effectively maintain the mechanical flexibility of silicone elastomer matrices [19]. Meanwhile, this way is beneficial for improving the systematic dielectric constant of composites according to the electrical percolation and microcapacitor theory [20,21]. Since the surprising milestone of the 2010 Nobel Physics Prize awarded for the preparation of graphene, the two-dimensional material has been widely used as a functional filler in nanocomposites for its extraordinary mechanical, thermal and electrical properties [22]. Graphene can cause a crucial improvement in the electrical, mechanical, and thermal properties of the resulting graphene/polymer composites at very low loadings. Graphene-based are being developed for promising applications in electromechanical systems. Kim et al. [23] proposed an actuator consisting of a dielectric elastomer substrate and multiple stacked graphene electrodes. The actuator had a stretch ability of 25% thanks to the mechanical properties of the graphene electrode. Tungkavet et al. [24] studied the electromechanical properties of graphene/gelatin hydrogel composites for use in actuator applications. They found that the graphene/gelatin composite with only 0.1 vol % filler loading delivered the greatest deflection distance and dielectrophoresis force. Chen et al. [25] demonstrated that polyurethane dielectric elastomer filled with titanium dioxide functionalized graphene displayed evident electric stimulus response and electric field induced strain. In this paper, we developed graphene nanosheets (GNS)/polydimethylsiloxane (PDMS) composite films through a layer-by-layer spin coating process for electroactive actuators. The effect of the number of GNS-PDMS layers on the mechanical, dielectric, and electromechanical actuation performance of the resulting multilayer GNS-PDMS/PDMS composite films was investigated. The addition of GNS to PDMS effectively improves the dielectric constant of the composite film, and the actuated displacement of the composite films is greatly enhanced by increasing the number of GNS-PDMS layers.

2. Materials and Methods

2.1. Preparation of Reduced Graphene Oxide/DMF Solution The preparation of graphene oxide (GO) was described in our previous work [26]. The obtained GO (30 mg) was dispersed into dimethylformamide (DMF) (100 mL) by a probe ultrasonicator for 1 h in an ice-water bath to get a uniform GO suspension (3 mg/mL). Then, hydrazine hydrate was added (weight ratio hydrazine hydrate/GO = 7/10). The reduction reaction was conducted at 90 ◦C for 24 h under magnetic stirring. After cooling to room temperature, the graphene nanosheets (GNS)/DMF solution was obtained. Polymers 2018, 10, 1243 3 of 10

2.2. Preparation of GNS/THF Solution The GNS/DMF suspension was vacuum filtrated through a funnel. The filtered GNS was washed with tetrahydrofuran (THF) five times to remove any residual DMF. Then the wet GNS was transferred into THF to get GNS/THF suspension with a concentration of 5 mg/mL.

2.3. Preparation of GNS/PDMS/THF Solution The polydimethylsiloxane (PDMS) prepolymer (10 g) (184 Silicone Elastomer, Dow Corning, Midland, MI, USA) was mixed with THF (5 mL) to obtain a mixture of PDMS/THF. The resulting PDMS/THF mixture was mixed with the GNS/THF solution (6 mL) using a vortex mixer for 3 min and followed by ultrasonication for 1 h. The curing agent (1 g) (Dow Corning, Midland, MI, USA) for PDMS was then added into the mixture. After 1 min of mechanical stirring with vortex mixer, and 10 min bath sonication treatment, the GNS/PDMS/THF solution was obtained.

2.4. Preparation of Multilayer GNS-PDMS Composite Films The PDMS prepolymer and curing agent (weight ratio 10:1) were mixed in a 50 mL centrifuge tube according to the manufacturer’s instructions. The centrifuge tube was placed in a vacuum oven and degassed for 5~10 min at room temperature. Then, the PDMS was spin coated on a silicon wafer at 3000 rpm for 30 s, and the silicon wafer was transferred to a hot oven immediately at 150 ◦C for 30 min to fully cure the PDMS. After the silicon wafer cooled down to room temperature, the GNS/PDMS/THF solution was spin coated on top of the PDMS layer with the same procedure described above and cured to get the double-layer GNS-PDMS/PDMS composite film, which is denoted as GNS-PDMS1/PDMS. The sample with two layers of GNS-PDMS films coated on top of the PDMS layer is denoted as GNS-PDMS2/PDMS. The sample with three layers of GNS-PDMS films coated on top of PDMS layer is denoted as GNS-PDMS3/PDMS. For comparison, two layers of PDMS film were also prepared by the same procedure and is denoted as double-layer PDMS film.

2.5. Characterization For all the tests described below, at least three replicates were measured for each sample, and the average results were reported [27]. The dielectric constant and dielectric loss (tan δ) of the films were measured using an E4980A LCR meter (Agilent Technologies, Inc., Santa Clara, CA, USA) at room temperature with a 16451B dielectric test fixture (Agilent Technologies, Inc., Santa Clara, CA, USA). Mechanical properties were characterized by an electronic tensile machine (JPL-2500, Jiangdu Jingcheng Testing Instrument, Yangzhou, China) at room temperature according to the ASTM D882 standard for tensile testing of thin plastic films. The breakdown field strength of the films was tested by an AMS-10B2 high voltage amplifier (Matsusada Precision Inc., Shiga, Japan). The out-of-plane actuation properties of the films were characterized under applied electric field. The testing set-up was schematically illustrated in Figure1. The electric field was applied by the high voltage amplifier. The voltage was turned on for 30 s and off for 30 s and repeated many times with gradual increase of the voltage until the film was broken or the displacement reached the extreme of the laser triangulation sensor. Both sides of the composite films were pasted with carbon paste electrodes (CPEs, EXLUB G310, Dongguan Excellent Lubrication Technology, Dongguan, China) in a circular shape to generate a uniform electric field throughout the active area. The displacement of the composite films was measured using a high-speed laser triangulation sensor (MICROTRACK LTC-025-02, MTI Instruments, Albany, NY, USA). The diameter of the circular active area was 16 mm. The films were biaxially stretched with 5% strain prior to electromechanical characterization to get a stable electromechanical response [28,29]. Polymers 2018, 10, x FOR PEER REVIEW 4 of 9

Figure 1. Schematic of the electromechanical response testing set-up for the composite films.

Polymers 2018, 10, 1243 4 of 10 3.Polymers Results 2018 and, 10, xDiscussion FOR PEER REVIEW 4 of 9 The in-plane actuation is the most typical electromechanical response for dielectric elastomers. Generally, when an electric field is applied, the dielectric elastomer film contracts along the thickness direction due to the electrostatic charges, and extends along the transverse direction, as shown in

Figure 2a. The compression force denoted as 휎푧 is the Maxwell stress [30], which is calculated by Equation (1),

푉 2 휎 = 휀 휀 ( ) , (1) 푧 0 푟 푡 where ε0ε0 is the dielectric constant of the air and ε푟ε푟 is the relative dielectric constant of the elastomer. V represents the applied voltage and t denotes the thickness of the elastomer. The field-induced strain S in the transverse direction is calculated by Equation (2), Figure 1. Schematic of the electromechanical response testing set-up for the composite films. 휎 Figure 1. Schematic of the electromechanical푆 = response푧 × 100% testing, set-up for the composite films. (2) 3. Results and Discussion 푌 where3. Results Y represents and Discussion the Young’s modulus of the elastomer. The in-plane actuation is the most typical electromechanical response for dielectric elastomers. From Equations (1) and (2) we can find that the elastomer with higher ε푟ε푟 and lower Y can Generally,The in when-plane an actuation electric field is the is applied,most typical the dielectric electromechanical elastomer response film contracts for dielectric along the elastomers. thickness generate larger strain with better actuation ability. Usually, a large pre-strain is needed to obtain the directionGenerally, due when to the an electric electrostatic field is charges, applied, and the extendsdielectric along elastomer the transverse film contracts direction, along asthe shown thickness in in-plane actuation [8]. The out-of-plane actuation is the electromechanical response of the elastomer Figuredirection2a. Thedue compressionto the electrostatic force denotedcharges, asandσ extendsis the Maxwellalong the stress transverse [30], which direction, is calculated as shown by in that deforms along the thickness direction, as shownz in Figure 2b. When the dielectric elastomer is EquationFigure 2a. (1), The compression force denoted as 휎 is the Maxwell stress [30], which is calculated by fixed by a frame with low or without pre-strain,푧 the out-of-plane actuation could be observed [23]. Equation (1),  V 2 When an elastomer film was prepared withσz = differentε0εr actuation, abilities on its two sides, the film (1) t 푉 2 would upheave to the side with greater actuation휎 = 휀 휀ability.( ) , The out-of-plane actuated displacement(1) 푧 0 푟 푡 wherecould εbe0 isimproved the dielectric by enhancing constant ofthe the dielectric air and constantεr the relative and reducing dielectric the constant Young’s of modulus the elastomer. of the Vfilm.whererepresents ε0ε0 theis the applied dielectric voltage constant and t denotes of the airthe andthickness ε푟ε푟 ofis the elastomer. relative dielectric constant of the elastomer. V represents the applied voltage and t denotes the thickness of the elastomer. The field-induced strain S in the transverse direction is calculated by Equation (2), 휎 푆 = 푧 × 100%, 푌 (2) where Y represents the Young’s modulus of the elastomer.

From Equations (1) and (2) we can find that the elastomer with higher ε푟ε푟 and lower Y can generate larger strain with better actuation ability. Usually, a large pre-strain is needed to obtain the in-plane actuation [8]. The out-of-plane actuation is the electromechanical response of the elastomer that deforms along the thickness direction, as shown in Figure 2b. When the dielectric elastomer is fixed by a frame with low or without pre-strain, the out-of-plane actuation could be observed [23]. When an elastomer film was prepared with different actuation abilities on its two sides, the film would upheave to the side with greater actuation ability. The out-of-plane actuated displacement could be improvedFigure 2.bySchematic enhancing of thethe (adielectric) in-plane constant actuation andand (breducing) out-of-plane the Young’s actuation. modulus of the Figure 2. Schematic of the (a) in-plane actuation and (b) out-of-plane actuation. film. The field-induced strain S in the transverse direction is calculated by Equation (2), The thickness, dielectric constant, dielectric loss (tan δ), and breakdown field strength of the σ GNS-PDMS/PDMS composite films are givenS = inz × Table100%, 1. The thickness of the double-layer PDMS (2) Y where Y represents the Young’s modulus of the elastomer. From Equations (1) and (2) we can find that the elastomer with higher εr and lower Y can generate larger strain with better actuation ability. Usually, a large pre-strain is needed to obtain the in-plane actuation [8]. The out-of-plane actuation is the electromechanical response of the elastomer that deforms along the thickness direction, as shown in Figure2b. When the dielectric elastomer is fixed by a frame with low or without pre-strain, the out-of-plane actuation could be observed [23]. When an elastomer film was prepared with different actuation abilities on its two sides, the film would upheave to the side with greater actuation ability. The out-of-plane actuated displacement could be improved by enhancing the dielectric constant and reducing the Young’s modulus of the film. The thickness,Figure dielectric 2. Schematic constant, of the (a dielectric) in-plane actuation loss (tan andδ), and(b) out breakdown-of-plane actuation. field strength of the GNS-PDMS/PDMS composite films are given in Table1. The thickness of the double-layer PDMS The thickness, dielectric constant, dielectric loss (tan δ), and breakdown field strength of the GNS-PDMS/PDMS composite films are given in Table 1. The thickness of the double-layer PDMS

PolymersPolymers2018 2018, 10, ,10 1243, x FOR PEER REVIEW 5 of5 10of 9

film is 78.2 μm, which is thicker than that of the GNS-PDMS1/PDMS composite film with a thickness film is 78.2 µm, which is thicker than that of the GNS-PDMS1/PDMS composite film with a thickness of 71.4 μm. This is because the viscosity of the PDMS prepolymer is higher than that of the of 71.4 µm. This is because the viscosity of the PDMS prepolymer is higher than that of the GNS/PDMS/THF solution. During the spin coating procedure, higher viscosity leads to a thicker film. GNS/PDMS/THF solution. During the spin coating procedure, higher viscosity leads to a thicker The dielectric constant of GNS-PDMS1/PDMS composite film at 1 kHz is 3.34, which is much higher film. The dielectric constant of GNS-PDMS1/PDMS composite film at 1 kHz is 3.34, which is much than that of the double-layer PDMS film with only 2.22, indicating the addition of GNS greatly higher than that of the double-layer PDMS film with only 2.22, indicating the addition of GNS greatly improves the dielectric constant of the composite film. The thickness and dielectric constant of the improves the dielectric constant of the composite film. The thickness and dielectric constant of the composite films increase with the number of layers. The dielectric constant of the GNS-PDMS3/PDMS composite films increase with the number of layers. The dielectric constant of the GNS-PDMS3/PDMS composite film at 1 kHz is 5.52, which is 1.7 times that of the GNS-PDMS1/PDMS composite film. This composite film at 1 kHz is 5.52, which is 1.7 times that of the GNS-PDMS1/PDMS composite film. indicates the dielectric constant of the composite films is enhanced by the GNS and strongly This indicates the dielectric constant of the composite films is enhanced by the GNS and strongly dependent on the number of layers. Besides, the dielectric loss of the GNS-PDMS/PDMS films at 1 dependent on the number of layers. Besides, the dielectric loss of the GNS-PDMS/PDMS films kHz decreases with the number of GNS-PDMS layers. The breakdown field strength of the double- at 1 kHz decreases with the number of GNS-PDMS layers. The breakdown field strength of the layer PDMS film is higher than 164 MV/m, because the real value is beyond the measuring range. The double-layer PDMS film is higher than 164 MV/m, because the real value is beyond the measuring breakdown field strength of the GNS-PDMS/PDMS composite films significantly decreases with an range. The breakdown field strength of the GNS-PDMS/PDMS composite films significantly decreases increase in the number of GNS-PDMS layers, which decreases from 109 to 64 MV/m when the number with an increase in the number of GNS-PDMS layers, which decreases from 109 to 64 MV/m when the of GNS-PDMS layers increases from one to three. number of GNS-PDMS layers increases from one to three. Table 1. Thickness, dielectric constant, dielectric loss, and breakdown field strength of the films. Table 1. Thickness, dielectric constant, dielectric loss, and breakdown field strength of the films. Thickness Dielectric Constant Dielectric Loss Breakdown Field Sample Dielectric Constant Dielectric Loss Breakdown Field Sample Thickness (µm) (μm) (at(at 1 1 kHz) kHz) (at(at 11 kHz) kHz) StrengthStrength (MV/m) (MV/m) Double-layer PDMS 78.2 ± 1.5 2.22 0.0064 >164 Double-layer PDMS 78.2 ± 1.5 2.22 0.0064 >164 1 GNS-PDMSGNS-PDMS1/PDMS/PDMS 71.471.4± ±1.2 1.2 3.34 3.34 0.00450.0045 118118± ±2 2 GNS-PDMSGNS-PDMS2/PDMS2/PDMS 96.496.4± ±1.3 1.3 4.32 4.32 0.00340.0034 8787± ±1 1 GNS-PDMSGNS-PDMS3/PDMS3/PDMS 111.6111.6± ±1.0 1.0 5.52 5.52 0.00210.0021 6464± ±1 1

TheThe dielectric dielectric constant constant and and dielectric dielectric loss loss of of the the films films as as a functiona function of of frequency frequency are are shown shown in in FigureFigure3. It3. can It becan found be found that the that dielectric the dielectric constant constant of each sampleof each slightly sample decreases slightly decreaseswith the increase with the 3 ofincrease frequency. of frequency. For instance, For the instance, dielectric the constant dielectric of theconstant GNS-PDMS of the GNS3/PDMS-PDMS composite/PDMS filmcomposite is 5.52 film at 1 kHz,is 5.52 and at it 1 slightly kHz, and decreases it slightly to5.44 decreases at 1000 to kHz. 5.44 The at 1000dielectric kHz. loss The of dielectric each sample loss increases of each sample with theincreases increasing with frequency. the increasing frequency.

Figure 3. Dielectric constant (a) and dielectric loss (b) of the films as a function of frequency. Figure 3. Dielectric constant (a) and dielectric loss (b) of the films as a function of frequency. The tensile stress-strain curves of the films are presented in Figure4, and the mechanical properties are givenThe in tensile Table 2 stress. It is-strain obvious curves that of the the addition films are of GNS presented to PDMS in Figure leads to4, a and softening the mechanical effect. Theproperties GNS-PDMS are 1given/PDMS in Table composite 2. It is film obvious showed that lower the addition Young’s of modulus GNS to andPDMS higher leads elongation to a softening at breakeffect. than The that GNS of the-PDMS double-layer1/PDMS PDMS composite film. filmThe softening showed effect lower of Young’s the GNS modulus to the PDMS and matrix higher canelongation be attributed at break to the than residual that of hydroxyl the double groups-layer on PDMS GNS film. that decreaseThe softening the crosslinking effect of the density GNS to of the thePDMS PDMS matrix matrix. can The be attributed crosslinking to reactionthe residual of the hydroxyl PDMS isgroups based on on GNS the hydrosilation,that decrease the which crosslinking occurs betweendensity the of the active PDMS hydrogens matrix. onThe the crosslinking crosslinking reaction agent andof the the PDMS vinyl is groups based on on the the linear hydrosilation, PDMS prepolymerwhich occurs in the between presence the of active a catalyst hydrogens [31]. The on GNS the were crosslinking obtained agent by the and chemical the vinyl reduction, groups which on the resultedlinear PDMS with a certainprepolymer amount in the of residualpresence hydroxyl of a catalyst groups [31] on. The the GNS surface were [32 obtained]. The hydroxyl by the groupschemical reduction, which resulted with a certain amount of residual hydroxyl groups on the surface [32]. The

Polymers 2018, 10, 1243 6 of 10 Polymers 2018, 10, x FOR PEER REVIEW 6 of 9 reactedhydroxyl with groups the vinyl reacted groups with on the the vinyllinear groups PDMS prepolymer. on the linear As PDMS a result, prepolymer the crosslinking. As a densityresult, the of thecrosslinking PDMS matrix density was of reduced. the PDMS Therefore, matrix the was more reduced. GNS-PDMS Therefore, layers the are more coated GNS on- topPDMS of thelayers PDMS are layer,coated the on greater top of the softening PDMS effectlayer, is the observed. greater softening The composite effect filmsis observed. exhibit The reduced composite Young’s films modulus exhibit andreduced enhanced Young’s elongation modulus at breakand enhanced with an increasing elongation number at break of GNS-PDMSwith an increasing layers. For number the composite of GNS- 3 filmPDMS with layer threes. For layers the ofcomposite GNS-PDMS, film i.e.,with GNS-PDMS three layers3/PDMS, of GNS-PDMS, the Young’s i.e., GNS modulus-PDMS decreases/PDMS, bythe 23.2%Young’s and modulus the elongation decreases at break by 23.2% increases and bythe 25.2% elongation in comparison at break toincreases that of theby GNS-PDMS25.2% in comparison1/PDMS 1 film.to that This of softening the GNS effect-PDMS is beneficial/PDMS film. for improvingThis softening the out-of-plane effect is benefi actuationcial for property improving because the out larger-of- strainplane can actuation be generated property for a because film with larger lower strain Young’s can modulus be generated according for ato film Equation with (2). lower In addition, Young’s themodulus GNS also according shows a to reinforcing Equation (2). effect In on addition, the composite the GNS film. also The shows tensile a reinforcing strength at effect break onof the the composite film. The tensile strength at break of the GNS-PDMS1/PDMS composite film is 2.82 MPa, GNS-PDMS1/PDMS composite film is 2.82 MPa, it increases to 3.71 MPa for the GNS-PDMS2/PDMS it increases to 3.71 MPa for the GNS-PDMS2/PDMS composite film, and further to 4.30 MPa for the composite film, and further to 4.30 MPa for the GNS-PDMS3/PDMS1 composite film. GNS-PDMS3/PDMS1 composite film.

FigureFigure 4.4. TensileTensile stress-strainstress-strain curves curves of of the the films. films. Table 2. Mechanical properties of the films Table 2. Mechanical properties of the films Tensile Strength at Tensile Strength Young’s Modulus Elongation at Sample Tensile Strength 100% Strain (MPa) at BreakTensile (MPa) Strength (kPa)Young’s ElongationBreak (%) Sample at 100% Strain Double-layer PDMS 2.07at 2.72Break (MPa) Modulus 14.6 (kPa) at B 111.3reak (%) GNS-PDMS1/PDMS 1.07(MPa) 2.82 11.2 156.8 GNS-PDMSDouble-2layer/PDMS PDMS 0.972.07 3.712.72 8.914.6 181.7111.3 GNS-PDMSGNS-PDMS3/PDMS1/PDMS 0.911.07 4.302.82 8.611.2 196.3156.8 GNS-PDMS2/PDMS 0.97 3.71 8.9 181.7 GNS-PDMS3/PDMS 0.91 4.30 8.6 196.3 Figure5 shows the out-of-plane actuation displacement of the samples as a function of time and electric field intensity. The displacement was obtained by measuring the distance that the center Figure 5 shows the out-of-plane actuation displacement of the samples as a function of time and point of the active area deviated from its original spot, as shown in Figure2b. The electric field electric field intensity. The displacement was obtained by measuring the distance that the center point intensity was calculated by dividing voltage by film thickness. The displacement of the films increases of the active area deviated from its original spot, as shown in Figure 2b. The electric field intensity withwas calculated an increase by in dividing electric fieldvoltage intensity, by film asthickness. shown inThe Figure displacement5. The actuated of the films displacement increases with of the an 1 GNS-PDMSincrease in electric/PDMS field composite intensity, film as is shown 0.16 mm in atFigure an electric 5. The field actuated of 39.4 displacement MV/m, and of it increases the GNS- to 0.39 mm at 69.1 MV/m, which is higher than that of the double-layer PDMS film exhibiting a PDMS1/PDMS composite film is 0.16 mm at an electric field of 39.4 MV/m, and it increases to 0.39 mm 0.31at 69.1 mm MV/m, displacement which at is 65.9 higher MV/m. than The that displacement of the double of the-layer composite PDMS films film is exhibiting greatly improved a 0.31 mm by 3 increasingdisplacement the number at 65.9 of MV/m. GNS-PDMS The displacement layers. For example, of the composite the displacement films is of greatly GNS-PDMS improved/PDMS by composite film at 63.4 MV/m is 1.12 mm, which is 2.9 times that of the GNS-PDMS1/PDMS composite increasing the number of GNS-PDMS layers. For example, the displacement of GNS-PDMS3/PDMS film at an even higher electric field of 69.1 MV/m. composite film at 63.4 MV/m is 1.12 mm, which is 2.9 times that of the GNS-PDMS1/PDMS composite film at an even higher electric field of 69.1 MV/m.

Polymers 2018, 10, 1243 7 of 10 Polymers 2018, 10, x FOR PEER REVIEW 7 of 9

Polymers 2018, 10, x FOR PEER REVIEW 7 of 9

FigureFigure 5. ActuatedActuated displacement displacement as as a function a function of oftime time and and electric electric field field intensity: intensity: (a) D (oublea) Double--layer 1 2 layerpolydimethylsiloxaneFigure polydimethylsiloxane 5. Actuated displacement (PDMS (PDMS);); (b as) GNSa (functionb)- GNS-PDMSPDMS of1 /PDMStime and/PDMS;; ( celectric) GNS (c )field-PDMS GNS-PDMS intensity:2/ PDMS (/a); PDMS;Dandouble (d- andlayer) GNS (d)- 3 1 2 GNS-PDMSPDMSpolydimethylsiloxane3/PDMS./PDMS. (PDMS); (b) GNS-PDMS /PDMS; (c) GNS-PDMS / PDMS; and (d) GNS- PDMS3/PDMS. The displacement of the films as a function of electric field intensity is also shown in Figure6 The displacement of the films as a function of electric field intensity is also shown in Figure 6 for for convenientThe displacement observation of the and films comparison. as a function The of displacement electric field intensity increases is also with show the increasingn in Figure electric6 for convenient observation and comparison. The displacement increases with the increasing electric field fieldconvenient intensity, observation as well as theand numbercomparison. of GNS-PDMS The displacement layers. increases The GNS-PDMS with the increasing3/PDMS composite electric field film intensity, as well as the number of GNS-PDMS layers. The GNS-PDMS3/PDMS composite film with withintensity, three layers as well of as GNS-PDMS the numberfilms of GNS exhibits-PDMS the layers. highest The displacement.GNS-PDMS3/PDMS According composite to the film dielectric with three layers of GNS-PDMS films exhibits the highest displacement. According to the dielectric constantthree layers and the of Young’sGNS-PDMS modulus films exhibits results, the GNS-PDMShighest displacement.3/PDMS composite According film to the possesses dielectric the constant and the Young’s modulus results, the GNS-PDMS3/PDMS composite film possesses the highestconstant dielectric and the constant Young’s and modulus lowest results, Young’s the modulus, GNS-PDMS which3/PDMS results composite in the highest film possesses displacement the highest dielectric constant and lowest Young’s modulus, which results in the highest displacement withhighest the best dielectric actuation constant ability. and The lowest results Young’s indicate modulus, that the which addition results of in GNS the improveshighest displacement the actuation with the best actuation ability. The results indicate that the addition of GNS improves the actuation propertywith the of best the actuation composite abilit film,y. andThe results the out-of-plane indicate that actuation the addition displacement of GNS improves is greatly the enhanced actuation by property of the composite film, and the out-of-plane actuation displacement is greatly enhanced by increasingproperty theof the number composite of GNS-PDMS film, and the layers. out-of-plane actuation displacement is greatly enhanced by increasingincreasing the the number number of of GNS GNS--PDMSPDMS layers.layers.

Figure 6. Displacement of the films as a function of electric field intensity Figure 6. Displacement of the films as a function of electric field intensity Figure 6. Displacement of the films as a function of electric field intensity

Polymers 2018, 10, 1243 8 of 10

4. Conclusions The GNS/PDMS composite films with out-of-plane actuation behavior were prepared by a layer-by-layer spin coating process. The GNS-PDMS/PDMS composite film shows higher dielectric constant than that of the double-layer PDMS film, indicating the addition of GNS greatly improves the dielectric constant of the composite film, which is beneficial for improving the actuation property. The dielectric constant of the GNS-PDMS/PDMS composite films increases dramatically with the number of GNS-PDMS layers. The composite films show reduced Young’s modulus and increased elongation at break with an increased number of GNS-PDMS layers. The increased dielectric constant and reduced Young’s modulus are beneficial to improve the actuation ability. Thus, the GNS-PDMS3/PDMS composite film with the highest dielectric constant and lowest Young’s modulus exhibits the highest displacement. This study offers a novel method to fabricate actuators with out-of-plane actuation behavior.

Author Contributions: conceptualization, C.Z. (Chunmei Zhang) and T.Z.; methodology, T.Z.; software, C.M.; formal analysis, C.Z. (Chao Zhan) and T.Z.; investigation, Q.F.; data curation, C.Z. (Chunmei Zhang); writing—original draft preparation, C.Z. (Chunmei Zhang); writing—review and editing, T.Z. and C.Z. (Chao Zhan); funding acquisition, C.Z (Chunmei Zhang) and T.Z. Funding: This research was funded by the Special Funding of Guiyang Science and Technology Bureau and Guiyang University (GYU-KYZ[2018]03-3); the Major Construction Project of First-rate University in Guizhou Province (No. 2017158134); the Natural Science Research Project of Guizhou Provincial Department of Education (No. [2016]089); Science and Technology Planning Project of General Administration of Quality Supervision, Inspection and Quarantine of China (2016QK041); and the National Natural Science Foundation of China (No. 51703038). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in: The design of the study; the collection, analyses, or interpretation of the data; the writing of the manuscript; or the decision to publish the results.

References

1. Yadav, S.K.; Kim, I.J.; Kim, H.J.; Kim, J.; Hong, S.M.; Koo, C.M. PDMS/MWCNT nanocomposite actuators using silicone functionalized multiwalled carbon nanotubes via nitrene chemistry. J. Mater. Chem. C 2013, 1, 5463–5470. [CrossRef] 2. Xia, N.; Zhao, P.; Xie, J.; Zhang, C.Q.; Fu, J.Z.; Turng, L.S. Defect diagnosis for polymeric samples via magnetic levitation. NDT&E Int. 2018, 100, 175–182. 3. Nie, J.; Gao, Q.; Qiu, J.J.; Sun, M.; Liu, A.; Shao, L.; Fu, J.Z.; Zhao, P.; He, Y. 3D printed Lego (R)-like modular microfluidic devices based on capillary driving. Biofabrication 2018, 10.[CrossRef][PubMed] 4. Terasawa, N.; Takeuchi, I. A mesoporous carbon polymer actuator with superior performance to that of single-walled carbon nanotube polymer actuators. J. Mater. Chem. C 2013, 1, 5272–5276. [CrossRef] 5. Madsen, F.B.; Daugaard, A.E.; Hvilsted, S.; Skov, A.L. The Current State of Silicone-Based Dielectric Elastomer Transducers. Macromol. Rapid Commun. 2016, 37, 378–413. [CrossRef][PubMed] 6. He, D.; Xie, Y.C.; Wang, X.; Zhang, Z.C. Significantly Enhanced Electromechanical Performance of PDMS Crosslinked PVDF Hybrids. Polymers 2018, 10, 714. [CrossRef] 7. Ruan, M.N.; Yang, D.; Guo, W.L.; Huang, S.; Wu, Y.B.; Wang, H.; Wang, H.M.; Zhang, L.Q. Improved electromechanical properties of brominated butyl rubber filled with modified barium titanate. RSC Adv. 2017, 7, 37148–37157. [CrossRef] 8. Pelrine, R.; Kornbluh, R.; Pei, Q.B.; Joseph, J. High-speed electrically actuated elastomers with strain greater than 100%. Science 2000, 287, 836–839. [CrossRef][PubMed] 9. Acquarelli, C.; Paliotta, L.; Proietti, A.; Tamburrano, A.; De Bellis, G.; Sarto, M.S. Electrical and Electromechanical Properties of Stretchable Multilayer-Graphene/PDMS Composite Foils. IEEE Trans. Nanotechnol. 2016, 15, 687–695. [CrossRef] 10. Xia, N.; Zhao, P.; Zhao, Y.; Zhang, J.F.; Fu, J.Z. Integrated measurement of ultrasonic parameters for polymeric materials via full spectrum analysis. Polym. Test. 2018, 70, 426–433. [CrossRef] Polymers 2018, 10, 1243 9 of 10

11. Zhao, H.M.; Chen, Y.S.; Shao, L.; Xie, M.J.; Nie, J.; Qiu, J.J.; Zhao, P.; Ramezani, H.; Fu, J.Z.; Ouyang, H.W.; et al. Airflow-Assisted 3D Bioprinting of Human Heterogeneous Microspheroidal Organoids with Microfluidic Nozzle. Small 2018, 14, e1802630. [CrossRef][PubMed] 12. Maffli, L.; Rosset, S.; Ghilardi, M.; Carpi, F.; Shea, H. Ultrafast All-Polymer Electrically Tunable Silicone Lenses. Adv. Funct. Mater. 2015, 25, 1656–1665. [CrossRef] 13. Rosset, S.; Niklaus, M.; Dubois, P.; Shea, H.R. Large-Stroke Dielectric Elastomer Actuators With Ion-Implanted Electrodes. J. Microelectromech. Syst. 2009, 18, 1300–1308. [CrossRef] 14. Racles, C.; Bele, A.; Dascalu, M.; Musteata, V.E.; Varganici, C.D.; Ionita, D.; Vlad, S.; Cazacu, M.; Dunki, S.J.; Opris, D.M. Polar-nonpolar interconnected elastic networks with increased permittivity and high breakdown fields for dielectric elastomer transducers. RSC Adv. 2015, 5, 58428–58438. [CrossRef] 15. Yu, L.Y.; Vudayagiri, S.; Zakaria, S.; Benslimane, M.Y.; Skov, A.L. Filled liquid silicone rubbers: Possibilities and challenges. In Electroactive Polymer Actuators and Devices (EAPAD) 2014; SPIE: Bellingham, WA, USA, 2014; Volume 9056. 16. Cazacu, M.; Racles, C.; Zaltariov, M.F.; Dumitriu, A.M.C.; Ignat, M.; Ovezea, D.; Stiubianu, G. Electroactive composites based on polydimethylsiloxane and some new metal complexes. Smart Mater. Struct. 2013, 22. [CrossRef] 17. Gharavi, N.; Razzaghi-Kashani, M.; Golshan-Ebrahimi, N. Effect of organo-clay on the dielectric relaxation response of silicone rubber. Smart Mater. Struct. 2010, 19.[CrossRef] 18. Zhao, H.; Yang, M.H.; He, D.L.; Liu, Y.; Bai, J.T.; Wang, H.; Chen, H.W.; Bai, J.B. Typical synergistic effect of graphite nanoplatelets and carbon nanotubes and its influence on polymer-based dielectric composites. High Volt. 2016, 1, 140–145. [CrossRef] 19. Goswami, K.; Daugaard, A.E.; Skov, L. Dielectric properties of ultraviolet cured poly(dimethyl siloxane) sub-percolative composites containing percolative amounts of multi-walled carbon nanotubes. RSC Adv. 2015, 5, 12792–12799. [CrossRef] 20. Yuan, J.K.; Yao, S.H.; Li, W.L.; Sylvestre, A.; Bai, J.B. Vertically Aligned Carbon Nanotube Arrays on SiC Microplatelets: A High Figure-of-Merit Strategy for Achieving Large Dielectric Constant and Low Loss in Polymer Composites. J. Phys. Chem. C 2014, 118, 22975–22983. [CrossRef]

21. Feng, Y.; Li, W.L.; Wang, J.P.; Yin, J.H.; Fei, W.D. Core-shell structured BaTiO3@carbon hybrid particles for polymer composites with enhanced dielectric performance. J. Mater. Chem. A 2015, 3, 20313–20321. [CrossRef] 22. Ahmadi-Moghadam, B.; Sharafimasooleh, M.; Shadlou, S.; Taheri, F. Effect of functionalization of graphene nanoplatelets on the mechanical response of graphene/epoxy composites. Mater. Des. 2015, 66, 142–149. [CrossRef] 23. Kim, U.; Kang, J.; Lee, C.; Kwon, H.Y.; Hwang, S.; Moon, H.; Koo, J.C.; Nam, J.D.; Hong, B.H.; Choi, J.B.; et al. A transparent and stretchable graphene-based actuator for tactile display. Nanotechnology 2013, 24. [CrossRef][PubMed] 24. Tungkavet, T.; Seetapan, N.; Pattavarakorn, D.; Sirivat, A. Graphene/gelatin hydrogel composites with high storage modulus sensitivity for using as electroactive actuator: Effects of surface area and electric field strength. Polymer 2015, 70, 242–251. [CrossRef] 25. Chen, T.; Qiu, J.H.; Zhu, K.J.; Li, J.H. Electro-mechanical performance of polyurethane dielectric elastomer flexible micro-actuator composite modified with titanium dioxide-graphene hybrid fillers. Mater. Des. 2016, 90, 1069–1076. [CrossRef] 26. Zhang, C.M.; Wang, L.W.; Zhai, T.L.; Wang, X.C.; Dan, Y.; Turng, L.S. The surface grafting of graphene oxide with poly (ethylene glycol) as a reinforcement for poly(lactic acid) nanocomposite scaffolds for potential tissue engineering applications. J. Mech. Behav. Biomed. 2016, 53, 403–413. [CrossRef][PubMed] 27. Zhao, P.; Peng, Y.Y.; Yang, W.M.; Fu, J.Z.; Turng, L.S. Crystallization Measurements via Ultrasonic Velocity: Study of Poly(lactic acid) Parts. J. Polym. Sci. Pol. Phys. 2015, 53, 700–708. [CrossRef] 28. Javadi, A.; Xiao, Y.L.; Xu, W.J.; Gong, S.Q. Chemically modified graphene/P(VDF-TrFE-CFE) electroactive polymer nanocomposites with superior electromechanical performance. J. Mater. Chem. 2012, 22, 830–834. [CrossRef] 29. Ren, K.L.; Liu, S.; Lin, M.R.; Wang, Y.; Zhang, Q.M. A compact electroactive polymer actuator suitable for refreshable Braille display. Sens. Actuators A Phys. 2008, 143, 335–342. [CrossRef] Polymers 2018, 10, 1243 10 of 10

30. Chang, M.Z.; Wang, Z.Q. A Numerical Method Charactering the Electromechanical Properties of Particle Reinforced Composite Based on Statistics. Polymers 2018, 10, 426. [CrossRef] 31. Ma, B.G.; Hansen, J.H.; Hvilsted, S.; Skov, A.L. Control of PDMS crosslinking by encapsulating a hydride crosslinker in a PMMA microcapsule. RSC Adv. 2014, 4, 47505–47512. [CrossRef] 32. Pei, S.F.; Cheng, H.M. The reduction of graphene oxide. Carbon 2012, 50, 3210–3228. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).