Article Dielectric Electroactive with Chemical Pre-Strain: An Experimentally Validated Model

Brittany Newell 1,*, Jose Garcia 1 and Gary Krutz 2

1 Purdue Polytechnic School of Engineering Technology, Adaptive Additive Technologies Lab (AATL), Purdue University, West Lafayette, IN 47907, USA; [email protected] 2 Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-765-494-7724

 Received: 15 June 2018; Accepted: 15 August 2018; Published: 22 August 2018 

Abstract: Dielectric electroactive materials represent a distinct group of smart materials that are capable of converting between electrical and mechanical energy. This research focuses on the modeling and testing of an industrial grade fluoropolymer material for its feasibility as a dielectric elastomer electroactive polymer. Through this process, a novel chemical pre-strain method was tested, along with a one-step process for application of pre-strain and addition of an elastomer conductive layer. Modeled and experimental actuators produced approximately 1 mm displacements with 0.625 W of electrical . The displacement of the actuators was characterized, and the effects of multiple parameters were modeled and analyzed.

Keywords: electroactive polymer; dielectric elastomer; finite element analysis; chemical pre-strain; manufacturing

1. Introduction Smart materials represent an ideology that the materials themselves, which are the building blocks of current mechanical and electrical structures, can react to presented stimuli, thereby improving safety and efficiency. Smart materials as a class represent a wide range of materials and applications including piezoelectrics, shape-memory alloys, ferroelectrics, thermoelectric materials, and . This research focuses on one sub-group: electroactive polymers. Electroactive polymers are categorized under the subclass of soft and actuators. Electroactive polymer materials are a unique class, capable of responding mechanically to an electric stimulus or of storing electrical energy from mechanical deformation in the same way as a capacitor. This makes these materials optimal for sensing, as well as actuation. The field of electroactive polymers can be further divided into two subsets: ionic and electric. Electric electroactive polymers generally require large electric fields for stimulation (between 80 V/µm and 180 V/µm) [1–3]. However, electric electroactive polymers have the ability to retain their activated state under a DC field, exhibit fast response times (microsecond range), and have no general environmental limitations excluding those that are polymer based [4]. Ionic , ionic polymer metal composites, conductive polymers, and specific nanotube based constructions comprise the ionic class. Ionic electroactive polymers require a smaller activation field—generally 1–5 V for conductive polymers—but due to their operating mechanism, an aqueous environment must be maintained [5]. While possible, a DC field maintained strain is more challenging to achieve in ionic polymers, but it can be achieved with certain subsets, such as conductive polymers. The focus of this research is on dielectric electroactive polymers which fall under the electric category, and more specifically, the utilization of these materials in industrial environments.

Actuators 2018, 7, 50; doi:10.3390/act7030050 www.mdpi.com/journal/actuators Actuators 2018, 7, 50 2 of 15

Dielectric elastomers were discovered in 1880 when Wilhelm Roentgen observed the movement of a by a 16 cm × 100 cm rubber band when an electric field was introduced to the rubber band. The rubber band had a fixed boundary at one end, with the mass remaining at the alternate end of the polymer. Roentgen studied the change in length of the rubber band [6,7]. In 1899, Sacerdote formulated that the strain response was due to an electric field [4,7]. In 1925, the first electroactive polymer was discovered and donned with the name “.” Eguchi discovered this material which was a mixture of carnauba, wax, beeswax, and , and documented the material’s piezoelectric characteristics [8]. In the 1970’s, Zhang and Bar-Cohen began research on poly(vinylidene fluoride), and noted its piezoelectric responses [4,9,10]. As a result of these observations, scientists began developing of a novel class of electroactive polymer materials. Then in the 1990s and the early 2000s, significant developments occurred through enhanced strains and the development of actuation strain enhancement through addition of mechanical pre-strain to the polymer matrix [11,12]. Goshkoderia and Rudykh [13] studied the effect of mechanical deformation on the stability of the dielectric material and proposed a model for predicting the electrostatic moduli of the dielectric material subjected to mechanical deformation. Furthermore, this work explored the effect of adding embedded inclusions on the dielectric matrix to improve the electrostatic properties of these materials. Other interesting uses of these materials include the study and use of spherically shaped objects made of EAP’s to model and produce micro-fluidic pumps [14]. Flouroelastomers are often used in EAP applications thanks to their ability to work as acceptable ferroelectric materials while maintaining a high resistance to high temperature, and good chemical resistance to degradation when exposed to a wide variety of solvents, acids, and bases. According to ASTM D1418 [15] Flouroelastomers are generally available in three specific compositions of the M class, which represents Rubbers having a saturated chain of the polymethylene type: (a) Non-substituted alkene monomers, (b) Partially substituted alkene monomers, and (c) fully substituted alkene monomers. Flouroelastomers are further classified into six different types based on their chemical composition, of which PMVE (Perflouromethilvinylether), a type 3, M rubber, is the preferred material for use in EAP applications thanks to its dielectric properties. Although the electrical properties of Flouroelastomers are less favorable than those of some acrylates, silicones, or urethanes, these materials are preferred for their high resistance to chemical degradation, high flexibility, and manufacturability [16]. Various researchers have attempted to improve the electric and mechanical properties of PDVF by synthesizing or combining it with diverse materials. Rat et al. [3] proposed and used P(VDF-TrFE) 70–30% mol as a dielectric material for EAP actuators and applications. In their work, they were able to homogenously disperse spin-crossover nanoparticles to enhance the mechanical flexibility and electric of the base P(VDF-TrFE) copolymer. Thakur et al. [17] investigated the use of aqueous functionalization of PVDF through bio-inspired proteins. They investigated the dielectric properties of regular PVDF and PDOPA@PVDF (dopamine-modified PVDF) films at various frequencies and temperatures. The PDOPA@PVDF resulted in a dielectric constant of 2.67 times higher than PVDF. According to Li et al. [15] conductive and strengthening particles of Silicon Dioxide SiO2, (nanofillers) can be used to facilitate crystallization of the polymer and have better control of the polymerization process. The addition of such particles resulted in an improved dielectric constant, low dielectric loss, excellent mechanical strength, toughness, and optical transparency. In this research, commercially available industrial grade fluoropolymer materials were adapted through the addition of graphite particles to adjust their electrical properties, and were used to produce dielectric EAPs through a one-step chemical pre-strain process.

2. Theoretical Background Dielectric elastomers are built as a parallel plate capacitor, comprised of a dielectric layer placed between two conductive layers. These materials actuate based on two primary mechanisms (1) Maxwell stress and (2) electrostriction [18]. As the capacitor which makes up the electroactive polymer circuit Actuators 2018, 7, 50 3 of 15 charges, a Maxwell stress is generated from changes in the electric field distribution as the dielectric is strained. This is also called Coulombic charge interaction. The second mechanism is electrostriction, wherein the dielectric material changes over time with strain, and often change can be seen in the materials permittivity [19]. The full relation to stress in the x and z directions can be seen in Equations (1) and (2). In these equations, σ represents mechanical stress, ε is the dielectric constant of −12 the insulator, εo is the permittivity of free space, 8.85 × 10 F/m, V represents the applied electrical potential, a1 is an electrostrictive coefficient which is the change in when shape is changed assuming constant volume, and a2 represents a density dependent permittivity [19]. However, the electrostriction effects are small in comparison to the Coulombic charge interaction response [18–20]. Therefore, for simplification, the following equations can be used to define the anticipated actuation and corresponding pressures seen by the electroactive material. The corresponding stresses can be seen in Equations (3) and (4). 1 a + a σ = − ε εV2(1 − 1 2 ) (1) zz 2 o ε 1 a σ = σ = ε εV2(1 + 2 ) (2) xx yy 2 o ε 1 σ = − ε εV2 (3) zz 2 o 1 σ = σ = ε εV2 (4) xx yy 2 o The pressure to achieve and maintain the designated strain can be found from calculating the differences between σyy and σzz as seen in Equation (5). In this equation σyy − σzz represents the change in normal stress, ε is the dielectric constant of the insulator material, εo is the permittivity of free space which is 8.85 × 10−12 F/m, and V represents the applied electrical potential.

2 σyy − σzz = εεoV (5)

Equation (6) shows the compressive pressure from the perspective of the dielectric polymer film in the z-axis through dividing the by the square of the thickness [18–21]. In this equation, P represents electrostatic pressure, ε is the dielectric constant of the insulator material, εo is the permittivity of free space which is 8.85 × 10−12 F/m, V represents the applied electrical potential, and D is the dielectric thickness. V2 P = εε (6) o D2 This formulated pressure is the foundation for actuation seen in the finite element model presented in this research and the correlating experimental data. This research expands upon this foundational knowledge to predict and test how a commonly used fluoropolymer industrial material can be manufactured and enhanced for use as a dielectric electroactive polymer in real world applications. This research produces a novel method of chemical pre-strain and uses an industrial polymer as the dielectric material, while also simplifying manufacturing through a single step process for pre-strain and electrode applications. Actuators produced using this method are evaluated for actuation response.

3. Materials and Methods The materials and methods section utilized in this research will be broken into the physical components for construction of experimental samples and the generation of the finite element analysis model.

3.1. Experimental Samples This research focused on testing and modeling of a novel industrial material, a proprietary fluoropolymer material used in sealing applications produced by Parker Hannifin Inc., Seals Division. Actuators 2018, 7, 50 4 of 15

However, these results can be expanded to other fluoropolymers and to other industrial grade materials. Fluoropolymer materials are utilized in numerous industrial and automotive applications due to their chemical resistivity particularly to solvents and strong acids and bases, and have ideal material properties for sealing and fluid conveyance applications. This, combined with their generally dielectric properties, made fluoropolymers an optimal choice for testing. Generally, electroactive polymer dielectric elastomer samples are pre-strained mechanically to enhance the actuation potential of the material under load [21,22]. However, this provides a significant challenge in real world manufacturing, since this mechanical pre-strain must be maintained throughout all subsequent processes and in application. Therefore, an alternative method, chemical pre-strain was investigated [23–25].

3.2. Experimental Samples: Chemical Pre-Strain In order to test the feasibility of using a chemical as a pre-strain agent, fluoropolymer samples that were initially pressed to their desired thickness ranging between 0.18 and 0.33 mm were formed. Then, the fluoropolymers were placed in a solvent, Methyl Ethyl Ketone (MEK) (Sunnyside® from Menards Inc., Eau Claire, WI, USA) for 5 min, removed, and allowed to dry for 12 h at room temperature in a fume hood. According to its MSDS, the evaporation rate of this product is approximately 4.6 per ASTM D3539 [26], which is slower than Acetone (with an evaporation rate of 7.8) but faster than 95% Ethyl alcohol (with an evaporation rate of 1.4) [27]; thus the 12 h period used was sufficient for drying the samples. The solvent interacted with the fluoropolymer material, causing intercalation within the polymer structure, resulting in swelling of the material. However, this process is primarily reversible. Therefore, during the dry time, the material returned to its native state. During the intercalation process, the fluoropolymer material expanded in the x, y, and z directions. These dimensional changes were measured using a standard ruler before, during, and after induced strain [23,24].

3.3. Experimental Samples: Addition of a Conductive Layer After completing chemical pre-strain testing, it was noted that pre-strain and electrode application could likely be combined into a one-step process. This provides a significant improvement in manufacturing by reducing time and the number of processing steps. Furthermore, this procedure utilized the same polymer material for both the conductive layer and the dielectric layer, providing a higher level of consistency in movement. In order to complete this combined manufacturing step, a mixture of the fluoropolymer material doped with conductive particles, Graphite Powdered Lubricant from MCM Electronics, and the pre-strain agent, MEK was applied to the surface of the dielectric fluoropolymer material [23–25,28,29]. This led to selective pre-strain in only the active region where the conductive material is present. This mixture was applied to the top surface of the dielectric fluoropolymer. The samples were allowed to air dry in a fume hood until the swelling was reduced, and then the sample was flipped so the mixture could be applied to the bottom of the samples to complete the capacitor design. The structures were left in the fume hood for 12 h to ensure full removal of the solvent. After this time, the samples were heat cured [23–25]. Copper foil tape with conductive adhesive purchased from McMaster-Carr Inc. (Elmhurst, IL, USA) were cut to approximately 10 mm in width, and placed on the top and bottom of the samples for testing.

3.4. Experimental Samples: Displacement Measurements Actuators were designed to move out of plane by adding a fixed boundary constraint to the perimeter of the material. This constraint was placed in the model and in the experimental set-up. Material z-axis displacement was then measured using a Keyence LK-G82 (Osaka, Japan) laser with a LK-GD500 controller. The laser was aligned and calibrated, and samples were then placed in line with the set-up. Samples were then subsequently activated with a high power supply in the range of 1000–5000 V and 125 µA, using the Biomimetics Laboratory (Auckland, New Zealand) four channel Artificial Muscle Control unit. Samples were powered using both AC and DC supply. The AC frequency was 0.2 Hz. The Keyence laser had a wavelenght of 650 nm and a spot diameter of Actuators 2018, 7, 50 5 of 15 Actuators 2018, 7, x FOR PEER REVIEW 5 of 15

AC70 frequencyµActuatorsm. The 2018 measurement was, 7, x FOR0.2 PEERHz. TheREVIEW range Keyence for this laser instrument had a wavelenght was ±15 mm. of The650 nm repeatability and a spot of diameter the system5 of 15 of was70 μwithinm. The 0.2measurementµm. Sample range measurements for this instrument were taken was in triplicate±15 mm. acrossThe repeatability four quadrants of the of system each sample. was AC frequency was 0.2 Hz. The Keyence laser had a wavelenght of 650 nm and a spot diameter of 70 withinThe recorded 0.2 μm. measurementsSample measurements were calibrated were taken to a 0in mm triplicate initial across starting four point. quadrants Data was of recordedeach sample. every μm. The measurement range for this instrument was ±15 mm. The repeatability of the system was The recorded measurements were calibrated to a 0 mm initial starting point. Data was recorded every millisecondwithin 0.2 forμm. one Sample minute measurements [23,24]. The were effect taken of in geometrytriplicate across and voltagefour quadrants was tested. of each Threesample. sample millisecondgeometriesThe recorded for were one measurements constructed minute [23 were for,24 this]. calibrated The testing. effect to Their aof 0 mmgeometry dimensions initial starting and canvoltage point. beseen Data was inwas tested. Table recorded1 ,Three one every full sample circle geometriesandmillisecond two ring were shapedfor constructed one minute actuators for [23 this, [2423]., 24 testing.The]. Aneffect Their image of geometry dimensions of the samplesand canvoltage be created seen was intested. can Table be Three seen 1, one sample in full Figure circle 1A. andFive twogeometries ring shaped were were constructed applied,actuators ranging for[23 this,24 ].testing. from An image 1000 Their V–5000 ofdimensions the samples V, in can 1000 becreated Vseen intervals in can Table be to 1, seen studyone fullin theFigure circle effects 1A of. Fiveapplied andvoltages two voltage. ring wer shapede applied actuators, ranging [23, 24from]. An 1 000image V– of5000 the Vsamples, in 1000 created V intervals can be seento study in Figure the effects 1A. of appliedFive voltage. voltages were applied, ranging from 1000 V–5000 V, in 1000 V intervals to study the effects of applied voltage. Table 1. Sample Dimensions [23,24]. Table 1. Sample Dimensions [23,24]. Inactive Region InactiveTable Region 1. SampleActive Dimensions Region [23,24Active]. Region Width of Set Number Active Area Set InactiveOuter Region Diameter InactiveInner DiameterRegion ActiveOuter DiameterRegion Outer InnerActive Diameter Region Inner Active RingWidth of Number Set OuterInactive Diameter Region InnerInactive Diameter Region ActiveDiameter Region Outer Active RegionDiameter Inner ActiveArea Width of Ring Number Outer Diameter Inner Diameter1 Diameter Diameter1 Area 2 Ring 1 1 15.7 cm 5.7 cm NANA 1 4.74.7 cm cm NA NA 1 17.3517.35 cm cm2 NANA 1 1 5.7 cm NA 1 4.7 cm NA 1 17.35 cm2 2 NA 1 2 25.7 cm 5.7 cm0.7 0.7 cm cm 4.74.7 cm cm 1.7 cm1.7 cm 15.0815.08 cm cm2 1.51.5 cm cm 2 5.7 cm 0.7 cm 4.7 cm 1.7 cm 15.08 cm22 1.5 cm 3 5.7 cm 2.7 cm 4.7 cm 3.7 cm 6.60 cm 2 0.5 cm 3 3 5.7 cm5.7 cm 2.72.7 cm cm 4.74.7 cm cm 3.7 cm3.7 cm 6.60 cm6.602 cm 0.5 cm 0.5 cm 1 1 Sample1 SampleSample 1 1is is 1a isacircular circular a circular actuator actuator not not not a a aring ring structure structure.structure. .

Figure 1. (A) Image of Standard Constructed Actuators. (B) Image of Powered Actuator [23,24]. Figure 1. (A) Image of Standard Constructed Actuators. (B) Image of Powered Actuator [23,24]. Figure 1. (A) Image of Standard Constructed Actuators. (B) Image of Powered Actuator [23,24]. 3.5. Model: Construction 3.5. Model: Construction 3.5. Model:In Construction order to predict the behavior of actuators produced using a fluoropolymer dielectric material andIn fluoropolymer order to predict conductive the behavior material, of actuatorsa new finite produced element analysis using a model fluoropolymer was created. dielectric The model material In order to predict the behavior of actuators produced using a fluoropolymer dielectric material andwas fluoropolymer completed in conductiveComsol Multiphysics material,TM a. newInitial finite inputs element to the system analysis included model a wasstress created.-strain curve The model and fluoropolymer conductive material, a new finite element analysis model was created. The model wasof completed the fluoropolymer in Comsol material Multiphysics with tensileTM test. Initial data taken inputs at toa rate the of system 20 in/min. included The stress a stress-strain-strain data curve was completed in Comsol MultiphysicsTM. Initial inputs to the system included a stress-strain curve of thewas fluoropolymer fit using a 5-order material Mooney with-Rivlin tensile fit with test dataa root taken mean at square a rate error of 20 of in/min. 0.272. The The model stress-strain fitting data of thewas fluoropolymer completed using material Creo Elements with tensile software test data for fitting taken of at a a hyperelastic rate of 20 in/min. model. MultipleThe stress forms-strain of data was fit using a 5-order Mooney-Rivlin fit with a root mean square error of 0.272. The model fitting was was fitstrain using energy a 5- ordercalculations Mooney exist-Rivlin including fit with Odgen, a root Yeoh, mean and square Arruda error-Boyce of 0.272.[22]. However,The model the fitting completed using Creo Elements software for fitting of a hyperelastic model. Multiple forms of strain was completedMooney-Rivlin using model Creo was Elements selected duesoftware to the outputfor fitting of the of lowest a hyperelastic Root Mean model. Square Multipleerror with formsthe of energy calculations exist including Odgen, Yeoh, and Arruda-Boyce [22]. However, the Mooney-Rivlin straindata energy fitting. calculations The 5th order exist Mooney including-Rivlin model Odgen, was Yeoh,constructed and usingArruda Equation-Boyce ( 7[22].) for calculationHowever, the model was selected due to the output of the lowest Root Mean Square error with the data fitting. Mooneyof strain-Rivlin energy, model W was[21,30 selected]. The strain due energyto the outputequation of is the dependent lowest Rootupon Meanthe invariants Square producederror with the Thefrom 5th the order left Mooney-RivlinCauchy Green deformation model was tensor. constructed The left Cauchy using Green Equation tensor (7) is forcalculated calculation from the of strain data fitting. The 5th order Mooney-Rivlin model was constructed using Equation (7) for calculation energy,deformationW [21, 30tensor,]. The F through strain energyFFT. The equationC values in is the dependent equation are upon based the on invariants material par producedameters, from of strain energy, W [21,30]. The strain energy equation is dependent upon the invariants produced theand left d Cauchyk is determined Green based deformation on the shear tensor. modulus. The The left material Cauchy is assumed Green tensor to be incompressible is calculated and from the frominitially the left isotropic. Cauchy GreenTherefore, deformation the resultingT tensor. invariants The leftcan Cauchybe calculated Green from tensor the eigenvaluesis calculated of fromthe the deformation tensor, F through FFT . The C values in the equation are based on material parameters, deformationdeformation tensor, gradient F through tensor. FFThese. The values C values are generally in the equationreferred to are as thebased stretch on materialratios, λ, andpar ametersare , and dk is determined based on the shear modulus. The material is assumed to be incompressible and and ddefinedk is determined in Equations based (8)– on(10 )the. In shearthe original modulus. undeformed The material state, λ is1 = assumed λ2 = λ3 = 1. to Therefore, be incompressible I1 = I2 = 3, and initially isotropic. Therefore, the resulting invariants can be calculated from the eigenvalues of the initiallywhich isotropic. results in Therefore, Equation ( 7the). Due resulting to incompressibility, invariants can J, which be calculated is the volume from ratio the, is eigenvalues set to 1. of the deformation gradient tensor. These values are generally referred to as the stretch ratios, λ, and are deformation gradient tensor. These values are generally referred to as the stretch ratios, λ, and are defined in Equations (8)–(10). In the original undeformed state,1 λ1 = λ2 = λ3 = 1. Therefore, I1 = I2 = 3, defined in Equations (8)–(10). In= the𝑁𝑁 original( 3undeformed) ( 3) + state,𝑁𝑁 ( λ1 = 1λ)2 = λ3 = 1. Therefore, I(17 =) I2 = 3, which results in Equation (7). Due to incompressibility,𝑖𝑖 𝑗𝑗 J, which is the2 volume𝑘𝑘 ratio, is set to 1. which results in Equation (7). Due to incompressibility,𝑖𝑖𝑖𝑖 1 2 J, which is𝑒𝑒𝑒𝑒 the volume ratio, is set to 1. 𝑊𝑊 � 𝐶𝐶 ḹ − ḹ − � 𝑘𝑘 𝐽𝐽 − Invariants (I): 𝑖𝑖+𝑗𝑗=1 𝑘𝑘=1 𝑑𝑑 1 = 𝑁𝑁 ( 3) ( 3) + 𝑁𝑁 ( 1) (7)(7) 𝑖𝑖 𝑗𝑗 2𝑘𝑘 𝑖𝑖𝑖𝑖 1 2 𝑒𝑒𝑒𝑒 𝑊𝑊 � 𝐶𝐶 ḹ − ḹ − � 𝑘𝑘 𝐽𝐽 − Invariants (I): 𝑖𝑖+𝑗𝑗=1 𝑘𝑘=1 𝑑𝑑

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Invariants (I): 2 2 2 I1 = λ1 + λ2 + λ3 (8) 1 1 1 = + + I2 2 2 2 (9) λ1 λ2 λ3 2 2 2 2 I3 = (λ1 )(λ2 )(λ3 ) = J (10) Additional inputs to the system included material properties for the fluoropolymer material, which can be seen in Table2[28].

Table 2. Material Property Model Inputs.

Material Property Value Dielectric Constant 10.7 Thickness 0.2549 mm Poisson Ratio 0.499 Bulk Modulus 2.2986 MPa Shear Modulus 2.29 kPa Young’s Modulus 1.8 MPa Electrical Conductivity (Dielectric Layer) 2.857 × 10−14 S/m Electrical Conductivity (Electrode Layer) 10 × 104 S/m Relative Permittivity 2.5 Density 1800 kg/m3 Thermal Conductivity 0.2 W/(m·K) Heat Capacity at Constant Pressure 700 J/(kg·K)

After inclusion of the material properties, the geometry was built in a 2-D axisymmetric format in order to improve computation efficiency. The construction can be seen in Figure2A, and the revolved version can be seen in Figure2C. The construction includes the dielectric area with an inactive dielectric perimeter, and the conductive electrodes which are produced from the same fluoropolymer material, but with added conductive particles. In Figure2B, an electrical potential is applied to the conductive electrode layers highlighting this region. Three geometries were created in the model. Each geometry had the same outer active diameter, but the inner diameter varied from a full circle to small ring, which was identical to the experimental constructions. The dimensions for each geometry can be seen in Table1. The thickness of the samples was measured in triplicate per sample, and the averaged value was used for model construction. The average experimental thickness as 0.2549 mm. The averages, standard deviation, and maximum and minimum values for each sample set can be seen in Table3. The variation in thickness of the dielectric material from the manufacturer was large, resulting in high standard deviations. The manufacturing process at the manufacturer’s site was lab-scale, due to production of one-off batches which resulted in lower sample repeatability in regards to thickness. After construction of the geometries and materials, the model was meshed using triangular elements. Four-node quadrilateral mesh elements were also tested, and the results were convergent between both element types. The mesh size ranged from 0.0124 mm–0.2877 mm. A denser mesh was applied to active interfaces where deformation was likely to occur, as seen in Figure3.

Table 3. Measured Sample Thickness.

Stencil Average (mm) Standard Deviation Maximum (mm) Minimum (mm) 1 0.234 0.054 0.305 0.152 2 0.284 0.030 0.330 0.254 3 0.255 0.050 0.330 0.152

Comsol Mutliphysics enabled use of multiple modalities, which is why this software was selected. Electric, mechanical, and hyperelastic effects were taken into account in the model. A square wave of Actuators 2018, 7, 50 7 of 15

Actuators 2018, 7, x FOR PEER REVIEW 7 of 15 0–5000 V was applied to the conductive material surface and converted to a mechanical pressure using Actuators 2018, 7, x FOR PEER REVIEW 7 of 15 Equationpressure (6). using At this Equation point, it(6 was). At this assumed point, thatit was heat assumed and energy that heat losses and energy associated losses with associated the conversion with werepressurethe minimal. conversion using The Equationwere applied minimal. (6 pressure). At thisThe point,applied was inducedit was pressure assumed in was a time-dependent that induced heat and in aenergy time manner- dependentlosses toassociated allow manner for with to matrix convergencetheallow conversion for asmatrix deformation were convergence minimal. occurred. as The deformation applied Ramping pressureoccurred. prevented was Ramping induced the prevented formation in a time the of-dependent formation inverted matricesofmanner inverted to in the solver.allowmatrices The for ramp inmatrix the function solver. convergence The was ramp set as withdeformationfunction a slope was occurred. set equal with to a Ramping theslope electromechanical equal prevented to the electromechanical the formation pressure of divided pinvertedressure by 100 to reachmatricesdivided the maximum byin the100 solver. to reach pressure The the ramp maximum resulting function pressure in was a slope set withresulting of a Pressure/100 slope in aequal slope to of (Pa/s).the Pressure/100 electromechanical A cutoff (Pa/s time). pA ofressure cutoff 100 s was time of 100 s was applied to the model. A MUlitfrontal Massively Parallel sparse direct solver applieddivided to the by model. 100 to reach A MUlitfrontal the maximum Massively pressure Parallelresulting sparsein a slope direct of Pressure/100 solver (MUMPS) (Pa/s). A was cutoff used to time(MUMPS) of 100 was s wasused appliedto resolve to the the solution. model. ThisA MUlitfrontal was selected Massively due to the solversParallel high sparse numeric direct stability solver resolve the solution. This was selected due to the solvers high numeric stability [23–25,28,29]. (MUMPS)[23–25,28,29 was]. used to resolve the solution. This was selected due to the solvers high numeric stability [23–25,28,29].

Figure 2. (A) Cross-Section of Modeled Actuator (B) Applied Electric Potential at Electrode Surfaces Figure 2. (A) Cross-Section of Modeled Actuator (B) Applied Electric Potential at Electrode Surfaces Figure(Sample 2 .4000 (A) CrossV) (C-)Section 2-D Axisymmetric of Modeled RevolutionActuator (B. ) Applied Electric Potential at Electrode Surfaces (Sample 4000 V) (C) 2-D Axisymmetric Revolution. (Sample 4000 V) (C) 2-D Axisymmetric Revolution.

Figure 3. Mesh Distribution. Figure 3. Mesh Distribution. Figure 3. Mesh Distribution.

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3.6. Model: Sensitivity Analysis Actuators 2018, 7, x FOR PEER REVIEW 8 of 15 In order to look at the importance and effects of different variables on the model, a sensitivity analysis3.6. Model: was conducted. Sensitivity Analysis A parametric sweep was utilized to test temperature, voltage, dielectric constant, and actuator size. A full factorial sensitivity analysis with 2k trials (k = 5) was conducted. In order to look at the importance and effects of different variables on the model, a sensitivity The variablesanalysis was tested conducted. and test A rangesparametric can sweep be seen was in utilized Table4 .to Thetest designedtemperature, experiments voltage, dielectric tested the maximumconstant and, and minimum actuator size. combinations A full factorial of all sensitivity variables. analysis The size with correlated 2k trials (k with= 5) was the conducted. minimum and maximumThe variables geometries tested used, and test and ranges the voltage can be valuesseen in correlated Table 4. The to designed achievable experiments ranges with tested the the voltage supplymaximum unit maximum and minimum (5000 combinations V), which was of alsoall variables. under the The dielectric size correlated breakdown with the of minimum the fluoropolymer and material.maximum The temperaturegeometries used, range and was the selectedvoltage values to be correlated within the toworking achievable range ranges for with the the fluoropolymer voltage material.supply The unit thicknessmaximum (5000 values V), werewhich basedwas also on under minimum the dielectric and maximumbreakdown of values the fluoropolymer measured from experimentalmaterial. The samples. temperature The dielectric range was constant selected was to be based within on the experimental working range measurements for the fluoropolymer and optimal valuesmaterial. for electroactive The thickness polymer values materials. were based on minimum and maximum values measured from experimental samples. The dielectric constant was based on experimental measurements and optimal values for electroactive polymer materials. Table 4. Sensitivity Analysis Variables and Ranges.

Table 4. Sensitivity Analysis Variables and Ranges. Variables Minimum Maximum Size (ActiveVariables Area) Minimum6.60 cm2 Maximum17.35 cm2 Size Voltage(Active Area) 6.60 1000 cm V2 17.35 5000 cm V2 TemperatureVoltage 1000 293 KV 5000 400 V K DielectricTemperature Constant 293 10.7 K 400 21.4 K Thickness 0.329 mm 0.654 mm Dielectric Constant 10.7 21.4 Thickness 0.329 mm 0.654 mm 4. Results 4. Results 4.1. Experimental Samples: Chemical Pre-Strain Ten4.1. Experimental samples of Samples: the dielectric Chemical fluoropolymerPre-Strain material were measured with a ruler, and the measurementsTen samples were recordedof the dielectric in Table fluoropolymer5. Then the samplesmaterial were were measured placed in with the chemicala ruler, and pre-strain the agent,measurements Methyl Ethyl were Ketone recorded (MEK) in Table for 5 min.5. Then After the thissamples time, were the placed sample in dimensions the chemical were pre-strain measured and recorded,agent, Methyl as seenEthyl in Ketone Table (ME5. AnK) examplefor 5 min. ofAfter the this material time, the prior sample to pre-strain dimensions can were be measured seen as in the left imageand recorded in Figure, as4 ,seen and in after Table pre-strain 5. An example can be of seenthe material in the right prior imageto pre-strain of Figure can 4be. Theseen outer as in the ring in left image in Figure 4, and after pre-strain can be seen in the right image of Figure 4. The outer ring Figure4 depicts the same material after incubation in the pre-strain agent, MEK for 5 min. The material in Figure 4 depicts the same material after incubation in the pre-strain agent, MEK for 5 min. The pre-strainmaterial averaged pre-strain 158.9%, averaged with 158.9% a standard, with a deviationstandard deviation of 6.3% of across 6.3% theacross 10 the samples 10 samples providing evidenceproviding that chemical evidence pre-strainthat chemical can producepre-strain strains can produce greater strains than 100% greater in dielectricthan 100% elastomers in dielectric [23 ,24]. The processelastomers is [ primarily23,24]. The process reversible. is primarily The chemical reversible. was The allowedchemical was to evaporate allowed to evaporate out of the out polymer of matrixthe for polymer 12 h in matrix a fume for hood12 h in at aroom fume temperaturehood at room temperature and the sample and the dimensions sample dimensions were re-measured. were The fluoropolymerre-measured. The expands fluoropolymer due to intercalationexpands due to of intercalation the solvent of within the solvent the polymer within matrix.the polymer The 12 h waitmatrix. time allows The 12 the h wait matrix time toallows re-crystallize the matrix afterto re-crystallize evaporation after of evaporation the solvent of from the solvent the fluoropolymer. from the The resultsfluoropolymer. showed The the results process showed was 97.5% the process reversible, was 97.5% with reversible a standard, with deviation a standard of 2.9%.deviation Therefore, of 2.9%. Therefore, the samples were stable for use as electroactive polymer materials [23–25,28]. Time the samples were stable for use as electroactive polymer materials [23–25,28]. Time left in solvent left in solvent controls the amount of pre-strain exhibited by the material. In mechanical pre-strain, controls the amount of pre-strain exhibited by the material. In mechanical pre-strain, generally generally 100–200% pre-strain is applied, and 5 min was selected in order to fall within this accepted 100–200%range. pre-strain is applied, and 5 min was selected in order to fall within this accepted range.

Figure 4. Chemical Pre-strain. Left o-ring is without treatment, right o-ring with treatment in solvent Figure 4. Chemical Pre-strain. Left o-ring is without treatment, right o-ring with treatment in solvent for 15 min. for 15 min.

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Table 5. Results of Chemical Pre-strain [23,24].

Actuators 2018, 7, x FOR PEER REVIEW 9 of 15 Initial Dimensions 5 min Chemical Strain 5 min after Removal % Swell % Reversal (cm) Dimensions (cm) Dimensions (cm) Table 5. Results of Chemical Pre-strain [23,24]. 5.10 8.17 5.40 160.13 94.44 Initial Dimensions5.13 5 min Chemical 8.00 Strain Dimensions 5 min after 5.13 Removal Dimensions 155.84 % 100.00% (cm)4.53 7.27(cm) 4.60(cm) 160.29Swell Reversal 98.55 4.775.10 6.978.17 4.735.40 146.15160.13 100.0094.44 4.635.13 7.638.00 4.875.13 164.75155.84 95.21100.00 4.604.53 7.177.27 4.974.60 155.80160.29 92.6298.55 4.604.77 7.506.97 4.654.73 163.04146.15 98.92100.00 4.604.63 7.607.63 4.504.87 165.22164.75 100.0095.21 4.60 7.17 4.97 155.80 92.62 4.60 7.50 4.65 163.04 98.92 4.2. Experimental4.60 Samples: Displacement7.60 Results 4.50 165.22 100.00

The4.2. Experimental constructed Samples: actuators Displacement produced Results using the process outlined in the Experimental Samples z section ofThe the constructed Materials and actuators Methods produced were testedusing the using process-axis outlined displacement. in the Experimental They were measuredSamples by the Keyencesection of laser the Materials system, and producing Methods the were displacement tested using z curves-axis displacement. seen in Figure They5 were. The measured actuators by were designedthe Keyence to actuate laser out system of plane, producing by placement the displacement of a boundary curves constraint seen in onFigure their 5. outerThe actuators perimeter, were forcing the motiondesigned into to theactuatez-axis. out Theof plane samples by placement shown in of Figures a boundary5–7 are constraint powered on withtheir 5000outer Vperimeter and 125, µA. Measurementsforcing the motion were taken into the in z all-axis. four The quadrants samples shown of each in Figures sample 5– and7 are at powered three locations with 5000 across V and each quadrant:125 μA. the Measurements inner diameter were (middle), taken in center,all four andquadrants outer of diameter each sample (edge). and The at three data locations shown inacross Figure 4 showseach the quadrant response: the of ainner sample diameter to a sudden(middle), increase center, and in voltage. outer diameter This experiment (edge). The wasdata performedshown in on a flatFigure ring sample 4 shows with the response a 57 mm of outside a sample diameter to a sudden and aincrease 0.5 mm ininner voltage diameter.. This experiment While there was was performed on a flat ring sample with a 57 mm outside diameter and a 0.5 mm inner diameter. While variability in the response to the voltage stimulus, it was found that in both experiments, the response there was variability in the response to the voltage stimulus, it was found that in both experiments, timesthe for response the displacement times for the was displacement similar. Sample was similar. 2 had Sample a time t2 =had 7.4 a stim toe achieve t = 7.4 s to the achieve 63.2% the of 63.2 maximum% steadyof statemaximum displacement steady state of displacement 0.638 mm. of Sample 0.638 mm. 1 had Sample a time 1 hadt = a 12.3 time s t to = 12.3 achieve s to achieve 63.2% 63.2 of the% full steadyof statethe full displacement steady state ofdisplacement 0.898 mm. of This 0.898 was mm further. This was reinforced further reinforced with the analysiswith the presentedanalysis in Figurepresented6A, which in representsFigure 6A, thewhich velocity represents of the the actuator; velocity theof the maximum actuator; speedthe maximum of the sample speed 2ofactuator the was 0.38sample mm/s 2 actuator at the timewas 0.38 where mm/s the at voltage the time was where applied, the voltage while was the applied, sample while 1 showed the sample a maximum 1 speedshowed of 0.57 a mm/s maximum at the speed same of instant. 0.57 mm/s The plotsat the in same Figure instant.6B show The theplots acceleration in Figure 6 ofB bothshow samples, the whereacceleration sample 2 hadof both an samples acceleration, where of sample 28.4 mm/s 2 had 2an, and acceleration sample of 1 had28.4 anmm/s acceleration2, and sample of 1 44.8 had mm/san 2. 2 The reasonacceleration for this of variability44.8 mm/s .is The unknown, reason for but this may variability be attributed is unknown to various, but factors,may be suchattributed as variation to various factors, such as variation in sample thickness and inconsistencies in the lab-scale in sample thickness and inconsistencies in the lab-scale manufacturing process. manufacturing process.

Figure 5. Set 2 Geometry Z-Axis Displacement. Figure 5. Set 2 Geometry Z-Axis Displacement.

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Figure 6. Set 2 (A) Velocity and (B) Acceleration. Figure 6. Set 2 (A) Velocity and (B) Acceleration. Figure 7 depicts the results from z-axis displacement of the three geometries studied. The Set Figurenumbers7 depictscorrelate theto the results sample from dimensionsz-axis displacementin Materials and of Methods the three Table geometries 1. The results studied. showed The Set numbersthat the correlate larger the to theactive sample sample dimensions area Set 1, the in Materialsgreater the and resulting Methods displacement. Table1. The The results average showed values that the largerfrom theeach active set can sample be seen area in SetTable 1, the6 [23 greater,24,28]. theDisplacements resulting displacement. ranged from 0.19 The mm average to 0.99 values mm from eachdepending set can be seenupon ingeometry. Table6[ The23,24 laser,28]. measuring Displacements spot was ranged located from at three 0.19 mmdistinct to 0.99radial mm locations depending within the samples; this was done by translating the horizontal position of the sample using a upon geometry. The laser measuring spot was located at three distinct radial locations within the graduated x-y table and leaving the laser head static, thereby assuring the three locations were samples; this was done by translating the horizontal position of the sample using a graduated x–y consistent between the sample. Figure 7 also shows that displacement is dependent upon tableme andasurement leaving thelocation laser; the head center static, region thereby of the assuring actuators the demonstrated three locations the greatest were consistent displacement between the sample.while regions Figure closer7 also to showsthe inactive that boundaries displacement experience is dependent less overall upon deflection. measurement Figure 7 location;A shows the centerthe region effect of of location the actuators for a ring demonstrated of 5.7 mm outside the greatestdiameter displacementand no inner diameter while regions(a flat circle) closer. For to the inactivethis boundariessample, the experienceedge location less corresponds overall deflection. to approximately Figure7 A2.85 shows mm from the effect the center, of location the middle for a ring of 5.7location mm outside corresponds diameter to 1.42 and mm no from inner the diametercenter, and (a the flat center circle). location For thiscorresponds sample, to the theedge center location of correspondsthe sample. to approximatelyIt showed that the 2.85 maximum mm from displacement the center, thewas middle achieved location at the center, corresponds the maximum to 1.42 mm fromdisplacement the center, and at the the center center was location 2.12 mm, corresponds 1.69 mm at the to themiddle center position of the and sample. 1.37 mm It showedat the edge. that the Figure 7B shows the effect of location for a ring of 5.7 mm outside diameter and an inner diameter of maximum displacement was achieved at the center, the maximum displacement at the center was 0.5 mm (a flat circle with a small centered hole); in this case the location of the edge measurement is 2.12 mm,the same 1.69 as mm before, at the butmiddle the middle position and edge and locations 1.37 mm are at found the edge. at 3.1 Figuremm and7B 0.5 shows mm from theeffect the of locationrelative for sample a ring ofcenter. 5.7 mmIt showed outside again diameter that the andmaximum an inner displacement diameter was of 0.5 achieved mm (a towards flat circle the with a smallcenter centered; the maximum hole); in displacement this case the at location the center of thewas edge 0.90 mm, measurement 0.64 mm at is the the middle same asposition before, and but the middle0.49 and mm edge at the locations edge. Lastly, are found Figure at 7 3.1C shows mm and the 0.5 effect mm of from location the relativefor a ring sample of 5.7 center.mm outside It showed againdiameter that the and maximum an inner displacementdiameter of 0.7 mm was (a achieved flat circle towardswith a medium the center; sized thecentered maximum hole). It displacement showed at theagain center that was the 0.90maximum mm, 0.64displacement mm at the was middle achieved position towards and the 0.49 center mm: the at maximum the edge. displacement Lastly, Figure 7C showsat the center effect was of location 0.64 mm, for 0.41 a ringmm at of the 5.7 middle mm outside position diameter and 0.26 mm and at an the inner edge. diameter In this case of, the 0.7 mm (a flatlocation circle withof the aedge medium measurement sized centered is the same hole). as before, It showed but the again middle that and the edge maximum locations are displacement found at 3.2 mm and 0.7 mm from the relative sample center. was achieved towards the center: the maximum displacement at the center was 0.64 mm, 0.41 mm at the middle position andTable 0.26 mm6. Averaged at the Experimental edge. In this Z- case,axis Displacement the location [23,26 of the–29] edge. measurement is the same as before, but the middle and edge locations are found at 3.2 mm and 0.7 mm from the relative Set Active Area Experimental Displacement sample center. 1 17.35 cm2 0.9876 mm 2 15.08 cm2 0.8475 mm Table 6. Averaged Experimental Z-axis Displacement [23,26–29]. 3 6.60 cm2 0.1891 mm

Set Active Area Experimental Displacement 1 17.35 cm2 0.9876 mm 2 15.08 cm2 0.8475 mm 3 6.60 cm2 0.1891 mm

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Figure 7. Displacement Curves across Multiple Geometries. (A) Set 1; (B) Set 2; (C) Set 3. Figure 7. Displacement Curves across Multiple Geometries. (A) Set 1; (B) Set 2; (C) Set 3.

4.3. Model Results and Experimental Comparison 4.3. Model Results and Experimental Comparison Figure 8 depicts standard displacement results generated from the model, and represents Figure8 depicts standard displacement results generated from the model, and represents predicted predicted results from Set 3 samples, with the black region representing the original location and the results from Set 3 samples, with the black region representing the original location and the multi-colored multi-colored region depicting modeled displacements. Table 7 depicts the numeric values for region depicting modeled displacements. Table7 depicts the numeric values for maximum actuation maximum actuation measured across the samples for each geometry compared to the predicted data measuredfrom the acrossfinite element the samples analysis for eachmodel. geometry Figure compared9A represents to the model predicted and experimental data from the data finite for element Set 2 analysisspecimens model. between Figure 10009A and represents 5000 V. model Three and samples experimental of Set 2 geometries data for Set were 2 specimens tested at between 1000–5000 1000 V andin 1000 5000 V.V Threeintervals samples, and ofthe Set values 2 geometries were averaged. were tested The at model 1000–5000 parameters V in 1000 for V Set intervals, 2 included and thea valuesstandard were temperature averaged. of The 293 model K and parametersa thickness value for Set of 2 0.2549 included mma and standard a dielectric temperature constant ofof 29310.7. K andThe a model thickness and valueexperimental of 0.2549 results mm anddemonstrate a dielectric the constant trend of ofincreasing 10.7. The actuation model and with experimental increasing resultsvoltage. demonstrate However, thethe trendmodel of g increasingenerally predicts actuation a withlarger increasing deformation voltage. compared However, to the the actual model generallyexperiment predicts produced. a larger This deformation is to be expected compared, because to the the actual model experiment assumes no produced. heat loss, Thisuniform is to bedistribution expected, becauseof charge, the and model material assumes uniformity. no heat In loss, the uniformactual experiments, distribution copper of charge, leads and are materialused to uniformity.transmit charge In the, and actual material experiments, geometry copper is not leads uniform. are used However to transmit, as seen charge, in Figure and material9A, at 5000 geometry V the isexperimental not uniform. results However, outperform as seen the in model. Figure9 ItA, was at 5000noted V that the experimentalmaterials did not results return outperform fully to their the model.initial start It was positions noted, likely that materials due to material did not creep return after fully deformation. to their initial In general, start the positions, model represents likely due tothe material theoretical creep performance after deformation. achievable In for general, this material the model under represents these supplied the theoreticalparameters. performance An analysis achievablecomparing for three this samples material of under the three these geometries supplied parameters. at 5000 V, 293 An analysisK, thickness comparing of 0.2549 three mm samples and a ofdielectr the threeic constant geometries of 10.7 at 5000 was V, performed 293 K, thickness, as seen of 0.2549in Figure mm 9 andB. The a dielectric results show constant similar of 10.7 trends was performed,between experimental as seen in Figure and model9B. The results, results but show the similarexperimental trends results between demonstrate experimental high and standard model results,deviations. but theA sensitivity experimental analysis results was demonstrate further performed high standard to analyze deviations. the discrepancies A sensitivity between analysis the wasmodel further and performedthe experimental to analyze data. theThickness discrepancies and size between had the thelargest model sensitivities and the, experimental as seen in Figure data. 10. The thickness of experimental samples varied significantly, as listed in Table 3; this is assumed to

Actuators 2018, 7, 50 12 of 15

ThicknessActuatorsActuators 2018 and 2018, 7, size, x7 ,FOR x FOR had PEER PEER the REVIEW REVIEW largest sensitivities, as seen in Figure 10. The thickness of experimental12 of12 15 of 15 samples varied significantly, as listed in Table3; this is assumed to be a leading cause of error in the be bea leadinga leading cause cause of of er errorror in in the the experimentalexperimental measures andand variationvariation between between the the model model and and experimental measures and variation between the model and experimental results. experimentalexperimental results. results.

Figure 8. Model Z-Axis Displacement Results for Set 3 at 5000 V. Figure 8. Model Z-Axis Displacement Results for Set 3 at 5000 V.

FigureTable 8. 7. Model Comparison Z-Axis between Displacement Model andResults Experimental for Set 3 atResults. 5000 V. Table 7. Comparison between Model and Experimental Results. SpecimenTable Active 7. Comparison Area Model between Displacement Model and Experimental Experimental Results. Displacement 2 Specimen1 Active17.35 Areacm Model1.1653 Displacement mm Experimental0.9876 mm Displacement Specimen2 Active15.08 Area cm2 Model0.633 Displacement mm Experimental0.8475 Displacementmm 1 17.35 cm2 1.1653 mm 0.9876 mm 1 3 17.356.60 cm cm22 1.16530.1870 mm 0.18910.9876 mm mm 2 15.08 cm2 0.633 mm 0.8475 mm 2 15.08 cm2 0.633 mm 0.8475 mm 3 6.60 cm2 0.1870 mm 0.1891 mm 3 6.60 cm2 0.1870 mm 0.1891 mm

Figure 9. Experimental and Model Displacement Data versus Voltage. (A) Stencil 2 Voltage Response; (B) Actuation versus Size. Figure 9. Experimental and Model Displacement Data versus Voltage. (A) Stencil 2 Voltage Response; Figure 9. Experimental and Model Displacement Data versus Voltage. (A) Stencil 2 Voltage Response; (B) Actuation versus Size. ( B) Actuation versus Size.

Actuators 2018, 7, 50 13 of 15 Actuators 2018, 7, x FOR PEER REVIEW 13 of 15

Figure 10. Finite Element Model Sensitivity Analysis. Figure 10. Finite Element Model Sensitivity Analysis. 4.4. Model: Sensitivity Analysis 4.4. Model: Sensitivity Analysis The radar plot seen in Figure 10 depicts the results of the sensitivity analysis from the finite Theelement radar analysis plot seenmodel. in The Figure results 10 showe depictsd the the actuator results size, of thickness, the sensitivity and voltage analysis supplied from played the finite elementthe analysislargest roles model. in the The final results displacement showed thevalues actuator, which size, directly thickness, correlates and with voltage the experimental supplied played the largestobservations. roles inSize the had final the displacement largest sensitivity values,, with whicha value directly of 0.685, correlates followed by with thickness the experimental, with a observations.value of 0.441, Size hadand thevoltage largest, with sensitivity, a value of with0.278. a The value effect of 0.685,of the material’s followed bydielectric thickness, constant with had a value a smaller effect, with a value of 0.054, while the effect of temperature was negligible, at −5.68 × 10−5. of 0.441, and voltage, with a value of 0.278. The effect of the material’s dielectric constant had a smaller −5 effect,5. with Discussion a value of 0.054, while the effect of temperature was negligible, at −5.68 × 10 .

5. DiscussionThe conducted research demonstrates both through modeling and experimentally that an industrial grade flexible fluoropolymer material can function as an electroactive polymer, and can Thealso conductedbe producedresearch in a single demonstrates process incorporating both through both modeling chemical andbased experimentally pre-strain and thatthe addition an industrial gradeof flexible a conductive fluoropolymer layer. The material effectiveness can function of the chemical as an electroactive agent in inducing polymer, chemical and can pre also-strain be produced was in a singlesuccessfully process demonstrated incorporating, and both shown chemical to be basedprimarily pre-strain reversible. and Model the addition and experimental of a conductive results layer. The effectivenessdemonstrate up of theto at chemical least 1 mm agent displacement in inducing in samples chemical with pre-strain an active was area successfully of 17.35 mm demonstrated,2. Sample and shownactuation to beshowed primarily an immediate reversible. fast Model response and experimentaltime, followed results by a slow demonstrate material creep. up to atMaterial least 1 mm displacementactuation was in samples also voltage, with active an active area, areaand measurement of 17.35 mm location2. Sample dependent. actuation Material showed displacement an immediate was greatest farthest away from the inactive boundaries of the materials. This trend was seen in both fast response time, followed by a slow material creep. Material actuation was also voltage, active area, experimental and modeled data. Variations in experimental results and discrepancies from model to and measurement location dependent. Material displacement was greatest farthest away from the experimental data were attributed to variation in material thickness and lacking uniformity due to inactivethe lab boundaries scale manufacturing of the materials. process. This process trend waswill be seen improved in both in experimentalfuture work to andprovide modeled a more data. Variationsuniform in experimentalresponse. In conclusion, results and a novel discrepancies material and from process model forto the experimental production of data industrial were grade attributed to variationelectroactive in material polymers thickness was demonstrated and lacking, and uniformity its effectiveness due totested. the lab scale manufacturing process. This process will be improved in future work to provide a more uniform response. In conclusion, a novel6. Patents material and process for the production of industrial grade electroactive polymers was demonstrated,1 Newell, and B.A. its; Krutz, effectiveness G.W. Electroactive tested. Actuators, Systems Equipped Therewith, and Methods of Use and Manufacture. U.S. Patent 9683663, 20 June 2017. 6. Patents2 Newell, B.A.; Krutz, G.W. Electroactive Polymers, Methods of Manufacture, and Structures Formed Thereof. U.S. Patent Application 15556696, 10 March 2016. 1. Newell, B.A.; Krutz, G.W. Electroactive Actuators, Systems Equipped Therewith, and Methods of Use and Manufacture. U.S. Patent 9683663, 20 June 2017. 2. Newell, B.A.; Krutz, G.W. Electroactive Polymers, Methods of Manufacture, and Structures Formed Thereof. U.S. Patent Application 15556696, 10 March 2016. Actuators 2018, 7, 50 14 of 15

Author Contributions: B.N. was responsible for design of the experiments, execution of the studies, data analysis and manuscript preparation. J.G. was responsible for formal data analysis, manuscript preparation, and reviewing and editing. G.K. was responsible for design of experiments, supervision, and reviewing. Funding: This research was funded by Parker Hannifin Inc. Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors provided technical expertise in regards to this research.

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