Synthesis and Electrorheological Response of Graphene Oxide/Polydiphenylamine Microsheet Composite Particles

polymers

Article Synthesis and Electrorheological Response of Graphene Oxide/Polydiphenylamine Microsheet Composite Particles

Chun Yan Gao, Min Hwan Kim, Hyoung-Joon Jin and Hyoung Jin Choi *

Department of Polymer Science and Engineering, Inha University, Incheon 22212, Korea; [email protected] (C.Y.G.); [email protected] (M.H.K.); [email protected] (H.-J.J.) * Correspondence: [email protected]; Tel.: +82-32-860-7486

Received: 10 August 2020; Accepted: 26 August 2020; Published: 31 August 2020

Abstract: Conducting graphene oxide/polydiphenylamine (GO/PDPA) microsheet nanocomposite particles were fabricated via in-situ oxidative polymerization using diphenylamine in the presence of GO. The morphological structures and dimensions of the fabricated GO/PDPA composites were evaluated using transmission electron microscopy and scanning electron microscopy. Electrorheological (ER) responses and creep behaviors of an ER fluid consisting of the GO/PDPA composites when suspended in silicone oil were evaluated using a rotational rheometer under input electric field. Three different types of yield stresses were examined along with dielectric analysis, demonstrating their actively tunable ER behaviors.

Keywords: conducting polymer; graphene oxide; polydiphenylamine; electrorheological

1. Introduction Intelligent and smart fluids have drawn widespread attentions from industries and academia because of their sensitivities to applied stimuli, including electric and magnetic fields, pressure, temperature, and light [1,2]. Among such fluids, electrorheological (ER) fluids, which are usually colloidal suspensions comprising polarizable or semiconductive solid materials dispersed in insulating phase media, have been one of the most investigated smart materials [3]. Their electrical responsive properties include agile and reversible transformations between a liquid- and a solid (or chain)-like phase with and without an input electrical field, respectively, in a period of the order of milliseconds [4,5]. Without an electric field, the particles in the suspension randomly dispersed in insulating liquids behave as Newtonian fluids. In contrast, by applying an electric field, the randomly suspended particles instantaneously arrange in a column-like form following the field direction owing to dipole–dipole interactions between the polarized particles. The aforementioned changes in the ER suspension state lead to significant differences in the rheological behaviors compared to those of the smart fluid without the input electrical field. The increase in the shear viscosities, yield stresses, and viscoelastic moduli depends on the magnitude of the external electrical field strength; in this case, the ER fluids generally behave similar to a Bingham fluid or other types of non-Newtonian fluids. Particularly, the tightly arranged chain-like structures can relax into an irregular liquid phase once the external electric field is removed; this process is repeatable, manageable, reversible, and instantaneous. Owing to these valuable properties, they have numerous industrial applications in various fields, including those involving smart robots, anti-vibration systems, actuators, and food processing [6–8]. Over the past several decades, numerous smart ER materials, including cellulose, oxide minerals, carbon nanotube, graphene, graphene oxide (GO), ionic liquids, and conducting and semiconducting polymers and their composites, have been studied [9–12]. Among these ER materials, GO is highly

Polymers 2020, 12, 1984; doi:10.3390/polym12091984 www.mdpi.com/journal/polymers Polymers 2020, 12, 1984 2 of 18 promising owing to its characteristic structure and properties such as electric conductivity, flexibility, large surface area, thermal stability, and good hydrophilic behavior. The oxygen associated parts on the surface of GO increase its dispersion stabilities in various solvents in addition to increasing the possibility of the formation of composites between GO and other substances. In contrast with neat graphene, 6 the poorly conductive oxidation state of GO (10− S/cm) is suitable for its ER applications. In their pioneering report, Zhang et al. [13] presented the synthesis of pure GO and the ER characteristics of GO when it is suspended in silicone oil. The ER performance of GO is lower than that of other conducting polymers, probably due to its flexible property even without an electrical field. Moreover, uniform-size core–shell-typed polystyrene (PS)/GO composite particles produced via the π–π interaction between the surfaces of the PS spheres and GO sheets, and their ER behaviors under an input electrical field, have been investigated [14]. Lee et al. [15] have reported the fabrication of Fe3O4/SiO2/GO spherical composites having dual stimulus responses in an electric and magnetic field-based smart fluid. Owing to the SiO2 and magnetic Fe3O4, the yield stress and modulus of the smart fluid under the electrical field were considerably smaller than those under the magnetic field. Other GO-based composites, such as polyaniline (PANI)/GO [16], poly(p-phenylenediamine)/GO [17], and poly(methyl methacrylate)/GO composites, were also studied [18]. For the high-conductivity PANI/GO composite, a dedoping step must be performed before the fabrication of the ER fluid. Conductive polymers with π-conjugated structures, such as PANI, have attracted considerable attention with regard to ER materials owing to their inherent polarizations, easy changes in conductivities, and low densities. Their conductivities can be adjusted through doping or dedoping to achieve a suitable semiconducting range to avoid electrical short circuits under an external electric field. Recently, we used polydiphenylamine (PDPA) to develop an ER material owing to its simple synthesis, low density of 0.96 g/cm3 [19], good optical properties, eco-friendliness, and good environmental stability [20]. In comparison to PANI, a typical conducting polymer used in ER fluids, PDPA provides a higher thermal stability, higher compatibility with organic solvents, and lower conductivity. Thus, it can be used directly for designing ER fluids without performing dedoping, which is required for most conductive polymers [21]. In this study, GO/PDPA microsheet particles were fabricated, with a density 1.45 g/cm3 lower than that of neat GO (1.78 g/cm3), via in-situ oxidative polymerization [13]. The numerous π bonds on the GO surface serve as a crucial factor for the occurrence of polymerization on the GO surface. Moreover, the ER responses and creep properties of a semiconducting GO/PDPA-composite-based ER suspension were tested.

2. Experimental

2.1. Materials and Synthesis Graphite (99.99%, powder size < 45 µm, Sigma-Aldrich, St. Louis, Missouri, USA) was adopted as a raw material for the synthesis of GO. Potassium permanganate (KMnO4, Sigma-Aldrich, St. Louis, MO, USA), sodium nitrate (NaNO3, Junsei Chem., Tokyo, Japan), and sulfuric acid (98%, DC Chem., Seoul, Korea) were employed, as purchased, without further purification treatment. Diphenylamine (DPA) (99%, Sigma-Aldrich, St. Louis, MO, USA) and ammonium persulfate (APS) (98%, Daejung Chem., Shiheung, Korea) were adopted as a monomer and initiator for the chemical oxidation polymerization, respectively. GO sheets were synthesized with graphite using the modified Hummers process [22]. NaNO3 (1.0 g) was initially dispersed in a concentrated sulfuric acid solution (140 mL) under continuous stirring at 450 rpm until the solution cooled to room temperature. Graphite (2 g) and KMnO4 (6 g) were then added into the sulfonic acid solution. After stirring for 30 min, the solution was sonicated for 15 min and stirred continuously for 2 h at 150 rpm. Subsequently, the suspension was slowly added to deionized water (350 mL). Finally, a 30% hydrogen peroxide solution (300 mL) was rapidly introduced. Stirring was continued for 0.5 h to obtain the final GO solution. All of the aforementioned Polymers 2020, 12, 1984 3 of 18 Polymers 2020, 12, 1984 3 of 18

GO/PDPA microsheet composites were fabricated via in-situ oxidative polymerization, as shownsteps, except in Scheme the sonication 1. The monomer process, were DPA carried (0.025 out mol) in a was ventilated dispersed facility. in ethanol The mixture (100 was mL) washed under magneticusing deionized stirring water for 20 via min centrifugation at 4 °C. The and solution freeze-dried of GO to (0.37 obtain g) puriﬁed dispersed GO. in ethanol (150 mL) pretreatedGO/PDPA using microsheet a sonicator composites for 2 h was were then fabricatedadded. The via initiator in-situ APS oxidative (0.025 polymerization, mol), in deionized as shownwater (50in SchememL), was1. Theintroduced monomer to the DPA above (0.025-mentioned mol) was suspension. dispersed in The ethanol polymerization (100 mL) under was allowed magnetic to continuestirring forfor 20a day min at at 4 4°C.◦C. The The product solution was of cleaned GO (0.37 with g) ethanol dispersed and in deionized ethanol (150water mL) several pretreated times, andusing the an sonicator dried in for vacuum. 2 h was Through then added. the polymerizationThe initiator APS-promoting (0.025 mol), π–π in interactions deionized water between (50 mL), the surfaceswas introduced of the GO to thesheets above-mentioned and the DPA monomer, suspension. PDPA The was polymerization uniformly attached was allowed to the to surface continue of GO.for a The day ER at 4fluid◦C. The(10 productvol.%) was was prepared cleaned withby suspending ethanol and the deionized GO/PDPA water microsheet several times, naocomposite and then particlesdried in vacuum.in silicone Through oil (100 thecS) polymerization-promotingthrough ultrasonication. π–π interactions between the surfaces of the GO sheetsOn the and other the hand, DPA it monomer, can be also PDPA noted was that uniformly the reduced attached GO/PDPA to the has surface been ofsynthesized GO. The ER via ﬂuid the electrochemical(10 vol.%) was prepared polymerization by suspending of DPA the GO on/PDPA reduced microsheet GO, naocomposite along with particles its various in silicone optical oil chara(100 cS)cterizations through ultrasonication.[23], while Muthusankar et al. [24] studied electrochemical behaviors of PDPA/phosphotungstic/GO hybrid.

Scheme 1. Schematic diagram of the synthetic process of graphene oxide/polydiphenylamine Scheme 1. Schematic diagram of the synthetic process of graphene oxide/polydiphenylamine (GO/PDPA) microsheet composites. (GO/PDPA) microsheet composites. On the other hand, it can be also noted that the reduced GO/PDPA has been synthesized 2.2.via Characterization the electrochemical polymerization of DPA on reduced GO, along with its various optical characterizationsThe morphological [23], while structures Muthusankar and sizes ofet the al. [neat24] studiedGO sheets electrochemical and GO/PDPA behaviors microsheet of compositesPDPA/phosphotungstic were evaluated/GO hybrid. using transmission electron microscopy (TEM) (CM-220, Phillips, Amsterdam, The Netherlands) and high-resolution scanning electron microscopy (HRSEM) (SU-8010, 2.2. Characterization Hitachi, Tokyo, Japan). The chemical structures were evaluated using the infrared spectra of mixtures of KBrThe pellets morphological and the samples structures via Fourier and sizes-transform of the infrared neat GO (FTIR) sheets spectroscopy and GO/PDPA (VERTEX microsheet 80 V, Bruker,composites Ettlingen, were evaluated Germany using). X transmission-ray diffraction electron (XRD) microscopy (DMAX (TEM)-2500, (CM-220, Rigaku, Phillips,Tokyo, Japan) Amsterdam, was performedThe Netherlands) to characterize and high-resolution the crystalline scanning structures electron of microscopythe neat GO (HRSEM) and GO/PDPA (SU-8010, composites. Hitachi, Tokyo, The electricalJapan). The conductivit chemicalies structures of pure GO were sheets evaluated and GO/PDPA using the composites infrared spectra were oftested mixtures by a low of KBr resistivity pellets meterand the (Loresta samples-GP via MCP Fourier-transform-T610, Mitsubishi infrared chem. (FTIR), Tokyo, spectroscopy Japan) equipped (VERTEX with 80a square V, Bruker, probe Ettlingen, (MCP- TPQPP,Germany). Mitsubishi X-ray di ﬀchemraction., Tokyo, (XRD) Japan). (DMAX-2500, The alignment Rigaku, Tokyo,of the GO/PDPA Japan) was-based performed ER suspension to characterize was testedthe crystalline directly structuresvia optical of microscopy the neat GO (OM) and GO(BX51,/PDPA Olympu composites.s, NY, USA) The electrical at a direct conductivities-current voltage of pure of 300GO V. sheets The anddielectric GO/PDPA properties composites of the wereGO/PDPA tested-composite by a low resistivity-based ER meterfluid were (Loresta-GP analyzed MCP-T610, using an inductanceMitsubishi– chem.,capacitance Tokyo,–resistance Japan) equipped (LCR) meter with a (HP square 4284A probe Precision (MCP-TPQPP,, Agilent, Mitsubishi Santa Clara, chem., CA,Tokyo, USA) withJapan). an TheHP16452A alignment liquid of the test GO fixture./PDPA-based The ER ER characteristics suspension was of the tested conducting directly via composite optical microscopy-based ER fluid(OM) were (BX51, measured Olympus, using NY, USA)a rotational at a direct-current rheometer (MCR voltage 300, of 300Anton V. The Paar dielectric-Physica, properties Graz, Austria) of the

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GO/PDPA-composite-based ER fluid were analyzed using an inductance–capacitance–resistance (LCR) meter (HP 4284A Precision, Agilent, Santa Clara, CA, USA) with an HP16452A liquid test fixture. ThePolymers ER 2020 characteristics, 12, 1984 of the conducting composite-based ER fluid were measured using a rotational4 of 18 rheometerPolymers 2020 (MCR, 12, 1984 300, Anton Paar-Physica, Graz, Austria) comprising a direct-current high-voltage4 of 18 generatorcomprising (HCP a direct 14-12-current 500 MOD, high FuG-voltage Elektronik generator GmbH, (HCP Rosenheim, 14-12 500 MOD, Germany) FuG with Elektronik a Couette-type GmbH, comprising a direct-current high-voltage generator (HCP 14-12 500 MOD, FuG Elektronik GmbH, cellRosenheim, (CC17). Germany) with a Couette-type cell (CC17). Rosenheim, Germany) with a Couette-type cell (CC17). 3. Results and Discussion 3. Results and Discussion 3.1. Material P Propertyroperty 3.1. Material Property FiguresFigures1 1 and and2 2show show the the surface surface morphologies morphologies and and dimensions dimensions of of the the microscale microscale pure pure GO GO and and GOGO/PDPA/PDPAFigure compositecomposites 1 and 2 show platelets.platelets. the surface The HRSEM morphologies images show and dimensions the pure GO of sheets the microscale of a few micron pure GO grades and (Figure(GO/PDPAFigure1 1).). TheircompositeTheir surfacessurfaces platelets. areare veryvery The smooth.HRSEMsmooth. images In the GO GO/PDPAshow/PDPA the pure sheet GO composite, sheets of aPDPA few micron is uniformly grades polymerized(Figure 1). Their on thesurfaces GO surface, are very yielding smooth. a rougher In the GO/PDPA appearance sheet ( (FigureFigure composite, 22)) thanthan thatthatPDPA ofof thethis euniformly purepure GOGO sheet.polymerized The The sizes sizes on of the the GO fabricated surface, sheets yielding are a slightly rougher smaller appearance when ( Figuredetermined 2) than using that the of TEMthe pure images GO thansheet. thosethose The measuredsizesmeasured of the using usingfabricated the the SEM SEM sheets images. images. are slightly This This might mightsmaller be explainedbe when explained determined by theby factthe using thatfact thethat the samples TEMthe samples images were ultrasonicatedwerethan thoseultrasonicated measured in ethanol in using ethanol to facilitatethe to SEM facilitate the images. TEM the observations, ThisTEM might observations, be which explained ledwhich to by small led the tochanges factsmall that changes in the the samples sizes in the of thesizeswere sheet ofultrasonicated the samples. sheet samples. in ethanol to facilitate the TEM observations, which led to small changes in the sizes of the sheet samples.

Figure 1. HRSEM images of pure GO microsheets. Figure 1. HRSEM images of pure GO microsheets.

Figure 2. TEM images of (a) pure GO and (b,c) GO/PDPA microsheet composites. Figure 2. TEM images of (a) pure GO and (b, c) GO/PDPA microsheet composites. Figure 2. TEM images of (a) pure GO and (b, c) GO/PDPA microsheet composites. XRD and FTIR spectroscopy experiments were performed to characterize the crystalline structuresXRD and and chemical FTIR spectroscopy components experimentsof the pure GO were and the performed GO/PDPA to microsheet characterize composite the crystalline (Figure 3).structures The GO and sheets chemical and GO/PDPA components composite of the pure exhibit GO the and typical the GO/PDPA (002) and microsheet (101) peaks composite at approximately (Figure 26.2°3). The and GO 42.0°, sheets respectively; and GO/PDPA the composite peaks are exhibit attributed the typical to the (002) graphitic and (101) framework. peaks at Theapproximately GO sheets exhibit26.2° and a sharp 42.0°, peak respectively; corresponding the peaks to the are (001) attributed plane at to a the 2θ graphiticof approximately framework. 11.8°, The whereas GO sheets the exhibit a sharp peak corresponding to the (001) plane at a 2θ of approximately 11.8°, whereas the

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XRD and FTIR spectroscopy experiments were performed to characterize the crystalline structures and chemical components of the pure GO and the GO/PDPA microsheet composite (Figure ??). The GO sheets and GO/PDPA composite exhibit the typical (002) and (101) peaks at approximately 26.2◦ and 42.0◦, respectively; the peaks are attributed to the graphitic framework. The GO sheets exhibit a sharp peak corresponding to the (001) plane at a 2θ of approximately 11.8◦, whereas the GO/PDPA composite exhibits peaks corresponding to both the (001) GO base plane (2θ = 10.0◦) and PDPA (2θ = 17.9◦), which are shiftedPolymers 2020 from, 12, 1984 those of the pure GO and PDPA [25,26]. The FTIR spectra of the GO,5 of 18 PDPA and 1 GO/PDPA composite show high and broad absorbance signals at about 3430 cm− owing to the O–H GO/PDPA composite exhibits peaks corresponding to both the (001) GO base plane (2θ = 10.0°) and stretching vibration. The neat GO sheets exhibit a distinctive signal at 1704 cm 1, originated to the C–O PDPA (2θ = 17.9°), which are shifted from those of the pure GO and PDPA [25,26].− The FTIR spectra 1 stretchingof vibration the GO, PDPA of carboxylic and GO/PDPA acid composite group. show The peakshigh and at broad 1616 absorbance and 1382 signals cm− atare about attributed 3430 to the O–H vibrationcm−1 owing in water to the and O–H to stretching CO–H groupsvibration. in The GO, neat respectively. GO sheets exhibit Peaks a distinctive corresponding signal at 1704 to the C–O–C cm−1, originated to the C–O stretching vibration of carboxylic1 acid group. The peaks at 1616 and 1382 and C–O bonds are observed at 1228 and 1054 cm− , respectively [14,27]. In the FTIR spectrum of cm−1 are attributed to the O–H vibration in water and to CO–H groups in GO, respectively. Peaks GO/PDPA composites, not only the characteristic peaks of GO but also the characteristic peaks of PDPA corresponding to the C–O–C and C–O bonds are observed at 1228 and 1054 cm−1, respectively [14,27]. 1 are observedIn the [19 FTIR]. The spectrum GO/PDPA of GO/PDPA composite composites, exhibits not peaksonly the at characteristic approximately peaks 2921of GO and but 833also the cm − , owing 1 to the aromaticcharacteri C–Hstic stretching peaks of PDPA vibrations. are observed The absorption [19]. The GO/PDPAsignals at composite 1595, 1494, exhibits 1170, peaks and 700 at cm− are attributedapproximately to N–H bending 2921 and vibrations, 833 cm−1, owing C=C stretchingto the aromatic of theC–H benzenoid stretching vibrations. ring, C–H The in-plane absorption bending of signals at 1595, 1494, 1170, and 700 cm−1 are attributed to N–H bending vibrations, C=C stretching of diphenoquinone, and C–H bending of the 1,4-substituted benzene ring, respectively [21,28]. These the benzenoid ring, C–H in-plane bending of diphenoquinone, and C–H bending of the 1,4- typical spectrasubstituted are attributed benzene ring, to respectively the contribution [21,28]. These of the typical PDPA. spectra are attributed to the contribution of the PDPA.

Figure 3. Figure(a) X-ray 3. (a) X- diffractionray diffraction profiles, (b) FTIR (b) data FTIR of pure data GO of, PDPA pure, and GO, GO/PDPA PDPA, microsheet and GO /PDPA microsheetcomposites. composites.

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3.2.3.2. ER ER and and Creep Creep Properties Properties TheThe ER ER fluidfluid was was formed formed using using the semiconducting the semiconducting GO/PDPA GO/PDPA microsheets microsheets (10 vol.%), (10 suspended vol.%), 8 suspendedin silicone in oil. silicone The electrical oil. The electrical conductivity conductivity of the GO of/PDPA the GO/PDPA microsheets microsheets was 8.0 was10− 8.0S ×/cm 10− without8 S/cm 6 × withoutdedoping, dedoping compared, compared to 1.0 10to −1.0S /×cm 10 of−6 theS/cm pure of the GO, pure which GO is, withinwhich theis wi idealthin range the ideal for ER range materials for 9 11 × ER(10 materials− –10− (10S/cm)−9–10 with−11 S/cm) regard with to avoidingregard to anavoiding electric an short electric circuit short [29 ].circuit [29]. FigureFigure 44 presents presents the the conducting conducting property property of of the the GO/PDPA GO /PDPA microsheet microsheet-based-based ER ER suspension. suspension. WithoutWithout an an electric electric field, field, OM OM shows shows that that the the GO/PDPA GO/PDPA sheets sheets were were randomly randomly dispersed dispersed in in the the carrier carrier oil,oil, whereas whereas the the composites composites start start to to move move to form to form flocculent flocculent fibrous fibrous microstructures microstructures along the along electrical the electricalfield direction field direction at an applied at an applied direct-current direct- voltagecurrent ofvoltage 300 V. of The 300 response V. The response sensitivity sensitivity and reversible and reversiblebehavior behavior of the microsheet-based of the microsheet ER- fluidbased are ER presented fluid are inpresented Figure5 (externalin Figure square-type 5 (external square voltage-type pulse 1 voltagetime: 20.0pulse s, time: constant 20.0 s, shear constant rate: shear 1.0 s −rate:). The1.0 s shear−1). The stress shear exhibits stress exhibits a square a squar pulsee responsepulse response almost almostwithout without hysteresis. hysteresis. The shear The shearstresses stresses instantaneously instantaneously change change at each at turning each turning point, and point, increase and increasewith the with electrical the electrical field, similar field, tosimilar that measured to that measured using flow using curves. flow Thiscurves. indicates This indicates that the GOthat/PDPA the GO/PDPAmicrosheet microsheet composite-based composite ER-based fluid isER highly fluid is sensitive highly sensitive to and reversible to and reversible under the under input the electrical input electricalfield. As field. shown As in shown Figure in5, underFigure a 5, stable under electric a stable field electric applied, field most applied of the, shearmost stressof the increases shear stress with increasestime, which with may time, be which because may the be longer because the electricthe longer field the is applied,electric field the strongeris applied, the the chain stron structureger the is. 1 chainIn addition, structure during is. In theaddition, shearing during process the at shearing a given shearprocess rate at of a 1.0given s− ,shear the chain rate structureof 1.0 s−1, is the destroyed chain structuredue to the is hydrodynamic destroyed due force to the and hydrodynamic the process of reorganization force and the due process to electrostatic of reorganization force occurs due at to all electrostatictimes. The decreaseforce occurs in shear at all stress times. may The be decr dueease to the in fact shear that stress the reorganized may be due chain to the structure fact that is notthe as reorganizedstable as the chain original structure chain is structure not as stable especially as the fororiginal lower chain electric structure fields. especially for lower electric fields.

FigureFigure 4. 4.OpticalOptical microscopic microscopic images images of ofGO/PDPA GO/PDPA microsheet microsheet composites composites-based-based electrorheological electrorheological (ER(ER)) fluid fluid (10 (10 vol.%) vol.%) when when the the electric electric field field was was (a ()a off) off andand (b (b) on) on (300 (300 V). V).

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Figure 5. Shear stress vs. time of the GO/PDPA microsheet composites-based ER fluid influenced by Figure 5. Shear stress vs. time of the GO/PDPA microsheet composites-based ER fluid1 influenced by alternating on–off state under different electric field at a fixed shear rate of 1.0 s− . alternating on–off state under different electric field at a fixed shear rate of 1.0 s−1. Dielectric characteristics (dielectric constant ε0 and dielectric loss ε”) are one of the crucial factors thatDielectric affect ER characteristics behavior. They (dielectric were measuredconstant ε’ usingand dielectric the LCR loss meter. ε’’) are The oneε 0ofand the εcrucial” values factors of the thatcomposite-sheet-based affect ER behavior. ER They fluid, were as functions measured of using the frequency, the LCR are meter. shown The in ε’ Figure and 6 ε”a. Furthermore, values of the ε0 composiand ε”te are-sheet related-based through ER fluid, the as Cole–Cole functions formula of the frequency, (Figure6b) are [30 ,shown31], in Figure 6a. Furthermore, ε’ and ε” are related through the Cole–Cole formula (Figure 6b) [30,31], ∆ε ε = ε + iε00 = ε + (1) ∗ε* = ε0’ + iε’’= ε∞ + 1 α (1) ∞ 1(+ )(iωλ ) − where ε0 is the dielectric constant when the frequency goes to zero and ε∞ is the permittivity when where ε0 is the dielectric constant when the frequency goes to zero and ε is the permittivity when ∞ thethe frequency frequency tends tends to toinfinity. infinity. The The difference difference Δε∆ andε and λ representλ represent the the polarizability polarizability and and polarization polarization relaxationrelaxation time time of ofthe the ER ERsuspension, suspension, respectively. respectively. The The index index 1−α 1in theα in range the rangeof 0 to of 1 0implies to 1 implies the − distributionthe distribution broadness broadness of the of relaxation the relaxation time. time. These These parameters parameters in the in the Cole Cole–Cole–Cole equation equation are are presentedpresented in inTable Table 1.1 .

TableTable 1. Optimum 1. Optimum values values of Cole of Cole–Cole–Cole formula formula for forGO/PDPA GO/PDPA sheet sheet composites composites-based-based ER ER fluid. ﬂuid.

Parameters ε0 ε∞ Δ ε = ε0 − ε∞ λ (s) α Parameters ε0 ε ∆ ε = ε0 ε λ (s) α value 4.45 2.91∞ 1.54 − 6∞ × 10−6 0.41 value 4.45 2.91 1.54 6 10 6 0.41 × −

PolymersPolymers2020 2020, 12, 12, 1984, 1984 88 ofof 1818

Figure 6. (a) Dielectric spectra (dielectric constant, ε : black square; dielectric loss, ε”: red circle) and Figure 6. (a) Dielectric spectra (dielectric constant, 0ε’: black square; dielectric loss, ε’’: red circle) and (b) Cole–Cole mode data of GO/PDPA microsheet composites-based ER ﬂuid (10 vol.%). (b) Cole–Cole mode data of GO/PDPA microsheet composites-based ER fluid (10 vol.%).

For ER behaviors of the ER suspension, both steady shear and dynamic oscillation measurements were successively carried out using a rotational rheometer. Figure 7 shows typical

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flow curve of shear stress vs. shear rate at different electric field strengths. Without the electrical field, the ER fluid exhibits Newtonian fluid-like behaviors, including a linear increase in shear stress with Polymersan2020 increase, 12, 1984 in shear rate, which is similar to the relationship between the dynamic modulus (storage9 of 18 or loss modulus) and angular frequency, observed in the oscillation test (Figure 8b). Under the electric field, the shear stress maintains relatively stable values regardless of the shear rate change at different Forapplied ER behaviors electrical field of the strengths, ER suspension, consistent both with steady the charact sheareristics and dynamic of Bingham oscillation fluids. measurements The solid werecurves successively in Figure carried 7 are outfitted using using a the rotational Bingham rheometer. fluid model Figure (Equation7 shows (2)) typical[32]. The flow corresponding curve of shear stressparameters vs. shear rateare given at diff inerent Table electric 2. τ and field ̇ strengths.are the shear Without stress and the shear electrical rate, field,respectively, the ER fluidand τ exhibits0 is Newtonianthe yield fluid-like stress. behaviors, including a linear increase in shear stress with an increase in shear rate, which is similar to the relationship between the dynamic modulus (storage or loss modulus) and τ = τ0 + η̇ (τ ≥ τ0) (2) angular frequency, observed in the oscillation test (Figure8b). Under the electric field, the shear stress maintains relatively stable values regardless of the shear rate change at different applied electrical field Table 2. Optimal values for Bingham model on the basis of the flow curves for GO/PDPA composites- strengths,based consistent ER fluid with (10 vol.%). the characteristics of Bingham fluids. The solid curves in Figure7 are fitted using the Bingham fluid model (Equation (2)) [32]. The corresponding parameters are given in Table2. . Electric Field Strength (kV/mm) τ and γ are the shear stressModel and shearParameters rate, respectively, and τ0 is the yield stress. 0.3 0.6 0.9 1.2 1.5 1.8 τ 8.8 .15.1 23.5 37.1 59.9 76.1 Bingham 0 τ = τ0 + ηγ (τ τ0) (2) η 0.17 0.14 ≥ 0.13 0.12 0.07 0.13

102 (Pa)

 , 101

0 kV/mm 0.3 1.2 0.6 1.5

Shear stress Shear 0.9 1.8 100 10-1 100 101 102 . Shear rate, (1/s)

FigureFigure 7. Shear 7. Shear stress stress vs. vs. shear shear rate rate of of the the GOGO/PDPA/PDPA microsheet microsheet composites composites-based-based ER fluid ER fluid (10 vol.%) (10 vol.%) with diwithfferent different electric electric fields. fields. The The solid solid lines lines in in graph graph are fitting fitting from from the the Bingham Bingham fluid fluid equation. equation.

The dynamic behaviors of the GO/PDPA microsheet-based ER fluid were characterized via amplitude sweep tests at a constant angular frequency of 6.28 rad/s (Figure 8a). Without the external

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electrical field, the storage modulus (G’) is lower than the loss modulus (G’’), exhibiting more viscous nature of the ER fluid. With an induced electric field, the ER fluid gives elastic characteristics (higher G’) originating from the closely aligned microsheets, as shown in Figures 4 and 5. Stepwise improvements in G’ and G’’ are observed with an increase in the applied electric field strength. In the low-strain region, the ER fluids tend to form stable chain-like structures; this region is referred to as the linear viscoelastic (LVE) region. Within the γLVE region, γ was fixed to 0.005% to perform the frequency sweep measurement at angular frequencies of 1 to 200 rad/s, as presented in Figure 8b. G’ is higher than G’’ with or without the input electrical field; thus, the elastic property precedes the Polymers 2020, 12, 1984 10 of 18 viscous property. When E = 0, the higher volume percent (10 vol.%) and lower constant strain (0.005%) of the ER fluid promote the fluid’s elastic behavior.

Figure 8. (a) Storage (closed) and loss (open) modulus vs. strain curves and (b) storage (closed) and Figure 8. (a) Storage (closed) and loss (open) modulus vs. strain curves and (b) storage (closed) and loss (open) modulus vs. angular frequency curves of the GO/PDPA microsheet composites-based ER loss (open) modulus vs. angular frequency curves of the GO/PDPA microsheet composites-based ER fluid (10 vol.%) with different electric fields. fluid (10 vol.%) with different electric fields. Table 2. Optimal values for Bingham model on the basis of the flow curves for GO/PDPA composites-based ER fluid (10 vol.%).

Electric Field Strength (kV/mm) Model Parameters 0.3 0.6 0.9 1.2 1.5 1.8 τ 8.8 15.1 23.5 37.1 59.9 76.1 Bingham 0 η 0.17 0.14 0.13 0.12 0.07 0.13 Polymers 2020, 12, 1984 11 of 18

The dynamic behaviors of the GO/PDPA microsheet-based ER fluid were characterized via amplitude sweep tests at a constant angular frequency of 6.28 rad/s (Figure8a). Without the external electrical field, the storage modulus (G0) is lower than the loss modulus (G”), exhibiting more viscous nature of the ER fluid. With an induced electric field, the ER fluid gives elastic characteristics (higher G0) originating from the closely aligned microsheets, as shown in Figures4 and5. Stepwise improvements in G0 and G” are observed with an increase in the applied electric field strength. In the low-strain region, the ER fluids tend to form stable chain-like structures; this region is referred to as the linear viscoelastic (LVE) region. Within the γLVE region, γ was fixed to 0.005% to perform the frequency sweep measurement at angular frequencies of 1 to 200 rad/s, as presented in Figure8b. G 0 is higher than G” with or without the input electrical field; thus, the elastic property precedes the viscous property. When E = 0, the higher volume percent (10 vol.%) and lower constant strain (0.005%) of the ER fluid promote the fluid’s elastic behavior. The elastic stress data in Figure9a were calculated based on τ0 = G0γ, obtained from Figure8a, where the elastic yield stress in the ER fluid is the maximum stress that enables complete restoration when the input electrical field is turned off; the maximum stress is obtained using the critical value, based on the changes in the slope. The static yield stress is the minimum shear stress necessary to ensure the fluidity of the ER fluid; this stress is observed using the controlled shear stress type (Figure9b). At the various applied electrical field strengths, we select the stress corresponding to the sharp decrease in the critical shear viscosity as the static yield stress; this is presented in Figure 10 along with the elastic and dynamic yield stresses. The power-law relations between the three different yield stresses and electric field strength (E) (Equation (3)) (m = 1.5) obey the conduction model for ER fluids. Note that the slope close to 1.5 has been also observed for ER fluids based on not only GO composite [33,34] but also other composite particles [35].

m τy E (3) ∝ The creep and recovery behaviors of the GO/PDPA composite-based ER fluid were tested under a fixed shear stress and fixed electric field, as shown in Figures 11–14. As a static test method, once a step change in stress is imposed during the creep test, immediate response of the strain is observed as a function of time. With this information, the creep compliance (J(t)), one of the indicators of creep characteristics, is defined as the ratio of the strain and shear stress (Equation (4)). For a constant shear stress of 7 Pa (Figures 11a, 12a and 13), the strain and creep compliance exhibit stepwise decreases with an increase in the applied electric field strength. This indicates that more stable chain-like structures are formed with the increase in electrical field to resist the constant shear stress. Notably, when the shear stress is applied or removed, the ER fluid immediately undergoes a strain or recovers from the strain without delay, which is different from the ER fluid creep phenomenon with time delay [36,37]. The creep test has been also performed for an ER elastomer [38]. The recovery ratio (χ) is the ratio of the recoverable strain (γR) to the maximum strain (γmax)[39],

J(t) = (γ(t))/τ (4)

X = γ /γmax 100% (5) R × Polymers 2020, 12, 1984 11 of 18

The elastic stress data in Figure 9a were calculated based on τ’ = G’γ, obtained from Figure 8a, where the elastic yield stress in the ER fluid is the maximum stress that enables complete restoration when the input electrical field is turned off; the maximum stress is obtained using the critical value, based on the changes in the slope. The static yield stress is the minimum shear stress necessary to ensure the fluidity of the ER fluid; this stress is observed using the controlled shear stress type (Figure 9b). At the various applied electrical field strengths, we select the stress corresponding to the sharp decrease in the critical shear viscosity as the static yield stress; this is presented in Figure 10 along with the elastic and dynamic yield stresses. The power-law relations between the three different yield stresses and electric field strength (E) (Equation (3)) (m = 1.5) obey the conduction model for ER fluids. Note that the slope close to 1.5 has been also observed for ER fluids based on not only GO composite [33,34] but also other composite particles [35]. Polymers 2020, 12, 1984 12 of 18 τy ∝ Em (3)

Figure 9. (aFigure) Elastic 9. (a stress) Elastic vs. stress strain vs. strain curves curves and and (b (b)) viscosity viscosity vs. vs. shear shear stress stress curves curves of the GO/PDPA of the GO /PDPA microsheetPolymersmicrosheet composites-based 2020, 12 , composites1984 - ERbased fluid ER fluid (10 vol.%)(10 vol.%) with with didifferentfferent electric electric fields. fields. 12 of 18

Figure 10. Yield stress vs. electric ﬁeld strength curves of the GO/PDPA microsheet composites-based Figure 10. Yield stress vs. electric field strength curves of the GO/PDPA microsheet composites-based ER ﬂuid (10 vol.%).ER fluid (10 vol.%).

The creep and recovery behaviors of the GO/PDPA composite-based ER fluid were tested under a fixed shear stress and fixed electric field, as shown in Figures 11–14. As a static test method, once a step change in stress is imposed during the creep test, immediate response of the strain is observed as a function of time. With this information, the creep compliance (J(t)), one of the indicators of creep characteristics, is defined as the ratio of the strain and shear stress (Equation (4)). For a constant shear stress of 7 Pa (Figures 11a, 12a, and 13), the strain and creep compliance exhibit stepwise decreases with an increase in the applied electric field strength. This indicates that more stable chain-like structures are formed with the increase in electrical field to resist the constant shear stress. Notably, when the shear stress is applied or removed, the ER fluid immediately undergoes a strain or recovers from the strain without delay, which is different from the ER fluid creep phenomenon with time delay [36,37]. The creep test has been also performed for an ER elastomer [38]. The recovery ratio (χ) is the ratio of the recoverable strain (γR) to the maximum strain (γmax) [39], J(t) = (γ(t))/τ (4)

Χ = γR/γmax × 100% (5) The recovery ratio gradually improves with an increased field strength; all cases correspond to reversible deformations at the fixed stress of 7 Pa. For the constant electric field strength of 1 kV/mm (Figures 11b, 12b, and 14), the strain and creep compliance steadily increase with the shear stress, up to 16 Pa. At a shear stress of 20 Pa, the ER fluid is immediately deformed and exhibits a continuously increasing strain delay until the stress is removed (it is almost irreversibly deformed). In other words, when E = 1 kV/mm, the shear stress of 20 Pa reaches the yield value, similar to the elastic yield stress in Figure 10. In addition, under a fixed electric field, with an increasing sustained shear stress, the recovery ratio gradually decreases until it approaches 0.

Polymers 2020, 12, 1984 13 of 18 Polymers 2020, 12, 1984 13 of 18

FigureFigure 11. Strain11. Strain vs. vs. time time under under ( (aa)) constantconstant stress stress and and (b ()b constant) constant electric electric field ﬁeld strength strength of GO/PDPA of GO/PDPA microsheetmicrosheet composites-based composites-based ER ER ﬂuid fluid (10 (10 vol.%).vol.%).

Polymers 2020, 12, 1984 14 of 18 Polymers 2020, 12, 1984 14 of 18

FigureFigure 12. 12.Creep Creep compliance compliance vs. vs. time under (a (a) )constant constant stress stress and and (b) (constantb) constant electric electric field stren ﬁeldgth strength of GOof /GO/PDPAPDPA microsheet microsheet composites-based composites-based ER fluid ﬂuid (10 (10 vol.%). vol.%).

Polymers 2020, 12, 1984 15 of 18

PolymersPolymers2020, 122020, 1984, 12, 1984 15 of 1518 of 18

FigureFigure 13. Recovery13. Recovery ratio ratio vs. vs. electric electric ﬁeld field strength strength of of GO GO/PDPA/PDPA microsheet microsheet composites-based composites-based ER ﬂuid fluid (10 vol.%). (10 vol.%).Figure 13. Recovery ratio vs. electric field strength of GO/PDPA microsheet composites-based ER fluid (10 vol.%).

FigureFigure 14. 14.Recovery Recovery ratio ratio vs. vs. stress stress ofof GOGO/PDPA/PDPA microsheet microsheet composites composites-based-based ER fluid ER ﬂuid (10 vol.%). (10 vol.%).

Figure 14. Recovery ratio vs. stress of GO/PDPA microsheet composites-based ER fluid (10 vol.%). The recovery ratio gradually improves with an increased field strength; all cases correspond to reversible deformations at the fixed stress of 7 Pa. For the constant electric field strength of 1 kV/mm

(Figures 11b, 12b and 14),Preparation of graphitic oxide. the strain and creep compliance steadily increase with the shear stress, up to 16 Pa. At a shear stress of 20 Pa, the ER fluid is immediately deformed and exhibits a continuously increasing strain delay until the stress is removed (it is almost Polymers 2020, 12, 1984 16 of 18 irreversibly deformed). In other words, when E = 1 kV/mm, the shear stress of 20 Pa reaches the yield value, similar to the elastic yield stress in Figure 10. In addition, under a fixed electric field, with an increasing sustained shear stress, the recovery ratio gradually decreases until it approaches 0.

4. Conclusions We successfully synthesized conducting composites of GO/PDPA microsheets, with a density lower than that of neat GO sheets, via in-situ oxidative polymerization. The PDPA uniformly adsorbed on the GO surface through π–π interactions. The electro-responsive ER behaviors of the GO/PDPA composites dispersed in silicone oil were analyzed using a rotational rheometer at various external electrical field strengths. In the steady shear tests, the GO/PDPA composite-based ER fluid showed a Newtonian fluid-like behavior without an electric field; these behaviors transformed into Bingham plastic characteristics under an input electric field. All three different types of yield stresses obtained from different rheological measurements showed a slope of 1.5, confirming its conductivity mechanism. Dielectric characteristics from the dielectric spectra were also found to fit well with Cole–Cole equation, being well correlated with the ER performance. According to dynamic oscillation tests, the storage modulus and loss modulus increased substantially under applied electric fields, which indicated the formation of chain-like structures in the ER fluid. Furthermore, from the creep and recovery tests under the applied electric field, it also showed distinctive solid-like properties in agreement with the observation from the steady shear and oscillation tests.

Author Contributions: Conceptualization, C.Y.G., M.H.K., and H.J.C.; investigation, C.Y.G. and M.H.K.; writing—original draft preparation, C.Y.G.; writing—review and editing, H.-J.J. and H.J.C.; funding acquisition, H.J.C. and H.-J.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Research Foundation of Korea (2018R1A4A1025169). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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