The Effect of Dielectric Polarization Rate Difference of Filler and Matrix on the Electrorheological Responses of Poly(Ionic

polymers

Article The Eﬀect of Dielectric Polarization Rate Diﬀerence of Filler and Matrix on the Electrorheological Responses of Poly(ionic liquid)/Polyaniline Composite Particles

Chen Zheng, Qi Lei, Jia Zhao, Xiaopeng Zhao and Jianbo Yin *

Smart Materials Laboratory, Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710129, China; [email protected] (C.Z.); [email protected] (Q.L.); [email protected] (J.Z.); [email protected] (X.Z.) * Correspondence: [email protected]; Tel.: +86-029-88431662

Received: 1 March 2020; Accepted: 20 March 2020; Published: 22 March 2020

Abstract: By using different conductivity of polyaniline as filler, a kind of poly(ionic liquid)/polyaniline composite particles was synthesized to investigate the influence of dielectric polarization rate difference between filler and matrix on the electrorheological response and flow stability of composite-based electrorheological fluids under simultaneous effect of shear and electric fields. The composite particles were prepared by a post ion-exchange procedure and then treated by ammonia or hydrazine to obtain different conductivity of polyaniline. Their electrorheological response was measured by dispersing these composite particles in insulating carrier liquid under electric fields. It showed that the composite particles treated by ammonia had the strongest electrorheological response and 1 1 most stable flow behavior in a broad shear rate region from 0.5 s− to 1000 s− . By using dielectric spectroscopy, it found that the enhanced electrorheological response with stable flow depended on the matching degree of the dielectric polarization rates between poly(ionic liquid) matrix and polyaniline filler. The closer their polarization rates are, the more stable the flow curves are. These results are helpful to design optimal composite-based electrorheological materials with enhanced and stable ER performance.

Keywords: composite particles; dielectric polarization rate; electrorheological responsive polymer

1. Introduction Polymer composites, consisting of at least two components, a polymer matrix and a filler, can exhibit enhanced physicochemical properties compared to neat polymers. Besides as structured materials, they have also been frequently used as functional and smart materials, such as high thermal conductivity materials, electromagnetic interface shielding or absorption materials, shape memory materials, field-responsive materials [1–5]. In these applications, the performance of the whole composites depends on not only the single properties of matrix or filler but also the matching degree of properties of matrix and filler. For example, the good elastic matching between matrix and filler can decrease the stress shielding behavior of composites [6]. The impedance matching of matrix and filler has an influence on the microwave absorption or shielding performance of composite [7]. Therefore, research on the property matching degree between matrix and filler is of great use to design polymer composites with optimal performance. Besides use with a bulk form, polymer composite particles have also been used as the active dispersed phase in some functional and smart fluids, such as polishing fluid, electrorheological (ER) fluid, and magnetorheological (MR) fluids [8–10]. Electrorheological fluid is an electro-responsive suspension, which is composed of polarizable particles and insulating liquid medium [11]. Without

Polymers 2020, 12, 703; doi:10.3390/polym12030703 www.mdpi.com/journal/polymers Polymers 2020, 12, 703 2 of 16 electric fields, the particles are freely dispersed. With external electric fields applied, the particles are polarized and attracted to form particle chains along electric fields. This orientated structure can enhance the viscosity of ER fluid and even make ER fluid transform from liquid-like state into solid-like state, the so-called ER effect [12,13]. Thanks to this tunable ER effect, ER fluid has many potential uses as an electrical-mechanical interface to develop smart devices, such as smart dampers, soft robotics, and microfluidic rectifier [14–17]. To promote real applications, ER fluids containing various kinds of particles have been developed, such as semiconducting polymer particles, inorganic particles, polyelectrolyte particles, and carbonaceous particles [18–22]. In addition, the ER fluid based on polymer composite particles has also been frequently studied because they often have enhanced properties, such as decreased low density, enhanced thermal stability, and ER activity [23,24]. For example, the ER fluid with polyaniline (PANI)/graphene composite particles shows increased yield stress due to graphene-induced enhancement of electric polarization [25]. The ER fluid with PANI/montmorillonite composite particles shows not only enhanced temperature stability but also good dispersion stability [26]. Recently, some poly(ionic liquid) (PIL)-based composite ER particles have also been developed in order to enhance the performance of some PIL-based ER materials, a new type of anhydrous polyelectrolyte-based ER system [27,28]. For example, Chen et al. used a multi-layer coating technique to prepare graphene oxide/polypyrrole/PIL composite ER particles showing enhanced yield stress. Zhao et al. used a Pickering emulsion polymerization to prepare PIL/nano- SiO2 composite ER particles showing enhanced temperature stability [29,30]. However, these studies have focused on yield stress or thermal stability. In many applications, besides yield stress, the flow stability of ER fluid under the simultaneous effects of shear and electric fields is also important. It has proposed that the flow stability is closely related to the dielectric polarization rate of dispersed particles [31,32]. A polarization rate that is too slow cannot provide enough time with particles to rebuild particle chains at a high shear rate, while polarization rate that is too fast easily leads to the interparticle repulsion as soon as the direction of particle chains is not consistent with the direction of electric fields under shearing. It is proposed that the polarization rate located in the frequency range of 102–105 Hz or 6.28 102–6.28 105 rad/s may be × × proper to fulfill a stable flow behavior under DC electric fields. However, this proposal is concluded based on single component ER particles. In recent PIL-based ER fluid, for example, the stable flow has been obtained when employing an appreciate pendant group to endorse PIL particles with suitable polarization rates [33]. In addition, changing the crosslinking degree or mesh size of crosslinking network has also adjusted the polarization rate and the resulting flow stability [34]. Whether this is the proposal available for composite ER particle systems, in particular for composite particles with multi-level polarization rates? The answer to this question is valuable for the choice and design of the composite-based ER materials with optimized performance. In this work, we prepared a kind of composite ER particles with PIL as matrix and PANI as filler and investigated the effect of dielectric polarization rate difference between matrix and filler on the ER response and flow stability of ER fluid under simultaneous effect of shear and electric fields. We prepared PIL/PANI composite particles by ion-exchange and then treated by ammonia or hydrazine to obtain different forms of PANI filler, such as emeraldine salt, emeraldine base, and leucoemeraldine base [35,36]. Because of their significantly different conductivities, we can adjust the difference in polarization rate between filler and matrix. The ER response was measured by a rheometer under electric fields. The dielectric polarization was analyzed by dielectric relaxation spectroscopy. It showed that the variations of the flow behavior of these ER fluids were consistent with the matching degree of dielectric polarization rate between PIL matrix and PANI filler. The closer their polarization rates were, the more stable the flow curve was in a broad shear rate region. Polymers 2020, 12, 703 3 of 16

2. Materials and Methods

2.1. Materials (Vinylbenzyl)trimethylammonium chloride ([VBTMA]Cl, 99%) was purchased from Sigma-Aldrich, St. Louis, MO, USA. Potassium hexaﬂuorophosphate (K[PF6], 99%) was purchased from J&K Chemical, Beijing, China. 2,2 -azobis(isoheptonitrile) (AIBN), aniline, ammonia (NH H O, 25%), hydrazine 0 3· 2 hydrate (N H H O, 80%), hydrochloric acid (HCl, 36%), and silver nitrate (AgNO , 99%) were 2 4· 2 3 purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Ammonium persulfate (APS) was purchased from Tianli Chemical Reagent Co., Ltd., Tianjin, China. Dimethyl silicone oil (KF-96) was purchased from Shin-Etsu Chemical Co., Ltd., Tokyo, Japan. All these reagents were used as received except that AIBN was puriﬁed by recrystallization in ethanol.

2.2. Preparation of PIL/PANI Composite Particles PIL/PANI composite particles were prepared by the modified method of Marcilla et al. [37]. The preparation details of P[VBTMA][PF6] particles and P[VBTMA][PF6]/PANI composite particles refer to our previous work [38]. The difference is that a spot of HCl was added into the reaction system to dissolve aniline at the outset. Then, a silver nitrate solution was used to detect the absence of chlorine ion to verify the ion exchange completely. What is more, in order to obtain the different forms of PANI, the product was equally divided into three parts for different post-processing. The one part was dipped in NH H O (50 mL, 3 wt%) for 5 min, the second part was reduced by N H H O (20 mL) 3· 2 2 4· 2 at 95 ◦C for 6 h, and the third part accepted no special treatment. After washing with deionized water and vacuum drying, the different polarization rates of PIL/PANI composite particles were obtained. To facilitate characterizing the structure of these composite particles, the PIL/SiO2 composite particles were prepared by the same method.

2.3. Preparation of ER Fluids The PIL, PIL/PANI (HCl), PIL/PANI (NH H O), and PIL/PANI (N H H O) particles were further 3· 2 2 4· 2 washed with ultrapure water, and dried in vacuum at 100 ◦C for 48 h. Then, the dry samples were added into insulating dimethyl silicone oil whose kinetic viscosity is 50 cSt at 25 ◦C and ground to uniform ﬂuids. The particle concentration of ﬂuids is 20 vol%.

2.4. Characterization The morphology of samples was observed by scanning electron microscopy (SEM, Hitachi TM3000, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, FEI Talos F200X, Hillsboro, OR, USA). The operating voltage of SEM is 15 kV and that of HRTEM is 200 kV. The element composition of the composite particle was characterized by the HRTEM equipped with energy dispersive X-ray spectroscopy (EDS, FEI Talos F200X’s own, Hillsboro, OR, USA). The chemical groups of particles, which were mixed with KBr and pressed into platelets, were detected by the Fourier transform infrared spectrum (FT-IR, JASCO FT/IR-470 Plus, Tokyo, Japan). The absorption spectrum was measured by using Ultraviolet and visible spectrophotometer (UV-vis, Hitachi U-4100, Tokyo, Japan).

2.5. Rheological Measurements The ER structure formed under an electric field was observed by an optical microscope (Nikon ALPHAPHOT-2, Tokyo, Japan). The electric field strength for the suspension in the gap was around 2 kV/mm to arrange well-balanced column-like ER structures. The ER response of fluids was tested by a stress-controlled rheometer (Thermal-Haake RS600, Karlsruhe, Germany) with a parallel plate system and an oil bath system. The electric field through two plates was applied by a DC high-voltage generator (WYZ-010, Beijing, China), and the electrode Polymers 2020, 12, 703 4 of 16 gap was 1.0 mm. The flow curves of the shear stress as a function of shear rate were tested by the 1 controlled shear rate mode within 0.1–1000 s− , and the test temperature range was from 25 ◦C to 120 1 ◦C. Before every measure started, the suspension was pre-sheared for 60 s at 100 s− to remove residual ER structures from the last test and then the electric field was applied for 10 s to form equilibrated gap-spanning ER structures.

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The dielectric dielectric propert propertiesies of of these these ER ERfluids fluids were were measured measured by an by impedance an impedance analyzer analyzer (HP 4284A (HP, Santa4284A, Clara, Santa CA, Clara, USA CA,) with USA) a with liquid a liquid measuring measuring fixture fixture (HP 16452A (HP 16452A,, Santa Santa Clara, Clara, CA, USA CA,) USA).. The Themeasurement measurement frequency frequency range range was 20 was–10 20–106 Hz. 6TheHz. bias The voltage bias voltage was 1 V was during 1 V duringmeasurements measurements,, which whichcould notcould cause not causefibrous fibrous-like-like structure structuress establishment establishment.. Hence Hence,, the thepolarization polarization characteristics characteristics of differentof different particles particles could could be be well well compare compared.d. Before Before every every temperature temperature measurement, measurement, the the liquid measuring fixture fixture was equilibratedequilibrated for at least 5 min to achieve thermal stabilization within 0.2 °C◦C by using a constant temperature oil bath.

3. Results Results and and Discussion Discussion

3.1. Morphology and Structure of PIL PANI Composite Particles 3.1. Morphology and Structure of PIL/PANI Composite Particles

Figure1A–D exhibits the SEM images of PIL, PIL /PANI(HCl), PIL/PANI(NH3), and PIL/PANI Figure 1A–D exhibits the SEM images of PIL, PIL/PANI(HCl), PIL/PANI(NH3), and (N2H4) particles. It can be seen that all the particles are irregular with a similar size distribution of PIL/PANI(N2H4) particles. It can be seen that all the particles are irregular with a similar size µ distribution4–10 m owing of 4 to–10 the μm same owing synthesis to the same process. synthesis Furthermore, process. thereFurthermore, is no PANI there structure is no PANI that looksstructure like thatbroccoli looks being like broccoli observed being which observed indicates which that theindicates PANI that is wrapped the PANI by is PIL wrapped during by the PIL ion during exchange the process. It is noted that the surface of PIL/PANI(N H ) particles is obviously diﬀerent from the others ion exchange process. It is noted that the surface of2 PIL4 /PANI(N2H4) particles is obviously different due to the high-temperature reaction by the reduction of N2H4 H2O. from the others due to the high-temperature reaction by the reduction· of N2H4·H 2O.

Figure 1. SEMSEM images images of of samples: samples: (A ()A poly(ionic) poly(ionic liquid liquid)) (PIL (PIL),), (B) ( BP)IL PIL/polyaniline/polyaniline (PANI (PANI)(HCl),)(HCl), (C) PIL/PANI(NH3), and (D) PIL/PANI(N2H4) (Scale bar = 20 μm). (C) PIL/PANI(NH3), and (D) PIL/PANI(N2H4) (Scale bar = 20 µm).

ForFor further further analysis, analysis, Figure Figure2A 2A shows shows the TEM the TEMimage image of an ultrathin of an ultrathin cross-section cross of-section the PIL of/PANI the particle.PIL/PANI An particle. overlay An of nitrogenoverlay of belonging nitrogen tobelonging the poly[VBTMA] to the poly[VBTMA] cation part andcation PANI, part and and phosphorus PANI, and phosphorusbelonging to belonging the [PF6] to anion the [PF part6] EDSanion maps part EDS is shown maps inis Figureshown2 inE. Figure It can be2E.clearly It can be found clearly that found the thatdistribution the distribution of elements of elements is not homogeneous.is not homogeneous. The phosphorusThe phosphorus tends tends to distribute to distribute on on the the edge edge of particles,of particles, which which indicates indicates the the unsymmetrical unsymmetrical structure structure by theby the post post ion-exchange ion-exchange procedure. procedure. This This can canbe confirmed be confirmed with wit ourh our previous previous work, work, in in which which we we proved proved the the PIL PIL/PANI/PANI structurestructure byby comparing the electrical response with PANI/PIL PANI/PIL mixture [38].]. However, However, it it is is still difficult difficult to distinguish the distribution of PANI due to that both PIL and PANI have nitrogen element. Ther Therefore,efore, the same mass fraction of SiO 2 isis applied applied to to take take the the place place of of PANI PANI by by using using the the same same prepar preparationation method. As shown in Figure 2 2G–L,G–L, it it can can be be clearly clearly seen seen that that the the SiO SiO2 2particles particles are are inlaid inlaid inside inside the the PIL PIL particle. particle. Hence, Hence, it itcan can be be inferred inferred that that the the distribution distribution of PANIof PANI is a is similar a similar situation. situation. In addition, In addition, the structurethe structure can becan also be verifiedalso verified by the by opticalthe optical photographs photographs in Figure in Figure 4, where 4, where PIL/ PANIPIL/PANI are dark are dark particles particles capsulated capsulated by a bytransparent a transparent shell. shell.

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FigureFigure 2 2.. TEMTEM images images,, HAAD HAADFF images images,, and and linear linear ED EDSS plots of ultrathin cross-sectionscross-sections onon holey holey

carboncarbon films: ﬁlms: ( (A,A ,B, FF)) PILPIL/PANI/PANI and and ( G(G,H, H,L,) L PIL) PIL/SiO/SiO2.2 EDS. EDS elemental elemental maps: maps (:C ()C N,) N (D, ()D P,) P, (E )(E N) N and and P, P,(I )(I F,) F, (J )(J Si,) Si, and and (K ()K F) andF and Si. Si. (B (–BE-Escale scale bar bar= =2 2µ μm,m,H H–K-Kscale scale bar bar= =5 5µ μm)m)

FigureFigure 33AA showsshows the the FTIR FTIR spectra spec oftra neat of PIL,neat PIL PIL/PANI(HCl),, PIL/PANI(HCl), PIL/PANI(NH PIL/PANI(NH3), and PIL3),/ PANIand PIL(N2/HPANI(N4) particles.2H4) particles. All the linesAll the show lines the show characteristic the characteristic peaks ofpeaks PIL, of which PIL, which peaks peaks designated designated to the 1 −1 1 −1 1 −1 1 −1 topoly[VBTMA] the poly[VBTMA] cation cation part are part at 3054are at cm 305− 4(CH cm3), (CH 29263), cm 292− 6(CH cm2), (CH 16252), cm 16−25, andcm 1491, and cm1491− (Ccm=C 1 −1 1 −1 (inC=C benzene in benzene ring), ring and), theand peaks the peaks designated designated to the to [PFthe6 [PF] anion6] anion part part at 838 at 83 cm8 −cmand and 558 558 cm cm− (P–F). (P– 1 −1 F).As As for for PIL PIL/PANI/PANI composite composite particles, particles, the the peaks peaks corresponding corresponding to PANI(HCl)to PANI(HCl) are are at 1585 at 158 cm5 −cm(C (C=C=C in 1 −1 1 −1 1 −1 inquinoid), quinoid 1477), 14 cm77− cm(C= C(C=C in benzene in benzene ring), 1300 ring), cm 1−300(C–N), cm and(C–N), 1126 and cm− 11(electronic-like26 cm (electronic absorption-like absorptionof N=Q=N of (Q N=Q=N representing (Q representing the quinoid)). the After quinoid)). treated After by NH treatedH O or by N NHH 3·HH 2O,O orthe N peaks2H4·H ascribed2O, the 3· 2 2 4· 2 peaksto the ascribed C=C stretching to the C=C of quinoid, stretching C= ofC quinoid stretching, C=C of benzenestretching ring, of benzene and electronic-like ring, and electronic absorption-like of 1 1 −1 −1 1 −1 absorptionN=Q=N show of N=Q=N an obvious show blue an shiftobvious to 1596 blue cm shift− , 1512 to 1596 cm− cm, and, 1512 1170 cm cm−, and. In addition,1170 cm the. In quinonoidaddition, thestructure quinonoid can be structure reduced can by Nbe2 Hreduced4 H2Oto by benzenoid N2H4·H 2O structure, to benzenoid which structure, leads to thatwhich the leads intensity to that ratio the of 1 · −1 1 −1 intensitythe peak atratio 1596 of cmthe− peakthen at to 1596 the peak cm at then 1512 to cm the− peakdecreasing. at 1512 Thesecm decreas changesing in. theThese PANI changes structure in the can PANIbe also structure confirmed can by be the also UV-vis confirmed spectra by (see the Figure UV-3visB). Afterspectra dedoping (see Figure by NH 3B). HAfterO, thededoping absorption by 3· 2 NHpeaks3·H 2 alsoO, the display absorption a slight peaks blue also shift display in theC a= slightC stretching blue shift vibration in the of C=C benzenoid stretching and vibration quinonoid of benzenoidrings from and 322/ 615quinonoid nm to315 rings/600 from nm. 322/615 After reducing nm to 315/600 by N H nm.H O,After the reducing intensity ofbyabsorption N2H4·H 2O, peak the 2 4· 2 intensityattributed of to absorption benzenoid peak ring attributed increases while to benzenoid that attributed ring increases to quinonoid while ring that decreases. attributed Theseto quinonoid changes ringin the decreases PANI chemical. These chang structurees in will the bring PANI the chemical differences structure in electrical will bring properties the differences for PANI filler,in electrical such as properconductivity,ties for polarization,PANI filler, such and as energy conductivity, gap [39,40 polarization,]. and energy gap [39,40].

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Figure 3.3. ((A)) FT FT-IR-IR spectra and ((B)) UV-visUV-vis spectraspectra ofof samples:samples: ((aa)) neatneat PIL,PIL, ((bb)) PILPIL//PANI(HCl),PANI(HCl), (c(c)) PILFigure/PANI(NH 3. (A)3 ),FT and-IR (spectrad) PIL/ PANI(Nand (B) UV2H4-).vis spectra of samples: (a) neat PIL, (b) PIL/PANI(HCl), (c) PIL/PANI(NH3), and (d) PIL/PANI(N2H4). PIL/PANI(NH3), and (d) PIL/PANI(N2H4). 3.2. Electrorheological Electrorheological Propert Propertiesies 3.2. Electrorheological Properties Figure4 4 displays displays the the optical optical photographs photographs of of these these ER ER fluids fluids between between two two electrodes. electrodes. Without Without the Figure 4 displays the optical photographs of these ER fluids between two electrodes. Without theelectric electric field, field, these these particles particles disperse disperse in the siliconein the silicone oil randomly. oil randomly The PIL particles. The PIL are particles white transparent, are white the electric field, these particles disperse in the silicone oil randomly. The PIL particles are white transparent,while the PIL while/PANI the composite PIL/PANI particles composite are black. particles These are composite black. These particles composite look uniform particles without look transparent, while the PIL/PANI composite particles are black. These composite particles look uniformwhite impurities, without white which impurities indicates that, which the PANIindicates is trapped that the by PANI PIL in isthe trapped counterion by PIL conversion in the counterion process. uniform without white impurities, which indicates that the PANI is trapped by PIL in the counterion conversionWith an electric process. field With applied, an electric the randomly field applied, distributed the randomly polarizable distributed particles polarizable quickly attract particles each conversion process. With an electric field applied, the randomly distributed polarizable particles quiotherckly and attract shape each into gap-spanningother and shape column-like into gap- structuresspanning attributed column-like to the structure interparticles attributed electrostatic to the quickly attract each other and shape into gap-spanning column-like structures attributed to the interparticleinteraction. Thiselectrostatic column-like interaction. structure This in column a direction-like structure perpendicular in a direction to shear perpendicular flow will increase to shear the interparticle electrostatic interaction. This column-like structure in a direction perpendicular to shear flowviscosity will ofincrease the fluids the andviscosity even generateof the fluid yields and stress even leading generate to ayield liquid-solid stress leading transition, to athe liquid so-called-solid flow will increase the viscosity of the fluids and even generate yield stress leading to a liquid-solid transitionER response, the [ 12so].-called We note ER thatresponse there [ are12]. someWe note diff thaterences there in are the some column-like differences structure in the betweencolumn-like PIL transition, the so-called ER response [12]. We note that there are some differences in the column-like and PIL/PANI particles. For PIL, the columns seem to be not asymmetric, and more particles tend structurestructure between between PIL PIL and and PIL PIL/PANI/PANI particles. particles. ForFor PIL, the columns seemseem to to be be not not asymmetric, asymmetric, and and to aggregate near the negative electrode. While for PIL/PANI, the columns seem to be more uniform moremore particles particles tend tend to to agg aggregateregate near near thethe negativenegative electrode. While forfor PILPIL//PANI,PANI, the the columns columns seem seem except Figure4D. This phenomenon has been discussed in our previous work in detail, so that is not toto be be more more uniform uniform except except Figure Figure 4 4DD. . ThisThis phenomenonphenomenon has been discusseddiscussed in in our our previous previous work work in in detailrepeateddetail, so, so anythat that moreis is not not inrepeated repeated this study any any [ 38more more]. inin thisthis studystudy [38].

FigureFigure 4 4. .4 Optical.Optical Optical photog photographsphotographsraphs of of of the the the e electrorheological electrorheologicallectrorheological (ER (ER)) fluids ﬂuids withoutwithout without and and and with with with electric electric electric field: field: ﬁeld: ( A(A) ) neat PIL, (B) PIL/PANI(HCl), (C) PIL/PANI(NH3), and (D) PIL/PANI(N2H4) (ϕ=3 vol%, Scale bar = 100 neat(A) neat PIL, PIL, (B) PIL (B)/ PILPANI(HCl),/PANI(HCl), (C) (PILC) PIL/PANI(NH/PANI(NH3), and3), and (D) ( DPIL) PIL/PANI(N/PANI(N2H42)H (ϕ4)(=3φ vol%,= 3 vol%, Scale Scale bar = bar 100 μm). μm).= 100 µm).

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Figure5 5shows shows the the variation variation of shear of shear stress stress with with shear shear rate for rate the for PIL the/PANI PIL ER/PANI fluids ER at fluiddifferents at electricdifferent fields. electric Without fields. an Without electric an field electric applied, field all applied these fluids, all exhibitthese fluids a shear exhibit thinning a shear phenomenon thinning withphenomenon a low viscous with a state. low viscous The off state.-field The viscosities off-field of viscosit these fluidsies of arethese very fluids close, arewhich very close, are around which 1 0.17are around Pa s at 10000.17 sPa·s− , owing at 1000 to the s−1, similarowing particle to the similar size and particle morphology size and for all morphology these particles. for all This these can · alsoparticles prove. This that can the also PANI prove is surrounded that the PANI by PIL is forsurrounded composite by particles. PIL for Withcomposite the electric particles field. With applied, the allelectric of the field fluids applied, show a all conspicuous of the fluids increase showin a shearconspicuous stress and increase possess in large shear yield stress stress and like possess a Bingham large fluid.yield stress The increase like a inBingham yield stress fluid can. The be adjustableincrease in with yield electric stress fieldcan strengthbe adjustable because with the electric polarization field intensitystrength ofbecause the dispersed the polarization particles canintensity be enhanced of the with dispersed the external particles electric can field be enhanced increasing, with leading the toexternal the stronger electric formation field increasing, level of column-likeleading to the ER stronger structures formation [41]. What level is more,of column there-like are manyER structures obvious di[41ff].erences What amongis more, the there flow are curves many of obvious these suspensions. differences Comparedamong the to flow the flowcurves curves of the ofse the suspensions. ER fluid of PILCompared particles, to thosethe flow of PILcurves/PANI(HCl) of the ER particles fluid of exhibitPIL particles, a wave those trough of PIL in high/PANI(HCl) shear rate particles region atexhibit high electrica wave field trough strength, in high those shear of PILrate/ PANI(NH region at 3 high) particles electric possess field much strength, larger those shear of stress,PIL/PANI(NH and those3) ofparticles PIL/PANI(N possess2H 4 much) particles larger look shear like continuous stress, and upward those of slopes. PIL/PANI(N It has been2H4) known particles that look the flow like behaviorcontinuous of ERupwa fluidrd slopes. is closely It has connected been known with the that rupture the flow and beh recombinationavior of ER fluid of field-induced is closely connected column ERwith structures the rupture under and the recombination electric fieldand of field shear-induced field simultaneously. column ER structure The stables under flow curvethe electric in a broad field shearand shear rate regionfield simultaneously requires that the. The dispersed stable flow polarizable curve particlesin a broad possess shear arate suitable region polarization requires that rate the to rebuilddispersed column polariza structuresble particles effectively. possess Hence, a suitable the structural polarization state of rate PANI to filler rebuild has acolumn distinct structures influence oneffectively. the electric Hence, response the structural of composite state of particles PANI filler leading has toa distinct these di influencefferences inon flowthe electric behavior response of the fourof composite suspensions. particles leading to these differences in flow behavior of the four suspensions.

Figure 5.5. Flow curves of shear stress vs.vs. shear rate forfor thethe ERER ﬂuids:fluids: ((AA)) neatneat PIL,PIL, ((BB)) PILPIL//PANI(HCl),PANI(HCl),

((C)) PIL//PANI(NHPANI(NH33),), andand ((DD)) PILPIL//PANI(NPANI(N22HH44))( (T = 2255 °◦C).C).

Figure6 6 plots plots thethe electric electric field field strength strength dependence dependence of of the the static static yield yield stress stress ( (ττss) and the dynamic yield stress stress ( (ττdd) forfor these these suspensions. suspensions. The The τss characterizescharacterizes the the solidified solidified level level of ER fluidfluid before flowing.flowing. While t thehe τd representrepresentss the the strength of suspension in the flow flow regime [[19,4219,42],], which can be obtained by extrapolating the fittingfitting lines ofof flowflow curves withwith thethe BinghamBingham modelmodel (see(see FigureFigure5 5).). It It isis needed to point out that the τdd of the ER fluid fluid of PIL /PANI(N22H4)) is is obtained obtained by fitting fitting the high shear rate data. Further, its τs can be used to represent the strength of suspension at the low shear rate region. As shown in Figure 6, both the τs and the τd of the suspensions of composite particles, except

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Polymers 2020, 12, x FOR PEER REVIEW 8 of 16 rate data. Further, its τs can be used to represent the strength of suspension at the low shear rate region. As shown in Figure6, both the τs and the τ of the suspensions of composite particles, except the τs of the τs of PIL/PANI(N2H4), are larger than dthat of PIL particles at 3.0 kV/mm of electric field, which PILindicat/PANI(Nes that2H the4), ER are fluids larger of than PIL that/PANI of PILcomposite particles particles at 3.0 kV ha/vemm an of enhanced electric field, ER response which indicates. For the thatpurpose the ER of fluidsquantify of PILing/PANI the comparison, composite particles the relationship have an enhanced between ER yield response. stress For and the electric purpose field of quantifyingstrength canthe be fitted comparison, by the power the relationship law function between 휏 ∝ 퐸 yield훼. As stressfor pure and PIL electric, its ER field respons strengthe originates can be fitted by the power law function τ Eα. As for pure PIL, its ER response originates from local ion from local ion motion. So it is inclined∝ to conform to the conduction model for the ER mechanism, α motion.whose α So value it is inclinedis no more to conformthan 1.5 to[43 the]. Here, conduction the α modelvalue of for PIL the is ER consistent mechanism, with whose the conductionvalue is α nomodel. more While than 1.5all [the43]. α Here, values the of ERvalue fluid of of PIL PIL/PANI is consistent composite with the particles conduction are more model. than While 1.5, allwhich the α indicatesvalues of that ER the fluid inside of PIL PANI/PANI filler composite make a particlescontribution aremore to increase than 1.5, polarization. which indicates Then thatthe composite the inside PANIparticles filler tend make to afollow contribution the polarization to increase model, polarization. whose α Then value the is composite two at most particles [42]. tendBesides, to follow the α the polarization model, whose α value is two at most [42]. Besides, the α values decrease gradually values decrease gradually from PIL/PANI(HCl) to PIL/PANI(N2H4) particles, reflecting the from PIL/PANI(HCl) to PIL/PANI(N H ) particles, reflecting the contribution of different degrees. contribution of different degrees. In addition,2 4 the τs is usually a little lager than the τd, but the τd of In addition, the τ is usually a little lager than the τ , but the τ of the ER fluid of PIL/PANI(N H ) are the ER fluid of PIL/PANI(Ns 2H4) are much differentd from the τds. These phenomena must be related2 4 to τ muchthe electro different-responsive from the propertiess. These phenomenaof PANI filler must, such be related as intensity to the electro-responsive of polarization and properties dielectric of PANIpolarization filler, such rate. as intensity of polarization and dielectric polarization rate.

FigureFigure 6.6. ((AA)) StaticStatic yieldyield stressstress andand ((BB)) dynamicdynamic yieldyield stressstress as as a a function function of of electric electric ﬁeld field strength strength for for

thethe ERER ﬂuidsfluids ((TT == 2525 ◦°C).

FigureFigure7 7 shows shows the the typical typical flow flow curves curves under under thethe electric electric fields fields of of 0.0 0.0 and and 3.0 3.0 kV kV/mm/mm atat variousvarious temperatures.temperatures. Without Without the the electric electric field, field, the o theff-field off- viscositiesfield viscosities of all the of suspensions all the suspensions decrease graduallydecrease withgradually temperature with tempera increasing.ture increasing. This is because This is the because viscosity the of viscosity silicone of oil silicone dispersion oil dispersion medium decreases medium withdecreases temperature with temperatu increasing.re Under increasing. the electric Under field the ofelectric 3.0 kV / fieldmm, ofthese 3.0 suspensions kV/mm, these exhibit suspensions different temperatureexhibit different dependence. temperature As for dependence. the ER fluid As of for neat the PIL ER particles, fluid of the neat flow PIL curves particles are, stablethe flow in a curves broad shearare stable rate region,in a broad and shear the shear rate stressregion, decreases and the littleshear by stress little decreases with working little temperature by little with increasing. working Thistemperature situation increasing. is similar to This that situation of the ER is fluid similar of PIL to /thatPANI(NH of the3 ER) particles, fluid of exceptPIL/PANI(NH that the yield3) particles, stress ofexcept composite that the particles yield decreases stress of even composite more. particlesAs for the decreases ER fluid of even PIL /PANI(HCl) more. As for particles, the ER the fluid wave of troughPIL/PANI(HCl) becomes particles, much deeper the wave and the trough flow becomes curves become much deeper more unstable. and the flow On the curves contrary, become the more flow curvesunstable. of theOn ERthe fluidcontrary, of PIL the/PANI(N flow curves2H4) particlesof the ER become fluid of more PIL/PANI(N and more2H stable4) particles in the become broad shearmore rateand regionmore stable with workingin the broad temperature shear rate increasing, region with though working there temperature is still an upwardincreasing slope, though below there 80 ◦ C.is Thesestill an di upwardfferences slope among below the ER 80 fluids°C . These of PIL differences/PANI composite among particles the ER indicatesfluids of thatPIL/PANI the flow composite stability dependsparticles onindicates the electro-responsive that the flow stability properties depends of the on PANI the filler.electro-responsive properties of the PANI fillerFigure. 8 plots the temperature dependence of τs and τd under the electric field of 3.0 kV/mm. It can be clearly seen that both the τs and the τd of all the suspensions can be obtained with a wide temperature range, but tend to decrease with increasing working temperature, except for the τs of the ER fluid of PIL/PANI(N2H4) particles. Furthermore, these downward trends are different between PIL and PIL/PANI particles, which indicates that the decrease is not only related to the decrease of off-field viscosity but also the variation of the inside PANI. The yield stress of the ER fluid of PIL/PANI(HCl) particles falls sharply and its difference value between τs and τd looks very large,

Polymers 2020, 12, x FOR PEER REVIEW 9 of 16

Polymers 2020, 12, 703 9 of 16

indicating an unstable flow behavior. The yield stress of the ER fluid of PIL/PANI(NH3) particles is the highest at low temperature, while it becomes near to that of PIL at high temperature. Moreover, the most obvious difference is from the ER fluid of PIL/PANI(N2H4) particles. From Figure8B, it looks like it possesses the best temperature stability, however, it is needed to point out that its τd at 25 ◦C, 40 ◦C, and 60 ◦C are obtained by only fitting the high shear rate data. Its small τs indicates that there is a steep hill in the low shear rate region. Nevertheless, its τs and τd become closer and closer with temperature increasing, indicating a gradually stable flow behavior. Overall, these differences mean that the rheological behavior and temperature stability are affected by the electro-responsive properties ofPolymers PANI 20 with20, 12, the x FOR temperature PEER REVIEW changes. 9 of 16

Figure 7. Flow curves of shear stress vs. shear rate for the ER fluids at various temperatures: (A) neat PIL, (B) PIL/PANI(HCl), (C) PIL/PANI(NH3), and (D) PIL/PANI(N2H4).

Figure 8 plots the temperature dependence of τs and τd under the electric field of 3.0 kV/mm. It can be clearly seen that both the τs and the τd of all the suspensions can be obtained with a wide temperature range, but tend to decrease with increasing working temperature, except for the τs of the ER fluid of PIL/PANI(N2H4) particles. Furthermore, these downward trends are different between PIL and PIL/PANI particles, which indicates that the decrease is not only related to the decrease of off-field viscosity but also the variation of the inside PANI. The yield stress of the ER fluid of PIL/PANI(HCl) particles falls sharply and its difference value between τs and τd looks very large, indicating an unstable flow behavior. The yield stress of the ER fluid of PIL/PANI(NH3) particles is the highest at low temperature, while it becomes near to that of PIL at high temperature. Moreover, the most obvious difference is from the ER fluid of PIL/PANI(N2H4) particles. From Figure 8B, it looks like it possesses the best temperature stability, however, it is needed to point out that its τd at 25 °C , 40 °C , and 60 °C are obtained by only fitting the high shear rate data. Its small τs indicates that there is a steep hill in the low shear rate region. Nevertheless, its τs and τd become closer and closer with temperature increasing, indicating a gradually stable flow behavior. Overall, these differences mean that Figure the rheologic 7.7. Flow curvesal behavior of shear and stressstress temperature vs.vs. shearshear raterate forfor stability thethe ERER ﬂuidsarefluids affectedat at various various by temperatures: temperatures: the electro ( A-(responsiveA)) neat neat properties of PANI with the temperature changes. PILPIL,, (B) PIL/PANI(HCl), ((CC)) PILPIL//PANI(NHPANI(NH33),), andand ((DD)) PILPIL//PANI(NPANI(N22HH44).

40 °C , and 60 °C are obtained by only fitting the high shear rate data. Its small τs indicates that there Figure 8. (A) Static yield stress and (B) dynamic yield stress at 3.0 kV/mm plotted as a function of is a steep hill in the low shear rate region. Nevertheless, its τs and τd become closer and closer with temperaturetemperature increasing, for the ER indicating fluids of PIL a gradually/PANI particles. stable flow behavior. Overall, these differences mean 3.3.that Dielectric the rheologic Propertiesal behavior and temperature stability are affected by the electro-responsive properties of PANI with the temperature changes. The rheological results above clearly show that the different structural states of PANI filler have a significant influence on the flow behavior and temperature stability. In order to understand this,

Polymers 2020, 12, 703 10 of 16 we investigate the dielectric properties of these suspensions due to the fact that it has been widely recognized that the polarization of particles in suspensions plays a critical role in ER response [31,44]. According to the ER mechanism, to obtain a good ER response, it is essential for suspended particles to have a large polarization intensity and a suitable polarization rate. The former influences the magnitude of interparticle interaction induced by an electric field, while the latter is closely related to the stability of interparticle interaction and the resulting flow stability under the simultaneous application of electric field and shearing field [32,42]. Figure9 displays the angular frequency ( ω) dependence of dielectric constant (ε’) and loss factor (ε”) for these ER fluids. All the fluids show an obvious dielectric relaxation peak within the measured frequency range except the ER fluid of PIL/PANI(HCl) particles. Owing to the fact that the ε’ and ε” of insulating carrier liquid are nearly independent of frequency within the measured range, the observed dielectric relaxation peak should be caused by the polarization process of dispersed particles in silicone oil. Although the ER fluids of PIL/PANI(NH3) and PIL/PANI(N2H4) show clear dielectric relaxations within the measured frequency range, their ε” values at the location of relaxation peak are lower than that of the ER fluid of neat PIL particles and the half-peak width of relaxation peak is also distinctly broader than that of the ER fluid of neat PIL particles. These also imply that there should be two relaxation processes attributed to outside PIL and inside PANI, respectively. Although no visible dielectric relaxation peak in the ER fluid of PIL/PANI(HCl) particles, the value of ε” increases in the high frequency region. Considering the high conductivity of PANI doping with HCl, we think there is a relaxation peak at higher frequency region from the fast polarization process of PANI filler. This can be supported by the dielectric data of previously reported composites containing high conductivity of filler inclusions [45,46]. In addition, the polarization of PIL composition in PIL/PANI(HCl) may be covered by large DC conductivity, as shown in Figure8B. This large DC conductivity maybe because there are some PANI(HCl) particles distributed at the edge of composite like the distribution of SiO2 in Figure2G–L. To fit the dielectric data and analyze the dielectric and electrical characteristics, therefore, we employ the following relaxation equation, including two Cole–Cole’s terms, a DC conductivity term, and an electrode polarization (EP) term [47–49]:

∆ε 10 ∆ε20 σ n ε∗ (ω) = ε0 + iε00 = ε0 + + + i + Aω− (1) β1 β2 ∞ 1 + (iωλ1) 1 + (iωλ2) ε0ω

The equations of ε0 and ε” are shown below, where subscript 1 for outside PIL matrix and subscript 2 for inside PANI ﬁller:

πβ ! πβ ! +( )β1 ( 1 ) +( )β2 ( 2 ) 1 ωλ1 cos 2 1 ωλ2 cos 2 n ε0 = ε0 + ∆ε0 πβ + ∆ε0 πβ + Aω− (2) 1 β1 1 2β1 2 β2 2 2β2 ∞ 1+2(ωλ1) cos ( 2 )+(ωλ1) 1+2(ωλ2) cos ( 2 )+(ωλ2)

πβ ! πβ ! β1 1 β2 2 (ωλ1) sin ( 2 ) (ωλ2) sin ( 2 ) σ ε00 = ∆ε0 β πβ β + ∆ε0 β πβ β + ε ω (3) 1 1 1 2 1 2 ωλ 2 2 ωλ 2 2 0 1+2(ωλ1) cos ( 2 )+(ωλ1) 1+2( 2) cos ( 2 )+( 2) = where ∆ε0 ε0 ε0 is the dielectric relaxation strength (ε0 and ε0 are the limit values of the − ∞ ∞ relative dielectric constant at the frequencies below and above the relaxation frequencies, respectively), λ = 1/ωmax is the dielectric relaxation time (ωmax is the local angular frequency of the dielectric loss peak), β (0 < β 1) is the Cole–Cole parameter leading to a symmetrical broadening for the spectrum, ≤ σ is the DC conductivity, A is related to the amplitude of EP, and n is related to the slope of EP’s high frequency tail. Polymers 2020, 12, 703 11 of 16 Polymers 2020, 12, x FOR PEER REVIEW 11 of 16

FigureFigure 9. 9Dielectric. Dielectric relaxation relaxation spectra spectra of of the the ER ER ﬂuids fluids (T (=T 30= 30◦C). °C). Solid Solid lines lines show show the the best best ﬁt fit of εof0 andε′ and ε”ε″ by by Equations Equations (2) (2) and and (3). (3).

AsAs shown shown in in Figure Figure9, Equations9, Equation (2) (2) and and (3) E canquation well fit(3) thecan experimental well fit the experimental dielectric data dielectric and Table data1 summarizesand Table 1 the summarizes dielectric characteristics. the dielectric Itcharacteristics is found that. two It is relaxation found that processes two relaxation in composites processes can be in clarified,composites which can can be be clarified respectively, which attributed can be to respectively PIL matrix and attributed PANI filler to PIL according matrix to and the magnitude PANI filler ofaλccording, and the to conductivity the magnitude of diff oferent λ, and forms the of conductivity PANI after doping of different or dedoping. forms However,of PANI after the magnitude doping or ofdedoping deviation. However, between twothe magnitude relaxation timesof deviation is different between among two these relaxation composites. times is The different value ofamongλ of PANI(NHthese composites.3) filler is The close value to that of ofλ of PIL PANI(NH matrix, while3) filler the is value close ofto λthatof PANI(HCl)of PIL matrix, filler while is far the from value that of ofλ PILof PANI(HCl) matrix. Compared filler is far it withfrom thethat flow of PIL curves matrix in.Figure Compared5, the it smaller with the the flow magnitude curves in of Figure deviation 5, the betweensmaller the two magnitude relaxation of times deviation is, the between more stable two relaxation the flow curvestimes is, are. theAmong more stable these the suspensions, flow curves theare flow. Among curves these of the suspensions, ER fluid of PILthe /flowPANI(NH curves3) of composite the ER fluid particles of PIL are/PANI(NH most stable3) composite in the broad particles shear rateare rangemost (seestable Figure in 5 theC). Accordingbroad shear to the rate proposal range (see ER mechanism, Figure 5C).the According polarization to the rate proposalof particles ER ismechanism, important to the the polarization stability of the rate flow of curveparticles of ER is suspensionsimportant to under the stability the simultaneous of the flow eff ectscurve of of both ER electricsuspensi andons shearing under the fields. simultaneous Under DC effects electric of fields, both itelectric is proposed and shearing that ER fields. fluids Under with a relaxationDC electric timefields, of 1.6it is proposed10 2–1.6 that10 ER6 s arefluids appropriate with a relaxation for achieving time of a stable1.6 × 10 flow−2–1.6 curve. × 10− Too6 s are slow appropriate or too fast for a × − × − relaxationachieving time a stable is not flow favorable curve to. Too the stabilityslow or duringtoo fast flow a relaxation because thentime too is not slow favorable a relaxation to the time stability easily resultsduring in flow insu becausefficient particle then too polarization slow a relaxation during time flow, easily while results too fast in a insufficient relaxation time particle easily polarization results in anduring increase flow of, repulsive while too interaction fast a relaxation between time particles easily due results to the in di anff erence increase between of repulsive the polarization interaction directionbetween and particl thees direction due to connecting the difference two particles between [ 31 the]. Here, polarization the λ of PILdirection matrix and in composites the direction is locatedconnecting in 1.6 two10 particles2–1.6 [1031].6 Here,s but the λ of PANIPIL matrix filler isin locatedcomposites in di isfferent located regions. in 1.6 The× 10−λ2–of1.6 PANI × 10−6 × − × − fillers but in the PIL λ/PANI(N of PANI2H filler4) is relativelyis located slow, in different which slows region downs. The the λ polarizationof PANI filler rate in ofPIL/PANI(N particles leading2H4) is torelatively insufficient slow, interparticle which slow interaction.s down The the λpolarizationof PANI filler rate in PIL of/ PANI(HCl) particles leading is too fast, to which insuffic canient resultinterparticle in an increase interaction. of repulsive The λ interactionof PANI filler between in PIL/PANI(HCl) particles. This is extreme too fast, mismatching which can result can cause in an thatincrease the polarization of repulsive of interactionPANI(HCl) betweenfiller and particles. PIL matrix This offsets extreme each other mismatching at a high can shear cause rate thatregion. the Aspolarization a consequence, of PANI(HCl) this counteraction filler and leadsPIL matrix to the offsets decline each of interparticle other at a high interaction, shear ra whichte region. further As a resultsconsequence, in the decline this counteraction of shear stress. leads So, to there the isdecline a deep of wave interparticle trough atinteraction, the high shear which rate further under results high electricin the decline filed (see of Figureshear stress.5B). However, So, there theis a deepλ of PANI wave fillertrough in at PIL the/PANI(NH high shear3) israte close under to the highλ ofelectric PIL matrix,filed (see which Figure results 5B). inHowev stableer, flow the curves.λ of PANI Moreover, filler in thePIL/PANI(NHλ values of3 PIL) is /closePANI to composite the λ of PIL particles matrix, arewhich not consistentresults in stable with thatflow of curves. PIL by Moreover, using the the same λ values preparation of PIL process./PANI composite This suggests particles that are there not isconsistent a synergistic with eff thatect like of PIL electrostatic by using interaction the same between preparation outside process PIL. and This inside suggests PANI, that or the there post is a processingsynergistic could effect aff likeect the electrostatic outside PIL interaction such as the between high-temperature outside PIL reaction and inside that changes PANI, the or thesurface post ofprocessing PIL/PANI(N could2H 4affect) particles the outside (see Figure PIL such1D). as In the addition, high-temperature we note that reaction the total that value changes of ∆theε’ surface of the ERof fluidPIL/PANI(N of PIL/PANI2H4) particles is not much (see Figure larger 1D). than I thatn addition, of the ERwe fluidnote ofthat neat the PIL,total which value of is not∆ε’ of enough the ER tofluid explain of PIL/PANI the enhanced is not yield much stress. larger This than enhancement that of the dueER fluid to the of improvement neat PIL, which of ER is not structure enough has to beenexplain investigated the enhanced in our yield previous stress. work, This and, enhancement therefore, isdue not to discussed the improvement here [38]. Comparedof ER structure to other has been investigated in our previous work, and, therefore, is not discussed here [38]. Compared to other PIL-based composite, like GO/PPy/PIL composite nanosheets or PIL/SiO2 composite particles [29,30],

Polymers 2020, 12, 703 12 of 16

PIL-basedPolymers 2020 composite,, 12, x FOR PEER like REVIEW GO/PPy /PIL composite nanosheets or PIL/SiO2 composite particles [1229 of,30 16], the ∆ε’ of PIL/PANI(NH3) composites is also relatively larger and λ is more suitable and thus result in enhancedthe ∆ε’ of ERPIL/PANI response(NH compared3) composites to them. is also relatively larger and λ is more suitable and thus result in enhanced ER response compared to them. Table 1. Dielectric characteristics of the ER ﬂuids of PIL/PANI. (T = 30 ◦C) Table 1. Dielectric characteristics of the ER fluids of PIL/PANI. (T = 30 °C ) ∆ε’ a λ (s) Sample ε’ σ (S/m) a ∞ ∆ε’1 ∆∆ε’ε ’2 λ1 λ (s)λ2 Sample ε’ σ (S/m) ∆ε’1 ∆ε’2 λ1 -4 λ2 10 PIL 2.89 3.65 3.7 10 ~2.48 10− 3 × -4 8 × 8 −10 PIL/PANI(HCl)PIL 2.782.89 0.323.65 2.7 1.55 10− 3.71.45 × 1010 − ~4.78 ~2.4810− × 10 × 3 × 2 × 11 PILPIL/PANI(HCl)/PANI(NH3 ) 2.802.78 2.780.32 0.8 2.7 1.40 1.5510− × 10−39.10 1.4510− × 10~7.97−8 10~4.78− × 10−8 × 4 × 1 × 11 PIL/PANI(N H ) 2.76 2.72 0.4 1.68 10 −34.10 10 ~7.08−2 10 −11 PIL/PANI(NH2 3)4 2.80 2.78 0.8 ×1.40− × 10 × 9.10− × 10 × ~7.97− × 10 a PIL/PANI(N2H4) Subscript2.76 1 for2.72 outside PIL0.4 and subscript1.68 2× for10 inside−4 PANI.4.10 × 10−1 ~7.08 × 10−11 a Subscript 1 for outside PIL and subscript 2 for inside PANI.

FigureFigure 10 showsshows the the dielectric dielectric relaxat relaxationion spectra spectra of these of these ER fluids ER fluids at various at various temperatures. temperatures. With Withtemperature temperature increasing, increasing, the dielectric the dielectric relaxation relaxation peak of peak all these of all fluids these fluidsshifts to shifts high to ω high, whichω, whichmeans meansthe increase the increase of polarization of polarization rate, and rate, the and ε the’ andε’ ε and” valuesε” values rise riseup quickly up quickly in the in thelow low ω rωegionregion duo duo to toelectrode electrode polarization polarization and and DC DC conductivity. conductivity. We We note note th thatat the the ER ER fluid fluid of PIL /PANI(HCl) particlesparticles alsoalso has electrode electrode polarization polarization owing owing to to the the quick quick increase increase of ofε’ (seeε’ (see Figure Figure 10B) 10, whichB), which indicates indicates that thatthere there are areions ions moving moving to the to the electrode electrode,, and and the the relaxation relaxation peaks peaks of of PANI PANI for for PIL /PANI(NH33) andand PILPIL//PANI(NPANI(N22HH44)) compositecomposite particlesparticles are are covered covered up up by by the the high high conductivity conductivity of of PIL PIL (see (see Figure Figure 10 C,D).10C– TheseD). Th areese further are further signs sign thats PIL that is onPIL the is outside.on the outside. Compared Compared to others, to we others, also note we thatalso thenote relaxation that the peakrelaxa oftion the peak ER of fluid the ofERPIL fluid/PANI(NH of PIL/PANI(NH3) becomes3) becomes wider withwider temperature with temperature increasing, increasing, which which also indicatesalso indicates that therethat there are two are relaxationtwo relaxation processes process fores the for composite the composite particles. particles.

FigureFigure 10.10. DDielectricielectric relaxation relaxation spectra spectra of of the the ER ER fluids ﬂuids at at various various temperatu temperatures:res: (A () Aneat) neat PIL PIL,, (B)

(PILB) PIL/PANI(HCl),/PANI(HCl), (C) ( CPIL) PIL/PANI(NH/PANI(NH3), 3and), and (D) ( DPIL) PIL/PANI(N/PANI(N2H24).H 4).

ByBy usingusing EquationsEquation (2) (2) and and E (3),quation we can (3), obtain we can the obtain dielectric the relaxationdielectric relaxation time of PIL time matrix of PIL and matrix PANI ﬁller.and PANI Figure filler 11A. Figure shows 1 the1A temperatureshows the temperature dependence dependence of the reciprocal of the ofreciproc their dielectrical of their relaxation dielectric relaxation time (λ−1). Owing to the fact that the relaxation peak shift to high ω is thermally promoted, the λ−1 can follow the Arrhenius equation:

λ−1 ∝ 푒−퐸a⁄푅푇 (4)

where Ea is the activation energy, R is the molar gas constant, and T is the absolute temperature. The Ea value can be calculated by the slope on the 1000/T coordinate. This Arrhenius-type temperature

Polymers 2020, 12, 703 13 of 16

1 1 time (λ− ). Owing to the fact that the relaxation peak shift to high ω is thermally promoted, the λ− can follow the Arrhenius equation: 1 Ea/RT λ− e− (4) Polymers 2020, 12, x FOR PEER REVIEW ∝ 13 of 16 where Ea is the activation energy, R is the molar gas constant, and T is the absolute temperature. dependenceThe Ea value of can Ea be for calculated PIL matrix by theindicates slope onthat the the 1000 ion/T motioncoordinate. of PIL This is from Arrhenius-type the thermal temperature diffusion, anddependence not coupled of E witha for PILthe chain matrix segment indicates [50,51 that]. the What ion is motion more, i oft can PIL be is found from the that thermal the Ea of di ffPILusion, for PILand/PANI not coupled composite with particles the chain is slightly segment larger [50,51 than]. What that of is more,pure PIL it can. This be may found be thatbecause the Ethata of after PIL wrappingfor PIL/PANI the PANI, composite the interfaces particles isbetween slightly PIL larger and thanPANI that can ofhinder pure the PIL. ion This motion may of be mobile because anion that leadaftering wrapping to the the increase PANI, of the E interfacesa of PIL. betweenAs for PANI, PIL and the PANI Ea corresponds can hinder the to ion the motion energy of gapmobile of semiconductinganion leading to polymer, the increase which of isE adeterminedof PIL. As by for its PANI, band thestructuEa correspondsre. The smaller to thethe energyenergy gap of PANIsemiconducting is, the better polymer, the conductivity which is determined is. The PANI by its doping band structure. with HCl The is closmallerse to the metallicity, energy gap so of it possessesPANI is, the the better smallest the conductivity Ea. Compared is. Theto the PANI PANI doping dedoping with HCl with is closeNH3, tomany metallicity, quinoid so units it possesses of the PANIthe smallest reducedEa. byCompared N2H4 transform to the PANI into dedoping benzene with rings. NH This3, many change quinoid can units increase of the the PANI energy reduced gap resultingby N2H4 transformin the largest into E benzenea for PANI(N rings.2H This4) [35, change40]. can increase the energy gap resulting in the largest Ea forThe PANI(N differences2H4)[ 35in ,E40a values]. between PIL matrix and PANI filler for composite particles can bring moreThe differenc differenceses in indielectricEa values polarization between PIL rate matrix with and increasing PANI filler temperature. for composite Figure particles 11B canplots bring the variationmore diff erencesof the ratios in dielectric of λPIL of polarization PIL matrix rateto λPANI with of increasing PANI filler temperature. with temperature Figure. This11B plotsratio can the reflectvariation the ofmagnitude the ratios of of differenceλPIL of PIL of matrix two polarization to λPANI of rates. PANI From filler Figure with temperature. 11B, it can be This seen ratio that canthe ratioreflect of thePIL magnitude/PANI(HCl) of is dimuchfference larger of twothan polarizationothers, which rates. exhibits From a huge Figure mismatch 11B, it caning between be seen thatPIL matrixthe ratio and of PIL PANI(HCl)/PANI(HCl) filler is. much As a larger result, than the others, flow curves which exhibits are not a stable huge mismatching within the measured between temperaturePIL matrix and (see PANI(HCl) Figure 7B). filler.As for As PIL a/PANI(NH result, the3), flow the curvesratio is areclosest not to stable one at within room the temperature measured, sotemperature it possesses (see the Figure most 7stableB). As flow for PIL curves/PANI(NH and the3), largest the ratio τd isat closestthis conditio to onen at. However, room temperature, the ratio startsso it possesses to depart thefrom most one stable with flowincreasing curves temperature, and the largest indicatingτd at this the condition. increase of However, the mismatching. the ratio Thisstarts change to depart leads from to the one rapid with decline increasing of τd (see temperature, Figure 8). indicatingOn the contrary, the increase the ratio of of the PIL mismatching./PANI(N2H4) becomesThis change clos leadser to toone the as rapid temperature decline of increasesτd (see Figure, which8). leads On the to contrary,more and the more ratio stable of PIL flow/PANI(N behavior2H4) (seebecomes Figure closer 7D). to oneTherefore, as temperature the temperature increases, modulatedwhich leads rheological to more and tests more fu stablerther flow support behavior the conclusion(see Figure 7thatD). Therefore,the flow behavior the temperature of the ER modulated fluids of rheologicalPIL/PANI composite tests further particles support de thepends conclusion on the matchingthat the flow degree behavior of dielectric of the polarization ER fluids of rate PIL /betweenPANI composite PIL matrix particles and PANI depends filler. on The the closer matching their polarizationdegree of dielectric rates are, polarization the more ratestable between flow curve PIL matrix is in a and broad PANI shear filler. rate The region closer under their polarizationan external electricrates are, field the. more stable flow curve is in a broad shear rate region under an external electric field.

1−1 PIL PANI FigureFigure 11. 11Temperature. Temperature dependence dependence of of (A (A) λ) −λ and and ( B(B))λ λPIL//λλPANI. .

44.. Conclusions Conclusions By using using different different structural states of semiconducting semiconducting PANI PANI as filler filler,, we investigate the influence influence of polarization polarization rate rate difference difference of of filler filler and and matrix matrix on on the the flow flow behavior behavior of ofelectrorheological electrorheological flu fluidsids of PILof PIL/PANI/PANI composite composite particles particles under under external external electric electric fields. fields. The PIL The/PANI PIL/PANI composite composite particles particles have been firstly prepared by a post counterion conversion procedure, and then treated by ammonia or hydrazine to achieve different electrical responses of PANI filler. Under electric fields, only the PIL/PANI(NH3) particles show a stable flow behavior in a broad shear rate region and an enhanced ER response at room temperature. Nevertheless, the flow curves of PIL/PANI(N2H4) become more and more stable with measure temperature increasing. By using dielectric spectroscopy, it can be found that the polarization rate of matrix should be located in an appropriate range, and then the flow stability depends on how closely the dielectric polarization rates of the outside PIL matrix and

Polymers 2020, 12, 703 14 of 16 have been firstly prepared by a post counterion conversion procedure, and then treated by ammonia or hydrazine to achieve different electrical responses of PANI filler. Under electric fields, only the PIL/PANI(NH3) particles show a stable flow behavior in a broad shear rate region and an enhanced ER response at room temperature. Nevertheless, the flow curves of PIL/PANI(N2H4) become more and more stable with measure temperature increasing. By using dielectric spectroscopy, it can be found that the polarization rate of matrix should be located in an appropriate range, and then the flow stability depends on how closely the dielectric polarization rates of the outside PIL matrix and inside PANI filler match. The closer they are, the more stable the flow curves are. This result may provide guidance to select suitable materials with matched polarization rates as matrix and filler to design optimal ER composite materials with enhanced and stable ER effect. In order to fix this conclusion, more studies, such as core-shell-like and Janus-like ER particles, are worth further investigation because of their tunable morphology and property.

Author Contributions: Conceptualization, C.Z. and J.Y.; methodology and investigation, C.Z.; validation, Q.L. and J.Z.; formal analysis and data curation, C.Z. and J.Y.; supervision, J.Y. and X.Z.; writing—original draft preparation, C.Z.; writing—review and editing, J.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51872243. Acknowledgments: We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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