shell composite nanospheres synthesized using a reactive surfactant and their electrorheology

Polymer 188 (2020) 122161

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Poly(diphenylamine)/polyaniline core/shell composite nanospheres synthesized using a reactive surfactant and their electrorheology Wen Jiao Han a, Jin Hyun Lee b, **, Hyoung Jin Choi a, * a b

Department of Polymer Science and Engineering, Inha University, Incheon, 22212, South Korea Polymer Research Center, Inha University, Incheon, 22212, South Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrorheological fluid Core/shell nanoparticle Reactive surfactant Polyaniline Poly(diphenylamine)

Relatively monodispersed poly(diphenylamine)/polyaniline (PDPA/PANI) core/shell composite nanospheres were fabricated by an oxidative polymerization, both in the dispersed phase of diphenylamine and at the interface between the suspended and continuous phases. A reactive surfactant synthesized with aniline and sodium dodecyl sulfate was adopted. The fine core/shell structure formation via π-π stacking between the aro­ matic rings of PDPA and PANI was confirmed by analyzing the chemical composition, morphology, and crys­ tallinity of the particle components. An electrorheological (ER) fluid was fabricated by suspending the PDPA/ PANI nanoparticles in silicone oil, and its ER behaviors were characterized by the controlled shear stress and rate, dynamic oscillation, and on-off electrical field tests. The ER fluid under an electric field exhibited an im­ mediate phase transition, viscoelasticity, and three measurement-dependent yield stresses. In addition, its stable shear stresses over a wide range of shear rates were fitted smoothly by the CCJ model and were shown to increase with increasing EF strength. Moreover, the dielectric characteristics and Cole-Cole analysis of the ER fluid confirmed the effective ER property. The PDPA/PANI-based ER fluid possessing an instantaneous and control­ lable electro-response is believed to be a promising system for intelligent control applications.

1. Introduction Smart materials made from actively controllable electrorheological (ER) fluids are prospective classes of materials whose mechanical and viscoelastic properties can be tuned by external electrical stimuli and used in numerous academic and industrial applications [1–3]. ER fluids are colloid systems of electrically polarizable particles suspended uni­ formly in an insulating liquid. Their desired phase state and properties are achieved by regulating the presence or absence of electric fields and by adjusting the electric field strength (E); thus, they can be used as very promising electro-responsive smart materials [4]. Without an applied E, the suspensions are in a liquid-like phase, exhibiting Newtonian fluid-like behavior. With an applied E, however, the particles tend to build up a highly organized structure aligned along the direction of the applied E due to the dipole-dipole interaction between the polarized particles. This process occurs rapidly and reversibly, resulting in ER fluids exhibiting a solid-like phase. Their rheological properties, such as shear viscosity, yield stress, and viscoelastic moduli, are dependent on the applied E [5–7]. Among these, the yield stress, as one of the critical

parameters of ER fluids for use in rheology studies and torque transfer applications, is the minimum stress value where the fluids begin to flow. When the applied shear stress is larger than the yield stress, the chain-like structure produced by the aligned particles in the fluids de­ forms and the hydrodynamic force is predominant over the electrostatic force [8]. These controllable and reversible intelligent behaviors of ER fluids have promoted the development of an extensive range of devices, such as active shock absorber, damper, clutch, and ER polishing [9–11]. Although the technology of many ER devices has entered the prototype stage, their commercialization has not yet been fully realized. The main limiting factor impeding the progress of ER technology is the lack of effective ER fluid systems with colloidal stability and high yield stress. Therefore, studies of ER fluids have been focused on developing suitable electro-responsive particles. Among the various materials for ER fluids, electron conductive polymers are forward-looking ER materials with extended π-conjugation on the polymer backbone with a semi­ conductor behavior [12]. Electrically conducting polymers that have not only similar electrical properties to metals, but also the features of organic polymers, such as low density, excellent flexibility, corrosion

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.H. Lee), [email protected] (H.J. Choi). https://doi.org/10.1016/j.polymer.2020.122161 Received 12 November 2019; Received in revised form 31 December 2019; Accepted 4 January 2020 Available online 6 January 2020 0032-3861/© 2020 Elsevier Ltd. All rights reserved.

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Polymer 188 (2020) 122161

resistance, and conductivity variability, have the potential to improve the ER performance [13]. Among conductive polymers, polyaniline (PANI) has been used for ER material research for many years, owing to its excellent environmental solidity, easy synthesis route, and control­ lable conductivity [14,15]. Nonetheless, the original PANI possesses some defects as an ER particle: irregular morphology, poor solubility in an organic solvent, colloidal instability, high current density, and easy electrical breakdown in high E. The composites containing PANI and other materials such as vana­ dium oxide, carbon and clay have been demonstrated to improve these limitations [16–18]. In particular, monodisperse spherical core/shell structures have attracted considerable attention because symmetrical polarization is induced in the particles with a spherical structure and the core/shell morphology leading interfacial polarization provides better ER performance [19,20]. PANI-based core/shell nanoparticles for ER fluids were synthesized using several methods, such as chemical oxidative polymerization [21], in-situ polymerization [22] in the pres­ ence of a surfactant, grafting polymerization [23], and seeded swelling polymerization [24]. In this study, oxidative polymerization with the introduction of a reactive surfactant that provides environmental and economic advantages is being utilized. Reactive surfactants with special two functions (concurrent roles of typical surfactants and a component for polymerization) have attracted considerable interest because they can effectively support the develop­ ment of novel nanoparticles, biofilms, drugs, drug targeted delivery systems, and catalysts [25]. Although they can stabilize the disperse phase, producing a good particle size distribution, such as traditional surfactants, they also have a moiety, such as a monomer that partici­ pates in polymerization, producing a part of the resulting polymer, the unlike traditional surfactants [26]. Therefore, reactive surfactants can induce polymerization at the interface of the suspended and continuous phases in the emulsion system as well as at the dispersed phase. The reactive surfactant used in this study for the fabrication of promising ER particles was synthesized with a component (here, aniline) constituting the shell part of the fabricated core/shell particles and a type of sur­ factant (here, SDS). Polydiphenylamine (PDPA) constituting the core part of the core/ shell nanoparticles prepared in this study is a PANI derivative that is structurally similar to PANI except that PDPA has the sequences of two aromatic rings in its backbone instead of one and can have multiple oxidation states. Owing to these structural differences between PDPA and PANI, PDPA exhibits higher thermal stability, electrochromism, and miscibility with organic solvents (e.g., tetrahydrofuran or chloroform), which are complementary features to PANI when they are used together [27]. In addition, PDPA has been reported to possess good redox prop­ erties, electrochemical, and optical properties, as well as environmental stability. Therefore, either homopolymers or composite forms are anticipated to have desirable properties [28,29]. Note that the con­ ductivity of polymers, such as PANI, is generally determined by the molecular structure of the polymer; hence, the copolymerization of aniline and another suitable monomer, helping control the conductivity, can be an effective way of controlling the molecular structure of poly­ mers [30]. Accordingly, PDPA possessing relatively lower electrical conductivity than PANI is used with PANI together, the resulting PDPA/PANI composite particles are predicted to have the appropriate and controllable conductivity for use in ER fluid systems without a de-doping process and have the advantage of avoiding the occurrence of electrical short circuits in operation. In this study, relatively monodisperse PDPA/PANI core/shell com­ posite nanospheres were synthesized by oxidation polymerization in an emulsion system using a reactive surfactant, called aniline dodecyl sulfate (ADS), which had been synthesized with a traditional surfactant, sodium dodecyl sulfate (SDS), and a monomer, aniline. The core/shell formation via π-π stacking between the aromatic rings of PDPA and PANI was anticipated and characterized by analyzing the chemical composition, morphology, and crystallinity of the particle components.

The spherical PDPA/PANI nanoparticles exhibited excellent ER prop­ erties that were most probably caused by uniform and interfacial po­ larization induced in the particles with a spherical shape and core/shell morphology. Accordingly, a PDPA/PANI-based ER fluid containing the PDPA/PANI nanoparticles was prepared, and their rheological proper­ ties were investigated by controlling the shear stress and shear rate, dynamic oscillation, and on-off electrical field tests as well as by two analytical models: the Bingham fluid model and the Cho–Choi–Jhon (CCJ) equation. Moreover, the polarizability of the nanoparticles affecting the ER performance was investigated by dielectric measure­ ments and by the Cole-Cole model. Few studies have assessed the use of the spherical PDPA/PANI core/shell nanoparticles, synthesized by oxidative polymerization using a reactive surfactant, as ER materials for ER fluids. Studies of the ER properties, including the change from a fluid-like to solid-like state, flow behavior, viscoelasticity, and yield stress of the PDPA/PANI-based ER fluid under an, confirmed the supe­ riority and potential of the spherical PDPA/PANI core/shell nano­ particle as the ER dispersed phase. The PDPA/PANI-based ER fluid exhibited an instantaneous controllable response to the change in E and possessed a stable shear stress (τ) platform region over a whole shear rate (_γÞ range, making it practical and promising for intelligent control systems. 2. Experimental 2.1. Materials Aniline (99%, DC Chem., Korea), sodium dodecyl sulfate (SDS) (�99.0% purity, Sigma-Aldrich), and hydrochloric acid (HCl, 1 mol/L, 35%, Duck San Chem. Korea) were used to synthesize the ADS reactive surfactant. Diphenylamine (DPA) (99% purity, Sigma-Aldrich), ammo­ nium persulfate (APS) (�98.0% purity, Daejung Chem., Korea), ethyl alcohol (95% purity, Samchun Pure Chem., Korea), and deionized (DI) water were used as received for the fabrication of the PDPA/PANI core/ shell nanoparticles. Silicone oil (KF-96-100cSt, the density of 0.96 g/ cm3, Shinetsu, Japan) was adopted as the base fluid of the PDPA/PANIbased ER suspension prepared in this study. 2.1.1. Synthesis of ADS reactive surfactant The ADS reactive surfactant was synthesized from a surfactant, SDS (colored in blue), and a monomer, aniline (colored in violet), via ion exchange. Under acidic conditions, the aniline ion replaces the sodium counter-ion binding to the dodecyl sulfate ion, so that the functionality is located on the aniline counter-ion group, as shown in Scheme 1(a). Compared to the traditional surfactant SDS, the synthesized reactive surfactant, ADS, plays important roles: acting as a surfactant to stabilize the disperse phase in an emulsion system owing to its amphiphilicity, and as a monomer to produce polymers (here, PANI) via participation in polymerization. Here, ADS was synthesized using the following detail steps. Aniline (2.79 g) and HCl (1 mol/L, 30 mL) were placed into DI water (80 mL) in a vial, with vigorously stirring for more than 40 min at room temperature. Simultaneously, SDS (8.6 g) was dissolved completely in DI water (200 mL) by intensive magnetic stirring for more than 30 min. The two solutions were mixed, and the mixture was then input to a 500 mL three-neck reactor. The mixture was heated to 50 � C for 3 h using a water circulator for salt purification because ion exchange between the sodium ion and aniline ion produced a white precipitate because of the much lower solubility of the resulting ADS salt in water at room temperature. After cooling the mixture to 4 � C with stirring for 12 h, the resulting white product was recovered via a centrifuge for 15 min at 5,300 rpm and dried in a vacuum oven at 65 � C. 2.1.2. Fabrication of spherical PDPA/PANI core/shell nanoparticles Chemical oxidation polymerization, which is one of the commonly used methods to fabricate polymers from aromatic compounds, was adopted for fabricating relatively monodisperse PDPA/PANI core/shell 2

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Scheme 1. Schematic diagrams of the synthesis processes of (a) aniline dodecyl sulfate (ADS) reactive surfactant and (b) spherical PDPA/PANI core/sell nanoparticles.

nanospheres [31]. DPA has low miscibility with DI water, but can be emulsified easily by ultrasonic treatment above its melting temperature (53 � C). The oxidative polymerization of DPA and aniline occurred not only in the disperse phase (DPA droplets), but also at the interface be­ tween the disperse phase and continuous phase (aqueous solution con­ taining aniline molecules) in the emulsified system. Scheme 1(b) shows the specific synthesis procedures of the spherical PDPA/PANI core/shell nanoparticles. The DPA monomers (0.25 g) were added to DI water (50 mL) in a three-neck glass vessel and heated up to 60 � C with mechanical stirring until the DPA monomers had melted completely. After ultrasonic treatment of the emulsion at 60 � C for 3 min, the oil-in-water emulsion system turned white. The synthesized ADS molecules containing aniline ions were then added to the emulsion, followed by ultrasonication for 30 min. During this process, the ADS molecules attached on the surface of DPA oil droplets. The emulsion system was heated for a further 3 h with mechanical stirring, and then cooled slowly to room temperature. Subsequently, the reactor was moved to an ice bath at 5 � C with mechanical stirring. The APS (0.6 g), as an initiator reagent, was dissolved in an aqueous HCl solution (0.25 mol/L, 10 mL), and the solution was inserted dropwise to the emulsion. The oxidative polymerization continued for 5 h and the color of the reaction mixture turned green. Polymerization occurred within the DPA emulsified droplets and also at the surface of the droplets where the ADS molecules containing aniline groups were placed, producing nano­ particles with the PDPA/PANI core/shell structure. After the reaction, the resulting product was washed with the solvent mixture of water and ethyl alcohol (1/3, v/v) and then separated using a centrifuge at 5,500 rpm for 20 min each time. The dark green particulates were gathered after filtering and drying in an oven at 60 � C. For characterization of the PDPA/PANI core/shell nanoparticles, pure PDPA and PANI were also prepared by oxidative polymerization using APS [32].

appropriate for ER studies without the need for further doping/dedoping step. To confirm the potential of practical application as an effective ER material of these PDPA/PANI core/shell particles, an ER fluid was fabricated by suspending 10 vol % of the PDPA/PANI nano­ particles in silicone oil (named the PDPA/PANI-based ER fluid), and its ER characteristics and performance were characterized. A uniformly dispersed suspension was achieved by mechanical stirring and ultrasonication. 2.2. Characterizations The morphology of the PDPA/PANI nanoparticles was measured by both scanning electron microscopy (HR-SEM) (SU-8010, Hitachi, Japan) and transmission electron microscopy (TEM) (CM200, Philips, Netherlands). Before the SEM observation, the sample was placed on a holder and coated with palladium, and then the SEM device was oper­ ated at a voltage of 15 kV. On the other hand, as for the TEM sampling, the nanoparticles were dispersed in alcohol and sonicated for 30 min. The solution was then dropped on the TEM grid with a dropper. The density was obtained using a gas pycnometer (Accupyc 1330, Gas pyc­ nometer, USA). The chemical structure and elemental composition were investigated by Fourier-transform infrared spectroscopy (FT-IR) (VER­ TEX 80V, Bruker) and X-ray diffraction (XRD) (DMAX-2500, Rigaku). In addition, the electrophoretic light scattering device (ELS-8000, Otsuka, Japan) was used to investigate the size distribution of the synthesized nanoparticles. Under a range of electric fields generated by a voltage generator, the change in the internal structure and ER characteristics of the PDPA/ PANI-based ER fluids were scrutinized by optical microscopy (OM, Olympus BX-51, USA) and a rotation rheometer (MCR300, Anton-Paar GmbH, Graz, Austria) with a concentric cylinder device (CC17, AntonPaar, Austria), respectively. During the ER measurements, an E was applied perpendicular to the flow direction of the sample, and prior to all the measurements, the nanoparticles in the fluids were dispersed thoroughly by sonication and vortexing to avoid aggregation. The dy­ namic oscillation, controlled shear stress, and controlled shear rate tests

2.1.3. Preparation of PDPA/PANI-based ER fluid Electrical conductivity of the PDPA/PANI nanoparticle tested by a resistivity meter was 2:3 � 10 8 S/cm. This value is within the window 3

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benzoid groups, respectively. The peaks at 1298 and 1112 cm 1 corre­ spond to the stretching vibration of C N in the aromatic amine, and inplane bending vibrations of aromatic C H, respectively [35,36]. In contrast, for the spectrum of PDPA (blue solid line), the main peaks at 1595 and 1504 cm 1 were associated with the C ¼ C stretching vibration modes in quinoid and benzene rings, respectively. The peaks for the stretching vibration of the aromatic secondary amine and in-plane bending vibration of C H were observed at 1317 and 1172 cm 1 [37,38]. The spectrum of the PDPA/PANI nanoparticles (red solid line) included the characteristic peaks of PANI and PDPA. The two different peaks at 1490 and 1595 cm 1, corresponding to the vibration of quinoid or benzoid C ¼ C in PANI and PDPA, respectively, also appeared in the PDPA/PANI composite nanoparticles. Furthermore, both peaks at 1298 and 1317 cm 1 were observed in the PDPA/PANI composite nano­ particle, whereas the former was only presented in PANI and the latter was seen only in PDPA. These results show that there is no poly (DPA-co-ANI) copolymer domain, but the PDPA and PANI domains coexist separately (here, as a core and shell) in the composite nano­ particles attaching to each other via π-π interactions (or π-π stacking) between the aromatic rings of PDPA and PANI. The crystallinity and orientation of the PDPA/PANI nanoparticles were analyzed by XRD. Fig. 4 shows the XRD patterns of the PANI (black solid line), PDPA (blue solid line), and PDPA/PANI composite (red solid line) nanoparticles. Two major diffraction peaks in the spectrum of the pure PANI nanoparticles at 20:6� ​ and ​ 25:3� 2θ were set to the peri­ odicities parallel and perpendicular to the PANI molecular chain, respectively [9]. For pure PDPA nanoparticles, however, the peaks at ​ 18:5� ​ and ​ 21:0� 2θ represent the periodicity parallel and perpendic­ ular to the PDPA molecular chain, respectively [39,40], regardless of the doping status. On the other hand, the XRD pattern of the PDPA/PANI composite nanoparticles showed peaks at 18:5� ​ and ​ 21:0� , 2θ, which were observed in the spectrum of the PDPA synthesized. This indicates the presence of a crystalline structure of PDPA in the composite parti­ cles. In addition, the overall spectrum of composite particles was broad with few intense peaks because of the existence of PANI. These obser­ vations also support the separate coexistence of the PDPA core and PANI shell domains in the PDPA/PANI composite particles. The output of the PDPA/PANI-based ER suspension to the E was observed by placing the ER fluid on a light-transmissive substrate con­ nected to two aluminum electrodes using OM. The internal structure of the ER suspension depends on the presence of an input E. Without an E, the ER fluid exhibited a liquid-like form, in which the electro-responsive particles were suspended irregularly (Fig. 5(a)). Under an input E of 2.5 kV/mm, however, the electrically polarized PDPA/PANI nanoparticles aligned instantly in the direction parallel to the applied EF, forming a fibrils-like (or chain-like) structure between the two electrodes (Fig. 5

of the fluid were assessed at 25 � 0:1� C, and their flow and viscoelas­ ticity behaviors were examined by measuring the shear viscosity, stor­ age modulus (G0 ), loss modulus (G00 ), and stress relaxation modulus. In addition, the three yield stresses of the ER fluid (dynamic, elastic, and static yield stresses) were determined from the flow curve, elastic stress curve, and curve of viscosity vs. controlled shear stress. Moreover, on-off tests of the fluids were carried out by periodically applying a square voltage pulse at a constant γ_ to measure the sensitivity and reversibility of the ER fluid to the electric field. Furthermore, analytical studies of the observed ER behaviors were carried out using the Bingham and CCJ models. Finally, the dielectric characteristics of the ER fluid were measured using an LCR meter (Agilent HP 4284A) over the frequency range of 20 Hz to 1 MHz. 3. Results and discussion Fig. 1(a) presents a HR-SEM image of the PDPA/PANI core/shell nanoparticles synthesized by oxidative polymerization using the ADS reactive surfactants. The nanoparticles were spherical and relatively monodisperse, even though their diameters were in the range of 60–120 nm (Fig. 1(b)). Their density was measured to be 1.3 g/cm3. Therefore, the size of the polymer particles prepared by emulsion polymerization depends on the dimensions and shape of the disperse phase. Moreover, the relatively monodisperse size distribution and spherical morphology of the PDPA/PANI nanoparticles could confirm not only that the ADS reactive surfactant synthesized in this study stabilized the emulsion well, but also that the size of the DPA droplets stabilized by the ADS showed a typical dimension of the disperse phase of the mini-emulsion (50–500 nm) [33,34]. Fig. 2 presents TEM image of the morphology of the PDPA/PANI composite nanoparticles. Fig. 2(b) shows the spherical shape and rela­ tive monodispersity of the PDPA/PANI nanoparticles, as well as their core/shell morphology. The dark black core of the sphere in the image was attributed to PDPA polymerized from the DPA mini-emulsified droplets, the gray shell part was made of PANI polymerized from the aniline ions of the ADS reactive surfactants adsorbed at the surface of the DPA droplets in the emulsion. The mean diameter and shell thickness of the particle core were approximately 59 nm and 13 nm, respectively, and the average volume ratio of the PDPA core and PANI shell parts constituting the composite nanoparticles was determined to be 11:1. Chemical structural study of the PDPA/PANI nanoparticles was carried out by FT-IR spectroscopy. Fig. 3 presents the infrared spectra of PANI, PDPA, and the PDPA/PANI composite nanoparticle in the mid infra-red range of 400–4000 cm 1. In the spectra of pure PANI (black solid line), the characteristic peaks at 1570 and 1490 cm 1 were assigned to the stretching vibration modes of C ¼ C in the quinoid and

Fig. 1. (a) HR-SEM image and (b) size distribution of the synthesized PDPA/PANI nanoparticles. 4

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Fig. 2. TEM images of the synthesized PDPA/PANI nanoparticles.

Fig. 3. FT-IR spectra of the PDPA/PANI (red solid line) nanoparticles, PANI (blue solid line), and PDPA (black solid line). (For interpretation of the refer­ ences to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4. XRD patterns of the PDPA/PANI (red solid line) nanoparticles, PANI (blue solid line), and PDPA (black solid line). (For interpretation of the refer­ ences to color in this figure legend, the reader is referred to the Web version of this article.)

(b). The ER suspension showed a solid-like state owing to the structure produced in the ER suspension. The effects of the E on the flow characteristics of the PDPA/PANIbased ER fluid (10 vol% concentration) were investigated via a controlled shear rate (CSR) testing method. Fig. 6(a) exhibits its τ as a function of the γ_ (0.01–200 s 1) for various applied E up to 2.5 kV/mm at 0.5 kV/mm intervals. At zero E, the τ was linearly dependent to the γ_ with a slope of approximately 1.0, indicating that the ER fluid possesses Newtonian-fluid like behavior. Under an applied E, however, the ER fluid exhibited clear yield behavior, showing a wide τ plateau over a γ_ range of 0.01–10 s 1. The value estimated by extrapolating the τ for the E to a zero shear rate is called as the dynamic yield stress (marked by the blue circle). Significantly, with increasing E, the dynamic yield stress increases gradually and the stress plateau region becomes wider, which is caused by the more enhanced electrostatic interactions among the polarized nanoparticles dispersed in the fluid. In other words, the ability of the ER fluid to resist a hydrodynamic force improved with increasing EF strength. Note that the rupture-reformation of the chain-like arrangement of the suspended particles in the ER fluid occurs repeat­ edly under a continuously applied shear because of the simultaneous roles of hydrodynamic and electrostatic forces [41]. Subsequently, in the high γ_ region, the fluid exhibits pseudo-Newtonian fluid property mainly because the reformation rate of the fractured form becomes slower than

Fig. 5. OM images of the PDPA/PANI-based ER fluid observed between two parallel electrodes (150μm) (a) in the absence of an electric field and (b) under an electric field (2.5 kV/mm).

the 5

shear

deformation

rate,

resulting

in

the

domination

of

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From the right side, the first term denotes the shear property of the ER fluid at a low γ_ range, and the second term implies the shear-thinning characteristics at a high γ_ range. t1 and t2 are the time constants, and η∞ represents the shear viscosity at an infinite γ_ . The index α is related to the reduction of the τ at a low γ_ range, and the index β (0 <β � 1, due to dτ=dγ_ � 0Þ, denotes the increase in τ at a high γ_ region. Table 1 shows the resulting physical parameters obtained for the above two models. As a result, the CCJ model fits the τ data of the PDPA/ PANI-based ER system more accurately than the Bingham fluid equation over the entire γ_ range. Nevertheless, the yield stresses (τy;B and τy;CCJ ) obtained by the Bingham and CCJ models were similar to each other at each E. Fig. 6(b) represents the shear viscosity of the ER system as a function of the γ_ for various E values. At a zero E, the fluid exhibited almost constant viscosity, indicating Newtonian-like fluid behavior. The ER fluid under an applied E exhibited typical shear-thinning behavior, and the degree of shear viscosity thinning became more significant as the E increased. The electro-response of the PDPA/PANI-based ER fluid was also evaluated by determining the ER efficiency (I), which was calculated using Eq. (3): I ¼ ​ ðηE

where ηE and η0 correspond to the shear viscosity with and without an input E, respectively. Fig. 7(a) shows the calculated ER efficiency of the ER system as a function of the γ_ for various E values. The ER fluid exhibited strong ER efficiency because of the high increment in viscosity under an input E, and the ER efficiency increased with increasing the E. In addition, the ER efficiency decreased with increasing shear rate, reflecting the gradual destruction of the chain-like form of the PDPA/ PANI nanoparticles in the fluids. The resistance to the structural destruction increased with increasing EF strength. Furthermore, the change in leaking current density with an electric field strength was recorded, as shown in Fig. 7(b). Higher leaking current density was found to occur in the stronger electric field strength range. The viscoelastic behaviors of the PDPA/PANI-based ER suspension were observed by studying the G0 and G00 as a function of frequency in the linear viscoelastic region (γLVE) through dynamic oscillatory tests. Prior to all the angular frequency (ω) sweep tests, the strain amplitude sweep test was first performed to find the γLVE. Fig. 8(a) presents the G’ (closed) and G’’ (open) of the ER fluid as a function of the strain amplitude (10 3–102%) at a fixed ω of 6.28 rad/s for various applied E (zero to 2.5 kV/mm). First, G0 is always larger than the G00 , indicating that the elastic part of the ER fluid is more dominant than the viscosity at a strain amplitude value less than 0.04%. In addi­ tion, the γ LVE can be determined from the region where the G0 values are constant regardless of the strain, and structural deformation is revers­ ible. The γ LVE and G values increased with increasing EF strength, respectively. In the strain range beyond the γLVE, both G0 and G00

Fig. 6. (a) Shear stress and (b) shear viscosity of the PDPA/PANI-based ER fluid as a function of shear rate for various applied electric field strengths. The solid and dashed lines are obtained from the numerical fitting by the CCJ and Bingham models, respectively.

hydrodynamic shear deformation. This τ of the PDPA/PANI-based suspension was analyzed as a func­ tion of γ_ using the Bingham fluid equation, as presented in Eq. (1) [42]. This model is generally used to examine semiconducting polymer-based ER fluids.

τ ¼ τy;B þ ηγ_ γ_ ¼ 0

τ � τy;B τ < τy;B

(3)

η0 Þ = η0

(1)

Here, two parameters, η (viscosity) and τy; ​ B (the yield stress deter­ mined from Bingham model), are related to the E. The Bingham model has been employed to describe two flow regimes: a pre-yield regime, where the field-dependent τy;B is smaller than τ, and the other is a postyield regime, where the fluid shows Newtonian fluid-like behavior beyond τy;B . Unfortunately, the curves obtained using the Bingham model (dashed lines) for the experimental data cannot explain the flow behavior of the PDPA/PANI-based ER fluid well, which is shown particularly at a high γ_ regime. The ER fluid exhibited pseudo-plastic flow behavior at a high γ_ beyond the yield point and steady shear flow characteristics over a wide range of the γ_ below the yield point. These flow behaviors were fitted rather accurately by the curves (solid lines) obtained by the CCJ model as follows [43]: � � τ 1 γ_ (2) τ ¼ y;CCJ α þ η∞ 1 þ 1 þ ðt1 γ_ Þ ðt2 γ_ Þβ

Table 1 Fitting parameters of the Bingham and CCJ models fitting the curves of the shear stress of the PDPA/PANI-based ER fluid over the whole range of shear rate for different applied electric field strengths. Model

Parameters

Bingham

τy;B ​ ðPaÞ η ðPa⋅sÞ

CCJ

0.5

1.0

1.5

2.0

10.0

22.6

43.7

71.5

2.5 103

0.209

0.160

0.096

0.040

0.004

τy;CCJ ðPaÞ

10.4

15.7

31.1

64.2

92.1

t1 ​ ðmsÞ

141

637

232

71.3

62.1

α η∞ ðPa⋅sÞ

0.352 0.186

0.718 0.190

0.832 0.154

0.900 0.117

0.971 0.436

t2 ​ ðmsÞ

30.2

10.7

4.09

1.48

0.340

0.744

0.931

0.901

0.773

0.702

β

6

Electric Field Strength [kV/mm]

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Fig. 8. (a) Strain-dependent viscoelasticity: storage modulus (G0 : closed sym­ bols), loss modulus (G00 : open symbols) of the PDPA/PANI-based ER fluid as a function of strain %, and (b) elastic stress of the PDPA/PANI-based ER fluid as a function of strain % for the various applied electric field strengths.

Fig. 7. (a) ER efficiency (I ¼ ðηE η0 Þ=η0 ) of the PDPA/PANI-based ER fluid as a function of shear rate (ηE : the shear viscosity in the present electric field, η0 : the shear viscosity in the absence of the EF) for the various applied electric field strengths; (b) Leaking current density with varying electric field strength for ER fluid.

200 rad/s. Both moduli G0 and G00 of the fluid increased with increasing E, meaning strong ER characteristics, and the G0 values were always higher than the G00 values, indicating solid-like properties. The electroresponse of the PDPA/PANI-based fluid was also verified by examining the stress relaxation modulus that was evaluated using dynamic tests with an inverse frequency, which is correlated with time. Note that stress relaxation is difficult to obtain because of the limitation of the mechanical measurements and the complexity of the calculation by applying a linear integral variation [47]. Therefore, the stress relaxation modulus (G(t)) of the ER system was estimated from G0 and G00 obtained from the frequency sweep tests using the Schwarzl equation shown in Eq. (4) [48].

decreased with increasing strain, which is probably caused by the chainlike arrangement in the ER fluid undergoing more irreversible destruc­ tion. The 8 � 10 3 strain % in the γLVE was selected as a fixed strain for continued ω tests [44]. In addition, this dynamic amplitude sweep data was analyzed further from a plot of the elastic stress as a function of strain. The elastic stress 0 0 (τ ) was calculated by the product of the strain amplitude (γ) and G’: τ ¼ 0 0 G γ [45]. Fig. 8(b) presents the τ of the ER fluid as a function of strain 0 [%] for a range of applied EF strengths. A linear relationship between τ and γ was observed at a low stain region, indicating an elastic response 0 of the ER fluid to structural deformation. The τ increased linearly up to 0 an inflection point from which the increase in τ began to decrease. Here, the inflection points (marked by underbars) referred to the elastic yield stresses below which the deformed chain-like form of the particles in the ER fluid can recover completely and reversibly [46]. With increasing 0 applied EF strength, the τ appeared to increase, and the yield strain, which is the strain value corresponding to the elastic yield stress, also increased slightly. Fig. 9(a) and (b) show G0 and G00 , respectively, of the PDPA/PANIbased ER fluid as a function of ω at a fixed strain % of 8 � 10 3 for a range of applied EF strengths. In the absence of EF, both G0 and G00 increased gradually with increasing ω, which is similar to the liquid-like behavior. In contrast, in the presence of an external E, they displayed a stable plateau region with high values over the entire ω range from 1 to

0

GðtÞ ffi G ðωÞ

0:566 G}ðω=2Þ þ 0:203 G}ðωÞ

(4)

Fig. 10 presents the calculated G(t) of the fluid as a function of time for different applied E. G(t) decreased linearly without the E, meaning Newtonian fluid-like behavior. In contrast, under an input E, the G(t) showed a plateau over the range of time and increased with increasing applied E, which indicates that the ER fluid exhibits a solid-like phase caused by the E -induced chain-like arrangement of the PDPA/PANI nanoparticles. Fig. 11 shows the shear viscosity of the PDPA/PANI-based ER fluid as a function of the τ for various applied EF strengths, which was measured using a controlled τ method (CSS). The viscosity decreased gradually with increasing τ up to the critical point and decreased drastically 7

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Polymer 188 (2020) 122161

Fig. 11. Shear viscosity of the PDPA/PANI-based ER fluid as a function of shear stress for various applied EF strengths.

various EF strengths are regarded as the static yield stresses, below which the ER fluids exhibit no flow and above which the fluids show liquid-like behavior with a plastic viscosity [49]. The yield stress is a significant parameter for the development and application of ER fluids and is related to the safety of the applications and energy for practical uses. Furthermore, the yield stress is strongly dependent on the measurement technique and condition. Therefore, there are various approaches to estimate the yield stress for different applications. In this study, three different yield stresses (static, dynamic, and elastic yield stresses) of the PDPA/PANI-based ER fluid were determined in three different ways and compared. The dynamic yield stresses of the ER fluid were determined by an extrapolation of the τ data to a zero γ_ in the flow curves (Fig. 6). In contrast, the elastic and static yield stresses were obtained from the inflection points in the elastic stress curves (Fig. 8) and from the drastic change points of the viscosity data obtained in controlled shear stress tests (Fig. 11), respectively. Fig. 12 shows the relationship between the three yield stresses (static, dynamic, and elastic yield stresses are denoted as blue, black, and red symbols, respectively) and the EF strength. In general, the correlation between the yield stress (τy Þ and E is

Fig. 9. (a) Storage modulus (G0 ) and (b) loss modulus (G00 ) of the PDPA/PANIbased ER fluid with varying angular frequency (ω) for the various applied electric field strengths.

Fig. 10. Stress Relaxation modulus G(t) of the PDPA/PANI-based ER fluid as a function of time, which was obtained from the frequency sweep data and by using the Schwarzl equation (Eq. (4)).

Fig. 12. Three different yield stresses (static, dynamic, and elastic yield stresses: blue, black and red symbols, respectively) vs. applied electric field strength of the PDPA/PANI-based ER fluid. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

immediately after the point, which is the minimum τ for the flow of an ER fluid. The viscosity decreased gradually again and reached an equi­ librium value. The critical points (marked by the blue circles) for the 8

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Polymer 188 (2020) 122161

described as follows:

τy ∝ E

two permittivity values is defined as Δε (Δε ¼ ε0 ε∞ ), being related to the electrostatic interaction between the particles dispersed in ER fluids because of the higher interfacial polarization of the particles. Δε of the PDPA/PANI-based ER fluid was determined to be 1.64. Furthermore, the dielectric relaxation time (λ) was determined to be 0.063 ms from the f point, where ε" reaches the maximum value (Fig. 14(a)), and α describes the broadness of the ε" peak, which was also found to be 0.501. Table 2 lists the determined Cole-Cole model parameters, including ε∞ and ε0 , of the PDPA/PANI-based ER system. Note that the higher Δε the ER in­ dicates the stronger chain-like structure of nanoparticles due to their stronger interactions and the short λ represents the rapid response to the applied EF [56].

(5)

n

For ER fluids, the index n generally varies in the range of 1.0–2.0 [50]. The PDPA/PANI-based ER fluid complied with the powder-law dependency of 1.5, according to the conduction model. In addition, the static yield stresses were highest. This is probably because the static condition (no controlled shear rate) induces the PDPA/PANI nano­ particles in the ER fluid to agglomerate more than the dynamic condi­ tion, resulting in a higher resistance to the shear. In addition, the dynamic yield stress, which is the minimum stress required for a PDPA/PANI-based fluid to flow in the dynamic condition with controlled shear rate, is larger than the elastic yield stress, which rep­ resents the maximum stress below which the chain-like form of the ER fluid can be completely reversible [51]. The reversibility of the PDPA/PANI-based ER fluid tested by the onoff tests by applying a periodic square voltage pulse at a γ_ of 1.0 (1/s) are described with the change in τ of the ER fluid with time (Fig. 13). At the instant of the application of an EF, the τ of the ER fluid increased rapidly and maintained a high-stress value. The shear stress then decreased rapidly to the low-stress value as soon as the EF is removed. In addition, the higher E value induces a higher stress value. The prompt change in τ at the voltage transition point without any hysteresis reflects the rapid response of the PDPA/PANI-based ER fluid to the E and the occurrence of a reversible phase transition [52]. Because the polarizability of materials is affected by their dielectric properties and influences the ER properties [53,54], the dielectric be­ haviors of ER fluids have been studied intensively to better understand the ER behaviors of the ER fluids. Fig. 14(a) shows the dielectric spectra, exhibiting the frequency dependence of the dielectric constant (ε0 ) and loss factor (ε"), of the PDPA/PANI-based ER fluid as a function of the frequency (f) range of 20–106 Hz, and the Cole-Cole plot of the dielectric spectra is given in Fig. 14.(b). With increasing frequency, ε0 decreased gradually, whereas ε" showed a peak at 2000 Hz. The experimental dielectric data were well fitted using the Cole-Cole model as follows [55]: 0

ε� ¼ ε þ iε00 ¼ ε∞ þ

Δε 1 þ ðiωλÞ1

α

ð0 � α < 1Þ

4. Conclusion PDPA/PANI core/shell composite nanospheres with an approxi­ mately 11:1 vol ratio of PDPA core and PANI shell parts were synthe­ sized successfully by oxidative polymerization. The polymerization that occurred in the disperse phase and at the interface between the sus­ pended and continuous phases was carried out using a reactive surfac­ tant with both features of a surfactant and a monomer. In this study, the reactive surfactant, called ADS, was synthesized with aniline and SDS. The fine core/shell structure formed by the separate coexistence of the PDPA core and PANI shell domains in the particles and the particle shape and size were confirmed by examining the chemical composition, morphology, crystallinity, and orientation of the polymers constituting the particles, using HR-SEM, TEM, FT-IR, and XRD. The electroresponse of the PDPA/PANI-based ER fluid was examined visually by OM, and the formation process of the chain-like structure of the nanoparticles was observed. In addition, the ER properties were characterized by the controlled shear stress and rate, dynamic oscilla­ tion, and electric field on-off tests. The shear stresses of the ER fluid under various applied EFs were stable over a wide range of shear rates, which was also analyzed using the Bingham and CCJ equations. The CCJ model fitted the shear stress data more accurately than the Bingham model because the PDPA/PANI-based ER fluid exhibited pseudo-plastic flow behavior in the high γ_ regime after the yield point. In addition, with increasing applied EF strength, the increase in shear stress, stress relaxation modulus, elastic stress, yield stress, G0 , and G00 highlights the strong ER characteristics of the fluid, which is most probably due to the formation of harder and firmer chain-like structures of nanoparticles. Moreover, the three different yield stresses (static, dynamic, and elastic yield stresses) of the ER fluid were determined in three different ways. The static yield stress of the ER fluid was obtained from the drastic change in the viscosity with increasing controlled shear stress, showing the highest value among them. The dynamic yield stress was determined by extrapolating the flow curve data to the zero γ_ , and the elastic yield stress was acquired from the inflection point in the elastic stress below which the structural deformation can be reversible. The increase in the yield stress was in proportion to the E to the power 1.5, following the conduction model. Furthermore, the dielectric spectra and the five fitting parameters obtained by the Cole-Cole model (λ, α, ε0 , ε∞ , and Δε) confirmed the effective ER performance, such as the rapid electroresponse of the ER fluid with a very short λ (0.063 ms). Finally, the PDPA/PANI nanoparticles are considered outstanding potential ER materials for ER fluids, exhibiting an instantaneously controllable response to the EFs and is promising for smart control systems.

(6)

where ε� represents the complex dielectric constant, and ε0 and ε" are the physical parameters related to the polarization of the ER suspension. In addition, ε0 is the relative permittivity at a frequency close to zero, and ε∞ is the permittivity at infinite frequency. The difference between the

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 13. Shear stress reversibility of the PDPA/PANI-based ER fluid to an alternatively applied electric field strength with the periodic time (T: 20 s) at the shear rate (1 s 1) as a function of time for various applied field strengths. 9

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Polymer 188 (2020) 122161

Fig. 14. (a) Dielectric spectra: dielectric constant (ε’: black) and loss factor (ε": red) as a function of frequency (f) and (b) the Cole-Cole plot of the PDPA/PANI-based ER fluid. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Acknowledgements

Table 2 Fitting parameters of the Cole-Cole model of the PDPA/PANI-based ER fluid (Eq. (6)). Sample

Parameters

PDPA/PANI-Based ER fluid

ε0

ε∞

Δε

α

λ (ms)

4.66

3.02

1.64

0.501

0.063

This work was supported by National Research Foundation of Korea (2018R1A4A1025169). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2020.122161.

CRediT authorship contribution statement

References

Wen Jiao Han: Investigation, Data curation, Writing - original draft. Jin Hyun Lee: Writing - review & editing, Supervision. Hyoung Jin Choi: Conceptualization, Supervision, Writing - review & editing, Funding acquisition.

[1] W. Wen, X. Huang, P. Sheng, Electrorheological fluids: structures and mechanisms, Soft Matter 4 (2008) 200–210. [2] H.J. Choi, M.S. Jhon, Electrorheology of polymers and nanocomposites, Soft Matter 5 (2009) 1562–1567. [3] M. Mrlik, M. Ilcikova, T. Plachy, R. Moucka, V. Pavlinek, J. Mosnacek, Tunable electrorheological performance of silicone oil suspensions based on controllably

10

W.J. Han et al.

[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

[21] [22] [23] [24] [25] [26] [27] [28] [29]

Polymer 188 (2020) 122161 [30] B. Massoumi, S. Najafian, A. Entezami, Investigation of conductivity and morphology of poly (diphenylamine-co-aniline) prepared via chemical and electrochemical copolymerization, Polym. Sci. Ser. B 52 (2010) 270–276. [31] N.Y. Abu-Thabit, Chemical oxidative polymerization of polyaniline: a practical approach for preparation of smart conductive textiles, J Chem. Educ. 93 (2016) 1606–1611. [32] S. Palaniappan, P. Manisankar, Mechanochemical preparation of polydiphenylamine and its electrochemical performance in hybrid supercapacitors, Electrochim. Acta 56 (2011) 6123–6130. [33] M. Willert, K. Landfester, Amphiphilic copolymers from miniemulsified systems, Macromol. Chem. Phys. 203 (2002) 825–836. [34] K. Landfester, The generation of nanoparticles in miniemulsions, Adv. Mater. 13 (2001) 765–768. [35] H. Liu, X. Hu, J. Wang, R. Boughton, Structure, conductivity, and thermopower of crystalline polyaniline synthesized by the ultrasonic irradiation polymerization method, Macromolecules 35 (2002) 9414–9419. [36] M.V. Kulkarni, A.K. Viswanath, Comparative studies of chemically synthesized polyaniline and poly (o-toluidine) doped with p-toluene sulphonic acid, Eur. Polym. J. 40 (2004) 379–384. [37] T. Permpool, A. Sirivat, D. Aussawasathien, Synthesis of polydiphenylamine with tunable size and shape via emulsion polymerization, Polym. Int. 63 (2014) 2076–2083. [38] H.S. Chae, W.L. Zhang, S.H. Piao, H.J. Choi, Synthesized palygorskite/polyaniline nanocomposite particles by oxidative polymerization and their electrorheology, Appl. Clay Sci. 107 (2015) 165–172. [39] T.-C. Wen, C. Sivakumar, A. Gopalan, Studies on processable conducting blend of poly (diphenylamine) and poly (vinylidene fluoride), Mater. Lett. 54 (2002) 430–441. [40] M. Mrlik, V. Pavlinek, Q. Cheng, P. Saha, Synthesis of titanate/polypyrrole composite rod-like particles and the role of conducting polymer on electrorheological efficiency, Int. J. Mod. Phys. B 26 (2012) 1250007. [41] M. Parthasarathy, D.J. Klingenberg, Electrorheology: mechanisms and models, Mater. Sci. Eng. R Rep. 17 (1996) 57–103. [42] G.M. Kamath, M.K. Hurt, N.M. Wereley, Analysis and testing of Bingham plastic behavior in semi-active electrorheological fluid dampers, Smart Mater. Struct. 5 (1996) 576–590. [43] M.S. Cho, H.J. Choi, M.S. Jhon, Shear stress analysis of a semiconducting polymer based electrorheological fluid system, Polymer 46 (2005) 11484–11488. [44] M. St� eni�cka, V. Pavlínek, P. S� aha, N.V. Blinova, J. Stejskal, O. Quadrat, Effect of hydrophilicity of polyaniline particles on their electrorheology: steady flow and dynamic behaviour, J. Colloid Interface Sci. 346 (2010) 236–240. � [45] J.C. Fern� andez-Toledano, J. Rodríguez-L� opez, K. Shahrivar, R. Hidalgo-Alvarez,

reduced graphene oxide by surface initiated atom transfer radical polymerization of poly(glycidyl methacrylate), J. Ind. Eng. Chem. 57 (2018) 104–112. J. Wu, L. Zhang, X. Xin, Y. Zhang, H. Wang, A. Sun, Y. Cheng, X. Chen, G. Xu, Electrorheological fluids with high shear stress based on wrinkly tin titanyl oxalate, ACS Appl. Mater. Interfaces 10 (2018) 6785. J. Jiang, Y. Tian, Y. Meng, Structure parameter of electrorheological fluids in shear flow, Langmuir 27 (2011) 5814–5823. M. Cabuk, T.A. Yesil, M. Yavuz, Halil Ibrahim Unal, Colloidal and viscoelastic properties of expanded perlite dispersions, J. Intell. Mater. Syst. Struct. 29 (2018) 32–40. B.X. Wang, C.J. Liu, Y.C. Yin, S.S. Yu, K.Z. Chen, P.B. Liu, B. Liang, Double template assisting synthesized core–shell structured titania/polyaniline nanocomposite and its smart electrorheological response, Compos. Sci. Technol. 86 (2013) 89–100. Y. Meheust, K.P.S. Parmar, B. Schjelderupsen, J.O. Fossum, The electrorheology of suspensions of Na-fluorohectorite clay in silicone oil, J. Rheol. 55 (2011) 809–833. Y. Xu, W. Qu, J. Ko, Seismic response control of frame structures using magnetorheological/electrorheological dampers, Earthq. Eng. Struct. Dyn. 29 (5) (2000) 557–575. O.Y. Gumus, O. Erol, H.I. Unal, Polythiophene/borax conducting composite II: electrorheology and industrial applications, Polym. Compos. 32 (2011) 756–765. Y. Tsai, C. Tseng, C. Chang, Development of a combined machining method using electrorheological fluids for EDM, J. Mater. Process. Technol. 201 (2008) 565–569. Y.D. Liu, H.J. Choi, Electrorheological fluids: smart soft matter and characteristics, Soft Matter 8 (48) (2012) 11961–11978. S. Palaniappan, A. John, Polyaniline materials by emulsion polymerization pathway, Prog. Polym. Sci. 33 (2008) 732–758. J. Yin, X. Wang, R. Chang, X. Zhao, Polyaniline decorated graphene sheet suspension with enhanced electrorheology, Soft Matter 8 (2012) 294–297. O. Quadrat, J. Stejskal, Polyaniline in electrorheology, J. Ind. Eng. Chem. 12 (2006) 352–361. S. Goswami, T. Brehm, S. Filonovich, M.T. Cidade, Electrorheological properties of polyaniline-vanadium oxide nanostructures suspended in silicone oil, Smart Mater. Struct. 23 (10) (2014) 105012. W.J. Han, S.H. Piao, H.J. Choi, Synthesis and electrorheological characteristics of polyaniline@attapulgite nanoparticles via Pickering emulsion polymerization, Mater. Lett. 204 (2017) 42–44. J. Santos, S. Goswami, N. Calero, M.T. Cidade, Electrorheological behaviour of suspensions in silicone oil of doped polyaniline nanostructures containing carbon nanoparticles, J. Intell. Mater. Syst. Struct. 30 (5) (2019) 755–763. R.A. Anderson, Electrostatic forces in an ideal spherical-particle electrorheological fluid, Langmuir 10 (9) (1994) 2917–2928. H. Kai, Q.K. Wen, C.W. Wang, B.X. Wang, S.S. Yu, C.C. Hao, K.Z. Chen, A facile synthesis of hierarchical flower-like TiO2 wrapped with MoS2 sheets nanostructure for enhanced electrorheological activity, Chem. Eng. J. 349 (2018) 416–427; [20a] M. Stenicka, V. Pavlinek, P. Saha, N.V. Blinova, J. Stejskal, O. Quadrat, The electrorheological efficiency of polyaniline particles with various conductivities suspended in silicone oil, Colloid Polym. Sci. 287 (2009) 403–412. B. Gercek, M. Yavuz, H. Yilmaz, B. Sari, H.I. Unal, Comparison of electrorheological properties of some polyaniline derivatives, Colloids Surf., A 299 (2007) 124–132. X.L. Tian, K. He, B.X. Wang, S.S. Yu, C.C. Hao, K.Z. Chen, Q.Q. Lei, Flower-like Fe2O3/polyaniline core/shell nanocomposite and its electrorheological properties, Colloid. Surf. Physicochem. Eng. Asp. 498 (2016) 185–193. W.L. Zhang, S.H. Piao, H.J. Choi, Facile and fast synthesis of polyaniline-coated poly(glycidyl methacrylate) core-shell microspheres and their electro-responsive characteristics, J. Colloid Interface Sci. 402 (2013) 100–106. S.D. Kim, Y. Tian, H.J. Choi, Seeded swelling polymerized sea urchin-like core-shell typed polystyrene/polyaniline particles and their electric stimuli-response, RSC Adv. 5 (2015) 81546–81553. M. Kaczorowski, G. Rokicki, Reactive surfactants–chemistry and applications Part I. Polymerizable surfactants, Polimery 61 (2016) 747–757. E.A. Zaragoza-Contreras, M. Stockton-Leal, C.A. Hern� andez-Escobar, Y. Hoshina, J. F. Guzm� an-Lozano, T. Kobayashi, Synthesis of core-shell composites using an inverse surfmer, J. Colloid Interface Sci. 377 (2012) 231–236. T.-C. Wen, J.-B. Chen, A. Gopalan, Soluble and methane sulfonic acid doped poly (diphenylamine)—synthesis and characterization, Mater. Lett. 57 (2002) 280–290. A.M. Showkat, K.-P. Lee, A.I. Gopalan, S.-H. Kim, Synthesis and chiro-optical properties of water processable conducting poly(diphenylamine) nanocomposites, Macromol. Res. 15 (2007) 575–580. C.-Y. Li, T.-C. Wen, T.-F. Guo, S.-S. Hou, A facile synthesis of sulfonated poly (diphenylamine) and the application as a novel hole injection layer in polymer light emitting diodes, Polymer 49 (2008) 957–964.

[46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56]

11

L. Elvira, F. Montero de Espinosa, J. de Vicente, Two-step yielding in magnetorheology, J. Rheol. 58 (2014) 1507–1534. W.L. Zhang, Y.D. Liu, H.J. Choi, S.G. Kim, Electrorheology of graphene oxide, ACS Appl. Mater. Interfaces 4 (2012) 2267–2272. M.J. Hato, K. Zhang, S.S. Ray, H.J. Choi, Rheology of organoclay suspension, Colloid Polym. Sci. 289 (2011) 1119–1125. F. Schwarzl, Numerical calculation of storage and loss modulus from stress relaxation data for linear viscoelastic materials, Rheol. Acta 10 (1971) 165–173. Y. Otsubo, K. Edamura, Static yield stress of electrorheological fluids, J. Colloid Interface Sci. 172 (1995) 530–535. K. Parmar, Y. M�eheust, B. Schjelderupsen, J.O. Fossum, Electrorheological suspensions of laponite in oil: rheometry studies, Langmuir 24 (2008) 1814–1822. J. de Vicente, D.J. Klingenberg, R. Hidalgo-Alvarez, Magnetorheological fluids: a review, Soft Matter 7 (2011) 3701–3710. K. Tan, A. Johnson, R. Stanway, W. Bullough, Model validation of the output reciprocating dynamic responses of a twin electro-rheological (ER) clutch mechanism, Mech. Mach. Theory 42 (2007) 1547–1562. Q. Cheng, V. Pavlinek, Y. He, A. Lengalova, C. Li, P. Saha, Structural and electrorheological properties of mesoporous silica modified with triethanolamine, Colloid. Surf. Physicochem. Eng. Asp. 318 (2008) 169–174. J.H. Lee, M.S. Cho, H.J. Choi, M.S. Jhon, Effect of polymerization temperature on polyaniline based electrorheological suspensions, Colloid Polym. Sci. 277 (1999) 73–76. Y.D. Liu, K. Zhang, W.L. Zhang, H.J. Choi, Conducting material-incorporated electrorheological fluids: core-shell structured spheres, Aust. J. Chem. 65 (2012) 1195–1202. F. Ikazaki, A. Kawai, K. Uchida, T. Kawakami, K. Edamura, K. Sakurai, H. Anzai, Y. Asako, Mechanisms of electrorheology: the effect of the dielectric property, J. Phys. D Appl. Phys. 31 (1998) 336–347.