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Magnetorheology of Core−Shell Structured Carbonyl Iron/ Polystyrene Foam Microparticles Suspension with Enhanced Stability Wei Huan Chuah,† Wen Ling Zhang,‡ Hyoung Jin Choi,*,‡ and Yongsok Seo*,† †

RIAM, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea

‡

S Supporting Information *

ABSTRACT: The sedimentation stability of a carbonyl iron (CI)-based magnetorheological (MR) fluid was improved by wrapping CI particles with a polystyrene (PS) foam layer. The PS layer on the CI particles was synthesized via conventional dispersion polymerization and was subsequently foamed using a supercritical carbon dioxide fluid to produce core−shell structured particles. The density of particles decreased after the PS-layer wrapping and subsequent PS-layer foaming. The surface morphology was observed by scanning electron microscope (SEM) and the specific surface areas were determined by Brunauer−Emmett−Teller (BET) adsorption measurements. Both modifications (PS-layer wrapping and foaming) increased the surface roughness of the particles, yet preserved particle’s spherical shape. The effect of the volume expansion after modification on the magnetorheological properties was investigated by using a vibrating sample magnetometer (VSM) and a rotational rheometer. All suspensions tested presented similar MR behaviors with the only difference in their yield stress strengths. Finally, the sedimentation properties of the synthesized particles was examined using a Turbiscan apparatus. MR fluids containing the newly developed CI particles wrapped with the foamed PS layer exhibited remarkably improved stability against sedimentation due to the reduced mismatch in density between the particles and the carrier medium.

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INTRODUCTION Magnetorheological (MR) fluids which are suspensions of fine particles in a magnetically insulating fluid are kind of smart materials because they can form a solid-like structure of fibril shapes along the magnetic field direction upon application of a magnetic field.1 When the magnetic field is on, randomly dispersed particles can rapidly form fibril shapes (mesostructure) along the field direction due to magneto-polarization between the suspended particles. Reverse structural transition happens once the applied field is off. The structural changes occur very quickly, on the order of milliseconds.2 The aligned structural changes formed in response to a magnetic field enable the apparent viscosities of the MR fluids to be increased by three to 4 orders of magnitude.1,2 The properties of magnetorheological (MR) fluids can be controlled over a wide range by varying the magnetic field intensity, which enables the material behavior to be fine-tuned. The field-responsive properties of MR fluids are quite useful for a variety of mechanical systems. In the automotive industry, for example, the variable properties of MR fluids have been used in vehicle suspension systems, clutches, power steering pumps, torque transducer.2,3 On a larger scale, an MR suspension system can be found in China’s Dong Ting Lake Bridge to counteract vibrations caused by sudden gusts of wind.4 The same principle has been applied to stabilize buildings against earthquakes.5 MR fluids and devices have made substantial progress toward commercialization. Nonetheless, the long-term stability of an MR fluid may be threatened by the sedimentation of magnetic © XXXX American Chemical Society

particles in an MR suspension due to the mismatch in densities of the particles and the carrier liquid, the poor redispersibility of settled particles, or the weak chemical degradation resistance of magnetic particles, all of which can limit MR fluids utility in industrial applications.6 Efforts to overcome these crucial restriction factors include the introduction of polymer coatings7 or passivation layers on magnetic particles,8 the use of viscoplastic medium as a carrier liquid, the addition of additives, fillers, or surfactants, and the use of nonspherical particles9 or a bidisperse MR suspensions.10 A recent study by Bell et al. demonstrated that iron nanofibers-based MR fluids show promise in reducing or preventing sedimentation, and they display improved yield stress.11 A common method for stabilizing heavy magnetic particles in MR suspensions involves the addition of thickening agents to the carrier liquid, which hinders particle settling. These types of systems unfortunately suffer from the trade-off between a high suspension stability and a high resistance to suspension flow. An ideal MR fluid should feature a low viscosity to facilitate a flow within a device. Introduction of a coating onto the magnetic particles using an appropriate surface layer turns out to be an efficient method for improving the suspension stability because such coating can improve the sedimentation stability by decreasing particle density, enhance the durability of the particles, and improve Received: June 30, 2015 Revised: September 16, 2015

A

DOI: 10.1021/acs.macromol.5b01430 Macromolecules XXXX, XXX, XXX−XXX

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temperature for 24 h. The final product was washed with excess methanol and distilled water, finally dried in a vacuum oven. Preparation of CI/Foamed Polystyrene Layer (CI/PSF) Particles. A schematic diagram of showing the scCO2 foaming experimental setup is presented in Figure 2. Distilled water (130 mL)

the surface free energy of magnetic particles, consequently yield superior mutual compatibility (a higher wettability) with the carrier liquid.12 Dual-step functional coatings have been shown to be more effective than single layer coating of a polymer or multiwalled carbon nanotubes.13 Other than these classical solutions, effective strategies for reducing the densities of a polymer-coated particles have not been extensively investigated. In this study, genuinely new core−shell structured particles composed of CI coated with polystyrene (PS) foam were fabricated in an effort to enhance the sedimentation stability using a supercritical carbon dioxide (scCO2) fluid as a physical foaming agent.14,15 The core−shell structure includes CI particles (composed of >99.5 wt % of Fe, ca. 0.18−0.35 wt % of O and max 0.05 wt % C) wrapped with a PS layer (CI/PSC)). After coating a PS layer, the density of CI particles decreased for about 5% and the subsequent foaming step through the introduction of the scCO2 fluid further reduced the density by as much as 35%. MR fluids composed of different particles (pure CI, CI/PSC and foamed PS-layer wrapped CI (CI/PSF)) were submitted to an MR flow analysis, particle element analysis, particle morphology analysis, particle surface area, saturation magnetization, and sedimentation stability tests. The advantage of creating foaming CI/PS core−shell particles were clearly demonstrated. The particle aggregation mechanism was also investigated by probing the universal yield stress behaviors using the Seo’s model proposed recently, as well as the Mason plot.

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Figure 2. Schematic diagram of the scCO2 foaming experimental setup for foaming processing: (1) CO2 gas cylinder, (2) sub chamber (3) pressure gauge, (4) heating unit with temperature indicator, (5) pump, (6) reaction chamber, (7) motor, (8) temperature indicator, (9) pressure gauge, (10) heating unit with temperature indicator, (11) container, (12) foaming product, and (13) V-1 to V-4 valve.

EXPERIMENTAL SECTION

Synthesis of CI/Polystyrene Coating (CI/PSC) Particles. A schematic diagram of the synthesis process for CI/PSC composite particles is illustrated in Figure 1. The affinity between the CI particles

was poured into the reaction chamber first, and a dispersing agent, hydrated magnesium carbonate (13 g) ((MgCO3)4Mg(OH)2·5H2O; Dae Jung Chemical, Korea), was then added and mixed thoroughly. CI/ PSC particles were subsequently dispersed in the mixture. Both chambers were then pressurized with CO2 (99.98% purity, Shin Yang Oxygen Ind. Co., Ltd., Korea) and heated. During the scCO2 foaming process to prepare the CI/PSF particles, the dispersing agent (hydrated magnesium carbonate) was added to stabilize the particles dispersion and prevent aggregation. As the scCO2 fluid diffused into the PS polymer, the fluid molecules permeated into the interstitial space between the polymer chains to increase the free volume and mobility of the polymer chains. The system was maintained at the preset pressure and temperature for a certain period of time. Subsequently, the reaction chamber was depressurized rapidly by opening the discharge valve (V-4) and venting the mixture (scCO2/distilled water/MgCO3/sample) to atmosphere. The CI/PSF sample was washed with excess distilled water and collected using a magnet. Finally, the sample was dried in a vacuum oven. To ensure that MgCO3 did not contaminate the samples, the EDS element analysis was carried out. A specific region on the surface of a CI/ PSF particle was selected to determine the chemical composition of its surface (Supporting Information, Figure S2 and Table S1). No dispersing agent remained after washing. After Soxhlet extraction, molar mass was measured by GPC (Young Lin SP930D, Column: GPC KD 806 M x2, Eluent: 0.01 M LiCl in DMF, Flow rate: 1.0 mL/min, Column Temperature: 40 °C). It turned out that number-average molar mass (Mn) was ca. 32400 Da, weight-average molar mass (Mw) was ca.73600 and PDI was ca.2.27. Characterization Methods. The densities of pure CI, CI/PSC, and CI/PSF particles were measured using a helium pycnometer (AccuPyc 1330, Micromeritics Instrument Corporation, Norcross, GA). The surface morphologies of each sample was monitored using a scanning electron microscope (SEM; SUPRA 55VP, Carl Zeiss, Germany) equipped with an energy-dispersive X-ray spectroscopy (EDAX) for the elemental analysis. The specific surface area of each sample was

Figure 1. Schematic diagram of the synthesis of CI/PSC particles. and the PS coating shell was improved by modifying the surfaces of the CI particles using a functional grafting agent, methacrylic acid (MAA). The carboxyl group anchors the MAA molecules onto the CI particle surface, and the vinyl group reacted with the vinyl radical of the styrene monomer, leading to a successful polymerization of PS on the surface of the CI particles. Magnetic CI particles (20 g) (mean particle size of 4.5 μm and density of 7.86 g/cm3, CC grade, BASF, Ludwigshafen, Germany) were treated with MAA (99% purity, Junsei Chemical, Tokyo, Japan) by dispersing the particles in a mixture of MAA (20 g)/ methanol (200 g). The mixture was vigorously agitated using a homogenizer, followed by ultrasonication. As a particles-aggregation preventing agent, homogeneous solution of polyvinylpyrrolidone (13.2 g) (PVP; Mw = 1 300 000 g/mol, Sigma-Aldrich, USA) in methanol (400 g) was prepared in a double-layered glass reactor (700 mL) fitted with a reflux condenser. The MAA-modified CI particles were subsequently placed into the reactor, and the mixture was stirred with a mechanical stirrer. A styrene monomer (20 g) (99% purity, Samchun Chemical, Korea) containing the initiator, azobis(isobutyronitrile) (0.2 g) (AIBN; Junsei Chemical, Tokyo, Japan), was slowly added to the reactor. The system was then heated to 65 °C and maintained at the same B

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Macromolecules determined by Brunauer−Emmett−Teller (BET) adsorption measurements on a Micromeritics TriStar analyzer (TriStar 3000, Micromeritics Instrument Corporation, Norcross, GA). The saturation magnetization was checked using a vibrating sample magnetometer (VSM; Model 7307, Lake Shore Cryotronics, Westerville, OH). Each MR fluid was prepared by dispersing the particles in silicone oil (KF-96, 50cS, Shin Etsu, Japan) at a particle concentration of 20 vol %. The rheological properties of the MR fluids were investigated using a commercial rotational rheometer (Physica MCR300, Stuttgart, Germany) equipped with a magnetic field generator (Physica MRD180). A parallel-plate measuring system with a diameter of 20 mm was employed at a gap distance of 1 mm. The tests were performed at room temperature. An optical microscope (Olympus BX-51, Japan) was used to observe the response of each CI-based MR fluid during exposure to an externally applied magnetic field. Finally, the sedimentation stability of the suspensions was analyzed using a Turbiscan (Classic MA2000, Formulation, France). MR fluids of 10 vol % particle concentration were tested.

particles were rough, unlike the smooth surfaces of the pure CI particles (Figure 3a). The PS polymer was successfully coated onto the CI particles. The grafting agent MAA facilitated the coating step on the CI particle surfaces.7 Figure 3c and 3d show the surface morphology of CI/PSF (6.36) and CI/PSF (5.10) particles after scCO2 foaming process. Evidently, the spherical shape of both CI/PSF particles was preserved as shown in the SEM images. The inset in Figure 3c, shows the enlarged view of the CI/PSF (6.36) particle surface at a magnification of 200 000 times. A mass of the polystyrene nanospheres compactly covered the surfaces of the CI/PSF particles. The CI/PSF (5.10) particles also appeared to have a similar surface texture. The particle surfaces of both CI/PSF (6.36) and CI/PSF (5.10) particles were noticeably rougher than the surfaces of the pure CI or CI/PSC (7.34) particles. Surface roughness due to voids, pores, steps, and other surface imperfections was checked using BET adsorption measurement. The specific surface area increased with the surface modification: 0.1907 m2 g−1, 0.2433 m2 g−1, and 0.7255 m2 g−1 for pure CI, CI/PSC (7.34) and CI/PSF (5.10) particles, respectively (Supplementary Figure S1). The PS-wrapped CI particles became porous due to the formation of PS nanoparticles on the CI surface (Figure 3c). The foamed particles were even more porous. The magnetic hysteresis were measured for each sample over the magnetic field range of −10 to +10 kOe (−795 kA/m to +795 kA/m), as illustrated in Figure 4. The maximum possible

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RESULTS AND DISCUSSION Particle Morphologies and Magnetic Hysteresis. The densities of the various CI particle samples after coating and foaming modification are presented in Table 1. Coating the CI Table 1. Density of CI Particles after Coating and Foaming Processing no.

samples

density [g cm‑3]

foaming condition

A B B1 C C1

pure CI CI/PSC CI/PSF CI/PSC CI/PSF

7.80 7.56 6.36 7.34 5.10

− − 128 °C/141 bar − 128 °C/144 bar

particles with PS layer reduced the density, which was further reduced by the foaming step. The particle morphology of each sample was investigated using the SEM. SEM images of (a) pure CI, (b) CI/PSC (7.34), (c) CI/PSF (6.36) (the inset shows a magnified view of the particle surface), and (d) CI/PSF (5.10) are presented in Figure 3. Figure 3b clearly shows that the formation of the PS polymer layer introduced distinct changes in the surface morphology. The CI particles were covered with many PS nanobeads so that the surfaces of the CI/PSC (7.34)

Figure 4. Vibrating sample magnetometer (VSM) data of pure CI, CI/ PSC (7.34), CI/PSF (6.36), and CI/PSF (5.10) particles (1kOe = 103/ (4π) kA/m).

magnetizations (magnetization saturation Ms) of the samples differed from sample to sample, but the intrinsic hysteresis behavior of the pure CI particles was maintained in the suspension fluids containing PS-coating and PS-layer foamed core−shell particles. The magnetization saturation values obtained from the pure CI, CI/PSC (7.34), CI/PSF (6.36), and CI/PSF (5.10) particle suspensions were 184, 171, 147, and 112 emu/g, respectively. The CI/PSC (7.34) particles demonstrated a slight reduction in the magnetization saturation upon the introduction of a nonmagnetic PS shell. The magnetization saturation decreased substantially during the foaming process which introduced a tremendous expansion in the volume of the PS shell. The presence of the PS shell increased the separation distance between the magnetic cores, which weakened their interaction force. These results agree well with the results reported by others.13,16,17 The MR fluid displayed a rapid transition in the particle suspension from a uniformly dispersed state to an aggregated

Figure 3. . SEM images of (a) pure CI particles (average particle size ∼4.5 μm), (b) CI/PSC (7.34) particles (average particle size ∼4.94 μm), (c) CI/PSF (6.36) particles (average particle size ∼5.16 μm, the inset shows a magnified view of the particle surface), and (d) CI/PSF (5.10) particles (average particle size ∼5.35 μm). C

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Macromolecules network-like state upon exposure to a magnetic field because the magnetic particles suspended in the fluid formed fibril chain-like structures that aligned along the applied magnetic field direction. These columnar structures restrained the fluid from flowing by spanning the gap of flowing channel. Figure 5 shows a

high shear rates due to the disruption and reaggregation of the mesostructures (chain-like structures).1 At very high shear rates, the strong hydrodynamic stress increased the shear stress, as demonstrated in Figure 7a. Figure 7a also reveals the strong dependence of the shear stress on the applied field strength. A pure CI suspension provided the highest shear stress, followed by the CI/PSC (7.34), the CI/PSF (6.36), and finally the CI/PSF (5.10) suspensions. The surrounding PS layer or foamed PS layer increased the distance between the core CI particles, which deteriorated the magnetization saturation and weakened the columnar mesostructures. The mesostructures were less stiff and could not withstand the shear stress applied on the columnar mesostructures. Mehdizadeh et al. reported that the magnetic interaction force (attraction) between magnetic particles diminishes exponentially as the distance increases.20 The attractive force between two soft magnetic spheres is inversely proportional to the fourth power of the distance between the centers of the particles. As a consequence, coating and foaming samples display a weaker magnetorheological performance due to the increased particle separation (Figure 3) and the reduced saturation magnetization (Figure 4). On the other hand, the shear stress in the coated particle suspensions exceeded the shear stress of a pure CI suspensions in the absence of a magnetic field. The CI/PSF (5.10) suspensions displayed the highest shear stress, followed by CI/PSF(6.36), CI/PSC and finally a pure CI suspension. In the absence of the magnetic field, the suspension viscosity increased linearly with the particle volume according to Einstein’s rule. Thus, a suspension formed by larger particles exhibited a high shear stress. Figure 7b shows the shear viscosity as a function of the shear rate for 20 vol % pure CI, CI/PSC (7.34), CI/PSF (6.36), and CI/PSF (5.10) suspensions under different magnetic field strengths. As the shear rate increased, the viscosity of the MR suspensions decreased linearly. The order of magnitude of the shear viscosity was the same as that of the shear stress. Yield Stress of the MR Suspensions. Under the applied magnetic field, the systems exhibited Bingham fluid behaviors (τ = τdy + ηplγ̇ where τ is the shear stress, τdy is the dynamic yield stress, and ηpl is the magnetic-field (M) dependent plastic viscosity, which approaches the suspension viscosity at a sufficiently high shear rate). All four suspensions exhibited a wide plateau region over the low and the medium shear rate range. This wide plateau was attributed to the magnetic field induced yield stress. In the high shear rate region, in which the shear stress increased with the shear rate, the shear stress dominated the magnetic polarization to induce the flow.21 Two

Figure 5. Optical microscope images of microstructural change for pure CI particles suspension (a) before and (b) after the application of the external magnetic field.

photograph of the MR fluid before and after the application of an external magnetic field. The formation of chain-like structures in response to the external stimulus (the magnetic field) is apparent. The application of a magnetic field triggered the magnetic polarization of each iron particle. Magnetorheological Behavior of Different Particle Suspensions. Oscillatory tests (amplitude and frequency sweep) were performed to investigate the suspensions’ viscoelastic behaviors. Figure 6a plots an amplitude sweep measurement that describes the changes in the storage modulus, G′, as a function of the strain for each MR suspensions. It is worth noting that the stable plateau in the graph terminated at strain of 10−2 %. This plateau region was identified as the linear viscoelastic region, in which the storage modulus was independent of the applied strain.18 In this region, the mesostructures formed under magnetic polarization remains undisturbed. As the strain amplitude increased, the chain structures began to break apart. The destruction of the chains after the plateau region was irreversible. As shown in Figure 6b, the storage moduli of the three MR fluids exceeded their corresponding loss moduli (G′ > G″), which verified the dominant solid-like elastic properties of the MR fluids arose from the formation of the columnar chain-like structures upon exposure to magnetic field.19 Both the storage modulus and loss modulus increase with the magnetic field strengths due to enhanced particles interaction. Strong magnetic polarization maintains those chain-like structures at a high shear rate. This effect contrasted with the properties of electrorheological (ER) fluids, which frequently produce a well in the stress curve at the

Figure 6. (a) Amplitude sweep dependence of storage modulus, G′ for each MR suspension under various magnetic field strengths. (b) Frequency dependence of storage modulus, G′, and loss modulus, G″, for pure CI and CI/PSF (6.36) based MR suspensions under various magnetic field strengths. D

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Figure 7. (a) Shear stress and (b) shear viscosity flow curves for 20 vol % of pure CI, CI/PSC (7.34), CI/PSF (6.36), and CI/PSF (5.10) suspensions under different magnetic field strengths.

Figure 8. (a) Static yield stress curve and (b) dynamic yield stress dependence on the applied magnetic field strength for each suspension for 20 vol % of pure CI, CI/PSC (7.34), CI/PSF (6.36), and CI/PSF (5.10) suspensions under different magnetic field strengths.

Table 2. Static Yield Stress and the Dynamic Yield Stressa yield stress (Pa) M = 343 (kA/m) CI CI/PSC(7.34) CI/PSF(6.36) CI/PSF(5.10)

M = 86 (kA/m)

M = 0 (kA/m)

experiment (figure 8)

static (τsy)

dynamic (τdy)

experiment (figure 8)

static (τsy)

dynamic (τdy)

static (τsy)

dynamic (τdy)

24800 21800 13700 8220

24000 21000 12300 8000

23800 19800 10800 7230

1990 1740 1130 630

2040 1780 1150 680

1930 1660 1100 580

10.8 12.3 14.0 16.1

8.4 9.4 10.5 12.3

a

The static yield stress values (τsy) are obtained from the Seo-Seo model eq 1 and the dynamic yield stress (τdy) values are obtained from the Bingham model after fitting the flow curves at various magnetic field strengths (Figure 7a).

yield stresses are present in MR fluids: a dynamic yield stress, τdy, corresponding to the stress of the MR fluid during complete breakdown under continuous shearing, and the static (or frictional) yield stress, τsy, which is the minimum stress required to induce the suspension to flow.22−25 Depending on the suspension behavior, the dynamic yield stress and the static yield stress can show marked differences; however, they were nearly coincident in suspensions of different CI particles that displayed a wide plateau stress over a wide shear rate range.24−26 The static yield stress was measured using the controlled shear stress (CSS) mode. Figure 8a shows the change in the viscosity as a function of the shear stress. The points marked with arrows indicate that all curves showed a fairly sharp decrease in viscosity, and values of the shear stress at these points (X-axis) were considered to indicate the static yield stress. In the current test, the viscosity of each MR fluid was measured by gradually increasing the shear stress exerted on the fluids. The shear viscosity remained almost constant in the low shear stress region,

but suddenly it plummeted by several orders of magnitude at the characteristic shear stress point where the shear stress applied to the fluid exceeded the yield stress required to initiate flow. This is the static yield stress. The results showed that pure CI had the highest static yield stress, following by the CI/PSC and then CI/ PSF suspensions. The decrease in yield stresses after applying the PS coating and PS-foaming modification in this study corroborated similar results obtained in the shear stress and shear viscosity flow curve. Recently, Seo and Seo proposed a new constitutive model for predicting the static yield stress as follows, ⎛ (1 − exp(−aγ )) ̇ ⎞ τ = τsy ⎜1 − ⎟ + ηpl γ ̇ α (1 + (aγ )̇ ) ⎠ ⎝

(1)

where τsy denotes the static yield stress at which the suspension initiates flow, ηpl is the plastic viscosity, a is the time constant (i.e., the reciprocal of the critical shear rate for an aligned mesostructure deformation), and α is the power-law index E

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Macromolecules used to decide the degree of shear thinning. The effects of the Seo−Seo model parameters on the stress have been fully described previously.21 This model predicts the static yield stress at a low shear rate, corresponding to the absence of motion or flow during mesostructure disruption. The model also predicts the presence of minima in the stress curves obtained from the ER fluids.21−26 The Bingham plastic model (τ = τdy + ηplγ̇) was used to obtain the dynamic yield stress value (τdy). The static yield stress predicted by the Seo−Seo model, the experimental data, and the dynamic yield stress by the Bingham model are compared in Table 2. The values agreed quite well within the measurement errors because the magnetic polarization dominated the shear stress across almost all shear rates. During the dramatic mesostructure change in MR fluids, the static yield stress and the dynamic yield stress displayed contrasting behaviors sometimes.22 Yield Stress Scaling Function in the MR Fluids. Figure 8b shows the dynamic yield stresses of the MR fluids as a function of the magnetic field strength. At low magnetic fields, the yield stress increases quadratically with the magnetic field strength, whereas it followed a 1.5 power-law under strong magnetic fields. At a sufficiently high magnetic field, magnetization saturation occurred, and the polarization force failed to increase with increasing magnetic field intensity. As a consequence, the exponent declined from 2.0 (optimal magnetization) to 1.5 due to the magnetization saturation in the magnetic particles.27−29 Choi et al. suggested that the experimental data could be normalized to collapse all the yield stress data into a single curve.28 The dynamic yield stress data obtained by fitting the experimental data to the Bingham model (Figure 8b) were normalized by the stress and magnetic field strength at the crossing point of the two power-law regimes. All data could be collapsed into a single curve, in accordance with the suggestion of Choi et al.28 Dynamic yield stress was used first instead of static yield stress because of the simplicity of Bingham model and the close coincidence between the dynamic yield stress values and the static yield stress values for MR fluids (Table 2). However, using the static yield stress presents the same behavior except the small difference of the absolute values at the critical magnetic field strength (Mc) which divides two power-law regions. Although the critical magnetic field strength, Mc, divided the stress dependence on the magnetic field strength into two regions, that is quite arbitrary. Seo proposed a single equation, eq 2, to describe the yield stress (τy) behavior over the whole magnetic field strength (M) without an arbitrary division, τy(M ) = αM3/2(1 − exp(−m′ M ))

Figure 9. Comparison of the predictions of two models of normalized dynamic yield stress τ̂ vs normalized magnetic field strength M̂ . The data are plotted on linear scale. The dashed line is the fit of two power law function (τ̂ = (1 − u(1 − Ê ))Ê 2 + u(1 − Ê )Ê 1.5, where u(x) is the unitary step function). The solid line is the fit by Seo’s eq (eq 4) where m was 2.85.

results agreed well with the normalized experimental data. The proposed model also agrees quite well with the original “bi-power law” model at high magnetic field strength.27 These results suggest that the behaviors of all magnetic fluids could be described by a common mechanism, although the yield stress values were different.29 This is a natural consequence of the lack of MR activity in the PS layer (foamed or not), and the entire MR response arises from the CI phase. Mechanism of Structure Evolution and the Suspension Stability. Under a magnetic field that is sufficiently strong to overcome the thermodynamic forces (Brownian motion), the primary forces that govern the behavior of an MR fluid are the magnetopolarization forces and the hydrodynamic forces.3,21,29 The MR fluid properties depend only on the ratio of those two forces. This dependence can be represented as the Mason number (Mn = 8η0γ̇/μ0μcβ2M2, where η0 is the medium viscosity, γ̇ is the shear rate, β is the contrast factor (=(μp − μc)/(μp + 2 μc)), μp is the particle relative permeability, μc is the relative permeability of the liquid medium phase, and μ0 = 4π × 10−7 N/ Å2 is the vacuum permeability, and M is the magnetic field strength) which is the ratio of the hydrodynamic drag and the magnetostatic forces acting on the particles.27,29 For a fixed particle volume fraction, the apparent viscosity η expressed as the ratio of the shear stress to the shear rates (=τ/γ̇) is proportional to inverse of the Mason number (Mn−1).9,29 This dimensional analysis is quite useful in that the shear rate- and field strengthdependence of an MR fluid’s rheological properties may be described in terms of a single independent variable that is proportional to the Mason number. The Mason number (equivalent to γ̇/M2) is then an independent variable that collapses the experimental data collected at various magnetic field strengths and shear rates onto a single master curve, as shown in Figure 10. The master curve was generated by plotting the specific viscosity, defined as the ratio of the apparent viscosity to the viscosity at a magnetic field strength of zero, and the volume fraction of the suspension used for nondimensionalization against γ̇/M2. A plot of the specific viscosity against γ̇/M2 agreed well with the experimental data for suspensions of pure CI particles or CI/PS(510) particles; all experimental data could be collapsed into a single curve for both suspensions. For clarity, only these two particle cases are described in detail, but other particle cases showed the same behavior. These results indicate that both suspensions followed the same flow behavior under the

(2)

where m′ is a fitting parameter. This equation followed two limiting behaviors at low and high magnetic field strengths, respectively. 27

τy = α m′M2 ∝ M2 τy = αM3/2 ∝ M3/2

for M ≪ Mc

(3a)

for M ≫ Mc

Normalization eq 2 with Mc and τy = α eq 4, 3/2 τ̂ = M̂ (1 − exp(−m M̂ ))

Mc3/2

(3b)

gives the following (4)

where τ̂ = τy/αMc , M̂ = M/Mc, and m = m′ Mc . Figure 9 shows that the eq 4 provided a good prediction of the yield stress behavior in MR fluids using a single parameter, m. The model 3/2

F

DOI: 10.1021/acs.macromol.5b01430 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules V(ϕ , d) =

|ρp − ρc | × g × d 2 18 × v × ρc

·

[1 − ϕ] ⎡ ⎣⎢1 +

4.6ϕ ⎤ ⎥ (1 − ϕ)3 ⎦

(5)

where V represents the particle migration velocity (m s−1), ρp denotes the particle density (kg m−3), ρc indicates the continuous phase density (kg m−3), ν designates the kinematic viscosity of continuous phase, g is gravity constant (9.81 m s−2), d stands for the particle diameter, and ϕ represents the volume fraction.30 Apparently, the dispersed particles in all four suspensions settled rapidly within 2 h and tend to reach steady (stable) state. A magnified view of the sedimentation profile during the initial 2 h was collected and is presented in Figure 11b. The settling rate (particle migration velocity) during the initial 2 h of each suspension was computed and the results are summarized in Table 3. Evidently, the sedimentation velocities of the core−shell

Figure 10. Dimensionless apparent viscosity of the MR fluids as a function of γ̇/M2 at various magnetic field strengths.

Table 3. Density and Particles Migration Velocity for Silicone Oil, Pure CI, CI/PSC (7.34), CI/PSF (6.36), and CI/PSF (5.10)

external forces, and their response followed the same scaling law. Suspensions composed of pure CI or CI/PS (foamed or not) particles, therefore, followed the same particle magnetization model.29 Differences between the core CI interparticle distance due to the surrounding PS layer only changed the yield stress magnitude, and the MR response remained the same. However, the PS layers were very helpful in ensuring that the particles remained suspended over a long period of time. Figure 11a shows the sedimentation profile as a function of time for each sample. The transmission measurement increased gradually over time due to the sedimentation of the particles. The pure CI suspension exhibited the highest sedimentation rate, reaching an 80% transmission within 24 h, which indicates that pure CI particles settled down rapidly. CI/PSC (7.34) suspension presented a lower sedimentation percentage, reaching a 64% of transmission because the PS coating reduced the particle density. Importantly, the PS foamed particle suspensions (CI/PSF (6.36) and CI/PSF (5.10)) achieved even lower sedimentation percentages, with only 44% and 32% of transmission values after 24 h, respectively. The low-density foaming particles exhibited better sedimentation stabilities. This phenomenon could be explained in terms of the reduced particle-fluid density mismatch. The general law of sedimentation predicts that a decrease in the particle density mismatch decreases the sedimentation velocity in the fluid, which consequently improves the sedimentation stability. The equation describing the general law of sedimentation is,

samples silicone oil (50cS) pure CI CI/PSC (7.34) CI/PSF (6.36) CI/PSF (5.10)

density [g cm‑3] particle migration velocity [mm min‑1] 0.96 7.80 7.34 6.36 5.10

− 0.070 0.067 0.063 0.056

structure particles (CI/PS layer (foamed or not)) were slower than that of the CI particles. Thus, the suspensions containing core−shell type particles were more stable against sedimentation. This stability was attributed to 3 key factors: a reduction in the particle-fluid density mismatch, the introduction of a rough surface on the PS layer, and improved compatibility (wettability and surface free energy) between the particles and the silicone oil fluid upon particle wrapping in a PS layer. The enhancement in the sedimentation stability renders the MR fluids particularly suitable for civil engineering applications, such as earthquake protection system for skyscrapers. The transient nature of a seismic event means that the dampers never see regular motions that remix the fluid. Therefore, the fluids require a high sedimentation stability.

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CONCLUSIONS Present study offers a viable method to reduce the sedimentation in CI-based MR fluids by first introducing a PS layer coating onto

Figure 11. (a) Change of transmission [%] as a function of time for pure CI suspension, CI/PSC (7.34), CI/PSF (6.36) and CI/PSF (5.10). (b) Magnified view of the sedimentation profile at an initial 2 h. G

DOI: 10.1021/acs.macromol.5b01430 Macromolecules XXXX, XXX, XXX−XXX

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(2) Liu, Y. D.; Lee, J.; Choi, S. B.; Choi, H. J. Silica-coated carbonyl iron microsphere based magnetorheological fluid and its damping force characteristics. Smart Mater. Struct. 2013, 22 (6), 065022. (3) Park, B. J.; Fang, F. F.; Choi, H. J. Magnetorheology: materials and application. Soft Matter 2010, 6 (21), 5246−5253. (4) Chen, Z.; Wang, X.; Ko, J.; Ni, Y.; Spencer, B. F., Jr; Yang, G. MR damping system on Dongting Lake cable-stayed bridge. Proc. SPIE 2003, 5057, 229−235. (5) Bitaraf, M.; Ozbulut, O. E.; Hurlebaus, S.; Barroso, L. Application of semi-active control strategies for seismic protection of buildings with MR dampers. Eng. Struct. 2010, 32 (10), 3040−3047. (6) (a) Quan, X.; Chuah, W.; Seo, Y.; Choi, H. J. Core-Shell Structured Polystyrene Coated Carbonyl Iron Microspheres and their Magnetorheology. IEEE Trans. Magn. 2014, 50 (1), 1−4. (b) Choi, H. J.; Zhang, W. L.; Kim, S. H.; Seo, Y. Core-Shell Structured Electro- and MagnetoResponsive Materials: Fabrication and Characteristics. Materials 2014, 7, 7460−7471. (c) Cvek, M.; Mrlik, M.; Ilcikova, M.; Plachy, T.; Sedlacik, M.; Mosnacek, J.; Pavlinek, V. A facile controllable coating of carbonyl iron particles with poly(glycidyl methacrylate): a tool for adjusting MR response and stability properties. J. Mater. Chem. C 2015, 3, 4646−4656. (7) Fang, F. F.; Choi, H. J.; Seo, Y. Sequential coating of magnetic carbonyliron particles with polystyrene and multiwalled carbon nanotubes and its effect on their magnetorheology. ACS Appl. Mater. Interfaces 2010, 2 (1), 54−60. (8) Sedlacik, M.; Pavlinek, V. A tensiometric study of magnetorheological suspensions’ stability. RSC Adv. 2014, 4 (102), 58377− 58385. (9) Shah, K.; Phu, D. X.; Seong, M. S.; Upadhyay, R. V.; Choi, S. B. A low sedimentation magnetorheological fluid based on plate-like iron particles, and verification using a damper test. Smart Mater. Struct. 2014, 23 (2), 027001. (10) Pramudya, I.; Sutrisno, J.; Fuchs, A.; Kavlicoglu, B.; Sahin, H.; Gordaninejad, F. Compressible Magnetorheological Fluids Based on Composite Polyurethane Microspheres. Macromol. Mater. Eng. 2013, 298 (8), 888−895. (11) Bell, R.; Zimmerman, D.; Wereley, N. In Magnetorheology: Advances and Applications; Wereley, N., Ed.; RSC Smart Materials: Cambridge, U.K., 2013; Chapter 2, pp 31−55. (12) Liu, Y. D.; Choi, H. J. In Magnetorheology: Advances and Applications; Wereley, N., Ed.; RSC Smart Materials: Cambridge, U.K., 2013; Chapter 7, pp 156−178. (13) Fang, F. F.; Liu, Y. D.; Choi, H. J.; Seo, Y. Core-shell structured carbonyl iron microspheres prepared via dual-step functionality coatings and their magnetorheological response. ACS Appl. Mater. Interfaces 2011, 3 (9), 3487−3495. (14) Huang, S.; Wu, G.; Chen, S. Preparation of open cellular PMMA microspheres by supercritical carbon dioxide foaming. J. Supercrit. Fluids 2007, 40 (2), 323−329. (15) Reverchon, E.; Cardea, S. Production of controlled polymeric foams by supercritical CO2. J. Supercrit. Fluids 2007, 40 (1), 144−152. (16) Mrlik, M.; Ilcikova, M.; Pavlinek, V.; Mosnacek, J.; Peer, P.; Filip, P. Improved thermooxidation and sedimentation stability of covalentlycoated carbonyl iron particles with cholesteryl groups and their influence on magnetorheology. J. Colloid Interface Sci. 2013, 396, 146−151. (17) Kim, Y. J.; Liu, Y. D.; Seo, Y.; Choi, H. J. Pickering-emulsionpolymerized polystyrene/Fe2O3 composite particles and their magnetoresponsive characteristics. Langmuir 2013, 29 (16), 4959−4965. (18) Upadhyay, R.; Laherisheth, Z.; Shah, K. Rheological properties of soft magnetic flake shaped iron particle based magnetorheological fluid in dynamic mode. Smart Mater. Struct. 2014, 23 (1), 015002. (19) Mrlik, M.; Sedlacik, M.; Pavlinek, V.; Bazant, P.; Saha, P.; Peer, P.; Filip, P. Synthesis and magnetorheological characteristics of ribbon-like, polypyrrole-coated carbonyl iron suspensions under oscillatory shear. J. Appl. Polym. Sci. 2013, 128 (5), 2977−2982. (20) Mehdizadeh, A.; Mei, R.; Klausner, J. F.; Rahmatian, N. Interaction forces between soft magnetic particles in uniform and non-uniform magnetic fields. Acta Mech. Sin. 2010, 26 (6), 921−929.

CI particle surfaces, followed by foaming of the PS coating layer using a scCO2 foaming process to produce core−shell structured particles (CI coated with PS (CI/PSC) or foamed PS (CI/PSF) particle). The incorporation of a polymer foam into the magnetic carbonyl iron particles markedly decreased the particle density. MR fluids that utilized CI particles coated with PS foam exhibited remarkable stabilities against the sedimentation owing to reduced particle-fluid density mismatch, improved surface roughness on the foaming particles, improved compatibility between the particles and the silicone oil fluid (better wettability and reduced surface free energy due to the presence of the organic PS layer). One drawback observed in suspensions containing this type of particles is that the maximum yield stress of the MR fluid is reduced in association with the enhanced interparticle separation distance between the CI cores. Therefore, the PS wrapped or PS foamed particles displayed lower MR performances under the applied magnetic fields due to a reduction in their magnetization saturation. However, this effect does not limit the applicability of the particles as long as the yield stress exceeds the application requirements, which can be met in most cases due to the strong MR activities compared to ER suspensions. MR fluid properties can be optimized by improving the sedimentation stability while controlling the yield stress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01430. Brunauer−Emmett−Teller (BET) sorption isotherms and BET plot of the samples, EDS spectra and elemental composition of the CI/PSF (5.10) particle (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: +82-32-8607486. Fax: +82-32-865-5178. *E-mail: [email protected]. Telephone: +82-2-8809085. Fax:+822-8859671. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NRF (RIAM NR03-09 (041720090027), MIKE (Original Material Technology Program (RIAM I-AC14-10 (0417-20100043)), the NRF (NRF2013R1A1A2057955), and the Fundamental R&D Program for Core Technology of Materials from MIKE (2013).

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ABBREVIATIONS CI/PSF (5.10) foamed-polystyrene coated CI particle of which the density was 5.10 g cm−3; CI/PSF (6.36) foamed-polystyrene coated CI particle of which the density was 6.36 g cm−3; CI/PSC (7.34) polystyrene coated CI particle of which the density was 7.34 g cm−3

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REFERENCES

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DOI: 10.1021/acs.macromol.5b01430 Macromolecules XXXX, XXX, XXX−XXX