Microstructure and magnetorheological properties of the thermoplastic magnetorheological

elastomer composites containing modified carbonyl iron particles and poly(styrene-b-ethylene-

ethylenepropylene-b-styrene) matrix

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IOP PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 21 (2012) 115028 (13pp) doi:10.1088/0964-1726/21/11/115028

Microstructure and magnetorheologicalproperties of the thermoplasticmagnetorheological elastomer compositescontaining modified carbonyl ironparticles and poly(styrene-b-ethylene-ethylenepropylene-b-styrene)matrixXiuying Qiao1, Xiushou Lu1, Weihua Li2, Jun Chen3, Xinglong Gong4,Tao Yang5, Wei Li1, Kang Sun1 and Xiaodong Chen5

1 State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240,People’s Republic of China2 School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522,Australia3 Intelligent Polymer Research Institute, University of Wollongong, NSW 2522, Australia4 CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of ModernMechanics, University of Science and Technology of China, Hefei, Anhui 230027,People’s Republic of China5 Shanghai Sunny New Technology Development Co., Ltd, Shanghai 201109,People’s Republic of China

E-mail: xyqiao@sjtu.edu.cn

Received 7 August 2012, in final form 26 September 2012Published 23 October 2012Online at stacks.iop.org/SMS/21/115028

AbstractNovel isotropic and anisotropic thermoplastic magnetorheological elastomers (MRE) wereprepared by melt blending titanated coupling agent modified carbonyl iron (CI) particles withpoly(styrene-b-ethylene-ethylene–propylene-b-styrene) (SEEPS) matrix in the absence andpresence of a magnetic field, and the microstructure and magnetorheological properties ofthese SEEPS-based MRE were investigated in detail. The particle surface modificationimproves the dispersion of the particles in the matrix and remarkably softens the CI/SEEPScomposites, thus significantly enhancing the MR effect and improving the processability ofthese SEEPS-based MRE. A microstructural model was proposed to describe the interfacialcompatibility mechanism that occurred in the CI/SEEPS composites after titanate couplingagent modification, and validity of this model was also demonstrated through adsorption testsof unmodified and surface-modified CI particles.

1. Introduction

Similar to the smart magnetorheological fluids (MRF),magnetorheological elastomers (MRE) consisting of the

micrometer-sized magnetically permeable particles in non-magnetic matrices also exhibit rapidly/reversibly controllablemechanical properties under an external magnetic field due tothe magnetic interaction of filler particles. However, problems

10964-1726/12/115028+13$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

Smart Mater. Struct. 21 (2012) 115028 X Qiao et al

of MRF such as particle sedimentation and fluid leakage canbe avoided for MRE [1]. In addition, the particle distributioncan be fixed in the fabrication process of MRE: the particlesare randomly dispersed (isotropic MRE) or aligned in chains(anisotropic MRE) when MRE is prepared in the absence orpresence of an external magnetic field. Besides the superiormagnetorheological (MR) effect, MRE are demonstrated topossess other special properties such as electrical resistance,piezoresistivity, thermoresistance, magnetoresistance andmagnetostriction [2–5]. These particular features have led toMRE receiving great attention and gaining a wide range ofapplication prospects in the fields of adaptive tuned vibrationabsorbers and mass dampers, sensors and actuators [6–8].

For the specific MRE applications mentioned above,the matrix and particles in MRE need to be properlychosen to satisfy the requirements for the mechanicalproperties and the magnitude of the MR effect. In general,the degree of freedom of movement for the magneticparticles is governed by the matrix elastomer and the MReffect is usually smaller in hard matrices than in softmatrices. However, the mechanical properties of soft siliconrubber actually fail to satisfy some practical applicationrequirements [9], and great efforts have been made to furtherimprove the performance of MRE by using other rubbers,such as natural rubber [9], polyurethane (PU) [10–12],isobutylene–isoprene [13] and cis-polybutadiene [14]. In thiscontext, thermoplastic elastomer (TPE) was focused on as theMRE matrix due to its good mechanical properties as well asgood recyclability and processability [15].

Carbonyl iron (CI) [16], Fe3O4 [17], Fe nanowires [18]and nickel [5] have been utilized as the magnetizableparticles in MRE preparation, in which the CI particle wasmost commonly used not only because of its excellentmagnetic properties (high level of saturated magnetizationand low remnant magnetization) but also due to its sphericalshape, which helps MRE applications. The chemical features,size and shape of the magnetizable particles obviouslyinfluence/govern the properties of MRE materials [16]. Underan applied magnetic field, the field-induced interactionsbetween the magnetizable particles can lead to a change in theparticle pre-configuration from isotropic random dispersionto anisotropic ordered chains or complex three-dimensionalstructures. The volume fraction of CI particles (φCI) inthe MRE can increase up to 50 vol% [19], but the bestvolume fraction for achieving good performance of MREwas reported to be ∼27 vol% with the aid of finite-elementanalysis [20]. Besides choosing a softer rubber matrix [21],adding plasticizer to soften the matrix [22] or introducingcompatibilizer to enhance the interactions between filler andmatrix [23], modification of CI particles [10, 24, 25] is anothermethod to improve the MR effect of MRE materials. It wasreported that when CI particles were modified by anionic,nonionic and compound surfactants, the relative MR effect ofMRE based on natural rubber can be increased up to 188%due to the perfect compatibility between particles and rubbermatrix and special self-assembly of the particle structure [24].However, when the CI particles were modified by a silanecoupling agent, the MR effect of the MRE based on silicone

rubber is unexpectedly decreased, although the modificationimproves the dispersion of CI particles and the interfacialinteractions between the particles and matrix [10, 25].

In our previous research, we prepared isotropicand anisotropic thermoplastic MRE composites basedon poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS)matrix and CI particles and examined their structures andproperties [26]. The equilibrium storage modulus (Ge) wasobviously enhanced on the addition of CI particles and alsoon the formation of a chain structure of the particles (inthe anisotropic MRE), and the increment of the modulusunder a magnetic field was found to exceed 150% forthe anisotropic SEBS-based MRE with φCI = 21 vol%. Inthis paper, we prepare novel thermoplastic MRE compos-ites containing poly(styrene-b-ethylene-ethylenepropylene-b-styrene) (SEEPS) matrix and CI particles modified by atitanate coupling agent and examine their microstructureand magnetorheological properties. Similar to SEBS, SEEPShas no C=C double bonds in the EEP block and exhibitsgood thermal stability and climate resistance (againstUV/ozone). However, SEEPS is softer and has a higherworking temperature than SEBS, so the SEEPS-basedMRE is expected to show a greater MR effect. Moreimportantly, usage of surface-modified CI particles enablesus to investigate the influence of the affinity of the particlestoward the matrix on the MR effect. In fact, it turns out thatthe surface modification of CI particles remarkably enhancesthe MR effect for both isotropic and anisotropic CI/SEEPScomposites without sacrificing the toughness of MRE evenat a very high filling of particles (φCI = 31 vol%). Thisenhancement of the MR effect is discussed in relation to theinterfacial compatibility mechanism occurring between theparticle and matrix after particle surface modification, and anadsorption test lends support to this argument.

2. Experimental details

2.1. Materials

A poly (styrene-b-ethylene-ethylenepropylene-b-styrene)(SEEPS) block copolymer (Septon 4055) was purchased fromKuraray Co. Ltd (Japan) and used as received. Its molecularcharacteristics were Mw = 2.4 × 105,Mw/Mn = 1.01, andS content wS = 30 wt% according to the manufacturer’sdesignation. White oil (Primol N 352; Exxon Mobil Co.,USA) was used to plasticize SEEPS. This oil, having anaverage molecular weight of 480 g mol−1, is a purifiedmixture of saturated hydrocarbons and is particularly suitedfor extension of thermoplastic elastomers (TPE) such asSEEPS; dissolving EEP block but precipitating S block.The SEEPS matrix used to prepare MRE composites was aSEEPS/oil 1/4 wt/wt mixture and contained glassy, sphericalS domains (at room temperature) that served as physicalcrosslinks at low T . The EEP mid-blocks formed a rubbery,continuous phase swollen by the white oil.

Commercially available carbonyl iron (CI) particles(FTF-1; Jianyou Hebao Nanomaterial Co. Ltd, China) wereused as received. These CI particles had an average particle

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Smart Mater. Struct. 21 (2012) 115028 X Qiao et al

Table 1. Composition of the SEEPS matrix and CI/SEEPS composites without/with titanate coupling agent modification.

Sample codeSEEPS powder(wt%)

White oil(wt%)

Carbonyl iron(wt%)

TC201(wt%)

SEEPSa 20 80 0 0SEEPS-70CI 6 24 70 0SEEPS-80CI 4 16 80 0SEEPS-70CI-2Ti 6 24 68.6 1.4SEEPS-80CI-2Ti 4 16 78.4 1.6

a Note: SEEPS matrix containing white oil; Ti represents the titanate coupling agent (TC201)contained in the CI/SEEPS composites.

diameter of 3.5 µm and Fe content of ∼98.3% (manufac-turer’s designation). The particle density was 7.64 g cm−3, asdetermined with a pycnometer method. The CI particles werespherical and had a relatively smooth surface, as noted fromthe scanning electron microscope (SEM) image. Isopropyltri(dioctylpyrophosphate) titanate (TC201, C51H112O22P6Ti),purchased from Nanjing Shuguang Chemical Group Co. Ltd(China), was used to modify the CI particle surface to enhancethe affinity between the CI particles and SEEPS matrix.

To verify the enhancement of the particle/matrix affinity,ethylene–propylene rubber (EPR, Vistalon 878) purchasedfrom Exxon Mobil Corporation (USA) was chosen forthe adsorption tests of CI particles in octane (SinopharmChemical Reagent Co. Ltd, China). This EPR, possessing asimilar molecular structure to the EEP block of SEEPS, had aMooney viscosity of 52MU and an ethylene concentration ofabout 60 wt%.

2.2. MRE preparation

Table 1 summarizes the composition of the SEEPS matrixand CI/SEEPS composites. The SEEPS matrix contained20 wt% SEEPS and 80 wt% white oil. The SEEPS-70CIand SEEPS-80CI composites were mixtures of CI particlesin this SEEPS/oil matrix at CI concentrations of wCI = 70and 80 wt% (φCI = 21 and 31 vol%), respectively. TheSEEPS-70CI-2Ti and SEEPS-80CI-2Ti composites containedthe titanate coupling agent at a concentration of 2 wt% tothe CI particle mass, and except for the modifier content thecompositions of these composites were the same as those ofSEEPS-70CI and SEEPS-80CI composites.

The CI particles were pretreated at room temperature withthe titanate coupling agent in a blender for 3 min at a lowmixing speed and then for 8 min at a high mixing speed.SEEPS powders were mixed with the white oil in a blender forabout 10 min at a low speed to allow SEEPS to fully absorbthe oil. The white oil selectively swelled the EEP phase of thecopolymer but slightly swelled the S phase to give a micellarnetwork structure with discretized, spherical S domainsworking as the physical crosslinks for the EEP blocks. TheCI/SEEPS composites were prepared by melt blending theSEEPS matrix and CI particles without/with the titanatecoupling agent modification in a rheometer (Rheocord 900;Haake, Germany), and the change of torque with time duringpreparation was monitored to evaluate the processibility ofthe CI/SEEPS composites. At T = 200 ◦C, a mixing time

of 15 min and a rotor speed of 40 rpm were found to bethe most suitable processing conditions for these CI/SEEPScomposites. The processibility of the CI/SEEPS compositeswas remarkably improved after the surface modification ofCI particles. This notable improvement may be attributed tothe formation of a monomolecular layer of titanate couplingagent on the particle surface that enhanced the particle/matrixaffinity and highly suppressed particle aggregation.

For the preparation of isotropic MRE, the CI/SEEPScomposites were molded into films of 2 mm thicknessat 200 ◦C for 10 min in an ordinary pressing machinein the absence of a magnetic field. For preparation ofanisotropic MRE, the CI/SEEPS composites were moldedinto sheets of 3 mm thickness at 200 ◦C for 10 min ina home-made magnet–heat-coupled device in the presenceof a magnetic field with the intensity ranging from 0to 1400 mT, just as done in our previous work [26].The anisotropic CI/SEEPS composites without/with titanatecoupling agent modification thus obtained were designatedas SEEPS-nCI-m and SEEPS-nCI-2Ti-m, where n representsthe CI concentration (in wt%), m represents the magneticfield intensity (in mT) during the fabrication, and 2Tirepresents the 2 wt% titanate coupling agent contained inthe composites. Similarly, the isotropic CI/SEEPS compositeswithout/with titanate coupling agent modification were namedas SEEPS-nCI and SEEPS-nCI-2Ti.

2.3. Morphology observation

The morphology of isotropic and anisotropic CI/SEEPScomposites were observed with a JSM-7500FA ScanningElectron Microscope (SEM; JEOL, Japan) operated at anaccelerating voltage of 15 kV. Before SEM observation, theCI/SEEPS composite samples, pretreated with liquid nitrogenand freeze-fractured, were coated with a thin gold layer toavoid surface charging.

2.4. Mechanical properties

The tensile test was performed on a CMT6104 UniversalTensile Tester (Sans Testing Machine, China) according to theGB/T 528-1998 standard. The dumbbell-shaped specimenswere stretched at a crosshead rate of 500 mm min−1 untilrupture. The hardness was measured by a TH200 HardnessTester (Beijing Time Technology, China) according to theGB/T531.1-2008 standard. Five specimens for each samplewere tested to obtain the average value for determination. Allthe measurements were made at room temperature.

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2.5. Magnetic dynamic mechanical analysis (MDMA)

The magnetorheological (MR) properties of the CI/SEEPScomposites were examined through a modified Tritec 2000Bdynamic mechanical analysis (Triton Technology, UK)system equipped with a home-made electromagnet thatcould generate magnetic fields of various intensities [26].This modified DMA system is referred to as MDMA,and the dimension of the sample specimen for tests was10 mm × 10 mm × 3 mm (length, width and thickness).The measurements were made at frequency of 1 Hz, strainamplitude of 0.0667% and room temperature under themagnetic field to record the field-induced dynamic shearmodulus response. The direction of the external magneticfield was perpendicular to the surface of the tested sample.This direction was parallel to the chain structure of the CIparticles in the anisotropic CI/SEEPS composites, thereforethe modulus measured under the magnetic field detected theresponse of the composites in the direction perpendicular tothose chains.

2.6. Magnetorheological measurements

The rheological behavior of the CI/SEEPS compositeswas measured at room temperature with a Physica MCR301 stress-controlled Rheometer (Anton Parr, Germany)possessing a magnetorheological device in the absence orpresence of magnetic field. A parallel plate geometry (ofnon-magnetic titanium) of 20 mm diameter was used. Duringthe measurements, the electromagnetic coil was activated witha current from 0 to 3.0 A, corresponding to a magnetic fluxdensity (ψ) from 0 to 0.66 T inside the gap. To ensurethe linear viscoelastic region, a dynamic strain sweep wasconducted in the range γ0 = 0.0001–1 (0.01%–100%), whereγ0 is the setting strain, at a frequency ω = 5 rad s−1. Magneticfield sweeps were performed from 0 to 0.66 T with a strainamplitude of 1% and a frequency ω = 5 rad s−1. Dynamicfrequency sweeps were performed from 100 to 0.1 rad s−1

with a strain amplitude of 1% in the linear regime, and thestorage modulus G′, loss modulus G′′ and complex viscosityη∗ were recorded as a function of frequency. Steady-staterotary shear measurements were carried out as a function ofshear strain amplitude from 0.01 to 25% at ω = 5 rad s−1,and the rheological parameter of shear stress τ was obtainedfor each shear strain.

2.7. Adsorption test for CI particles

In order to characterize the effect of the titanate couplingagent on the interfacial microstructure in the CI/SEEPScomposites, the EPR molecules (having similar molecularstructure to the EEP block of SEEPS) were allowed toadsorb on the CI particle (without/with titanate coupling agentmodification) surface in octane. In this adsorption test, the CIparticles were dispersed in EPR/octane solution with the aidof supersonication and then mechanical shaking for 5 min.After that, the particles were dried to weigh the residue andcompared with the initial mass before dispersion, allowingthe amount of EPR adsorbed on the particle surface to bedetermined.

3. Results and discussion

3.1. Morphology observation

The MR effect of the CI/SEEPS composites should obviouslybe dependent on the state of the CI particle distribution inthe SEEPS matrix. Figure 1 shows the SEM images of theisotropic and anisotropic SEEPS-70CI and SEEPS-70CI-2Ticomposites prepared in the absence of a magnetic field and inthe presence of a magnetic field of different intensities ψpre asrepresentative for the CI/SEEPS composites. The CI particles(white dots) are uniformly dispersed in the SEEPS matrixwithout forming large aggregates, and clearly the titanatecoupling agent modification further improves the dispersionof the particles in the matrix with the enhancement of theircompatibility and interactions. The pretreatment of titanatecoupling agent does not obviously change the morphologyof the CI/SEEPS composites, but the external magneticfield loading during the anisotropic composite fabricationindeed changes the CI particle distribution greatly, with theformation of chain-like structures aligned in the direction ofthe magnetic field due to mutual attraction between thesemagnetizable particles. Clearly, when the SEEPS matrix lostits elasticity and behaved as a soft liquid at 200 ◦C, the CIparticles were reorganized from the random dispersion intoan ordered orientated distribution in the matrix. It can alsobe noted that the degree of alignment and orientation of CIparticles is enhanced and the particle chains become longerat larger ψpre. This behavior is naturally expected because theparticles are more strongly driven by an increasing ψpre (up toa saturation threshold of ∼=600 mT; cf figure 2).

3.2. Mechanical properties

Table 2 lists the mechanical property parameters of the SEEPSmatrix and isotropic CI/SEEPS composites. During thetensile test, different from SEEPS-80CI, the SEEPS-80CI-2Ticomposite exhibits superior elastomer behavior close tothat of the SEEPS matrix, and it is evident that aftertitanate coupling agent modification such a high filling ofmagnetic particles (φCI = 31 vol%) has almost no influenceon the strength and toughness of the SEEPS matrix. Thesignificant difference of the mechanical properties betweenthe SEEPS-80CI and SEEPS-80CI-2Ti composites lies in thechange of the interfacial structure with the interaction of thetitanate coupling agent. The thicker the interfacial layer is, thefewer soft SEEPS molecular segments (plasticized by whiteoil) exist, and the harder the CI/SEEPS composites behave.The introduced titanate coupling agent greatly enhances theinterfacial compatibility and reduces the thickness of theinterfacial layers in the CI/SEEPS composites, thus makingthe CI/SEEPS composite much softer, and even similar tothe SEEPS matrix. Moreover, the titanate coupling agentpretreatment can also significantly reduce the amount ofparticle aggregation in the matrix, which further decreasesthe hardness. The detailed compatibility mechanism andmicrostructural model of the interface formed between theCI particles and SEEPS matrix will be described anddemonstrated in section 3.5.

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Smart Mater. Struct. 21 (2012) 115028 X Qiao et al

Figure 1. SEM images of the isotropic and anisotropic CI/SEEPS composites without/with titanate coupling agent modification:(a) SEEPS-70CI, (b) SEEPS-70CI–500 mT, (c) SEEPS-70CI-2Ti, (d) SEEPS-70CI-2Ti–100 mT, (e) SEEPS-70CI-2Ti–300 mT,(f) SEEPS-70CI-2Ti–500 mT, (g) SEEPS-70CI-2Ti–700 mT and (h) SEEPS-70CI-2Ti–1400 mT. The white arrow indicates the direction ofthe external magnetic field. The scale bar in (a) is 20 µm, and the scale bar in the other images from (b) to (h) is 10 µm.

Table 2. Mechanical properties of the SEEPS matrix and isotropic CI/SEEPS composites without/with titanate coupling agent modification.

Sample code Elongation at break (%) Tensile strength (MPa) Hardness

SEEPS >1300 >1.1 0SEEPS-70CI 1150 1.5 5SEEPS-80CI 1000 1.48 13SEEPS-70CI-2Ti 1400 1.82 2SEEPS-80CI-2Ti 1127 0.67 4

3.3. Magnetic dynamic mechanical properties

One of the most important parameters describing the MReffect of MRE is the shear modulus change caused bythe magnetic field. In this study, a modified MDMA wasutilized to measure the field-induced dynamic shear modulusresponse under the magnetic field (ψ 6 1 T) for theisotropic and anisotropic CI/SEEPS composites without/with

titanate coupling agent modification, just as illustrated infigure 2. The anisotropic CI/SEEPS composites, coded asSEEPS-nCI(-2Ti)-m with n = wCI in wt% (= 70 or 80),m = ψpre (= 100–1400 mT) and 2Ti as titanate couplingagent, were pre-structured at 200 ◦C for 10 min under amagnetic field of intensity ψpre and had a chain-like structureof CI particles (cf figure 1), while the isotropic CI/SEEPScomposites, coded as SEEPS-nCI(-2Ti) were hot-pressed in

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Smart Mater. Struct. 21 (2012) 115028 X Qiao et al

Figure 2. Plots of shear storage modulus against magnetic field intensity obtained from MDMA measurements for the isotropic andanisotropic CI/SEEPS composites: (a) without titanate coupling agent modification and ((b), (c)) with titanate coupling agent modification.

Table 3. Shear storage modulus, MR effect and model parameters of the isotropic and anisotropic CI/SEEPS composites without/withtitanate coupling agent modification obtained from the MDMA measurements. (Note: G′0 is the initial storage modulus without an externalmagnetic field. G′M,E is the equilibrium storage modulus under a saturated magnetic field. 1G′ is the value of G′M,E subtracting G′0,corresponding to the absolute MR effect. 1G′/G′0 is the relative MR effect. GM,∞

′ and aψ are the model parameters estimated from theslopes and the intercepts of the plots of ψ2/G′ (ψ) versus ψ2.)

Sample code G′0 (MPa) G′M,E (MPa) 1G′ (MPa) 1G′/G′0 (%) G′M,∞ (MPa) aψ

SEEPS-70CI 0.053 0.152 0.099 187 0.17 0.41SEEPS-80CI 0.390 1.300 0.910 233 1.39 0.38SEEPS-70CI–1400 mT 0.166 0.802 0.636 383 1.44 0.11SEEPS-80CI–1400 mT 0.660 1.800 1.140 173 1.94 0.33

SEEPS-70CI-2Ti 0.020 0.213 0.193 944 0.28 1.30SEEPS-70CI-2Ti–100 mT 0.057 0.404 0.347 609 0.46 0.61SEEPS-70CI-2Ti–300 mT 0.069 0.542 0.473 686 0.59 0.37SEEPS-70CI-2Ti–500 mT 0.073 0.600 0.527 722 0.65 0.35SEEPS-70CI-2Ti–700 mT 0.074 0.621 0.547 739 0.67 0.32SEEPS-70CI-2Ti–1400 mT 0.076 0.619 0.543 714 0.67 0.34SEEPS-80CI-2Ti 0.271 1.775 1.504 555 2.28 1.26SEEPS-80CI-2Ti–1400 mT 0.355 2.320 1.965 554 2.76 0.75

the absence of a magnetic field and had a random and evendispersed structure of CI particles. The shear storage modulusand MR effect of these isotropic and anisotropic CI/SEEPS

composites are listed in table 3, in which G′0 is the initialstorage modulus without an external magnetic field, G′M,E isthe equilibrium storage modulus under a saturated magnetic

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Smart Mater. Struct. 21 (2012) 115028 X Qiao et al

field, 1G′ is the value of G′M,E subtracting G′0, correspondingto the absolute MR effect, and 1G′/G′0 is the relative MReffect.

From figure 2, it can be observed that for theseisotropic and anisotropic CI/SEEPS composites, the storagemodulus G′ exhibits a similar tendency in its change undera magnetic field, increasing rapidly with increasing fieldintensity ψ up to a threshold value of ψc ∼= 600–700 mTand then leveling off, whether without or with titanatecoupling agent modification. Moreover, the threshold valueof ψc decreases while G′ increases with an increase ofCI particle concentration in the composites and ψpre usedfor anisotropic composite preparation, especially for theCI/SEEPS composites with larger n and m having moreordered and longer chain structures of magnetic particles.This increasing trend of the shear storage modulus with ψbefore reaching magnetic saturation is also found for MREbased on natural rubber [27], silicon rubber [28] and PU [22],which agrees well with the theoretical analysis based on thefield-induced dipole magnetic forces between the particles.It can be noted from table 3 that, as expected, the relativeMR effect (1G′/G′0) of the CI/SEEPS composites is muchhigher than that of our previous CI/SEBS composites [26]and other MREs reported before, greater than 180% forthe isotropic CI/SEEPS composites and extremely high, asmuch as 700%, for the anisotropic CI/SEEPS composites.The pre-structuring under a magnetic field not only enhancesthe degree of alignment and orientation of CI particles inthe composites but also elevates the initial storage modulusof the CI/SEEPS composites, thus resulting in a slightlyweaker relative MR effect of the anisotropic composites thanthat of isotropic composites, sometimes unfavorably. Thesame situation also occurs when the filling of CI particlesis high in the SEEPS matrix, and with φCI increasingthe relative MR effect of the CI/SEEPS composites issomewhat decreased. However, the absolute MR effect (1G′)of both the isotropic and anisotropic CI/SEEPS compositesindeed continuously increases remarkably with the increaseof φCI and ψpre (before saturation), due to the increasingordered degree of the magnetic particle alignment and thestrengthening of the mutual particle interactions and resultantchain structures. After titanate coupling agent modification,G′ of the CI/SEEPS composites is obviously decreased,which should be attributed to the remarkable increase in thesoftness of the composites originating from the improvementof interfacial compatibility. The resultant significant decreaseof G′0 and increase of 1G′ makes the SEEPS-nCI-2Ticomposites possess a much higher absolute and relativeMR effect than the SEEPS-nCI composites, whether for theisotropic or anisotropic composites. It is the strong surfaceactivity of titanate coupling agent that allows its moleculesto reside at the interface of the CI particles and SEEPSmatrix and thus decrease the interfacial tension, which resultsin a better alignment of CI particles and an enhancementof the MR effect [24]. However, the threshold value of ψcdoes not change much after particle modification, because themagnetization saturation is related to the magnetic particleinteractions and a lesser amount of surface coating will

not obviously change the magnetic properties of these CIparticles.

It was found that the magnetically induced modulusGM (ψ) is proportional to the square of the magneticfield intensity ψ (GM(ψ) ∝ ψ

2) at low field intensity andapproaches the maximum value GM,∞ at high field intensityas the magnetization saturates; these two limiting cases can bephenomenologically described by the following equation:

ψ2

GM(ψ)=

aψGM,∞

+ψ2

GM,∞(1)

in which aψ is a material parameter [28]. Through the linearrelationship of the plots of ψ2/GM (ψ) versus ψ2, values ofGM,∞ and aψ can be estimated from the slopes and intercepts.Figure 3 gives the plots of ψ2/GM

′(ψ) versus ψ2 obtainedfrom MDMA measurements (figure 2) for the isotropicand anisotropic CI/SEEPS composites without/with titanatecoupling agent modification, and the evaluated parametersof G′M,∞ and aψ are listed in table 3. It can be clearlyseen from figure 3 that the MDMA experimental dataof the CI/SEEPS composites can be fitted quite well bythe phenomenological description in equation (1), with avery good linear relationship when ψ ranges from 200 to700 mT, whether the composite is isotropic or anisotropic andunmodified or modified by titanate coupling agent. It shouldalso be noted from table 3 that the calculated G′M,∞ values ofthe isotropic and anisotropic CI/SEEPS composites are veryclose to the experimental G′M,E data, which further verifiesthe applicability of equation (1) for describing the dependenceof the elastic modulus on the induced magnetic field forthese SEEPS-based MRE materials. Opposite to the changeof G′M,∞, aψ decreases with the increase in φCI and ψpre andincreases after particle modification, but attains a steady valueafter magnetization saturation, similarly to G′M,∞. It is evidentthat both G′M,∞ and aψ depend on the concentration anddistribution of the magnetic particles as well as the strengthof the applied magnetic field.

3.4. Magnetorheological behaviors

The MDMA results clearly demonstrate that the MR effectof the CI/SEEPS composites is significantly improvedafter particle surface modification. In this section, theisotropic CI/SEEPS composites with titanate couplingagent modification were taken as examples to investigatethe magnetorheological behaviors, including the dynamicoscillatory shear and steady-state rotary shear responses. Asusual, dynamic strain sweep experiments were first carried outto determine the linear viscoelastic region of the CI/SEEPScomposites under different ψ (data are not shown here).It is evident that the CI/SEEPS composite with lower φCIand under weaker ψ is tougher, with higher deformation foryielding. The strain amplitude of 1% in the linear regimewas chosen for all the following magnetic field and frequencysweep tests of the CI/SEEPS composites.

The dynamic magnetic field sweep results of theisotropic CI/SEEPS composites with titanate coupling agentmodification are given in figure 4 as the change of storage

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Figure 3. Plots of ψ2/G′M(ψ) versus ψ2 obtained from MDMA measurements for the isotropic and anisotropic CI/SEEPS composites: (a)without titanate coupling agent modification and ((b), (c)) with titanate coupling agent modification.

modulus G′ and damping factor tan δ with ψ . It is clearthat both φCI and ψ greatly affect G′ and tan δ of theCI/SEEPS composites. On the one hand, G′ and tan δ increasewith an increase in φCI, particularly at high ψ . More CIparticles help to enhance the strength in the ordered structuresof particles formed by their mutual magnetic attractionsunder magnetic field and elevate the elastic modulus ofthe CI/SEEPS composites correspondingly. The resultantthickening or complexity of the chain-like particle bundlesoriented in the magnetic field further improves the dampingand vibration isolation effect of the CI/SEEPS composites,with more energy dissipated. Undoubtedly, the increase ofψ obviously strengthens the effect of φCI on the elasticand damping behaviors of the SEEPS-based MRE. On theother hand, G′ increases continuously with ψ , following afirst-order exponential power law, while tan δ first increaseswithψ and then drops rapidly after experiencing its maximumvalue following a second-order power law when ψ > 0.03 T.All the values of coefficient squared (R2) are larger than

0.98, indicating the good fitting of these experimental data.Actually, the existence of a tan δ maximum has also beenreported before for the MRE based on natural rubber [27], butthe tan δ maximum, ψ for tan δ maximum and the increasingrate of tan δ with ψ are much higher for our CI/SEEPScomposites. The absolute and relative MR effect of theCI/SEEPS composites calculated from the storage moduluschange caused by the magnetic field are shown in figure 5as a function of ψ . It can be found in figure 5 that 1G′ andthe relative MR effect of the isotropic CI/SEEPS compositesexhibit the same change tendency with ψ , increasing linearlywhen 0.1 < ψ < 0.4 and then exponentially when ψ >

0.4. The greater ψ is, the stronger the magnetic particleassociations are, and the greater the MR effect of theSEEPS-based MRE is. It appears that the rate of increaseof the MR effect with ψ greatly depends on both theconcentration and surface features of the particles and the typeof matrix. Different from the MRE based on silicone rubber,

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Figure 4. Change of (a) the storage modulus G′ and (b) dampingfactor tan δ with the magnetic field intensity ψ , obtained from thedynamic magnetic field sweeps for the isotropic CI/SEEPScomposites with titanate coupling agent modification.

our SEEPS-based MRE shows a greater rate of increase in theMR effect at higher ψ above 400 mT than at lower ψ [25].

Figure 6 shows the linear viscoelastic behaviors ofisotropic CI/SEEPS composites with titanate coupling agentmodification under different ψ obtained from the dynamicfrequency sweeps. All the isotropic CI/SEEPS compositesexhibit similar changes of storage modulus G′ and complexviscosity η∗ with frequency ω at different ψ , so just theresults of SEEPS-80CI-2Ti composite are shown here as arepresentative. Similar to the results of the SEBS matrixand CI/SEBS composites [26], when ω varies from 100to 0.1 rad s−1,G′ of the SEEPS matrix and CI/SEEPScomposites is always one order of magnitude greater thanG′′, shows an elastic plateau at low ω with a much lowerslope than 2 (G′ ∝ ω0.006–0.061), either in the absence orpresence of magnetic field, and increases with an increasein φCI and ψ , as it does in MDMA measurements. Theelastic plateau of G′ should be attributed to the entanglementnetwork of the middle EEP blocks swollen with the white

Figure 5. Change of (a) absolute MR effect (1G′) and (b) relativeMR effect with the magnetic field intensity ψ , obtained from thedynamic magnetic field sweeps for the isotropic CI/SEEPScomposites with titanate coupling agent modification.

oil and physically crosslinked by the spherical domains ofthe end-S blocks. The addition of the magnetic field makesthe particles reorganize to be chain-like clusters along themagnetic field direction, and the enhancement in the strengthof these oriented particle bundles at stronger magnetic fieldendows this kind of MRE with a greater elastic responseand higher modulus and viscosity. The particle associationsorientated in the magnetic field direction can be clearly seenin the microscope images in figure 1, especially at highψ . Theincrease of modulus and viscosity is more remarkable for theSEEPS-80CI-2Ti composite due to the increase in the strengthof the particle interconnections at higher φCI. Moreover, thecomplex viscosity of all the CI/SEEPS composites decreaseslinearly with frequency, following a power law η∗ ∝ ω0.94–0.98

in the absence or presence of a magnetic field.Figure 7 shows the shear stress–shear strain curves of the

isotropic CI/SEEPS composites with titanate coupling agentmodification under different ψ obtained from the steady-stateshear measurements. All the CI/SEEPS composites exhibit

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Figure 6. Linear viscoelastic properties obtained from the dynamicfrequency sweeps for the isotropic SEEPS-80CI-2Ti compositeunder different magnetic field intensity: (a) log G′ and log G′′ versuslogω and (b) log η∗ versus logω.

stress growth in the absence of a magnetic field and slightstress overshoot before reaching steady state in the presenceof a magnetic field, similar to the steady shear response of theMRE based on silicone rubber [29]. In the linear viscoelastictest (figure 6), the CI/SEEPS composites behave as a solidunder small strain, and the steady state seen at large strainshere (figure 7) should characterize highly nonlinear, plasticflow as well as slippage in the composites, especially whenthe CI particle forms rigid chain-like structures in the presenceof a magnetic field. The peak strain for the stress overshootdoes not change much with ψ , centering around 20% forthe SEEPS-70CI-2Ti composite, but decreases remarkablyfrom 13.8% to 6.9% when ψ increases from 0.055 to 0.22 Tfor the SEEPS-80CI-2Ti composite. The stress overshoot ofthe CI/SEEPS composites appears to be greatly affected bythe shear orientation of the particle chains formed undera magnetic field and the slippage between the particlechains and the SEEPS matrix. As for the SEEPS-70CI-2Ticomposite, the amount of SEEPS seems to be sufficient tofirmly grip the shear-oriented particle chains and suppresstheir slippage, so that the peak strain is hardly affected by thechange inψ . However, as for the SEEPS-80CI-2Ti composite,

Figure 7. Change of the shear stress with shear strain obtainedfrom the steady-state shear measurements for the isotropicCI/SEEPS composites with titanate coupling agent modificationunder different magnetic field intensity: (a) SEEPS-70CI-2Ti and(b) SEEPS-80CI-2Ti.

the reduction of amount of SEEPS makes some particle chainslose their firm grip in the rubber matrix, and the resultantslippage of particle chains leads to an obvious decrease ofthe peak strain. Similar to the dynamic modulus and viscosity,the shear stress still significantly increases with an increase ofφCI and ψ . The equilibrium shear stress (σe) in steady stateincreases with ψ exponentially, and the increasing rate of σewithψ is much faster for the SEEPS-80CI-2Ti composite withhigher φCI.

3.5. Interfacial compatibilization mechanism

An interphase exists between the CI particles and SEEPSmatrix in the CI/SEEPS composite, and it is apparent thatthe modification of titanate coupling agent on the CI particlesurface obviously improves the interfacial compatibilitybetween the particle and matrix, thus remarkably softening

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the CI/SEEPS composites and enhancing their MR effect.The difference of the interfacial microstructure directly resultsin a difference in the mechanical, DMA and rheologicalproperties between the unmodified and modified CI/SEEPScomposites. The adsorption of matrix layers on the polarCI particle surface would have occurred with the S blocksbeing polar compared to the EEP block and white oil, just asmentioned in our previous study of CI/SEBS composites [26].After modification, the titanate coupling agent moleculesadsorbed on the particle surface significantly enhance theaffinity of CI fillers with the SEEPS matrix, so that theinterfacial layer thickness and possible particle aggregationsare then greatly reduced. The oxygen atoms contained inthe hydrophilic groups of titanate coupling agent can formchemical adsorption on the surface of CI particles andstrengthen the particle hydrophobicity, and the hydrophobicgroups of the titanate coupling agent can combine with theSEEPS molecules by nonpolar interactions, which results ina significant improvement in the compatibility between theparticles and matrix.

In order to describe the interfacial compatibilitymechanism occurring in the SEEPS-based MRE after particlesurface modification, we put forward the microstructuralmodels for both unmodified and modified CI/SEEPScomposites in figure 8. When the carbonyl iron particlesare dispersed in the SEEPS matrix, PS hard segmentsself-assemble to form PS micro-domains anchored on theCI particle surface, and EEP soft segments that couldbe swollen by white oil are adsorbed on the CI particlesurface too. We believe that the adsorption mode of EEPsoft segment on the particle surface should be differentfor the unmodified and modified CI/SEEPS composites. Forunmodified CI particles, EEP soft segments are adsorbedon the particle surface in a single point mode, and moreSEEPS molecular chains are involved to form a soft andthick interfacial layer, thus leading to a remarkable increasein the modulus and stiffness (strength and hardness) anda corresponding decrease in the toughness and softness ofSEEPS-based MRE. The resultant obvious gaps between theCI particles and SEEPS matrix due to their poor interactionscauses loss of magnetic energy and the microstructure is lesschanged under the magnetic field [25], as represented bythe lower absolute and relative MR effect. For modified CIparticles, a monomolecular layer of titanate coupling agentis formed on the particle surface, EEP soft segments areadsorbed on the particle surface in a covering mode dueto the affinity of EEP segments and titanate coupling agentand the resultant van der Waals interactions, therefore lessSEEPS molecular chains are involved to form a hard andthin interfacial layer. Besides improving the dispersion ofCI particles in the SEEPS matrix, the enhancement of theinterfacial compatibility between the particles and matrix alsomakes the CI/SEEPS composites retain the original toughnessand softness of the SEEPS matrix, which allows the CIparticles to align in the magnetic field direction easily andmakes the SEEPS-based MRE exhibit superior MR effectunder a magnetic field. It can also be clearly seen fromthe SEM images under higher magnification of the isotropic

Figure 8. Microstructure illustration of the interphase betweencarbonyl iron particles and the SEEPS matrix in the isotropicCI/SEEPS composites: (a) carbonyl iron particles and SEEPS matrixand (b) titanate coupling agent modified carbonyl iron particles andSEEPS matrix. White oil is not shown here. The top plots are SEMimages of isotropic SEEPS-70CI (left) and SEEPS-70CI-2Ti (right)composites with a magnification of 10 000× and scale bar of 5 µm.

SEEPS-70CI and SEEPS-70CI-2Ti composites in figure 8that the gaps between the SEEPS matrix and CI particleshave been greatly reduced and even disappear after particlemodification.

EPR (ethylene–propylene rubber, non-crosslinked) hasa similar structure to that of EEP segment, and octane hassimilar chemical properties to those of white oil. In orderto check the adsorption mechanism of EEP segments onthe CI particle surface, by supersonication we dispersedthe unmodified CI particles (CI) and titanate couplingagent modified CI particles (CI-2Ti) in EPR/octane solutionrespectively for comparison, analogous to the dispersion of CIand CI-2Ti particles in EEP/white oil phase for the CI/SEEPScomposites. After shaking for enough time, drying andweighing the residue solid, it is found that about 3.4 mg EPRmolecules is adsorbed on the unmodified CI particles (40 mg),while 0.6 mg EPR molecules is adsorbed on the modifiedCI particles (40 mg). This result further demonstrates ourconclusion that more SEEPS molecules are adsorbed on theunmodified CI particle surface than those on the modified CIparticle surface described above in the microstructural model.

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4. Conclusion

Similar to the CI/SEBS composites we previously reported,the CI particles are uniformly dispersed in the SEEPS matrix,and reorganize into an ordered chain-like structure alongthe magnetic field direction during the anisotropic compositefabrication, with a higher degree of orientation and longerparticle chains at larger ψ . However, superior to the CI/SEBScomposites, the CI/SEEPS composites exhibit a much highermagnetorheological (MR) effect, greater than 180% for theisotropic CI/SEEPS composites and extremely high, as muchas 700%, for the anisotropic CI/SEEPS composites. Afterthe CI particles were modified by the titanate coupling agentto enhance their compatibility with the SEEPS matrix, it isfound that the obtained CI/SEEPS composites become muchsofter with a higher MR effect and show almost the samemechanical properties as the SEEPS matrix does, withoutsacrificing toughness, even with such a high filling of the solidCI particles as 80 wt%. These phenomena should be attributedto the change in the interfacial structure formed between theCI particles and the SEEPS matrix. The introduced titanatecoupling agent enhances the affinity of the CI particlestoward the SEEPS matrix, changes the adsorption of softEEP segments on the particle surface from single point modeto cover mode, and obviously decreases the thickness ofthe interfacial layers in the CI/SEEPS composites. Theseinfluences are very helpful in avoiding the magnetic energyloss originating from the gaps, allowing the CI particles toalign in the magnetic field direction more easily, and makingthe SEEPS-based MRE exhibit a superior MR effect under amagnetic field.

Acknowledgments

This research was supported by the Science and TechnologyResearch and Development Programme from the Ministry ofRailways of China (grant No. J2011J002), Minhang District-Shangai Jiao Tong University Science and TechnologyCooperation Special Fund. The authors also thank theInstrumental Analysis Center of Shanghai Jiao TongUniversity for assistance with the measurements.

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