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Microstructure–property relationships of urethane magnetorheological elastomers

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2007 Smart Mater. Struct. 16 1924

(http://iopscience.iop.org/0964-1726/16/5/049)

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IOP PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 16 (2007) 1924–1930 doi:10.1088/0964-1726/16/5/049

Microstructure–property relationships ofurethane magnetorheological elastomersAnna Boczkowska1, Stefan F Awietjan and Rafal Wroblewski

Faculty of Materials Science and Engineering, Warsaw University of Technology,02-507 Warsaw, Woloska 141, Poland

E-mail: abocz@meil.pw.edu.pl

Received 29 January 2007, in final form 25 July 2007Published 7 September 2007Online at stacks.iop.org/SMS/16/1924

AbstractStudies on the structure of urethane magnetorheological elastomers (MREs),with respect to their magnetic and mechanical properties, are reported. MREswere obtained from a mixture of polyurethane gel and carbonyl-iron particlescured in a magnetic field of 100 and 300 mT. Samples with different numbersof particles (1.5, 11.5 and 33 vol%) were produced. The microstructure andmagnetic properties of the obtained MREs were studied. Also, thedisplacement of the samples in an external magnetic field was examinedusing a specially designed experimental set-up. The influences of the numberof ferromagnetic particles and their arrangement in relation to the externalmagnetic field were investigated.

It was found that the microstructure of the MREs depends on the numberof ferrous particles and the fabrication conditions. The orientation of the ironparticles into aligned chains is possible for a lower volume content of theferromagnetic fillers. The high carbonyl-iron volume content in the matrixleads to the formation of more complex microstructures, similar tothree-dimensional lattices. The magnetic measurements also confirmed theexistence of the microstructure anisotropy for the MREs with 1.5 and11.5 vol% of iron particles. The structural and magnetic anisotropy has notbeen found in the MREs with 33 vol% of Fe. To evaluate the effect of theexternal magnetic field on the magnetorheological properties, thedisplacement under magnetic field, the compressive strength, and therheological properties were measured. The experiments showed that both theparticle content and the field strength used during curing have a significanteffect on the microstructure of the MREs and, in consequence, on theirproperties.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Interest in magnetorheological elastomers (MREs) has recentlyincreased due to their prospective applications in smartsystems. They are solid analogues of magnetorheologicalfluids, in which the fluid component is replaced by a cross-linked material such as a rubber or gel. They both consistof micron sized magnetically permeable particles in a non-magnetic matrix material. Iron powder is used as the mostcommon magnetic material with a high purity. In the case

1 Author to whom any correspondence should be addressed.

of MREs the magnetically permeable particles are added to aviscoelastic polymeric material prior to cross-linking [1, 2]. Ina manner similar to the case of MRFs, the particles tend toalign themselves in the direction of the magnetic field [3–5].But in MREs after the matrix curing process, the ferromagneticparticles are fixed in their positions and form chain-likestructures. Magnetorheological (MR) materials change theirrheological properties continuously, rapidly, and reversiblyunder the influence of an applied magnetic field [6, 7].

It is reported that the MREs contain ferromagneticparticles having sizes from a few to a few hundredmicrometres [8]. Pure iron has the highest saturation

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Microstructure–property relationships of urethane magnetorheological elastomers

magnetization of the known elements and it also has a highpermeability and low remanent magnetization, providing high,short-term interparticle attraction [9]. It is known from theavailable literature that the number of particles oscillatesbetween a few and 50% by volume [10, 11]. The high ironconcentration may influence the long-term stability of MREmaterials [7].

Different elastomers and fillers can be used for MREpreparation [3, 4, 9, 12]. An external magnetic field isapplied before the curing process of the polymer is finished.The field induces dipole moments within the particles, whichtend to achieve minimum energy states. Particle chains withcollinear dipole moments are formed and the curing of thepolymeric host material locks the chains in place [3, 6, 13]. Inthe orientation, the particles can form separate chains, three-dimensional simple lattice structures (consisting of separatechains), or even more complex structures where particles havemultiple interaction points [12].

The advantage of the MRE, with respect to the MRF,is that ferrous particles in the former do not undergosedimentation. The thermal stability of the MRE is greaterthan that of the MRF and its resistance for degradation ishigher. In comparison with MRFs, the MREs have a field-responsive modulus [3, 14]. The amount of the particlefiller can also be seen to be lower when it is compared tothe MRF. As a result, the weight of sensing and actuatingdevices based on the MRE is lower. Due to the characteristicMRE microstructure, the time of response and strain valueversus magnetic field strength could be shortened. MREs holdpromise for enabling variable stiffness devices and adaptivestructures in aerospace, automotive, and civil and electricalengineering applications [15–17]. The relative change of theMRE modulus in the magnetic field is higher in the caseof soft elastomers [18]. Urethane elastomers are promisingas an MRE matrix, because they have better degradationstability, compared with natural rubbers. The high iron volumefraction may influence the long-term stability of the MRE.The selection of a suitable matrix material is important whenconsidering the possible applications and long-term stability ofthe MRE.

In this paper studies on the microstructure of urethaneMREs with respect to their magnetic and mechanical propertiesare reported.

2. Materials and methods

Urethane MREs were manufactured using polyurethane gel,supplied by Dow Chemical Company. Polyurethane (PU)gel was synthesized from polyether polyol Voralux® HF 505used in a blend with 14 922 polyol and isocyanate compoundHB 6013. The mixing of substrates and the curing processwere conducted at room temperature. PU gel is characterizedby low density (1.03 g cm−3), low viscosity before curing(ca 1600 mPa s), low hardness (below 10◦ ShA), and lowYoung’s modulus (below 0.2 MPa). The low hardness andstiffness of the polyurethane matrix can lead to higher relativeproperty changes of the MRE under an external magnetic field.A relatively low viscosity during the processing of the MREmakes the arrangement of the particles into aligned chains easy.

Figure 1. Scheme of the experimental set-up for MRE investigationsin the magnetic field.

The ferromagnetic particles used in the MRE werecarbonyl-iron powders with particle sizes of 6–9 μm, producedby Fluka.

The samples were produced with randomly dispersed oraligned carbonyl iron particles, respectively. The amount of thecarbonyl iron particles was equal to 1.5, 11.5 and 33.0 vol%,respectively. The samples were subjected to a magnetic fieldduring the curing reaction to produce carbonyl chains withinthe elastomer aligned with the long sample axis. Two differentmagnetic field strengths were used: 100 and 300 mT.

The microstructure observations were carried out usinglight (LM) and scanning electron (SEM) microscopy.Investigations of the brittle fractures of the MRE were doneusing a LEO 1530 Zeiss microscope. Thin slices of the MRE,for LM observations, were cut using a Leica RM2165 rotarymicrotome with LN21 cooling device. The microstructure ofthin MRE slices was examined using a Biolar PI (PZO) typelight microscope equipped with a polarization head.

The magnetic measurements of the magnetic momentversus field were carried out using a Lake Shore vibratingsample magnetometer (VSM) up to maximum field 1 T. Toevaluate the magnetorheological materials’ response to theapplied magnetic field experiments were conducted usingcylindrical samples (� = 8 mm, h = 18 mm) in aspecially designed experimental set-up shown in figure 1. Anelectromagnet was used to produce the magnetic field. Thefield strength was constantly monitored using a Hall probe.

The experiment’s objective was to measure the samples’response to deflection under a magnetic field. The sampleswere placed parallel to the magnetic field lines and deflectedprior to the application of the magnetic field. Deflection,which is analogous to three point bending, was applied tochange the orientation of particle paths in the material. Afterthe application of the magnetic field, the sample tends tostraighten, which is measured by a displacement sensor. Amagnetic field in the range of 0–0.9 T has been applied.

The compression tests were performed using an MTSQTest/10 machine with magnetic coil device. Thisconfiguration allows performance of tests in magnetic field upto 0.3 T. Cylindrical samples 20 mm in diameter and 25 mm inheight were compressed at 5 mm min−1 speed.

Rheological properties of the MRE samples weremeasured with the application of an Ares rheometer plate–platesystem with plate diameter 20 mm, gap 2 mm, and magneticfield range 0–0.2 T. The elastic (storage) modulus G′ wasmeasured as a function of angular frequency ω under constantstrain of 0.1% at RT (25 ◦C).

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Figure 2. Microstructure of PU cured under a magnetic field of 300 mT, filled with carbonyl-iron particles: (a) 11.5 vol%, (b) 33 vol%.

Figure 3. SEM images of PU filled with 11.5 vol% of carbonyl-iron cured without (a) and under a magnetic field of (b) 100 mT and(c) 300 mT. White arrows show the magnetic field direction.

3. Results and discussion

Orientation of the ferromagnetic particles occurs during curingof the polymer matrix under a magnetic field. After curing,the position of the ferrous particles is fixed. Microstructureobservations carried out with light microscopy showed thatthe MRE fabrication method used in this work leads to anorganization of the carbonyl-iron particles into chains alignedwith magnetic field lines (figure 2(a)) if the particle content isequal to 11.5 vol% or less. When the ferrous particle content isequal to 33 vol% it is hard to distinguish particle chains whichare aligned with the magnetic field direction (figure 2(b)).

SEM examinations also confirmed the existence of alignedparticle chains in the elastomer matrix, when the magnetic fieldis applied during curing (figures 3(b) and (c)). Such particlechains are visible on the fractured surfaces of the MRE. If the

reactive mixture of the substrates with the particles is curedwithout the magnetic field, a homogeneous distribution of theparticles is observed (figure 3(a)).

It seems that the alignment of the particle chains dependson the strength of the magnetic field applied during curing.Application of a stronger magnetic field (e.g. 300 mT) leadsto the formation of wider particle chains, consisting of highernumbers of particles.

Observations of the influence of the particle volumefractions on the MRE microstructure were also carried out.As shown in figure 4, the orientation of the iron particlesinto aligned chains is possible for 1.5% and 11.5% volumefraction of the ferromagnetic fillers. A high carbonyl-ironvolume content in the PU matrix leads to the formation ofmore complex microstructures, similar to three-dimensionallattices. It is reported in the literature that the particles can

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Figure 4. SEM images of PU cured under a magnetic field of 300 mT, filled with carbonyl-iron particles: (a) 1.5 vol%, (b) 11.5 vol%,(c) 33 vol%. White arrows show the magnetic field direction.

Figure 5. Magnetic properties of PU filled with 1.5 vol% ofcarbonyl-iron, cured under a magnetic field of 100 mT.

form separated chains, three-dimensional simple lattices oreven more complex structures, where particles have multipleinteraction points [3–5].

The particles’ orientations and their arrangement were in-vestigated using a Lake Shore vibrating sample magnetome-ter (VSM). Studies of the magnetic properties of the obtainedMRE were carried out parallel and perpendicular to the samplelong axis, which is appropriate to the magnetic field directionduring curing. The correlation between the microstructure ofthe MREs and their magnetic properties was found. Selectedhysteresis loops for the MREs with different volume fractionsof the carbonyl-iron, cured under different strengths of themagnetic field, are shown in figures 5–8. Hysteresis loops forthe MREs with 1.5 and 11.5 vol% of carbonyl-iron give evi-

Figure 6. Magnetic properties of PU filled with 11.5 vol% ofcarbonyl-iron, cured under a magnetic field of 100 mT.

dence for the structural and magnetic anisotropy. As found inthe SEM observations, the hysteresis loops also indicate that,for the higher ferrous particle contents (33 vol%), an isotropicnetwork of particles is formed (figure 8).

The mass susceptibility was calculated from the hysteresisloops at magnetic field strength equal to 0.2 T. Also theanisotropy coefficient was evaluated with respect to thecarbonyl-iron particle volume content and the strength of themagnetic field applied during curing of the polymer matrix.The results are shown in table 1.

It was found that there is no visible effect of the aligningmagnetic field strength on the anisotropy of the specimenscontaining 1.5 vol% of carbonyl-iron (samples 1 and 2). Thematerial anisotropy coefficient Ab grows from 1.42 for the

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Table 1. Magnetic properties of the obtained MREs.

Samplenumber

Fe content(vol%)

Aligningfield (mT)

Magnetic fielddirection

Mass susceptibility,χ(m3 kg−1)

Anisotropycoefficienta , Ab

1 1.5 100 Parallel 0.476 12 1.46Perpendicular 0.325 64

2 1.5 300Parallel 0.564 49

1.42Perpendicular 0.396 50

3 11.5 100Parallel 1.699 05

1.50Perpendicular 1.134 55

4 11.5 300 Parallel 1.810 51 1.63Perpendicular 1.113 06

5 33 100 Parallel 3.054 94 1Perpendicular 3.054 94

6 33 300 Parallel 3.022 29 1Perpendicular 3.022 29

a Anisotropy coefficient is expressed by the ratio of magnetic moments measured, respectively,parallel and perpendicular to the alignment direction.

Figure 7. Magnetic properties of PU filled with 11.5 vol% ofcarbonyl-iron, cured under a magnetic field of 300 mT.

Figure 8. Magnetic properties of PU filled with 33 vol% ofcarbonyl-iron, cured under a magnetic field of 300 mT.

1.5 vol% Fe to 1.63 for the 11.5 vol% Fe specimen. Moreover,for the latter samples, a significant effect of the aligningmagnetic field is visible; a higher magnetic field leads to abetter alignment (samples 3 and 4).

The structural and magnetic anisotropy has not been foundin the specimens having 33 vol% of carbonyl-iron (samples 5and 6). This is supposed to be due to the formation of a networkof the particles instead of the particle chains.

Figure 9. Displacement in the external magnetic field for the MREswith different volume contents of carbonyl-iron particles.

Figure 10. Displacement in an external magnetic field for MREswith 11.5 vol% of carbonyl-iron particles, cured without and withdifferent magnetic field strengths.

The materials response to the applied magnetic field wasmeasured using a specially designed set-up for experiments.The results of the measurements of the samples’ displacementas a function of the magnetic field strength are shown infigures 9 and 10.

As is visible in figures 9 and 10, the samples’ responseis stronger with the rise of the magnetic field in all cases.

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Microstructure–property relationships of urethane magnetorheological elastomers

Figure 11. Compressive curves for samples with different amountsof carbonyl iron, performed without and with 0.3 T externalmagnetic field.

As shown in figure 9, the displacement changes with varyingparticle content. The greatest displacement under a magneticfield is observed for the sample with 11.5 vol% of the particles(greater than for the higher number of iron particles). It seemsthat, if the particles are organized into anisotropic chains ratherthan isotropic ones, the response of the MREs to the magneticfield is stronger. This indicates that, under an external magneticfield, the properties of the MREs depend not only on theparticle content but also on the arrangement of the particles.

The displacement of MREs in the magnetic field alsodepends on the field strength applied during curing (seefigure 10). For constant particle volume fractions, thedisplacement rises with an increasing magnetic field strengthduring curing. For the samples with isotropic particledistributions the response to an applied field is weak. Theresponse of the MREs to the external magnetic field is thestrongest in the sample with the highest anisotropy coefficient,equal to 1.63. It also turns out that the properties of the MREsdepend on their microstructure.

In figure 11 the results of compression tests of sampleswith different numbers of carbonyl iron particles aligned under0.1 T magnetic field are shown. It can be seen that under anexternal magnetic field the samples are distinguished by highercompressive strengths. This effect is stronger with the rise ofthe ferromagnetic particle numbers in the material.

The results of the studies of the rheological propertiesof the MREs are shown in figure 12. The storage modulus,G′, represents the ability of the viscoelastic material to storethe energy of deformation, which contributes to the materialstiffness.

It was found that the elastic modulus grows with theincrease of angular frequency. Application of an externalmagnetic field leads to a significant increase in elastic modulus.As shown in figure 12, values of G′ increase with increasingmagnetic field for both MREs with 11.5 and 33 vol% of Fe.Due to the higher amount of ferromagnetic particles in the PUmatrix, higher values of G′ for MREs with 33 vol% of Fe areobserved.

As is known, the magnetorheological effect can beexpressed by the relative change of elastic modulus �G ′

G ′0

undermagnetic field. The relative changes of elastic modulus shown

Figure 12. Elastic modulus versus angular frequency for MREsfilled with 11.5 and 33 vol% of Fe, cured under a magnetic field of0.3 T, subjected to an external magnetic field of 0, 0.1 and 0.2 T.

Table 2. Relative changes of MRE elastic modulus under a magneticfield.

�G′100 mTG′

0

�G′200 mTG′

0

Fe content (vol%) at 0.1 Hz at 63 Hz at 0.1 Hz at 63 Hz

11.5 1.04 1.21 2.57 2.9333 0.54 0.69 1.5 1.81

where: �G′100 mT = G′

100 mT–G′0 mT; �G′

200 mT = G′200 mT–G′

0 mT

in table 2 were calculated from data shown in figure 12 for thelowest (0.1 Hz) and the highest (63 Hz) angular frequency usedin the experiment.

It was found that the relative change of the elastic modulusunder magnetic field of 100 and 200 mT is considerablyhigher for the MRE with 11.5 vol% of Fe than for the MREwith 33 vol% of Fe and increases with the increase of themagnetic field strength. This is due to the better alignmentof the ferromagnetic particles into chains for the sample with11.5 vol% of Fe, which was confirmed by SEM observationsand magnetic measurements. Also growth of the relativechange of elastic modulus with the increase of frequencyis observed in each case. The magnetorheological effect issignificantly higher for the MRE with better alignment ofthe particles although the Fe particle content is lower. Themost important factor influencing the MRE properties undermagnetic field seems to be the particle arrangement.

4. Conclusions

Magnetorheological elastomers consisting of polyurethanegel and carbonyl-iron particles were fabricated and studied.Samples with different numbers of particles (1.5, 11.5 and33 vol%) were cured under two different magnetic fieldstrengths (0.1 and 0.3 T). The microstructure and magneticproperties of the obtained MREs were studied. Thedisplacement of the samples in an external magnetic field wasexamined using a specially designed experimental set-up.

It was found that the microstructure of the MREs dependson both the number of ferrous particles and fabricationconditions. The orientation of the iron particles into alignedchains is possible for a lower volume content of ferromagnetic

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fillers. A high carbonyl-iron volume content in the matrixleads to the formation of more complex microstructures.Microscopic observations proved that the iron particles formchains aligned with the magnetic field direction when theparticle content is 11.5 vol% or less. If the particle contentreaches 33 vol%, a microstructure similar to three-dimensionallattices of iron particles is formed.

Also, the magnetic measurements confirmed the existenceof the microstructure anisotropy for the MREs with 1.5 and11.5 vol% of iron particles. The structural and magneticanisotropy has not been found in the MREs with 33 vol%of carbonyl-iron. Moreover, the results of the studies provethat the anisotropy coefficient expressed by the ratio ofthe magnetic moments measured, respectively, parallel andperpendicular to the alignment direction depend on the strengthof the magnetic field used during fabrication for the MREs with11.5 vol% of particles. The application of higher magneticfields (equal to 300 mT) during curing leads to higher values ofthe anisotropy coefficient and to the formation of wider particlechains, consisting of a larger number of particles. The spacesbetween the chains seem to be greater.

The magnetorheological material response to the appliedmagnetic field measured using a specially designed set-upconfirmed that the displacement of the MREs in the externalmagnetic field depends mainly on the number of particlesand their arrangement. The highest displacement is observedfor samples with 11.5% of particles, which is higher thanfor samples with 33 vol% Fe. This can be explained by adifference in their microstructures.

Compression test results showed that under external mag-netic field samples are distinguished by higher compressivestrengths. The stiffness growth is more significant with the riseof the ferromagnetic particle numbers in the material.

The microstructure of aligned particle chains of theMREs has a significant influence on the elastic propertiesof the composite material. The storage modulus can beincreased significantly by the applying of a magnetic field.The magnetorheological effect depends on the particles’arrangement expressed by anisotropy coefficient.

Acknowledgment

This study is financed as a Targeted Research Project fromfunds of the Ministry of Science and Higher Education withinthe years 2006–2008.

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