FeCo3

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Magnetorheology of Polydimethylsiloxane Elastomer/FeCo Nanocomposite Bablu Mordina, Rajesh Tiwari, Dipak Kumar Setua, and Ashutosh Sharma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507005s • Publication Date (Web): 13 Oct 2014 Downloaded from http://pubs.acs.org on October 16, 2014

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The Journal of Physical Chemistry

Magnetorheology of Polydimethylsiloxane Elastomer/FeCo3 Nanocomposite Bablu Mordina,1, 2 Rajesh Kumar Tiwari,1 Dipak Kumar Setua,1, # Ashutosh Sharma2,* 1

Defence Materials and Stores Research and Development Establishment, Kanpur-208013,

India 2

Department of Chemical Engineering, Indian Institute of Technology, Kanpur-208016, India

Abstract We investigate for the first time the magnetorheological (MR) properties of bi-metallic alloy nanocomposites based on cross-linked polydimethylsiloxane (PDMS) elastomer and ferromagnetic FeCo3 nanoparticles. The nanoparticles (~ 30 nm), with a saturation magnetization value of 166 emu/g, are synthesized by hydrazine reduction of Fe2+ and Co2+ metal ions. Isotropic and anisotropic nanocomposite films are prepared by solution casting technique with 5, 10, 20 wt. % FeCo3 in the absence and presence of 0.2 T magnetic field, respectively. The structural, morphological and magnetic properties of nanoparticles and their composites are characterized by X-ray diffraction (XRD), TEM, FESEM, confocal and optical microscopy, and vibrating sample magnetometer (VSM). Steady state and dynamic mechanical properties of the nanocomposite under a magnetic field are evaluated by rotary shear, strain amplitude sweep, angular frequency sweep and magnetic flux density sweep tests using a parallel plate rheometer. Effect of particle concentration, particle alignment on the magnetic properties and anisotropic coefficient of the nanocomposites are determined by measuring the hysteresis property parallel and perpendicular to the particle chain alignment. The anisotropic nanocomposites show higher saturation magnetization than the isotropic one, except for the particle concentration at 20 wt. %. Magnetorheological (MR) study reveals that the isotropic nanocomposites have higher absolute and relative MR effect than their anisotropic counterpart. Under 1.098 T magnetic field, the highest absolute MR effect of ~21600 Pa is found for 5 wt. %, whereas the highest relative MR effect of ~8.4 % is obtained with 20 wt. % isotropic composites. Key words: Nanocomposites, FeCo3, PDMS, magnetorheological elastomer *

Corresponding author

E-mail addresses: #[email protected] , *[email protected]

ACS Paragon Plus Environment


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1. Introduction Smart materials have physical or chemical properties which are controlled by the external stimuli such as the magnetic field, stresses, electric field, temperature, moisture, pH, etc. in a reversible way. Magnetorheological elastomers (MRE) are the materials with tuneable rheological and viscoelastic properties under the applied magnetic field, with significant applications in several emerging areas, e.g., tuneable vibration absorber in mechanical devices,1-3 actuators,4-5 magneto-active valve for air flow control,6 isolators in vehicle seat vibration control,7-8 sensors,9 rubber bushing,10 etc. The research in the field of MRE are mainly focused on the study of the effect of different elastomer matrix, 11-15 types of magnetic particles,4,16,17 size, shape and particle orientation,18,19 surface modification of magnetic particles,20-24 effect of addition of plasticizer,25,26 and carbon based materials.27-31 These studies are generally focused on determination of mechanical, viscoelastic and magnetorheological properties of the MRE. The MREs studied in the literature are mainly based on soft magnetic micro-particles, e.g. iron, cobalt, nickel and their oxides.11,32-34 Pure iron shows highest saturation magnetization and high permeability with low remnant magnetization which makes it a popular magnetic particle in most of the MREs.11,14,35-43 Carbonyl iron particle is another widely explored magnetic particle in this field.12,13,36,44-46 Magnetorheological elastomers with hard magnetic material like BaFe12O19 or SrFe12O194,47 and Nd2Fe14B, SmCo54,16 have also been reported. Giant magnetostriction of MREs containing Terfenol D as magnetostrictive particles have been reported by Wang et al. & Yin et al.48,49 Magnetic shape memory particles (such as Ni-Mn-Ga) containing MREs have also been reported in the published literature.50,51 However, most of above investigations have been carried out with relatively large magnetic particles of micron sizes at high loadings between 60 and 80 wt. %, and even as high as 90 wt. %.52 Stepanov et al. studied the elastic modulus of silicone rubber composite containing different filler loading (7.1-27.6 vol.%) of metallic ferrous powder (size 2-3 µm) and magnetite (size 0.2-0.3 µm). The study demonstrated that the shear modulus increases with the higher particle content and size 41. In another study Stepanov et al. investigated the effect of different volume fraction (viz. 30, 37, 35 and 35) of 2-4 µm and 2-70 µm iron microparticles in the mass ratios 100:0, 50:50, 75:25, and 50:50, respectively on the magnetorheological effect of the silicon based MRE. The experimental results revealed that the anisotropic sample containing higher fraction of larger particle showed larger increment in storage and loss moduli 42.


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However, high loading of the magnetic particles increases the material cost and viscosity of the composites significantly, may cause non-uniformities by particle aggregation and makes their processing more difficult. Higher particle loading leads to the severe particle aggregation and may destroy the continuance of the elastomer matrix which ultimately has detrimental effect on the elasticity of elastomer matrix.46,53 As MR performance is dependent on elasticity of the polymer matrix, deterioration in elasticity may lead to inferior MR properties. Li et al.27 investigated 3 to 5 µm carbonyl iron filled PDMS composites and reported only 4.5 and 4.8 % relative MR effect for 62.5 wt.% isotropic and anisotropic composites at 0.44 T magnetic field. In another study Li et al.52 achieved very small relative MR effect of 1.27, 1.30, 1.74 and 1.81% with 60, 70, 80 and 90 wt. % filler loading, respectively for isotropic composites of same elastomer-filler combination and magnetic field value. For successful designing of magnetorheological (MR) elastomer, the service life has also to be taken into account along with high MR effect. Considering the practical application, the magnetorheological elastomer based devices are generally used in alternating magnetic field. Loss of energy per cycle is larger for micron sized magnetic particle based MRE because of their greater remnant magnetization value. Therefore heating of the MRE is a problem due to the higher hysteresis loss of micron sized magnetic particles in the alternating magnetic field. Due to this continuous heating, ageing of the elastomer matrix takes place which in turn deteriorate the elastic and damping properties of the elastomer and finally reduces the functional properties as well as service life of the device under operation. Zrinyi et al. reported small energy loss per cycle for 10 nm Fe3O4 particle filled cross-linked polyvinyl alcohol based ferrogel subjected to the alternating magnetic field because of narrow hysteresis loop of the ferrogel.54 It is well known that decreasing the size of the magnetic filler to the submicron level decreases the remnant magnetization considerably. Thus the thermal fatigue and deterioration of elastic properties of the MRE due to hysteresis could be minimized. To this end, several investigations have also been carried out in the literature with small magnetic filler loading of nano, rather than micro-particles. Zrinyi et al. in their pioneering work studied the unidirectional deformation behaviour of ferrogels with isotropic distribution of 2.45-4.9 wt. % Fe3O4 nanoparticles (average size 10-12 nm) in polyvinyl alcohol (PVA). The study revealed that modulus of the ferrogel was not affected by the presence of inhomogeneous magnetic field and the elongation of ferrogel vertically suspended in electromagnet axis was varied with the square of steady current intensity.55 In another study Zrinyi et al. investigated the shape transition of 10 nm Fe3O4 nanoparticles filled polyvinyl alcohol (PVA) ferrogel and observed that the shape distortion was



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instantaneous and it occurred via shift of equilibrium from one local energy minimum state to another. The experimental results also revealed that deformation degree remained almost same in both 0.01and 0.05 vol. % filler loaded ferrogels but there was significant increase in sensitivity of the ferrogel at higher filler loading.54 Mendes et al. studied the transient rheological response of magnetic gel consisting varying content of SEBS triblock copolymer and 6 vol. % carbonyl iron microparticle (average particle diameter1µm) and compared the results with the rheological response of MRE and MR fluid based on PDMS and paraffin oil, respectively. Their experimental results revealed that similar to MR fluid the chaining and clustering process took place in the MR gel at a smaller length scale. However, no particle network evolution was observed in MRE up to 410 mT magnetic field due to high resistance of the PDMS elastomer. The formation of long particle chains and clusters were observed to be hindered due to the resistance offered by the polymer network in the copolymer gel.56

For higher MR performance, formation of three dimensional network of magnetic particle within the polymer matrix is necessary to assist better specific interaction induced by the external magnetic field 42. In this regards nano-magnetic materials have attracted tremendous attention of the researchers as they can form strong interconnected three dimensional networks at relatively lower filler loading. Submicron sized magnetic particles additives were reported to exhibit synergistic effect on the yield stress of MR fluid due to increased dipoledipole interaction between the particles forming strengthened particles chain structure under the externally applied magnetic field.57,58 Choi et al. showed that the addition of small amount (1 wt. % with respect to the carrier fluid) of carbonyl iron nanoparticles (particle size4 nm) enhanced the yield stress of 75 wt. % carbonyl iron microparticle (particle size-7 µm) based magnetorheological fluid by ~43 %.57 Hence, the nanoparticles filled the gap between the microparticles and easily formed strong interconnected three dimensional particle networks by acting as a bridge which otherwise was not possible by the micron sized particles. Landa et al.59 studied 2 wt. % ~10 nm Ni nanoparticles and 360 nm nanochains filled PDMS based magnetorheological elastomer and showed that even at low magnetic filler loadings one could obtain magnetorheological elastomer with larger anisotropic properties. Several other investigations with lower filler loading of small magnetic particles were also studied for magnetorheological elastomer say for instance 5-15 vol. % of 100 nm Ni particle in PDMS matrix by Denver et al.23, 20-80 mass % of 89 nm SrFe12O19 nanoparticle in nitrile rubber matrix by Tian et al.60, etc. Padalka et al.61 studied the stiffness and dynamic damping properties of the 10 wt. % Fe, Co, Ni nano wire filled silicone rubber


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under magnetic fields. Balasoiu et al.62 considered the micro and nano Fe3O4 filled stomaflex elastomer and showed that the average size of the crystallite block, quasi-crystalline phase concentration and ordering distance were increased when the polymerization was carried out under the magnetic field in case of doped elastomer. However, these trends were reversed when polymerization was carried out in absence of the magnetic field. Antonel et al.63 studied the

magnetic

and

elastic

properties

of

2-10

nm

CoFe2O4

nanoparticle

filled

polydimethylsiloxane (PDMS) composites at around 5-10 wt. % loading and obtained young modulus of 5 KgF/cm2 and 15-20 KgF/cm2, respectively for the 10 wt. % particle filled nanocomposite when measured parallel and perpendicular to the aligned nanoparticles needles but they did not measured the magnetorheological properties of these nanocomposites. There are only a limited number of studies with nano-magnetic materials/PDMS composites and their magneto-rheological characterization. Therefore, efforts have been taken to investigate the effect of small loading of magnetic nanoparticles on the rheological behaviour of the PDMS nanocomposite. It is apparent that in order to obtain a significant magnetorheological effect at low particle concentrations, the magnetic particles should have characteristics like large saturation magnetization, high permeability and high surface to volume ratio. Among the different bimetallic alloy nanoparticles, FeCo3 shows high saturation magnetization value64 and is therefore

a

promising

candidate

for

development

of

MREs.

In

this

study,

polydimethylsiloxane/FeCo3 elastomeric nanocomposites with different magnetic particle loading have been synthesized and investigated for their magnetorheological properties for the first time. The morphological, structural and magnetic properties of the nanoparticles and the nanocomposites have been evaluated by TEM, FESEM, XRD and VSM. The rheological behaviour of the nanocomposites under magnetic field in a parallel plate rheometer was also studied to correlate the magnetorheological properties with the microstructure and particle alignment.

2. Experimental 2.1 Materials FeSO4.7H2O (purity 99.5%), CoCl2.6H2O (purity 98 %) and Ethanol (purity 99.5%) were obtained from E. Merck. Hydrazine hydrate (99-100 %) was supplied by s.d.fine-chem pvt. Ltd., India and Sodium hydroxide (purity 98 %) from Ranbaxy Laboratories Ltd., India.



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Silicone Elastomer (Sylgard 184) was obtained from Dow Corning Corporation, USA and acetone (purity 99+ %) from Alfa Aesar, USA.

2.2 Synthesis of FeCo3 nanoparticles by hydrazine reduction method Initially 50 ml 0.3 M FeSO4.7H2O and 50 ml 0.9 M CoCl2.6H2O solutions were prepared in deionised water and mixed in a 500 ml beaker so that molar ratio of the Fe2+ and Co2+could be maintained in 1:3 ratio. Then 16 g of hydrazine hydrate solution was added drop wise in the above mixture for about 2 h and at pH 8 under continuous stirring and ultrasonication with the help of a magnetic stirrer and ultrasonic processor (Cole-Parmer India, Model no. GEX 750, Power 750 Watt, Frequency 20 kHz). Afterwards 15 ml of 12 N NaOH solution was added drop wise in the resultant solution with bluish black colour precipitate for another 2 h to raise the pH to 11 under continuous stirring and ultrasonication. Finally the solution was kept at 50 °C for 4 h to complete the reduction process. The nanoparticles were separated by applying a bar magnet and washed with the deionised water several times followed by ethanol and finally dried at 100 °C for 3 h to get FeCo3 nanoparticles.

2.3 Preparation of MRE Nanocomposites Sylgard 184 was used as the elastomer matrix. It is a two parts system, part A contains vinyl terminated silicone oligomer viz. dimethylvinyl-terminated dimethyl siloxane which acts as the base polymer. Part B, dimethyl methylhydrogen siloxane serves as curing agent. Pt based catalyst is present as the additional component in Part A. When these two parts are mixed together Pt catalyst aids in the hydrosilylation reaction and addition of Si-H bond of the part B take place across the double bond of the terminal vinyl group of part A forming three dimensional Si-CH2-CH2-Si linkage as shown below in scheme I.65


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Me R Part B Me Si O Si O Si Me m Me

Me

Me

m= ~10

Me

R is -Me, sometimes H H2 O C Si C Si Me Me H 2 Me O O

H Si

Me H2C

Si Me

Me

O

Me

Pt Catalyst

O

Si Me

O Me

n

CH2

+

Part A n= ~60

Me

Me

Si Me O

Me

Si Me O H

Me O

Si Me

Si Me O Si Me O

H2 C Si Me C O H2

Si Me O

Scheme I: Crosslinking reaction of Sylgard184. The magnetic nanocomposites with varied nanoparticles concentration of 5, 10 and 20 wt. % with respect to elastomer content are prepared. To this end, 0.4 g, 0.8 g and 1.6 g of the magnetic particles were weighed with an accuracy of ±0.1 mg into a 25 ml beaker and dispersed in 5 ml acetone for 15 minutes with an ultrasonicator for homogeneous dispersion without agglomeration. 8 g of part A was then added in the above dispersion and mixed thoroughly for another 15 minutes. During this mixing process under ultrasonication, the acetone evaporated and finally 0.8 g (10 % with respect to the part A) of part B (cross-linking agent) was added and the mixing was continued for another 2 minutes. The beaker was then kept under vacuum for 10 minutes to eliminate any air bubble generated by the cross linking process. Finally the composite mixture was divided into two parts and casted on two separate Petridis. For isotropic composites, the Petridis 1 is left for 48 h at room temperature. For anisotropic composites, the Petridis 2 is placed under the magnetic field of 0.2 Tesla for 16 minutes between the two poles of an electromagnet (Walker Scientific Inc., Worcester, USA, Model No. HV-5H). After aligning the magnetic particles, the magnetic field is turned off and

the Petridis is left for 48 h at room temperature to obtain the cured composite.

3. Characterization 3.1 X-ray Diffraction (XRD)



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The synthesized magnetic nanoparticles were characterized by XRD spectra in X’Pert PRO, PAN-analytical, Netherland, using Cu-Kα radiation at room temperature to obtain the crystal structure and phase component in the scan range from 30° to 100°. Angle 2θ was varied with scan step size of 0.0295431° per second during the analysis.

3.2 Vibrating Sample Magnetometer (VSM) Study VSM study was carried out at room temperature in GMW Magnet System (Model no. - 347270), San Carlos, USA. In this study both the magnetic nanoparticle and the nanocomposites were weighed in an electronic balance with accuracy ±0.05 mg and wrapped in a wax paper. The sample was mounted on the top of the vibrating probe kept between two magnetic poles and continuously vibrated mechanically during the analysis. The magnetic field was varied continuously and cycled from maximum negative field to zero and then increased to positive maximum and reversed again to complete the hysteresis loop. Maximum magnetic field applied for nanoparticles and nanocomposites was ~15 kOe and ~16.718 kOe, respectively. The temperature dependent characteristics of the magnetic moment at low magnetic field were measured in Zero Field Cooled (ZFC) and Field Cooled (FC) curves in the temperature range between 4 K to 300 K. The Physical Property Measurement System (PPMS; Model 6000, Quantum Design, USA, Software version-MultiVu) used for these measurements was equipped with the VSM and a superconducting coil capable of producing maximum magnetic field of 15 T. In ZFC experiment, the nanoparticles sample was cooled to 4 K under zero magnetic field. Afterwards a very low magnetic field of 20 Oe is applied on the sample and the magnetic moment is recorded while heating the sample to the maximum temperature of 300 K. The temperature ramp for the heating was 2 K/minute from 4 K to 50 K and 5 K/minute from 50 K to 300 K temperature. But in FC, the sample is cooled to 4 K under the applied magnetic field of 20 Oe and subsequently heated to 300 K with simultaneous recording of magnetic moment under similar temperature ramp as mentioned above.

3.3 Microscopic Analysis The morphological analysis of the magnetic nanoparticles was carried out in Field Emission Scanning Electron Microscope (FESEM, Quanta 200, Zeiss, Germany). The sample was first sputter coated with gold for FESEM studies. Optical images of the magnetic elastomer nanocomposites were taken using a Leica optical microscope at a depth of 1 µm to 2 µm from the surface of the sample to check the particle distribution within the composites. Confocal microscopic images were obtained from WI Tec focus innovations (Model-Alpha


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SNOM, Germany) to acquire further insights into the organization of nanoparticles in the elastomer matrix. Nanoparticles are also analyzed in Tecnai, USA Transmission Electron Microscope (TEM) (Model no.FEIG2 T2) to investigate the particle size and selected area electron diffraction pattern to acquire understanding of the crystal structure.

3.4 Rheological Studies The MR properties of the nanocomposites and virgin PDMS were investigated under variable magnetic field in a Rotational rheometer (Physica MCR 301, Anton Paar, Germany) using a magnetorheological device (MRD 180) equipped with magnetic field generating assembly. This setup can produce variable magnetic field perpendicular to the shear flow by controlling the current through the coil. The circular sample (thickness of film ~1 mm) was placed in the cavity of the base plate. The shear force was applied on the sample using a rotary disc of diameter 20 mm which was directly connected to the instrument by means of air bearing. Each sample was tested both in the steady-state rotary shear as well as oscillatory shear mode to investigate its static and dynamic rheological properties. For both the modes, the current was varied from 0 to 4 A with an increment of 1 A. The magnetic flux density was first calibrated against the current and the magnetic field applied on the sample during the testing was calculated for each value of current from the calibration curve. The magnetic flux density calculated corresponding to the 1 A, 2 A, 3 A and 4 A current are 0.345 T, 0.654 T, 0.914 T and 1.098 T, respectively.

3.5 Thermomechanical Analysis (TMA) Crosslink density of neat PDMS and nanocomposites was measured in a Thermo Mechanical Analyser (TMA 2940, TA Instrument, USA). A square film sample of dimension 5 mm × 5 mm × 1.7 mm was placed inside the instrument and 50 g load was applied on the sample by means of a spherical indenter of radius 0.089 cm. Change in dimension with respect to time was recorded at room temperature.

4. Results and Discussion 4.1 XRD characterization Figure 1 represents the XRD spectra of the synthesized magnetic nanoparticles. The spectra shows three peaks at 2θ value at 45, 65.66, 83.20 degree of which the peak at 2θ =45° is dominant. The positioning of this intense peak also matches with the database. All the peak



positions are matched with the Pearson’s Crystal Data (Entry prototype-W,cI2, 229) and can be accounted for (110), (200) and (211) planes as shown in Figure 1. Therefore, the above spectral analysis clearly confirms the presence of body centred cubic atom arrangement FeCo3 phase in the synthesized magnetic nanoparticles. Cell parameters calculated based on (110) plane appears to be a=2.846 A° which is similar to the lattice parameter (a=2.842 A°) reported in the database. The average crystallite size (D) of the nanoparticles is evaluated from the highest intensity peak using the Debye-Scherrer equation D=kλ/βcosθ, where k the Scherrer constant (0.9), λ the wavelength of the used X-ray radiation in nm, β the full angular line width in radians at half maximum intensity, θ the diffraction angle in degree. The calculated average crystallite size of the nanoparticles using this equation appears to be 31.22

(110)

nm.

1700 1600 1500 1400 1300

1100 1000 900 30

(211)

1200 (200)

Intensity (a.u)

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40

50

60

70

80

90

100

2 θ (Degree)

Figure 1: X-ray diffraction spectra of magnetic nanoparticles.

4.2 VSM measurements Figure 2(a) represents the specific magnetization curve (M vs. H curve) of FeCo3 nanoparticles at room temperature. The FeCo3 nanoparticles show a very small hysteresis loop with saturation magnetization (Ms) value as high as 166.3 emu/g and hysteresis ferromagnetism in the field range ±3000 Oe. Figure 2(b) is the zoom of the M vs. H plot across the origin, which exhibits the remnant magnetization and coercive field more clearly visible in field ranges between +500 to -500 Oe. From the figure, the coercive field (Hc) and remnant magnetization (Mr) appear to be 85.886 Oe and 5.875 emu/g,


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respectively. Therefore, synthesized FeCo3 nanoparticles possess very low remnant magnetization and coercive force characteristic of soft ferromagnetic behaviour. Figure 2 (c) reveals that the ZFC curve exhibits progressively increasing magnetic moment with the increase in temperature without showing any transition in the temperature range between 4 K to 300 K. This rise is attributed to the increasingly aligned magnetic spin to yield a higher magnetic moment under the small applied field. In contrast FC curves show a slow decrease in the magnetic moment from 4 K to 114.64 K temperature followed by a sharp reduction at 115.83 K. Beyond this temperature, the magnetic moment again exhibits a slow rise and at 300 K it becomes almost similar to that of the magnetic moment obtained at 4 K. In field cooling, magnetic moments are partly aligned under the influence of small magnetic field which is applied during the cooling process. When the temperature of the nanoparticle is increased, the aligned magnetic spin of the nanoparticles becomes disordered due to flipping of magnetic spin originating from the thermal activation. Consequently, magnetic moment of the nanoparticles decreases gradually. From the figure, we can observe that after 114.64 K, the magnetic spin flipping due to thermal activation becomes predominant leading to the sharp decline in the magnetic moment with a transition at 115.83 K. Further slow increase in the magnetic moment beyond this temperature is not well understood and may be because of the transient alignment of some magnetic spin with the magnetic field. 2(a) 200

150

Magnetization (emu/g)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 50 0 -50 -100 -150 -200

-15 -10 -5 0 5 10 15 3 Applied Magnetic Field (Oe) × 10

2(b) 50 40 30 20 10 0 -10 -20 -30 -40 -50 -500-375-250-125 0 125 250 375 500 Applied Magnetic Field (Oe)



2(c) 115.83 K

0.64 FC Moment (emu/g)

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0.60 0.56 0.52 ZFC 0.48 0.44

0

50

100

150

200

250

300

Temperature (K)

Figure 2: (a) M vs. H curve in the applied field range -15 kOe to +15 kOe (The hysteresis curve contains two closely spaced line which are not visible in this figure); (b) Higher resolution M vs. H curve in the applied field range -500 Oe to +500 Oe, clearly showings the coercivity and remnant magnetization value; (c) M vs. T curve of FeCo3 nanoparticles. The PDMS-FeCo3 nanocomposite samples (rectangular shape) are also characterized by VSM. In this study M versus H curve (i.e. hysteresis loop) of all anisotropic samples are recorded by applying the field at 0° (i.e. parallel to the particle chain alignment) and 90° (i.e. perpendicular to the particle chain). All isotropic samples are also investigated in a similar way by applying the field at 0° (i.e. along the length) and 90° (i.e. along the width) to compare the result between both the samples. These conventions are same throughout the text unless otherwise stated. Figure 3(a), (c) and (e) represent the hysteresis curve of the 5 wt. %, 10 wt. % and 20 wt. % isotropic PDMS-FeCo3 nanocomposites. While Figure 3(b), (d) and (f) show the hysteresis curves of the 5 wt. %, 10 wt. % and 20 wt. % anisotropic PDMSFeCo3 nanocomposites, respectively. All of these nanocomposites show soft ferromagnetic behaviour. The saturation magnetization (Ms) and remnant magnetization (Mr) values of the nanocomposites are observed to increase monotonically with the increase in particle concentration. This can be accounted for by the reduced inter-particle distance and antiferromagnetic layer of PDMS on the surface of the FeCo3 particles with increasing particle loading.


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10

M1

5

0° 90°

0 -5 -10 -15

3(b)


10

M2

5

M1 0° 90°

0 -5 -10

-20 -15 -10 -5 0 5 10 15 20 Applied Magnetic Field (Oe) × 103 3(d) 20 M2



3(a) 10 8 M2 6 4 M1 2 0° 0 90° -2 -4 -6 -8 -10 -20 -15 -10 -5 0 5 10 15 20 3 Applied Magnetic Field (Oe) × 10 3(c) 20 15 M2

-20 -20 -15 -10 -5 0 5 10 15 20 Applied Magnetic Field (Oe) × 103

M1

10

0° 90°

0 -10 -20

-20 -15 -10 -5 0 5 10 15 20 3 Applied Magnetic Field (Oe) × 10

3(e)

3(f)

30 M2

30 M2

20 10 0



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M1 0° 90°

-10 -20 -30 -20 -15 -10 -5 0 5 10 15 20 3 Applied Magnetic Field (Oe) × 10

20 10 0

M1 0° 90°

-10 -20 -30 -20 -15 -10 -5 0 5 10 15 20 3 Applied Magnetic Field (Oe) × 10

Figure 3: M vs. H curve: (a) 5 wt. % Isotropic, (b) 5 wt. % Anisotropic, (c) 10 wt. % Isotropic, (d) 10 wt. % Anisotropic, (e) 20 wt. % Isotropic, and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites (Note: All the hysteresis curves contain two closely spaced lines not visible at this scale).

It has been observed from Table 1 that the saturation magnetization and remnant magnetization of the nanocomposites measured parallel (i.e. parallel to the particle chain



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alignment in anisotropic nanocomposites and along the length for isotropic nanocomposites) are higher compared to the corresponding perpendicularly (i.e. perpendicular to the particle chain in anisotropic nanocomposites and along the width for isotropic nanocomposites) measured values for both isotropic and anisotropic nanocomposites. This difference arises due to the different directional exposure of the rectangular composite samples during the VSM study. Interaction of magnetic field with the sample is always more during 0° measurement than the 90° measurement irrespective of the nature of the composite sample as larger number of magnetic flux lines are encountered along the length than the width of sample. However, for preferential alignment of the magnetic particles in polymer matrix in case of the anisotropic sample, the relative difference of the saturation magnetization values between two orientations (i.e. 0° and 90°) will be more than the isotropic sample. This additional increment can be attributed to the columnar particle chain formation in the anisotropic nanocomposites during the fabrication under magnetic field. This clearly indicates the magnetic anisotropy of the composites. Further, the difference between the saturation magnetization between the two measurements (i.e. 0° and 90°) for isotropic and anisotropic nanocomposites approaches close to each other with increased particle concentration of 20 wt. % in the nanocomposites. This is because of the transformation of magnetic anisotropy to the isotropic behaviour at higher magnetic particle loading. Similar observation has also been reported earlier.66 The coercivity (Hc) value of the each nanocomposite is observed to be higher for 90° than the 0° measurement. All the anisotropic nanocomposites exhibited lower coercivity than their corresponding isotropic counterpart during 0° measurement but opposite results are obtained in 90° measurement. Furthermore increase in nanoparticle loading from 5 wt. % to 20 wt. % have no significant effect on the coercivity values of the nanocomposites as the nature of the magnetic particles remains unaltered in all the samples. Table 1: VSM results of the FeCo3 -PDMS nanocomposites Sample Name

0°

90°

0°

90°

0°

90°

Difference between the Ms values (emu/g)

0.17± 0.006

0.13± 0.004

157.5± 0.20

157.7± 0.24

8.0± 0.08

6.5± 0.06

1.5± 0.02

5 wt. % anisotropic FeCo3- 0.32±

0.12±

151.4±

160.4±

10.4±

7.0±

3.4±

5 wt. % isotropic FeCo3PDMS nanocomposite

Mr (emu/g)

Hc (Oe)

Ms (emu/g)


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PDMS nanocomposite

0.01

0.006

0.18

0.26

0.10

0.07

0.03

0.31± 0.012

0.24± 0.007

156.4± 0.22

157.0± 0.24

15.2± 0.11

11.7± 0.10

3.5± 0.01

10 wt. % anisotropic FeCo3- 0.67± PDMS nanocomposite 0.12

0.27± 0.008

148.2± 0.18

162.0± 0.30

20.1± 0.12

16.1± 0.10

4.0± 0.02

0.62± 0.10

0.45± 0.014

156.7± 0.24

157.7± 0.26

28.6± 0.16

22.2± 0.13

6.4± 0.03


0.35± 0.012

148.7± 0.18

161.0± 0.28

27.4± 0.14

20.5± 0.12

6.9± 0.02



The anisotropic coefficient of the nanocomposite is expressed by the ratio of the magnetization values measured parallel (M2) and perpendicular (M1) to the particle chain in the anisotropic nanocomposites and along the length and width, respectively for isotropic nanocomposites. From Table 2 it has been observed that anisotropy coefficient decreases from 1.68 for 5 wt. % FeCo3 to 1.54 for 20 wt. % FeCo3 filled anisotropic nanocomposite. This decrement in anisotropy coefficient is attributed to the gradual transformation of the anisotropic to the isotropic behaviour at higher magnetic particle loading. The isotropic nanocomposites have shown very close anisotropic coefficient values indicating similar nature of particle distribution within the nanocomposites at all particle concentration. Table 2: Anisotropy coefficient of the FeCo3 -PDMS nanocomposites Sample Name

Value of magnetization (M1) in emu/g at 103 Oe magnetic field

Value of magnetization (M2) in emu/g at 103 Oe magnetic field

Anisotropic coefficient (M2/M1)


5.67±0.07

7.2±0.08

1.27±0.002

5 wt. % anisotropic FeCo3PDMS nanocomposite

5.91±0.08

9.91±0.10

1.68±0.010


10.38±0.08

13.67±0.09

1.317±0.002


13.38±0.11

19.24±0.13

1.438±0.002


19.25±0.12

25.56±0.14

1.327±0.001


17.13±0.14

26.38±0.16

1.54±0.003



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4.3 Microscopic analysis Figure 4(a), (b) show the FE-SEM images of bulk powder sample (magnetic nanoparticles) at 7 KX and 175 KX magnifications, respectively. The magnetic nanoparticles in powder form at lower magnification (7 KX) exhibit agglomeration. The image 4(c) shows that the primary nanoparticles in the size range of 22 to 37 nm form agglomerates. Figure 5 (a) is the TEM image of the FeCo3 nanoparticles showing the existence of two types of microstructures, viz., Fe-rich grain (X) with bcc phase structure and Co-rich grain (Y) with fcc phase structure. Due to the co-existence of these two phase structures, the selected area electron diffraction pattern taken over the large area exhibits the mixed diffraction pattern as shown in Figure 5(c). It consists of both the ring diffraction pattern and ordered diffraction spots. The ring patterned diffraction reveals the presence of randomly oriented nano polycrystals with bcc phase structure as the dominant constituent. The most intense ring comes due to the diffraction at (110). Middle and outer rings originated because of diffraction at (200) and (211), respectively.

Figure 4: FE-SEM images of (a) FeCo3 nanoparticles at 7 KX magnification, (b) FeCo3 nanoparticles at 175 KX magnification, (c) FeCo3 nanoparticles at 137 KX magnification showing particle size.


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Figure 5 (b) is the high magnification TEM images of the same FeCo3 nanoparticles. It reveals that the size of the nanoparticles mostly falls within the 20 to 30 nm range. This result has also good agreement with the FESEM results and average crystallite size calculated by the Debye-Scherrer equation (i.e. 31.22 nm). Figure 5 (d) exhibits the selected area electron diffraction pattern of the nanoparticles with Co-rich fcc phase structure. Similar type of bcc and fcc microstructure has been reported in case of the Co0.73Fe0.27 alloy thin film produced by co-sputtering-spread approach.67

Figure 5: (a), (b) TEM images of FeCo3 nanoparticles at two different magnification, (c) selected area electron diffraction pattern of FeCo3 nanoparticles taken over large area showing both bcc and fcc phase structures, (d) [011] electron diffraction pattern of FeCo3 particles showing the fcc phase structure.

Figure 6 (a), (b) and (c) represent the digital pictures of 5 wt. %, 10 wt. %, and 20 wt. % isotropic FeCo3-PDMS nanocomposite specimens. While Figure 6 (d), (e), and (f) are the digital pictures of 5 wt. %, 10 wt. %, and 20 wt. % anisotropic nanocomposite specimens of



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the same. The direction of parallel aligned particle chains due to externally applied magnetic field is shown by the arrows in case of anisotropic nanocomposite samples. From the picture we can observe that the concentration of magnetically aligned particles chains in the anisotropic specimen increases gradually from 5 wt. % to the 10 wt. % FeCo3 filled nanocomposites. The particle lines become more densely packed in the 20 wt. % FeCo3 filled nanocomposite.

Figure 6: Digital picture of (a) 5 wt. % Isotropic, (b) 10 wt. % Isotropic, (c) 20 wt. % Isotropic, (d) 5 wt. % Anisotropic, (e) 10 wt. % Anisotropic and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites. The confocal microscopic images of the nanocomposites containing 5 wt. %, 10 wt. %, and 20 wt. % of FeCo3 in isotropic and corresponding anisotropic conformation in PDMS have been shown in Figure 7(a), (b), (c) and Figure 7 (d), (e), and (f), respectively. The images were recorded at a depth of 10 µm to 50 µm from the surface of the sample. The arrows show the direction of particle chain formation under the magnetic field in case of anisotropic samples. The magnetic particles are found to be homogeneously distributed throughout the matrix at 10 wt. %, and 20 wt. % of FeCo3 in isotropic nanocomposites but not in the case of 5 wt. % FeCo3 filled sample. The anisotropic nanocomposites, however, have shown clear evidence of columnar chain formation between the particles. The chains are discontinuous in 5 wt. % FeCo3 filled nanocomposite due to insufficient particle concentration (Figure 7d).


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Some particles lie separately within the matrix without being a part of the chain structure. A similar finding has also been reported in the literature for another micro-composite.68

Even in a carefully prepared MRE, the isolated particles, their random clusters and the aligned chains of particles coexist within the elastomer matrix. Agglomeration of nanoparticles is encouraged both by inter particle attractive forces and by the externally applied magnetic field which induces dipole magnetic moments. Interestingly, it is evident that the average size of these agglomerates in 5 wt. % FeCo3 is larger that of other nanocomposites containing 10 and 20 wt. % of FeCo3. This may be due to the lower viscosity of the polymer nanocomposites at 5 wt. % particle content which in turn favors the particle agglomeration by the inter particle attraction and thereby lead to poor dispersion of the particles within the nanocomposite. A similar phenomenon is observed in the preparation of polymer blends where 5-10 wt. % of dispersed polymer in the host polymer matrix often leads to the heterogeneous morphology, but a homogeneous morphology results at 20-30 wt. %.

Figure 7: Confocal Microscope images of (a) 5 wt. % Isotropic, (b) 10 wt. % Isotropic, (c) 20 wt. % Isotropic, (d) 5 wt. % Anisotropic, (e) 10 wt. % Anisotropic and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites (Scale bar is 10 µm). Figures 8 (a), (b) and (c) exhibit the optical microscope images of 5 wt. %, 10 wt. %, and 20 wt. % isotropic FeCo3-PDMS nanocomposites at 50X magnification and Figures 8 (d), (e), and (f) represent the corresponding anisotropic nanocomposites. The arrows indicate the direction of particle chains formed during the fabrication of the anisotropic nanocomposites



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under the magnetic field. From these images it is observed that the magnetic particles are distributed homogeneously throughout the polymer matrix in case of isotropic nanocomposites. It is also noteworthy that the density of the distributed particle rises significantly with increased filler loading. The gap between the particle chains become smaller with increased particle loading from 5 wt. % to 20 wt. %.

Figure 8: Optical microscope images of (a) 5 wt. % Isotropic, (b) 10 wt. % Isotropic, (c) 20 wt. % Isotropic, (d) 5 wt. % Anisotropic, (e) 10 wt. % Anisotropic and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites.

4.4 Magneto-Rheological Analysis 4.4.1 Static test Steady state shear stress versus strain curves are obtained by subjecting the sample under rotary shear test with variable magnetic field at room temperature. Shear modulus of the sample at different magnetic field is obtained from the slope of the linear viscoelastic region of the corresponding curve. Figure 9 (a) to (f) represent the stress versus strain curves of the FeCo3-PDMS nanocomposites at five different magnetic field ranging from 0 to 1.098 T. Figure 9 (g) depicts the same for virgin PDMS at zero magnetic field. For Virgin PDMS stress vs. strain curves are not recorded at variable magnetic field as it does not possess any magnetic properties.


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(a)

(b)

5

5

1.4x10

1.4x10

5

5

1.2x10

Shear Stress (Pa)

Shear Stress (Pa)

1.2x10

5

1.0x10

4

8.0x10

4

0T 0.345 T 0.654 T 0.914 T 1.098 T

6.0x10

4

4.0x10

4

2.0x10

0.0 0

50

100

150

5

1.0x10

4

8.0x10

0T 0.345 T 0.654 T 0.914 T 1.098 T

4

6.0x10

4

4.0x10

4

2.0x10

0.0

200

0

50

Strain (%)

(c)

5

5

4

1.0x10 Shear stress (Pa)

Shear Stress (Pa)

200

1.2x10

8.0x10

4

6.0x10

0T 0.345 T 0.654 T 0.914 T 1.098 T

4

4.0x10

4

2.0x10

0.0

4

8.0x10

0T 0.345 T 0.654 T 0.914 T 1.098 T

4

6.0x10

4

4.0x10

4

2.0x10

0.0

0

5

100 150 Strain (%)

(d)

5

1.0x10

50

100 150 Strain (%)

200

0

(e)

50

100 150 Strain (%)

200

(f)

1.2x10

4

5

4

8.0x10

4

0T 0.345 T 0.654 T 0.914 T 1.098 T

6.0x10

4

4.0x10

4

2.0x10

0.0 0

50

100 150 Strain (%)

200

Shear stress (Pa)

1.0x10

Shear stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9x10 4 8x10 4 7x10 4 6x10 4 5x10 4 4x10 4 3x10 4 2x10 4 1x10 0

0T 0.345 T 0.654 T 0.914 T 1.098 T 0

50


100 150 Strain (%)

200


(g) 4

8x10

4

7x10 Shear stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4

6x10

4

5x10

4

4x10

4

3x10

4

2x10

4

1x10 0

0

50

100 150 Strain (%)

200

Figure 9: Static test: (a) 5 wt. % Isotropic, (b) 5 wt. % Anisotropic, (c) 10 wt. % Isotropic, (d) 10 wt. % Anisotropic, (e) 20 wt. % Isotropic, and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites; (g) Virgin PDMS.

All the nanocomposites show an appreciable amount of MR effect which is reflected in the shear modulus enhancement with increasing magnetic field. The increment in the shear modulus is more when the magnetic field is increased from 0 T to 0.345 T, which is evidenced by a steep rise in the slope of the stress-strain curves. Further increment in the magnetic field from 0.345 T to 0.654 T causes relatively small increment in the shear modulus and finally beyond 0.914 T the curves almost level off as the saturation magnetization value of the magnetic particle is reached. As can be seen from the figures [compare Figure 9 (a) & (b), 9(c) to (f)], the stiffness enhancement of the 5 wt. % isotropic FeCo3-PDMS nanocomposite under 1.098T magnetic field is higher than the other nanocomposites which can be attributed to the extent of agglomeration of the magnetic nanoparticles{Figure 7(a)}. Static friction between sample and upper plate surface, sample roughness and normal stress also contribute to the ultimate resultant stress of the sample. Due to these factors sometime overshoot may occur as seen in the Figure 9 (b), (c), (e) and (f). It can also be seen from the figures that shear stress of each nanocomposite exhibits linear dependence with the strain % within some specified strain limit, called linear viscoelastic region.


Page 22 of 48

Page 23 of 48

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Table 3: Linear range of the stress-strain response at various applied magnetic field Sample Name

Strain (%) 0T

0.345T

0.645T

0.914T

1.098T

21.74± 1.80

42.33± 2.20

42.33± 2.25

50.77± 2.40

50.77± 2.60


52.59± 2.48

52.62± 2.53

52.61± 2.62

52.60± 2.65

31.58± 2.00

42.10± 2.36

42.10± 2.42

52.62± 2.50

52.64± 2.68


63.15± 2.50

63.15± 2.54

73.67± 2.66

84.19± 2.70

78.96± 2.50

126.2± 2.80

126.2± 2.92

156.2± 3.20

157.1± 3.46


94.71± 2.72

94.71± 2.83

94.71± 2.89

110.5± 2.94

-

-

-

-




Virgin PDMS

≥ 200

Table 3 represents the linear range of all the nanocomposites at different magnetic field in terms of the strain percentage. Virgin PDMS shows maximum linear range (≥ 200 %) than all the nanocomposites. Moreover, linear range of the nanocomposites show an increasing trend with enhancement in applied magnetic field due to the strain induced crystallinity as well as magnetic field induced stiffness enhancement. All the anisotropic nanocomposites exhibited greater linear range than their corresponding isotropic counterpart except 20 wt. % FeCo3 filled composites because of oriented chain structure. Figure 10 (a), (b) represent the shear modulus (at 42 % shear strain) vs. FeCo3 loading plots of the isotropic nanocomposites and virgin PDMS, anisotropic nanocomposites, respectively. The 42 % strain value is chosen for comparison of shear modulus since most of the nanocomposites (except 5 wt. % isotropic FeCo3-PDMS nanocomposite at 0 T magnetic field, for which linear range exist up to 21.74 %) exhibits linear stress-strain behaviour at this particular shear strain as can be seen from Table 3. Figure 10 (c), (d) shows the maximum shear stress achieved under the variable magnetic field for isotropic nanocomposites and virgin PDMS, anisotropic nanocomposites, respectively as a function of FeCo3 loading in the PDMS matrix. The error in shear modulus and maximum stress values are within ±5 %. It can be seen from the figures that in general both modulus and maximum shear stress show decreasing trend with the increasing filler



loading. The highest enhancement in shear modulus and maximum stress are obtained with 5 wt. % isotropic FeCo3-PDMS nanocomposites. Moreover, all the nanocomposites show enhanced shear modulus and maximum shear stress value compared to the virgin PDMS at zero as well as other experimentally applied magnetic fields.

(b) 200

Modulus (kPa)

240

Virgin PDMS 0T 0.345T 0.645T 0.914T 1.098T

210 180 150 120 90

Modulus (kPa)

(a) 270

0

5 10 15 FeCo3 loading (wt.%)

120

(c) 130

Virgin PDMS 0T 0.345T 0.654T 0.914T 1.098T

120 110 100 90 80 70 0

5 10 15 20 FeCo3 loading (wt.%)

0

20


20

(d) 130 Maximum Stress (kPa)

30

0T 0.345T 0.654T 0.914T 1.098T

160

80

60

Maximum Stress (kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 48

0T 0.345T 0.654T 0.914T 1.098T

120 110 100 90 80 70

0


20

Figure 10: Modulus (at 42 % shear strain) vs. FeCo3 loading plots (a) Isotropic PDMS-FeCo3 nanocomposites and virgin PDMS (b) Anisotropic PDMS-FeCo3 nanocomposites; Maximum Shear Stress vs. FeCo3 loading plots (c) Isotropic PDMS-FeCo3 nanocomposites and virgin PDMS (d) Anisotropic PDMS-FeCo3 nanocomposites.

4.4.2 Dynamic test The FeCo3-PDMS nanocomposites are characterized for their dynamic mechanical behaviour by performing both strain amplitude sweep test and angular frequency sweep test at room temperature. The strain amplitude sweep test are carried out under five different magnetic field by supplying different input current (viz. 0 A, 1 A, 2 A, 3 A, 4 A) through the magnetic coil at constant angular frequency of 1 Hz. The storage and loss moduli are recorded by varying the strain amplitude from 0.01 % to 100 %. Similarly in case of angular frequency


Page 25 of 48

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sweep test, all the nanocomposites are subjected to five aforesaid magnetic fields at constant strain amplitude of 1 %. The storage and loss moduli are recorded in the frequency range of 1 to 200 rad/s.

4.4.2.1

Strain amplitude sweep test

Figure 11 (a) to (f) show the log-log plot of the storage and loss moduli versus strain amplitude of the nanocomposites at ambient temperature. Whereas Figure11 (g) shows the same for virgin PDMS at zero magnetic field. From the above figures it can be observed that the changes in storage and loss moduli are relatively less for virgin PDMS than the other nanocomposites. Moreover, both the initial zero field storage and loss moduli are higher for isotropic nanocomposites than the corresponding anisotropic nanocomposites except for 20 wt. % FeCo3. This can be attributed to the better particle and matrix interaction in isotropic nanocomposites which make them more resistant to deformation than the corresponding anisotropic nanocomposites under dynamic condition. It is also noteworthy that under increasing magnetic field the nanocomposites have shown increased storage and loss moduli at 0.01 stain %. This indicates obvious MR effect in all the nanocomposites. The storage modulus exhibits a slow decline up to around 10 % shear strain, except for 5 wt. % isotropic FeCo3-PDMS nanocomposite where this region extends only to about 1 % strain. The storage modulus starts to fall rapidly beyond this region. The severe drop in the storage modulus value beyond 1 % critical strain amplitude is an indication of the transition of MRE from elastic to nonlinear viscoelastic material. Similar type of transition from linear range to nonlinear range has been also reported in MR fluid.69 This observation can be explained by considering the origin of the magnetorheological effect. As we know that the MR effect is governed by two parallel phenomena. MRE exhibits a net magnetic moment and remains in the minimum dipolar energy state by creating a chain like columnar structure during the fabrication. When MRE is subjected to the magnetic field the magnetic particles experience a dipole magnetic moment and hence magnetic field tries to orient the particle along its own direction and causes displacement in the particles with respect to their original position without external magnetic field. This displacement or movement under magnetic field is opposed by the magnetic particles since the initial low dipolar energy state is disturbed by their movement. Therefore modulus of the MRE is increased under magnetic field. In the magnetic field each magnetic particle exists in a new equilibrium state by the dual action of dipole-dipole interaction forces and internal stresses of the polymer. Moreover, the composite sample as a whole remains in one of the local minimum energy state of the system. Each time



magnetic field is changed particles assumes new equilibrium state. Hence, because of dipoledipole interaction between the magnetic particles composite sample uphold newly formed state in the external magnetic field and exist in a new local minimum energy state of the system. When the magnetic field is turned off the magnetic particles returned back to their original positions since they are fixed in space within the polymer matrix by means of chemical crosslinking. The driving force behind this phenomenon is the elastic forces exerted by the polymer matrix as well as elimination of dipole-dipole interaction forces between the particles.70 7

(b)

(a)

5

4x10

10

5

G'' (Pa)

10

4

10

0.01

0.1

1 Strain (%)

10

G' (Pa)

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

6

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

5

3x10

5

2x10 4 4.5x10 4 3x10

G'' (Pa)

G' (Pa)

10

4

1.5x10

0.01

100

(c)

1 Strain (%)

10

100

(d)

10

5

G'' (Pa)

0.1

1 Strain (%)

10

G' (Pa)

5

10

4

10

10 0.01

100

0.1

1

10

100

Strain (%) (f)

(e)

5

5

1.2x10

4

4

10

3

5x10

0.1

1 Strain (%)

10

100

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098T 1.098 T

5

10

4

2x10

G'' (Pa)

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

G' (Pa)

2x10

6x10

0.01

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098T 1.098 T

3

4

10 0.01

G'' (Pa)

G' (Pa)

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098T 1.098 T

10

G' (Pa)

0.1

6

6

10

G'' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 48

4

10

0.01

0.1


1 Strain (%)

10

100

Page 27 of 48

G' (Pa)

(g) 5 10

G' G"

4

10

G'' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

10 0.01

0.1

1 Strain (%)

10

100

Figure 11: Strain sweep test: (a) 5 wt. % Isotropic, (b) 5 wt. % Anisotropic, (c) 10 wt. % Isotropic, (d) 10 wt. % Anisotropic, (e) 20 wt. % Isotropic, and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites; (g) Virgin PDMS. Another important source of MR effect is that under the magnetic field magnetic particles come closer by the magnetic dipolar interaction and this induces deformation in the neighbouring elastomer matrix and consequently the stiffness of the materials is increased significantly. The value of the magnetic dipole moment depends on the size as well as the distance between the magnetic particles. The distance between the magnetic particles is also increased with increase in the strain. The above factors engender a rapid fall in the storage modulus after a critical strain. This type of decrease in storage modulus with the rise in strain % has also been reported by the Li et al.27,52 for carbonyl iron filled PDMS microcomposites. The loss modulus in contrast follows a decreasing trend up to around 10 % strain amplitude for all nanocomposites except 5 wt. % isotropic FeCo3-PDMS for which this region extends up to about 1% shear strain. After this region, the loss modulus shows a sharp rise in magnitude. This illustrates that the tanδ value, which is the measure of the damping properties of the nanocomposite, in general decreases initially and then increases rapidly with the increment in strain value as shown in Figure 12 (a) to (g).



(b) 0.35

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0.30

0.15 0.10

20

40 60 Strain (%)

80

0.00

100

(c) 0.6

(d) 0.56

0.5

0.48

0

20

40 60 Strain (%)

80

100

0.40

0.4

0T 0.345T 0.654T 0.914T 1.098T

0.3 0.2 0.1 0.0

0T 0.345T 0.654T 0.914T 1.098T

0.20

0.05

0

Tanδ

0.25 Tanδ

0T C1 E1 0.345T G1 0.654T I1 0.914T B 1.098T

Tanδ

Tanδ

(a)

0T 0.345T 0.654T 0.914T 1.098T

0.32 0.24 0.16 0.08

0

20

40 60 Strain (%)

80

100

0

20

40 60 Strain (%)

80

100

(f) 0.40

(e) 0.28

0.35

0.24 0T 0.345T 0.654T 0.914T 1.098T

0.16 0.12 0.08 0.04

Tanδ

0.30 0.20 Tanδ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 48

0T 0.345T 0.654T 0.914T 1.098T

0.25 0.20 0.15 0.10 0.05

0

20

40 60 Strain (%)

80

100

0.00

0

20


40 60 Strain (%)

80

100

Page 29 of 48

(g) 0.13

0.12 0.11 0.10 Tanδ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.09 0.08 0.07 0.06 0.05

0

20

40 60 Strain (%)

80

100

Figure 12: Tanδ versus strain plots: (a) 5 wt. % Isotropic, (b) 5 wt. % Anisotropic, (c) 10 wt. % Isotropic, (d) 10 wt. % Anisotropic, (e) 20 wt. % Isotropic, and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites; (g) Virgin PDMS. Figure 13 (a) and (b) show the magnetic field dependent behaviour of storage and loss moduli with filler loading at 0.1 % shear strain for isotropic and virgin PDMS, anisotropic nanocomposites, respectively. All the nanocomposites show increased storage and loss moduli than the virgin PDMS at zero as well as other applied magnetic fields. FeCo3-PDMS nanocomposites with 5, 10 and 20 wt. % FeCo3 have shown storage modulus value of 334.2, 204.4 and 94.11 kPa, respectively at zero magnetic field compared to the 47.75 kPa value of virgin PDMS. The storage modulus first increases from 0 to 5 wt.% filler loading and then decreases gradually as the particle content is increased further to 20 wt.%. The initial increase in the storage modulus at lower loadings is due to the incorporation of hard magnetic particles into the PDMS matrix. The decline in storage modulus at higher filler loading could be attributed to the retardation effect of FeCo3 magnetic filler on the cross linking reaction of PDMS. Crosslinking density of different samples measured by the thermomechanical analyser (see section “4.5 Thermomechanical Analysis” for results) also firmly supports the experimental observation. Similar type of retardation behaviour of Fe3O4•Fe and γ-Fe2O3 magnetic powder on the curing of PDMS elastomer were reported by the Zrinyi et al.70,71 According to the report only 10 wt. % curing agent could be utilized from 20 wt. % curing agent for Fe3O4•Fe magnetic powder based PDMS composite. The lower crosslinking density results in lowering of the modulus. Moreover, increase in particle loading may lead to the agglomeration of nanoparticles and thus storage modulus of the nanocomposites is decreased gradually from 5 to 20 wt. % FeCo3 loading. The increment in the storage modulus under the variable magnetic field is observed to be the highest for 5 wt. % isotropic PDMS-FeCo3



nanocomposite compared to the other nanocomposites. This result is also supported by the highest absolute MR effect of the same. Inset of the Figure13 (a) is the zoom of the storage modulus vs. FeCo3 loading plot in the modulus range 60 kPa to 360 kPa, which clearly shows the change in storage modulus at 10 and 20 wt. % filler loading. On contrary the loss modulus first increases from 0 to 10 wt. % filler loading and then declines sharply at 20 wt. % for both isotropic and anisotropic nanocomposites.

G' (kPa)

6000 4500 3000 1500

(b)

240 180 120 60

5 10 15 20 FeCo 3 loading (wt.%)

G'' (kPa)

60 40 20 0 0


Virgin PDMS Virgin PDMS 0T 0T 0.345T 0.345T 0.654T 0.654T 0.914T 0.914T 1.098T 1.098T

G' (kPa)

(a)

300

300 0T 0T 0.345T 0.345T 0.654T 0.654T 0.914T 0.914T 1.098T 1.098T

250 200 150

G'' (kPa)

G ' (kPa)

360

100 50 0

20

5

10 15 FeCo3 loading (wt.%)

20

Figure 13: Storage Modulus (G') and Loss Modulus (G'') (at 0.1 % shear strain) vs. FeCo3 loading plots (a) Isotropic PDMS-FeCo3 nanocomposites and virgin PDMS (b) Anisotropic PDMS-FeCo3 nanocomposites

4.4.2.2

Angular frequency sweep test

Figure 14 (a) to (f) exhibit the log-log plot of storage and loss moduli versus angular frequency of the FeCo3-PDMS nanocomposites at room temperature. While Figure14 (g) shows the same as that of virgin PDMS at zero magnetic field. (b)

(a)

G' (Pa)

10

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

6

10

5

10

4

10

1

10 100 Angular Frequency (rad/s)

G' (Pa)

6

10

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

5

10

4

10 G" (Pa)

7

G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 48

3

10

1



Page 31 of 48

(c)

(d)

6

10

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

5

G'' (Pa)

10

4

10

3

10

1

10

G' (Pa)

10

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

5

10

4

10

G'' (Pa)

G' (Pa)

6

3

10

100

1


Angular Frequency (rad/s)

(f)

(e)

6

6

5

10

4

10

3

10

4

10

3

10 1

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

5

10

G'' (Pa)

G' (Pa)

0T 0T 0.345 T 0.345 T 0.654 T 0.654 T 0.914 T 0.914 T 1.098 T 1.098 T

G' (Pa)

10

10

G'' (Pa)


1

10

100

Angular Frequency (rad/s)

(g) 5

G' (Pa)

10

G' G"

4

10 G'' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

10

1

10 Strain (%)

100

Figure 14: Frequency sweep test: (a) 5 wt. % Isotropic, (b) 5 wt. % Anisotropic, (c) 10 wt. % Isotropic, (d) 10 wt. % Anisotropic, (e) 20 wt. % Isotropic, and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites; (g) Virgin PDMS. From these figures it is evident that similar to strain amplitude sweep test, all the isotropic nanocomposites have greater initial zero field storage and loss moduli than the corresponding anisotropic nanocomposites except for 20 wt. % FeCo3-PDMS nanocomposites. This



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 48

difference in magnitudes of the storage and loss moduli in isotropic and anisotropic nanocomposites originates mainly from the interaction of the magnetic nanoparticles with the polymer matrix and distribution of the nanoparticles within the nanocomposites. Both storage and loss moduli of the isotropic nanocomposites have shown increasing trend when the samples are subjected to the increasing magnetic field, whereas for the anisotropic nanocomposites, only slight changes are observed. This illustrates that isotropic nanocomposites have grater obvious MR effect than their corresponding anisotropic nanocomposites which is also supported by the magnetic flux density sweep test results, described in the next section. Moreover, both storage and loss moduli have been found to increase linearly for all nanocomposites with angular frequency.

4.4.2.3 Magnetic flux density sweep test Magnetic flux density sweep test is performed to obtain understanding about the absolute and relative MR effect by subjecting the nanocomposite samples under increasing magnetic flux density from 0 T to 1.1 T at 5 Hz angular frequency and 1 % strain amplitude. Figure 15 (a) to (f) depict the curves for storage and loss moduli versus magnetic flux density of the nanocomposites at room temperature. However, Figure15 (g) demonstrates the same as that of virgin PDMS. The absolute magnetorheological effect is expressed by the equation: ∆G = GMAX –G0

(1)

Where GMAX is the maximum value of storage modulus achieved when the MREs are exposed in the magnetic field and G0 is the value of storage modulus in absence of magnetic field (i.e. zero field modulus). Relative MR effect in % is expressed by the equation, ∆Gr = {(GMAX –G0) ×100}/ G0 = (∆G×100) / G0

(2)

Absolute and relative MR effects of a particular magnetorheological elastomer depend on the percentage loading of magnetic particles, frequency of oscillations and strength of the external magnetic field.11,20,46,72-74 Amplitude of the applied strain also influences the MR effect since the magnetic interaction force strongly depends on the distance between the dipoles (e.g., the iron particles).11,75 The values of the absolute and relative MR effect are given in the Table 4.Virgin PDMS shows negligible absolute and relative MR effect.


Page 33 of 48

5

(b)

(a)

5

3x10

G' (Pa)

G' (Pa)

4.0x10

5

3.6x10

5

2.94x10

5

2.88x10 2.82x10

4

G" (Pa)

2.8x10

4

4.0x10

0.0

G' (Pa)

0.2 0.4 0.6 0.8 1.0 1.2 Magnetic Flux Density (T)

4

2.0x10

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Magnetic Flux Density (T)

5

1.6x10

5

2.2x10

5

G' G"

2.0x10

5

5

1.4x10

G' G"

5

1.2x10 G" (Pa)

1.8x10

G" (Pa)

4

2.4x10

(d)

(c)

2.4x10

G' (Pa)

G" (Pa)

8.0x10

5

4

2.0x10

0.0

4

3x10

4

2x10

4

1x10 0


0.0 0.2 0.4 0.6 0.8 1.0 1.2

Magnetic Flux Density (T)

(e)

(f)

5

5

1.6x10

G' (Pa)

1.2x10

5

1.0x10

4

8.0x10 4 1.6x10

G' G"

3

8.0x10

0.0

5

1.4x10

G' G"

5

1.2x10

G" (Pa)

G' (Pa)

G' G"

5

G' G"

4

G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

2.0x10

0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Magnetic Flux Density (T)




4

(g)

G' (Pa)

5.0x10

4

4.8x10

G' G"

4

4.6x10 G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 48

3

4x10

3

3x10


Figure 15: Magnetic sweep test: (a) 5 wt. % Isotropic, (b) 5 wt. % Anisotropic, (c) 10 wt. % Isotropic, (d) 10 wt. % Anisotropic, (e) 20 wt. % Isotropic, and (f) 20 wt. % Anisotropic PDMS-FeCo3 nanocomposites; (g) Virgin PDMS. As expected, in our study the relative MR effect is observed to increase with increasing concentration of the magnetic particles except 20 wt. % anisotropic FeCo3-PDMS nanocomposite. From the table we can also observe that all the isotropic nanocomposites show higher absolute and relative MR effect than their corresponding anisotropic nanocomposites. This may be because of less interaction of the magnetic particles and polymer matrix under the external magnetic field in the case of anisotropic nanocomposites.

Table 4: Absolute and relative MR effect of the FeCo3-PDMS nanocomposites Sample Name

Absolute MR

Relative MR

effect in (Pa) at

effect in (%) at

1.098 T

1.098 T

21600±20

5.856±0.006

5 wt. % anisotropic FeCo3-PDMS nanocomposite

3400±8

1.166±0.003

10 wt. % isotropic FeCo3-PDMS nanocomposite

14880±14

7.05±0.008


9600±10

6.852±0.007


7750±12

8.404±0.013


6600±8

4.789±0.006

14±6

0.029±0.012


Virgin PDMS

As can be viewed from the digital picture of the 5 wt. % anisotropic FeCo3-PDMS nanocomposite (Figure 6d), the particle chains are not continuous throughout the matrix and


Page 35 of 48

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are localized in some position of the matrix without interconnecting to each other due the low magnetic particle loading. Therefore, when the anisotropic nanocomposites are subjected to the external magnetic field, most of the magnetic flux lines pass through without any interaction with the magnetic particles. Owing to localized chains, the deformation of the matrix induced by the particles movement exhibits lower modulus increment under the external magnetic field. With the increasing magnetic particle content in the anisotropic nanocomposites, the particle chains become continuous but still remain non- interconnected. The gap between the parallel particle chain lines decreases from 10 wt. % to the 20 wt. % particle loading. This results in the increased interaction between the magnetic particle and polymer matrix under magnetic field thereby showing increased modulus at higher particle loading. On the other hand isotropic nanocomposites have homogeneous magnetic particle distribution throughout the matrix forming a three dimensional interconnected network. Hence when they are subjected to the external magnetic field, whole matrix experiences magnetic field induced deformation by the movement of magnetic particles thereby exhibiting higher increment in the modulus value. With increased magnetic particle content, the inter particle distance decreases and the particles experience more magnetic dipolar interaction resulting in a greater push for the particles to get closer. Again magnetic dipolar interaction force is proportional to the amount of the magnetic particles present in the polymer matrix. Therefore combining effect of these contributions to the enhancement of the magnetic dipolar interaction force increases the modulus with the increasing particle content.

Anisotropic coefficient of 10 wt. % and 20 wt. % anisotropic FeCo3-PDMS nanocomposites are 1.438 and 1.54, respectively. This clearly indicates the less particle chain alignment in these 10 wt. % anisotropic FeCo3-PDMS nanocomposite. Hence it is closer to the isotropic nanocomposites in terms of particle distribution. This near isotropic structures could be the reasons for having higher relative MR effect for 10 wt. % anisotropic FeCo3-PDMS nanocomposite than the 20 wt. % anisotropic FeCo3-PDMS nanocomposite. Furthermore fabrication of anisotropic nanocomposite containing greater particle content under magnetic field resulted in the particle agglomeration.53 Choi et al. reported that severe particle aggregation destroyed the continuance of the elastomer matrix which ultimately showed detrimental effect on the elasticity of elastomer matrix and lead to the lower MR effect.53 Thus, at the highest particle content of 20 wt. % anisotropic FeCo3-PDMS, the observed particle aggregation may have resulted in the lower MR effect.



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Page 36 of 48

Mendes et al. studied the effect of particle alignment (isotropic, parallel and perpendicular to the plane of gel film) and concentration (varied from 1 to 30 vol. %) on the rheological response of spherical carbonyl iron (average particle diameter 930±330 nm) filled SEBS copolymer gel. The experimental results revealed that perpendicular particle arrangement (i.e. magnetic field and particle chain orientation are in the same direction) in gel matrix resulted in highest increase of the moduli. However, other two configurations exhibited almost same response for each different particle content (4-30 vol. %).76 Zrinyi et al. also reported that elastic modulus of the 10 to 30 wt. % carbonyl iron filled PDMS composites were not influenced by the external magnetic field when the applied magnetic field was perpendicular to the particle chain.70 In our study the particle chain alignment was also parallel to the plane of the nanocomposite film in the anisotropic sample. Moreover, both isotropic and anisotropic samples were analysed for magnetorheological properties by applying the field perpendicular to the plane of composite film i.e. perpendicular to the particles chains of nanocomposites. This could be a possible reason for lower interaction of magnetic field with the particle chain during the rheological analysis. Hence there was not much improvement in the MR effect with increasing particle content.

4.5 Thermomechanical Analysis Figure 16 (a) represent the dimension change versus time plot of neat PDMS and nanocomposite samples. According to the Gent equation77 penetration depth of the indenter under normal load and Young’s modulus of the sample is in relation as follows Em = (F/P3/2)(9/16 r1/2)

(3)

Where, Em is elastic modulus of the sample in dyne/cm2, F is load in dyne, P is the penetration in cm, r is the probe radius in cm. Crosslink density is related to the Young’s modulus of the sample by the following equation υe = Em/3RT

(4)

υe is crosslink density in mole/cm3, T is experimental temperature in K and R is universal gas constant in force unit i.e.8.314×107 erg mole-1K-1.


Page 37 of 48

(b)

(a)

-50 -100 -150 -200 -250 -300 -350 -400 -450

24 Crosslink density× 105 (moles/cm3)

Dimension Change (µ m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Neat PDMS 5 wt. % FeCo3-PDMS 10 wt. % FeCo3-PDMS 20 wt. % FeCo3-PDMS

22 20 18 16 14 12

0 10 20 30 40 50 60 70 80 90 Time (minute)

0


20

Figure 16 (a) Dimension change versus time plot of TMA (b) Crosslinking density versus FeCo3 loading (wt. %) plot for virgin PDMS and nanocomposite samples. Figure 16 (b) demonstrates the crosslink density of the samples as a function of FeCo3 loading in PDMS matrix. It shows that with increase in filler loading crosslinking density of the sample gradually decreases. FeCo3 nanoparticles hinder the covalent bond formation between Part A and Part B by the chemical reaction and ultimately lead to the weak mechanical properties in the nanocomposites. As the modulus of the sample is directly proportional to the crosslink density therefore increase in filler loading lead to the decrease in storage modulus of the sample. Hence this explains the experimental observation on storage modulus in Figure 13.

5. Conclusions Both isotropic and anisotropic FeCo3-PDMS nanocomposites have been developed here for the first time and found to be promising for MRE applications based on their high saturation magnetization and magnetic permeability. FeCo3 nanoparticles prepared by hydrazine reduction method show mixed (bcc and fcc) diffraction pattern due to the presence of Fe-rich and Co-rich phase structures. The nanocomposites were characterized for their microstructure,

magnetic

properties

and

magnetorheological

properties.

Saturation

magnetization values of the anisotropic nanocomposites measured parallel to the plane of particle alignment is higher than the value obtained perpendicular to the aligned structure of the nanocomposites. VSM study reveals that anisotropic coefficient of the nanocomposites mostly depends on the extent of particle alignment and particle content in the nanocomposites. Highest magnetic anisotropy of 1.68 is observed for 5 wt. % anisotropic nanocomposite, which decreases with further increase of magnetic particle content as anisotropic texture is gradually converted into isotropic structure at higher particle loading.



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All the nanocomposite samples show linear stress-strain behaviour to certain critical strain which is observed to increase with increasing applied magnetic field. Highest shear modulus and maximum stress is achieved for isotropic nanocomposite with 5 wt. % FeCo3. Interestingly, this is despite the fact that the magnetic nanoparticles used also aggregate into larger size at this composition as evidenced by confocal microscopy. The optimal properties are thus likely because of the nanoparticles retaining their identity and larger surface area in the aggregates compared to the individual similar micron sized magnetic particles. MR effects of the anisotropic nanocomposites are lower than their corresponding isotropic nanocomposites because of the lower interaction of the magnetic nanoparticles with polymer matrix acting as spacer under the external magnetic field. This lower interaction mainly results from the non-continuous particle chain formation and localized presence of magnetic particle in the anisotropic nanocomposites at low particle loading. Highest relative MR effect of ~8.4 % is achieved in case of 20 wt. % isotropic FeCo3-PDMS nanocomposite, which is much higher than the micron sized magnetic particle containing magnetorheological elastomers reported in the literature. All the nanocomposites have shown increased damping properties with increasing strain amplitude which suggests that these types of nanocomposites can be of potential use in tuneable vibration absorber applications.

Acknowledgements The authors would like to acknowledge support from DST to the Nanoscience Center at IITK and Nano Science Division and Fuel & Lubricant Division of DMSRDE for the sample fabrication and magnetorheological analysis.


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Magnetorheology of Polydimethylsiloxane Elastomer/FeCo3 Nanocomposite TOC Graphic


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FeCo3

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