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Journal of Magnetism and Magnetic Materials 322 (2010) 2956–2968

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jmmm

Synthesis and characterization of magnetite particles covered witha-trietoxysilil-polydimethylsiloxane

Anamaria Durdureanu-Angheluta a,b, Lucia Pricop a, Iuliana Stoica a, Catalina-Anisoara Peptu b,Andrei Dascalu a,b, Narcisa Marangoci a, Florica Doroftei a, Horia Chiriac c, Mariana Pinteala an,Bogdan C. Simionescu a,b

a ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, 700487 Iasi, Romaniab ‘‘Gh. Asachi’’ Technical University, 700050 Iasi, Romaniac National Institute of Research and Development in Technical Physics, 700050 Iasi, Romania

a r t i c l e i n f o

Article history:

Received 1 October 2009

Received in revised form

3 May 2010Available online 15 May 2010

Keywords:

Magnetite

Hydrophobic magnetic particle

Magneto-rheological fluid

Polydimethylsiloxane

Silicon ferrofluid

53/$ - see front matter & 2010 Elsevier B.V. A

016/j.jmmm.2010.05.013

esponding author.

ail address: maripint@yahoo.com (M.R. Pintea

a b s t r a c t

New silicon magnetite ferrofluids were prepared by dispersing siloxane-coated magnetite particles in

polydimethylsiloxane with low or high molecular weights. Ferrofluids are stable colloidal dispersions

of ultra fine covered magnetite particles, which may be selected for a specific application. We

demonstrated new methods of stabilizing the magnetic particles by reacting the hydroxyl groups on the

surface of magnetite particles with terminal ethoxy groups of polydimethylsiloxane, followed by their

dispersion in silicon fluids. The new silicon ferrofluids were tested from the morphology, magnetic

properties/losses, and rheological properties point of view.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Magnetite nanoparticles coated with polymers have beenwidely studied because of their applications in ultrahigh densitymagnetic storage media [1], biological labeling, tracking, imaging,detection, separations [2], and ferrofluids [3]. Magnetic nanopar-ticles colloidal dispersions, or ferrofluids, contain iron oxideparticles covered with polymers to ensure their compatibilitywith the fluids as carriers. Ferrofluids are very stable colloidaldispersions of ultra fine particles of magnetic materials withvarious technical and newer utilizations in the life sciences [4,5].

In order to prepare stable magnetite dispersions, the attractiveforces born between nanoparticles must be overcome. For that themagnetite surface can be grafted with monomers or polymers(covalent bonds) or can be modified by adsorption, electrostaticinteraction, hydrogen bonding or by other types of interactionwith stabilizers in order to obtain stable particles. In this contextthe synthesis of stable silicon magnetite fluids can be achievedwith hydrophobic polymers as stabilizers; the polymers contain-ing a functionalized portion that can bind to the particle surfaceand a hydrophobic non-reactive part solvated in the dispersionmedium or carried fluid. Most of the work recently published in

ll rights reserved.

la).

the literature has involved oleic acid as stabilizer (oleic acid is aneighteen carbon surfactant chain with one terminal carboxylicgroup) for preparing hydrocarbon-based magnetite fluids [6,7].Furthermore, the covered magnetite particles dispersed in poly-dimethylsiloxane (PDMS) carrier fluids using anionic surfactants[8,9] or nitrile containing PDMS triblock copolymers [10,11] wereprepared. So in order to obtain a magnetic fluid it is important todevelop a steric stabilizer, efficient in preventing the coagulationof the magnetite particles. Polydimethylsiloxane homopolymersare recognized to be efficient as dispersion stabilizers [12].Magnetic silicon fluids are comprised of magnetite nanoparticlessterically stabilized in a low molecular weight silicon carrier fluidwith promising biomedical applications due to their naturecombining the oxidative stability of magnetite and the biocom-patibility of PDMS [11].

Considering that large aggregates of magnetic particles lead toprecipitation and the colloidal suspension destabilization [13],various chemistry-based processing routes have been developedto synthesize nanosized magnetite particles, including co-pre-cipitation or precipitation [14,15], sol–gel method [16], emulsionstechnique [17], mechanochemical processing [18], hydrothermalpreparation [19] and DC thermal arc-plasma method [20].

In previous papers we described the synthesis and characteriza-tion of magnetite particles functionalized with organotriethoxysi-lanes on their surface with either hydrophobic or hydrophiliccharacter induced by the nature of the organic radical found on the

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particle surface [21]. Also, we used poly{dimethylsiloxane-g-[carboxyester-poly(ethylene oxide)]} (PDMS-PEO-COOH) copolymeras stabilizer to obtain double-shell magnetite particles [22].

This paper is centered on the synthesis of stable magnetitesilicon fluids. The fluids were prepared by reacting magnetitenanoparticles with a-triethoxysilil-polydimethilsiloxane (PDMS-TES) stabilizer dispersed into silicon, as carrier. It should bementioned that the siloxanes exhibit a good waterproof perfor-mance, solubility in nonpolar solvents, good resistance againstradiation, high thermal and chemical resistance, low viscosity vs.temperature coefficient, good compressibility and frost resistancethat recommend them as performing ferrofluid carriers.

2. Experimental

2.1. Materials

Materials were used without advanced purification: 1,1,3,3-tetramethyldisiloxane 97%, (TMDS) (Aldrich); octamethylcyclote-trasiloxane 98% purity (D4) and polydimethylsiloxane withmolecular weight 10 000 g/mol (PDMS10 000) (Aldrich); penta-methyldisiloxane Z95.0%, (PMDS) (Aldrich); allyl glycidyl ether99%, (AGE) (Aldrich); an ion exchange resin, styrene-divinylben-zene with sulfonic groups (exchange capacity ¼4.2 mval/g,porosity¼39.42%, granulation¼0.4–0.65 mm, specific surface¼35 m2/g) (VIONIT CS 34C, Romanian product); H2PtCl6 �

�6H2O (Aldrich); 3-aminopropyltriethoxysilane (APTES) (Al-drich); FeCl3 �6H2O 97% (Aldrich); FeCl2 �4H2O 99.99% (Aldrich);HCl 36.5% (Reactivul-Bucuresti); NH4OH 25% (Aldrich); citric acidtripotasium salt anhydrous (Sigma); dichloroethane (Reactivul-Bucuresti); dibutyltin dilaurate (CH3CH2CH2CH2)2Zn[OCO(CH2)10

CH3]2 (Carom-Onesti).

2.2. Methods

Fourier transform infrared (FT-IR) spectra were obtained on aBruker Vertex 70 instrument using KBr disc method. Scanningwas done from 4000 to 310 cm�1.

Thermogravimetric (TGA) curves were obtained on a MettlerToledo TGA-SDTA851e Derivatograph, under N2 atmosphere witha 20 ml/min flow and 15 K/min (25–900 K) heating rate; thesample weight was 4–6 mg. All these parameters were useduniformly for all samples.

Nuclear magnetic resonance (1H NMR) studies were performedon a DRX 400 Advance Bruker device.

The size and the polydispersity of the particles were deter-mined after ultrasonication of the particles in octamethylcyclote-trasiloxane (D4) or polydimethylsiloxane with molecular weight10 000 g/mol (PDMS10 000) for 5 min. Particle size and particle sizedistribution were evaluated on a Microtrac Nanotrac 250 ParticleSize Analyzer.

AFM images of the films were obtained on a SPM Solver PRO-Minstrument produced by NT-MDT, Russia, in semicontact modeusing a commercially available cantilever (silicon cantileverNSG10) with working frequencies between 230 and 246 kHz.The scan velocity was 12.07 mm/s. The samples were prepared bysuspending the uncoated and coated magnetic particles in waterand in either D4 or PDMS10 000, respectively. These systems weresonicated for 5 min and small drops of coated magnetic particlesin water were placed on top of clean glasses followed by theirdrying overnight. The coated particles dispersed in polydimethyl-siloxane were separated with a magnet and were washed withtoluene in order to remove the unreacted PDMS and were dried ina vacuum oven overnight. Additional microscopic investigation

was performed on an environmental scanning electron micro-scope QUANTA 200 coupled with an energy dispersive X-rayspectroscope (ESEM/EDX). The dried particle samples wereexamined in low vacuum mode operating at 30 KV using an LFDdetector.

The molecular weight and the polydispersity were determinedby gel permeation chromatography (GPC) using a GPC PL-EMD950 evaporative mass detector instrument. The system columnswere thermostated at 25 1C. Calibration was performed withnarrow polydispersity polystyrene standards (Polymers Labora-tories Ltd.). The samples were eluted with chloroform; the flowrate was 0.7 mL min�1.

Vibrating sample magnetometer analyses (VSM) were donewith a Lake Shore 7410 device.

Determination of hydrogen content was performed byZerewitinoff method [23].

Determination of epoxy content was performed from epoxycontent analysis [24].

2.3. Synthesis

2.3.1. Synthesis of a-triethoxysilil-polydimethylsiloxane (PDMS-TES)

PDMS-TES was obtained by a multistep procedure (Scheme 1).The first step is represented by the synthesis of a-hydride-

polydimethylsiloxane (H-PDMS). H-PDMS was prepared by theequilibration reaction of octamethylcyclotetrasiloxane (D4) 43.6 g(0.1472 mol) with penthamethyldisiloxane (PMDS) 7.42 g(0.05 mol) in a molar ratio D4/PMDS¼2.94. The reaction wasconducted for 4 h at 90 1C temperature, in the presence of 2.5%dried poly(styrene-divinylbenzene) with sulfonic groups (VIONITCS 34C) as catalyst [25].

H-PDMS molecular weight of 1430 g/mol was determinedfrom content of hydrogen [23] and 1H NMR spectrum.

1H NMR (CDCl3): d¼0.1 ppm (CH3–Si); 4.9 ppm (H–Si).The second step is represented by the synthesis of a-

glycidoxypropil-polydimethylsiloxane (PDMS-AGE) [26]. PDMS-AGE was prepared by hydrosilation reaction of 3.158 g(0.0276 mol) AGE with 37.727 g (0.0270 mol) H-PDMS (Si–H/AGE¼1/1.05 molar ratio), in the presence of H2PtCl6 as catalyst(2% solution in isopropanole; 10 ml/mol Si–H). The reaction wasconducted in toluene (50% w/w) for 6 h at 100–110 1C [18]. Theformation of PDMS-AGE was monitored by evaluating thedisappearance of the characteristic Si–H absorption band at2160 cm�1 in FT-IR spectrum. After the complete hydrosililationthe reaction product was separated by vacuum distillation of thesolvents and AGE in excess, and was purified by dissolution in n-hexane, filtration and vacuum evaporation of n-hexane.

Molar ratio of a-/b-isomers was E1/6 as calculated from the1H NMR spectrum.

PDMS-AGE molecular weight of 1540 g/mol was calculatedfrom epoxy content [24] and 1H NMR spectrum.

1H NMR (CDCl3) (Fig. 1a): d¼0.1 ppm (Si–CH3); 0.5–0.7ppm (Si–CH2); 1.1–1.2 ppm (CH3 of a-adduct); 1.3–1.7ppm (CH2CH2CH2, Si–CH(CH3)(CH2); 3.4–3.7 ppm (CH2–O);3.15 ppm (C–H of epoxy cycle); 2.6–2.8 ppm (CH2 of epoxy cycle).

PDMS-AGE was reacted with 3-aminopropyltriethoxysilane(APTES) [25] giving a-triethoxysilil-polydimethylsiloxane (PDMS-TES) in the last step of the reaction scheme. 10 g PDMS-AGE(0.0064 mol) and 1.4377 g (0.0065 mol) APTES (epoxy/NH2

¼1/1.05 molar ratio) were reacted in isopropanol (OH/NH2¼1/1molar ratio) at 60 1C for 8 h. After 8 h the reaction was stoppedand the excess of APTES was removed by vacuum distillation.

Molecular weight PDMS-TES 1800 g/mol was determined by GPC.1H NMR (CDCl3): d¼0.1–0.7 ppm (Si–CH3 and Si–CH2);

1.15 ppm (CH2–CH3); 1.5–1.7 ppm (CH2CH2CH2, Si–CH(CH3(CH2);

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Scheme 1. Synthesis of a-triethoxysilil-polydimethylsiloxane (PDMS-TES).

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2.5 ppm (HN–CH2); 3.2–3.8 ppm (O–CH2, CH(OH)–CH2–O); 3.9–4.1 ppm (CH–OH).

2.3.2. Preparation of silicon ferrofluid based on magnetite particles

with hydrophobic polydimethylsiloxane shell (Ma-PDMS)

The preparation of silicon magnetic fluids was performed in twodifferent ways (Scheme 2): the first one implied the separation ofneutral magnetite particles followed by the reaction of HO– groups ofmagnetite particles with the ethoxy groups of polymer indichloroethane at room temperature, followed by their dispersioninto a hydrophobic fluid (a) and the second one was based on thepreparation of magnetite particles by water/solvent interfacial intera-ctions (Ma-PDMS-TES2) when the uncovered magnetite particleswere extracted into the organic phase during covering process (b).

This organic phase dispersion was mixed with a hydrophobic fluidand then the solvent was removed by vacuum distillation.

2.3.2.1. Preparation of covered magnetite particles by a two step

procedure (Ma-PDMS1). As seen from Scheme 2 the first step isrepresented by magnetite particles preparation by co-precipitation ofFe2+ and Fe3+ ions according to the method of Massart [27]. 40 mlFeCl3 �6H2O (1 M) and 10 ml FeCl2 �4H2O (1 M), both prepared in HCl(2 M), were added in 500 ml NH4OH solution (0.7 M) under vigorouscontinuous mechanical stirring and inert nitrogen atmosphere, atroom temperature. The color of the solution shifted from orange toblack and a black precipitate started to appear indicating thatthe magnetite was formed. The reaction mixture was kept understirring for 24 h. 0.553 g (0.008 mol) citric acid tripotassium salt

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Fig. 1. AFM images of Ma particles: (a) 2D image; (b) 3D image and (c) height profile indicated by the white line in panel (a).

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(C6H5O7K3 �H2O) was then added (molar ratio of citric acid tripo-tassium salt to metal species (Fe2+ +Fe3+)¼0.135, representing 15%from total metal species). The precipitate was isolated using a mag-netic field, was washed twice with distilled water, and was dried at40 1C under vacuum for 24 h. The dry magnetite particles were pre-served in a desiccator on calcium chloride.

The covered particles (Ma-PDMS1) were prepared in a 100 ml3-neck flask, equipped with reflux condenser, mechanical stirringand nitrogen inlet tube on a thermostated silicone bath. 20 mldichloroethane and 0.6658 g magnetite particles (2.8 OH mmol;the content of –OH groups was calculated from TGA curve) wasfirst sonicated for 10 min and after that was mixed with a 10 mldichloroethane solution of 1.42 g PDMS-TES (0.9 mmol; Ma-OH/C2H5O–Si–E1/1 molar ratio). The reaction was performed in inertatmosphere, at 81 1C (dichloroethane reflux temperature) for24 h. The Ma-PDMS1 particles were separated by evaporating thedichloroethan and ethanol subproduct under reduced pressureusing a rotary evaporator. The magnetic particles were washedwith small amounts of toluene for removing of the unreactedpolydimethylsiloxane prepolymer and were collected with a

permanent magnet. After that Ma-PDMS1 particles were sus-pended in chloroform and centrifuged until no significant amountof magnetite precipitated from dispersion. The chloroform wasthen removed under vacuum and the complex was dried undervacuum for 3 days at 40 1C. After complete drying the core-shellmagnetite particles were dispersed by sonication for 5 min in a10000 g/mol polydimethylsiloxane (10 ml) as hydrophobic fluidwith the formation of silicon ferrofluid.

2.3.2.2. Preparation of covered magnetite particles by a one step

procedure (Ma-PDMS2). The preparation of covered magnetiteparticles (Ma-PDMS2) in situ was performed by chemical co-pre-cipitation of Fe2+ and Fe3+ ions in a mixture of water–PDMS-TESdichloroethane solution. 2 g FeCl3 �6H2O (2 M) and 0.76 gFeCl2 �4H2O (1 M) were dissolved separately in 25 ml deoxygenatedwater and the obtained solutions were mixed together under argonatmosphere into a three-necked flask equipped with a mechanicalstirrer. The aqueous mixture was vigorously stirred and after totalmixing, a solution containing 3.71 g of a-triethoxysilil terminated

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Scheme 2. Preparation of magnetite particles.

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polydimethylsiloxane stabilizer (PDMS-TES) and 40 ml di-chloroethane was added. The solution was kept under vigorousstirring in inert atmosphere while adding drop by drop ammoniumhydroxide solution (30%), until the pH of the reaction mixtureturned to 9. A color shift from orange to black indicating the ap-pearance of the magnetite particles was observed immediately afteradding the ammonia solution. After 6 h under vigorous continuousstirring the dispersion of covered particles in dichloroethane wasseparated and was centrifuged in order to remove all uncoveredmagnetite particles. In the final stage 50% from centrifuged dis-persion was suspended in 10 ml octamethylcyclotetrasiloxane and50% in 10 ml of 10000 g/mol polydimethylsiloxane (PDMS10 000) inorder to form the silicon ferrofluids. The solvent was removed byvacuum evaporation in a rotary evaporator.

3. Results and discussions

3.1. Morphology, size and structure of magnetite particles

Considering the significant impact of the coated magneticparticles size onto the stability of the silicon ferrofluids, the shapeand size of Ma-PDMS1 dispersed in PDMS10000 Ma-PDMS2 in D4

and PDMS10000 were established by atomic force microscopy

Fig. 2. AFM images of Ma-PDMS1 particles in PDMS10 000: (a) 2D image; (b)

techniques. AFM images of uncoated and coated particlessuspended in polydimethylsiloxane (10000 g/mol, PDMS10 000)or octamethylcyclotetrasiloxane (D4) are presented in Figs. 1–4.As known, the AFM-2D images (Figs. 1a, 2a, 3a, 4a) give usefulinformation about the shape of the particles and the presence orabsence of inhomogeneous aggregates, indicating the stability ofthe analyzed silicon ferrofluids. In addition, AFM-3D images(Figs. 1b, 2b, 3b, 4b) provide quantitative and qualitative information.They show the variable height of particles and aggregates, which arereflected in the height profiles (Figs. 1c, 2c, 3c, 4c).

The uncoated magnetite particles (Ma) (Fig. 1a, c) have aspherical shape with an average diameter of about 600–700 nm. Itshould be mentioned that the magnetite particles have thetendency to agglomerate. The spherical shape is also maintainedfor all coated magnetic aggregates Ma-PDMS1dispersed inPDMS10000 (Fig. 2a) and Ma-PDMS2 dispersed in D4 or inPDMS10000 (Figs. 3 and 4), the average diameter being around800 nm for Ma-PDMS1, and significantly smaller for the coatedparticles prepared in situ (around 350 nm for Ma-PDMS2 in D4

and 200 nm for Ma-PDMS2 in PDMS10000) indicating a good abilityof dispersion and a higher homogeneity of the silicon ferrofluidsobtained in situ.

As can be seen from AFM images, the height profiles aresmaller for the coated particles, prepared in situ, suspended in

3D image and (c) height profile indicated by the white line in panel (a).

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Fig. 3. AFM images of Ma-PDMS2 particles in PDMS10 000: (a) 2D image; (b) 3D image and (c) height profile indicated by the white line in panel (a).

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either D4 or PDMS10 000, indicating the efficiency of thisalternative synthesis route. From the height profile across of anindividual particle the apparent radius of a particle (ri) and itsrelative height (hi) can be determined. The profiles of Ma in water(sample surface: 10�10 mm), Ma-PDMS1 in PDMS10000 (samplesurface: 10�10 mm), Ma-PDMS2 in D4 (sample surface: 5�5 mm)and Ma-PDMS2 in PDMS10000 (sample surface: 2�2 mm) grainscollected were analyzed and the average radius (rAFM) and height(hAFM) were calculated. The average values were then used tocalculate the true radius (r) of individual particles or aggregates(Table 1) by applying the following equation (1) [28]:

r¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2

AFM�2RtiphAFM

qð1Þ

where Rtip represents the radius of curvature of the AFM tip (Rtip ofNSG10 cantilever is 10 nm).

The results obtained by AFM on particle size and shape weresupported by the results obtained by SEM (Fig. 5).

The stability in time of the coated particles silicon suspensionswas evaluated by analyzing the effect of aging onto theprecipitation in solution and the size of the particles in bothoctamethylcyclotetrasiloxane and polydimethylsiloxane by laserdiffractometry measurements (Fig. 6).

After 3 months approximately 1–2 wt% of Ma-PDMS2 sus-pended in PDMS10 000 was precipitated from solution. The laserdiffractometry measurements of supernatants showed that thecolloids have a low polydispersity with an average diameter of250 nm. For Ma-PDMS2 in D4 (2–3 wt% precipitated from silicon

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Fig. 4. AFM images of Ma-PDMS2 particles in D4: (a) 2D image; (b) 3D image and (c) height profile indicated by the white line in panel (a).

Table 1Calculated average values of the true radius of uncoated and coated magnetite particles dispersed in PDMS10 000 or in D4.

Sample Scan size (lm2) The number ofparticles collected

Particle characteristics

Average diameter (nm) Average height (nm) Average true radius (nm)

Ma 10�10 47 679720 59720 338720

Ma-PDMS1 in PDMS10 000 10�10 19 885720 48720 442720

Ma-PDMS2 in PDMS10 000 2�2 20 214720 22720 104720

Ma-PDMS2 in D4 5�5 17 376720 85720 183720

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solution), the analysis of the supernatant indicated the same lowpolydispersity, but higher dimensions reaching almost 500 nm.Ma-PDMS1 aggregates dispersed in PDMS10000 precipitated al-most 5 wt%, the size of the aggregates in supernatant was almost800 nm, quite similar with the value observed at the end of theexperiment (Figs. 2 and 5). From these results it can be concludedthat the dispersion of coated particles prepared in situ in both

high and low molecular weight fluid ensures a good homogeneityand stability of colloidal solution, without agglomerationtendency.

Fig. 7 shows the IR spectra of uncovered and coveredmagnetite particles (Ma-PDMS-TES). Ma spectrum (Fig. 7a)presents absorption peaks at around 629, 590 and 440 cm�1

characteristic absorptions of Fe–O bond, which confirmed the

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Fig. 5. SEM micrographs of: Ma (a); Ma-PDMS1 (b); Ma-PDMS2 in PDMS10 000 (c) and Ma-PDMS2 in D4 (d).

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presence of magnetite nanoparticles [29]. The bands near 3396,1617 and 1400 cm�1 are ascribed to the hydroxyl characteristicpeaks of water absorbed in the surface or the OH-stretching bandsand its bending vibration peak [30]. FT-IR spectra of the particlescovered with PDMS-TES obtained by both methods showed thediminishing of the magnetite –OH bands and possess new bandsat about 1094 and 1024 cm�1 of Si–O–Si bonds from PDMS-TES.After functionalization the characteristic bands of Fe–O of themagnetite particles are shifted at around 628, 588 and 448 cm�1

due to the formation of Si–O–Fe bonds (reaction between –OHgroups of magnetite and ethoxy group of the polymer). Also, threenew representative absorption bands at 1260, 2923 and2853 cm�1, corresponding to Si–CH3 bond and stretchingvibration of C–H and C–H2 bonds, respectively, were observed.Furthermore, the two broad bands at around 3400 and 1628 cm�1

can be ascribed to the N–H stretching vibration which is

superposed with unreacted hydroxyl groups of Ma particles[29]. In conclusion, from the IR spectra of both samples it canbe seen that the magnetite particles were indeed coated by thesiloxane polymer.

3.2. Magnetic properties and losses

The TGA and DTA curves of Ma particles (not shown) indicatedecomposition phenomena in the 50–450 1C temperature interval[21]. The uncoated particles are covered by the ferroferrichydroxyl complexes and by adsorbed water as evidenced by theweight loss of around 20% in the 70–120 1C interval, characteristicfor water elimination. The second weight loss (�6%) at 250–450 1C is characteristic for the transformation of ferroferric oxidein ferric oxide [28]. The coated Ma-PDMS particles (Fig. 8) are

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Fig. 6. Size of coated particles determined by diffractometry measurements.

Fig. 7. FT-IR spectra for Ma (a) and Ma-PDMS1 (b) particles.

Fig. 8. TGA and DTA evolution

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characterized by a continuous weight loss of about 1–2% (water)up to 200 1C, which is much less as compared to pure Ma particles(Fig. 8), confirming the results obtained by FT-IR analysis. Asecond and a third weight loss of about �7 and 8%, in 220–320 1Cand 320–450 1C temperature intervals, respectively, are due to thesimultaneously occurring decomposition of attached PDMS andtransformation of magnetite in maghemite.

To study the magnetic behavior of covered magnetite,magnetization measurements were performed and compared tothe results obtained on uncovered magnetite. As one can observein Fig. 9, the saturation magnetization for the uncoated magnetiteis 64 emu g�1, in agreement with previously reported results [21].Lower saturation magnetizations of around 46 and 45 emu g�1

were registered for Ma-PDMS1 and Ma-PDMS2, respectively,which are smaller than the typical value for bulk materials(93 emu g�1) [30]. The decreased saturation magnetization can beattributed to surface effects, such as magnetically inactive layerproducing disordered surface [31]. The oxidation of magnetiteparticles during the reaction with PDMS-TES could also occur[32]. Low hysteresis values were observed for all three samples(Ma, Ma-PDMS1, Ma-PDMS2) and their characteristic curves werereversible at 300 K with coercivity around 1 G.

The rheological properties of the Ma-PDMS2 dispersed inPDMS10000 and in D4 supernatants after three months aging timewere investigated as a function of shear rate, temperature, andapplied magnetic field (current intensity). Fig. 10 shows theviscosity dependence of the shear rate for supernatants(suspensions of 250 nm and 500 nm of Ma-PDMS2 in PDMS10000

and Ma-PDMS2 in D4, respectively) at different magnetic fields.As seen from Fig. 10, the viscosity of unloaded and loaded

fluids in the absence of applied magnetic field is depending on thenature of the fluid. Certainly, the increase in molecular weight ofthe fluid leads to an increase in viscosity. Viscosities (especially atlow shear rates) of loaded fluids are quite high, suggesting theformation of some clusters.

Increasing the shear rate, the viscosity is decreasing up to acertain specific value of the applied magnetic field, indicating thatthe clusters are broken up. In Fig. 10b it can be observed that theviscosity of Ma-PDMS2 dispersed in PDMS10000 is increasing whenthe magnetic field is up to 64.9 kA/m and beyond this value of theapplied magnetic field the viscosity value is decreasing to values

curves for the Ma-PDMS1.

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Fig. 9. Magnetization curves of Ma (a) and Ma-PDMS1 (b) particles.

Fig. 10. Effect of shear rate on the viscosity behavior for Ma-PDMS2 suspension in D4 (a); suspension in PDMS10 000 (b) after 90 days aging time; ’ D4 and m PDMS10 000 (c).

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Fig. 11. Effect of magnetic field on the viscosity behavior for Ma-PDMS2 in D4 (a) and in PDMS10000 (b) after 90 days aging time. Shear rate¼25 s�1 and gap¼0.5 mm.

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even smaller than the sample in the absence of magnetic field,suggesting that an increase of applied magnetic field beyond64.9 KA/m breaks the clusters. These conclusions cannot besustained by the viscosity variation vs. magnetic field (Fig. 11)when the viscosity is increasing with the increase of the magneticfield for a shear rate of 25 s�1. This can be explained due to achange of properties that can occur during the experiment,because the experiment takes place much faster than previously.By contrast, for the dispersion of Ma-PDMS2 in D4 the viscosityvariation as a function of the shear rate is increasing withincreasing the magnetic field value (Fig. 10). In this last case, theincreasing of the magnetic field could determine the formation ofsome clusters. However, if some clustering occurs, the formedclusters could be broken up by applied shear, under anappropriate magnetic field value (higher than 64 kA/m for thedispersions in PDMS10 000 and lower than 134.3 kA/m fordispersions in D4). It should be also underlined that themagnetic field values increase while raising the intensity. Bytaking into account this observation, to obtain a certain amount ofmagnetic field a certain intensity of current should be applied sothat viscosity is kept at a minimum value (without clusters). Theviscosity vs. temperature curve showed that the viscosity isdecreasing with the increase in temperature in a very smallinterval (25.2–25.8 1C).

4. Conclusions

Stable magnetic colloids, prepared by using polydimethylsiloxaneas stabilizer, were prepared in silicon fluids. The stabilization processwas induced by reacting –OH groups of magnetite surfaces withPDMS-TES by two different preparation methods (Scheme 2). Thecovered and uncovered magnetite particles have spherical shapes andsizes depending on the synthesis method. The magnetite particleswith polydimethylsiloxane presented saturation magnetization ofaround 45 emu/g, due to the interaction with the surfactantmolecules. The magnetic colloids were stable in time; only up to5 wt% losses were recorded after 90 days aging. The laser diffracto-

metry measurements of aging dispersions showed that the particleshad a low polydispersity with an average diameter varying from 200to 500 nm (prepared in situ) and 800 nm when the magnetite wasobtained separately. Rheological studies on aging dispersions illu-strated that the viscosity value depends on the molecular weight ofthe carrier, the shear rate, magnetic field value, applied field intensityand on temperature. Increasing the shear rate decreases the viscosityto a characteristic value of the applied magnetic field.

Acknowledgments

This work was supported by the Bilateral Project IntegratedAction ‘‘Brancusi’’ on the synthesis and characterizationof magnetic nanoparticles PEG-magnetite (PEGMag) betweenCEMEF, Sophia Antipolis, France and ‘‘Petru Poni’’ Institute ofMacromolecular Chemistry, Iasi, Romania.

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