Ternary magnetic nanocomposites based on core-shell Fe 3 O 4 /polyaniline nanoparticles distributed...

Phys. Status Solidi A, 1–6 (2009) / DOI 10.1002/pssa.200824421 p s sa

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applications and materials science

Ternary magnetic nanocompositesbased on core–shell Fe3O4/polyaniline
nanoparticles distributed in PVDF matrix
Mykhaylo Petrychuk1, Valery Kovalenko*,1, Alexander Pud2, Nikolay Ogurtsov2, and Alexey Gubin3

1Taras Shevchenko National University, Volodymyrska 60, 01033 Kyiv, Ukraine2 Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 02160 Kyiv, Ukraine3A. Usikov Institute of Radiophysics and Electronics, National Academy of Sciences of Ukraine, 61085 Kharkiv, Ukraine

Received 13 October 2008, revised 10 July 2009, accepted 16 September 2009

Published online 18 November 2009

PACS 62.23.Pq, 72.80.Le, 72.80.Tm, 75.50.Dd, 75.50.Tt

*Corresponding author: e-mail [email protected], Phone: þ38 044 526 0550, Fax: þ38 044 526 0531

Magnetic, electric, and radioprotector properties of hybrid

nanocomposites of Fe3O4 nanoparticleswith andwithout a shell

of polyaniline (PANI), which is doped with dodecylbenzene-

sulfonic acid (DBSA), dispersed in polyvinylidene fluoride

(PVDF) matrix have been studied. It has been found that the

presence of PANI–DBSA as a separate filler in the ternary

nanocomposite film, which also contains as another filler the

core–shell Fe3O4/PANI–DBSA nanoparticles, facilitates dis-

persion of the magnetic filler due to the improvement of its

compatibility with the PVDF matrix. This leads both to the

decrease in coefficient of squareness of the hysteresis loop and

to the increase in electromagnetic energy (EME) absorption of

the nanocomposite film.

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction During the last decade a significantgrowth of interest in hybrid composite materials based onhighly dispersed ferromagnetic powders and intrinsicallyconducting polymers (ICP) namely polypyrrole or polyani-line (PANI) has been observed [1–4]. Obviously, this isconnected with expectations of researchers for a synergeticcombination of electrical andmagnetic properties of the bothcomponents in such composites. This combination canmovesuch materials to their applications in practically importantfields of electromagnetic interference (EMI) shielding [5],microwave absorption [6], metamaterials [7, 8], photoniccrystals [9], etc. The studies in this field are mainlyconcentrated on either investigation of core–shell nanopar-ticles prepared by polymerization of a monomer (aniline orpyrrole) in dispersion of ferromagnetic nanoparticles (e.g.,Fe3O4 [10], NiZn [11], or MnZn ferrites [4, 5], etc.) or onnanocomposites formed by mixing a solution of an iron saltwith a solution of emeraldine base followed by a treatmentwith a base/acid solution [1, 12, 13]. Interestingly, the

magnetic properties of such materials can depend on PANIconductivity, e.g., the dedoped (nonconducting) form of thecomposites PANI–Fe3O4 was characterized by significantvalues of saturation magnetization (up to about 80 emu/g)[13]. The redoping of this composite inHCl solution led to anincrease in its conductivity up to �10�4 S/cm, but itssaturation magnetization decreased to 73 emu/g [13].

Naturally, both electrical andmagnetic characteristics ofsuch composites should depend on the kind and loading of aferromagnetic filler, dopant nature, doping degree of PANI,and the method of their preparation. Specifically, Tang et al.[12] prepared nanocomposites of g-Fe2O3with doped PANI,which had conductivities in the range of 10�3–180 S/cmdepending on the formation technologies. An increase incontents of ferromagnetic filler (Fe nanoparticles) from 0 to70wt.% in a matrix of PANI doped with dodecylbenzene-sulfonic acid (DBSA) led to a decrease of conductivities ofpellets of the prepared Fe-PANI–DBSA nanocomposites from0.25 to 0.07 S/cm [14]. All these nanocomposites showed


2 M. Petrychuk et al.: Ternary magnetic nanocomposites based on core–shell Fe3O4/polyanilinep

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Figure 1 TEM image of pristine Fe3O4 nanoparticles.

Table 1 Composition of the film composites.

sample composition

1 15mg PVDF2 15mg PVDF/PANI3 15mg PVDFþ 12mg Fe3O4

4 15mg PVDFþ 1.8mg Fe3O4

5 15mg PVDFþ 13mg Fe3O4/PANI6 15mg PVDFþ 2mg Fe3O4/PANI7 15mg PVDF/PANIþ 13mg Fe3O4/PANI8 15mg PVDF/PANIþ 2mg Fe3O4/PANI

hysteresis loop and hard magnetization characteristics.Specifically, their saturation magnetization monotonicallygrew from 32 to 78 emu/g when the Fe content increasedfrom 30 to 70wt.%. However, their saturation field andcoercivity were almost independent of the Fe nanoparticlecontent and were about 5500 and 385Oe, respectively [14].

Obviously, the main drawbacks of PANI, being lowmechanical strength and poor processibility [15], should bealso typical of its nanocomposites with ferromagneticnanopowders. Therefore, the mixing of these nanocompo-sites with a dielectric polymer matrix is not only theinstrument to govern their magnetic losses m00 [16] but thisapproach can also be considered as themethod of preparationof the mechanically strong ternary composite materials withsuitable processibility, magnetic properties, and conduc-tivity. From our point of view, use of ferroelectric polymerpolyvinylidene fluoride (PVDF) as the polymer matrix insuch the materials is of significant interest because PVDFcomposites with PANI possess good conductivity andinteresting nonlinear electrical characteristics [17, 18].However, to our knowledge, it is still unknown how such akind of polymer matrix will affect the properties of magneticfiller whose particles are covered with the PANI shell. Inorder to fill this gap in our knowledge the main goal of ourwork was evaluation of a contribution of each of thesecomponents into magnetic and electrophysical properties ofthe ternary composite. To realize this goal we comparedconductivity, magnetic, and UHF shield behaviors ofcomposites of PVDF with Fe3O4 nanoparticles being bareor covered by PANI doped with DBSA (PANI–DBSA) andin the presence of a separate phase of PANI–DBSA. Forconvenience, PANI–DBSA will be referred hereinafter to asPANI except for cases when an effect of DBSA is discussed.

2 Materials and methods As amagnetic filler of thecomposites commercial Fe3O4 nanopowder (Nanostructuredand Amorphous Materials, Inc.) consisting of almostspherical nanoparticles with diameters in the range of 20–40 nm (Fig. 1) was used. Powder of core–shell nanocompo-site Fe3O4/PANI (19.5wt.% of PANI) was prepared inaccordance with the technique for hybrid core–shell SiC/PANI nanoparticles described elsewhere [19]. In short, asolution of 2mLof aniline and 10.3mg ofDBSA in 1.5mLofwater was prepared and stirred for 1 h to form the aniliniumsalt. 60mg of Fe3O4 were then added to this solutionfollowed by stirring this dispersion for 30min. Then, asolution of 8.2mg of ammonium persulfate in 2mL of waterwas added to the anilinium salt and Fe3O4 dispersion underconstant stirringto run the monomer chemical oxidativepolymerization at 22 8C for 24 h. After the polymerizationhad completed, the final nanocomposite was cleaned bydialysis through cellophane membrane against distilledwater followed by drying under vacuum at 70 8C to separatethe Fe3O4/PANI nanopowder for further studies.

We have studied the characteristics of both of thisnanocomposite powder or pristine Fe3O4 nanopowder and oftheir composite films. Specifically, there were prepared one


film sample of pristine PVDF and seven film samples(2 cm� 2 cm� 0.004 cm) were prepared, which includedonly PANI or only Fe3O4 or only Fe3O4/PANI or mixture ofFe3O4/PANI and PANI as fillers dispersed in PVDF-matrix(Table 1). These films were made by compression molding(using press Specac at 200 8C under a load of 3 ton) of PVDFpowder Kynar 1000 (Arkema) (sample 1); PVDF powderwhose particles were covered by PANI in accordance withthe polymerization technique described elsewhere [17](sample 2); and mixtures of these powders with differentcontents of Fe3O4 (samples 3, 4) or Fe3O4/PANI (samples 5,6 and 7, and 8).

The film compositions allowed evaluation of thecontribution of each component into the magnetic, conduc-tivity, and radiooptical properties of the compositematerials.

Conductivity–temperature dependences of the sampleswere measured with a multimeter HP34401A when using aStirling cooler and a LakeShore 340 controller.

Radiooptical properties of the film samples are charac-terized by absorption of electromagnetic energy (EME) inthe frequency range from 22 to 40GHz using an HP 8722Cspectrum analyzer. Values of the EME absorption areestimated by transmission coefficients of the samples, whichare calculated from a simple ratio (1)

K ¼ I=I0; (1)

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Phys. Status Solidi A (2009) 3

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Figure 3 SEM images of cross-sections of the samples 1 (a), 3 (b),5 (c), and7 (d).Thewhite arrowsdesignatedifferent agglomeratesofFe3O4/PANI–DBSA nanoparticles.

where I is the intensity of the transmitted radiation; I0 is theintensity of the incident radiation.

To measure these EME intensities we used theexperimental setup including the film samples placed in awaveguide, which connected the generator output with thedetector, at right angles to the direction of electromagneticwave propagation and to completely overlap the cross-section of the waveguide.

It was found in previous tests that the transmission valueof the samples did not undergo significant collisions in theused frequency range. This allowed us to determine theirtransmission coefficient as an averaged value in the wholefrequency range. However, the physical mechanism, whichis responsible for such behavior, is not completely clear yetand will be studied in detail in a separate work.

The hysteresis loops were measured with a homemadevibration magnetometer.

The size and morphology of the pristine and nano-composite nanoparticles was evaluated with a JEOL JEM-100CXII transmission electron microscope. The conditionof these nanoparticles in the nanocomposite films wasestimated by scanning electron microscopy (SEM) of theircross-sections performedwith the help of a Super Probe 7333JEOL XR Microanalyzer.

3 Results3.1 Electron microscopy measurements Com-

paring transmission electron microscopy (TEM) images ofpristine Fe3O4 nanoparticles and the Fe3O4/PANI compositenanoparticles (Figs. 1 and 2), one can distinguish a core–shellstructure of the latter formed due to the used polymerizationapproach. The estimated thickness of the PANI shell variesin the range 2–10 nm (Fig. 2). A part of these core–shellnanoparticles are pooled in the aggregates of different sizeincluding from units to a few tens (see bottom of Fig. 2) ofnanoparticles.

The condition of these nanoparticles in the nanocompo-site films in comparison with pristine PVDF matrix can be

Figure 2 TEM image of Fe3O4 nanoparticles covered with PANI.Black arrows designate examples of PANI shells surrounding theFe3O4 nanoparticles and their agglomerates.

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roughly estimated from SEM images of their cross-sections(Fig. 3).

As compared with the homogeneous pristine PVDF filmsample 1 (Fig. 3a), one can distinguish in the cross-sectionsof the other nanocomposites the presence of a dispersionphase whose dispersivity depends on the presence,morphology, and quantity of PANI. Specifically, sample 3without PANI contains the 50–200 nm aggregates of thepristine Fe3O4 magnetic nanoparticles that are accumulatedin the PVDF matrix cavities of different size (Fig. 3b). Theobserved sharp interface between these aggregates and thematrix (Fig. 3b) suggests the absence or aweak interaction ofthe non-modified inorganic and the polymer phases of thenanocomposite.

Addition of PANI to this unmodified nanocompositedramatically improves the situation (compare the cross-section images in Figs. 3b–d). Thus, the sample 5, whichcontains the core–shell Fe3O4/PANI–DBSA nanoparticlesassociated into different aggregates (50–500 nm), does nothave the sharp interface between the dispersion phase andPVDF matrix (Fig. 3c). This fact testifies to an improved



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Table 2 Transmission coefficients of the nanocomposite films.

sample transmissioncoefficient (%)

1 992 963 924 965 916 977 588 77

interaction between Fe3O4/PANI–DBSA nanoparticlesand PVDF matrix in the sample 5 as compared with thesample 3 containing pristine Fe3O4 nanoparticles (Fig. 3b).Obviously, the improvement became possible due to theknown compatibilizing effect of the dopant DBSA [15]. Atthe same time the samples 3 and 5 display not veryhomogeneous distribution of the magnetic aggregatesindependently of the absence or presence of the PANI shell(Figs. 3b and c).

Unlike these samples, the sample 7, which contains inaddition to Fe3O4/PANI–DBSA aggregates a separate phaseof PANI–DBSA, is characterized by a more homogeneousdistribution of these aggregates being distant one fromanother and better dispersed than in the absence of theseparate phase of PANI–DBSA (see the case of the sample 5)(compare images in Figs. 3c and d). This can result in adecrease in contribution of their magnetic interaction intoenergy of magnetic anisotropy (see below).

3.2 Electrical characterization of the filmsamples As one can see from Fig. 4, conductivity–temperature dependences (Arrhenius plots) of the PANI-containing film samples with and without magnetic nano-particles are similar. The curves display two linear parts withdifferent slopes at T< 130K and T> 130K. This corre-sponds to an exponential (activation) dependence ofconductivity on temperature and allows determination ofthe activation energies E1¼ 0.021 eV and E2¼ 0.043 eV[20] for the low and high-temperature ranges, respectively.

The existence of these activation energies suggests thatthe conductivity of the samples is determined by twoimpurity levels in the band gap of the semiconductor-dopedpolymer PANI. Judging by data presented in Fig. 4, acontribution of the level E1 to conductivity of the sampleswith Fe3O4/PANI is less (by�7%) than a contribution of thelevel E2 comparatively to the sample 2 without the magneticpowder. This suggests that the presence of Fe3O4 in Fe3O4/PANI nanoparticles leads to a change of structure of thePANI energy levels. These changes are stronger in the filmsample 5 with higher loading of Fe3O4/PANI (Fig. 4) thatconfirms this suggestion.

Figure 4 Temperature dependences of conductivity of the sam-ples: without themagnetic nanoparticles (sample 2, solid line), withhigh loadingofFe3O4/PANI (sample5, dashed line) and low loadingof Fe3O4/PANI (sample 6, dotted line).


3.3 Characterization of radiooptical propertiesof the nanocomposite film samples In order tocharacterize these properties the amount of EME passedthrough a sample was measured. A dispersion of trans-mission in the range of 22–40GHzwas not found. This setupand the fact that thicknesses of all these samples werepractically the same allowed us to consider that the absolutevalue of the transmission characterized the radioopticalproperties of the materials.

Results of measurements of EME absorption (Table 2)showed that the samples 3–6, containing only Fe3O4 orFe3O4/PANI nanoparticles without a separate PANI phase,absorbed nomore than 4% (samples 4 and 6with low contentof Fe3O4) and no more than 9% (samples 3 and 5 with highcontent Fe3O4) of the EME flux. Sample 2, containing onlyPANI phase weakly absorbed EME – 4% that was similar tothe samples with low loading of magnetic nanoparticles.

The absorptance of the nanocomposites was dramatic-ally enhanced in the case of the film samples 7 and 8 (42 and23% of the EME flux, respectively), which contained themixture of two different fillers namely separate phases ofboth PANI and the core–shell Fe3O4/PANI nanoparticles(Table 2).

The discovered difference in radiooptical characteristicsof the film samples is probably connected with thesynergistic effect of the two separate fillers: (i) magneticnanoparticles Fe3O4/PANI–DBSA and (ii) PANI–DBSA.This synergistically increased absorption of the samples 7and 8 stems obviously from the magnetic nanoparticlesinteraction with PANI–DBSA phase, specifically withsurface-active long tails of DBSA anions that resulted inbetter separation of the nanoparticles confirmed by SEMmeasurements (Fig. 3).

3.4 Magnetic measurements Measurements ofmagnetic hysteresis loops were performed to evaluateinfluence of composition of the film and nanopowdercomposite samples (Table 1) on their magnetic properties.The results of measurements are given in Table 3 andFigs. 5–7.

Based on the magnetic measurements, we determinedthe coercive force HC of the investigated materials andevaluated the coefficient of squareness of hysteresis loops

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Table 3 Magnetic properties of film samples in accordance withTable 1 and powders.

sample HC (Oe) kp

sample 3, film 7.9 0.41sample 5, film 6.8 0.44sample 7, film 7.4 0.21Fe3O4, powder 6.5 0.61Fe3O4/PANI, powder 6.3 0.68

Figure 5 Hysteresis loops of the magnetic composite films ofsamples 3, 5, 7 (compositions see Table 1).

Figure 6 Hysteresis loops of the magnetic composite films withlow loading of the magnetic nanopowder (samples 4, 6, 8 forcomposition see Table 1).

kp¼M0/Mmax, whereM0 is a residualmagnetization,Mmax isa value of magnetization in saturation. Values of HC and kpfor nanopowders and composite films with high content ofFe3O4 are given in Table 3. The obtained results allow us toconclude that PANI coating ofmagnetic Fe3O4 nanoparticlesin the nanocomposite film (samples 3, 5, and 7) increases thecoercive force from 6.8 to 7.4–7.9Oe. However, in the caseof the magnetic nanopowders there is an inverse situation –PANI coating of Fe3O4 nanoparticles weakly decreases HC

from 6.5 to 6.3Oe. The reasons of such changes are stillunclear and should be additionally studied.

Values of squareness of the hysteresis loops ofmagnetiteFe3O4 both in powder and film composite states have apredictable difference. Specifically, squareness of hysteresisloops of film sampleswith Fe3O4 and the PANI separate filler

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(Table 3, sample 7) is significantly lower (by a factor of 2)than that of the films without the PANI separate filler(Table 3, samples 3 and 5). This testifies the existence of asubstantial demagnetizing factor in the case of the film withtwo separate fillers (sample 7): magnetic core–shellnanoparticles Fe3O4 (pristine or covered with PANI) andPANI. Obviously, this fact demonstrates the synergisticbehavior of the sample 7 as it was also confirmed in theexample of its comparatively high EME absorption (seeabove, Table 2)

4 Discussion and conclusions The obtained hyster-esis loops of the nanocomposite films with Fe3O4 nanopar-ticles (pristine or covered with PANI) (Figs. 5 and 6) showthat squareness kp of the sample 7 is less than that of samples3 and 5 and tends to the value being characteristic ofthe samples 4 and 6, and 8 with less loading of Fe3O4. Thislogically moves us to an assumption that a small squarenesscoefficient is a feature of the samples with a large distancebetween magnetic nanoparticles. Therefore this allows us toconclude that addition of PANI–DBSA as a separate filler tomagnetic nanocomposite of PVDF filled with Fe3O4/PANI–DBSA (sample 7) leads to the fact that the magneticnanoparticles are better separated and distributed in thepolymermatrix due to a specific interaction between chargedPANI shells with large DBSA anions at the surface of Fe3O4

nanoparticles and the same counterparts in a separate phaseof PANI–DBSA filler. As a consequence, a magneticinteraction between these nanoparticles becomes weaker.On the other hand, due to this separation the interfacebetween magnetic nanoparticles and PANI–DBSAincreases. Obviously, these interactions change the state ofPANI–DBSA at the interface comparatively with the state ofPANI–DBSA in the composite bulk. This supposition agreeswith the above-mentioned changes in structure of energylevels of PANI–DBSA being responsible for the samplesconductivity.

Magnetic nanoparticles are monodomain, i.e., they donot contain domain walls. This means that they possess asignificant uncompensated magnetic moment [21]. In thisrespect their behavior is analogous to the behavior ofmagnetic ions of transition and rare-earth metals. In the lattercase, when exchange interaction is present, magneticmoments rearrange and this leads to the appearance offerromagnetism and open hysteresis loop with a coefficientof squareness kp> 0. If the energy of exchange interaction isless than the energy of heat motion, ferromagnetismdisappears and a substance displays only paramagneticproperties, i.e., magneticmoment appears only in an externalmagnetic field. In this case magnetic hysteresis does notoccur and magnetization is proportional to intensity of theapplied external magnetic field. Similarly, the magneticproperties of magnetic nanoparticles and based on themclusters are described in terms of superferromagnetism andsuperparamagnetism. Here a magnetic dipole–dipole inter-action plays the role of an exchange interaction [22].Depending on the ratio between superferromagnetic and



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Figure 7 Hysteresis loops of the magnetic nanopowder Fe3O4

with and without PANI–DBSA coating.

superparamagnetic contributions to thewholemagnetizationand on the nanocomposite anisotropy, the kp value can varybetween 0 and 1. When the magnetic cluster size decreasesits magnetic energy drops and approximates to the energy ofheat movement that causes an enhancement of the super-paramagnetic contribution into magnetization. This resultsin a decrease in the kp value, that is observed for the samples4, 6, 7, and 8. In turn this decrease indicates a diminution ofan average size of the clusters and, respectively, displays anincrease in the nanocomposite dispersivity.At the same time,the increase in the cluster size leads to a growth in itsmagnetic energy and to a corresponding enhancement of itssuperferromagnetic properties. The hysteresis loop herebecomes more rectangular and kp increases. This is the caseof the samples 3 and 5 and powders of Fe3O4 and Fe3O4/PANI. These considerations suggest that the kp value can beconsidered as a measure of the dispersivity of a magneticnanocomposite.

Comparing results of radiooptical and magneticmeasurements, one can deduce that composite films ofPVDF matrix embedding two separate phases of PANInanoparticles and Fe3O4/PANI nanoparticles differ by theirproperties from those films, which include a phase of only ofpristine Fe3O4 nanoparticles or Fe3O4/PANI nanoparticles.An increase in dispersivity leads to the growth of the totalsurface of the magnetic nanoparticles interacting with aseparate PANI phase. As a consequence, the absorbingability of the nanocomposite material increases by a factorof about 5. It is possible that presence of array of conduct-ing shells at magnetic nanoparticles here plays a significantrole.

Therefore, addition of PANI as separate filler to thecomposite PVDF film containing Fe3O4/PANI facilitates theincrease in dispersivity of the magnetic filler and growth ofthe EME absorption.

At the same time, nanopowder of Fe3O4 both with andwithout PANI coating displays high values of kp (Fig. 7).


This testifies to the presence of large aggregates of thenanoparticles in the both cases due to interaction of theirPANI shells.

Acknowledgements This work is executed due to projectM/175-2007 of the Ministry of Science and Education of Ukraine.

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