In situ Production of Polystyrene Magnetic Nanocomposites through a Batch Suspension Polymerization...

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In situ Production of Polystyrene MagneticNanocomposites through a Batch SuspensionPolymerization Process

Juliete S. Neves, Fernando G. de Souza, Jr., Paulo A. Z. Suarez,Alexandre P. Umpierre, Fabricio Machado*

This work presents the synthesis of micro-sized polystyrene magnetic beads by in situincorporation of oleic acid-modified Fe3O4 magnetic nanoparticles via a suspension polymeri-zation process. Fe3O4 nanoparticles with superparamagnetic characteristics were obtainedthrough a coprecipitation technique. These particles present an average diameter equal to7.4� 4.6 nm, as determined by AFM. This result is in agreement with the crystallite size ofsingle domains calculated by using Scherrer’s equation, which was equal to 7.7 nm, based onXRD measurements. The obtained materials were also studied using TGA. The weight lossbehavior was independent of the Fe3O4 content and the stability to the thermal degradationwas also not improved bymagnetic nanoparticles present in the composite. Polystyrene/Fe3O4

magnetic nanocomposites exhibited the same diffraction peaks observed in the pure Fe3O4,which indicates that nanoparticles inside the composites preserved the structure and proper-ties of pure Fe3O4. It was also shown that nanosizedpolystyrene particles, dispersed in the aqueous phase,are obtained due to the stabilization effect of the oleicacid on the styrene droplets. A cross-section of poly-styrene magnetic particles showed empty sphericalregions, attributed to the encapsulation of watermicrodroplets during the polymerization reaction.The obtained polymeric materials also presented goodmagnetic behavior, indicating that the modified Fe3O4

nanoparticles were successfully dispersed in the poly-styrene particles.

F. Machado, J. S. Neves, P. A. Z. Suarez, A. P. UmpierreInstituto de Quımica, Universidade de Brasılia, CampusUniversitario Darcy Ribeiro, CP 04478, 70910-900 Brasılia, DF,BrazilFax: þ55 61 3273 4149; E-mail: [email protected]. G. de Souza, Jr.Instituto de Macromoleculas Professora Eloisa Mano – IMA/UFRJ,Universidade Federal do Rio de Janeiro, Cidade Universitaria, CP68525, Rio de Janeiro CEP 21945-970, Brazil

Macromol. Mater. Eng. 2011, 296, 1107–1118

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Introduction

Nanotechnology is afield thathas attracted theattentionof

many researchers in recent years. When related to the field

of nanocomposite materials, the idea of enhancing proper-

ties and improving features of materials through the

creation of multiple-phase nanocomposites is widely

used in order to produce efficient materials for several

technological application purposes, as e.g., medicine

elibrary.com DOI: 10.1002/mame.201100050 1107

1108

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J. S. Neves, F. G. de Souza, Jr., P. A. Z. Suarez, A. P. Umpierre, F. Machado

and healthcare, information technology, biotechnology,

manufacturing, nanoelectronics, national energy, and

security.[1–3]

Magnetic particles are extensively used for the immo-

bilization of biological protein and enzyme, enzyme-linked

immunosorbent assays, magnetizable implants for tar-

geted drug delivery, immunomagnetic separations, nanop-

robes for biomedical imaging in vivo, environmental and

analysis, stimuli-responsive systems, magnetic recording,

catalysis, ultrahigh frequency microelectronic application,

magnetic refrigeration, electromagnetic wave absorbers,

contrast agents in magnetic resonance imaging, and

hyperthermia agents.[4–13]

Arruebo et al.[7] have published a review article, where

the potential use of magnetic micro- and nanoparticles for

drug delivery is explored, addressing issues related to the

intravenous, oral, subcutaneous and intratumoral admin-

istrations, therapy, diagnosis, influence of the magnetic

field, and toxicity.

Recently,Medeiros et al.[13] have performed an extensive

survey of some potential stimuli-responsive magnetic

materials employed in biomedical applications. This work

focuses on the features related to the response of stimuli-

responsivepolymers to changes inprocessvariables suchas

temperature, pH, electric, and magnetic field, in combina-

tion with magnetic properties of nanoparticles. Addition-

ally, theuse thesestimuli-responsivematerials asmagnetic

resonance imaging agents,mediators of hyperthermia; and

for drug delivery, chemotherapy, and cancer treatment

purposes are explored. The reader is referred to theseworks

for more detailed information.

Several kinds of magnetic nanoparticles presenting

different magnetic metals can be used for magnetic

purposes, as for instance, magnetite (Fe3O4), maghemite

(g-Fe2O3), iron cobalt oxide (CoFe2O4), cobalt zinc ferrite

(Co0.5Zn0.5Fe2O4), barium ferrite (BaFe12O19), MnZn

ferrite, NiZn ferrite, strontium ferrite (SrFe12O19),

Zn0.6Cu0.4Cr0.5Fe1.5–xLaxO4 (0 � x � 0:06) ferrites, andman-

ganese perovskite La1–xSrxMnO3 (0:2 � x � 0:3).[9–11,14–21]

In spite of the large number of magnetic nanoparticles,

special attention is given to superparamagnetic magnetite

(Fe3O4)nanoparticlesdue to its remarkable features, suchas

biocompatibility, low toxicity, low susceptibility to

changes due to oxidation, magnetic retention only when

exposed to an external magnetic field and strong ferro-

magnetic behavior.[22–25]

Themagnetic nature of the polymeric material is closely

associated to the magnetic nanoparticle size. For practical

reasons, superparamagnetic nanocomposites are useful in

several technological applications, as particle aggregation

induced by magnetic attraction is avoided during proces-

sing of the material.[23]

Several experimental methods can be employed to

preparation of magneto-polymeric materials, for instance,

Macromol. Mater. Eng. 20

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solvent evaporation followed by grinding the polymer into

small particles, bulk, suspension, dispersion, microemul-

sion, miniemulsion, and emulsion polymerizations, poly-

mer-pyrolysis route.[14,20,26–34] The success of the produc-

tion of magnetic nanocomposites depends fundamentally

on the compatibility of themagnetic nanochargeswith the

polymeric matrix. In the particular case of hydrophobic

polymeric matrix, the surface modification of the hydro-

philic nanomagnetic particles plays a key role in the

morphology of the final magneto-polymeric material. The

synthesis of polymeric materials presenting both the

regular morphology and the good magnetic features in

heterogeneous aqueous processes can successfully be

accomplished, as described elsewhere.[35–40]

Joumaa et al.[34] have demonstrated the use of mini-

emulsion polymerization system to encapsulate iron oxide

nanoparticles into submicrometer polystyrene particles.

According to the observed experimental results, the

adopted experimental procedure based on the miniemul-

sion polymerization was not able to encapsulate the g-

Fe2O3 nanoparticles, leading to the both the inhomoge-

neous distribution of magnetic nanoparticles in the

polymeric matrix and the phase segregation, probably

due to aggregation phenomenon of the maghemite

nanoparticles as a result of the low compatibility between

the compatibilizing agent used tomodify the nanoparticles

surface and the thermoplastic matrix of polystyrene.

Montagne et al.[41] have also illustrated the undesirable

formation of polystyrene/g-Fe2O3 particles presenting a

hemisphere-likemorphology during emulsion polymeriza-

tions of styrene. According to the authors, the polymer

particleswith irregularmorphology showing aggregates of

maghemite are obtained due to the ionic nature of the

hydrosoluble initiator, which significantly influences the

interfacial properties of particles dispersed in the poly-

merization medium.

Horak et al.[42] have published an interesting review

article focusing on the main aspects related to the

experimental methods for the preparation of magneto-

polymeric materials and properties of magnetic particles

intended to be used for biological and environmental

separations, and the reader is encouraged to read the

original reference for more detailed information.

Water-based process like suspension polymerization can

be regarded as an environmentally friendly reaction

system, being widely employed to manufacture polymeric

materials due to several advantages, such as low levels of

impurities and additives in the polymer and easy separa-

tion of the polymer particles, avoiding undesirable

problems of water wastage and contributing to the

recycling of this process stream.[43] Additionally, polymer-

izations performed in suspension processes are character-

ized by presenting easy removal of the heat of reaction,

easy temperature control. This polymerization process

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Figure 1. Photographs of (A) Fe3O4 and (B) surface modified Fe3O4nanoparticles dispersed in ethanol and styrene, respectively.

In situ Production of Polystyrene Magnetic Nanocomposites through . . .

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allows for the formation of micro-sized spherical and very

simple control of both the average size and the distribution

of particle size, low cost of separation of the polymer beads

and low viscosity of the reaction medium.

It is well-known that in suspension polymerization

processes each aqueous-dispersed monomer droplets

behaves as a bulk microreactor.[43] For this reason, the

surfacemodification of Fe3O4magnetic particles with oleic

acid permits a good dispersion of these inorganic nano-

chargeswithin the final polymeric particles, which leads to

the formation of materials with uniformmagnetic proper-

ties.

Somedesirable characteristics such as spherical particles

with regular morphology, low density, control of final

particle size distribution, uniform surface area for coating

purposes, etc. are only attained when heterogeneous

polymerization processes like suspension process are

employed.

This work focuses on the synthesis of micro-sized

polystyrene magnetic beads by in situ incorporation of

surface modified Fe3O4 nanoparticles through suspension

polymerization process intended for biotechnological

applications.

Experimental Section

Distilledwaterwas used as the continuous phase. Styrene (Sigma–

Aldrich Brasil Ltda, Sao Paulo, Brazil) was used as monomer in the

polymerizations. The suspending agent [poly(vinyl alcohol) (PVA),

DENKA POVAL B-24] with a degree of hydrolysis of 86–89% and

viscosity in the range of 40–48 mPa � s was kindly supplied by

DENKA, Tokyo, Japan. The initiator [benzoyl peroxide (BPO),

LUPEROX 78]with aminimumpurity of 99.4%was kindly donated

by Arkema Quımica Ltda, Sao Paulo, Brazil. Nitrogen was supplied

by White Martins Ltda, Rio de Janeiro, Brazil, with 99.5% purity.

Sodium hydroxide (NaOH) with purity of 97% was provided by

F. MAIA (Rio de Janeiro, Brazil), ferric chloride hexahydrate

(FeCl3 � 6H2O) with purity of 97% and ferrous sulfate heptahydrate

(FeSO4 �7H2O) with purity of 99% were provided by VETEC (Rio de

Janeiro, Brazil), hydrochloric acid (HCl, 36.5–38.0% w/w) was

providedby ISOFAR (Sao Paulo, Brazil) andoleic acid extrapurewas

provided by Merck (Rio de Janeiro, Brazil). All chemicals were used

as received, without further purification.

Synthesis of Magnetic Nanoparticles

Magnetite (Fe3O4) nanoparticles were prepared by chemical

coprecipitation of aqueous Fe2þ and Fe3þ salt solution and

NaOH solution, as described elsewhere.[44,45] Initially, 6.1 g of

FeCl3 �6H2O and 3.1 g of FeSO4 �7H2O were dissolved in 125mL of

distilled water and 5mL of hydrochloric acid and heated to 60 8Cwith bubbling of nitrogen gas. In order to guarantee stoichiometric

formationof the Fe3O4, itwasused amolar ratio of Fe2þ–Fe3þ equal

to 2.0. Separately, 37.5 g of NaOH was dissolved in 625mL of

distilled water, followed by heating to 60 8C with bubbling of

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nitrogen gas. The salts and basic solution were mixed under

vigorous stirring. The temperature was kept constant and equal to

60 8C for 30min under atmosphere of nitrogen to keep the reaction

environment free of oxygen. The resulting black magnetic

nanoparticles were isolated by magnetic decantation and washed

with distilled water until neutral pH. After that, the material was

washed three times with ethanol. Then, the magnetic nanopar-

ticles were sonicated for 30min and kept dispersed in ethanol (see

Figure 1).

Surface Modification of Magnetic Nanoparticles

The surface of Fe3O4 nanoparticleswasmodifiedbyusing oleic acid

as reported elsewhere:[31,44,46] 5 g of magnetic nanoparticles were

dispersed with mechanical stirring in 170mL of distilled water

under nitrogen and heated to 85 8C. Then, 5.6mL of oleic acid were

added dropwise at a constant rate 0.5 mL �min�1 under an inert

nitrogen atmosphere in order to keep the reaction environment

free of oxygen to avoid oxidation of the magnetic particles. After

that, the resulting mixture was maintained under mechanical

stirring for 30min at 85 8C. The surface modified Fe3O4 magnetic

nanoparticles were washed several times with distilled water,

followed by washing with ethanol. Then, the resulting surface

modified Fe3O4 were dispersed in styrene and finally sonicated for

30min (see Figure 1).

Batch Suspension Polymerizations: In Situ Formation

of Magnetic Nanocomposites

The polymerization reactions were carried out in a 250-mL glass

reactor (Quickfit, England) at 85 8C, with a total organic load of

25wt.%, under an inert nitrogen atmosphere in order to maintain

the reaction environment free of oxygen. A hotplate IKAC-MAGHS

7 (IKA Works, Inc.) equipped with an integrated temperature

control, includingaPt1000 temperature probewasused toheat the

reaction medium. The reactor was also equipped with a reflux

condenser, which was linked to cold water feed stream.

Initially, the reactorwas fedwith distilledwater, containing the

specified amount of suspending agent (PVA). The systemwas kept

under isothermal conditions with a constant agitation of

1 000 rpm. Themechanical agitator (Fisatom 713, Sao Paulo, Brazil)

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Table 1. Basic recipe for styrene polymerizations.

Species Weight [g]

Distilled water 195.0

Styrene 65.0

Fe3O4 nanoparticles 0.0–6.5

LUPEROX 78 2.28

DENKA POVAL B-24 0.98

Stirred: 1 000 rpm Temperature: 85 8C

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was equipped with a helix-type impeller. When the desired

temperaturewas reached, the reagentsand the initiator (BPO)were

added and the reaction conducted for �4.5 h. The basic polymer-

ization recipe is presented in Table 1.

Material Characterization

The thermal stability of materials was determined by DTG

measurements on a Thermogravimetric Analyzer Shimadzu

DTG-60 (Shimadzu Scientific Instruments, Maryland, USA) at

heating rates of 10 8C �min1, under nitrogen atmosphere with a

flow rate of 50 mL �min�1.

The X-ray diffraction (XRD) patterns of the polymeric materials

were determined on a Bruker D8 FOCUS X-ray diffractometer

(Bruker AXS, Inc., Wisconsin, USA), using CuKa radiation (l¼1.5418 A, 40 kV, and 30mA) under the same conditions. The

diffractionpatternswere obtained in the angular range 2� 2u�80

with steps of 0.058 at a rate of 0.258 �min�1.

EnergydispersiveX-rayspectroscopy (EDX)measurementswere

carried out on an EDX-720 fluorescence spectrometer (Shimadzu

Europa GmbH, Duisburg, Germany). The EDX spectra of the Fe3O4/

polystyrenenanocompositeswereacquiredwithstepsof0.02anda

live time of 100 s.

Atomicforcemicroscopy(AFM)wasperformedusingaNanoSurf

EasyScan 2 (Nanosurf Inc., Massachusetts, USA) in dynamic mode.

AFM image was treated with help of Gwyddion (GNU General

Public License, Czech Metrology Institute.) software in order to

determine the particle size distributions of the magnetic particles.

Particle morphology and particle size distributions analyses

were performed by using a Carl Zeiss microscope Axiophat (Carl

Zeiss MicroImaging, Inc., New York, USA), equippedwith AxioCam

ICc3 camera and Plan Neofluar objectives, using dried polymer

samples. Additional particle size distribution characterizations

were also performed through light scattering analyses using a

particle size analyzer Zetasizer Nano-S equipmentModel ZEN1600

(Malvern Instruments Ltd, Worcester, UK).

The surface morphology of polymer particles was analyzed

through scanning electron microscopy (SEM). The images were

recordedwithanFEIQuanta200 ScanningElectronMicroscope (FEI

Company, Oregon, USA).

Magnetic force measurements were carried out by using a

conventional analytical balance and a digital caliper, as illustrated

elsewhere.[23,47] According to the experimental methodology, two

removable supports were placed on the plate of the balance and



outside the balance. The distance between the lower and higher

topswere equal to 25.9� 0.2mm.Material sampleswere placed on

the support located on the plate of the balance. Then, the balance

was unset and a neodymium N42 magnet with an external Gauss

strengthequal to13.2 kgandenergydensityof42MGOewasplaced

on the higher support. Mass variation of the sampleswas recorded

andmagnetic force opposite to gravitational forcewas determined

given by:

11, 296

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FM ¼ Dm � g (1)

where FM is the magnetic force, Dm corresponds to the apparent

variation ofmass in the presence of themagnetic field, and g is the

acceleration of gravity.

The relative magnetic force is determined as follows:

RFM ¼ FMfNP

(2)

where RFM is weight fraction of nanoparticles dispersed in the

polystyrene matrix.

Magnetic susceptibility wasmeasured as volume susceptibility

using a Johnson Matthey magnetic susceptibility balance (Penn-

sylvania, USA), as described elsewhere.[48] The mass susceptibility

(xg) of the samples was calculated by the following equation:

xg ¼ CB � L � R�Roð Þ109 �m (3)

where CB is the calibration constant of the balance (equal to 1 166),

L the length of sample in cm (L> 1.5 cm),m is the mass of sample

expressed in grams, R corresponds to the balance reading for

sample in tube and Ro is the balance reading of empty tube.

Results and Discussion

Figure 2 shows both the AFM image of the Fe3O4

nanoparticles and the particle size distribution. As can be

seen the Fe3O4 nanoparticles present an average particle

diameter equal to 7.4� 4.6 nm. Based on the average

particle size, it is reasonable to expect that Fe3O4

nanoparticles present a superparamagnetic behavior,

feature considered as the first requirement for biomedical

purposes,[7,22] which indicates that magneto-polymeric

materials, such as polystyrene/Fe3O4 produced in thiswork

are very suitable for biological applications.

These magnetic nanocomposites present great potential

to be used in in vitro biomedical separation, purification,

diagnosis applications, and as support for immobilization

of biomolecules. When in vivo biomedical applications are

considered, Fe3O4/polystyrene nanocomposites may be

employed as contrast agents in magnetic resonance

imaging, in nanodiagnosis, ultrasonic imaging, and mag-

netic drug delivery. One has to keep in mind that the

spherical nature (regular morphology ensure easy flow of

the polymer particles via the catheter and proper occlusion

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0 100 200 300 400 500 600 700 80060

65

70

75

80

85

90

95

100

Fe3O4

Fe3O4 / Oleic Acid

Wei

ght L

oss

(%)

Temperature (°C)

Figure 3. Thermogravimetric analysis of the Fe3O4 nanoparticles.

0 100 200 300 400 500 600 700 8000

10

20

30

40

50

60

70

80

90

100

Polystyrene

Polystyrene/Fe3O4 (1.7 wt-%)



Wei

ght L

oss

(%)

Temperature (°C)

Figure 4. Thermogravimetric analysis of the polymer samples.

Figure 2. Atomic force microscopy (AFM) analysis and particle sizedistribution of Fe3O4 nanoparticles.


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of blood vessels, which are responsible for irrigation of the

injured area) of Fe3O4/polystyreneparticlesmight allow for

intravenous administration in vascular embolization pro-

cedures to the treatment of arteriovenous malformations,

tumors and aneurysms. As an additional advantage, if used

as magnetic embolic agent, Fe3O4/polystyrene may be

successfully used for the hyperthermia treatment and

prevention.

It iswell-knownthat superparamagnetismisobserved in

nanoparticles composed of single magnetic domains,

whose particle diameters are normally below a critical

size of �25nm, depending on the ferromagnetic nature of

the material.[15,29,37,49]

Thermogravimetric evaluationof the thermal stabilityof

thematerials was carried out, as depicted in Figure 3 and 4.

Figure 3 illustrates the behavior of theweight loss observed

for both Fe3O4 and oleic acid-modified Fe3O4 magnetic

nanoparticles. In the particular case of the dried Fe3O4

nanoparticles, itwasdeterminedanweight loss of�6wt.%,

which can be attributed to the presence of adsorbed water

and ethanol.[27]

The TG thermogram of the oleic acid-modified Fe3O4

magnetic nanoparticles is also shown in Figure 3. As can be




observed, based on the weight loss due to the decomposi-

tion of the hydrophobe agent, the oleic acid content on the

nanoparticle surface is of �30wt.% (the amount of

adsorbedwater and ethanol is not considered). Considering

the carbonyl linkage distance equal to 0.122nm and

assuming that they are aligned due to the electrostatic

interactions, each single nanoparticle is covered with

�6 100 oleic acid molecules.

TG curve of oleic acid-modified nanoparticles represents

two significant weight-losing behaviors within the tem-

perature range from 130 to 300 8C and 350 to 750 8C.According to Yan et al.[40], the first reflects the release of

physisorbed oleic acid whereas the second one is related to

the oleic acid chemisorbed onto magnetic nanoparticles

surface.

According to Figure 4, polystyrene homopolymer and

Fe3O4/polystyrene nanocomposites undergo complete

thermal degradation at temperatures ranged from 250 to

500 8C (a strong weight loss was observed in the narrow

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10 20 30 40 50 60 70 80

(533)(400)

(220)

(311)

(422)

(440)(511)

Polystyrene

2θ (°)

Fe3O4




Figure 5. XRD patterns of the polymer samples.

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range of 350–450 8C due to degradation of the organic

phase). The weight fraction of nanoparticles (fNP) deter-

mined by thermogravimetric measurements was 1.7, 4.8,

and 10.1wt.%, respectively.

As can be observed the behavior of the weight loss was

independent of the Fe3O4 content and the thermal stability

was not improved by magnetic nanoparticles distributed

in the polymeric matrix. In addition, the similar behavior

observed to weight loss indicates that the polymeric

materials present average molecular weight very close,

whose values were also not dependent on the final Fe3O4

amount,as thesameexperimental conditionwasemployed

(see Table 1). It is very important to point out that all

analyseswere performedunder inert nitrogen atmosphere,

which avoid oxidation of Fe3O4 nanoparticles and conse-

quently eliminates apotential increase inweight as a result

of the magnetic nanoparticles oxidation.

Figure 5 shows typical XRD patterns of the polymeric

samples. The characteristics peaks of themagnetic particles

were observed at 2u¼ 30.2, 35.5, 43.5, 53.6, 57.3, 62.7, and

74.68, corresponding to the (220), (311), (400), (422), (511),

(440), and (533) reflections of a spinel crystal structure,

which indicates that magnetic particles are pure Fe3O4

crystalline without impurity phases.[22,23,28] The character-

istic peak of the amorphous polymeric material was found

to be equal to 2u¼ 18.48. Polystyrene/Fe3O4 nanocompo-

sites exhibits all reflections found in the pure Fe3O4, where

the intensity of the characteristics peaks increase with the

magnetic particles content, suggesting that the magneto-

polymeric materials preserve the magnetic properties of

the nanocharges of Fe3O4.

The crystallite size (jNP) of the polymeric materials was

estimated using the well-known Scherrer’s equation,

expressed as follows:

Tab

Sam

I

II

III

IV

jNP ¼k � l

b � cos uð Þ (4)

where k is a constant (equal to 1.0, value of k normally falls

in the range from 0.87 to 1.00), l the X-ray wavelength

(equal to 0.1542nm), u the Bragg diffraction angle, and b is

the full width at half maximum (FWHM) of the more

intense peak, corresponding to the (311) reflection.

le 2. Hydrophobic effect, crystallite size, crystallinity, and magne

ple Fe3O4 [wt.%] OA/styrene [wt.%]

0.0 0.0

1.7 0.7

4.8 2.2

10.1 4.8



Based on the Scherrer’s model [Eq. (3)] pure Fe3O4

presents average crystallite size equal to 7.7 nm. According

to Table 2, polystyrene/Fe3O4 nanocomposites materials

present crystallite sizes very similar to the one observed for

Fe3O4 nanoparticles. These values are in complete agree-

ment with the average particle diameter determined by

AFM. Table 2 also illustrates the crystallinity (ANP) of the

polymeric materials. It is noted that the crystallinities

presents a linear relationship with the content of Fe3O4

nanocomposites dispersed in the polystyrene matrix. The

magnetic susceptibility values measured in response to an

applied magnetic field is also showed in Table 2. Based on

the susceptibility classification, the polystyrene produced

tic susceptibility of the final products.

dPSty [mm] jNP [nm] ANP [%] xg [c.g.s]

106.6� 21.7 0.0 0.0 0.00

69.1� 17.6 4.2 2.6 0.15

54.7� 15.9 4.6 10.3 0.31

38.8� 15.7 6.9 20.0 0.38

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Figure 6. Energy dispersive X-ray spectra of the polymer particles.(A) Polystyrene/Fe3O4 (1.7wt.%); (B) polystyrene/Fe3O4 (4.8wt.%);(C) polystyrene/Fe3O4 (10.1wt.%).


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here presents a paramagnetic behavior since the magnetic

susceptibility values are positive when a magnetic field is

applied externally. As illustrated in Table 2, the magnetic

susceptibility increases as the concentration of Fe3O4

nanoparticles dispersed in the polystyrene matrix is

increased.

The energy dispersive X-ray spectra of the Fe3O4/

polystyrene nanocomposites particles observed on the

micrographs of Figure 7 show strong characteristic peaks

Ka and Kb of iron at the interval from 6.38 to 7.04 keV, as

illustrated in Figure 6. The average amount of iron in the

samples was determined to be equal to 98%. These results

indicate that the inorganic material dispersed in the

polystyrene matrix is essentially composed by Fe in the

form of iron oxide (Fe3O4). EDX analyses also show small

amounts of Si, Mn, S, and Cr, which could be attributed to

typical contaminant species normally present in the

reagents and glassware used in the synthesis of the

magnetic nanoparticles.

It is generally agreed that the particle morphology plays

an important role in the product application, because it

exerts a strong effect on the finalmaterial processability. In

this scenario, the suspension polymerization processes are

most often employed due to the capacity of synthesizing

spherical particles with good morphological features.

It is observed that the polymeric particles presenting

smooth surface and regular morphology can be obtained

independently of the amount of Fe3O4 distributed in the

polystyrene matrix, as shown in Figure 7.

From the point of view of the development of the

morphology of the polystyrene/Fe3O4 magnetic nanocom-

posites, it seems that a multi-granular particle structure

was formed, which means that a single polymer particle

might be represented by a macrostructure composed of

agglomerates of much smaller substructures, as can be

noticed more clearly in Figure 7C.

In order to enhance reliability of theanalysismethod, the

particle size distributions of polymeric materials were

determined by the measuring and counting of at least 400

particles photographed with the optical microscope.

Figure 8 presents a typical particle size distribution of the

final product. The particle size of the final polymer particle

lies in the interval of 40–160mm, considering a full range of

values observed for polystyrene homopolymer and poly-

styrene/Fe3O4 nanocomposites.

As can also be observed the particle size distribution of

thepolymericmaterials isunimodalandnarrow, indicating

that the impeller used in the polymerizations is efficient at

keeping the suspension dispersed in the reaction medium.

The average particle size diameter decreases as the

amount of Fe3O4 increases (see Table 2). This behavior takes

place probably due to the increase of the oleic acid

concentration in the reaction medium in comparison to

the organicmonomerphase. It is very reasonable to assume




that oleic acid improves stabilization ofmonomer droplets,

which normally leads to a decrease in the average particle

size. According to Table 2, the oleic acid strongly affects the

final particle diameter, as weight fraction of 0.7% accounts

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Figure 7. Morphology of the final polymeric materials. (A) Polystyrene; (B) polystyrene/Fe3O4 (1.7wt.%); (C) polystyrene/Fe3O4 (4.8wt.%);(D) polystyrene/Fe3O4 (10.1wt.%).

Figure 8. Particle size distributions of polymericmaterials. (A) Polystyrene; (B) polystyrene/Fe3O4 (1.7wt.%); (C) polystyrene/Fe3O4 (4.8wt.%).

1114 Macromol. Mater. Eng. 2011, 296, 1107–1118

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Figure 9. Scanning electron microscopy (SEM) of polystyrenemagnetic particles with 1.7wt.% of Fe3O4.

Figure 10. Particle size distribution of polystyrene dispersed in theaqueous phase as stable colloidal dispersion.

Figure 11. Cross-section of the polystyrenemagnetic particles with1.7wt.% of Fe3O4.


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for a reduction of the average diameter in 65%, when the

Experiments I and II are compared.

Figure 9A and B illustrates a typical particle size of the

polystyrene samples. In the particular case of the suspen-

sion polymerization system studied here, it was observed

the formation of a distinct population of polystyrene

particles, exemplifying the formationofverysmallpolymer

particles. The average particle size of the polystyrene

presented in Figure 9A was determined as 2:3� 1:3mm,

which is 30 times smaller than the average diameter

observed for the standard polystyrene (see Figure 8b and

Table 2). The pronounced effect of the oleic acid on the

stabilization of the styrene droplets is also illustrated in

Figure 10. In all experimental conditions evaluated, a small

amount of nanosized polystyrene particles was formed,

whose average particle diameter was determined to be

equal 281.7, 293.5, and 308.9 nm for polymer samples with

1.7, 4.8, and 10.1wt.% of Fe3O4 nanoparticles, respectively.

As additional information, it is important to say that the

amount of nanosized polystyrene dispersed in the aqueous

phase, behaving as stable colloidal dispersion, contribute to

an average conversion of 6%.

Figure 11 shows an SEM of a cross-sectioned polystyrene

magnetic particle with 1.7 of Fe3O4. It is observed the

occurrence of spherical empty regionsprobably formeddue

to the encapsulation of water microdroplets during the

polymerization reaction. Empty regions with different

diameterareobtained, as shown inFigure11B. It seems that




the encapsulated water microdroplets are surrounded by

the modified Fe3O4 nanoparticles (see Figure 7B–D).

Figure 12 shows themagnetic behavior of the polymeric

samples. As illustrated in Figure 12A and C, the magnetic

force is proportional to the amount of Fe3O4 nanoparticles

dispersed in the polystyrene matrix. When the relative

magnetic force is considered, one can observe that the

11, 296, 1107–1118


Figure 12. Magnetic behavior of the polymeric material samples. Magnetic force (A) and (C); Relative magnetic force (B) and (D).

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polymeric sample with 1.7wt.% of Fe3O4 nanoparticles

presents the best response to the employed magnetic field.

Although all magnetic polystyrene samples present the

same magnetic profile, the maximum relative magnetic

forces are relatively different, as depicted in Figure 12B

andD. The observedmagnetic behavior canbe attributed to

dispersionof theFe3O4nanoparticles inthestyrenedroplets

during the polymerization. It is important to take into

account that each polystyrene (macro)particles contains

several water microdroplets with different diameters

surrounded by the modified Fe3O4 nanoparticles. It is

reasonable to assume that the amount of magnetic

nanoparticles on the water microdroplets significantly

depends on the volume of water microdroplets, and as

consequence the concentration of Fe3O4 nanoparticles

dispersed in the polystyrene matrix can be affected.

Figure 7 and 11 clearly indicate that the water micro-

droplets exhibit different sizes,whichcancontribute for the

magnetization observed for the samples.



Conclusion

In the present work micro-sized polystyrene/Fe3O4 mag-

netic nanocomposites beadswere successfully produced in

situ through suspension polymerization process.

Itwasshownthatpolymericparticlespresenting smooth

surface and regular morphology could be obtained inde-

pendently of the amount of Fe3O4 present in the

polystyrene matrix. Magneto-polymeric particles with

multigranular structure composed of agglomerates of

much smaller substructures were also observed.

It was also observed the pronounced effect of the oleic

acid on the stabilization of the styrene droplets, leading to

formation of nanosized polystyrene particles dispersed in

the aqueous phase, presenting behavior of stable colloidal

dispersion.

Fe3O4 nanoparticleswere obtainedwith average particle

diameter equal to 7.4� 4.6 nm, determined by AFM. The

crystallite size (jNP) of the magnetic nanoparticles using

11, 296, 1107–1118



www.mme-journal.de

XRDwasestimated tobeequal to7.7 nm,agreeingverywell

with the AFM results.

All diffraction peaks found in the pure Fe3O4 appeared in

polystyrene/Fe3O4 nanocomposites, suggesting that the

magnetic composites preserve the structure of the nano-

particles, which is related to themagnetic properties of the

Fe3O4.

Cross-section of polystyrene magnetic particle showed

the occurrence of spherical empty regions, which can be

attributed to the encapsulation of water microdroplets

during the polymerization reaction. These water micro-

droplets canbedeterminant to themagneticbehaviorof the

polymeric materials as the distribution of the Fe3O4

nanoparticles may depends on the water microdroplets

diameter. The obtained polymeric materials presented

goodmagnetic behavior, indicating that themodified Fe3O4

nanoparticles were successfully dispersed in the polystyr-

ene particles.

Acknowledgements: The authors thank Coordenacao de Aper-feicoamento de Pessoal de Nıvel Superior (CAPES), ConselhoNacional de Pesquisa e Desenvolvimento (CNPq), and Fundacao deAmparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) for thefinnantial support and the scholarships. Particularly, F. M. thanksArkema for providing benzoyl peroxide and DENKA for providingPoly(vinyl alcohol), and Laboratorio de Processos comMembranas(Prof. Helen Ferraz) and Laboratorio de Sistemas Coloidais eDispersoes (Prof. Montserrat Fortuny) for SEM and light scatteringanalyses, respectively.

Received: February 4, 2011; Revised: April 18, 2011; Publishedonline: July 4, 2011; DOI: 10.1002/mame.201100050

Keywords: magnetic nanoparticles; nanocomposites; polystyr-ene; suspension polymerization

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In situ Production of Polystyrene Magnetic Nanocomposites through a Batch Suspension Polymerization...

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