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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: fmachado@unb.brF. 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,
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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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J. S. Neves, F. G. de Souza, Jr., P. A. Z. Suarez, A. P. Umpierre, F. Machado
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
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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:
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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-%)
Polystyrene/Fe3O4 (4.8 wt-%)
Polystyrene/Fe3O4 (10.1 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.
In situ Production of Polystyrene Magnetic Nanocomposites through . . .
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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
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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
Polystyrene/Fe3O4 (1.7 wt-%)
Polystyrene/Fe3O4 (4.8 wt-%)
Polystyrene/Fe3O4 (10.1 wt-%)
Figure 5. XRD patterns of the polymer samples.
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J. S. Neves, F. G. de Souza, Jr., P. A. Z. Suarez, A. P. Umpierre, F. Machado
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
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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.%).
In situ Production of Polystyrene Magnetic Nanocomposites through . . .
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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
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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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J. S. Neves, F. G. de Souza, Jr., P. A. Z. Suarez, A. P. Umpierre, F. Machado
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.
In situ Production of Polystyrene Magnetic Nanocomposites through . . .
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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
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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
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Figure 12. Magnetic behavior of the polymeric material samples. Magnetic force (A) and (C); Relative magnetic force (B) and (D).
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J. S. Neves, F. G. de Souza, Jr., P. A. Z. Suarez, A. P. Umpierre, F. Machado
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.
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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
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In situ Production of Polystyrene Magnetic Nanocomposites through . . .
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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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