IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 18 (2007) 335704 (8pp)
doi:10.1088/0957-4484/18/33/335704
Flexible high-loading particle-reinforced
polyurethane magnetic nanocomposite
fabrication through
particle-surface-initiated polymerization
Zhanhu Guo1 , Sung Park1 , Suying Wei1 , Tony Pereira1 ,
Monica Moldovan2, Amar B Karki2 , David P Young2 and
H Thomas Hahn1
1
Multifunctional Composites Lab, Mechanical & Aerospace Engineering Department and
Materials Science & Engineering Department, University of California Los Angeles,
Los Angeles, CA 90095, USA
2
Department of Physics and Astronomy, Louisiana State University, Baton Rouge,
LA 70803, USA
E-mail: [email protected]
Received 1 May 2007, in final form 25 June 2007
Published 25 July 2007
Online at stacks.iop.org/Nano/18/335704
Abstract
Flexible high-loading nanoparticle-reinforced polyurethane magnetic
nanocomposites fabricated by the surface-initiated polymerization (SIP)
method are reported. Extensive field emission scanning electron microscopic
(SEM) and atomic force microscopic (AFM) observations revealed a uniform
particle distribution within the polymer matrix. X-ray photoelectron
spectrometry (XPS) and differential thermal analysis (DTA) revealed a strong
chemical bonding between the nanoparticles and the polymer matrix. The
elongation of the SIP nanocomposite under tensile test was about four times
greater than that of the composite fabricated by a conventional direct mixing
fabrication method. The nanocomposite shows particle-loading-dependent
magnetic properties, with an increase of coercive force after the magnetic
nanoparticles were embedded into the polymer matrix, arising from the
increased interparticle distance and the introduced polymer–particle
interactions.
(Some figures in this article are in colour only in the electronic version)
mechanical, magnetic and conductive properties [5–9]. High
particle loading and flexibility of the composites are required
for certain applications, such as electromagnetic wave
absorbers [10, 11], photovoltaic cells [12], photodetectors and
smart structures [13–15]. However, high particle loading
always causes brittleness and rigidity of the nanocomposite,
and even introduces artificial defects, such as air voids, which
limits its applications.
The reported methods for incorporating inorganic nanoparticles into polymeric matrices include ex situ methods, i.e. dispersion of the synthesized nanoparticles into an aqueous or organic polymeric solution [16–18], in situ monomer polymer-
1. Introduction
Magnetic nanoparticles (NPs) have attracted much interest
due to their special physicochemical properties, such as
enhanced magnetic moment [1] and larger coercivity [2],
which are different from their bulk and atomic counterparts [3].
Incorporation of the inorganic nanoparticles into a polymer
matrix has extended the particle applications (for example,
as high-sensitivity chemical gas sensors [4]).
This is
primarily due to the advantages of polymeric nanocomposites
possessing high homogeneity, flexible processability and
tunable physicochemical properties, such as improved
0957-4484/07/335704+08$30.00
1
© 2007 IOP Publishing Ltd Printed in the UK
Nanotechnology 18 (2007) 335704
Z Guo et al
ization methods in the presence of the nanoparticles [19–23]
or in situ nanoparticle formation in the presence of the polymer [6]. The interactions between the polymer and the
nanoparticles for the ex situ formed composites are normally
steric interaction forces, van der Waals forces or Lewis acid–
base interactions. However, in situ synthetic methods can create strong chemical bonding within the nanocomposites and are
expected to produce more-stable and higher-quality nanocomposites [16–18]. Nevertheless, the former has the advantage of
being able to utilize a large variety of nanoparticles that have
become available recently.
The existing challenge in the composite fabrication
is to provide a high tensile strength due to local stress
within the nanocomposite. In other words, the response
of a material to an applied stress is strongly dependent
on the nature of the bonds within its microstructure.
The interfacial interactions between nanoparticles and the
polymer matrix play a crucial role in determining the
quality and properties of the nanocomposites [16]. The
poor bonding linkage between the fillers and the polymer
matrix such as the composites made by simple mixing [16]
will introduce artificial defects, which consequently result
in a deleterious effect on the mechanical properties of the
nanocomposites [24]. Introducing good linkages between
the nanoparticles and the polymer matrix is still a challenge
for specific composite fabrication [16–18].
However,
appropriate chemical functionalization of the nanofiller surface
by introducing proper functional groups could improve both
the strength and toughness, with an improved compatibility
between the nanofillers and the polymer matrix [25], and
make the nanocomposites stable in harsh environments as
well [26]. Thus, surface functionalization of nanoparticles
with a surfactant or a coupling agent is important not only to
stabilize the nanoparticles [27] during processing but also to
render them compatible with the polymer matrix.
In this paper, the nanoparticle surface-initiated polymerization (SIP) method [28–30] was adopted to fabricate highloading Fe2 O3 nanoparticle-reinforced polyurethane magnetic
nanocomposites. This method utilized the physicochemical adsorption of an initiator onto the iron-oxide (Fe2 O3 ) nanoparticle surface for urethane polymerization in a tetrahydrofuran
(THF) solution. Strong chemical bonding between nanoparticles and polymer matrix was formed without any additional
surfactant or coupling agent. The obtained nanocomposites
exhibit high flexibility as compared with the brittle and rigid
counterparts obtained from the conventional direct mixing
(DM) method used for the micron particle (iron carbonyl) reinforced polyurethane composites. The particle loading can be
tuned up to 65 wt%.
O
O
C N R N
C
diisocyanate
HO R OH
diol
Chart 1. Chemical formulae of the two-part polyurethane monomers
used.
propyl ketone) and accelerator (polyurethane STD-102, containing 1 wt% organotitanate and 99 wt% acetone) were provided by CAAP Co. Inc. Iron oxide (γ -Fe2 O3 , Nanophase
Technologies) nanoparticles with an average diameter of 23 nm
and a specific surface area of 45 m2 g−1 were used as
nanofillers for the nanocomposite fabrication. All the chemicals were used as-received without further treatment.
2.2. Nanocomposite preparation
The nanocomposites were fabricated by two different methods.
One was the conventional direct mixing (DM) used currently
for the micron particle (iron carbonyl) reinforced polyurethane
composites and the other was based on the surface-initiated
polymerization (SIP) approach. The fabrication procedures
were as follows. In the DM method, 7.7 g monomers,
1.03 g catalyst and 1.42 g accelerator were added into 30 ml
tetrahydrofuran (THF) with ultrasonic stirring for half an hour.
Then nanoparticles were added into the above solution and
ultrasonically stirred for 10 min. The suspended solution was
poured out into a mould container for curing and evaporation
of solvent at room temperature.
Surface-initiated polymerization (SIP) was used to
provide physicochemical adsorption of the initiator onto the
iron-oxide (Fe2 O3 ) nanoparticle surface in a tetrahydrofuran
(THF) solution. This method utilizes physicochemically
adsorbed moisture (as seen from the FT-IR spectrum in
figure 4) on the nanoparticles as a linking site between the
nanoparticle and polymer chain. Figure 1 illustrates the entire
SIP process. The nanoparticles were added into the catalyst
and THF promoter solution. The above nanoparticle suspended
solution was then sonicated for about 30 min (figure 1, step 1).
The sonication energy should enhance the reactivity of the
surface and promote the adsorption of catalyst and promoter
onto the nanoparticle surface while evaporating the physically
adsorbed moisture. The monomers were then introduced
into the above solution dropwise within half an hour and the
polymerization was continued for another 6 h (figure 1, step 2).
The final solution was poured into a mould and the solvent
allowed to evaporate naturally at ambient conditions. The
obtained composite was then pressed at 127 ◦ C and 10 psi for
10 min in a hot press (figure 1, step 3).
2. Experimental details
2.1. Materials
2.3. Characterization
The polymeric matrix used was a commercial, clear
polyurethane coating (CAAPCOAT FP-002-55X, manufactured by the CAAP Co., Inc.), which contains two-part urethane monomers, i.e. 80 wt% diisocyanate and 20 wt%
diol; the chemical structures are shown in chart 1. The liquid resin has a density of 0.83 g cm−3 . Polyurethane catalyst (a liquid containing ∼20–65 wt% aliphatic amine, 1–
50 wt% parachlorobenzotrifluoride and 10–35 wt% methyl
The physicochemical attachment of catalyst–accelerator (CA)
mixture onto the nanoparticle surface was initially verified
by thermo-gravimetric analyses (TGA, Perkin Elmer). The
sample was prepared by dispersing the nanoparticles into
THF, adding the catalyst and accelerator, stirring for half an
hour and washing the precipitated nanoparticles with excessive
THF to remove the extra CA. TGA was carried out from
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Nanotechnology 18 (2007) 335704
Z Guo et al
Physically
adsorbed moisture
Step 1.
NP
5.1 g
NP
NP
+
C: 1.03g
NP
P:1.42 g
Ultrasonic bath: 30min
Step 2.
Physicochemical
adsorption of catalyst
and promoter
P
NP
Fe 2O 3
7.7g monomers
Ultrasonic bath: 6 Hours
Step 3.
THF
NP
Fe2O3
NP/PU
composite
SIP NP/PU
Aluminum mold:
3 days
o
Hot press: 130 C
10 minutes
Figure 1. Scheme of the surface-initiated polymerization process.
25 to 600 ◦ C with an argon flow rate of 50 ccpm (cubic
centimetres per minute) at a heating rate of 10 ◦ C min−1 . The
thermal stability of the nanocomposites with different loadings
was characterized by TGA. Polyurethane chemically bound
onto the nanoparticle was determined by TGA and derivative
thermal gravimetric (DTG). The samples were polymerized
nanocomposites in solution after excessive THF washing.
Fourier transform infrared (FT-IR) spectra were recorded
in a FT-IR spectrometer (Jasco, FT-IR 420) in transmission
mode under dried nitrogen flow of 10 ccpm. The FT-IR
samples were prepared by mixing with powder KBr, grinding
and compressing into a pellet.
The as-received nanoparticles, nanoparticles treated with
CA and the Fe2 O3 /PU nanocomposites underwent surface
analysis with an Axis 165 x-ray photoelectron spectrometer
using a monochromatized Al Kα (1486.6 eV) x-ray source
with a power of 150 W. Both survey and high-resolution
spectra were obtained using pass energies of 40 eV. Curvefitting of the high-resolution spectra of all the samples
involved in the Fe2 O3 /PU nanocomposite fabrication was
used to investigate the binding between the polymer and the
nanoparticle surface. The curve fitting was performed using
a sum of Gaussian–Lorentzian profile peak shapes (GL(30))
after subtraction of a linear background.
Atomic force microscope (AFM) (multimode, Digital
Instruments, Veeco Company) in tapping mode was utilized
to characterize the dispersion quality of nanoparticles in
the polyurethane matrix. The probes used were tappingmode etched silicon probes with resonant frequency of about
300 kHz and spring constant of around 40 N m−1 . The
samples were prepared by embedding the nanocomposite in
a cured vinyl-ester-resin tab and polishing the nanocomposite
cross section. Particle dispersion in polyurethane was further
characterized with a scanning electron microscope (SEM,
JEOL field emission scanning electron microscope, JSM6700F). The SEM sample was the same as that used in the
AFM analysis except that a thin gold coating was sputtered
to improve the electrical conductivity needed for high-quality
images. Transmission electron microscopy (TEM, JEOL 2000)
was used to characterize the morphology of the nanoparticles.
The samples were prepared by dropping the nanoparticlesuspended THF solution onto a holey carbon coated copper
grid and drying naturally.
The mechanical properties of the fabricated nanocomposites were evaluated by tensile tests following an American Society for Testing and Materials standard (ASTM, 2002, standard D 412-98a). The samples were prepared according to the
standard. A crosshead speed of 15 mm min−1 was used and
strain (mm mm−1 ) was calculated by dividing the crosshead
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Nanotechnology 18 (2007) 335704
Z Guo et al
Figure 2. TGA analysis of the as-received Fe2 O3 nanoparticles,
mixture of the catalyst and accelerator (CA), and the CA-treated
nanoparticles.
Figure 4. FT-IR spectra of the as-received Fe2 O3 nanoparticles, neat
PU and the Fe2 O3 /PU nanocomposite.
of the nanoparticles remaining on the wall as shown in figure 3.
However, the 6 h cured nanocomposite-particle-suspended
THF solution turns red and has no effect on the vial. In other
words, the polymerization turned the brown nanoparticles into
red and the wall still kept translucent white with a trace of
red residue when the vial was emptied. The colour change
after polymerization is an indicator of the interaction between
the nanoparticles and the polymer matrix. The hydrogen
bonding between the moisture on the as-received nanoparticle
surface (NP–OH) and the PET wall (C=O) favours the stable
attachment of nanoparticles onto the wall. On the other side,
the polyurethane coating on the particle has a very weak
hydrogen bond and favours affinity to the THF rather than
to the PET wall. The different interactions between the
nanoparticle and the different polymers can also explain the
quick sedimentation of as-received nanoparticles in THF and
the stable suspension of the nanocomposite particles in THF.
This is also consistent with the polymer serving as a stabilizer
for certain nanoparticles in some solvents.
Figure 4 shows the FT-IR spectra of the as-received
Fe2 O3 nanoparticles, neat cured polyurethane and the prepared
Fe2 O3 /polyurethane nanocomposites. The peak at 3429 cm−1
corresponds to the OH stretching band, indicating that the asreceived nanoparticles were partially hydrolyzed, consistent
with the TGA observation. The density of the hydrolyzed
OH group was estimated to be 16 μmol m−2 based on the
weight loss from TGA and the particle-specific surface area.
The estimated value is of the same magnitude as that of
nanoparticles with a comparable size and specific surface area
of ZrO2 (∼20 μmol m−2 ) and Al2 O3 (∼10 μmol m−2 ) [31].
The spectrum of the Fe2 O3 /PU nanocomposite was observed
to have the peaks characteristic of both Fe2 O3 (e.g. polyhedral
Fe3+ –O2− stretching vibrations [32] in the range of 450
and 800 cm−1 in both the as-received nanoparticles and the
prepared nanocomposites as seen in figure 4) [33] and of
polyurethane (e.g. amide II bands in the range of 1570–
1515 cm−1 due to the CO–NH motion, which is seen in
both the neat polyurethane and the nanocomposites). The
relative intensity of the peaks of polyurethane and ironoxide nanoparticles was observed to be different from the
Figure 3. As-received Fe2 O3 nanoparticles remaining on the PET
vial wall (left) and composite nanoparticles almost completely
removed from the PET vial wall (right); chemical structure of PET.
displacement by the gauge length. The effect of the polymer
matrix on the magnetic properties of the nanoparticles was investigated in a 9 T physical properties measurement system
(PPMS) by Quantum Design.
3. Results and discussion
Figure 2 shows the thermal behaviour of the as-received
nanoparticles, mixture of the catalyst and the accelerator (CA),
and the nanoparticles treated with CA. The CA mixture was
observed to get completely decomposed at around 200 ◦ C
and the as-received nanoparticles show continuous weight
loss arising from the dehydration of the physicochemically
adsorbed moisture. However, the CA-treated nanoparticles
showed a similar weight loss as the CA mixture by itself.
The delayed complete decomposition temperature in the CA
attached onto the nanoparticle surface indicates the chemical
adsorption of CA mixture on the nanoparticle surface, and that
the presence of the nanoparticles increased the stability of CA
with a higher complete-decomposition temperature.
When the as-received nanoparticle suspended THF
solution was removed from a polyethylene terephthalate (PET)
vial, the wall turned from translucent white to brown as a result
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Nanotechnology 18 (2007) 335704
Z Guo et al
Figure 5. XPS O 1s spectra of the as-received Fe2 O3 NP, CA-treated
Fe2 O3 nanoparticles and the final PU–Fe2 O3 nanocomposite.
Figure 6. TGA of pure polyurethane and composites made by
conventional and surface-initiated polymerization methods measured
at different sample locations (centre and edge); left inset shows
corresponding DTG and right inset shows the bright-field TEM
microstructures of the treated nanoparticles (scale bar 190 nm).
Table 1. Deconvolution data of O 1s high-resolution spectra of the
as-received Fe2 O3 nanoparticles, CA-treated Fe2 O3 nanoparticles
and the PU–Fe2 O3 nanocomposite. The relative sensitivity factor
(RSF) is 0.78 for O 1s.
Peak
Mass
Position
conc. (%)
(eV)
RSF Fe2 O3 NPs
1 O 1s 527.5
2 O 1s 529.9
3 O 1s 531.5
0.78 11.70
0.78 58.54
0.78 29.76
from the ultrasonic stirring. As a result, the intensity of the
peak at 527.5 eV for the Fe–O–Fe contribution was observed
to increase dramatically. The absorbed catalyst accelerator
(CA) by the nanoparticles was observed to have a significant
effect on the thermal stability of the nanoparticles as shown
in figure 1 of the TGA analysis. The intensity of the Fe–O–
Fe decreased after the polymerization in solution, which is
due to the polyurethane coating on the nanoparticle surface.
The corresponding peaks at 529.9 and 531.5 eV were observed
to be relatively higher. The –OH and moisture parts were
observed to have relative higher intensities arising from the
presence of diols. The enhanced peak at 531.5 eV is due to
the polyurethane carbonyl groups (C=O).
The transmission electron microscope (TEM) brightfield microstructure (inset of figure 6) of the nanoparticles
after polymerization in THF for 6 h showed a size of
26 nm, similar to the reported as-received nanoparticles
(Nanophase Technologies), indicating that no Oswald ripening
phenomena [37] happened during the composite fabrication.
Figure 6 depicts the weight percentage loss from different
locations of the nanocomposite sample (centre and edge)
in TGA. The uniform particle loading was indicated in the
SIP samples as observed by the almost overlapping weight
percentage variation. However, a dramatic difference was
observed in the DM samples. This shows that the SIP method
improves particle dispersion as compared with the composites
fabricated with the DM method.
Different thermal behaviours of pure polyurethane and
composites fabricated by SIP and DM, respectively, were
further noticed in the derivative thermal gravimetric (DTG)
curves shown in the inset of figure 6. The difference
was due to the present status of the polymer, i.e. free
chains in the pure polyurethane, weak interaction in the
DM Fe2 O3 /PU nanocomposite and strong interaction in the
SIP Fe2 O3 /PU nanocomposite. Only one peak is observed
in pure polyurethane, and the existence of nanoparticles
promotes the decomposition of bulk polyurethane with a lower
Mass
Mass
conc. (%)
conc. (%)
Fe2 O3 NPs AC PU–Fe2 O3
63.01
29.86
7.13
29.36
50.24
20.40
corresponding spectra of polyurethane and iron oxide. This
indicates a physicochemical interaction between the two
components, iron-oxide nanoparticles and neat polyurethane.
Figure 5 shows the high-resolution XPS spectra and
table 1 summarizes quantification data for O 1s of the asreceived Fe2 O3 nanoparticles, CA-treated Fe2 O3 nanoparticles
and the final Fe2 O3 /PU nanocomposite, respectively. Three
oxygen components were observed in all three spectra with
varying ratios. The oxygen high-resolution spectrum can
be deconvolutionalized into three components. The O 1s at
531.5 eV is from the adsorbed moisture (H2 O) or carbonyl
groups (C=O) after the polymerization, O 1s at 529.9 eV
originates from hydrolyzed OH groups and O 1s at 527.5 eV
comes from the Fe–O–Fe group [34–36]. In the spectra of
the as-received nanoparticles, the peaks at 531.5 and 529.5 eV
reflect the presence of the partially hydrolyzed nanoparticles
and physically adsorbed moisture. The spectra still have three
characteristic peaks after the nanoparticles were treated with
CA under ultrasonication. However, the intensity (counts
per second, cps) of different peaks is completely different
from the as-received nanoparticles as shown in table 1. The
moisture portion at 531.5 eV in the nanoparticles decreased
(O 1s = 531.5 eV, reduced from 29.76% to 7.13%) due to
the ultrasonic stimulated natural evaporation. The hydrolyzed
–OH (at 529.9 eV) amount was also observed to decrease
dramatically due to the condensation of the hydrolyzed groups
from within the nanoparticles. The evaporation of the most
physically adsorbed moisture and the condensation of the
hydrolyzed FeO–OH was due to the local hot-spots arising
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Nanotechnology 18 (2007) 335704
HO
OH
NP
HO
Z Guo et al
+
O
O
C N R N
C + HO R OH
OH
HO
diisocyanate
diol
HO
OH
NP
HO
HO
O
O H
O
C N R N
C
O R O
n
H
OH
polyurethane
Scheme 1. Nanocomposite formation by the SIP method.
Figure 7. AFM tapping mode phase images of the 65 wt% composites fabricated by (a) the DM method and (b) the SIP method.
dispersion quality shown in figures 8(a) and (c) was observed
with extensive SEM examinations. The observed spherical
particles in the nanoscale within the polymer matrix are a little
larger than the iron-oxide nanoparticles due to the polymer
wrapping. As shown in figures 8(b) and (d), larger clusters
with agglomerated nanoparticles wrapped with a continuous
polymer matrix were observed for the nanocomposite by the
conventional DM method. This indicates that the SIP method
favours the high-quality nanocomposite fabrication due to the
polymer layer chemically bound to the nanoparticle during the
SIP process in solution, which prevented agglomeration via the
steric hindrance forces.
Large cracks were observed in the nanocomposites
fabricated by a conventional DM method as shown in the inset
of figure 9, whereas no sign of cracking were observed in the
pure polyurethane and Fe2 O3 /PU nanocomposites fabricated
by the SIP method, especially in the high particle loading. The
latter two specimens were much more flexible than the first.
The flexibility of high-loading nanocomposites was further
quantitatively evaluated by the tensile test. Figure 9 shows
the tensile stress–strain curves of the Fe2 O3 /PU composites
fabricated from the DM and the SIP method, respectively. The
Young’s moduli and tensile strengths are almost the same for
both composites. However, the elongation is about four times
greater than that of the DM composite. The strong chemical
bonding between nanoparticles and polyurethane, polymer
structure variation [14] with particle addition, and uniform
decomposition temperature as observed in the DM sample.
However, SIP composites show three different peaks. The first
two peaks are from the bulk matrix and the intermediate phase,
and the latter one is from polyurethane, chemically attached
to the nanoparticle surface, respectively. This observation
indicates that the SIP method further favours good bonding
between nanoparticles and the polyurethane matrix except for
a more uniform particle distribution, as indicated by the XPS
and TGA analysis.
All the above analyses indicate the physicochemical
adsorption of catalyst and accelerator (CA) on the nanoparticle
surface and the subsequent surface-initiated polymerization
for composite fabrication. Scheme 1 shows the reaction
of the composite formation. Diisocyanate monomers with
the aid of the catalyst attached to the particle surface
reacted with physicochemically attached moisture and further
copolymerized with diols to form a composite unity.
Particle dispersion within the polymer matrix was further
analysed by microscopic observation. Figure 7 shows the
typical atomic force microscope (AFM) phase image under
tapping mode of a composite with 65 wt% loading. Clear
agglomeration is observed in the composite fabricated by the
conventional DM method, similar to the reported extrusion
process [14]. However, discrete nanoparticles without obvious
agglomeration are observed in the SIP composite, indicating
good particle dispersion consistent with the TGA analysis.
Here, scanning electron microscopy was further utilized to
study the particle dispersion quality. Similar uniform particle
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Nanotechnology 18 (2007) 335704
Z Guo et al
Figure 8. Scanning electron microscope micrographs of the composites by the SIP method: (a) magnification 220 000 and (c) 44 000; and the
DM method: (b) low magnification 22 000 and (d) high magnification 44 000.
Figure 10. Magnetic hysteresis loop of the nanocomposite by SIP
and pure Fe2 O3 nanoparticles (inset shows the enlarged loop at low
field).
Figure 9. Tensile stress–strain curves of 65 wt% nanocomposites
fabricated by the DM and SIP methods; insets show the optical
micrographs of composites by DM (left), SIP (right) and pure
polyurethane (middle).
100 Oe) materials [38]. With the magnetic nanoparticles
uniformly dispersed into the polyurethane matrix, Hc increased
as compared with the pure nanoparticles assembly, while
maintaining the same saturation magnetization (Ms ). This
indicates that the incorporation of nanoparticles into the
polymer matrix in the polymer nanocomposites induced the
materials much harder. The coercivity is reported to be
strongly dependent on the size of particles and to be the largest
in the nanoparticles with a single-domain size (166 nm for
the γ -Fe2 O3 ) [38]. In other words, the coercivity decreases
when the nanoparticles are larger or smaller than this singledomain size. The increased coercivity as shown in the inset
of figure 10 is attributed to the decreased interparticle dipolar
particle distribution within the polymer matrix contribute to the
observed flexible behaviour in the SIP composites.
The effect of the polymer matrix on the magnetic property
of the nanoparticles was investigated in a 9 T physical
properties measurement system by Quantum Design. Figure 10
shows the hysteresis loops of the pure as-received Fe2 O3
nanoparticles and the SIP Fe2 O3 /PU nanocomposites with a
loading of 65 wt%, respectively. The observed non-zero values
of the coercive force (coercivity, Hc ) in both the as-received
nanoparticles and the polymeric nanocomposite indicate the
ferromagnetic state. Coercivity was used to classify the
materials into soft (less than 100 Oe) and hard (larger than
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Nanotechnology 18 (2007) 335704
Z Guo et al
interaction arising from the increased interparticle distance
within the single-domain as compared with the close contact
of the pure nanoparticles, and also attributed to the polymer–
particle interfacial effect [39] due to the large portion of the
atoms exposed to the polymer.
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nanoparticle filled polyurethane nanocomposites 10th Joint
MMM/Intermag Conf. (Baltimore, MD)
[41] Guo Z, Park S, Hahn H T, Wei S, Moldovan M, Karki A B and
Young D P 2007 J. Appl. Phys. 101 09M511
4. Conclusion
In conclusion, flexible magnetic Fe2 O3 /PU nanocomposites
with high particle loading were fabricated by utilizing
a surface-initiated polymerization (SIP) method. Surface
engineering the nanoparticles with the SIP method effectively
improves the quality of the high particle loading Fe2 O3 /PU
nanocomposite with a uniform distributed particle and strong
chemical bonding. As a result, high flexibility was observed
in the SIP nanocomposites, which renders possible coating
industrial applications in the areas of electromagnetic wave
absorbers and communication systems. The reported SIP
nanocomposite fabrication method is general and has the
potential to be used in other nanoparticle and polymer
systems. Flexible nanocomposites filled with higher saturation
magnetization zero-valence metallic nanoparticles, such as
metallic iron, have been achieved. The initial results show
fairly well selective electromagnetic reflection loss in the
SIP fabricated nanocomposite [40, 41] and make them good
candidate materials for giant magnetoresistance (GMR) sensor
fabrication [15].
Acknowledgments
This work was supported by the Air Force Office of
Scientific Research through grant FA0550-05-1-0318 managed
by Dr B Les Lee. DPY kindly acknowledges the support from
the National Science Foundation under grant no. DMR 0449022.
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