a novel approach for mixing zno nanoparticles into poly(ethyl methacrylate)
TRANSCRIPT
Communication
A Novel Approach for Mixing ZnONanoparticles into Poly(ethyl methacrylate)a
Mukesh Agrawal,* Nikolaos E. Zafeiropoulos, Smrati Gupta,Ekaterina Svetushkina, Jurgen Pionteck, Andrij Pich, Manfred Stamm*
M. Agrawal, N. E. Zafeiropoulos, S. Gupta, E. Svetushkina,J. Pionteck, M. StammLeibniz-Institut fur Polymerforschung Dresden e.V., Hohe Strasse6, 01069 Dresden, GermanyFax: þ49-351-4658281; E-mail: [email protected];[email protected]. PichInstitut fur Makromolekulare Chemie, Technische UniversitatDresden, Zellescher Weg 19, 01069 Dresden, GermanyN. E. ZafeiropoulosCurrent Address: Department of Materials Science & Engineering,University of Ioannina, GreeceS. GuptaCurrent Address: Institut fur Makromolekulare Chemie,Technische Universitat Dresden, Zellescher Weg 19, 01069Dresden, GermanyA. PichCurrent Address: DWI an der RWTH Aachen e.V., Pauwelsstr. 8,52056 Aachen, Germany
a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.
A novel and versatile approach for the mixing of ZnO nanofillers into a host polymer matrix,poly(ethyl methacrylate) (PEMA), is reported. Firstly, ZnO nanoparticles are deposited onto thesurface of polystyrene (PS) colloidal particles in a ‘‘raspberry-like’’ fashion and subsequentlyobtained PS/ZnO composite particles are mixed into the PEMA matrix in the range of 0.5 to5wt.-%. Microscopic analyses reveal a homogenous distribution of PS/ZnO domains into thePEMA matrix even at 5wt.-% loading level. Thermogravimetric analysis and differentialscanning calorimetry results indicate an improvement in thermal stability of PEMA matrixafter mixing with PS/ZnO filler particles. A significant enhancement in mechanical propertiesof PEMA matrix in the presence of PS/ZnOparticles has been evidenced by dynamic mech-anical analysis and three point bending measure-ments.
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Introduction
In recent years, organic–inorganic nanocomposites have
emerged as interesting materials for a wide spectrum of
applications because of their unique combination of
advantages, for example, characteristics of organic compo-
nents (flexibility, low dielectric constant and easy proces-
sability) and inorganic components (rigidity, durability,
and thermal stability).[1] A great deal of the work has been
done on the preparation of a variety of polymer-inorganic
nanocomposites, which possess interesting mechanical,[2]
electrical,[3] optical,[4] and magnetic[5] properties usually
superior to those of the parent polymer or inorganic species.
In general, two strategies have been employed for the
fabrication of nanocomposite materials, which allow a
versatile design of their physical and chemical properties to
meet the needs of the final end user application. The first
approach involves mixing of nanoparticles into a polymer
matrix by means of physical methods including melt
blending, solid grinding, soaking in liquids or gas diffusion.
In these processes, physical interactions such as van der
Wall’s forces, Lewis acid-base interactions or electrostatic
interactions are the driving forces for the binding of
nanoparticles with polymer matrices.[6] The second
DOI: 10.1002/marc.200900584 405
M. Agrawal et al.
406
approach involves either in-situ preparations of nanopar-
ticles inside the polymer matrices[7] or polymerization of
the monomers in the presence of inorganic nanoparticles.[8]
Physical methods are easy to handle but not efficient to
disperse the filler homogenously in polymer matrices
because of the high surface energy of nanoscale particles.
On the other hand, chemical methods offer the advantage of
producing a variety of nanocomposites owing to the
diversity of chemical synthesis with strong chemical
interactions between the components but suffer from
some limitations including difficulty in control of particles
size or contamination by unreacted educts or byproducts,
and so on. Therefore, seeking new strategies for the
preparation of nanocomposite materials, which allow for
a good dispersion of filler nanoparticles without the above
mentioned difficulties are currently of great significance.
In this study, we report on a novel approach of the mixing
of filler particles into a polymer matrix. Firstly, filler
nanoparticles are decorated on the surface of colloidal
beads in a ‘‘raspberry-like’’ fashion and subsequently the
resulting organic-inorganic composite particles are mixed
into a host polymer matrix. In the described study, this
approach has been demonstrated by decorating the ZnO
nanoparticles on polystyrene (PS) colloidal beads followed
by the mixing of resulting PS-ZnO composite particles into
the poly(ethyl methacrylate) (PEMA) matrix. This approach
not only ensures the thermodynamically favorable mixing
of ZnO nanoparticles into the PEMA matrix because of the
good compatibility of low molecular weight PS (used as the
core of PS/ZnO composite particles) with PEMA matrix,[9]
but also changes the nanoscale mixing of the very small
ZnO nanoparticles (4–5 nm) into the micro scale mixing of
PS/ZnO composite particles (0.1mm), reducing the possibi-
lities of the aggregation of ZnO nanoparticles in the
polymer matrix. To the best of our knowledge, this is the
first report on the exploitation of latex particles as filler-
carrier for the preparation of nanocomposite materials.
However, some studies have been reported on the
improvement of the physical properties of polymer
matrices by mixing polystyrene beads as filler particles.[10]
Experimental Part
Materials
Sodium dodecylsulfate (SDS) (98 wt.-%), zinc acetate dihydrate
(Zn(Ac)2 �2H2O) (99 wt.-%), 2-propanol (99.5 wt.-%), NaOH (98 wt.-%)
and ethanol (99 wt.-%) were purchased from Aldrich and have been
used without additional purification. PEMA was also purchased
from Aldrich and re-precipitated from THF using distilled water as
precipitating agent before use.
Synthesis of PS/ZnO Composite Particles
Polystyrene colloidal particles were synthesized by surfactant free
emulsion polymerization as described elsewhere.[11] In order to
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achieve PS beads of smaller diameter (80 nm), SDS (2 wt.-% of
styrene) was added into the reaction media at the beginning of the
polymerization process. To achieve the PS/ZnO composite particles,
a controlled hydrolysis of Zn(Ac)2 �2H2O salt was carried out in the
presence of PS beads as described in our previous study.[12] In a
typical process, 0.22 g (1 �10�3M) of Zn(Ac)2 �2H2O salt was added
into 80 mL of 2-propanol and resulting mixture was stirred for 1 h at
55 8C. Then 5 g dispersion of polystyrene latex particles (with
10 wt.-% solid content) were added into the reaction mixture
followed by vigorous stirring for 20 min. The reaction mixture was
cooled to 20 8C and 2 mL of 0.2 M aqueous NaOH solution were
added. After stirring for another 20 min at 40 8C, the solvent was
removed under reduced pressure and the obtained PS/ZnO particles
were washed 3 times with distilled water through centrifugation
and dried in a vacuum oven at 25 8C.
Preparation of PEMA-PS/ZnO Nanocomposites
PS/ZnO composite particles were mixed with a dispersion of re-
precipitated PEMA powder using water as a common dispersing
agent. For this purpose, 1 g of the re-precipitated PEMA powder was
added into 150 mL of distilled water and sonicated by an ultrasonic
tip for 20 min to get a good dispersion. Similarly, the calculated
amount of the PS/ZnO composite particles (0.5–5 wt.-% of PEMA
matrix) were added into 50 mL of water and sonicated for 15 min.
Subsequently, both water dispersions were mixed with each other
and the resulting mixture was agitated by an ultrasonic tip to
achieve good mixing of the components with each other. After
30 min, the dispersion was filtered and the obtained cake was dried
in a vacuum oven at 55 8C for 3 d. The obtained dried powders were
turned into discs of 25 mm diameter and 1 mm thickness by means
of compression molding at 105 8C.
Characterization
Particle size measurements were carried out with a Zetasizer 2000,
Malvern Instruments. The average value of at least 3 measure-
ments was taken as the size of composite particles. The powdery
samples were moulded into discs of 25 mm diameter and 1 mm
thickness by using a PO-Weber 10-HS Gard 3 compression molding
machine under a load of 50 kN at 105 8C. Three point bending
measurements were performed according to the ISO 178 with a
DSM Micro-5 (Holland) testing machine under the constant load of
10 N. Dynamic mechanical analysis was carried out on a 2980 DMA
V1.7B instrument by using a single cantilever clamp at 1 Hz
frequency and a heating rate of 3 K �min�1. Differential scanning
calorimetry (DSC) measurements were carried out on a DSC Q 1000;
TA Instruments at 10 K �min�1 heating rate in nitrogen atmo-
sphere. Thermo-gravimetrical analyses (TGA) were performed with
TGA 7 (Perkin-Elmer) instrument at heating rate of 5 K �min�1 in
the 25–700 8C temperature range in air and nitrogen atmospheres.
Atomic force microscopy (AFM) images were taken with a
Dimension 3100 microscope (Digital Instruments, Inc., Santa
Barbara, CA). Samples were prepared by embedding the cuts of
the disc into the resin and then smoothing them with an ultra-
microtome. Transmission electron microscopy (TEM) images were
obtained on a Zeiss Omega 912 at 120 kV. Samples were sectioned
into thin slices of 100 nm thickness by an ultra-microtome.
DOI: 10.1002/marc.200900584
A Novel Approach for Mixing ZnO Nanoparticles into . . .
Figure 1. AFM phase images of a) neat PEMA (4mm�4mm) andb) PEMA-PS/ZnO nanocomposite film filled with 5 wt.-% PS/ZnOparticles (10mm� 10mm). TEM images of c) PEMA and PEMA-PS/ZnO nanocomposite films loaded with d) 0.5, e) 1, f) 2.5, and g)5 wt.-% PS/ZnO particles.
Results and Discussion
PS/ZnO composite particles with ‘‘raspberry-like’’ morphol-
ogy were prepared by the controlled precipitation of ZnO
nanoparticles on the surface of ß-diketone functionalized
PS beads. A detailed description of the preparation and
characterization of these particles can be found in our
previous study.[12] The PS/ZnO composite particles used in
this study were 80 nm in diameter with the ZnO content of
9.6 wt.-%. ZnO nanoparticles are attached on the surface of
PS beads by means of electrostatic interaction. Owing to the
high polarity of the C¼O groups of the ß-diketone, highly
electron negative O atoms interact with the electron
deficient Zn of the ZnO nanoparticles. The PEMA was
selected as the model host matrix because it possesses a
glass transition temperature (62–65 8C) below that of the PS
core (100–110 8C) of composite particles, which allowed the
processing of PEMA-PS/ZnO nanocomposites without
deformation of the PS/ZnO filler particles. Moreover, PEMA
has been reported to have a good compatibility with low
molecular weight PS.[9,13] The raspberry-like morphology of
PS/ZnO particles allows the exposure of a fraction of the
surface of PS core with the host matrix and thus facilitates
the distribution of filler particles into PEMA matrix. PEMA-
PS/ZnO nanocomposites have been prepared by mixing the
PS/ZnO particles with re-precipitated PEMA powder in
dispersion followed by the compression moulding of the
dried powder mixture at 105 8C. The schematic presentation
of the preparation and mixing of PS/ZnO particles into
PEMA matrix is shown in Scheme 1.
The distribution of PS/ZnO composite particles in PEMA
matrix has been investigated by microscopic analyses.
Figure 1a and b shows AFM phase images of the neat PEMA
matrix and PEMA-PS/ZnO nanocomposites with 5 wt.-%
filler content, respectively. These results reveal that the
surface of a slice from PEMA matrix is quite smooth with a
rms (root mean square) roughness of 3.4 nm. On the
contrary, nanocomposites with filler particles are very
rough indicating the presence of the embedded PS/ZnO
composite particles within the PEMA matrix. As expected, a
drastic increase in the rms roughness to 175 nm has been
observed after the loading of 5 wt.-% filler particles. In
addition, Figure 1b reveals that PS/ZnO domains are
homogenously distributed into the PEMA matrix. A closer
inspection of this image indicates presence of the fine pores
Scheme 1. Preparation of PS/ZnO composite particles and mixinginto the polymer matrix.
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in the polymer matrix (marked by arrows), which probably
appeared because of the removal of PS/ZnO particles,
loosely associated with the surface of sectioned slice of the
sample, by the diamond knife during microtoming. The
observed pore sizes are in the range of 80–100 nm, which
succinctly demonstrates the presence of single and
spherical PS/ZnO nanoparticles in the PEMA matrix.
However, large aggregates with 450 nm size can also be
observed. In order to investigate the effect of the filler
contents on the resulting domain size of filler particles,
samples were characterized with TEM analysis.
Figure 1c–g shows the TEM micrographs of the PEMA
matrix loaded with different weight fractions of PS/ZnO
composite particles. Unlike to the neat PEMA matrix,
presence of the dark areas in TEM micrographs of PEMA-PS/
ZnO nanocomposites can be noticed revealing the incor-
poration of ZnO nanoparticles in PEMA matrix. Moreover,
one can observe a gradual increase in the average domain
size of PS/ZnO composite particles from 160 nm to 450 nm
with increasing filler content from 0.5 to 5 wt.-%. These
aggregates consist of loosely associated individual 80 nm
PS/ZnO composite particles and clusters are aggregating
into the ramified rather than compact structures. It can be
noticed that apparent dark areas are not spherical in shape,
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M. Agrawal et al.
408
which can be correlated to the cutting of the samples with a
diamond cutter during the TEM sample preparation. This
deviation from spherical shape further increases the mean
size of the aggregates and thus appeared clusters do not
seem to be composed of more than 2–3 PS/ZnO particles.
The influence of filler particles on the thermal stability of
the PEMA matrix has been investigated by thermo
gravimetric analysis (TGA). Figure 2a shows typical TGA
scans for PEMA with and without PS/ZnO particles taken in
air and nitrogen atmospheres. The TGA curve of neat PEMA
matrix taken in nitrogen atmosphere shows a main
degradation stage in the temperature range of 330 to
485 8C with a maximum weight loss rate at 385 8C. It can be
attributed to the unzipping of monomers and oligomers
initiated by random scission of polymer chains. It is
noticeable that degradation of PEMA in air begins at 288 8Cand seems to proceed in a one-reaction stage. A comparison
of the degradation behaviour of neat PEMA in air and
nitrogen atmospheres reveals that the oxidative random
Figure 2. a) TGA analysis of neat PEMA (solid line) and PEMA-PS/ZnO nanocomposites loaded with 5 wt.-% filler particles (dottedline) in air and nitrogen atmospheres. b) DSC scans of PEMAmatrix loaded with different weight fractions of PS/ZnO compo-site particles a) 0, b) 0.5, c) 1, d) 2.5, and e) 5 wt.-%.
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scission is significantly enhanced compared with the
thermally initiated one as the temperature at maximum
weight loss is shifted by 75 8C.
The TGA scan of the PEMA-PS/ZnO nanocomposite is
nearly identical to that of the neat PEMA in nitrogen
atmosphere. However, the temperatures at which degrada-
tion of the polymer begins and the maximum weight loss
occurs, have both been observed to be increased by 5 and
10 8C, respectively, for the PEMA-PS/ZnO nanocomposite
with 5 wt.-% filler particles. Presence of the PS/ZnO
composite particles has been found to significantly increase
the oxidative thermal stability also. One can observe from
Figure 2a that mixing of the 5 wt.-% PS/ZnO particles into
the PEMA leads to an increase in its maximum weight loss
temperature by 32 8C. A comparison of the oxidative
thermal stability of the PEMA-PS/ZnO nanocomposites
loaded with various filler contents is depicted and discussed
in Supporting Information 1.
DSC traces of neat PEMA and PEMA-PS/ZnO nanocom-
posites filled with different amounts of composite particles
are shown in Figure 2b. In order to obtain a precise and
quantifiable measure of the shift in glass transition
temperatures of nanocomposites with loaded amounts of
PS/ZnO particles, we used the points of inflection obtained
from the maxima in the first derivative curves of heat flow
versus temperature curves (Figure 2b). The pure PEMA
matrix shows glass transition temperature at 75 8C while
PEMA-PS/ZnO nanocomposites exhibit it in the range of 76–
77 8C. Pu et al.[14] also reported only a slight increase in Tg of
a polymer matrix after addition of the silica particles. It is
known from the literature that an increase in filler content
can cause the Tg of a composite to increase, decrease, or stay
unchanged depending on the specific polymer-filler
system.[15]
To analyze the effect of the filler concentration on the
mechanical properties of the nanocomposites, samples
have been characterized by the three point bending
analysis and dynamic mechanical analysis. As shown in
Figure 3a, a significant improvement in the flexural
properties of the PEMA matrix has been observed after
the mixing of filler particles. The highest enhancement
(215%) in the flexural stiffness has been achieved at the
loading of 2.5 wt.-% PS/ZnO composite particles. However, a
further increase in filler content from 2.5 to 5 wt.-% led to a
decrease in the flexural strength of the PEMA matrix, which
can be ascribed to the agglomeration of PS/ZnO particles at
the highest loading level. Nevertheless, it was still 145%
higher than that of neat PEMA.[16] These results are
consistent with the observations made by the Pu et al.[14]
It should be underlined here that employed approach
enables us to achieve a significant increase in flexural
properties of the polymer matrix in the presence of a small
amount of filler particles. Harper at al.[17] reported a
marginal increase of 12 MPa in flexural strength of PEMA
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A Novel Approach for Mixing ZnO Nanoparticles into . . .
Figure 3. a) Three point bending measurements and b) DMAanalysis of PEMA matrix filled with different weight fractionsof PS/ZnO composite particles a) 0, b) 0.5, c) 1, d) 2.5, ande) 5 wt.-%.
matrix by mixing of 40 wt.-% hydroxyapetite powder. In
another study, Tee et al.[18] improved the flexural strength
of epoxy resin only by 45 MPa by mixing of about 35 wt.-%
Ag nanoparticles. On the contrary, in the described
approach an introduction of only 0.24 wt.-% ZnO nanopar-
ticles (2.5 wt.-% PS/ZnO with 9.6 wt.-% ZnO) could lead to
the improvement in flexural strength by 215%. This can be
attributed to the effective mixing of ZnO nanoparticles into
PEMA matrix.
Figure 3b shows the variation in storage moduli and tan d
as a function of temperature for the PEMA matrix filled with
different amounts of PS/ZnO particles. These experiments
were performed from �100 to 150 8C at a frequency of 1 Hz.
As expected, the storage moduli of all the samples were
found to decrease with increasing the temperature because
of the softening of the matrix and initiation of relaxation
processes and melting.[19] Furthermore, it can be seen that
the addition of the PS/ZnO composite particles into the
PEMA matrix results in the increase in the storage modulus
in the glassy state (T< Tg). This leads to the conclusion that
PEMA matrix can be reinforced in the glassy state by
addition of PS/ZnO composite particles. We believe that the
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presence of the filler particles induces the reduction in the
chain mobility and the deformation of the PEMA matrix. In
addition, it can be seen that this improvement in storage
modulus is proportional to the added contents of the filler
particles from 0.5 to 2.5 wt.-%. In agreement with the three
point bending analysis, a further increase in filler level to
5 wt.-% leads to the sharp decrease in modulus, which can
be ascribed to the large domain size of filler particles.
Similar results have been reported by Trabelsi et al.[19] for
titanium-oxo cluster based hybrid materials. Presence of
large agglomeration in polymer matrix renders the high
free volume to the polymer chains, present around the filler
domains offering the easiness in their mobility. In a marked
contrast, above the Tg no effect of the filler particles was
observed.[20] This can be attributed to the softening of the PS
core of the filler particles at higher temperature.
In order to further investigate the gravity of the
employed approach, we investigated the mechanical
properties of the PEMA matrix loaded with neat ZnO
nanoparticles and results are shown in Supporting
Information 2. One can clearly observe from these data
that incorporation of 0.24 wt.-% neat ZnO nanoparticles led
to more than 20% decrease in storage modulus of PEMA
matrix, which can be attributed to their high degree of the
agglomeration in host PEMA matrix. On the contrary,
mixing of the PS stabilized ZnO nanoparticles at same
loading level (2.5 wt% PS/ZnO particles with 9.6 wt% ZnO
content) resulted into a 28.3% increase in storage modulus,
succinctly demonstrating that the employed approach can
result in a better dispersion of ZnO particles and hence offer
an enhancement in mechanical properties of host polymer
matrix.
As shown in Figure 3b, variation in tan d as a function of
temperature revealed an intense peak, which can be
associated with the glass–rubber transition of the amor-
phous phase of the PEMA matrix. The temperature
corresponding to the maximum of this peak can be
assigned to the glass transition temperature (Tg) of
the polymer matrix. For all samples, the glass transition
temperature has been observed at 88 8C. However, this
value is higher than those obtained by the DSC measure-
ments of PEMA-PS/ZnO nanocomposites (as shown in
Figure 2b). This can be attributed to the fact that DMA
measures the glass transition temperature on the basis of
extrinsic mechanical properties, unlike to the DSC which is
based on the intrinsic heat capacity. Similar results have
been reported by Bilyeu et al.[21] for fiber reinforced epoxy
resins. In addition, one can observe that tan d values of all
the PEMA-PS/ZnO nanocomposites have been found to be
smaller than that of pure PEMA matrix. This can be
attributed to the so-called ‘‘volume effect,’’ which suggests
that filler particles reduce the effective volume of host
polymer matrix.[22] Since the mechanical loss of rigid ZnO
nanoparticles is much smaller than that of the PEMA
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M. Agrawal et al.
410
matrix, a net decrease in the tan d is observed with an
increase in filler content. Interestingly enough, all the
composite materials exhibit a damping factor of tan
d >0.45. Usually polymeric materials with tan d >0.3 are
considered as having very good damping properties.[23] The
broadening of the glass transition peak can be attributed to
the distribution of the mobility (i.e., relaxation times) of
polymer segments.[24] In addition, it can be taken as a
measure of the degree of the structural heterogeneity of the
system. It is evident from Figure 3b that the composites are
more heterogeneous as compared to the neat polymer and
that the degree of heterogeneity increases with an increase
in the filler contents, especially at 2.5 and 5 wt.-%.
Conclusion
In summary, a new protocol for introducing inorganic filler
particles into a host polymer matrix has been demon-
strated. A significant improvement in mechanical and
thermal properties of the host polymer matrix has been
observed at a low level, i.e., 0.05–0.24 wt.-% of inorganic
filler content. Although this approach has been illustrated
by taking into consideration the ZnO as filler and PEMA as
the host matrix, however it is speculated that same protocol
can be exploited for mixing a wide range of filler particles
into a suitable polymer matrix.
Acknowledgements: We acknowledge the help from Dr. RudigerHaßler, Mr Holger Scheibner, Dr. Liane Haußler, Mr Axel Menschand Mr Andreas Janke for TGA, three point bending measure-ments, DSC, TEM and AFM analyses, respectively. Authors arethankful to Mrs Uta Reuter for ultra-microtoming of the samplesand Dr. Konrad Schneider for fruitful discussions.
Received: August 14, 2009; Revised: October 12, 2009; Publishedonline: December 3, 2009; DOI: 10.1002/marc.200900584
Keywords: colloids; filler; mixing; nanocomposites; nano-particles
Macromol. Rapid Commun. 2010, 31, 405–410
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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