a novel approach for mixing zno nanoparticles into poly(ethyl methacrylate)

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.

Macromol. Rapid Commun. 2010, 31, 405–410

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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



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.



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,

www.mrc-journal.de 407

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.-%.



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

DOI: 10.1002/marc.200900584

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



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

www.mrc-journal.de 409

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



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DOI: 10.1002/marc.200900584

a novel approach for mixing zno nanoparticles into poly(ethyl methacrylate)

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