nano-level mixing of zno into poly(methyl methacrylate)

Full Paper

Nano-Level Mixing of ZnO into Poly(methylmethacrylate)a

Mukesh Agrawal,* Smrati Gupta, Nikolaos E. Zafeiropoulos, Ulrich Oertel,Rudiger Haßler, Manfred Stamm*

A simple, facile and versatile approach is presented for the preparation of PMMA/ZnOnanocomposite materials, which possess high transparency, no color, good thermal stability,UV absorption and improvedmechanical properties. The employed process involvedmixing ofZnO nanoparticles dispersed in DMAc with the PMMAmatrix dissolved in the same solvent. The effect of ZnOcontent on the physical properties of the PMMAmatrix isstudied. A significant improvement in mechanical prop-erties was observed with the incorporation of 0.5 wt.-%ZnO particles. The beauty of the described approach liesin the fact that despite being a simple and facileapproach, it offers nano-level (2–5nm) mixing of ZnOnanoparticles into a polymer matrix.

Introduction

The development of polymer-based nanocomposite mate-

rialshasbeenattracting immense interest fromresearchers

M. Agrawal, S. Gupta, N. E. Zafeiropoulos, U. Oertel, R. Haßler,M. StammLeibniz-Institut fur Polymerforschung Dresden e.V., Hohe Strasse6, 01069 Dresden, GermanyFax: þ49 351 4658 281; E-mail: [email protected],[email protected]. GuptaCurrent address: Institut fur Makromolekulare Chemie,Technische Universitat Dresden, 01069, Dresden, GermanyN. E. ZafeiropoulosCurrent address: Department of Materials Science andEngineering, University of Ioannina, Greece

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.mcp-journal.de, or from theauthor.

Macromol. Chem. Phys. 2010, 211, 1925–1932

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

as the mixing of nanoscale inorganic fillers into organic

polymers offers the potential to create newmaterials with

unusual combinations of optical, mechanical, physical and

chemical properties.[1,2] The most vital challenge in the

preparation of these nanocompositematerials is to achieve

a uniform distribution of filler particles in the polymer

matrix. It has been realized that, regardless of the nature of

filler material, nanoscale particles tend to agglomerate in a

polymer matrix because of the high surface energy. A

number of strategies have been employed to avoid the

agglomeration of nanoparticles in the host matrix. One

method is the in situprecipitationof particles in theorganic

phase, which may consist of a bulk polymer, polymer

solutionormonomer.[3] It is believed thatpolymersashosts

do not provide a sufficiently fluid environment to allow

individual particles to meet with each other by diffusion;

thereby, the aggregation of the particles may be prevented

for reasons of kinetics.[4–6] However, the drawback is that

thepolymermaystay contaminatedbyunreactedeductsor

by-products of the precipitation reaction. In addition, it is

difficult to control the particle size distribution of fillers in a

DOI: 10.1002/macp.201000191 1925

M. Agrawal, S. Gupta, N. E. Zafeiropoulos, U. Oertel, R. Haßler, M. Stamm

1926

polymer matrix. The second method involves the blending

of pre-formed inorganic particles into the organic med-

ium.[7] In an ex situ synthesis, the particles are prepared

separately, isolated and purified. Subsequently, they are

dispersed into the monomer solution and in situ poly-

merization is carried out.[8] The challenges of this approach

involvesynthesisofnanoparticles in largeenoughamounts

with good dispersity in the monomer and long term

stability against aggregation. In order to facilitate the

dispersion of prepared nanoparticles in polymerization

media, they are modified with organic coupling agents.

Therefore, cost becomes the prohibiting factor for themass

production of polymer/inorganic nanocomposites, as

nanoscale particles modified with organic coupling agents

are quite expensive. The third approach is the mixing of

ex situ prepared nanoparticles in a pre-formed polymer

matrix, for example by solution mixing.[9] In this protocol,

the polymer matrix is dissolved and nanoparticles are

dispersed separately, in a common solvent, or in two

different solvents which are soluble in each other. There-

after, both the solutions are mixed into each other and

nanocomposite films are achieved by solution casting or

spin coating. However, sometimes this approach also

requires the modification of the particle surface in order

to make them disperse into the suitable solvent. This has

been realized as a simple, fast and cost effective way to

achieve nanocomposite films with a good dispersion of

filler particles, provided a suitable solvent is available

for solution mixing of both components. In the context

of nanocomposite materials, the immediate advantage

offered by solution mixing over conventional methods

like melt compounding includes better dispersion of the

nanoparticles in the polymer matrix. A dramatic decrease

in the viscosity of the polymer matrix in the presence of

a suitable solvent allows the nanoparticles to distribute

homogeneously.Unlike inmelt compounding, thepresence

of the solvent in solutionmixing enables the nanoparticles

to overcome the viscosity driven hindrance in their

distribution into the polymer matrix. In addition, solution

mixing can be performed at relatively low temperatures

when compared tomelt compounding as the polymer does

not need to melt but is dissolved in a solvent.

Poly(methyl methacrylate) (PMMA) is an optically clear,

amorphous thermoplastic. It is widely used as a substitute

for inorganicglass, because it showshigher impact strength

and undergoes ductile rather than brittle fracture. It

has favorable processing conditions, and a wide range of

additives have been shown to further improve its proper-

ties.[7a,10] Zinc oxide (ZnO) is awell-knownmultifunctional

inorganicfiller andanenvironmentally friendlymaterial. It

is a colorless wide band gap semiconductor with an optical

band gap in the UV region that makes it useful as an

efficient absorber of UV radiation. It has a refractive index

of 2.02 at 589nm,[11] a thermal conductivity of 1.16W



�m�1 K�1 at 50 8C,[12] and an electrical conductivity of 0.02 S

� cm�1.[13] All of these values are considerably higher than

those of PMMA, 1.49, 0.19W �m�1 � K�1 and 10�16 S � cm�1,

respectively, at thegiven conditions.[14] The combinationof

these two materials should have many potential applica-

tions, such as in anti-reflection coatings, UV protecting

films, transparent barrier/protective layers and flame

retardantmaterials.While composites of polyacrylatewith

SiO2 and TiO2 have received considerable attention, the

polyacrylate/ZnO pair has been rarely studied. Hung and

Whang[15] have reported on the luminescence of ZnO/

poly(hydroxyethyl methacrylate) films where the ZnO

particles had been produced by a sol/gel method and

treated with a silane coupling agent before the polymer-

ization was achieved. Khrenov et al.[16] produced surface

modified ZnO particles by employing a mini-emulsion

precipitation procedure and incorporating them into the

PMMA matrix by blending. Liufu et al.[17] investigated

the thermal degradation of polyacrylate/ZnO blends

and proposed that the ZnO particles have both a

role in stabilization and destabilization, depending on

the temperature region. Liu et al.[18] and Demir et al.[19]

prepared PMMA/ZnO composite films through in situ

polymerization of methyl methacrylate (MMA) monomers

in the presence of organically modified ZnO nanoparticles.

Thermal stability and UV absorption properties were

reported to increase with increasing the ZnO content.

Herein, we report on a simple, fast and facile approach for

the preparation of PMMA/ZnOnanocomposite films, exploit-

ing a solution mixing approach. In first step, 2–5nm sized

ZnO nanoparticles were prepared by the hydrolysis of

Zn(Ac)2O � 2H2O salt in 2-propanol. Thereafter, these

nanoparticlesweremixed into the PMMAmatrix by solution

mixingusingN,N-dimethylacetamide(DMAc)asthecommon

solvent. Thenoveltyof thepresentedstudy lies in the fact that

a nano level (2–5nm) mixing of the ZnO filler particles in a

polymer matrix has been demonstrated by a simple and

versatile approach. Unlike in previously reported studies,

the described approach employs direct mixing of the filler

particles and polymer of interest through a common solvent.

No pre-functionalization of the nanoparticles or grafting of

the initiator is required. A comparison of the presented

findings with those reported in the literature[20] on solution

mixing of filler particles would reveal that the aggregate size

offillerparticles in thepolymermatrix is significantly smaller

(below 10nm), indicating nano-level mixing.

Experimental Part

Materials

PMMA, 2-propanol (>99%) and NaOH (>99%) were all purchased

from Aldrich and used as received. Zinc acetate dihydrate [Zn(Ac)2

DOI: 10.1002/macp.201000191

Nano-Level Mixing of ZnO into Poly(methyl methacrylate)

� 2H2O, 99%] and DMAcwere obtained from Fluka and also used as

received.

Synthesis of ZnO Nanoparticles

In a round bottom flask, 0.22 g of Zn(Ac2)2 � 2H2O was added to

80mL of 2-propanol and stirred vigorously at 22 8C for 10min.[21]

Thereafter, the reaction mixture was heated to 55 8C for 1 h to

obtain a transparent solution and cooled to room temperature. In

order to precipitate the ZnO nanoparticles, 2mL of 1M aqueous

NaOH solutionwas added into the reactionmixture over 8–10min

followed by heating at 55 8C for another 5–7min. Solvent was

removed at reduced pressure and the obtained ZnO nanoparticles

were washed 3 times with distilled water via centrifugation to

remove an excess of NaOH and dried in a vacuum oven at room

temperature.

Preparation of PMMA/ZnO Composite Films

PMMA/ZnOcompositefilmswerepreparedvia the solutionmixing

of ZnOnanoparticles and PMMAmatrix. DMAcwas selected as the

solvent for the PMMA because ZnO particles were also found

to disperse well in this solvent. In a typical synthesis, ZnO

nanoparticles were added into 15mL of DMAc and the resulting

solution was subjected to ultrasonic vibration for 30min to break

apart the agglomerates. Then, the resulting dispersion was heated

to 70 8C and1 g of PMMAwas added to it. The reactionmixturewas

allowed to stir for another 2 h at 70 8C. The final concentration of

PMMA inDMAcwas 7wt.-%. The ZnO contentwas varied from0.1,

0.5, 1 and 2 wt.-% based on the polymer matrix. Nanocomposite

filmswith2–3mmthicknesswere obtainedviafilmcasting at 80 8Cfor 4 d.

Characterization and Instrumentation

Transmission electron microscopy (TEM) images were obtained

using a Zeiss Omega 912 microscope at an accelerating voltage of

200 kV. Samples were prepared via cutting the samples into a few

nanometer thick slices by ultramicrotoming. Dynamicmechanical

analysis (DMA) was carried out on a 2980 DMA V1.7B instrument

using a single cantilever clamp at 1Hz frequency and 3K � min�1

heating rate. Differential scanning calorimetry (DSC) measure-

ments were carried out on a DSC Q 1000 from TA Instruments at a

Figure 1. TEM images of PMMA/ZnO nanocomposite films with (a) 0



10K �min�1 heating rate in a nitrogen atmosphere. Thermogravi-

metric analysis (TGA) was performed using a TGA 7 (Perkin-Elmer)

instrument at a heating rate of 5K � min�1 over a 25–700 8Ctemperature range in an air and nitrogen atmosphere. UV-Vis

measurementswere carried out onaPerkin-Elmer Lambda800UV-

Vis spectrometer. Wide angle X-ray scattering (WAXS) patterns

were collected on a HZG 4/A-2 (Seifert FPM) X-ray diffractometer

using a Cu Ka monochromatic beam (l¼1.54 A).

Results and Discussion

The morphology of the PMMA/ZnO nanocomposite films

loadedwithdifferentweight fractionsof ZnOnanoparticles

was investigated with TEM and the results are shown in

Figure 1. These images revealed that the samples exhibit a

nearlyuniformglobal distributionof theZnOnanoparticles

in the polymer matrix. However, distinct local aggregation

of the ZnO nanoparticles can also be observed. The size of

these local aggregates has been found to increase when

increasing the loaded amount of ZnO nanoparticles, and

ranges from the apparent single ZnOnanoparticle (2–5nm)

to 20nm. This local aggregation is not entirely unexpected

given the high surface energy of ZnO nanoparticles. A

strong interaction of ZnO nanoparticles with the polar

solvent DMAc, used as the common solvent formixing into

the polymer matrix, plays a crucial role in breaking the

agglomerates and produces the uniform dispersion of ZnO

in the final nanocomposite films. In addition, we believe

that an electrostatic interaction between the Zn2þmoieties

of ZnO nanoparticles and the C¼O bonds of the PMMA

molecules also facilitates the mixing of filler particles into

the polymer matrix. We have observed a similar type of

interaction in our previous study during the preparation of

polystyrene/ZnO composite particles.[21] It is well known

that the efficiency of the nanoparticles in improving the

propertiesof thepolymermaterials isprimarilydetermined

by the degree of its dispersion in the polymer matrix. The

beautyof thedescribedapproach lies inthe fact that itoffers

the same or an even better level of dispersion of ZnO

nanoparticles into the polymer matrix, compared to what

has been achieved using previously reported complex,

time-consuming and costly protocols.[16,19b,22]

.5, (b) 1 and (c) 2 wt.-% filler content.

www.mcp-journal.de 1927


1928

Since ZnO nanoparticles have a high thermal stability

compared to the PMMAmatrix, it can therefore be expected

that the incorporation of these particles would lead to the

improvement in thermal properties of the polymermatrix.

Figure 2(a) illustrates TGA scans of the pure PMMAfilmand

PMMA/ZnOnanocompositefilmloadedwith0.5wt.-%filler

content. In order to study the effect of the environment,

samples were analyzed in air and nitrogen atmospheres.

As expected, the presence of oxygen accelerates the

degradation of both bare and nanocomposite films, as

the temperature required for the 50%degradation (T0.5) has

been found to decrease by 47 and 24 8C for bare PMMA and

PMMA/ZnO nanocomposite films, respectively, when

switching the environment from inert to air. The presence

of ZnOnanoparticles suppresses the effect of oxygen on the

thermal degradation of the polymer matrix. The degrada-

tion onset temperature (T0.1), measured as the temperature

required for 10% degradation, has been observed as 228 8Cfor a bare PMMAfilm,while it is 326 8C for PMMA/ZnOfilm

(in a nitrogen atmosphere), suggesting a delay in the

thermal degradationof the PMMAmatrix in thepresence of

Figure 2. (a) TGA scans of PMMA films before and after theloading of 0.5 wt.-% ZnO nanoparticles, taken in air and nitrogenatmospheres and (b) DSC scans of PMMA/ZnO films loaded withdifferent amounts of ZnO nanoparticles.



ZnO nanoparticles. Similarly, the values of T0.5 have been

found to shift from 316 to 350 8C (in air) and from 362 to

375 8C (in nitrogen) after the incorporation of 0.5% ZnO

nanoparticles into the PMMAmatrix. This enhancement of

the thermal stability can be attributed to the nanoparticles

preventing out-diffusion of the volatile decomposition

products. TGA scans of PMMAfilms incorporating different

amounts of ZnO nanoparticles are shown in Figure S1(a)

(Supporting Information). These results illustrate the

increase in thermal stability of the PMMA matrix after

mixing in 0.1–2 wt.-% of ZnO nanoparticles. In the

derivative TGA curve [shown in Figure S1(b)], a peak (Tmax)

occurs when the rate of mass change is a maximum. An

increase in Tmax from 313 8C for the bare PMMA film to the

range 342–355 8C for PMMA/ZnO nanocomposite films

suggests that ZnOnanoparticles cause the delay in thermal

degradation of the polymermatrix. Themaximumvalue of

Tmax was observed with 0.5 wt.-% filler content. A further

increase in the same to 1 and 2 wt.-% causes a decrease in

Tmax value to 347 8C and 344 8C, respectively, which can be

attributed to the aggregation of ZnO nanoparticles at this

composition. Moreover, the absence of the shoulder in the

TGA derivative curves indicates that ZnO nanoparticles are

compatible with the PMMA matrix.[23] This small bend in

the TGA curves of PMMA and PMMA/ZnO (0.5 wt.-%)

nanocomposite samples can be attributed to the oxidative

degradation of the polymer,which typically starts at 250 8Cin the presence of air. On the other hand, there is no

oxidation of the polymer in inert media (nitrogen) and

hence no such bends in TGA scans have been observed.

Figure 2(b) shows DSC scans of PMMA/ZnO composite

films taken at 10K � min�1 heating rate in a nitrogen

atmosphere. One can observe that the incorporation of ZnO

nanoparticles led to an increase in the glass transition

temperatureof thePMMAmatrix, asobservablebythepeak

maxima of the DW/DT curve. The inset in Figure 2(b)

illustrates the variation in glass transition temperature (Tg)

as a function of the ZnO contents and reveals that the

mixing of 0.5 wt.-% ZnO leads to a maximum shift in Tgtowards higher temperature. In agreement with the TGA

results, a decrease in Tg with a further increase in filler

content can be ascribed to a slight increase in the degree of

aggregation at a higher loading of ZnO. The presence of

large agglomeration in the polymer matrix renders a high

free volume to the polymer chains, present around the filler

domains, offering easiness in their mobility. Typically, the

filler serves to increase the Tg via hindering the mobility of

polymer chains in the vicinity of the surface of nanopar-

ticles. As reported by Savin et al.,[24] tethering and chain

confinement are the two dominant contributions that can

affect the mobility of polymer chains. An increased Tg of

the PMMA/ZnO nanocomposite films seems to be affected

mainly by confinement of the polymer chains adjacent to

the ZnO, rather than by tethering due to little interaction

DOI: 10.1002/macp.201000191


between PMMA and ZnO. It is worth mentioning that,

in contrast to the results reported by Chae et al.,[25] we

observed a significant increase in Tg even at 0.5% filler

content, which again strongly indicates the effective

mixing of ZnO nanoparticles in the polymer matrix.

Another remarkable observation is that the peak area per

gram of the nanocomposite films also increased after

mixing in the ZnO nanoparticles. This can be explained by

the larger surface area in intimate contactwith theDSCpan

for PMMA/ZnO nanocomposite films compared to that of

pure PMMA films. Sircar et al.[26] reported that most

elastomers are nervy in the raw state and it is very difficult

to make a filmwith a large surface area that can lay flat in

a DSC pan with these nervy samples.

The crystallinity of PMMA/ZnO films was investigated

with wide angle X-ray diffraction measurements. Figure 3

shows the X-ray diffraction patterns for nanocomposite

films loaded with different ZnO contents. All the patterns

exhibited the broadnon-crystalline peak (13.38) of PMMA[27]

and sharp diffraction peaks of ZnO nanoparticles. The peak

intensities have been found to increasewith increasing ZnO

contents inthepolymermatrix.[28]Thesediffractionpatterns

exhibit the characteristic peaks for crystalline ZnO of the

Wurtzite structure. All these peaks can be indexed to

the hexagonally structured ZnO with cell constants of

a¼ 0.324nm and c¼ 0.519nm, which are consistent with

the standard values for bulk ZnO.[29] The incorporation of

ZnO nanoparticles produces neither a new peak nor a peak

shiftwithrespecttoPMMA,indicatingthatZnO-filledPMMA

nanocomposites consist of two phase structures, i.e.,

polymer and nanoparticle. The average grain size (D) of

ZnO was estimated from the well-known Scherrer formula

Figloa

Macrom

� 2010

D ¼ 0:89 l =b cos u (1)

where l¼ 1.5418 A (CuKa) and b is the full width at half

maximum (FWHM) at a diffraction angle of u. From the

ure 3. XRD patterns for PMMA/ZnO nanocomposite filmsded with different amounts of ZnO nanoparticles.

ol. Chem. Phys. 2010, 211, 1925–1932

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

(002) diffraction peak, one can calculate the average grain

size as 13nm for nanocomposite films with 2 wt.-% ZnO

filler particles.

PMMA is usually used as a transparent material and

therefore it is important to retain the optical properties of

this polymer matrix after mixing in reinforcing filler. In

order to investigate the optical properties, nanocomposite

films were characterized by UV-Vis spectroscopy. These

films had a typical thickness of ca. 1.0mm. These results

reveal, not surprisingly, the influence of the ZnO nano-

particles on the optical properties of PMMA films.

Figure 4(a) illustrates the onset of absorbance for these

samples in the range 340–346nm,which can be ascribed to

the excitation of electrons from the valance band to the

conductionbandofZnO.Moreover, onecanobserve that the

intensity of this band increases with an increase in ZnO

content. The onset of the absorption shows a red shift and

hence a decrease in the band gap of ZnO with increasing

the filler content in composite films. Similar behavior was

reported by Yuwono et al.[30] for TiO2 nanoparticles

dispersed into a polymer matrix. It should be noted that

the absence of the absorption band in the visible region of

Figure 4. (a) UV-Vis spectra of PMMA/ZnO nanocomposite filmsloaded with different amounts of ZnO nanoparticles. (b) Trans-mittance of PMMA/ZnO nanocomposite films as a function offiller content in the UV (350 nm) and visible regions (550 nm).



Figure 5. (a) Variation in storage moduli of PMMA matrix filledwith different weight fractions of ZnO nanoparticles (a) beforeand (b) after annealing at 80 8C for 30 min. (c) Tan d as a functionof temperature for PMMA/ZnO nanocomposite films afterannealing at 80 8C for 30 min.

1930

these UV spectra indicates that the prepared PMMA/ZnO

nanocomposite films do not possess any color and can

readily be used as transparent UV-absorbing materials.

Figure 4(b) illustrates the optical transmissions, T, of the

nanocompositefilmsat twodifferentwavelengths, namely

at 550 and 350nm, as a function of the ZnO content. As we

know that ZnO does not absorb in the visible region,

therefore we can assume that the loss in transparency in

this visible region will be caused only by the scattering of

nanoparticles.Onecanobserve fromFigure4(b) that there is

no significant decrease in the transparency of composite

filmsuntil the loading level is 0.5%. Indeed, these values are

quite similar to that of the pure PMMA film, namely

92%.[19a] It suggests that, in this concentration range, our

particles are small enough so that scattering losses are

negligible, and the composite films are almost as trans-

parent as PMMA itself. It is worthmentioning that that the

transmission of the pure PMMA films for 550nm light at

normal incidence is about 92% only as a result of the

reflection losses on air/film, film/substrate and substrate/

air interfaces and not because of absorption and scattering

losses. A further increase in the ZnO content of composite

films from 0.5 to 2 wt.-% causes a significant decrease in

transparency as, at higher filler contents, particles aremore

likely to be agglomerated in the polymermatrix and hence

inevitably cause optical scattering. It should be noted that

transmission loss for such amultiphase systemdepends on

several parameters, such as the concentration of particles,

the refractive index difference between the particles and

polymer matrix, the size of the inorganic domains and the

thickness of the films studied. In addition, Figure 4(b)

reveals that PMMA/ZnO nanocomposite films strongly

absorb in theUVregion, in contrast to thatof thevisibleone.

This can be attributed to the wide band gap and hence the

highabsorption coefficient of ZnO in this spectral region.[31]

An exponential decay in transmission at 350nm can be

observed with increase in filler content. These results

clearly indicate that the obtained PMMA/ZnO composites

can be used as UV-protecting materials, which are reason-

ably transparent in the visible region.

The effect of ZnO nanoparticles on the mechanical

properties of PMMA was studied by dynamic mechanical

analysis (DMA). Figure 5(a) shows the variation in the

storage moduli of the PMMA matrix filled with different

amount of ZnOnanoparticles, as a function of temperature.

For the pure PMMAmatrix, the storagemoduluswas found

to decrease with increasing the temperature, because of

softening of the matrix and initiation of relaxation

processes and melting.[32] However, for ZnO-filled PMMA

nanocomposite films, an increase in the same with

temperature has been observed above 30 8C. This can be

ascribed to the crosslinkingof PMMAchains in thepresence

of ZnO nanoparticles. It is noteworthy that the maximum

increase in storagemodulus was observed for the 0.5 wt.-%



filler content where nanoparticles are finely dispersed into

the matrix. Figure 5(b) displays the variation in storage

moduli as a function of temperature for the same samples,

but after annealing at 80 8C for 30min. This data reveals

that pre-annealing has drastically suppressed the increase

in storagemoduli above 30 8C, confirming the curing effect

of ZnO filler on the PMMA matrix. A slight increase in

storage moduli in the same temperature range after

DOI: 10.1002/macp.201000191


annealing canbeascribed to the crosslinkingof theuncured

polymer chains. Similar types of DMA results have been

observed by Shen et al.[33] for polybenzoxazines. Wewould

like to emphasize here that we repeated these measure-

ments 2–3 times and found these peaks in DMA scans at

the respective positions. This data, for the first time, reveals

that one can determine the crosslinking temperature and

respective change in storage modulus by DMA analysis.

Storage moduli increases with increasing the filler content

from0to0.5wt.-%andsubsequentlydecreaseswith further

increase in filler contents up to 1 and 2 wt.-%. A careful

inspection of the presented set of results reveals that this

data is in agreement with the thermal analysis results,

which showa steady increase in Tg until 0.5wt.-%, and then

a decrease for 1 and 2wt.-% filler levels. It can be attributed

to the homogeneous mixing of the ZnO nanoparticles into

the PMMA matrix until a 0.5 wt.-% filler level. A further

increase in the filler level causes aggregation of particles in

the matrix, deteriorating the physical properties of the

PMMAmatrix. These DMA curves suggest that the PMMA/

ZnO has a 500–600% higher storage modulus than neat

PMMA at room temperature.

Furthermore, it can be seen that the addition of the ZnO

nanoparticles into the PMMAmatrix results in an increase

in the storagemodulus in the glassy state (T< Tg). It leads to

the conclusion that the PMMAmatrix can be reinforced in

the glassy state by the addition of ZnO nanoparticles. The

presenceoffillerparticles induces thereduction in thechain

mobility and the deformation of the PMMA matrix,

improving its stiffness. In addition, one can observe that

this improvement in storagemodulus is proportional to the

added content of filler particles from 0.1 to 0.5 wt.-%. A

further increase to 1 and 5wt.-% leads to the sharp decrease

inmodulus, which can be ascribed to the large domain size

of filler particles at these loading levels. Similar results

have been reported by Trabelsi et al.[31] for the preparation

of titanium-oxo cluster based hybrid materials. The

presence of large agglomeration in the polymer matrix

renders a high free volume to the polymer chains, present

around the filler domains, offering the easiness in their

mobility. In marked contrast, above the Tg, no effect of the

filler particles was observed.[34] A closer look at the DMA

results (Figure 5) reveals that, for most of the PMMA/ZnO

compositions, the storagemodulus decreased after anneal-

ing the samples at 80 8C for 30min. However, the pure

PMMAmatrix showsapproximately samestoragemodulus

before and after the annealing process. These results

indicate that annealing has caused the change in distribu-

tion of the nanoparticles inside the matrix. It is quite

obvious that as the annealing temperature is higher than

that of the glass transition temperature of the polymer

matrix, movement of the polymer chains at 80 8C would

result in the variation in distribution of these particles. A

slight decrease in mechanical properties after annealing in



the DMA results indicates that particles have moved a bit

closer to each other at 80 8C, disturbing their homogeneous

distribution.

Thevariation in tan dasa functionof temperature reveals

a relaxation peak, which corresponds to the glass-rubber

transition of the amorphous phase and the temperature of

the maxima of the peak is assigned as the glass transition

temperature (Tg). As observed in Figure 5(c), the tan d peak

maxima have shifted to higher temperature values after

mixing in the ZnOnanoparticles, suggesting the increase in

the glass transition temperature of the PMMAmatrix. After

heating beyond 90 8C, tan d data became noisy, as perhaps

samples were not recovering from the deformation.

Interestingly enough, all these nanocomposite materials

exhibitedadampingfactorof tan d> 0.4.Usually,polymeric

materials with tan d> 0.3 are considered as having very

good damping properties.[35]

Conclusion

In summary,wedemonstratedasimpleandfacileapproach

for themixing of ZnO nanoparticles into a polymermatrix.

The described method offers a good dispersion of filler

particles into the host matrix in a time and cost effective

way. A reasonable increase in thermal, optical and

mechanical properties of the PMMA matrix was observed

after the loading of a small amount of ZnO nanoparticles.

Thenanocomposite films are highly transparent, even after

the mixing of ZnO nanoparticles, and possess good

absorption of light in the UV region, suggesting that these

nanocomposites can be used as transparent and UV

shielding materials.

Acknowledgements: The authors are thankful to Mr. Alex Mensch(Technical University Dresden) and Dr. Dieter Jehnichen and Mr.Torsten Hofmann for help with TEM and XRD measurements,respectively. We acknowledge the financial support by DeutscheForschungsgemeinschaft (DFG) and European Union (EU) for theproject.

Received: April 12, 2010; Revised: June 10, 2010; Published online:August 3, 2010; DOI: 10.1002/macp.201000191

Keywords: films; mixing; nanocomposites; nanoparticles; struc-ture-property relations

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DOI: 10.1002/macp.201000191

nano-level mixing of zno into poly(methyl methacrylate)

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