polypropylene + boehmite nanocomposite fibers

DOI 10.1515/polyeng-2012-0010 J Polym Eng 2012; 32: 445–451

Lubica Š kovranov á , Eberhard Borsig *, Rouven Streller , Ralf Thomann , Rolf M ü lhaupt ,

Anna Ujhelyiov á , Du š an Berek and Robert A. Patsiga

Polypropylene + boehmite nanocomposite fibers

Abstract: The influence of nanosized filler particles on

mechanical properties of composite fibers has some speci fic

features related to the fiber matrix, its preparation and the

filler content, as well as the orientation of the fibers at their

drawing. This paper discusses the manner of dispersion of

chemically unmodified boehmite type nanofiller, Disperal

40 (D40) or Disperal 60 (D60) in polypropylene (iPP) fibers,

and the influence of filler and its particle size on the tenac-

ity, modulus and elongation of the iPP fibers. It was found

that filler D40 was effective in increasing the tenacity only

up to a content of about 1 wt % and in fibers prepared only

at higher drawing ratios. Transmission electron microscopy

(TEM) pictures of the cut iPP + D40 or iPP + D60 fibers have

shown the presence of aggregation of nanosized boehmite

particles in all cases. A comparison of the effects of two dif-

ferent particle sized fillers on the mechani cal properties of

the isotactic PP (iPP) fibers showed that a small difference

between the particle sizes of both kinds of filler plays some

role regarding the tensile strength of the fibers. The iPP

fibers containing a mixture of D40 and D60 showed some

deviations from the expected sum of the effects of both

fillers on their mechanical properties.

Keywords: inorganic nanofiller; isotactic polypropylene

fibers; polymer nanocomposite fibers.

*Corresponding author: Eberhard Borsig, Slovak University of

Technology , Faculty of Chemical and Food Technology, Department

of Polymer Materials, Radlinsk é ho 9, 81 237 Bratislava , Slovakia ,

e-mail: [email protected]

Eberhard Borsig: Polymer Institute , Slovak Academy of Sciences,

D ú bravsk á cesta 9, 845 41 Bratislava , Slovakia

Lubica Š kovranov á : Slovak University of Technology , Faculty of

Chemical and Food Technology, Department of Polymer Materials,

Radlinsk é ho 9, 81 237 Bratislava , Slovakia

Rouven Streller: Freiburger Materialforschungszentrum und Institut

f ü r Makromolekulare Chemie der Albert-Ludwigs-Universit ä t , Stefan

Meier Str. 31, D-79104 Freiburg i. Br ., Deutschland

Ralf Thomann: Freiburger Materialforschungszentrum und Institut



Rolf M ü lhaupt: Freiburger Materialforschungszentrum und Institut



Anna Ujhelyiov á : Slovak University of Technology , Faculty of

Chemical and Food Technology, Department of Polymer Materials,

Radlinsk é ho 9, 81 237 Bratislava , Slovakia

Du š an Berek: Polymer Institute, Slovak Academy of Sciences,

Dúbravská cesta 9, 845 41 Bratislava, Slovakia

Robert A. Patsiga: Indiana University of Pennsylvania , Department

of Chemistry, Indiana, PA 15705 , USA

1 Introduction Measurement of the mechanical and physical proper-

ties of polymer + nanofilled fibers is not only of practical,

but also of theoretical importance. Mainly, the values of

mechanical properties (like tenacity, toughness elonga-

tion, etc.), of a polymer nanocomposite obtained for bulk

plastics, e.g., polypropylene (iPP) [1, 2] cannot be directly

transformed to fibers prepared from the same polymer

nanocomposite, because the latter is measured as an

oriented polymer [3, 4] . Moreover, the values for photo-

oxidation, or barrier properties, will be different for fibers

[4, 5] .

Our research on polymer + inorganic nanocomposite

fibers has been focused on iPP matrices. This is due to the

ease of spinning of isotactic polypropylene (iPP) alone

and the fact that iPP fibers have good mechanical prop-

erties, which makes them prevalent in the textile indus-

try. The aim of preparing iPP fibers filled with inorganic

nanofiller is to increase the mechanical, barrier or other

properties, which would make the fibers applicable for

technical purposes.

While the tensile strength properties of unfilled iPP

fibers are highly correlated with molecular weight and

orientation in the crystalline and amorphous regions,

iPP + filler nanocomposite fibers exhibit properties that

reflect the important role played by the enhanced inter-

facial bonding between the particles and the matrix [1, 6] .

Such interactions allow for the attainment of desirable

objectives, like increased toughening in these reinforced

polymer + filler nanocomposite fibers [2] .

Early experiments involved the preparation of iPP

fibers containing organoclay nanofiller, NANOFIL, and

iPP grafted with maleic anhydride – iPP-g-MAN as a com-

patibilizer [5] . The fibers containing NANOFIL in the range

0.16 – 2.76 wt % showed a higher tensile strength than

unfilled iPP fibers.

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446 L. Škovranová et al.: Polypropylene + boehmite nanocomposite fibers

Syndiotactic polypropylene (sPP), fibers filled with

layered silicate, containing bonded octadecyl ammo-

nium chains (M-ODA) allowed the preparation of

sPP + M-ODA + iPP-g-MAN nanocomposite fibers at a higher

drawing ratio. These sPP nanocomposite fibers exhibited a

higher tensile strength than neat sPP fibers [7] . However, the

crystallinity of nanocomposite fibers was found to be lower

than neat sPP fibers [8] . In the case of iPP + calcium phos-

phate nanocomposites, the overall crystallization rate, G,

was found to be dependent on the particle size as logG α (1/d)

[9] . Also, carbon nanotubes (CNTs) are a promising nano-

filler material for the preparation of polymer nanocomposite

fibers; these are comprised of several concentric graphilic

cylinders that are capped with carbon pentagons at each

end [10] . These nanotubes have diameters ranging from 2 to

25 nm and lengths ranging up to several micrometers [11] .

CNTs are very effective as nanofillers, in the improvement of

mechanical properties of polymer nanocomposites (the 10

times modulus increase was obtained in PMMA) [12] .

Recently, boehmite type fillers, of natural or synthetic

origin, based on aluminum oxide AlO(OH) x , without treat-

ment or modification, were used for the preparation of iPP

or PE nanocomposites [9, 13 – 15] . The crystal nanoparticles

in iPP exhibited a higher stiffness, strength and dimen-

sional and thermal stability of the nanocomposite [16 – 18] .

The use of the boehmite type nanofiller, Disperal

40 (D40), in preliminary experiments, showed positive

effects on the fiber tenacity at a low content of 0.1 wt %

[19] . A rheological study of iPP + D40 nanocomposite

fibers showed that the melting temperature dependence

on filler content passes through a maximum, which cor-

relates with the highest tenacity values [19] .

The aim of this work is to study the influence of the

amount of boehmite filler and its dispersibility on pro-

perties in these nanocomposite fibers. We examined the

boehmite labeled as D40 or D60 blended into an iPP (iso-

tactic) matrix without using a compatibilizer, what could

have some advantage in fibers preparation. The iPP + D

nanocomposite fibers were prepared by melt spinning in a

laboratory spinning machine and the obtained fibers were

investigated with regard to their mechanical and morpho-

logical properties.

2 Experimental

2.1 Materials

iPP was obtained from Basell Moplen 561 N and inorganic

aluminum oxide filler boehmite, based on γ -AlO(OH), D40

and D60, products of Sasol GmbH (Hamburg, Germany),

were used for the preparation of iPP nanocompo sites. Both

boehmites consist of agglomerates of nanosized crystal-

lites. The average size of the agglomerates exceeds 20 μ m.

The crystallite size of D40 is 40 nm and of D60 is 53 nm [20] .

2.2 Preparation of iPP + Disperal nanocomposites

IPP powder and the inorganic filler Disperal were mixed

in a tumbling mixer with 0.25 wt % of mixture of stabiliz-

ers (Irganox 1100 + Irgafos 168 in the ratio 4:1, Ciba, Basel,

Switzerland). This mixture was melted in a co-rotating

twin-screw extruder (Werner-Pfleiderer, ZS K25, Stuttgart,

Germany) at 190 – 230 ° C and at 300 rpm, during which the

D40 or D60 particles were dispersed in the iPP matrix [21] .

In this way, iPP nanocomposite samples in granulated

form, containing 0.5, 1.0, 2.0 and 5.0 wt % of D40, were

prepared.

2.3 Spinning of iPP + Disperal 40 (60) nanocomposites

The iPP + Disperal nanocomposite samples containing 0.5,

1.0, 2.0, 3.0 and 5.0 wt % Disperal 40 or Disperal 60 were

spun with a laboratory TS ϕ 16 mm extruder using a spin-

ning nozzle with 13 spinnerets of ϕ 0.05 mm each, under

the following conditions: homogenization zone, 250 ° C

and spinning speed, 150 m/min. In this way, partially

oriented isotropic fibers were prepared and drawn with

different draw ratios: λ = 2, 3, 4, 5, and partly 7.8.

2.4 Tensile strength measurements

The mechanical properties of the iPP + Disperal nanocom-

posite fibers were determined with an Instron ISO 3343

apparatus at ambient temperature. The clamping length

and deformation rate were 100 mm and 500 mm/min,

respectively. Ten specimens of each composition were

tested and the average values of 10 measurements of each

sample are reported. The tenacity of the fibers was deter-

mined at breaking strength of the fibers, in N.tex -1 .

2.5 Transmission electron microscopy (TEM)

TEM measurements were carried out with a LEO OMEGA

912 transmission electron microscope applying an



L. Škovranová et al.: Polypropylene + boehmite nanocomposite fibers 447

0

20

40

60

0 1 2 3 4 5Content of disperal 40 (wt%)

Ten

acity

(cN

/tex)

λ=2 λ=3 λ=4 λ=5

Figure 1 Tenacity of polypropylene (iPP) + Disperal 40, (iPP + D40)

nanocomposite fibers: Influence of nanofiller content and drawing

ratio of fibers.

acceleration voltage of 120 keV. The specimens were pre-

pared by embedding the fibers in “ LR white ” embedding

media and sectioning with an ultra microtome equipped

with a cryochamber [21] .

3 Results and discussion

3.1 The mechanical properties of iPP ++ boehmite nanocomposite fibers

3.1.1 Tenacity and Young ’ s modulus of iPP ++ D40 fibers

The synthetic inorganic filler D40, which has an alumina

composition similar to natural boehmite, Al(OH)O

was compounded with iPP Moplen 561N to produce an

iPP + D40 nanocomposite. Five samples of the iPP nano-

composite were prepared by the extruder method. These

samples contained different contents of nanofiller D40:

0.5; 1.0; 2.0; 3.0 and 5.0 wt % . These five iPP + D40 samples

were spun and then drawn at the following drawing ratios:

λ = 2, 3, 4, 5 and 7.8. All prepared iPP + D40 nanocomposite

fibers, were examined by measurements of tenacity and

Young ’ s modulus (Figures 1 and 2 ). The tenacity measure-

ments depicted in Figure 1 show an unambiguous positive

influence of the nanofiller on tenacity of the fibers only for

fibers which underwent higher drawing ratios, i.e., λ = 4

and 5, and had a lower content of nanofiller. The maximal

tenacity was attained at about 1 wt % of nanofiller. The

maximum tenacity was only about 10 % greater than the

neat iPP fibers (Figure 1). At a lower drawing ratio, i.e.,

λ = 2 and 3, the prepared iPP nanocomposite fibers with

0

1

2

3

4

5

6


Mod

ulus

(N/te

x)

λ=2 λ=3 λ=4 λ=5

Figure 2 Modulus of polypropylene (iPP) + D40 nanocomposite

fibers: influence of filler content and drawing ratio λ .

increasing content of filler (up to about 2 wt % ) showed a

slight increase (at λ = 3) or retained the tenacity (at λ = 2) in

comparison with unfilled iPP fibers (Figure 1).

The course of the Young ’ s modulus dependency at λ = 5

exhibits a maximum at about 1 wt % of nanofiller (Figure 2).

If one considers that a greater drawing ratio should produce

a higher order of orientation in the amorphous phase, the

positive influence of the filler on the tensile strength and

Young ’ s modulus is unexpected. By contrast, this result

indicates that the formation of a physical network involving

the nanoparticles in the more oriented phase is more effec-

tive than in the less oriented phase of iPP. Probably, in this

case, the physical network better resists the mutual sliding

of the tight parallel oriented iPP molecules or crystallites.

Similarly, at lower drawing ratios, the dependency of

the Young ’ s modulus on the content of nanofiller of the

fibers also has a similar tendency to decrease, as in the case

of tenacity (Figure 2). This means that the presence of the

nanofiller D40 at these low drawing ratios produces a neg-

ative, rather than a positive, contribution to the mechani-

cal properties of the fiber. The Young ’ s modulus of fibers

containing 5 wt % of D40 decreased from 15 to 30 % (for

λ = 3 and λ = 2, respectively). From these results, it follows

that the final effect of the nanofiller on the mechanical

properties is a balance of positive and negative contribu-

tions [22] . As is shown on TEM micrographs (Figure 3 ), the

nanoparticles have an irregular shape and they form small

aggregates at a relatively low content of filler (2 wt % ). So,

the gradual decrease of the mechanical properties with an

increasing content of filler, seen in Figures 1 and 2, can be

caused by an increasing formation of such aggregates. The

aggregates would be expected to hinder the orientation of

crystallites in the iPP. This explains why a positive effect

on the mechanical properties of the iPP + D40 fibers is




observed only at a low content of the nanofiller and a high

drawing ratio. This also means that the expected positive

effect of the nanofiller, caused by the physical network

formation in the iPP matrix, is countered by the negative

effects of aggregation of nanoparticles.

The negative effect of the filler on mechanical proper-

ties can be also expected from the known increase of the

polymer melt viscosity that is observed when nano-parti-

cles are present in iPP matrix. This was also observed in

our previous experiments [22] . Higher viscosity of the melt

can contribute to the thermo-mechanical degradation of

the iPP fiber matrix during the spinning process and so

cause a worsening of mechanical properties of fibers.

3.1.2 Influence of nanofiller D40 on % elongation of iPP ++ D40 fibers

The elongation measurement of the iPP + D40 nano + com-

posite fibers showed that the nanofiller D40 greatly influ-

enced the elongation of the fibers. This was also observed

in the rheological study of iPP + boehmite nanocomposites

[23] ; evaluation of basic rheological parameters showed

non-Newtonian behavior of the polymer melt [17] . The

fibers prepared at λ = 4 and 5 exhibited a very low elonga-

tion at break, of only a few percent over the whole filler

content range investigated (from 0.5 to 5 wt % of filler,

Figure 4 ). This effect was also found with other fillers, e.g.,

organoclay types [2] . However, the fibers prepared at λ = 2

and 3 showed a remarkable decrease of % elongation only

at contents between 0 and 1 wt % of D40, which represents

about 50 to 85 % elongation of all samples prepared (from

0 to 5 wt % of the filler).

Figure 3 TEM micrograph of the vertical cut of the polypropylene

(iPP) + D40 nanocomposite fiber containing 2 wt % of filler.

0

50

100

150

200

250


Elo

ngat

ion

(%)

λ=5 λ=4 λ=3 λ=2

Figure 4 Elongation of polypropylene (iPP) + D40 nanocomposite

fibers: influence of filler content and drawing ratio λ .

At the higher content of filler, > 1 wt % , the % elon-

gation decrease is more moderate. This means that the

contribution of the filler to the % elongation of the fibers

is most effective at ≤ 1 wt % . This can be understood as

being a consequence of the formation of the physical

network by the filler. By contrast, at higher content of the

filler, > 1 wt % , a very small effect of filler on % elonga-

tion was observed. This is also true in the case of tenac-

ity and Young ’ s modulus measurements. The effects can

be caused by an aggregation of the particles at increased

amounts of filler, which causes a displacement of filler

particles and their aggregates from the oriented part of

the iPP fiber matrix. These processes will decrease the

efficiency of particles as agents for formation of physi-

cal networks and so also the elasticity of the iPP matrix.

3.2 Influence of the size of Disperal nanofiller particles on morphology and mechanical properties of the iPP + D composite fibers

3.2.1 TEM study of the morphology of iPP + D40 and iPP ++ D60 nanocomposite fibers

Two kinds of nanofiller, D40 and D60, were used for the

study of the influence of particle size on the morphology

and mechanical properties of the iPP + boehmite nano-

composite fibers. According to the leaflet of the com-

mercial supplier, Sasol Production, the original particle

size of about 45 wt % of D40 is <45 μm. The particles of

D40 dispersed in iPP fibers consist of small crystallites of

size 356 Å .




The filler particles of D40 or D60 after compounding

with iPP (without a compatibilizer) and spinning, were

physically reduced (crumbled) into a much smaller size

than the starting particles. This is shown in the TEM

photos (Figures 3 and 5 ).

The iPP fibers containing 2 wt % D40 or D60, or a

mixture of 1 wt % of D40 and 1 wt % of D60 (Figures 6 and

7 ) show that the iPP matrix still contains aggregates of

Disperal crystallites. Figure 6 provides a TEM picture of

the parallel cut of the fibers and shows that the orien-

tation of the filler follows the drawing direction of the

fibers.

In the case of iPP + D40 fibers, it can be seen that the

filler was physically reduced (crumbled) into small parti-

cles of a maximal size of about 200 nm, but also with a rel-

atively large amount of particles <100 nm (Figure 3). The

detail of the smaller particles in Figure 7 shows aggregates

of crystallites and independent nano sized crystallites.

In the case of the fiber sample containing the filler

D60 (Figure 5), the dispersed particles, are, as expected,

mostly > 200 nm and also, the ratio of larger to smaller

particle size is much higher than that of D40 (Figure

5). The process of partial destruction of large boehmite

filler particles can also be seen, which can be caused by

drawing of the fibers. The boehmite fragments could not

diffuse far from the original D40 particle, because of the

high viscosity of the iPP matrix. The temperature of the

iPP composite fiber during drawing was about 80 ° C.

The parallel cut of the iPP + D60 fiber shows how the

larger filler particles are dispersed in the iPP fiber (Figure

8 ). It also shows that there are only a few particles as large

as 200 nm, which is in agreement with the interpretations

of Figure 5.

3.3 Influence of the mixture of different size of Disperal particles on the mechanical properties of the iPP ++ D composite fibers

From the discussion in the previous section, it follows

that the Disperal 40 particles, of a mean size of 40 nm,

have a positive influence on the mechanical properties

up to a content of about 1.0 wt % and at a high drawing

ratio ( λ = 5). The higher content of D40 probably hinders

the orientation of the iPP crystalline lamellas and polymer

chains, which are mainly responsible for providing the

mechanical properties of the fibers.

Using the filler of the boehmite type Disperal, with

a different mean size of particles, we investigated how

Figure 5 TEM micrograph of the vertical cut of the polypropylene

(iPP) + D60 fiber containing 2 wt % of filler.

Figure 6 TEM micrograph of the parallel cut of the polypropylene

(iPP) + (D40 + D60) fillers (the arrow shows the direction of fiber

drawing).

Figure 7 TEM micrograph of the detail of the vertical cut of the

polypropylene (iPP) + D40 nanocomposite fiber containing 2 wt % D40.




relatively low size differences of the filler can influence

the properties of iPP nanocomposite fibers.

For this study, three sets of samples of iPP fibers based

on two different mean sizes of Disperal filler particles,

40 and 60 nm, using D40 and D60, were prepared. The

first sample contained 2 wt % D40, the second contained

2 wt % D60 and the third contained a mixture of 1 wt %

D40 and 1 wt % D60. All samples underwent the drawing

ratios: λ = 3, 5 and 7.8 (the maximal drawing ratio at which

the nanocomposite fibers could be rolled up a reel) and

their mechanical properties were compared with the neat

iPP fibers (Figure 9 ). The fibers containing D40, D60 and

a mixture of D40 and D60, all prepared at λ = 3, showed

a slight decrease in tenacity with increasing content of

D60, as expected. However, at higher drawing ratios, the

iPP fibers with the larger particle size filler, D60, showed

a different influence on tenacity. At a drawing ratio λ = 5,

the iPP + D60 fibers showed a little higher tenacity than

the iPP fibers containing a mixture of D40 and D60 fillers

(Figure 9). However, at the highest drawing ratio, λ = 7.8,

the tenacity of all the iPP fiber samples containing D40

or D60 was lower than the tenacity of the neat iPP fibers.

That could be caused by: a) the filler at these conditions

does not achieve the degree of orientation of the neat iPP

fiber or b) the lower orientation degree of the iPP fiber

matrix affected by the filler is not compensated by the

reinforcement effect of the filler. By contrast, the tenac-

ity of iPP + D40 fibers was again higher than fibers based

on iPP + D60. At this drawing ratio, there was a not very

expressive synergetic effect of the mixed filler system

iPP + D40 + D60 (1:1) at which the value of the tenacity was

higher than both the iPP fibers containing D40 or D60

(Figure 9). This could mean that at the highest orientation

of the iPP fibers, the Disperal particles of both average

sizes, after drawing of the fibers, are more effective when

located in free volumes of the amorphous part of iPP, than

any kind of dispersal filler alone.

It is also interesting to compare the effect of the

drawing ratio separately on the elongation of iPP fibers

containing D40 and D60. At the lower drawing ratios

λ = 3 and λ = 5, the values obtained for the elongation of

iPP + D40 fibers were a little higher than for neat iPP fibers

(Figure 10 ). It seems that, at this elongation, the physi-

cal network formed in the presence of D40 nanoparticles

causes a partial perturbation of the inter-molecular attrac-

tive power among the iPP lamellas and also among the

chains in the amorphous part of the polymer matrix, and

Figure 8 TEM micrograph of the parallel cut of polypropylene

(iPP) + D60 nanocomposite fiber showing the dispersion of the filler

in the fiber (content = 2 wt % D60).

0

20

40

60

80

100

iPP (neat) iPP+2%D40 iPP+1%D40+1%D60 iPP+2%D60

Tena

city

at t

he b

reak

(cN

/Tex

)

λ=3 λ=5 λ=7.8

Figure 9 Comparison of tenacities of the nanocomposite fiber

containing mixture of the D40 + D60 with the fibers containing

separately D40 or D60 filler.

0

10

20

30

40

50

iPP (neat) iPP+2% D40 iPP+1% D40+1% D60 iPP+2% D60

Elo

ngat

ion

at th

e br

eak

(%)

λ=3 λ=5 λ=7.8

Figure 10 Comparison of the elongation of the polypropylene

(iPP) fibers containing different sizes of filler particles (all samples

contain 2 wt % of filler).




this allows for a little higher elongation than for the neat

iPP fibers. The elongation of the iPP fibers with 2 wt % of

D60 and prepared at λ = 7.8, was significantly lower than

that of iPP fibers prepared without and with 2 wt % D40

(Figure 10); this means that at the highest drawing ratio,

the larger nanoparticles of D60 affect the stiffness and

also the brittleness of iPP fibers much more than do the

particles of D40.

The comparison of tenacity and elongation of fibers

containing a mixture of 1 wt % of D40 and 1 wt % of D60

(Figures 9 and 10) with the fibers containing the same

amount of individual filler, showed some deviations from

the expected values. Also, the presence of D60 in the iPP

fibers prepared only at λ = 7.8 proved to have a greater influ-

ence on the tenacity than D40. We believe that, besides the

drawing ratio at the preparation of fibers, the distribution

of particle size also plays an important role; the tenacity of

the mixture of D40 + D60 (Figure 9) illustrates this.

4 Conclusions From the study of the morphology and the mechanical

properties of the iPP + boehmite nanocomposite fibers,

the following conclusions can be drawn:

– The TEM micrographs of the iPP + boehmite nano-

composite fibers prepared using two kinds of

boehmite fillers, Disperal D40 and D60, show that

the original fillers with a particle size > 300 μm, were

without additives, physically degraded (crumbled)

to the nanosized particles. During the compounding

with iPP and subsequent spinning.

– The aggregates of D40 or D60 crystallites in iPP fibers

formed short oriented linear “ chains ” of crystallite

aggregates, following the direction of spinning.

– The highest tenacity was obtained with the

iPP + boehmite fibers ( λ = 5) containing 1 wt % of filler.

The tenacity decreased if > 1 wt % of filler is used. This

is explained by aggregation of the filler particles.

– The drawing ratio of the fibers plays a decisive role

on the efficiency of the filler on mechanical proper-

ties of the iPP nanocomposite fibers. In our case,

the “ optimal drawing ratio ” was at about λ = 3 to 5.

The iPP fibers containing D40, D60 or their mixture,

prepared at a drawing ratio of λ = 7.8, showed no

positive effect on the mechanical properties of the

nano-composite iPP fibers.

– The mixture of both types of fillers, D40 and D60,

in the iPP fibers, has shown some deviations of

mechanical properties from the expected contribu-

tions of the individual fillers.

Acknowledgements: The experimental work was sup-

ported by EU FP6 project: NMP3-CT-2005-516972 “ NANO-

HYBRID ” and Agency VEGA, project VEGA-1/0444/09.

Received February 21, 2012; accepted August 17, 2012

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