Electrical, Thermal, and Mechanical Characterizationof Novel Segmented-Leg Thermoelectric Modules

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Electrical, Thermal, and MechanicalCharacterization of Poly(propylene)/CarbonNanotube/Clay Hybrid Composite Materials

Humberto Palza,* Boris Reznik, Manfred Wilhelm, Oscar Arias,Alejandro Vargas

A set of hybrid composite materials based on a PP matrix with multiwalled CNTs andclay particles is prepared and characterized. The incorporation of clay particles into apercolated composite with 3 wt% CNT disrupts the percolation, decreasing dramaticallythe electrical conductivity. As expected for layered fillers, PP/CNT/clay hybrid compositematerials and PP/clay composites display increases as high as 100 8C in the temperaturefor the maximum rate of weight loss. Surprisingly, these temperatures are justslightly higher than those of PP/CNTcomposites. PP/CNT composites dis-play viscosities that are considerablylower than those of PP/clay compo-sites. A synergistic effect of both fillersis observed in the viscoelastic responseof PP/CNT/clay materials.

1. Introduction

Carbon nanotubes (CNTs) have attracted a large interest due

to their extraordinary and unique properties.[1–7] In

particular, the addition of low amounts of CNTs into

polymer matrices can modify several of its properties such

as mechanical,[8–10] glass transition,[11,12] crystallization

processes,[13–16] melt flow instabilities,[17,18] die-swell,[17,18]

thermal stability[19,20] and viscoelasticity,[17,21,22] among

H. Palza, O. Arias, A. VargasFacultad de Ciencias Fisicas y Matematicas, Departamento deIngenieria, Quimica y Biotecnologia, Universidad de Chile,Beauchef 861, Casilla 277, Santiago, ChileE-mail: [email protected]. Reznik, M. WilhelmInstitut fur Technische Chemie und Polymerchemie, KarlsruheInstitute of Technology (KIT), Engesserstrasse 18, 76131 Karlsruhe,Germany

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others. However, because of the low electrical percolation

threshold found in composites based on CNT, they are

mainly studied to produce conductive polymeric materi-

als.[5–7,16,17,19,23] The outstanding results obtained in these

composites have recently motivated studies about the

effect of adding a third component on the polymer/CNT

behavior in order to further improve the performance of the

resulting hybrid composite material.

Zhang et al.[24] grew CNT in clay-supported iron

nanoparticles producing a hybrid material that was added

into a nylon-6 matrix. The resulting hybrid composite

material was successfully dispersed in the matrix allowing

significant improvements in the mechanical properties.

Noteworthy, these improvements were larger than those

from nylon-6/clay and nylon-6/CNT composites. Liu and

Grunlan[25] found that the electrical percolation threshold

of CNT is reduced by a factor of 5 when clay particles are

added into epoxy/single-walled CNT (SWCNT) composites.

One of the reasons explaining this improvement is the

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H. Palza, B. Reznik, M. Wilhelm, O. Arias, A. Vargas

excluded volume created by the mm-scale clay clusters

forming a segregated network of nanotubes.[25] Another

hybrid composite material was developed by Tjong

and co-workers[26] preparing poly(propylene) (PP)/CNT

materials with silver metal nanoparticles. The addition

of these metal particles promoted significantly the

electrical properties of the resulting material. The

mechanism for this improvement was the anchor of Ag

particles among CNT clusters facilitating the electron

transfer through the carbon filler. This model is supported

by the results of Balik and co-workers[27,28] studying hybrid

materials composed of blend matrices with carbon fiber

and graphite particles. A different approach to improve

the properties of PP/CNT composites was the addition of

CaCO3 microparticles obtaining a significant reduction in

the electrical resistivity that was explained by the

concept of effective concentration of CNT in the polymer

similar to the excluded volume theory.[29] Similar

results were reported by the same authors using other

fillers (e.g. talc and wollastonite) and polymer matrices

(e.g. polyoxymethylene and polyamide).[29] This theory

is further supported by the results from He and co-

workers[30] in epoxy/clay/vapor-grown carbon fiber

hybrids materials. Despite the above mentioned, a higher

electrical percolation threshold in PP/clay/CNT hybrid

composite materials than in PP/CNT composites has

been reported.[31] The disruption of the interconnecting

conduction paths from the CNT as caused by the clay

particles can explain this behavior. These results show that

the effect of a third component, for example clay particles,

on the performance of polymer/CNT composites is neither

simple nor completely understood justifying further

investigations.

Based on the above mentioned, the goal of this article is to

study the effect of clay particles on the behavior of PP/CNT

composites. In particular, the electrical conductivity,

thermal behavior and viscoelastic properties in the melting

state are discussed within this article. To analyze the real

contribution of the clay particles on PP/CNT composites, PP/

clay and PP/CNT composites were further prepared and

characterized.

2. Experimental Section

A commercial-grade PP from Petroquim S. A. (PH1310) with a melt

flow rate (190 8C/2.16 kg) of 13 g � (10 min)�1 was used as matrix.

The multiwalled CNTs are commercially available from Bayer

Material Science AG (Baytubes C150P). The montmorillonite clay

filler is a Cloisite 15A from Southern Clay Product Inc. (USA) with a

cation exchange capacity (CEC) of 125 mequiv. per 100 g clay.

The composites and the master batch were prepared by using a

Brabender plasticorder internal mixer at 190 8C and a speed of

110 rpm during 10 min. For PP/CNT composites, predetermined

amounts of the nanofiller, antioxidant and neat polymer were

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mixed under nitrogen atmosphere in order to obtain the

composites with filler content ranges from 1 to 8 wt%. For PP/

clay composites a commercial PP grafted with maleic anhydride

(Polybond 3200) was further used as compatibilizer. In this case, a

master batch containing a mixture of silica layered particles and

the compatibilizer with a weight ratio of 1:3 was prepared. By

mixing predetermined amounts of the master batch, antioxidant

and neat polymer, PP/clay composites containing 1, 3 and 5 wt% of

clay were prepared. For the PP/CNT/clay hybrid materials, first the

polymer was added to the mixer followed by the proper amount of

CNT to produce a composite with 3 wt% of filler. Afterward, the

desired amount of master batch to prepare hybrid materials with:

1, 5 and 10 wt% of clay is added. The conditions are the same as for

the composites.

The dielectric spectra were obtained by an Alpha-Analyzer

(Novocontrol Technologies). The values reported in this contribu-

tion correspond to the conductivity at the lowest frequency studied

(0.1 Hz) that can be approximately considered as the DC

conductivity. Rheological measurements were carried out on a

strain controlled rheometer (ARES, TA Instruments) at 190 8C in

parallel plate geometry (25 mm diameter). Transmission electron

microscopy (TEM) images were recorded in a Philips model CM 100

at 80 kV. Ultrathin sections of about 70 nm were obtained by

cutting the samples with an Ultracut Reichert-Jung microtome

equipped with a Diatome diamond knife. Thermal gravimetric

analysis (TGA) were carry out in a DuPont 951 equipment under

oxidative conditions (air) with a flow of air rate of 100 mL �min�1.

The heating scan was 10 8C �min�1. Several measurements were

carried out for the pure polymer in order to evaluate the standard

deviation of TGA. In these measurements the experimental error in

the temperature for the maximum rate of weight loss for the pure

sample was about 2% that it can be extrapolated to the other

samples. Therefore, changes in this temperature in more than 15 8Cbetween samples can be considered as significantly different by, for

example, the ANOVA statistical analysis.

3. Results

3.1. Electrical Conductivity

Figure 1 displays some representative images of a PP/CNT

composite with 5 wt% of nanotubes. A homogeneous

dispersion of CNT clusters of different sizes and shapes is

mainly observed showing that the shear forces produced

during the melt-blending are able to break-up the initial

micrometric aggregates of CNT. Similar morphologies have

been found in other systems as described elsewhere.[32,33]

TEM images from this sample do not show an inter-

connected 3D network of CNT. However, this sample

displays an electrical conductivity that is four orders of

magnitude higher than either pure sample or composites

with lower filler concentration as displayed in Figure 2

showing the electrical conductivity of the PP/CNT compo-

sites. Therefore, the presence of a continue conduction path

of CNT through our composite with 5 wt% of CNT exists

although not observed by our TEM images.[5,23] This

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Figure 1. TEM images of a PP/CNT composite with 5 wt% of filler. The picture on the leftis a magnification of representative CNT agglomerates.

Electrical, Thermal, and Mechanical Characterization . . .

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hypothesis is supported by single particles of CNT forming

an interconnecting network between some agglomerates

as observed in TEM micrographs at larger magnification

(see left-side of Figure 1). Figure 2 further shows that the

composite with 3 wt% of filler is above the percolation

threshold and a slightly increase in the conduction is

observed at higher filler contents. Based on this information

about PP/CNT composites, a set of hybrid composite

materials containing 3 wt% of CNT and between 1 and

5 wt% of clay were prepared and characterized.

Figure 3 displays the effect of adding clay particles on the

electrical conductivity of PP/CNT composites. A drastic

decrease in the conduction behavior is observed when

layered particles are incorporated even at 1 wt%. This

tendency cannot be predicted by the excluded volume

Figure 2. Effect of the CNT content on the electrical conductivityof PP/CNT composites.

Figure 3. Effect ofCNT with 3 wt%

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theory stating that the volume available

for the CNT is reduced by the presence of a

third inert component shortening the

CNT/CNT distance. These shorter dis-

tances increase the electrical conductiv-

ity as explained by models based on

random mixtures of resistors and capa-

citors.[23,34,35] Figure 4 displays represen-

tative TEM images of the PP/CNT/clay

hybrid composite material with 5 wt% of

clay showing that the CNT networks are

disrupted by the well disperses clay

particles. In particular, single or agglom-

erated CNT are contacted with the clay

forming CNT islands surrounded by clay

particles. This mechanism is explained in

the schemes of Figure 4 (right side) and it

is further supported by the affinity that

CNT seem to have for clay particles as

reported previously.[25] The original com-

posite displays well disperses and inter-

connected CNT clusters that become isolated by the

presence of clay particles forming islands represented by

the grey circle in the scheme of Figure 4. These isolated

islands could be characterized by shorter CNT/CNT

distances as the excluded volume theory states. Therefore,

in PP/CNT/clay hybrid composite materials two phenom-

ena could occur simultaneously: (a) a decrease in the

effective volume accessible for the CNT increasing the

conductivity as explained by the theory based on mixtures

of resistors and capacitors and (b) a disruption of the CNT

network by the layered particles forming highly conductive

islands isolated by clay platelets. The competition between

these two phenomena can explain our results showing a

decrease in electrical conductivity when clay particles are

added to PP/CNT samples and those results from previous

clay content on the electrical conductivity of PP/of CNT.

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Figure 4. Left: TEM images of PP/CNT/clay hybrid composite material with 3 wt% CNTand 5 wt% clay. Right: The conduction path of CNT in a polymer matrix (upper part) thatis disrupted by the presence of clay (lower part). The gray circle represents an island ofCNT isolated by clay platelets.

Figure 5. TGA results for PP/CNT composites under oxidativeconditions.

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publications showing an opposite tendency.[25] If these

islands become interconnected an increase in the con-

ductivity will be observed because of the shorter filler-filler

distances allowing tunnel effect for electrons.[25]

To understand when one mechanism becomes more

relevant than the other is a complex task as it can depend of

several interrelated factors such as concentration of fillers,

dispersion degree and size of inert particles. Based on

previous results we can assume that the main parameter

governing the effect of add a second particle into polymer/

CNT composites is the size of the inert particle. Larger

particles such as a second epoxy matrix or calcium

carbonate microparticles are not as effective as well-

dispersed clay particles in enhance the conductivity of

polymer/CNT composites.[29,30] In particular, a much higher

concentration of these inert particles than clay is needed to

see an improvement in the electrical conductivity.[29,30]

Therefore, we can assume that when clay is the second

particle added to polymer/CNT the main parameter

governing the conductivity behavior will be its dispersion

degree in the polymer matrix. Under this hypothesis, the

mechanism driven the effect of clay will further depend on

the affinity of the matrix with the clay, processing

conditions, compatibilizer, concentration of the different

components and molecular weight of the polymer matrix,

among other variables.[36–38]


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3.2. Thermal Analysis under

Oxidative Conditions

Figure 5 displays TGA results from PP/

CNT composites. Despite platelet parti-

cles of high aspect ratio, for example clay,

are claimed to be necessary for thermal

stabilization, CNTs are able to render

improvements as high as 100 8C in the

temperature for the maximum rate of

weight loss (Tpeak) under oxidative con-

ditions. This improvement is propor-

tional to the filler content as observed

in Figure 5.

Independent of the specific conditions

of degradation, high aspect ratio silicate

layers reduce the out-diffusion of volatile

decomposition products within the

nanocomposites increasing the thermal

stability of the material.[39,40] Under

thermo-oxidative conditions, the pre-

sence of high-contact-area clay particles

could also hinder the penetration of

oxygen molecules from the gas phase

to the polymer bulk, protecting the

polymer.[41] Based on this information,

the addition of clay particles into PP/CNT

composites should further increase the

thermal stability of the sample. However, as observed in

Figure 6, PP/CNT/clay hybrid composite materials have a

thermal stabilization just slightly larger than PP/CNT

composites when the total amount of filler is plotted

against Tpeak. Moreover, PP/clay composites have a Tpeak

that is just 20 8C larger than PP/CNT composites at low filler

contents confirming that there is a weak effect of the

particle aspect ratio.

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Figure 7. Dynamic complex viscosity against the shear-strainfrequency for PP/CNT composites and pure PP measured at190 8C.

Figure 6. Effect of the filler content on the temperature for themaximum rate of weight loss (Tpeak) as measured by TGAmeasurements for all the samples studied. In the particularcase of PP/CNT/clay particles, the total amount of filler is dis-played.


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Therefore, based on our findings the effect of the particle

aspect ratio on the thermal stabilization of polymer

composites should be reduced. Plate-like particles of high

aspect ratio are not the only fillers suitable for thermal

improvements in polymer composites.[42] Nanoparticles of

high specific area can adsorb radicals or high polar groups

improving the thermal stability independent of the aspect

ratio as recently reported for PP/silica nanosphere compo-

sites.[42] This is due to the physical/chemical adsorption of

volatile degradation products on the particle surface. This

mechanism can be more important for thermal stabiliza-

tion than others such as the nanoconfinement (decrease in

the diffusion processes as the polymer increase its stiffness

due to the nanoparticles) or the labyrinth effect (decrease in

the diffusion processes due to the presence of particles

increasing the tortuosity of the system)[42] The presence of

oxygen at high temperatures produces peroxide chains,

macro-radicals, oxidative dehydrogenation chains, oxi-

dized volatile products, unsaturated hydrocarbons, alco-

hols, ketones, esters, etc., during the oxidative degradation

of PP.[43] The hydroxyl and other polar or functional groups

from the surface of the silica nanoparticle can adsorb these

degradation products explaining the stability in polymer/

clay composites.[42,44] In the case of CNTs, the stability could

also be associated with the adsorption of these highly

volatile products on the surface of the particles.[23,45,46] In

this way, CNTs can act as a scavenger for these gaseous

products.[23,47,48]

3.3. Viscosity Analysis

Figure 7 displays the effect of the shear frequency on the

complex dynamic viscosity of the pure PP and the

composites with CNT. From this Figure 7 is concluded that





composites with concentration of filler higher than 3 wt%

present a rheological percolation associated with a solid-

like behavior evidenced by a drastic increase in the viscosity

at low shear frequencies. Therefore, a three-dimensional

network is formed restraining the polymer motion as the

CNT/CNT distance becomes comparable to the size of the

polymer chain.[23,49] These physical interactions between

the polymer and the nanoparticle are confirmed observing

that the effect of the filler is much more important at low

frequencies where large length scales are probed.[18,49]

By adding clay particles into PP/CNT composites, the

rheological percolation is also observed but much larger

viscosities are displayed. For example, the hybrid composite

material containing 3 wt% of CNT and 5 wt% of clay

presents a viscosity that is larger by a factor of 2 than the

composite with 8 wt% of CNT. This effect is better observed

in Figure 8 where the dynamic complex viscosity measured

at the lowest shear frequency used is plotted for both

composite and hybrid composite materials.

To deeply understand these results, the values from PP/

clay composites are also displayed. Figure 8 shows that

composites based on these layered particles have viscosities

that are as large as six times the viscosity of PP/CNT

composites at the same filler concentration. Therefore, high

aspect ratio layered particles render stronger restrains to

polymer molecules showing that the particle aspect ratio is

a very relevant variable in the viscoelastic behavior of

composites. In diluted particle/solvent systems, the effect

of the particle aspect ratio on the viscosity or on the

geometrical percolation is clear.[50] Prolate (disc-like) and

oblate (fiber-like) ellipsoid particles render the highest

increase in the viscosity while spheres display the

minimum.[50] In melt state polymer composites, the

physical interaction of macromolecules with the particles

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Figure 8. Effect of the filler content on the dynamic complexviscosity measured at the lowest shear-strain frequency for allthe samples studied. In the particular case of PP/CNT/clayparticles, the total amount of filler is displayed.

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should be further considered. The interfacial region controls

new structural arrangements on the molecular scale and is

responsible for the efficient transfer of stress across the

composite components.[51] For a cylindrical filler of radius r

and length L, the surface to volume ratio of the filler is:

Ac

Vc¼ 2

rþ 2

L

where Ac and Vc are the area and the volume of the

particle, respectively. This relationship demonstrate that

platelet-like particles (r> L) present higher specific area

than fiber-like particles (L> r). In particular, if clay

particles are assumed as cylinder-like platelets, they will

have a surface to volume ratio about 0.5 nm�1 (L¼ 5 nm

and r¼ 300 nm)[36] while CNT (L¼ 1 mm and r¼ 15 nm)

have a value about 0.1 nm�1. Therefore, the higher specific

area of platelet particles explains the mechanical response

of our samples. It is stressed that this analysis is only valid

for physical interactions such as those involved in the

rheological response of a nanocomposite based on a low

polar matrix. Equation 1 cannot be used to analyze TGA

results as in this case the chemical interaction between the

filler and the degradation products is the driven force that

are different in clay and in CNT.

However, the larger viscosities of the hybrid composite

materials cannot be explained by a linear superposition of

the contribution of clay and CNT. For example, the viscosity

of the hybrid composite sample containing simultaneously

3 wt% of CNT and 5 wt% of clay is larger than the sum of the

viscosities of the composites with 3 wt% of CNT and 5 wt%

of clay. This tendency is understood as a synergistic effect of

the layered and tube-like particles in the hybrid composite

material.



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Figure 7 and 8 give further information about the

mechanism explaining the thermal stabilization of the

composites. Samples with low filler content display a

thermal stabilization (see Figure 6) but do not have any

relevant increase in the viscosity. This means that the filler

network is not formed and the chains do not have constrains

to move in the melt. Therefore, the thermal stabilization

observed in Figure 6 is not associated with the nanocon-

finement phenomenon stressing that the physical/chemical

adsorption of volatile products is the plausible mechanism.

4. Conclusion

By adding different amounts of clay particles into a PP/CNT

composite with 3 wt% of filler, the effect of a third inert

component on the properties of the resulting hybrid

composite material is studied. Although the original PP/

CNT composite displays an electrical percolation transition,

the presence of well-dispersed clay particles drastically

decreases the electrical conductivity of the material. This

behavior is related with the disruption of the conduction

path of CNT through the composite because of the inorganic

filler. This disruption forms isolated CNT islands sur-

rounded by the clay layers in the polymer matrix explaining

the reduced conductivity. Regarding the thermal stability

under oxidative conditions, PP/CNT composites present

increases in Tpeak as large as 100 8C as compared with the

pure polymer. Noteworthy, the addition of clay particles to

these composites slightly improve the thermal stability

confirming that the particle aspect ratio is not the main

parameter for changes in the thermal stability. Rheological

tests in the melting state otherwise show that the high

aspect ratio platelet-like particles render larger increases in

the rheological behavior of the matrix than tube-like

particles. Noteworthy, PP/CNT/clay hybrid composite

materials display the highest viscosities showing a

synergistic effect between both kinds of filler.

Acknowledgements: The authors gratefully acknowledge thefinancial support of CONICYT, projects FONDECYT INICIACIONEN INVESTIGACION 11075001. The authors also thank Dr W.Sierralta for the TEM images and Dr R. Quijada for the supportduring this research. The support of Prof H. Bockhorn and A. Unalwith TGA analysis is also acknowledged.

Received: July 27, 2011; Revised: September 22, 2011; Publishedonline: DOI: 10.1002/mame.201100249

Keywords: carbon nanotubes; clays; hybrid composite materials;nanocomposites; poly(propylene)

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Electrical, Thermal, and Mechanical Characterizationof Novel Segmented-Leg Thermoelectric Modules

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