Electrically Conductive Multi-walled CarbonNanotube-Reinforced Amorphous PolyamideNanocomposites

N. Aranburu, J.I. Eguiaz�abalDepartamento de Ciencia y Tecnolog�ıa de Pol�ımeros and POLYMAT, Facultad de Ciencias Qu�ımicas,Universidad del Pa�ıs Vasco UPV/EHU, P. O. Box 1072, 20080 Donostia, Spain

Nanocomposites (NCs) based on an amorphous poly-amide (aPA) and multi-walled carbon nanotubes(MWCNTs) were obtained by melt-mixing. As individualnanotubes were mostly observed, dispersion of thecarbon nanotubes was deemed good. The electricalpercolation threshold (pc) occurred at 2.97 wt%MWCNTs and as a result, electrical conductivityimproved by nine orders of magnitude upon addition of6 wt% MWCNTs. The 6 wt% MWCNTs also led to anincrease in both thermal stability (measured by thedegradation temperature) and Young’s modulus (19%)for the NCs, and ductility remained the same. POLYM.COMPOS., 35:587–595, 2014. VC 2013 Society of PlasticsEngineers

INTRODUCTION

The increasingly widespread incorporation of nanofil-

lers in polymeric matrices has created a new family of

polymeric materials: polymer nanocomposites (NCs).

Depending on the nature of the nanofiller, NCs are known

to enhance mechanical, thermal and barrier properties,

among others. Consequently, they are used successfully

for engineering applications [1, 2]. Carbon nanotubes

(CNTs) offer a high aspect ratio, a large specific surface

area, good mechanical strength, and electrical properties

[3, 4]. Therefore, small amounts of CNTs, when properly

dispersed in polymer matrices, appear to be able to

improve mechanical and electrical conductivity properties

substantially [5–8]. The “network-like structure” [9, 10]

of CNTs—formed once the so-called electrical percola-

tion threshold concentration has been reached—is usually

responsible for electrical conduction in insulating polymer

matrices [11]. Also efficient dispersion of the CNTs in

the matrix may help produce enhanced mechanical prop-

erties [12, 13].

Polymer/CNT nanocomposites are generally prepared

by “in situ” polymerization, solution dispersion and melt

mixing [14]. It can sometimes be difficult to disperse

nanotubes effectively using melt mixing which is more

suited for use with low concentrations as higher nanotube

loadings lead to higher viscosity [15]. Despite this, melt

mixing offers some basic advantages over other methods,

e.g., it is quick and easy to implement as the necessary

manufacturing technology is already available in the plas-

tics industry, and more importantly, the process does not

require the use of non-environment-friendly solvents or

monomers [16].

There are many studies in the bibliography concerning

melt mixed NCs based on polyamides (PAs) and MWCNTs

using both non-functionalized MWCNTs [16–21] and func-

tionalized MWCNTs when improved dispersion is required

[11, 22–29]. Among them, melt-mixed PA6/MWCNT NCs

show a percolation threshold between 2.5 and 5 wt%

MWCNT [21], and when amine-functionalized MWCNTs

were used [30], the results included enhanced interfacial

interaction, a more homogeneous dispersion of the

MWCNTs, and larger improvements in the Young’s

modulus [27].

As a semicrystalline matrix, in PA6, the dispersion of

the CNTs is affected by the presence of crystalline phase

microparticles which are insulating, because the advancing

crystalline fronts force the CNTs into the remaining amor-

phous phase. This usually leads to undesirably high electri-

cal percolation thresholds, which, for example, jump to

concentrations of between 4 and 6 wt% in MWCNT-filled

PA6 NCs [31, 32]. In amorphous polyamides (aPAs), how-

ever, this does not occur and while they are more difficult

to process than semicrystalline ones, they show attractive

properties such as good dimensional stability, favorable

dielectric and barrier properties, and reduced water sorption

[33]. However, no work on any aPA/CNT NC has been

published so far to our knowledge.

Correspondence to: J. I. Eguiaz�abal; e-mail: josei.eguiazabal@ehu.es

Contract grant sponsor: Spanish “Ministerio de Econom�ıa y Competitividad”;

contract grant number: MAT2010-16171; contract grant sponsor: Basque

Government; contract grant number: IT-611-13; contract grant sponsor:

University of the Basque Country; contract grant number: UFI11/56.

DOI 10.1002/pc.22699

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2013 Society of Plastics Engineers

POLYMER COMPOSITES—2014

For this study, NCs based on an aPA and MWCNTs

were obtained by melt-processing. MWCNT concentra-

tions were varied from 1 to 6 wt% to determine the per-

colation threshold. The phase behavior and the thermal

properties of the NCs were studied by dynamic

mechanical-thermal analysis (DMTA) and thermogravi-

metric analysis (TGA), respectively. The characterization

of the dispersion and the nanostructure was carried out by

transmission electron microscopy (TEM) and scanning

electron microscopy (SEM). The electrical properties of

the NCs were determined by electrical conductivity meas-

urements and the mechanical properties were measured

by tensile tests.

EXPERIMENTAL

The polymer used in this study was an aPA, GRILA-

MID TR55, from EMS Grivory. It is a random copoly-

mer comprising isophthalic acid, 12-aminododecanoic

acid, and bis-(4-amino-3-methylcyclohexyl)methane.

The carbon nanotubes were multi-walled nanotubes

with an outside diameter of 20–30 nm, inside diameter

of 5–10 nm, length of 10–30 lm and >95% purity

(Cheaptubes).

Drying before processing was performed at 100�C in a

vacuum oven for 24 h for both the neat aPA and the

aPA/MWCNT NCs. The NCs, with compositions ranging

from 1 to 6 wt% MWCNTs (aPA-X hereafter, where X

represents the MWCNT wt%), were first mixed by extru-

sion and afterwards, injection molded to obtain standard

testing specimens.

The extrusions were performed in a Collin ZK25 co-

rotating twin screw extruder-kneader at a melt tempera-

ture of 265�C and a screw rotation rate of 200 rpm. The

screw diameter and the L/D ratio were 25 mm and 30,

respectively. The extrudates were cooled in a water bath

and pelletized. Injection molding was carried out in a

Battenfeld BA-230E reciprocating screw injection mold-

ing machine to obtain tensile (ASTM D638, type IV,

thickness 1.84 mm) and impact (ASTM D256, thickness

3.1 mm) specimens. The screw of the plasticization unit

was a standard screw with a diameter of 18 mm, L/Dratio of 17.8, and a compression ratio of 4. The melt

temperature was 265�C and the mold temperature was

15�C. The injection speed and pressure were 10.2

cm3�s21 and 2750 bar, respectively. The specimens wereleft to condition for 24 h in a desiccator before analysis

or testing.

Dynamic mechanical-thermal analysis was carried out

in a TA Instruments DMTA Q800 in flexion mode (single

cantilever), that provided the plot of the loss modulus

(E00) against temperature. The scans were carried out from

2100 to 170�C at a constant heating rate of 4�C/min and

at a frequency of 1 Hz. The samples were taken from the

central part of the tensile specimens, and measured 17.5

3 5.9 3 2.0 mm. Two samples were tested for each com-

position, and the estimated standard deviation of the

curves was 66 MPa.

Cryogenically broken surfaces of the tensile specimens

were observed by SEM after gold-coating, using a Hitachi

S-2700 electron microscope operated at an accelerating

voltage of 15 kV. The transmission electron microscopy

(TEM) samples were ultrathin-sectioned at 30–40 nm

using an ultramicrotome. The micrographs were obtained

in a Tecnai G2 20 Twin microscope at an accelerating

voltage of 200 kV.

AC conductivity measurements were performed in the

frequency range 1012106 Hz and at room temperature

using a Novocontrol impedance analyzer adapted to an

ARES Rheometric rheometer with plate–plate geometry

of 25 mm diameter and 1 mm gap.

Thermogravimetric analysis was carried out on a TA

Instruments TGA-Q500 thermobalance. The scans were

carried out from 30 to 800�C under air atmosphere at a

flow rate of 80 mL/min. A heating rate of 10�C/min was

employed.

Tensile testing was carried out using an Instron 4301

machine at a cross-head speed of 10 mm/min, at 2362�Cand 50 6 5% relative humidity. Young’s modulus was

determined using an extensometer at a cross-head speed

of 1 mm/min. A minimum of five tensile specimens were

tested for each reported value.

RESULTS AND DISCUSSION

Phase Behavior and Structure

Figure 1 and Table 1 shows the loss modulus curves

and the glass transition temperatures of the NC matrix,

measured by DMTA, respectively. As can be observed,

the Tg of the aPA decreased slightly on addition of the

MWCNTs and, as was observed in other matrices [34],

the Tg bore no relation to the MWCNT content. This is

not an overall behavior because both lack of variations

and increases in Tg—attributed to the hindering effect

FIG. 1. Loss modulus of the (a) aPA, (b) aPA-1, (c) aPA-2, (d) aPA-3,

(e) aPA-4, (f) aPA-5, and (g) aPA-6 NCs.

588 POLYMER COMPOSITES—2014 DOI 10.1002/pc

of the nanotubes on the mobility of the polymeric

chains—have been reported [19, 35–38]. When

decreases like those in this study were observed in lit-

erature [16, 25, 28], they were attributed to the interfer-

ence of the MWCNTs in the interactions among the

polymer chains, leading to a larger free volume and to

a lower Tg [26].

The structure of the NCs was analyzed by SEM and

TEM. Figure 2 shows the SEM micrographs of the aPA

NCs with 1, 3, and 5 wt% MWCNTs, while Fig. 3 shows

the TEM micrographs of the aPA NCs with 2 and 5 wt%

MWCNTs. The rest of the NCs showed similar character-

istics. As Fig. 2 shows, some MWCNT ends (white spots)

appear to be protruding from the matrix. These nanotube

ends are uniformly distributed throughout the observed

area. This indicates that melt mixing was efficient enough

to achieve adequate distribution even at a high concentra-

tion of MWCNTs.

As can be seen in Fig. 3, the MWCNTs are well dis-

tributed and even though occasional MWCNT bundles

can be detected, most of nanotubes are found individu-

ally indicating that the quality of dispersion is also

good. A highly magnified nanotube can be seen in

Fig. 4 showing the shape of the nanotubes and how

they were pulled from the matrix. The white spots seen

on the SEM micrographs are, in fact, nanotubes. More-

over, there are two other features that suggest low

interfacial adhesion: (i) the protruding nanotubes which

did not break, and (ii) their surface which appears clean

without any adhered polymer residue. This low interfa-

cial adhesion [22, 26, 39] is attributed to the non-polar

nature of the MWCNTs and the polarity of the aPA

amide groups.

TEM testing gave a first indication of the formation of

a nanotube network, implying that the percolation thresh-

old was reached. As Fig. 3b shows (low MWCNT con-

tents), there is no contact between the nanotubes;

however, at high MWCNT contents (Fig. 3d), most CNTs

are touching; this suggests that a nanotube network capa-

ble of causing the percolation threshold probably exists.

This possibility was tested by means of electrical testing

and is discussed below.

TABLE 1. Glass transition temperatures of the aPA matrix.

Sample Tg (�C)

aPA 155.0

aPA-1 151.5

aPA-2 153.0

aPA-3 152.5

aPA-4 152.5

aPA-5 152.5

aPA-6 152.5

The average standard deviation is 0.5�C.

FIG. 2. SEM micrographs of the (a) aPA-1, (b) aPA-3, and (c) aPA-5 NCs.

DOI 10.1002/pc POLYMER COMPOSITES—2014 589

Electrical Conductivity

Figure 5a shows the AC electrical conductivity of the

NCs as a function of both frequency and MWCNT con-

centration, at room temperature. Only a few values for

the conductivity of the aPA-5 and aPA-6 samples are

reported because the values attained for these concentra-

tions were close to the resolution of the apparatus (e00 �109 pF/m). The conductivity plot for aPA-1 increased

linearly with frequency. That of aPA-2 shows a

frequency-independent plateau which is found below a

critical frequency. Linear frequency dependence was

observed when this value was exceeded. This behavioris consistent with the “Johnscher Universal Power Law”

for frequency dependent conductivity of solids [40]. At

higher MWCNT contents, conductivity was almost inde-

pendent of frequency, indicating the formation of a per-colating “network-like-structure” [11]. Thus, these

FIG. 3. TEM micrographs of aPA-2 (a and b) and aPA-5 (c and d) NCs.

FIG. 4. SEM micrograph of a carbon nanotube pulled out from the

matrix at high magnification.

590 POLYMER COMPOSITES—2014 DOI 10.1002/pc

results give us a first indication of the percolation

threshold, which seems to be located between 2 and 3

wt% MWCNTs.

The dependence of the DC conductivity at 20 Hz on

the MWCNT content is shown in Fig. 5b. As the figure

indicates, conductivity increased greatly with the

MWCNT concentration. The electrical conductivity

improves by nine orders of magnitude between aPA-1

and aPA-6 NCs (from 2 3 10211 to 2 3 1022 S/cm,

respectively). This latter conductivity value means that

these NCs would be suitable for electrostatic painting or

electrostatic dissipation applications [15].

According to other works in bibliography [17, 41] the

electrical percolation threshold (pc) was fitted using the

power law function for NC conductivity near the percola-

tion threshold [42]:

r pð Þ5A pc2pð Þ2s(1)

for concentrations of filler below percolation threshold

(p< pc), and

r pð Þ5B p2pcð Þt (2)

above percolation threshold (p> pc), where r(p) is the

experimental DC conductivity, A and B are the propor-

tionality constants, and t and s are the critical exponents.

For better comparison with other works in the literature,

Eq. (2) was used in order to determine the pc. The experi-

mental results were fitted in the subfigure by plotting

log(r) versus log(p 2 pc) incrementally varying pc until

the best linear fit was obtained at pc 5 2.97, B 5 2.8 3

1023 and t 5 1.80 [43, 44]. This t value is within the the-

oretical 1.6–2.0 range for three-dimensional (3D) perco-

lating systems [45, 46] (consistent with the percolation

threshold on the AC conductivity versus frequency plot),

indicating that a 3D percolating network was formed

at 2.97 wt% MWCNTs.

Regarding this noteworthy pc value, many parameters

have been suggested to influence the percolation thresh-

old in polymer/CNT NCs, such as the aspect ratio of the

nanotubes [44, 47], the degree of dispersion of the CNTs,

whether they are found separately or in agglomerates

[48], the processing method and parameters [49, 50],

the viscosity [18, 51], molecular weight [51, 52] or crys-

tallinity [11, 53] of the polymer matrix and the pure or

functionalized nature of the nanotubes [39, 54]. As some

of the stated parameters do not apply to the NCs of this

study, the rather good dispersion observed in Figs. 2

and 3, exhibiting mostly individual carbon nanotubes, in

addition to the high aspect ratio of the MWCNTs

(roughly 800) would cause a very low percolation thresh-

old. The obtained value is not so low; a possible decrease

in the aspect ratio of the carbon nanotubes during extru-

sion, that has been seen often to occur [41, 55], would

lead to a percolation threshold higher than expected based

on the SEM and TEM results.

To analyze the significance of the obtained results,

Table 2 shows a comparison with those of other works on

melt mixed polyamide/MWCNT NCs. As can be

observed, the percolation threshold in this study is compa-

rable with those reported in Refs. [11] and [31]. This is

attributed to the mutually offsetting effects of using the

purified state/masterbatch on the one hand, which should

lead to better dispersion (positive effect), and, on the

other, the negative effect (higher percolation values) of

crystallinity on the percolation threshold. Socher et al.

[18] obtained a comparable percolation threshold for

medium viscosity PA12 which may have been because

the positive effect of the lower viscosity of the PA12,

FIG. 5. (a) AC conductivity of the NCs as a function of the frequency and of the MWCNT content at

room temperature and (b) DC conductivity of the aPA/MWCNT NCs as a function of the MWCNT content.

The dashed line corresponds to the power law fit (inset: log–log plot of electrical conductivity versus

p 2 pc).

DOI 10.1002/pc POLYMER COMPOSITES—2014 591

compared to aPA, counteracted the negative effect of the

crystallinity. In the case of reference [24], where a lower

percolation threshold was obtained, a negative effect of

the higher viscosity of the aPA may take place.

Thermal Stability

The thermal stability of the aPA/MWCNT NCs was

studied by TGA under air environment. The results are

displayed on Fig. 6, where the curves of the aPA-6 NC

are shown as well as those of the neat aPA, as a refer-

ence. The plots of the rest of the NCs showed similar

characteristics. As can be observed, the decomposition of

the aPA was complete at approximately 550�C, and at

630�C for the aPA-6 NC. Moreover, thermal degradation

temperatures were higher for the NCs in all the ranges

studied. This is consistent with previous results [56, 57].

Table 3 shows the T10% and Tmax of the NCs as well as

the corresponding increases in relation to the values of

the matrix. The improvement in the degradation tempera-

ture was slight at the initial stages of degradation (T10%),

but it was considerable at intermediate stages (Tmax). In

the latter case, stabilization seems to increase with the

MWCNT content and an increase in Tmax of approxi-

mately 20� was detected.

A number of mechanisms have been suggested as

being responsible for improving thermal stability [15]: (i)

dispersed nanotubes might hinder the flux of degradation

products through a barrier effect, thereby delaying the

onset of weight loss [58], (ii) polymer chains near the

CNTs might degrade more slowly and form protective

layers [59] shifting Tmax to higher temperatures, and (iii)

greater thermal conductivity in the NCs facilitates heat

dissipation within the composite resulting in improved

thermal stability [15, 60]. Given that the flux of degraded

products is insignificant at low temperatures, and that

most polymer chains are not in contact with the CNTs,

the most probable cause for the behavior observed in

Table 3 is the significant increase in thermal conductivity

[15], which rose to as much as 100% upon the addition

of 3 vol% CNTs [60].

Mechanical Properties

Young’s modulus of the NCs is shown in Fig. 7 and

summarized in Table 4 as a function of the CNT content.

The yield strength showed behavior qualitatively similar

to that of the modulus but with lower increases, as seen

previously in other polymer/MWCNT NCs [23, 61]. As

Fig. 7 reflects, the modulus showed a linear increase with

the CNT content. The improvement in the elastic modulus

of the NCs is the result of the very high modulus of the

CNTs and the large contact surface area that cause a rein-

forcing effect. The addition of 6 wt% MWCNTs led to a

19% increase in the elastic modulus. To assess the mean-

ing of these results, the mechanical properties obtained in

other melt mixed PA NCs with 1 wt% MWCNTs are

summarized together with those of this study in Table 5.

As can be seen, the modulus improvements obtained with

purified MWCNTs or MWCNT masterbatches were

clearly higher than those obtained with untreated/directly

mixed MWCNTs indicating that dispersion of the nano-

tubes is better in the former. However, the values

TABLE 2. Percolation threshold concentrations for melt-mixed

PA/MWCNT NCs.

NC pc (wt%) References

aPA/MWCNT 2.97 This study

PA6/MWCNT (purified) 2–3 [11]

PA6/MWCNT (masterbatch) 4–6 [31]

PA12/MWCNT

Low viscosity PA12 1

Medium viscosity PA12 2–2.5 [18]

High viscosity PA12 3.5

PA12/MWCNT 1.33 [24]

FIG. 6. (a) TGA and (b) DTG curves of the neat aPA and the aPA-6 NC.

592 POLYMER COMPOSITES—2014 DOI 10.1002/pc

obtained from procedures similar to those in this study

[26, 31] show slight differences despite the different

chemical nature of the matrix.

The elongation at break of the NCs in this study

(Table 4) was very similar to that of the matrix (all broke

in the cold drawing stage); the ductile character and the

high elongation at break remained constant despite the

non-functionalized nature of the nanotubes and the result-

ing low degree of interaction between the aPA and the

MWCNTs. Individual CNTs often tend to bundle together

due to the intrinsic Van der Waals attraction which, com-

bined with their high aspect ratio and surface area, leads

to agglomeration. The agglomerates may act as stress

concentrators preventing an efficient load transfer to the

matrix [7, 62]. Although the nanometric scale of the

CNTs means this effect on the NCs is small, the absence

of a significant decrease in ductility, points to a reduced

number of large CNT agglomerations in the system of

this study. To give a more general idea of the significance

of these results, some ductility results from the literature

are reported in Table 5. As can be seen, the ductility of

the NCs in this study is better than those observed in

NCs with both non-functionalized and purified/master-

batch MWCNTs. This is consistent with the slightly lower

modulus values recorded, because modulus and ductility

are mostly contrary properties. Nevertheless, for most

potential applications, the fact that in our study, there was

no significant decrease in ductility would seem to out-

weigh the slight increase detected in modulus.

CONCLUSIONS

The MWCNTs were uniformly distributed throughout

the matrix, whatever the MWCNT content, in the form of

mostly individual nanotubes.

MWCNT concentrations over 2 wt% led to constant

conductivity over a wide range of frequencies, indicating

that a percolating network of carbon nanotubes was

formed at 2.97 wt% MWCNTs. The electrical conductiv-

ity was improved by nine orders of magnitude upon

addition of 6 wt% MWCNTs. The absence of the nega-

tive effect of crystallinity in the matrix allows the con-

ductivity threshold to appear at low MWCNT

concentrations.

The improvement in the degradation temperature upon

addition of MWCNTs was slight in the initial stages of

degradation (T10%), but considerable at intermediate

stages (Tmax). This is mainly attributed to the increase in

thermal conductivity in the NCs.

Young’s modulus of the NCs linearly increased with

the MWCNT content (19% increase with 6 wt%

MWCNT). However, the elongation at break remained

almost constant as both the aPA and the NCs broke in

the cold drawing stage, and thus maintained ductile

behavior attributable to the absence of large CNT

agglomerations.

TABLE 3. Degradation temperatures of the NCs on TGA analysis.

Sample T10% (�C) DT Tmax (�C) DT

aPA 408 – 436 –

aPA-1 413 5 446 10

aPA-2 413 5 454 18

aPA-3 419 11 459 23

aPA-4 416 8 461 25

aPA-5 418 10 458 22

aPA-6 418 10 457 21

FIG. 7. Young’s modulus for the aPA NCs as a function of the

MWCNT concentration.

TABLE 4. Mechanical properties of the aPA/MWCNT NCs.

Sample Young’s modulus (MPa) (650) Ductility (%) (65)

aPA 2150 80

aPA-1 2250 80

aPA-2 2300 75

aPA-3 2300 45

aPA-4 2350 65

aPA-5 2500 75

aPA-6 2550 55

TABLE 5. Mechanical properties from bibliography in melt-mixed PA/

MWCNT NCs compared with those in this study.

NC

Increase in

Young’s

modulus (%)

Ductility

decrease (%) References

aPA/1 wt% MWCNT 5 0 This study

PA6/1 wt% MWCNT

(purified)

24 47 [28]

PA6/1 wt% MWCNT

(masterbatch)

19 23 [27]

PA6/1 wt% MWCNT 6 48 [26]

PA6/1 wt% MWCNT 8 75 [31]

The properties have been interpolated when necessary to obtain the

value for 1 wt% MWCNT.

DOI 10.1002/pc POLYMER COMPOSITES—2014 593

ACKNOWLEDGMENTS

Technical support provided by SGIker (UPV/EHU,

MICINN, GV/EJ, ERDF, and ESF) is gratefully

acknowledged. N. Aranburu acknowledges the Basque

Government for the award of a grant for the

development of this study.

REFERENCES

1. E.T. Thostenson, Z. Ren, and T.W. Chou, Compos. Sci.Technol., 61, 1899 (2001).

2. R.H. Baughman, A.A. Zakhidov, and W.A. de Heer, Science(Washington, DC), 297, 787 (2002).

3. S. Iijima, Nature (London), 354, 56 (1991).

4. M. Naraghi, Carbon Nanotubes: Growth and Applications,

In Tech, Rijeka (2011).

5. S. Subramoney, Adv. Mater. (Weinheim, Germany), 10,

1157 (1998).

6. J.N. Coleman, U. Khan, W.J. Blau, and Y.K. Gun’ko, Car-bon, 44, 1624 (2006).

7. R. Andrews and M.C. Weisenberger, Curr. Opin. Solid StateMater. Sci., 8, 31 (2004).

8. T. McNally and P. P€otschke, Polymer-Carbon Nanotube

Composites: Preparation, Properties and Applications,

Woodhead Publishing, Cambridge (2011).

9. T. Kashiwagi, F. Du, J.F. Douglas, K.I. Winey, R.H. Harris,

and J.R. Shields, Nat. Mater., 4, 928 (2005).

10. S.B. Kharchenko, J.F. Douglas, J. Obrzut, E.A. Grulke, and

K.B. Migler, Nat. Mater., 3, 564 (2004).

11. P.V. Kodgire, A.R. Bhattacharyya, S. Bose, N. Gupta,

A.R. Kulkarni, and A. Misra, Chem. Phys. Lett., 432, 480

(2006).

12. G. Pandey and E.T. Thostenson, Polym. Rev. (Philadelphia,PA), 52, 355 (2012).

13. B.P. Grady, Carbon Nanotube-Polymer Composites:

Manufacture, Properties and Applications, Wiley, Hoboken

(2011).

14. Z. Spitalsky, D. Tasis, K. Papagelis, and C. Galiotis, Prog.Polym. Sci., 35, 357 (2010).

15. M. Moniruzzaman and K.I. Winey, Macromolecules, 39,

5194 (2006).

16. E. Logakis, C. Pandis, V. Peoglos, P. Pissis, C. Stergiou,

J. Pionteck, P. P€otschke, M. Micusik and M. Omastova,

J. Polym. Sci., Part B: Polym. Phys., 47, 764 (2009).

17. R. Socher, B. Krause, R. Boldt, S. Hermasch, R. Wursche,

and P. P€otschke, Compos. Sci. Technol., 71, 306 (2011).

18. R. Socher, B. Krause, M.T. Mueller, R. Boldt, and

P. P€otschke, Polymer, 53, 495 (2012).

19. H. Mahfuz, A. Adnan, V.K. Rangari, M.M. Hasan,

S. Jeelani, W.J. Wright, and S.J. DeTeresa, Appl. Phys.Lett., 88, 083119/1 (2006).

20. I. Alig, D. Lellinger, M. Engel, T. Skipa, and P. P€otschke,

Polymer, 49, 1902 (2008).

21. E. Logakis, C. Pandis, V. Peoglos, P. Pissis, J. Pionteck,

P. P€otschke, M. Micusik, and M. Omastova, Polymer, 50,

5103 (2009).

22. J. Li, L. Tong, Z. Fang, A. Gu, and Z. Xu, Polym. Degrad.Stab., 91, 2046 (2006).

23. T. Liu, I.Y. Phang, L. Shen, S.Y. Chow, and W.-D. Zhang,

Macromolecules, 37, 7214 (2004).

24. S. Chatterjee, F.A. Nuesch, and B.T.T. Chu, Nanotechnol-ogy, 22, 275714/1 (2011).

25. A.-C. Brosse, S. Tence-Girault, P.M. Piccione, and L.

Leibler, Polymer, 49, 4680 (2008).

26. H. Meng, G.X. Sui, P.F. Fang, and R. Yang, Polymer, 49,

610 (2008).

27. H. Deng, E. Bilotti, R. Zhang, K. Wang, Q. Zhang, T. Peijs,

and Q. Fu, J. Appl. Polym. Sci., 120, 133 (2011).

28. H. Liu, X. Wang, P. Fang, S. Wang, X. Qi, C. Pan, G. Xie,

and K.M. Liew, Carbon, 48, 721 (2010).

29. W.D. Zhang, L. Shen, I.Y. Phang, and T. Liu, Macromole-cules, 37, 256 (2004).

30. G.-X. Chen, H.-S. Kim, B.H. Park, and J.-S. Yoon, Poly-mer, 47, 4760 (2006).

31. O. Meincke, D. Kaempfer, H. Weickmann, C. Friedrich, M.

Vathauer, and H. Warth, Polymer, 45, 739 (2004).

32. B. Schartel, P. P€otschke, U. Knoll, and M. Abdel-Goad,

Eur. Polym. J., 41, 1061 (2005).

33. M. Garcia, J.I. Eguiazabal, and J. Nazabal, Polym. Compos.,24, 555 (2003).

34. K. Kelar and B. Jurkowski, J. Appl. Polym. Sci., 104, 3010

(2007).

35. Y. Yan, J. Zhang, J. Cui, J. Cheng, and J. Liu, ColloidPolym. Sci., 290, 1293 (2012).

36. A. Szymczyk, J. Appl. Polym. Sci., 126, 796 (2012).

37. Z. Jin, K.P. Pramoda, G. Xu, and S.H. Goh, Chem. Phys.Lett., 337, 43 (2001).

38. R.A. Kalgaonkar and J.P. Jog, Polym. Int., 57, 114 (2008).

39. J. Li, Z. Fang, L. Tong, A. Gu, and F. Liu, J. Appl. Polym.Sci., 106, 2898 (2007).

40. A.K. Jonscher, Nature (London), 267, 673 (1977).

41. P. P€otschke, S.M. Dudkin, and I. Alig, Polymer, 44, 5023

(2003).

42. D. Stauffer and A. Aharony, Introduction to Percolation

Theory, Taylor & Francis, London (1994).

43. B.E. Kilbride, J.N. Coleman, J. Fraysse, P. Fournet,

M. Cadek, A. Drury, S. Hutzler, S. Roth, and W.J. Blau,

J. Appl. Phys., 92, 4024 (2002).

44. W. Bauhofer and J.Z. Kovacs, Compos. Sci. Technol., 69,

1486 (2009).

45. M. Weber and M.R. Kamal, Polym. Compos., 18, 711

(1997).

46. G. Hu, C. Zhao, S. Zhang, M. Yang, and Z. Wang, Poly-mer, 47, 480 (2006).

47. S. Bose, A.R. Bhattacharyya, A.P. Bondre, A.R. Kulkarni

and P. P€otschke, J. Polym. Sci., Part B: Polym. Phys., 46,

1619 (2008).

48. J. Li, P.C. Ma, W.S. Chow, C.K. To, B.Z. Tang, and J.-K.

Kim, Adv. Funct. Mater., 17, 3207 (2007).

49. T. Villmow, P. P€otschke, S. Pegel, L. Haeussler, and B.

Kretzschmar, Polymer, 49, 3500 (2008).

50. I. Alig, P. P€otschke, D. Lellinger, T. Skipa, S. Pegel, G.R.

Kasaliwal, and T. Villmow, Polymer, 53, 4 (2012).

594 POLYMER COMPOSITES—2014 DOI 10.1002/pc

51. H. Ha, S.C. Kim, and K.R. Ha, Macromol. Res., 18, 512 (2010).

52. G.R. Kasaliwal, A. Goeldel, P. P€otschke, and G. Heinrich,

Polymer, 52, 1027 (2011).

53. J.H. Du, J. Bai, and H.M. Cheng, eXPRESS Polym. Lett., 1,

253 (2007).

54. Y. Pan, L. Li, S.H. Chan and J. Zhao, Compos., Part A,

41A, 419 (2010).

55. B. Krause, R. Boldt, and P. P€otschke, Carbon, 49, 1243 (2011).

56. J. Pascual, F. Peris, T. Boronat, O. Fenollar, and R. Balart,

Polym. Eng. Sci., 52, 733 (2012).

57. A.K. Barick and D.K. Tripathy, Mater. Sci. Eng. B, 176,

1435 (2011).

58. B.M. Amoli, S.A.A. Ramazani, and H. Izadi, J. Appl.Polym. Sci., 125, E453 (2012).

59. F.-C. Chiu, B.-H. Li, and J.-Y. Jiang, Compos. Part A, 43,

2230 (2012).

60. E. Lizundia, A. Oleaga, A. Salazar, and J.R. Sarasua, Poly-mer, 53, 2412 (2012).

61. K. Prashantha, J. Soulestin, M.F. Lacrampe, P. Krawczak,

G. Dupin, and M. Claes, Compos. Sci. Technol., 69, 1756

(2009).

62. M.S. Dresselhaus, G. Dresselhaus, and P.H. Avouris, Car-

bon Nanotubes: Synthesis, Structure, Properties, and Appli-

cations, Springer, Berlin (2001).

DOI 10.1002/pc POLYMER COMPOSITES—2014 595