IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 275703 (11pp) doi:10.1088/0957-4484/18/27/275703

Carbon nanotube induced structural andphysical property transitions ofsyndiotactic polypropyleneGiuliana Gorrasi1, Valentina Romeo1, Diana Sannino1,Maria Sarno1, Paolo Ciambelli1, Vittoria Vittoria1,3,Biagio De Vivo2 and Vincenzo Tucci2

1 Department of Chemical and Food Engineering, University of Salerno, via Ponte donMelillo, 84084 Fisciano (SA), Italy2 Department of Electrical and Information Engineering, University of Salerno, via Ponte donMelillo, 84084 Fisciano (SA), Italy

E-mail: vvittoria@unisa.it

Received 5 March 2007, in final form 9 May 2007Published 12 June 2007Online at stacks.iop.org/Nano/18/275703

AbstractIn this paper we have studied the effect of increasing carbon multi-wallednanotube (CNT) concentration in composites of syndiotactic polypropylene(sPP) having the same crystalline form but different morphologies. Theattention was focused on the form I of sPP with different degrees ofperfection (in terms of percentages of chains in helical conformation, crystaldimensions and crystallinity) obtained using two different quenchingtemperatures from the melt, i.e. 25 and 100 ◦C. We observed a decreasingeffect of the crystallization temperature on increasing the nanotube contentup to the samples with 10% of CNT, that show a very similar structuralorganization independent of the undercooling. Only the amorphous phaseturns out more relaxed in the samples crystallized at the highest temperature.Either the thermal or the mechanical properties are improved on increasingthe CNT content in both series of samples. The electrical conductivityincreases in a similar manner in both series of samples and between 1 and3 wt% it shows a sizable step of about eight orders of magnitude, aphenomenon that can be regarded as the onset of a percolating structure forwhich charge transport may take place.

1. Introduction

In the last years the development of new materials, togetherwith advances in computational power, sensors, actuators,and microelectronic architectures, have enabled an evolvingshift from single-purpose materials to multifunctional systemsthat can provide greater value than the base materials alone.This shift is fundamental, and it is more and more resultingin performances that are truly much better than the sumof individual parts, components or materials. By choosingthe appropriate synthetic polymers as well as the inorganicfillers, unprecedented morphological control down to the

3 Author to whom any correspondence should be addressed.

nanoscale is obtained. For this purpose tailoring of thepolymer–inorganic interface is of key importance since thestructures generated at the nanoscale are a result of afine balance of competing interactions with those at theinterphases being the most important ones. The potentialfor new multifunctional materials lies in the versatility ofboth the polymer and the inorganic chemistry that canbe exploited in the materials synthesis. With emergingavailability of nanometre feature materials such as carbonnanotubes, fibres, plates, and particles, coupled to a growingability to characterize and manipulate systems at this scale,nanoengineered multifunctional materials could hold the keyto future development and use of advanced superperformingcomposites for several advanced applications [1–7]. In

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Nanotechnology 18 (2007) 275703 G Gorrasi et al

particular, the very appealing physical and mechanicalproperties of carbon nanotubes (CNT) [8–12], such as highelastic modulus, as well as remarkable thermal and electricalconductivity, have stimulated great interest due to the widerange of possible technological applications. Among thecurrent studies a research line of particular impact is indeedfocused on the manufacturing of CNT reinforced polymermatrix composites [13–16]. By combining the right choiceof base materials with the appropriate processing method,the excellent properties of the CNT may be transferred tothe polymeric composites. Moreover the good electricaltransport properties and high aspect ratios of CNT havethe ability, at a very low threshold of particle content, toinduce a tunable electrical conductivity in the compositesmaterials. Several studies have dealt with the electricalproperties of either single-walled (SW) or multi-walled (MW)CNT based composites. The composites exhibit a percolationtype behaviour depending on the processing method, polymermatrix and nanotube structural properties. On the other hand,there has been a great interest in the procedures of preparationof these materials and they are also considered a uniquemodel for studying the structure and dynamics of polymers inconfined environments [17–19]. A practical advantage of usingcarbon nanotubes in olefinic polymers, such as polyethyleneor polypropylene, is their organophilic nature that favoursdispersion into the polymer matrix. In contrast, since clayand silica are hydrophilic they often require both an organictreatment and a compatibilizing polymer modifier, e.g. PPgrafted with maleic anhydride [20].

In a previous paper we investigated syndiotactic polypropy-lene (sPP) and 1% (wt/wt) multi-walled carbon nanotube com-posites, prepared by melt mixing. We achieved a good disper-sion without any organic treatment of the nanotube surface orthe use of a compatibilizer. Moreover, an enhanced crystalliza-tion of the helical chains was found in composites crystallizedunder the same conditions as the pure sPP samples, indicatingthat the crystallization in a confined environment favours thehelical polymorph of sPP with respect to the trans-planar one.Carbon nanotubes improve the thermal stability of either poly-morph of sPP in nitrogen as well as in air, although the complexbehaviour of the degradation in air was interestingly found tobe dependent on the structural organization and morphology ofthe composite samples [21]. This first result showed the impor-tance of a careful structural and morphological study to derivea right correlation with the physical properties.

The purpose of this paper was to study the effect ofincreasing nanotube concentration (1; 3; 10% wt/wt) incomposites having the same crystalline form but differentmorphologies. The attention was focused on the form I of sPPwith different degrees of perfection (in terms of percentages ofchains in helical conformation and crystallinity) obtained usingtwo different quenching temperatures from the melt.

We have analysed the structure of sPP samples crystallizedin the presence of CNT, and their thermal, mechanical andelectrical properties. We have found interesting effects onthe crystallization in the presence of nanotubes. Moreover,a dramatic increase of the electrical conductivity has beenobserved in correspondence with the percolation thresholdaround 2–3% of CNT weight loading. The conductivityvalues obtained may be appropriate for a potential use of these

composites in several electronic applications, for exampleelectromagnetic interference (EMI) shielding components.

2. Experimental details

2.1. Carbon nanotube preparation

Carbon nanotubes (CN) have been prepared by ethylenecatalytic chemical vapour deposition (CCVD) [22, 23] onCo/Fe-modified gibbsite (γ -Al(OH)3). The catalyst wasprepared by wet impregnation of Al(OH)3 powder with cobaltacetate (2.5 wt%) and iron acetate (2.5 wt%) aqueous solution.Ethylene pyrolysis was carried out in a continuous flowmicroreactor fed by an ethylene–helium gas mixture obtainedfrom 99.9998% ethylene and 99.9990% helium cylinder gas.The reactor temperature was increased from 298 K up to 973 K(10 K min−1) under helium flow and then 90 (stp) cm3 min−1

flow rate of 10% ethylene in helium passed through the reactorfor 60 min. After this time, the ethylene–helium flow wasstopped and the reactor was cooled to room temperature underhelium flow. The as produced material, containing carbonnanotubes together with a small amount of amorphous carbonand catalyst, was treated with 50 wt% HF aqueous solution todissolve the catalyst support. The remaining solid was oxidizedwith HCl to remove metal particles and amorphous carbon(metal encapsulated at various locations inside the nanotubesis unattached). High purity multi-walled carbon nanotubes(MWNT) were finally obtained.

2.2. Materials and processing

2.2.1. Materials. Syndiotactic poly(propylene) (sPP pellets)was obtained from Aldrich Polymer Products. The polymerdensity, molecular weight, and melt index were 0.900 g ml−1,127 000, and 4.5 g (10 min)−1 (ASTM 1238), respectively.The tacticity was 89% syndiotactic pentads.

A reference sPP sample and sPP composite materialscontaining multi-walled carbon nanotubes (sPP–CNT) wereprepared using a laboratory-scale conical twin-screw extruderThermoHaake Micro-Compounder Minilab Rotor. The rotorspeed and blending temperature were kept at 200 rpm and180 ◦C, respectively. An integrated bypass valve allowed eitherrecirculation of the sample in a slit capillary backflow channelor extrusion of the material for further tests. This instrumentallows compounding and viscosity tests to be combined. Aftera transient of about 2 s, the pressure drop along the slit (i.e.,a direct measure of the material viscosity) in the test madewith pure sPP attained a constant value (±1%). This valuewas maintained for a mixing time longer than 30 min. Thecomposite showed a longer transient (about 10 min), afterwhich a constant value of the pressure drop was maintainedfor a mixing time longer than 15 min. In this case the plateauvalue in the pressure drop revealed an equilibrium state inthe inorganic dispersion (i.e., a homogeneity in the nanotubesdispersion along the polymer melt).

Both the pure sPP and the three sPP–CNT blends weresubjected to melting for 15 min. The composition of the blendswas 1%, 3%, 10% wt/wt of CNT. Nitrogen gas was purgedinto the extruder during the compounding to prevent thermaldegradation at high temperature.

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Upon completion of blending, the molten polymer wasejected from the extrusion and allowed to cool at ambienttemperature.

The extruded samples were moulded in a hot press (CarverInc.) at 170 ◦C, forming 100 ± 5 μm thick films, which wererapidly quenched in a bath either at room temperature or at100 ◦C and stored for 1 h. The samples are coded as 25sPPCNX or 100sPPCNX , where 25 and 100 are the temperatures ofthe quenching bath, and X is the weight percentage of carbonnanotubes.

2.3. Methods

Field emission scanning electron microscopy (FESEM)pictures were obtained with a LEO 1525 microscope. Thesamples were covered with a 250 A thick gold film using asputter coater (Agar mod. 108 A).

Transmission electron microscopy (TEM) images wereobtained with a Jeol 1200 EX2 microscope.

X-ray diffraction measurements (XRD) were performedwith a Brucker diffractometer (equipped with a continuousscan attachment and a proportional counter) with Ni-filteredCu Kα radiation (λ = 1.540 50 A).

The infrared spectra were obtained at room temperatureby using a Nexus Thermo Nicolet FTIR (32 scans collected).Powdered materials (100 mg, 0.5 wt% of CNT in KBr) werepelleted (pressure 2 × 104 kg cm−2) in self-supporting discs of0.1–0.2 mm.

Differential scanning calorimetry (DSC) measurementswere carried out using a thermal analyser Mettler DSC 822/400under N2 atmosphere at a heating rate of 10 ◦C min−1 between−50 and 200 ◦C.

Thermogravimetric analysis (TGA) was carried outwith a Mettler TC-10 thermobalance. sPP–CNT samples(about 17 mg) were heated from 25 to 800 ◦C at 10 ◦C min−1

heating rate under air flow. The weight loss was recorded as afunction of temperature.

The mechanical properties of the samples (10 mm initiallength) were evaluated from stress–strain curves obtainedusing a dynamometric apparatus INSTRON 4301. Themeasurements were carried out at room temperature with10 mm min−1 deformation rate. The initial length of thesamples was 10 mm.

The room temperature electrical conductivity was mea-sured by using a HV generator (FUG HCN35-6500) and apicoammeter (Keithley 6514) which enables high accuracyDC measurements of I–V characteristics over a wide range(10−15–10−3 A). Small strips of sPP/CNT with Pd/Pt sputtercoated ends as electrodes were used. A DC voltage was appliedalong the length direction of the strips and the correspondingcurrent was measured after stabilization. Particular care wastaken to avoid electrical disturbs and leakage currents.

3. Results and discussion

3.1. Microscopy observation of nanotubes

The high nanotube purity is clearly evidenced by TEM(figure 1) and a typical bundle of nanotubes is shown in theinset of the same figure. The tubes have an entangled structurepermitting them to better anchor in the embedding matrix than

Figure 1. TEM image of purified carbon nanotubes; the inset showsa bundle of CNT.

straight nanotubes. The average diameter and length of thetubes are 10–15 nm and up to 120 μm, respectively.

The morphology of the CNT bundle is clearly shownin figure 2(a) and at higher resolution in figure 2(b). Thetypical SEM picture of CNTs (figure 2(c)) recovered fromsample 25sPPCN1 after thermal oxidation up to 370 ◦C showswell separated carbon nanotubes (compare with figure 2(b)),implying good dispersion of CNTs in the sPP matrix.

3.2. Structural organization of sPP

Syndiotactic polypropylene presents a complex polymorphicbehaviour related to different chain conformations (helical ortrans-planar) as well as to different crystalline cells with thesame chain conformation [24–31]. A fraction of mesophasewith chains in trans-planar conformation can be formed attemperatures lower than 50 ◦C, even in samples crystallized inhelical chains, depending either on the syndiotactic index or onprocessing conditions, such as cooling rate and temperature.A careful structural analysis is therefore needed in order tocorrelate the structural organization to the physical properties.Moreover, the well established picture of processing conditionsagainst structural organization for the pure sPP samples canindeed be different for the crystallization in the presence ofcarbon nanotubes. We have prepared two series of samples,crystallized at 25 and 100 ◦C, in order to have the samecrystalline form, that is form I, with different crystallinitydegrees and different amounts of trans-planar mesophase.The structural organization has been studied with differenttechniques, reported in the following.

3.2.1. Differential scanning calorimetry. In figure 3the calorimetric curves of samples 25sPPCN0, 25sPPCN1,25sPPCN3 and 25sPPCN10 are reported. In all the sampleswe can observe between 0 and 15 ◦C a deflection from thebaseline due to the glass transition, and between 40 and70 ◦C a small endotherm due to the melting of the trans-planar mesophase. The main endotherm of melting appearsbetween 105 and 138 ◦C, indicating well crystallized samplesin all the cases. In figure 4 we show the calorimetriccurves of samples crystallized at 100 ◦C. We observe that

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(a)

(b) (c)

Figure 2. FESEM images of CNT ((a), (b)); FESEM image of sample recovered after thermal oxidation of polymer composites up to370 ◦C (c).

Figure 3. Calorimetric curves of samples quenched at 25 ◦C.

for the pure 100sPPCN0 sample the endotherm of the trans-planar mesophase is very reduced, as expected for a samplecrystallized at a temperature of 100 ◦C, that is higher than themelting of the trans-planar mesophase. The small trace presentin the curve is probably due to a very small formation of the

Figure 4. Calorimetric curves of samples quenched at 100 ◦C.

mesophase when the sample is extracted at room temperature.Moreover, increasing the content of carbon nanotubes thefraction of mesophase slightly increases, becoming more

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Figure 5. The glass transition temperature (Tg (◦C)), the meltingenthalpy of the mesophase (�H1 (J g−1)), the melting enthalpy ofthe crystalline phase (�H2 (J g−1)) for samples quenched at 25 ◦C(•) and at 100 ◦C (��).

evident. All the calorimetric data are collected in figure 5,where the glass transition temperature, Tg (◦C), the meltingenthalpy �H1 (J g−1) of the mesophase, and the meltingenthalpy �H2 (J g−1) of the crystalline phase are reportedand compared for the two series of samples. The mainresult regarding the glass transition temperature is that, besidessmall variations on increasing the CN content, the glasstemperature of 25sPP samples is always consistently higherthan that the Tg of the 100sPP samples. This indicates thatthe higher temperature of crystallization of 100 ◦C influencesthe amorphous phase, making the chains in this phase morerelaxed with respect to the samples crystallized at 25 ◦C.Also the melting enthalpy of the mesophase is consistentlyhigher in samples crystallized at 25 ◦C, and it increases onincreasing the CNT content. A lower value characterizesthe pure 100sPPCN0, although it increases on increasingthe nanotube content. However the fraction of mesophasealways remains lower in 100sPP samples than in 25sPPsamples. As for the melting enthalpy of the crystallinephase, it is higher for the sample crystallized at 100 ◦C, asexpected, although a fluctuation is observed for the 100CN1sample. Moreover a shoulder on the low temperature side

Figure 6. The crystallization temperature from the melt for samplesquenched at 25 ◦C (•) and at 100 ◦C (��).

is present in the melting curve, indicating a broad crystaldistribution. The samples crystallized at 25 ◦C show a decreaseof the melting enthalpy on increasing the CN content. Inconclusion, the calorimetric curves show that in the presenceof CN an increasing fraction of trans-planar mesophase isformed even in samples crystallized at 100 ◦C, whereas thecrystallinity decreases for both samples. The amorphous phaseis more relaxed in all the samples crystallized at the highertemperature. Studies of the crystallization isotherms are inprogress to better understand this behaviour.

All the samples, after melting up 200 ◦C, were kept atthis temperature for 5 min and cooled at a constant rateto determine the crystallization temperature. In figure 6we show this parameter for all the samples. We observethat both the pure samples crystallize at 80 ◦C, whereas allthe composite samples with carbon nanotubes crystallize athigher temperatures, around 100 ◦C, slightly increasing onincreasing the CN content. This result is in agreement withmany literature results pointing to the enhanced crystallizationof polymers in the presence of CN, although, in our case,this is reflected neither in better crystallization nor in highercrystallinity.

3.2.2. X-ray analysis. Figure 7 shows the x-raydiffractograms of samples quenched at 25 ◦C. All the samplescrystallized in the usual form I, characterized by the mostintense peaks at 12.3◦, corresponding to the (200) reflection,15.9◦ corresponding to the (010) reflection, and 20.8◦ of 2ϑ ,corresponding to the (210) reflection. The absence of the(211) reflection at 2ϑ = 18.9◦ indicates that we obtained,in each sample, the disordered modification of form I, asexpected for samples crystallized at low temperatures. Infact the preferential crystallization of the disordered formwas always found in samples of low syndiotacticity or inpowder samples crystallized from the melt at temperatureslower than 120 ◦C [25]. In this case, as is well documentedin other cases, departures from the fully antichiral packingboth along a and b axes may occur, leading to a less orderedform. Moreover, the peak at 15.9◦ of 2ϑ is not symmetric,

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Figure 7. The x-ray diffractograms of samples quenched at 25 ◦C.

Figure 8. The x-ray diffractograms of samples quenched at 100 ◦C.

showing a shoulder at 17◦ of 2ϑ , typical of the trans-planarmesophase [30]. It was previously shown that the trans-planar mesophase can be formed keeping the sample for along time at 0 ◦C, and this phase is stable up to 60 ◦C [30];the present results show that it can be formed, although ina small fraction, even at 25 ◦C. This result is in agreementwith the calorimetric DSC curves, where the melting of themesophase is apparent. No evident qualitative differences canbe seen comparing the diffractograms of the pure polymerand the nanocomposite samples. The very high undercoolingdetermined a so rapid and disordered crystallization that thepresence of the nanotubes did not further affect the process.

In figure 8 the diffractograms of samples quenched at100 ◦C are shown. Comparing the diffractograms of the purepolymers it is evident that at 100 ◦C the sample is muchbetter crystallized, with a reduced amorphous fraction andan almost absent peak at 17◦ of 2ϑ , corresponding to the

trans-planar mesophase, in agreement with the DSC results.From the diffractograms it is also evident that the amorphousbackground decreases going from 25sPPCN0 to 100sPPCN0.Beside the crystallinity, the sharpness of the peaks in thediffractogram of 100sPPCN0 indicates much larger and betterformed crystals. However, introducing the carbon nanotubesin the sample and increasing their concentration we observe apoorer and poorer capacity for crystallization and crystallineperfection. Indeed both the amorphous fraction and theshoulder at 17◦ of 2ϑ increase going from 1% to 10% CNTcontent and this last sample is not very much different from thesame sample crystallized at 25 ◦C (see figure 7). This clearlyindicates that, although the nanotubes act as nucleating agents,greatly increasing the crystallization kinetics, their presenceleads in any case to more disordered forms, eliminatingthe effect of a lower undercooling for samples crystallizedat 100 ◦C.

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Figure 9. The D200 parameter, evaluated using x-rays, for samplesquenched at 25 ◦C (•) and at 100 ◦C (��).

To obtain a better understanding of the crystallizationbehaviour in the presence of carbon nanotubes we determinedfor all the samples the crystal thickness. The crystaldimensions, as determined using the Scherrer equation, arereported in figure 9, where the values of D200, the coherentcrystalline domain size in the direction perpendicular to the(200) planes, obtained from the full width at half-height(fwhm) of the (200) profiles, are reported as a function of thenanotube content for the two series of samples. Comparingthe pure samples, 25sPPCN0 and 100sPPCN0, we observe thatthe thickness of the crystals is much higher for the samplecrystallized at the higher temperature, as expected. However,on increasing the nanotube content, although an irregular trendcan be observed, in particular for both samples with 3% ofnanotubes, the crystal dimensions converge toward similarvalues for both series of samples, in spite of a very differentcrystallization temperature. In the samples with 3% and 10%of CNT the irregular behaviour is related to the previouslyunderlined effect, that is the independence of the undercooling.An unpredictable value, related neither to the temperaturenor to the previous values is observed. This means thatat higher CNT content the crystallization occurs in such aconfined volume that the sample does not follow the usual rulesof crystallization. The crystal dimensions no longer dependon the undercooling, only on the CNT concentration. Thisvery interesting result needs a deeper analysis and will beinvestigated in forthcoming papers.

3.2.3. Infrared analysis. The infrared analysis is very usefulfor the chain conformation determination. In particular somebands are characteristic of the chains either in helical or trans-planar conformation [31]. We have investigated the bandsat 993 and 963 cm−1, assigned to helical and trans-planarconformations in the amorphous phase, respectively. Thesebands are very close together, and their ratio gives an indicationof the relative presence of the different conformations. Weshow in figure 10 the ratio of the helical to the trans-planarband intensities in the amorphous phase, as a function of theCN content. We observe that in the pure samples, the fractionof helical chains is higher in the sample crystallized at 100 ◦C,

Figure 10. The ratio of the helical to the trans-planar bands in theamorphous phase for samples quenched at 25 ◦C (•) and at 100 ◦C(��).

as expected. However, this ratio decreases on increasing thenanotube content, and converges to a similar value for thesamples with the highest concentration. This result is inagreement with the decrease of crystallinity and confirms theoverall convergence of the structural parameters, independentof the undercooling.

3.3. TG–DTG analysis

Thermal degradation in air of pure sPP and composite samples(figure 11) shows that oxidation of 25sPPCN0 occurs in thetemperature range 200–315 ◦C. The Lorentzian fitting of theDTG profiles (figure 11(c)) shows two curves, centred at 236and 258 ◦C. The peaks are probably due to the oxidationof trans-planar and helical fractions of sPP, respectively,residing in the melt and deriving from the previous structure.The DTG peak temperatures of 100sPPCN0 oxidation profileare quite close to 25sPPCN0 DTG peak values (237 and255 ◦C, respectively). However, different areas under the twocurves are calculated, suggesting that 100sPPCN0 had, beforemelting, a greater amount of the fraction attributed to helicalconformations with respect to 25sPPCN0, in agreement withthe XRD results.

The incorporation of CNT in the polymer matrix resultsin increased oxidative stability. Moreover, DTG curves ofsPP/CNT composites show complex profiles, suggesting thepresence of new contributions coming from the presence ofCNT, probably due to more or less strong interactions betweenCNTs and polymorphic forms.

The sample 25sPPCN1 exhibits a broad peak ofdegradation. The onset temperature is close to that of25sPPCN0, while the completion temperature is shifted to345 ◦C where also all the original helical fraction is completelyoxidized.

By comparing the DTG profiles of 25sPPCN1 and100sPPCN1 it appears that even if their oxidation occurs in thesame temperature range, a greater contribution to the oxidationat higher temperature is given by the sample 100sPPCN1,

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Figure 11. TG–DTG of samples 100sPPCNs (a) and 25sPPCNs (b); Lorentzian fitting of the asymmetric profile of the DTG peak (c).

showing, before melting, a greater helical fraction (see the x-ray section).

The enhancement of oxidative stability becomes evengreater as the CNT content increases. In particular, theonset temperature for 25sPPCN3 and 25sPPCN10 shifts tohigher temperature with respect to that for 25sPPCN1, whilethe completion temperature remains substantially unmodified,suggesting that the termination of the antioxidant effect istemperature dependent. However, the relative amount ofCNT in the sPP does not seem to proportionally affect theoxidative stability, requiring further investigation and study tobe clarified.

The oxidation profiles of 100sPPCN3 and 100sPPCN10are quite similar to those of the corresponding samplesquenched at 25 ◦C, suggesting that the oxidation behaviourof the polymer in the composite could be correlated withits morphology. Indeed we have shown that, when theCNT content in the composite increases, the x-ray diffractionpatterns of the composites quenched at different temperatures,but having the same CNT concentration, are very similar.

The increased oxidative stability of the compositematerials can be essentially explained as an effect of the trapaction of the CNT on the polymer peroxyl radicals, preventingtheir recombination and determining a radical terminationduring polymer degradation [32]. The behaviour is very similareven for different polymorphs or different morphologies.

3.4. Mechanical properties

The enhancement of the mechanical properties of thecomposites relies on many factors, the most important being:

(i) Morphology of the polymeric matrix, in terms of crys-tallinity, crystal dimensions and principal interconnectionbetween the amorphous and the crystalline phases. In gen-eral a strong interconnection as well as the orientation ofthe phases along the draw direction yield both higher mod-uli and better mechanical response.

(ii) Good dispersion of the nanotubes into the polymericmatrix. Indeed in the case of poor dispersion, thenanotubes will fail by separation of the bundle rather than

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Figure 12. The elastic moduli for samples quenched at 25 ◦C (•)and at 100 ◦C (��).

by failure of the nanotube itself, resulting in significantlyreduced strength [33–35].

(iii) Strong interfacial adhesion between the phases, leading toa high degree of load transfer between the matrix and thenanotubes. With a weak or non-homogeneous adhesion,the nanotubes behave as holes or nanostructured flaws,introducing local stress concentrations, and the benefits ofthe CNT properties are lost [36].

Figure 12 shows the elastic moduli of all the samples of sPP,as a function of the CNT content. As expected, the modulusof the 25sPPCN0 is lower than that of 100sPPCN0, due to thehigher crystallinity degree of the latter sample, that seems tobe the predominant factor. However, an inversion is observedfor the nanocomposites. As a matter of the fact in both caseswe observe an increase of the modulus with the CNT content;indeed such behaviour is more relevant for the nanocompositesquenched at 25 ◦C, while a less dramatic effect is observed forthe samples quenched at 100 ◦C. This result must be correlated

with the morphology of the amorphous phase in both samples.The carbon nanotubes are dispersed in this phase and theirbehaviour is dependent both on their properties (dispersionand interaction) and on the amorphous phase interconnectionwith the crystalline phase. In the samples crystallized atlower temperature (25 ◦C) the crystals are initially smaller andthe amorphous phase is better dispersed between the smallcrystals, forming a more interconnected matrix. Moreoverthe glass transition temperature is higher in these samples andthe fraction of trans-planar mesophase is also more consistent.These two characteristics, a more rigid amorphous phase anda higher fraction of trans-planar mesophase, make the wholesystem more rigid and justify the higher value of the modulus.It is worth noting that the increase of modulus is higherbetween 0 and 1% of CNT (167%) than up to 10%, where itis 47%. In spite of different mechanisms for modulus increaseand electrical conduction, in the following we will show thatthe electrical percolation threshold is just around the 1% value.

In the samples crystallized at higher temperature (100 ◦C)the crystals are initially bigger and the amorphous phase ismore relaxed and segregated in not completely interconnecteddomains. The results of the elastic modulus behavioursuggest that the development of two different morphologies,in spite of similar crystallinity and crystal dimensions, yieldsa different dispersion of the carbon nanotubes making theirpresence more efficient in the case of a more interconnectedamorphous matrix. This calls for great attention whencomparing the effect of filler content on the physical propertiesof polymer composites without considering the morphology ofthe polymeric matrix, that can be a dominant effect.

3.5. Electrical properties

In figure 13 the isochronal values of the conductivity versuswt% of the CNT loading are reported for samples quenchedeither at 100 or at 25 ◦C (solid lines). The reported values areobtained when the current through samples reaches a steadyvalue for an applied voltage of 2 kV. Between 1 and 3 wt%,the conductivity has a sizable step of about eight orders ofmagnitude, a phenomenon that can be regarded as the onsetof a percolating structure on which charge transport may take

Figure 13. Isochronal (60 s) conductivity σ at 2 kV versus CNT (%) content for samples: 100SPP—2 kV (�); 25SPP—2 kV ( ); ratio of theconductivity σ for the samples quenched at 100 and at 25 ◦C (�).

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Figure 14. Current density versus applied electric field for samples with 1% of CNT content, T = 23 ◦C: 100SPPCN1 (�); 25SPPCN1 ( ).

place. This range of wt% is comparable with those obtainedin the literature for other composites and blends based onMWCNT [37–39]. However, our composites reach the highestvalues of conductivity (>0.1 S m−1) with a loading (10 wt%)appreciably lower than those reported for other systems. Forexample, comparable values of the conductivity are obtainedwith 15 wt% for CNT–polycarbonate/polyethylene compositesproduced by melt mixing [38] or with 20 wt% for polyamide-6blended with acrylonitrile/butadiene/styrene [39].

As regards the effect of the quenching temperature onthe conductivity of the systems, the processing temperature ofsPP–CNT composites has a limited influence on the electricaltransport properties. Samples of pure sPP quenched at 100 ◦Cshow a greater value of the conductivity with respect tothose quenched at 25 ◦C. This finding is in accordancewith conclusions achievable from either x-ray diffractometryand thermal analysis and with other results concerning thesPP reported in the literature where the current density incrystallized sPP is found to be only slightly higher than thatin the pristine material characterized by a lower crystallizationlevel [40]. This result can be inferred from the dashed linealso reported in figure 13, where the ratio of the conductivityfor the samples quenched at 100 and 25 ◦C is reported asa function of the CNT content. Such a ratio decreasesfor higher CNT content, thus confirming that the nanotubesaffect the crystallization behaviour, leading to more disorderedforms independently of the quenching temperature. Thus aconductivity almost independent of the thermal processing ofthe matrix is observed.

In figure 14 the current density versus applied field forthe two samples with 1% of CNT is reported, also showingthe effect of the quenching temperature. In accordance withliterature data [40], the conduction mechanism at low fieldlevel is, with a high confidence (R2 > 0.98), quasi-ohmic forboth systems.

4. Concluding remarks

The purpose of this paper was to study the effect of carbonnanotube concentration in composites of sPP having the samecrystalline form but different morphologies. The attention

was focused on the form I of sPP with different degreesof perfection (in terms of percentages of chains in helicalconformation, crystal dimensions and crystallinity) obtainedusing two different quenching temperatures from the melt,i.e. 25 and 100 ◦C. The two pure samples 25sPP0 and 100sPP0confirmed that the sample crystallized at higher temperature(100 ◦C) has higher crystallinity, more helical chains and largercrystal dimensions, as reported in many papers. Moreover thissample shows a lower glass transition temperature, indicatingthat the amorphous phase is more relaxed.

Incorporating the carbon nanotubes and increasing theirconcentration, we observed many effects, different for theamorphous and crystalline phases:

• From the melt phase we observe a crystallizationtemperature higher than that for the pure sample andincreasing on increasing the CNT content, indicating thatthe crystallization process is faster in the presence of CNT.

• The glass transition temperature remains almost constantin both series of samples, crystallized at 25 and 100 ◦C,and its value is higher for the samples crystallizedat 100 ◦C, indicating in these samples a more relaxedamorphous phase.

• The fraction of trans-planar mesophase is almost constantin the samples crystallized at 25 ◦C, while it increases insamples crystallized at 100 ◦C.

• The crystallinity values, after a small increase for thesamples with 1% of CNT, decrease for both series ofsamples.

• The crystal dimensions, very different for the puresamples, decrease in the presence of CNT, andinterestingly they converge toward the same value. Theinverse dependence of the undercooling on the crystaldimensions is not respected in the presence of CNT.

• Either the thermal or the mechanical properties areimproved on increasing the CNT content in both series ofsamples.

• The conductivity increases in a similar manner in bothseries of samples and between 1 and 3 wt% it showsa sizable step of about eight orders of magnitude, aphenomenon that can be regarded as the onset of a

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Nanotechnology 18 (2007) 275703 G Gorrasi et al

percolating structure on which charge transport may takeplace.

The crystallization behaviour, in particular the observationthat all the crystalline parameters converge toward a similarvalue, independently of the undercooling, is worth noting andneeds a deeper investigation of the crystallization mechanismin the presence of CNT.

Other interesting features concerning the electricalcharacteristics of these systems have been observed and willbe discussed in a forthcoming paper specifically devoted tothe analysis of such properties which appear to be verypromising for several engineering applications, for example,in electromagnetic interference (EMI) shielding components,sensors, compact and lightweight surge arresters, to citejust some.

Acknowledgment

This work was supported by the INSTM Italian Consortium onScience and Technology Materials through the PRISMA 2005project.

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