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Materials Chemistry and Physics 117 (2009) 313–320

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

mprovement of mechanical and thermal properties of carbon nanotubeomposites through nanotube functionalization and processing methods

anda Gopal Sahooa, Henry Kuo Feng Chenga,b, Junwei Caia,b, Lin Lia,∗, Siew Hwa Chana,ianhong Zhaob, Suzhu Yub

School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, SingaporeSingapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore

r t i c l e i n f o

rticle history:eceived 16 January 2009eceived in revised form 22 April 2009ccepted 11 June 2009

a b s t r a c t

The effect of multi-walled carbon nanotubes (MWCNTs) and processing methods on the morphological,dynamic mechanical, mechanical, thermal and electrical properties of MWCNT/nylon 6 (PA6) compositeshas been investigated. The MWCNTs have been functionalized covalently and noncovalently for betterdispersion in the polymer matrix. A homogeneous dispersion of MWCNTs was achieved in the PA6 matrix

eywords:arbon nanotubeylon 6unctionalizationechanical strenghth

rystallization

as evidenced by scanning electron microscopy. The strong interaction between the functionalized MWC-NTs and the PA6 matrix greatly enhanced the dispersion as well as the interfacial adhesion. As a result, theoverall mechanical performance of the composites could be improved. The incorporation of the MWCNTseffectively enhanced the crystallization of the PA6 matrix through heterogeneous nucleation. The presentinvestigation revealed that the mechanical, thermal as well as electrical properties of MWCNT-filled poly-mer composites were strongly dependent on the state of dispersion, mixing and processing conditions,

polym

lectrical conductivity and interaction with the

. Introduction

Since the carbon nanotubes (CNTs) were reported by Iijima in991 [1], they have attracted as ideal reinforcing fillers in hightrength, light weight polymer composites due to their uniquetructural, mechanical, electrical, and thermal properties [2–5]. Thempressive and unique properties of CNTs make them promising

aterials for a wide variety of applications such as nanoelectronicnd photovoltaic devices [6,7], superconductors [8], electrome-hanical actuators [9], electrochemical capacitors [10], nanowires11], and composite materials [5,12].

However, the pure CNTs are generally insoluble in common sol-ents and polymers, and they usually form stabilized bundles due toan der Waals force. So, it is extremely difficult to align and dispersehe CNTs in a polymer matrix.

A significant challenge for developing high performance poly-er/CNT composites is to introduce the individual CNTs in a

olymer matrix in order to achieve better dispersion, possible align-

ent and stronger interfacial interactions, which improve the load

ransfer across the CNT–polymer matrix interface. The function-lization of CNT is an effective way to prevent nanotubes fromggregation, which helps to achieve better dispersion and stabilize

∗ Corresponding author. Tel.: +65 6790 6285; fax: +65 6794 2035.E-mail address: mlli@ntu.edu.sg (L. Li).

254-0584/$ – see front matter © 2009 Published by Elsevier B.V.oi:10.1016/j.matchemphys.2009.06.007

eric matrix.© 2009 Published by Elsevier B.V.

the CNTs within a polymer matrix. There are several approachesfor functionalization of CNTs, including defect functionalization,covalent functionalization, and noncovalent functionalization [13].

Noncovalent functionalization of nanotubes is of particularinterest because it does not spoil the physical properties of CNTsbut improves solubility and processability. This type of function-alization mainly involves surfactants or wrapping with polymers.In the search for nondestructive purification methods, nanotubescan be transferred to the aqueous phase in the presence of sur-factants such as sodium dodecylsulfate (SDS) [14,15]. In this case,the CNTs are surrounded by hydrophobic moieties of the corre-sponding micelles of surfactants. The main potential disadvantageof noncovalent attachment is that the forces between the wrap-ping molecule and the nanotube might be weak, thus as a filler ina composite the efficiency of the load transfer might be low.

The covalent functionalization of CNT can improve solubility aswell as dispersion in solvents and polymer. The functional groupsat the surface of CNT make the strongest type of interfacial bond-ing with the polymer matrix. The covalent functionalization can beaccomplished by either modification of surface bound carboxylicacid groups on the nanotubes or direct reagents attached to the

side walls of nanotubes.

Currently various techniques are used to incorporate CNTs intoa polymer matrix, e.g., solution casting, melt mixing, electron spin-ning, and in situ polymerization [16–23]. Melt mixing is a commonand simple method, which is particularly useful for thermoplastic

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raw MWCNTs are shown in Figs. 1 and 2. The FTIR spectra of the rawMWCNTs showed the peaks with very low intensity at 3440, 1640,and 1182 cm−1, corresponding to OH, C O, and C C O stretching,respectively. In the case of our modified MWCNTs, these character-istic bands appeared with significantly higher intensity, according

14 N.G. Sahoo et al. / Materials Chem

olymers. In melt processing, CNTs are mechanically dispersed intopolymer matrix using a high temperature and high shear forceixer or compounder [24]. This approach is simple and compatibleith current industrial practices.

Various polymer matrices are used for making composites, suchs thermoplastics [25,26], thermosetting resin [27,28], liquid crys-alline polymers [29,30], water-soluble polymers [31], conjugatedolymers [7], and so on. In this research, we have used nylon-6polyamide 6 (PA6)] and multi-walled carbon nanotubes (MWC-Ts) to prepare high performance polymer composites throughmelt mixing process. Nylon-6 has been widely used as an

mportant engineering plastic due to its excellent chemical andbrasions resistance, toughness, and low-coefficient of friction. Var-ous researchers incorporated CNT into nylon-6 in order to studyheir effect on the properties such as thermal, mechanical, electri-al, and crystallization behavior of nylon-6 [19,32,33].

The objective of the present study is to investigate in detailhe thermal, dynamic mechanical, morphological and electricalroperties of MWCNT/PA6 composites. The MWCNTs have beenunctionalized covalently and noncovalently for better dispersion inhe matrix. The effects of processing methods and nanotube contentn the properties of the composites have also been examined.

. Experimental

.1. Materials

The matrix polymer, PA6 (Ultramid® B36 LN 01) used in this study, wasurchased from BASF, Singapore. Its density is about 1.12–1.15 g cm−3. SDS was pur-hased from Aldrich. The MWCNTs were purchased from Bayer Materials Science.he diameter and length of the MWCNT were 13–16 nm and 10 �m respectively. Twoypes of methods were used to functionalize the raw MWCNTs. First, MWCNT–COOHas prepared by oxidation of raw MWCNTs with concentrated H2SO4/HNO3 (volu-etric ratio 3:1) at 90 ◦C for 10 min with vigorous stirring as described elsewhere

reviously [17]. Second, 100 ml aqueous solution containing SDS (1 wt%) was son-cated with 1 g of MWCNTs for 2 h. PA6 pellets were mixed with this solutionontaining required amount of MWCNTs. The mixture was heated and magneticallytirred continuously until the fluid was evaporated and coated on the PA6 pellets.hese MWCNT-coated pellets of PA6 were used for the subsequent compoundingrocess.

.2. Preparation of MWCNT/PA6 composites

We have employed two methods for processing the composites. First (method), the composites of PA6 with MWCNT were prepared using a Haake internal mixer

ith a rotor speed of 30 rpm at a mixing temperature 245 ◦C for 20 min. The moldedlabs were prepared in compression molding at 245 ◦C for 8 min. The molded samplesere cut into required shapes for investigation of different properties. The secondethod (method II) is to prepare the MWCNT/PA6 composites by an extrusion pro-

ess and subsequently mould the compounded products by injection molding. In thisase, samples were melt-mixed using a co-rotating twin screw extruder (LEISTRITZSE 27 HP). The temperature zones starting from the hopper to the die were sett 230, 245, and finally, 240 ◦C at the die and the screw speed was about 50 rpm.he extrudate was immediately quenched in a water bath at room temperature.he quenched samples were pelletized by cutting. After being pelletized and dried,he composite samples were injection-molded using an injection molding machine

Netstal HP 1000). The temperature of the barrel was set at 245 ◦C. The compoundingormulations of the composites prepared in this work are tabulated in Table 1. Theotations I and II represent the method I and method II, respectively. For example,RC10-I means the composite prepared by method I.

able 1ompounding formulations.

Method I and method II Method I

ngredients PRC1 PRC3 PRC5 PRC10 PRCS10 PAC10

ylon 6 (wt%) 99 97 95 90 90 90aw MWCNTs 1 3 5 10 10 –odified MWCNTs – – – – – 10

DS – – – – 10 –

and Physics 117 (2009) 313–320

2.3. Characterization

Fourier transform infrared (FTIR) spectroscopic measurements were performedusing a FTIR spectrometer (Thermo Nicolet Magna-IR 560 spectroscopy, USA). TheFTIR spectra of raw MWCNTs and acid modified MWCNTs were obtained in trans-mittance mode by placing a small amount of the materials in KBR pellets. Ramanspectroscopy (Renishaw, RM1000) was used to investigate the structural changes ofMWCNTs by the acid treatment. A 632.8 nm He–Ne laser was used as the light source.Transmission electron microscopic (TEM) analysis was performed on a JEM-2010F(Jeol Co.) electron microscope. For analysis, the MWCNT samples were prepared bysonicating about 1 mg of the MWCNTs powder in 10 ml of ethanol. A few drops ofthe resulting suspension were deposited on a TEM grid (200 mesh).

The surface morphology of the tensile fractured samples was observed by fieldemission scanning electron microscopy (FESEM), after gold coating. The analysis wasdone using a Jeol JSM-5800 SEM at an accelerating voltage of 20 kV. X-ray diffraction(XRD) was studied using an X-ray diffractometer (PW1830 series, Philips) with Cu K�radiation at a scan rate of 2◦ min−1. The percentage crystallinity �c was determinedusing the equation:

�c = IcIa + Ic

× 100% (1)

where Ic and Ia are the integrated intensities corresponding to the crystalline andamorphous phase respectively.

Differential scanning calorimetric (DSC) measurements were carried out using athermal analyzer (PerkinElmer DSC 7) in a temperature range of −50 ◦C to 300 ◦C, ata heating rate of 10 ◦C min−1 in nitrogen. The first cooling and heating thermogramsof DSC were used for the analysis. Thermal stability of the composites was examinedunder dry air using a thermal gravimetric analyzer (TGA 2950, TA Instruments) from25 to 700 ◦C at a heating rate of 10 ◦C min−1.

Tensile tests were carried out for the compression molded dumb-bell shapedsamples on an Instron machine (Instron 5569) at room temperature with an exten-sion speed of 5 mm min−1 and an initial gauge length of 35 mm. For each compositefour measurements were repeated within an experimental error of ±2%.

Dynamic mechanical analysis of composites was conducted using a dynamicmechanical analyzer (PerkinElmer DMA 7) under single cantilever clamp arrange-ment in a bending mode at a frequency of 1 Hz from 25 to 300 ◦C at a heating rateof 10 ◦C min−1. The storage modulus (E′) and loss tangent (tan ı) were measured foreach sample in the temperature range. Electrical conductivity measurement wascarried out using a four-point test fixture combined with a Keithley electrometermodel 230.

3. Results and discussion

3.1. Functionlization of MWCNTs

The FTIR and Raman spectra for the functionalized MWCNTs and

Fig. 1. FTIR spectra for (a) the raw MWCNTs and (b) carboxylically functionalizedMWCNTs.

N.G. Sahoo et al. / Materials Chemistry

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ig. 2. Raman spectra for the raw and carboxylically functionalized MWCNTs.

o the degree of modification. This was attributed to the increasedumber of carboxylic acid groups generated at the surface of theWCNTs after the acid treatment in H2SO4/HNO3. From Fig. 2, itas clearly seen that the two bands around 1579 and 1325 cm−1 in

he spectra were assigned to the tangential mode (G-band) and theisorder mode (D-band), respectively [17,34]. The D-band intensityas increased in the modified MWCNTs compared to raw MWCNTs.

he peak intensity ratio (ID/IG = 1.85) at D-band and G-band for theodified MWCNTs exceeded those of raw MWCNTs (ID/IG = 1.40).

his result indicates that some of the sp2 carbon atoms (C C) wereonverted to sp3 carbon atoms (C C) at the surface of the MWCNTsfter the acid treatment in H2SO4/HNO3. This may also be evi-enced by the TEM observation as shown in Fig. 3. The uniformurfaces of raw MWCNTs have been observed, owing to the per-ect lattice structure of carbon–carbon bonds, while the modified

WCNTs have showed some defects in the carbon–carbon bond-ng associated with the formation of carboxylic acid groups on theurface.

Fig. 4 shows the TEM images of the raw MWCNTs and SDS-

reated MWCNTs used in this work. The existence of highlyntangled network-like structure of MWCNTs is well evident fromhe micrograph. Fig. 4(b) illustrates the free ends of the MWCNTseing unbounded from the clusters after treatment with SDS. Theurfactant adsorbed on the nanotube surface during the disper-

Fig. 3. TEM images of (a) raw MWC

and Physics 117 (2009) 313–320 315

sion procedure, ultrasonication may help a surfactant to debundlenanotubes by steric or electrostatic repulsions.

3.2. Dispersion of MWCNTs

The representative FESEM photographs of the cross-sectionalfracture of composites with the achieved dispersion of the inves-tigated MWCNTs are shown in Fig. 5. Fig. 5 illustrates that thedispersed MWCNTs are present as the bright dots and some linesare the ends of the broken MWCNTs. The dispersion of raw MWC-NTs was found to be poor in the PA6 matrix as prepared by methodII as shown in Fig. 5(a) for the 10 wt% CNT composite (PRC10-II).Furthermore, the agglomerates of MWCNTs were observed in thePA6 matrix, reducing the reinforcing effects of the MWCNTs. Thedispersion was somewhat better in the composite prepared bymethod I with the same MWCNT’s loading (PRC10-I). It was clearlyobserved from Fig. 5(b) that the MWCNTs were broken (as indi-cated by arrow), which is of great importance for making MWCNTreinforced polymer composites.

The SDS-treated MWCNTs showed the better dispersion in thePA6 matrix (composite PRCS 10) as compared to the composite con-taining the same amount of raw MWCNTs. The surfactant moleculescould serve as a link between the nanotubes and the polymer, pro-viding hydrophobic interactions that can enhance the contact atthe interface [35]. The homogeneous dispersion in the compos-ites was also achieved by the addition of 10 wt% of MWCNT–COOHin the composite (PAC10) (Fig. 5(d)). Moreover, the MWCNTs werebroken rather than pulled out due to the strong interfacial bond-ing between the MWCNTs and the polymer matrix. The carboxylicgroups seemed to stabilize the MWCNT dispersion by strong inter-actions with the PA6 matrix. This can be attributed to the increasedpolarity of the MWCNTs by the functional groups and the possi-ble interaction of the carboxylic groups with NHCO of the PA6matrix.

It is very interesting to note that a belt like nanotube wasobserved which interconnected polymer lumps as indicated byarrow in Fig. 5(d). This typical phenomenon also indicated that astrong interfacial adhesion between MWCNTs and PA6 matrix anda sufficient load transfer from the polymer to nanotubes.

3.3. Crystallization

The DSC thermograms and XRD patterns of pure PA6 andMWCNT/PA6 composites are presented in Figs. 6–9. The crystalliza-

NT and (b) MWCNT–COOH.

316 N.G. Sahoo et al. / Materials Chemistry and Physics 117 (2009) 313–320

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Fig. 4. TEM images for (a) raw MW

ion temperature (Tc), melting temperature (Tm) and heat of fusion�Hf) obtained from DSC studies are summarized in Table 2. The

SC melting endotherms (Fig. 6) indicated that PA6 had meltingeak at 220 ◦C, corresponding to the melting event of the �-formrystals [32,36]. The same melting peak was also observed in theurves of MWCNT/PA6 composites. It is also confirmed by theRD results. The X-ray patterns of the PA6 and composites (Fig. 8)

Fig. 5. FESEM images of the cross-sectional fracture of compos

(a) and (b) SDS-treated MWCNTs.

displayed the presence of two main peaks at 2� = 20.1 and 23.9 cor-responding to the (2 0 0), (0 0 2) and (2 2 0) reflections, indicating

that PA6 and all the composites had the �-form structure under ourexperimental conditions. The raw MWCNT/PA6 composites showedthe higher diffraction intensity compared to PA6. As a result, forthe raw MWCNT/PA6 composites, the percent crystallinity becamehigher with increasing concentration of raw MWCNTs. It indicates

ites: (a) PRC10-II, (b) PRC10-I, (c) PRCS10 and (d) PAC10.

N.G. Sahoo et al. / Materials Chemistry and Physics 117 (2009) 313–320 317

Fig. 6. DSC thermograms on heating at 10 ◦C min−1 in nitrogen for: (a) PA6, (b) PRC1,(c) PRC3, (d) PRC 10-II, (e) PRC10-I, (f) PRCS10, and (g) PAC10.

Fig. 7. DSC thermograms on cooling from melt at 10 ◦C min−1 for: (a) PA6, (b) PRC1,(c) PRC 10-I, (d) PRCS10, (e) PRC10-I, and (f) PAC10.

Fig. 8. X-ray diffractrograms for: (a) PA6, (b) PRC3, (c) PRC5, and (d) PRC10-I.

Fig. 9. X-ray diffractrograms for: (a) PRC10-I, (b) PRCS10, (c) PAC 10, and (d) PRC10-II.

that MWCNTs promoted the crystallization of PA6 crystals, whichwas more prominent in the case of MWCNT/PA6 composite with10 wt% raw MWCNTs prepared by method I.

The melting peak temperature of PA6 was affected by incorpora-tion of MWCNTs in the matrix, which was more evident in the caseof the composites prepared by method I. The PRCS10 composite didnot exhibit any significant change in the melting temperature ofPA6 compared to the composites with the raw MWCNT. It is alsoobserved that the melting temperature of PA6 was shifted higherby 10 ◦C for the acid treated MWCNT/PA6 composites. This is dueto the existence of a strong interaction between the acid treatedMWCNTs and the PA6 matrix.

From Fig. 7, it can be seen that the pure PA6 showed only a crys-tallization peak temperature at about 183 ◦C. The crystallizationtemperature shifted to a higher temperature and the crystalliza-tion temperature range became broader, when both acid treatedMWCNTs and raw MWCNTs were incorporated in the PA6 matrix;the effect being more dominant in the presence of raw MWCNTs.At the same loading of raw MWCNTs (10 wt%), the composite inthe presence of SDS surfactant (PRCS10) did not exhibit signifi-cant change in the crystallization behavior of PA6 compared to thePRC10-I and PRC10-II composites. The fillers often have a positiveeffect on the crystallization of a polymer in the composite sys-tem [37]. That is, they can have a nucleating effect, resulting inan increase in crystallization temperature. It is very interesting toobserve that the new crystallization peak appeared at a higher tem-perature for the MWCNT/PA6 composites and this peak positionshifted to the higher temperature side with increasing content ofMWCNTs in the composites. The magnitude of this crystallization

peak was also increased with increasing MWCNTs in the compos-ites. These observations are in good agreement with the findingsby Phang et al. [38] while the DSC results showed two crystal-lization exotherms for MWCNT/PA6 composites instead of a singleexotherm for the neat matrix. They reported that the formation

Table 2Thermal properties of composites prepared in this study.

Sample Tc (◦C) Tm (◦C) �Hf (J g−1) Percent crystallinity (%)(from XRD)

PA6-I 183 220 74 53PRC1-I 188 223 78 56PRC3-I 189 224 80 58PRC10-I 191 227 88 66PRC10-II 189 224 68 48PRCS10-I 190 227 86 64PAC10-I 191 230 71 52

3 istry and Physics 117 (2009) 313–320

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18 N.G. Sahoo et al. / Materials Chem

f the higher-temperature crystallization peak is closely related tohe addition of MWCNTs. When the amount of MWCNT increasesn the PA6 matrix, more heterogeneous nucleation sites are avail-ble, and hence more polymer chains are induced to crystallizend thus, result in a more significant second crystallization peak.he composite processing methods also affected the crystallizationemperature. The composites prepared by method I were of higherrystallization temperature than those prepared by method II. Ouresults suggest that the incorporation of the MWCNTs effectivelynhanced the crystallization of the PA6 matrix through heteroge-eous nucleation, and the nucleation effect was more evident forhe PA6 composites with the raw MWCNTs, as compared to otheromposites. The heat of fusion (�Hf) and heat of crystallization �Hc

ncreased with increasing the raw MWCNTs in the composites. Theeat of fusion is proportional to the amount of crystallinity in theample, which was also supported by the crystallization behaviorf composites. However, the heat of fusion (�Hf) for the composite,AC10-I, which contained the functionalized MWCNTs, was lowerhan those of pure PA6 and the composite (e.g. PRC10-I) with theaw MWCNTs at the same loading. This can be explained by the facthat the functionalized MWCNTs had stronger interaction with PA6o result in a better dispersion of MWCNTs but a higher barrier forA6’s mobility.

Therefore, the crystallinity for those composites containing theunctionalized MWCNTs decreased as compared to the pure PA6.owever, it was noted that the heat of fusion for the compositeRC10-II was also lower than that of the pure PA6. This might beecause the PRC10-II was prepared using method II. In method II,he sample was rapidly cooled down from the melt state to roomemperature, so that there was no sufficient time for the compos-te to form more crystallites than the composites prepared with

ethod I that allowed slower cooling.

.4. Thermal stability

In order to investigate the thermal stability of the MWCNT/PA6omposites, TGA measurements were carried out, and the resultsre shown in Fig. 10. In this study, the criteria for thermal stabil-ty was taken as the temperature at which 10% and 50% weightoss occurred in the system. It was observed from Fig. 10 thathe 10% and 50% decomposition of pure PA6 occurred at 424 ◦C

nd 467 ◦C, respectively. Compared to the pure PA6, the 1 wt%aw MWCNT-composite (PRC1-I) prepared by method I showedhe slightly delayed decomposition. However, for the 10 wt% raw

WCNT-composite (PRC10-I) prepared by method I, the 10% and

ig. 10. TGA thermograms of composites at a heating rate of 10 ◦C min−1: (a) PA6,b) PRC1, (c) PRC 10-II, (d) PRC10-I, (e) PRCS10, and (f) PAC10.

Fig. 11. Tan � for the MWCNT/PA6 composites at 1 Hz.

50% decomposition temperatures extended to higher temperaturesat 435 ◦C and 498 ◦C with a slower decomposition rate and almost87.2% of the sample was degraded in this step. So, the thermalstability of the composites increased with increasing MWCNTs inthe composites. Polymer chains near the nanotubes may degrademore slowly, which helps to shift the decomposition temperatureto the higher side. Other reason is the increased thermal stabilityof the polymer composites due to the effect of higher thermal con-ductivity of MWCNTs that facilitates heat dissipation within thecomposite [39]. It was also observed that composite containing10 wt% MWCNTs prepared by method I showed the more delayeddecomposition as compared to the same MWCNT compositionmade by method II. Such an improvement can be associated withthe better dispersion of MWCNTs by method I. The crystallinityalso influenced the thermal decomposition of the composites pre-pared by method I. The 10% and 50% decomposition temperatures ofPRCS10 composites increased by 21 ◦C and 36 ◦C compared to purePA6 and by 10 ◦C and 5 ◦C compared to PRC10-I composite. Dis-persed nanotubes might hinder the flux of decomposition productand hence delay the decomposition. The acid treated MWCNT/PA6composite showed the highest decomposition temperature as com-pared to the composites with the same loadings of raw MWCNTs(e.g. PRC10-I). The presence of the more carboxylic groups on thenanotube surface is likely to give the strong interfacial interactionbetween the polymer matrix and the nanotubes in polymer com-posites. Since the polymer thermal degradation begins with chaincleavage and radical formation, the MWCNT–COOH in the com-posite acted as radical scavengers, delayed the onset of thermaldegradation and hence improved the thermal stability of PA6 [40].It is important to point out that the extent of interaction betweenfunctionalized MWCNTs and PA6 matrix could be responsible forthe higher thermal stability of the PAC10 composite.

3.5. Dynamic mechanical properties

The loss tangent (tan ı) versus temperature curves are presentedin Fig. 11. The tan ı of each composite had a wider distributionthan that of pure PA6 because of the incorporation of MWCNT intothe PA6 matrix. In the presence of MWCNTs, the mobility of thepolymer chain became more restricted by the MWCNTs [41]. It iswell known that the temperature corresponding to the maxima of atan ı peak is normally associated with the glass transition temper-

ature Tg. From the variation of tan ı with temperature (Fig. 11), Tg

of pure PA6 appeared at approximately 63 ◦C. The glass transitiontemperature of PA6 was shifted to the higher temperature side bythe addition of MWCNTs, which was more evident for the function-alized MWCNT/PA6 composites. The glass transition temperature

N.G. Sahoo et al. / Materials Chemistry and Physics 117 (2009) 313–320 319

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composites increased with increasing MWCNT content. The elec-trical conductivity of the 5 wt% MWCNTs composite produced bymethod II was too low to be measured but the electrical conductiv-ity of 10 wt% MWCNT composite prepared by method II was greatlyincreased to 3.1 × 10−5 S cm−1. At low concentrations, conducting

Table 3

ig. 12. Storage modulus for the MWCNT/PA6 composites at 1 Hz: (a) PA6, (b) PRC1c) PRC10-II, (d) PRC10-I, (e) PRCS10, and (f) PAC10.

ncreased from 63 ◦C for the pure PA6 to 75 ◦C for the compositeith 10 wt% MWCNT–COOH. This is due to the presence of MWC-Ts which may have imposed restrictions on molecular mobility.his effect can also be explained in terms of decreasing free volumef polymer. From the concept of free volume, with the addition ofWCNTs, this free volume is evidently reduced.Fig. 12 shows the storage modulus versus temperature curves of

WCNT/PA6 composites. Generally, storage modulus can describehe stiffness of materials. The storage modulus of the compositesramatically increased when MWCNT–COOH or raw MWCNTs were

ncorporated into the PA6 matrix. The composite PRC10-II showedbout 42% and 75% increment of storage modulus at the glassylateau region (−20 ◦C) and the rubbery plateau region (100 ◦C),espectively, as compared to pure PA6. But this improvement oftorage modulus was even higher at the same level of MWCNTsn the composite PRC10-I, i.e. 52% and 267% at the glassy plateauegion (−20 ◦C) and the rubbery plateau region (100 ◦C), respec-ively, over the pure PA6. The presence of SDS surfactant in theomposite (PRCS10) exhibited higher storage modulus comparedo the PRC10 composite. The improvement in storage modulus of

WCNT/PA6 composites is attributed to the effect of good disper-ion and high performance of MWCNT filler. Generally, the storageodulus of composites strongly depends on their microstructure

nd crystallinity. From XRD and DSC results, we conclude that themprovement of storage modulus of PRCS10 composite is also influ-nced by crystallinity.

The effect of MWCNT–COOH on the increase in storage modu-us was more dominant than the unmodified MWCNTs. The storage

odulus for the composite containing 10 wt% acid treated MWCNTsPAC10) showed an increase by 74% and 375% at the glassy plateauegion (−20 ◦C) and by 15% and 30% at the rubbery plateau region100 ◦C), as compared to the pure PA6 and the PRC10, respectively.he enhanced storage modulus is considered to be due to the effectf the fine dispersing ability of the functionalized MWCNTs intohe PA6 matrix and the enhanced interaction between the COOHroups of MWCNTs and the polymer chain. In this case, MWCNTsispersion in the polymer matrix is more pronounced than therystallization effect.

.6. Tensile strength

The tensile strength for pure PA6 and MWCNT/PA6 composites

re illustrated in Fig. 13. As can be seen, the addition of MWCNTsnto the PA6 matrix improved the tensile strength of the composites.he tensile strength of the composite with 1 wt% MWCNTs (PRC1-I)as enhanced by 22.5% as compared to the pure PA6. The tensile

trength of PRC10-I composite was enhanced by 77% as compared

Fig. 13. Tensile strength for the MWCNT/PA6 composites.

to pure PA6, while an increase of 50% was achieved by incorpo-rating the same amount of raw MWCNTs into the PA6 matrix bymethod II. From SEM, it was clearly showed that the better dis-persion of MWCNTs was achieved throughout the PA6 matrix bymethod I. In this method, most MWCNTs were separated into indi-vidual tubes by shear force from the internal mixer, which was moreimportant to make CNT reinforced PA6 composites. The PRCS10composite showed a higher tensile strength than PRC10-I compos-ite and PA6 because of the better dispersion and higher interactionin this composite due to the presence of the surfactant. In addi-tion, the crystallinity of PRCS10 was higher than PA6 which alsoaffected the mechanical properties of PRCS10 composite. Finally,the tensile strength of the composites increased from 16.0 MPain the PA6 to 36.2 MPa (an increase of 126%) when the 10 wt%functionalized MWCNTs were incorporated to the PA6 matrix. Thefunctionalized MWCNTs prepared by acid treatment containedmany COOH groups. The incorporation of functionalized MWC-NTs into the polymeric matrix creates some interaction betweenMWCNTs and polymer chains, thus being favorable to stress trans-fer to MWCNTs. Consequently, the hydrophilic functional groupson the MWCNTs were helpful in improving the interaction with

CONH groups in PA6. Therefore, the strong interaction betweenthe functionalized MWCNTs and the PA6 matrix greatly enhancedthe dispersion as well as the interfacial adhesion. As a result,the overall mechanical performance of the composites could beimproved.

3.7. Electrical conductivity

CNTs exhibit the high aspect ratio and high conductivity, whichmakes CNT excellent candidate for fabrication of conducting com-posites. The electrical conductivity of the MWCNT/PA6 compositesas a function of MWCNTs is shown in Table 3. The electrical con-ductivity of pure PA6 is 10−14 S cm−1 [42]. The PA6 composite filledwith containing 5 wt% raw MWCNTs by method I showed the elec-trical conductivity of 3.7 × 10−6 S cm−1. The conductivity of the

Conductivity of MWCNT/PA6 composites.

Method PRC5 PRC10 PRCS10

I 3.7 × 10−6 S cm−1 4.9 × 10−4 S cm−1 1.2 × 10−3

II – 3.1 × 10−5 S cm−1 –

3 istry

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20 N.G. Sahoo et al. / Materials Chem

llers were dispersed within the polymeric matrix as isolated clus-ers. Above the percolation threshold concentration, independentllers tended to link together to form conductive networks. This

ed to significant increase in the electrical conductivity of the com-osite. From the experimental results, the electrical conductivity ofhe composites prepared by method II was lower than that of theorresponding composites prepared by method I. This observations quite in agreement with the findings by Lee [43] while using car-on black as conducting filler for polymer composites. This is due tohe break-down of the black aggregates under high shear and alsohe existence of a polymer rich skin layer. Since injection-moldedarts normally shows a layered structure (i.e. skin-core structure),he conductivity distribution in injection-molded composites isxpected to be more complicated. It was observed that the skinayer, i.e., high shear zones, had a substantially lower conductivityhan the core zone. The high shear near the surface of the sam-le disrupted the formation of conductive networks in the polymeratrix, resulting in poor electrical conductivity in the skin layer. The

oor conductive layer, which surrounds the more electrical conduc-ive core, contributes to a higher bulk resistivity. On the other hand,

ethod I was capable of producing better electrical conductivity bydopting an internal mixer to mix the polymer and MWCNTs andorming the samples by compression molding. The higher conduc-ivity obtained with method I could be attributed to the importanteatures of the Hakkae internal mixer, such as the screw geome-ry, shear stress and shear rate, which may cause more efficientller breakage in the mixing process [44]. The agglomerated MWC-Ts in the polymer could be broken down by the mixing process

n the internal mixer leading to better dispersion of the MWCNTss compared to that by method II. In addition, compression mold-ng allowed more time for the MWCNTs to align themselves in theolymer matrix than in the injection molding where the coolingrocess was rapid, severely lowering the mobility of the CNT parti-les.

The presence of SDS surfactant in the composites exhibited theigher electrical conductivity of 0.012 S cm−1 which was 2.5 timeshat of the composites with the same MWCNTs loading. The mixingf a surfactant with MWCNTs played an important role in the dis-ersion of the MWCNTs in the polymer matrix. Our experimentalesults confirmed the ability of the surfactant to unravel individualanotubes from the clusters so as to allow conductive networks toe formed more easily.

. Conclusions

The raw MWCNTs, which were treated by an anionic surfactantSDS) or carboxylically functionalized, have been incorporated intoA6 by two methods of mixing in order to examine their effects onhe crystalline, thermal, morphological, and mechanical propertiesf MWCNT-reinforced composites of PA6. The PA6 containing func-ionalized MWCNTs showed the improved mechanical and thermalroperties over the composites filled with the raw MWCNTs. Theomposites fabricated by method I exhibited the better crystalline,

echanical, thermal as well as electrical properties than those

repared by method II. It has been proved that the crystalline,echanical, thermal as well as electrical properties of MWCNT-

lled polymers are strongly dependent on the state of dispersionnd interaction of MWCNTs with the polymeric matrix.

[

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and Physics 117 (2009) 313–320

Acknowledgment

This work was supported by the A*STAR SERC Grant(0721010018), Singapore.

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