Journal of Non-Crystalline Solids 358 (2012) 1339–1344

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Journal of Non-Crystalline Solids

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Electrical properties and glass transition temperature of multiwalled carbonnanotube/polyaniline composites

Mansoor Farbod ⁎, Somayeh Khajehpour TadavaniPhysics Department, Shahid Chamran University, Ahvaz, Iran

⁎ Corresponding author. Tel./fax: +98 6113331040.E-mail address: farbod_m@scu.ac.ir (M. Farbod).

0022-3093/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.jnoncrysol.2012.03.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 January 2012Received in revised form 1 March 2012Available online 4 April 2012

Keywords:PANI;Electrical properties;Functionalized carbon nanotube;Glass transition temperature

Polyaniline (PANI) was synthesized and doped with 0, 2, 4 and 16 wt.% of pure and functionalized multiwallcarbon nanotubes (MWCNTs) by “in-situ” polymerization. Measurement of temperature dependence of elec-trical resistivity showed a reduction in the resistivity of the composites at all temperatures. The reductionwas increased by increasing the wt.% of MWCNTs. This decrease was more for the composites containingfunctionalized MWCNTs and was more prominent for temperatures below 150 K. The glass transitiontemperature (Tg) of the pure and doped PANI was measured using electrical resistivity measurements. Itwas observed that by increasing the amount of functionalized MWCNTs in PANI, its Tg increases. Temperaturedependence of resistivity of pressed pure PANI showed that by increasing the pelletization pressure, the bulkelectrical resistivity decreased but the Tg increased.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) have been widely studied due to theirsuperb properties such as unusually high strength and stiffness, flex-ibility, thermally stable and low density. These properties accompa-nied by their high aspect ratio make CNTs ideal for various potentialapplications such as filler for polymeric composites. Comparing withcomposites containing conducting nanoparticles, CNT compositesshow high electrical conductivity at a low concentration of CNTs. Inorder to use the CNTs as dopant in various substances, they requireto be well dispersed. This is due to their large surface area and strongvan der Waals forces which can form strongly bound aggregates[1–3]. The chemical functionalization of CNTs is a technique to im-prove their dispersion in different matrices such as polymers. Func-tionalized CNTs are also easier to disperse in organic solvent andwater, which can improve the dispersion and homogeneity of theCNTs within a polymer matrix [4–10].

Many polymers have been doped with CNTs for various target ap-plications. Among them, polyaniline (PANI), a conducting polymerwith molecular formula C6H7N, has achieved extensive importancefor having good environmental stability, electrical conductivity andreversible control of conductivity both by charge transfer doping andprotonation [4,11]. PANI is also used in electromagnetic shielding,corrosion inhibitors and “smart windows” due to its electrochromic

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properties. It shows a whole range of colors as a result of their manyprotonation and oxidation forms [12].

Polymer molecules can have different kinds of motions dependingon the polymer structure. The Tg of polymers is a way of understand-ing the molecular motion of a polymeric system. The degree of molec-ular motion is of fundamental concern when considering adhesion,cohesion, and other properties of polymers. Many physical propertiesincluding thermal expansion coefficient, heat capacity, resistance tomechanical shock and electrical characteristics can be changed bypassing through Tg. PANI has high intra-chain rigidity coupled withstrong inter-chain hydrogen bonding and dipolar interaction due toits crystalline structure. Thus, the detection of its molecular motionand therefore its glass-to-rubber transition temperature (Tg) be-comes quite difficult with the available methods of measurement.

The Tg is determined using different techniques such as differen-tial scanning calorimetry (DSC), dynamic mechanical analysis(DMA) and dilatometry [13]. It seems that the measurement of tem-perature dependence of resistivity is a very good and near accuratetechnique for Tg measurement of conductive polymers because anymotion of polymer chains can affect the motion of electrons resultingin an increase in resistivity.

MWCNT/PANI composites with different wt.% ratios of pure andfunctionalized MWCNTs were synthesized by “in-situ” polymeriza-tion and the properties of two sets of samples were compared. Thesamples were characterized using XRD, SEM and Raman spectrosco-py. By measuring the electrical resistivity of the composites as a func-tion of temperature using a standard four-probe technique, theeffects of pure and functionalized MWCNTs on the electric resistivityof PANI were investigated. The Tg of composites was deduced fromthe resistivity curves.

Fig. 1. Raman spectroscopy of pure and functionalized MWCNTs.

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2. Experimental

2.1. Composite preparation

MWCNTs used in this study were synthesized using chemicalvapor deposition (CVD). The MWCNTs were acid treated as describedbelow to remove impurities such as metallic catalysts and to intro-duce functional groups such as carboxylic acid and hydroxyl groupsto their surfaces. Initially, the MWCNTs were ultrasonically treatedin concentrated HCl for 1 h in an ultrasonic bath. The samples werethen filtered using a 0.45 μm Teflon filter paper and washed withdeionized water until the rinsing water was of neutral and thendried in an oven at 100 °C for 24 h. Afterward the MWCNTs were ul-trasonically treated with a 3:1 mixture of concentrated H2SO4:HNO3

at room temperature for 30 min. Then, the oxidation reaction wascarried out in a one necked, round bottomed glass flask, equippedwith reflux condenser, magnetic stirrer and thermometer whichwas mounted in the preheated water bath. The reaction was carriedout at 80 °C and refluxed for 8 h. After that, the sample was filteredusing a 0.45 μm Teflon filter paper and was washed with deionizedwater until the pH value was around 7.0. The samples were thendried at 100 °C in 24 h. The composites of doped PANI with pureand functionalized MWCNTs were synthesized via in situ chemicaloxidation polymerization. In a typical synthesis experiment, 0, 2, 4and 16 wt.% of pure or functionalized MWCNTs were dissolved in50 ml of 1.0 M HCl solution and ultrasonicated for 2 h, separately.Then, they were transferred into one necked, round bottomed glassflask with an ice bath. Aniline monomer also dissolved in 100 ml of1.0 M HCl solution and then was added to the above MWCNT suspen-sion. A 50 ml of 1 M HCl solution containing 1.96 g ammonium per-sulfate (APS) was slowly added dropwise into the individualsuspension with constant mechanical stirring at a reaction tempera-ture of 0–5 °C for 30 min. The reaction mixture was stirred for 8 h at0–5 °C. In the next step, the resultant green suspension, indicatingthe formation of insoluble PANI in its emeraldine form, was filteredand rinsed several times with deionized water and acetone. The pow-der obtained was dried at 70 °C in 20 h.

2.2. Structural and morphological analysis

The Raman spectroscopy, X-ray diffraction and SEM measure-ments were utilized to characterize the structure of MWCNTs, func-tionalized MWCNT/PANI and pure MWCNT/PANI composites.Raman spectra were performed under Almega Thermo Nicolet disper-sive Raman spectrometer using 532 nm of an Nd:YLF laser, at thespectra range of 500–4200 cm−1, resolution 4 cm−1 and 32 scans in3 min. The powder X-ray patterns were carried out using a PW-1840 Philips diffractometer at room temperature utilizing Cu Kα radi-ation wavelength of λ=1.5418 Å. The peak position and intensitieswere obtained between 10–80° with a velocity of 0.02° per second.The structural and morphological characterization of composites,were performed by measuring SEM using a Philips XL30 scanningelectron microscope.

2.3. Electrical resistivity measurements

The temperature dependence of resistivity was measured usingthe standard four-probe technique from 427–113 K using a liquid ni-trogen cryostat. In order to measure the resistivity, powders of PANI,pure MWCNT/PANI and functionalized MWCNT/PANI compositeswere pressed into bar shaped pellets under 380 MPa. The crosssection and the length of all samples were the same. The effect ofpelletizing pressure on Tg of PANI was also investigated by measuringthe resistivity of samples which were pressed under 96, 380, 580, 770and 1150 MPa.

3. Results and discussion

3.1. Raman spectroscopy analysis

Raman spectroscopy is a very useful technique to characterize ma-terial composition, sample temperature, and strain from analysis ofthe material specific phonon mode energies. The most prominentRaman active modes in CNTs are radial breathing modes (RBM) inthe low frequency region and D, G and G′ modes in the higher fre-quency region. While D, G, and G′ modes are also found in graphite,the RBM mode which is seen at 100–300 cm−1 is a unique CNTmode and is only seen in single-walled carbon nanotubes. The Dmode or disorder mode originates from structural defects and isseen at 1200–1400 cm−1. The G mode or graphite mode correspondsto planar vibration of carbon atoms and is seen at 1500–1600 cm−1.This mode appears in most graphite like materials. Therefore, theratio of the ID/IG represents the degree of disorder in CNT structures.The G′ mode is a second overtone of the defect induced D mode andis seen at 2500–2900 cm−1 [4,6,11,14,15].

Fig. 1 shows the Raman spectrum of pure and functionalizedMWCNTs. Both spectra are the same which means the functionaliza-tion does not affect the graphite structure of the MWCNTs. As canbe seen the ratio of ID/IG is bigger for the functionalized MWCNTs asexpected. The oxidation of MWCNTs breaks some bonds and insertschemical groups that can be interpreted as defects on thestructure. Table 1 shows the peak position and the intensity ratio ofID/IG of the samples.

Fig. 2 shows the Raman spectroscopy of functionalized MWCNTs,PANI and the composite containing 2 and 4 wt.% of functionalizedMWCNTs. For comparison, Fig. 2 also includes the spectrum of func-tionalized MWCNT, which contains D and G modes. For PANI andfunctionalized MWCNT/PANI composites, C\H bending of the quin-oid ring at 1170 cm−1, C\H bending of the benzenoid ring at1230 cm−1, C\N+ stretching at 1330 cm−1, and C\C stretching ofthe benzene ring at 1480 and 1590 cm−1 were observed, revealingthe presence of the doped PANI structures [4,16,17]. On the otherhand, with the increase of the wt.% of MWCNTs in the polymer ma-trix, the intensity of the peaks at 1330 and 1590 cm−1 was increasedbecause they are very close to the D and G modes, respectively. Thisindicates a strong interaction between MWCNTs and PANI.

The formation mechanism of functionalized MWCNT/PANI com-posites is believed to arise from the strong interaction between ani-line monomer and MWCNTs. The interaction could be due to boththe π–π* electron interaction between MWCNT and the anilinemonomer and the hydrogen bond interaction between the carboxylicgroups of functionalizedMWCNTs and amino groups of anilinemono-mer. Such a strong interaction causes the adsorption of aniline mono-mer on the surface of MWCNTs. MWCNTs therefore serve as thetemplate during the formation of composites. The polymer chainsduring their growing can possibly disentangle the MWCNT bundles

Table 1The peak position and the intensity ratio of ID/IG of the samples in Raman spectroscopy.

Sample D mode(cm−1)

ID G mode(cm−1)

IG G′ mode(cm−1)

IG′ ID/IG

Pure MWCNTs 1350 2270 1580 2930 2960 3120 0.77FunctionalizedMWCNTs

1360 3560 1590 3490 2700 3800 1.09

Fig. 3. XRD patterns of (a) pure MWCNTs, (b) functionalized MWCNTs, (c) PANI,(d) composite with16 wt.% of pure MWCNTs (e) composite with 16 wt.% of functiona-lized MWCNTs.

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and then separate them into individual MWCNTs, so the MWCNTs canbe dispersed uniformly into the PANI matrix [4,16–18].

3.2. X-ray diffraction studies

The structural information can be deduced from X-ray diffraction(XRD) pattern of samples. Fig. 3 shows X-ray diffraction pattern ofMWCNTs, PANI and MWCNT/PANI composite. As can be seen, theXRD pattern of pure MWCNTs and functionalized ones (Fig. 3a andb) is the same, which means the functionalization doesn't have anyeffects on the tube like structure of MWCNTs. The peaks were ob-served at 25.95° and 43.53° for pure MWCNTs and at 25.47° and43.42° for functionalized MWCNTs corresponding to graphite likestructure and small amount of catalytic particle encapsulated insidethe walls of MWCNTs, respectively [4,16,17]. For PANI (Fig. 3c) thepeaks at 2θ=14.63°, 20.47°, 25.31°, 26.00°, and 28.47° belong to thePANI in emeraldine salt form and correspond to the (011), (020),(200), (121) and (022) reflections, respectively [4,10,15,16,19,20]. Thepeaks at 20.47° and 25.31° can be ascribed to the periodicity paralleland periodicity perpendicular to the polymer chain, respectively [20].The XRD patterns of composites with 16 wt.% of pure MWCNT(Fig. 3d) and 16 wt.% of functionalized MWCNT (Fig. 3e) are the same.These patterns are similar to the pattern of PANI except, the intensityof (200) reflection increases.We observed, the intensity of (200) reflec-tion enhances with increasing the wt.% of MWCNTs possibly due tooverlapping the main peaks of PANI and MWCNTs indicating that noadditional crystalline introduces to the composites.

3.3. SEM measurements

The SEM images of the pure and functionalized MWCNTs areshown in Fig. 4A and B. The images were used to detect possible mor-phological changes on MWCNT after the functionalization process. Asit is observed, the functionalization breaks the nanotube bundles andcauses some decrease in their length. The SEM image of the functio-nalized CNTs (Fig. 4B) seems a little blurry due to the presence oforganic functionalities [21].

Fig. 5A, B and C shows SEM images of PANI, PANI composite dopedwith 16 wt.% of pure and functionalized MWCNTs, respectively. In

Fig. 2. Raman spectroscopy of (a) functionalized MWCNTs, (b) PANI, composite using(c) 2 wt.% and (d) 4 wt.% of functionalized MWCNTs.

Fig. 5A, the nanoparticles of PANI are observable while in Fig. 5B and Cthe polymer chains are observed to form around the MWCNTs. Theshortened length of functionalized MWCNTs is observed as well.

4. Electrical properties

4.1. Resistance measurements

The measurement of temperature dependence of electrical resis-tivity is a way to illustrate the conduction mechanism in a material,so the researchers usually paid particular attention to the

Fig. 4. SEM images of (A) pure MWCNTs, (B) functionalized MWCNTs.

Fig. 5. SEM images of (A) PANI, (B) PANI composite doped with 16 wt.% of pureMWCNT and (C) 16 wt.% of functionalized MWCNTs.

Fig. 6. Normalized electrical resistance of PANI composite doped with (A) 2 wt.%,(B) 4 wt.% and (C) 16 wt.% of pure and functionalized MWCNTs.

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temperature dependence of electrical resistivity [22]. The study ofdoping and temperature dependence of electrical resistivity of con-ductive polymers such as PANI can show the effect of dopant likeMWCNTs in the polymer matrix. So the electrical resistivity of PANIand MWCNT/PANI composites was measured using the standardVan Der Pauw DC four-probe method [4,15,23]. The sample heatingrate for electrical resistance measurements was about 5 K/min.

Fig. 6A, B and C shows the normalized resistance as a function oftemperature for 2, 4 and 16 wt.% pure and functionalized MWCNTcomposites, respectively. As can be observed from the figures, the re-sistivity of samples increase with decreasing the temperature almostmonotonically down to 150 K, below which the slope increases.Mizoguchi et al. [24] have measured the temperature dependence of

diffusion rate of charge transfer, along and across the PANI chainsusing electron spin resonance experiment. They showed that the dif-fusion rate along the chains decreases below 150 K, so the increase inresistivity below 150 K could be due to weak interchain coupling ofPANI. Also it can be observed that the resistivity of the compositesis lower than that of the pure PANI at all temperatures. The reductionof resistivity enhances by increasing the wt.% of MWCNTs. This de-crease was more for the composites containing functionalizedMWCNTs and was more prominent at temperatures below 150 K.For better comparison, the electrical resistivities of functionalizedMWCNT/PANI composites are plotted in Fig. 7.

Using Arrhenius plot of resistivity, we measured the activation en-ergy of composites and found that the activation energies changefrom 44.53 meV for the sample doped with 0.25 wt.% CNTs to31 meV for the sample doped with 16 wt.%. The reason can be attrib-uted to both the charge transfer from PANI to MWCNTs and the effectof MWCNTs which can bridge between the conducting domains ofPANI because of their large aspect ratio and surface area. The resultsare compatible with the published data [4,16,20]. The resistivity ofpure MWCNT composites is higher than that of the functionalizedMWCNT composites due to the lack of appropriate distribution,agglomeration and the higher length of pure MWCNTs in matrix.

Fig. 7. Normalized electrical resistance of PANI composite doped with different wt.% offunctionalized MWCNTs. Fig. 9. Tg of composites versus MWCNT concentrations.

Fig. 10. Normalized electrical resistance versus temperature for PANI samples pressedunder different pressures.

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4.2. The glass transition temperature measurement

It is known that by increasing the temperature of a polymer, themobility of the polymer chains gradually increases and at glass tran-sition temperature (Tg), the chain wiggling increases rapidly. So it isexpected that, the electrical resistivity will increase at Tg due to theinteraction of polymer chains with the conduction electrons. There-fore, the Tg as a kinetic parameter can be measured by measuringthe resistivity's temperature dependence at a suitable heating rate.Based on our knowledge this procedure is a fairly new way on deter-mining the Tg of conducting polymers. Fig. 8 shows the normalizedresistance of PANI composite doped with 0, 2, 4 and 16 wt.% of func-tionalized MWCNTs from room temperature to about 430 K. As can beobserved, the resistance initially decreases but, at a certain tempera-ture which is shown by an arrow, it increases rapidly. The tempera-ture, at which the resistance increases, was chosen as the glasstransition temperature Tg. The Tg of PANI deduced using Fig. 8 was378 K which is very close to the reported Tg (373 K) for PANI [25]which means this method is a reliable method for measuring the Tgof electrical conducting polymers. The Tg of functionalized MWCNT/PANI composites were determined and observed that the Tg for 2, 4and 16 wt.% composites are 388, 393 and 394 K, respectively. Theerror in the determination of Tg was less than 1°. Fig. 9 shows the Tgversus MWCNT concentrations which indicates by enhancing thewt.% of MWCNTs, the Tg increases. Such an effect can be attributedto the presence of MWCNTs in the polymer matrix which limits themobility of polymer chains and so as a result, the transition happenedat the higher temperatures. Reduction in the increase rate of Tg for16 wt.%, can be due to the presence of MWCNTs, as a hot conductivityfactor in polymer matrix. On the other hand, by increasing wt.% ofMWCNTs, especially at higher temperatures, the electrical resistivitywas decreased.

Fig. 8. Normalized electrical resistance versus temperature for functionalized MWCNT/PANI composites. The arrows indicate theupturn point of resistance (heating rate 5 K/min.).

4.3. The effect of pelletizing pressure on Tg

As mentioned in Section 2.3, for resistance measurement, thepowders of samples were pressed into pellet form. The pelletizingpressure can affect the amount of resistivity and the freedom of thepolymer chains as well. So it is expected that the Tg will be affectedby the pelletizing pressure. Fig. 10 shows the normalized resistanceof the pure PANI samples which were pressed into the pellets underdifferent pressures. As can be seen from Fig. 10, the Tg or the temper-ature at which the resistance increases, shifts to higher values by in-creasing the pelletizing pressure. The Tg for the samples which werepressed under pressures of 96, 380, 580, 770 and 1150 MPa weremeasured 373, 378, 381, 384 and 387 K, respectively. Fig. 11 showsthe Tg versus the pelletizing pressure.

Fig. 11. Tg of PANI versus the pelletizing pressure.

1344 M. Farbod, S. Khajehpour Tadavani / Journal of Non-Crystalline Solids 358 (2012) 1339–1344

5. Conclusions

CNTs were purified and functionalized using a mixture of sulfuricand nitric acids. The resistivity data showed that by doping PANIwith CNTs, the activation energies change from 44.53 meV for thesample doped with 0.25 wt.% CNTs to 31 meV for the sample dopedwith 16 wt.%, so the resistivity of composites was lower than that ofthe pure PANI at all temperatures. This decrease was more for thecomposites containing functionalized MWCNTs due to the better dis-persion of MWCNTs in the matrix. The glass transition temperature(Tg) of the pure PANI was measured 378 K using electrical resistivitymeasurement. The Tg increased up to 394 K when the PANI is dopedup to 16 wt.% of functionalized MWCNTs. The Tg of pure PANI was ob-served to be dependent on the pelletizing pressure and increasedfrom 373 K to 387 K when the pelletizing pressure is increased from96 to 1150 MPa.

Acknowledgments

The authors would like to acknowledge Shahid Chamran Universityof Ahvaz for the financial support of this work, Dr. Kiasat andDr. Rafieepour for their help and discussion.

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