dr kiani, 7001 - ippijournal.ippi.ac.ir/manuscripts/ipj-2011-08-7619.pdf · japan) equipped with a...

Iranian Polymer Journal20 (8), 2011, 623-632

single-walled carbon nanotubes; conducting polymers;in situ chemical oxidativepolymerization;nanocomposites;electrical conductivity.

(*) To whom correspondence to be addressed.E-mail: [email protected]

A B S T R A C T

Key Words:

Highly Enhanced Electrical Conductivity andThermal Stability of Polythiophene/Single-walled

Carbon Nanotubes Nanocomposite

Gholamreza Kiani1*, Hossein Sheikhloie2, and Ali Rostami1

(1) School of Engineering-Emerging Technologies, University of Tabriz, Tabriz, Iran(2) Department of Chemistry, Islamic Azad University, Maragheh Branch,

Maragheh, Iran

Received 14 March 2011; accepted 6 July 2011

Herein, we synthesized and characterized polythiophene/single-walled carbonnanotubes nanocomposite. Thiophene (Th) and 2-(2-thienyl) pyrrole (TP) wereselected as interfacial modifiers for a SWNT-poly(Th-TP) nanocomposite. The

electrical conductivity and thermal stability can be dramatically improved by in-situ polymerization of a thin layer of self-doped conducting polymer around and along thesingle-walled carbon nanotubes (SWNTs) beside the bulk polymer. The resulting cable-like morphology of the SWNT-poly(Th-TP) composite structure was character-ized with Fourier transform infrared (FTIR), ultraviolet-visible spectroscopy (UV-Vis),field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis(TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and Ramanspectroscopy. Also, the morphology of the film was investigated using a scanning tunneling microscopy (STM). The characterization of the molecular structure has indicated that thiophene and TP molecules are adsorbed onto the surface of SWNTs topolymerize and SWNTs have been used as the core in the formation of a hybrid SWNT-poly(Th-TP) composite. Transmission electron microscopy analysis revealed that in theSWNT-poly(Th-TP) composite, SWNT constitutes the core and poly(Th-TP) acts as theshell. TGA data confirm that the presence of SWNT in the composite is responsible forthe high thermal stability of the whole material in comparison with pure poly(Th-TP).The standard four-point-probe method was utilized to measure the conductivity of thesamples. The conductivity through SWNT-poly(Th-TP) is as high as 38 Scm-1, wellabove the normal conductivity value of bulk poly(Th-TP) films (~1.67 × 10-1 Scm-1). Inaddition numerous physical properties of the nanocomposite were measured and theobtained results are discussed.

INTRODUCTION

Carbon nanotubes (CNTs) [1] arethe focus of considerable researchinterest due to their potential appli-cations. Their unique propertiesinclude splendid mechanical, thermal and electrical conductivityperformances. They offer trem-endous opportunities for the devel-

opment of fundamentally newmaterial systems for application innanoelectronic devices [2], scan-ning probe microscopy [3], fieldemitters [4] and so forth. Similarly,composites based on polymers andnanotubes have ability to makeimpacts on variety of applications

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ranging from general low-cost circuits and displays topower devices, micro-electromechanical systems,super capacitors and solar cell sensors [5,6].

Alternatively, π-conjugated conducting polymerssuch as polypyrrole (PPy), polyaniline (PANi), andpolythiophene (PTh) or their derivatives have attracted a great deal of attention because of theirunique properties, such as environmental stability andease of preparation [7,8].

Dispersion of carbon nanotubes into polymers canbe improved by either using surfactants as processingaids [9], functionalization [10] or in-situ polymeriza-tion [11]. The ability to combine CNTs with conduct-ing matrices such as poly(3-octylthiophene) [12]poly(3,4-ethylenedioxythiophene) [13] to producenanocomposite materials has stimulated muchresearch interest, specifically in organic electronics.

The novel one-dimensional structures of single-walled carbon nanotubes (SWNTs) lead to excellentelectrical properties as well as extremely highmechanical and chemical stability, but they are hardto be shaped into film. Meanwhile, conducting poly-mers show high transparency and flexibility and canbe easily processed into films. By combination ofthese two materials, we can obtain high performanceSWNTs/conducting polymer composite films.Recently, a variety of methods have been reported forproducing composites from CNTs and conductingpolymers [14-17].

The idea of using carbon nanotubes (CNT) as conductive filler to improve the electrical propertiesof polymer composites has drawn great attention bothfrom industrial and academic aspects. In our previouswork, 1-(2-pyrrolyl)-2(2-thienyl) ethylene (PTE) and2-(2-thienyl) pyrrole (TP) showed a considerableincrement in the electroactivity and redoxability ofpyrrole, N-methyl pyrrole and thiophene [18-20].

In our present work, considering the suitableeffects of TP on the polymerization of thiophene andthe corresponding effects of SWNTs on nano-composite, modified SWNTs coated with conductingpoly(Th-TP) can form complex SWNT-poly(Th-TP)com-posite. It was assumed to be able to observe various enhancements in nanocomposite propertiesincluding electrical conductivity and thermal stabili-ty. The molecular structure of the resulting SWNT-poly(Th-TP) composite has been characterized.

EXPERIMENTAL

Materials and MethodsSWNTs (AP-grade, diameter 1-1.2 nm and length 2-20 μm) were supplied by Iljin Nanotech Co. Ltd.,South Korea). The thiophene monomer, chloroform,anhydrous iron (III) chloride (FeCl3, oxidant) andother organic solvents were bought from Aldrich(reagent-grade) and used without further purification.2-(2-thienyl) pyrrole (TP) was prepared by themethod of Engle and Steglich [18].

A typical in situ chemical oxidative poly-merization of thiophene and TP was carried out in thepresence of SWNT. The routine synthesis of the SWNT-poly(Th-TP) composite was as follows:100 mL of chloroform containing SWNTs (0.2 g) wasadded to a 500-mL, double-necked, round-bottomflask equipped with a magnetic Teflon-coated stirrer.The mixture was sonicated for 30 min at room temperature to disperse the SWNTs. FeCl3 of 0.1 gdissolved in 100 mL chloroform was added to thesolution, and sonicated further for 30 min at roomtemperature.

The thiophene monomer (1 mL, 0.25 M) and TP(0.186 g ,0.025 M) with 50 mL chloroform wereplaced in a small portion line of the double neckedflask and added gradually (dropwise) to the suspension solution with constant stirring. The reaction mixture was stirred for an additional 24 hunder the same conditions.

The resultant SWNT-poly(Th-TP) powder wasprecipitated in methanol, filtered with a Buchner funnel, and then carefully washed with methanol,hydrochloric acid (0.1 M), distilled water, and acetone. The obtained black powder was dried undera vacuum dryer at room temperature for 24 h.Poly(Th-TP) was synthesized with a similar method,and the same molar ratios of monomer to dopant wereincorporated into the polymerization.

Infrared spectra were recorded on a ShimadzuFTIR-8400S, Japan. We also performed X-ray diffraction (XRD) experiments (Rigaku X-Ray dif-fractometer, Japan). SEM and TEM images wereobtained with a field emission scanning electronmicroscope (FE-SEM) (Hitachi model S- 4700,Japan, operating voltage; 15 kV) and Jeol JEM-2100,Japan, respectively. TGA curves were obtained with a

Highly Enhanced Electrical Conductivity and Thermal ... Kiani GR et al.

Iranian Polymer Journal / Volume 20 Number 8 (2011)624

Perkin Elmer Pyris Diamond, USA under nitrogenatmosphere with a heating rate of 10°C/min and temperature range of 25-800°C. Also a laser Ramanspectrometer (model HR800, Jobin-Yvon Horiba,Japan) equipped with a Coherent Innova 90C Fredargon ion (Ar+) laser source and an ultraviolet-visible (UV-Vis) spectrophotometer (Cary 50, Varian, USA) were used.

The morphology of the films was investigatedusing a scanning tunneling microscopy (STM) Nanosystem Pars (SS-1), Iran. STM images were obtainedin constant current mode using Pt/Ir tips, previouslytested on an HOPG (highly ordered pyrolyticgraphite) standard sample in order to verify their suitability for topographic performances. The STMmeasurements were performed at a tunneling currentof 0.1 nA and at bias of +0.1 V relative to the tip and

characterization was performed in air and at roomtemperature. The room-temperature conductivity ofthe pressed pellets was measured with the standardfour point-probe method with a Jandel Engineeringmodel CMT-SR1060N instrument, Japan.

RESULTS AND DISCUSSION

Structural CharacterizationFigure 1a shows SEM image of SWNTs. In Figure 1b,it is evident that pure poly(Th-TP), synthesized without SWNTs, shows a bulk morphology. Also thefield emission scanning electron microscopy image in Figure 1c shows a morphological view of the SWNT-poly(Th-TP) composite. The tubular morphology of the SWNT-poly(Th-TP) composite

625Iranian Polymer Journal / Volume 20 Number 8 (2011)

Highly Enhanced Electrical Conductivity and Thermal ...Kiani GR et al.

(a) (b)

(c)

Figure 1. SEM Images of: (a) SWNTs, (b) poly(Th-TP), and (c) SWNT-poly(Th-TP) composite samples.

Figure 2. TEM Image of SWNT-poly(Th-TP) composite.

has been imaged with TEM, as shown in Figure 2. Structural characterizations have shown that the

SWNTs are used as self-assembly hard templates forthe formation of one-dimensional, coaxial SWNT-poly(Th-TP) nanostructures, and the polymer is tightly coated on the surface of the SWNTs. Also, thissuggests that the interaction between polymer molecules and SWCNTs overcomes the Van derWaals interaction between SWCNTs, which general-ly would result in separate growth or aggregates ofpoly(Th-TP).

Figure 3 is a STM topographic image of SWNT-poly(Th-TP) composite showing a boundary regionbetween nanotubes and polymer. However, no exactinformation about the relative location of two materials can be extracted from this 3-dimensionalimage. STM topographic images show the compositenanostructure in which all the individual aligned nanotubes were fully or partially coated with a

Figure 3. Imaging by STM of SWNT bundles embedded in

the conducting polymer.

concentric layer of conducting polymers. The STM topographic image also indicates that the nan-otubes have not been damaged during the chemical polymerization process.

To examine the mixing-induced interactionbetween SWNTs and poly(Th-TP), FTIR and Ramanspectroscopic analyses were carried out. The FTIRspectra of poly(Th-TP), SWNTs, and SWNT-poly(Th-TP) composite are shown in Figures 4, spectra a-c, respectively. There are several low- intensitypeaks in the range of 2800-3100 cm-1 that can beattributed to the aromatic C-H stretching vibrationsand C=C characteristic band (1637 cm-1) [ 21,22].The absorption in this region is obscured by the bipolaron absorption of the doped poly(Th-TP).

The range of 600-1500 cm-1 is the fingerprintregion of poly(Th-TP). The peak at 786 cm-1 is usually ascribed to the C-H out-of-plane deformationmode, whereas other peaks in this region are attributed to the ring stretching, and C-H in-planedeformation modes [23,24]. The C-S bending modehas been identified at approximately 690 cm-1 andindicates the presence of a thiophene and TPmonomer [25]. The SWNT-poly(Th-TP) compositeshows nearly identical numbers and positions of themain FTIR bands in the range of 600-3200 cm-1. The

Figure 4. FTIR Spectra of: (a) poly(Th-TP), (b) SWNTs, and(c) SWNT-poly(Th-TP) composite samples.

Iranian Polymer Journal / Volume 20 Number 8 (2011)626


C-H stretching vibrations and C=C characteristicpeaks can be identified almost in the same range at2800-3100 cm-1 and 1637 cm-1, respectively. Onlypeaks of ring-deformation modes are shifted becauseof the probable polaron/π-transition interactionbetween the planar poly(Th-TP) backbone andSWNTs surface, indicating that some of the SWNTsdo not fully interact with the poly(Th-TP) molecules.Also, Table 1 shows some FTIR characteristic peakpositions (cm-1) in detail.

To detect the structure of nanocomposites of carbon nanotubes and conducting polymers Ramanscattering was employed. The Raman analysis of nanotubes samples, pure and composite polymerfilms have been performed in the 150-2000 cm-1

spectral region, where one can observe the peaks produced by the tangential modes of carbon nanotubes as well as the peaks ascribed to differentforms of conducting polymers. For Raman spectrum,the strong peak at 1583 cm-1 represents the backbonestretching mode of C=C bonds and the peaks at ~1377and 1083 cm-1 belong to the ring stretching and the N-H in-plane deformation of the oxidized (doped)species of the poly(TP), respectively.

In Figure 5, the typical Raman spectra of SWNTs,SWNT-poly(Th-TP) composite, and Poly(Th-TP) arepresented. In general, the Raman spectra of the poly-thiophene family show three main lines coupled to theelectronic transition. The main dominant lines of thepoly(Th) powder (spectrum c in Figure 5,) are around1500 and 1450 cm-1 (C-C stretching region) whichare totally symmetric, in-phase vibrations of the thiophene rings that spread over the whole polymerchain [25]. A weaker feature can be seen within therange of 1070-1040 cm-1 (C-C stretching plus a C-Hwagging component) and at 697 cm-1 for C-S-C ring

deformation [26,27].This behaviour is a vibrational signature that

proves that in PTh, π electrons are mostly confinedwithin each thiophene ring or within a very restricteddomain of the chain [28,29]. The bands located atabout 970 and 930 cm-1 are belonged to ring deformations associated with dication (bipolaron) andradical cation (polaron), respectively [30,31].

We have observed two phonon modes for theSWNTs (spectrum a in Figure 5) that give strongRaman scattering including low-energy peaks whichare attributed to a radial breathing mode (R band) atapproximately 163 cm-1 and 177 cm-1, and the higher energy peaks, with neighboring C atoms, vibrating

Figure 5. Raman spectra of: (a) SWNTs, (b) poly(Th-TP),and (c) SWNT-poly(Th-TP) composite samples measuredby 514-nm laser source.


Iranian Polymer Journal / Volume 20 Number 8 (2011) 627

Band assignments SWNTs Poly(Th-TP) SWNT-poly(Th-TP)

Ring deformationC-H in-plane bendRing breathingC-H in-plane bendRing breathing with contribution from C=C/C-C and C-NC=C/C-C stretching

79110261203129614191542

77910361210129814301550

78510231198130014271446

Table 1. Peak positions (cm-1) of FTIR absorption bands for poly(Th-TP), SWNTs and SWNT-poly(Th-TP) composite.

out of phase and parallel to the surface of the cylinder (tangential modes, T band) are related to the E2gphonon at about 1555 cm-1 and 1581 cm-1 in graphite[32-34]. The band at 1336 cm-1 appears only in theslightly disordered graphite (D band) [35]. Acombination mode has also been observed at 1727 cm-1 [36].

The spectrum c in Figure 5, the Raman spectra areshown for the SWNT-poly(Th-TP) composite, andthey are clearly identical to those of poly(Th-TP) andSWNTs, demonstrating that the SWNTs serve as thecore in the formation of SWNT-poly(Th) nanotubularcomposites. For instance, the so-called T band centered at 1581 cm-1 and R band of low-energypeaks (163 and 177 cm-1) dominate the nanotubesspectra. On the other hand, the peaks at 1044 and 697 cm-1 demonstrate the effects of poly(Th-TP)spectra. Consequently, this result further confirmedthat the poly(Th-TP) layer was deposited within thenetwork of the SWNTs.

XRD patterns of SWNTs, poly(Th-TP) andSWNT-poly(Th-TP) composite were depicted inFigure 6. The pure SWNTs have characteristic peaksat 23.4°, 44.1°, and 51°, which are in good agreementwith other reports [37]. In contrast, no peaks (broad pattern) were observed in the spectrum of

Figure 6. XRD Analysis data for: (a) SWNTs, (b) poly(Th-TP), and (c) SWNT-poly(Th-TP) composite samples.

poly(Th-TP), amorphous diffraction peak at approximately 2θ= 20-24°, suggesting an amorphousstructure of the polymer prepared by in-situ oxidative polymerization. A peak centered around 22°, corresponding to intermolecular π-π stacking [38].For the SWNT-poly(Th-TP) composites, the XRDdata show characteristic poly(Th-TP) broad peaks andstrong SWNTs peaks. There are, however, no newpeaks.

UV-Vis spectroscopy was used to observe theinterfacial interaction between poly(Th-TP) andSWNTs. Figure 7 shows the UV-Vis spectra ofSWNTs, SWNT-poly(Th-TP) composite, and bulkpoly(Th-TP) in tetrahydrofuran (THF) solutions. Forthe SWNTs, there is no absorption peak in the 350-800 nm range. The characteristic absorption band ofpoly(Th-TP) at approximately 441 nm is attributed to the transition from the valence bond to the antibonding polaron state (π-π* electronic transition),indicating that the resultant poly(Th-TP) is in thedoped state [39]. When the tubular nanostructure ofSWNT-poly(Th-TP) is formed, the characteristicbroad peak is slightly shifted to a higher wavelength(463 nm) as the SWNTs content loads inside closepacking by poly(Th-TP), revealing the interactionpossibility between the quinoid rings of poly(Th-TP)and the π transition of SWNTs [40].

This could be described by the percolation theory

Figure 7. UV-Vis Spectra for: (a) SWNTs, (b) poly(Th-TP),and (c) SWNT-poly(Th-TP) composite samples.


628 Iranian Polymer Journal / Volume 20 Number 8 (2011)

as the insulator-to-conductor transition in compositemade of conductive filler in an insulating matrix [41].

TGA curves of SWNTs, poly(Th-TP) and SWNT-poly(Th-TP) composite has been exhibited in Figure 8 upon heating in a nitrogen atmosphere. TheSWNTs are comparatively more stable in the range of25-800°C. It can be seen that SWNTs were stable anddid not show a dramatic decomposition in the tested temperature range. A small amount of weight lossoccurred due to water or volatile impurities evaporation.

In the case of poly(Th-TP), the TGA curve showsa weight loss at two stages as shown in curve c ofFigure 8. The first stage ranges between room tem-perature and 100°C, may be attributed to the loss ofadsorbed and bound water. The second stage ofweight loss starts at about 230°C for poly(Th-TP) thatcould be due to the large scale thermal degradation ofthe poly(Th-TP) chains. Addition of SWCNTs to thepoly(Th-TP) increased the thermal stability ofpoly(Th-TP) between 25 and 500°C around 6% massloss (comparing curves b and c of Figure 8). Also, theremained mass of the composite is higher than that ofpoly(Th-TP) itself at temperatures higher than 400°Cwhich was obviously related to the existence of thermally stable SWNTs.

As a result, these data confirm that the presence ofSWNTs in the SWNT-poly(Th-TP) composite isresponsible for the high thermal stability of the

Figure 8. TGA Curves for: (a) SWNTs, (b) SWNT-poly(TP-Th) composite, and (c) poly(TP-Th) samples.

composite material in comparison with pure poly(Th-TP). For instance in comparison with TGA data of Karim et al. [42] the thermal stability of our com-posite in the presence of TP is improved.

It is well-known that the electrical conductivity ofany material depends on the carriers' concentrationand mobility. In conducting polymers (CPs) carriers(polaron and bipolaron) concentration is determinedby the level of oxidation or doping. Alternatively, thecarrier mobility depends on the frequency of interchain hopping. Therefore, it is related to the conjugation length and the interchain distance.

As a consequence, any method of synthesis of CPsthat enhances molecular and super molecular orderleads to an increase in carrier mobility and enhances electrical conductivity. We believe that increasedmobility and conductivity can further improve theperformance of the electronic device. Thus, introduction of CNs into a polymer matrix canimprove the electric conductivity as well as themechanical properties of the original polymer matrix[43-45] while it possibly provides an additional activematerial for capacitive energy storage.

How CNTs enhance the electrical conductivity ofthe composite may be attributed to two reasons. Onereason is the introduction of effective conductingpaths to polymeric matrix [7,28]. The other reason isa good interfacial combination between CNTs andmatrix. A powder sample of 0.02 g was loaded and pressed into a pellet of 1.2 cm diameter and at170 atm of pressure by a manual hydraulic press for 10 min. The electrical conductivity measurements ofSWNTs, poly(Th-TP) and SWNT-poly(Th-TP) composite were carried out using a four point-probeapparatus at the room-temperature.

The conductivity of the resulting SWNT-poly(Th-TP) composite, at room temperature, is 38 Scm-1, which is higher than that of the purepoly(Th-TP) (~1.67×10-1 Scm-1) synthesized withoutCNTs under the same conditions. The obtained conductivity of our composite in the presence of TP isapproximately 90 times of the one previously reported in the absence of TP [42]. For each of theconductivity values reported, the results are an average of five measurements.

A different behaviour is observed for the SWNT-poly(Th-TP) composite which in any case, exhibits a



higher conductivity than the pure polymer. Moreoverit is clear that, whether chemically treated or untreat-ed, SWNTs are found to enhance charge transport andelectrical properties of the composite significantlydepending on the treatments to which the SWNTswere undergone.

The CNTs may also serve as "conducting bridges"connecting the conducting domains of poly(Th-TP).Such enhanced conductivity of polymeric chains, dueto an increased degrees of electron delocalization andCNT bridging, has been reported for macroscopicPAn/CNT [46] and PPy/ CNT [47] films. The one-dimensional structure of CNTs may also induceand promote oriented polymerization, hence yieldingan enhanced supramolecular order and higher conductivity.

CONCLUSION

We have developed a new composite of SWNTs and conducting poly(Th-TP). The composite SWNT-poly(Th-TP) has been synthesized successfully by thein situ chemical oxidative polymerization method ofthe thiophene monomer in the presence of TP ontoSWNTs. The characterization of the molecular structure has indicated that thiophene and TPmolecules are adsorbed onto the surface of SWNTs,and then polymerized. Thus SWNTs have been usedas the core in the formation of hybrid SWNT-poly(Th-TP) composite. The conductivity through SWNT-poly(Th-TP) is as large as 38 Scm-1 which is muchlarger than the usual value of bulk poly(Th-TP) films (~1.67 ×10-1 Scm-1). The SWNT-poly(Th-TP) composite shows improved thermogravimetric stability in comparison with the poly(Th-TP).Consequently, the improvements made in the variousphysical properties of SWNT-poly(Th-TP) compositeare paving the way for enhancement in the application potential of conducting poly(Th-TP) without hampering its chemical properties. The conductivity and thermal behaviour of compositehave been improved in comparison with other similarcomposite. The method described here may be utilized in developing new applications for SWNT-poly(Th-TP) composite in molecular electronics andother fields.

ACKNOWLEDGEMENT

The authors wish to thank University of Tabriz for allthe supports provided. We would like to thank Mr. A.Haddadpour for his kind cooperation.

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