Materials Letters 64 (2010) 1313–1315

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Materials Letters

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Expanded graphite/polyaniline electrical conducting composites: Synthesis,conductive and dielectric properties

Chen Xiang, Liangchao Li ⁎, Suyong Jin, Baiqun Zhang, Haisheng Qian, Guoxiu TongZhejiang Key Laboratory for Reactive Chemistry on Solid Surface, Zhejiang Normal University, Jinhua 321004, China

⁎ Corresponding author. Tel.: +86 579 82282384; faxE-mail address: sky52@zjnu.cn (L. Li).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.03.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 November 2009Accepted 8 March 2010Available online 17 March 2010

Keywords:GraphiteElectrical propertiesSemiconductorsComposite materials

Expanded graphite/polyaniline electrical conducting composites were prepared by emulsion polymerizationof aniline in the presence of expanded graphite. The phase composition andmorphology of the composites werecharacterized by X-ray diffractometer and scanning electron microscopy, respectively. The electricalconductivities were measured by a four-probe resistivity instrument. The dielectric loss factor was performedon an impedance/material analyzer. The electrical conductivities of the composites were enhanced dramaticallyto 298.51 S cm−1 compared to polyaniline. The composites also showed excellent dielectric loss behavior in thefrequency range from 1 MHz to 3 GHz, and the maximum of the dielectric loss factor approximated to 15,000.

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1. Introduction

Electrical conducting functional composites have recently at-tracted much attention. Due to the improved physical and chemicalproperties, they have an important potential application in micro-wave shielding and absorption. Among the family of conductingpolymers, polyaniline is unique and has been most studied becauseof its easy synthesis, environmental stability, and simple doping/dedoping chemistry [1,2]. Much effort has been made to design therational methods to synthesize polyaniline [3–5]. It's well known thatexpanded graphite (EG) is much cheaper, lighter, and has a higherspecific surface area than carbon powder and carbon nanotubes [6].EG is of great importance to apply in manufacturing nanofibers [7],electrolyte membrane fuel cells [8], hydrogen storage [9,10] andammonia adsorption [11]. To the best of our knowledge, the synthesisof expanded graphite/polyaniline (EG/PANI) functional compositesfor microwave shielding and absorption have not been studiedthoroughly so far. Considering their high conductivities and dielectricproperties, graphite/polyaniline composites would have potentialapplications in rechargeable batteries [12–14], biosensor monitoring[15,16], conductive inks [17], radar evasion and anti-static textiles[18], etc.

In the present study, we developed emulsion polymerization toprepare expanded graphite/polyaniline (EG/PANI) functional compo-sites. The mophology, conductivity and dielectric properties of thecomposites have been studied.

2. Experimental

2.1. Materials

Aniline (An), dodecyl benzenesulfonic acid (DBSA), and ammoni-um persulfate (APS) were all bought from Sinopharm ChemicalReagent Co., LtdS. Expandable graphite were bought from QingdaoBaichuan Graphite Company Ltd. All of reagents were analyticallypure. The aniline was purified by vacuum distillation; the others wereused as received.

2.2. Sample preparation

2.2.1. The synthesis and pretreatment of expanded graphiteExpanded graphite (EG) was obtained after thermal treatment of

expandable graphite at 700 °C for 1 min in muffle furnace. A mixtureof 1 g EG and 60 mL ethanol was stirred vigorously by mechanicalraking for 1 h in a flask; and the above solution was treated using anultrasonic cleaner for 10h; the final expanded graphite was collectedby vacuum filtration and dried at 80 °C.

2.2.2. The synthesis of expanded graphite/polyaniline (EG/PANI)composites

In a typical experimental process, 0.2 g EG and 1 g aniline (An), wereadded into 60 mL 0.01 mol/L dodecyl benzenesulfonic acid (DBSA)aqueous solution. It was kept by mechanical raking for 0.5h, resultinginto emulsion. Subsequently, a solution of ammonium persulfate (APS)wasdropwisely added into the emulsion. The reactionwas carriedout inan ice-bath for 12h. The products were collected by vacuum filtration;washed with ethanol and distilled water for three times and dried at80 °C for 12 h. The expanded graphite/polyaniline (EG/PANI) composite

Fig. 1. XRD patterns of PANI (a); EG/PANI composites with different mass ratios:mEG/An=0.2 (b), 0.4 (c), 0.6(d), 0.8(e), and 1.0 (f), respectively; and EG (g).

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with the mass ratio of EG/An (mEG/An)=0.2 was obtained. In anothersynthesis, the mass ratio of EG and aniline (mEG/An) was changed from0.4 to 1.0, while other experimental conditions were kept the same. Forcomparison, the pure polyanilinewas also preparedby the sameprocessin the absence of expanded graphite.

2.3. Measurements

The phase composition of the obtained samples were character-ized by powder X-ray diffraction, which is operated on a Philips-PW3040/60 X-ray diffractometer equipped with graphite monochro-matized Cu Kα radiation (λ=0.154 nm). The morphologies of thecomposites were investigated by scanning electron microscopy(Hitachi S-4800) with an accelerating voltage of 5.0 kV. The electricalconductivities were carried out on a four-probe resistivity instrument(SDY-4) at room temperature. The dielectric loss factor tests wereperformed on an impedance/material analyzer (E4991A) in thefrequency range of 1 MHz–3 GHz.

3. Results and discussion

3.1. X-ray diffraction

Fig. 1 shows the X-ray diffraction patterns of the as-preparedpolyaniline (PANI), expanded graphite (EG), and EG/PANI composites.

Fig. 2. SEM images of PANI (a), EG (b), EG/PA

All the patterns (shown in Fig. 1g) can be easily indexed as graphitematerials, which is in good agreementwith the literature value (JCPDSCard number 75-2078). Fig. 1a shows the diffraction patterns of as-prepared polyaniline, in which there are two diffraction peaks oflocated at 20.07 ° and 25.52 °. All the diffraction peaks shown in Fig. 1b–f, which can confirmed that EG/PANI composites were preparedsuccessfully. We found that the relative intensity of EG peaks arestronger as the content of EG increases.

3.2. Morphology

Fig. 2 shows the typical morphology of PANI, EG, and EG/PANIcomposites with mEG/An of 0.8. Polyaniline bulks (shown in Fig. 2a)obtained by this method appears as irregular particles with more than500 nm in diameter. As shown in Fig. 2b, the expanded graphite ismicroflaked with tens of micrometers in diameters, which is composedby some multilayer graphite structures along the c axis. There is a bigenough interspace between two layers, which provides the possibilityfor aniline monomer to occupy and polymerize. So, aniline monomersare not only polymerized on the surface, but also in the interspace ofEG, resulting into thedelaminating and scrolling of thegraphite layers toform regular EG/polyaniline microrods (Fig. 2c). High magnificationSEMimages (Fig. 2d ande) clearly reveal that theEG/PANImicrorods arearranged in both horizon and vertical orientation, and some of themlinked to each other. Most of them have the same diameter (around250 nm), and the surface looks like thorns, which are atactic sheetswith20–25 nm thickness (Fig. 2f). It can be speculated from Fig. 2e and f thatPANI molecular chains are intertwining in screw type when growing,and PANI microrods are formed by several PANI chains, which sug-gests that EG promoted aniline monomer to polymerize regularly.The possible reasons for the special morphology of PANI are that themultilayer structures of EG are propitious to the dispersing of anilinemonomer and the oligomers, and causes the PANI chains to be oriented.

3.3. Electrical conductivity

The electrical conductivity of EG/PANI composites are shown inTable 1. The conductivities of pure EG and pure PANI are 110.50 and0.17 S cm−1, respectively. The conductivities of the composites in-crease along with the increase of EG content, and when mEG/An=0.8,it reached to the maximum, 298.51 S cm−1. The possible reasons aregiven as follows: the conduction mechanism of EG includes tworespects, the electron directional movement in the EG layers and the

NI composites with mEG/An of 0.8 (c–f).

Table 1The electrical conductivities of PANI, EG, and expanded graphite/polyaniline composites.

Sample σ (S cm−1)

PANI 0.17EG 110.50mEG/An=0.2 6.18mEG/An=0.4 18.00mEG/An=0.6 93.46mEG/An=0.8 298.51mEG/An=1.0 34.42

Fig. 3. Frequency dependence of Tan δm of PANI and EG/PANI composites.

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electron transition between the layers. On one hand, the PANIconjugated chains offer a lot of good conductive paths on the surfaceof EG layers, which activates more electrons to participate in thedirectional movement. On the other hand, the embedding of PANIparticles into the interspace of EG layers, as seen in Fig. 2, may causesome interactions between PANI chains and EG layers. The existence ofPANI chains play a part as bridged linkage,which enables the electron totransfer between the EG layers easier, and so the forbidden band ofEG are minished. The synergistic effect of these two parts lead to thedramatically increase of the conductivity of EG/PANI composites. AsmEG/An increases further, the effect of the PANI chains as bridged linkagebecomes weaker, the equalization of EG and PANI for conductivitiesresults in the decrease of composites' conductivities. So, the conductiv-ities of the composites are determined by the content of EG or PANI,and is better than the polyaniline–graphite composites reportedbefore [18–20].

3.4. Dielectric property

The dielectric loss of the EG/PANI composites were tested withinthe frequency of 1 MHz–3 GHz, as shown in Fig. 3. The EG/PANIcomposites have high dielectric loss in the range of 1.0 GHz to 3.0 GHz,and the peak values increase at first, and then decrease, as the contentof EG increases. The dielectric loss factor of EG/PANI composites withmEG/An of 0.2 is the highest of all EG/PANI composites, which is up to14,634.8. The dielectric loss peaks of EG/PANI composites with mEG/An

of 0.2, 0.4, and 0.6 move to the higher frequency region, but the peaksbecome weaker obviously whenmEG/An=0.8 and 1. It can be indicatedthat the dielectric property of the composites could be enhancedwithin the specific limits of mEG/An. The same as natural flake graphite,the dielectric loss property of EG is mainly ascribed to interfacialpolarization. Meanwhile, as a conjugated conducting polymer, thedielectric loss property of PANI ismostly attribute to presence of bound/localized charges (polarons/bipolarons) leading to strong polarizationand relaxation effects [21,22]. Therefore, the dielectric loss of compo-sites is a summation of the polarization relaxation and the dipolarreorientation of PANI, the interfacial polarization of EG layer, andinterface relaxation between PANI polymer and EG layers. Thus, thedielectric property of EG/PANI composites could be adjusted bychanging the content of EG.

4. Conclusions

In conclusion, the conducting composites of expanded graphite/polyaniline have been fabricated via emulsion polymerization success-

fully. The electrical conductivities of the composites were enhanceddramatically compared topolyaniline. The composites showedexcellentbehavior of dielectric loss in the frequency regionof 1 MHz to3 GHz. Theexpanded graphite/polyaniline functional composites could be used aspotential materials for microwave shielding and absorption.

Acknowledgement

This work was supported by the Natural Science Foundation ofZhejiang Province (Y4080417).

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