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Materials Science in Semiconductor Processing 10 (2007) 246–251

Polypyrrole–montmorillonite clay composites:An organic semiconductor

Anuar Kassima, H.N.M. Ekramul Mahmudb,�, Fariz Adzmia

aDepartment of Chemistry, Faculty of Science and Environmental Studies, Universiti Putra Malaysia, 43400 Serdang, Selangor, MalaysiabFaculty of Chemical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia

Available online 6 May 2008

Abstract

The conducting polypyrrole–montmorillonite clay (Ppy–MMT) composites were prepared by chemical polymerization.

The prepared composites were subjected to structural, thermal and morphological characterizations and dc conductivity

measurement. The dc conductivity of Ppy–MMT composites measured at room temperature was found to decrease from

2.25 to 0.31 S/cm with an increase in the percentage of montmorillonite (MMT) from 1% to 7%. The surface morphology

of the prepared composites is denser and more compact compared to pure montmorillonite as can be evidenced from SEM

micrographs. The formation of Ppy–MMT composites was supported by Fourier transform infrared (FTIR) spectra of the

composites.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: Polypyrrole; Conducting polymer; Chemical polymerization; Morphology; Semiconductor

1. Introduction

Conducting polypyrrole (Ppy) polymer has beenextensively researched due to its varied potentialapplications in batteries [1], super-capacitors [2],and microwaves shielding and corrosion protectionbecause of its environmental stability to oxygen andwater, high conductivity and ease of synthesis [3–6].The positively charged Ppy, the electron holesavailable from longer polymer chains and the co-planarity between interchains are favorable for ahigher conductivity performance [7]. One advantage

e front matter r 2008 Elsevier Ltd. All rights reserved

ssp.2008.02.001

ing author. Tel.: +6 3 55 436 343;

6 300.

esses: ekramul@salam.uitm.edu.my,

hoo.com (H.N.M.E. Mahmud).

of Ppy concerns the low oxidation potential ofpyrrole [8]. The typical Ppy, which is insoluble andinfusible, exhibits poor possibility and lacks essen-tial mechanical properties. Efforts to overcomethese drawbacks have led to numerous researcheson the synthesis of Ppy by both electrochemical andchemical routes [9–12].

Clay minerals have been adapted to the fieldof nanocomposites because of their small particlesize and intercalation property, especially in theapplication of reinforcement materials with poly-mers. Among various clay materials, montmorillo-nite (MMT) clay, whose lamellae are constructedfrom an octahedral alumina sheet and sandwichedbetween two tetrahedral silica sheets, exhibits a netnegative charge on lamellae surface, and causesit to absorb cations such as Na+ or Ca+ [13].

.

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The interlayer spacing becomes large enough to bepenetrated by relative charge size micelles withswelling of the clay in water.

In this study, we synthesized and characterizedthe composites of conducting Ppy polymer andMMT clay through the chemical method in aqueousmedium. We report the composites based on resultsof dc conductivity, scanning electron microscopy(SEM), Fourier transform infrared (FTIR) andthermogravimetric analysis (TGA).

Table 1

Conductivity of Ppy–MMT composites with the increase in

MMT clay

Percentage of MMT used Conductivity of Ppy–MMT

composites (S/cm)

1 2.25

2 2.07

3 0.95

4 0.64

5 0.60

6 0.54

7 0.38

2. Experimental

In this study, the monomer pyrrole (supplied byFluka) was distilled prior to use. FeCl3 � 6H2O(supplied by APS Ajax Finechem) was used as anoxidant, while MMT clay (supplied by Fluka) wasused as a clay mineral. Distilled water was used asthe solvent (50mL). For the synthesis of PPy–MMTclay composites in aqueous medium, a calculatedamount of MMT (1% to 7% v/w) was added to anaqueous solution of 0.4M of FeCl3 � 6H2O undercontinuous stirring. To this mixture, 0.1M ofpyrrole was added directly. The mixture wascontinuously stirred for 6 h at room temperature(25 1C). The addition of pyrrole monomer to theMMT was accompanied by a gradual color changefrom light gray to greenish-blue to black, indicatingthe formation of Ppy [14]. Finally, the black mass(composite) present in the system was filtered andwashed thoroughly with distilled water until it wascompletely free from FeCl3. The black powder ofthe composite was then dried in the oven for 6 hat 45 1C.

0

100

200

300

400

1000200030004000

(c) Ppy/MMT

(b) pure Ppy

(a) pure MMT

Wavenumber (cm-1)

Tra

nsm

ittan

ce (

%)

Fig. 1. FTIR spectra for pure MMT, pure Ppy and Ppy–MMT

composites.

3. Characterization of the composites

The dc conductivity of the composites wasmeasured at room temperature using a four-pointprobe. The composites were pelletized beforemeasuring the conductivity. The conductivity meterwas calibrated by using a standard silicon wafer(Standard Reference Material no. 2545, suppliedby the National Institute of Standard and Technol-ogy, USA). The infrared spectra of variousPpy–MMT composites were taken on a Perkin-Elmer FTIR spectrophotometer. The thermal sta-bility of the samples was measured by TGA (TAinstrument Mettler Toledo SW 7.01). Surfacemorphology of the samples was studied by SEM(model Philip).

4. Results and discussion

The dc conductivity of the composites wasmeasured by using the following equation: s ¼(I ln 2)/(Vpt), where s ¼ conductivity, I ¼ current inamperes, V ¼ voltage in volts and t ¼ thickness ofthe pellets. As shown in Table 1, the conductivityvalues of the composites decreased from 2.25 to0.35 S/cm with the increase in the percentage ofMMT clay. Since the conductivity measured by thefour-point probe was done on the surface, thedecrease in conductivity indicates that the MMTclay is distributed on the surface of the compositekeeping the conducting Ppy in the galleries of theclay. The conductivity of the composites varied withthe Ppy loading in the MMT. This may be due tothe increase in more electron holes available alongthe longer polymer chain and the creation of moreco-planarity between the interchain. Increasing theamount of MMT clay, the tetrahedral structure of

ARTICLE IN PRESSA. Kassim et al. / Materials Science in Semiconductor Processing 10 (2007) 246–251248

MMT is likely to stop the elongation of the Ppypolymer chain, resulting in the limited electrontransfer between the interchain, which in turnaffected the conductivity of the composites. Thus,significant changes in the structure occur due to theintercalation of Ppy in the MMT clay galleries [15].

FTIR spectroscopy reveals the functional groupsof Ppy and MMT in the Ppy–MMT composites.Fig. 1 shows the FTIR spectrum of Ppy. A broadpeak at 3437 cm�1 is the characteristic of the N–Hstretching mode. The peak at 1539 cm�1 is forCQC stretching of pyrrole. A small peak at1454 cm�1 is due to C–N stretching of pyrrole.The peak at 1168 cm�1 is observed for C–Cstretching of pyrrole. The C–H stretching and

Wei

ght L

oss

(%)

Fig. 2. TGA analyses for (a) Ppy, (b) MM

N–H bending of pyrrole can be observed at1046 cm�1. The peak at 780 cm�1 is for C–Hbending. The peak observed at 672 cm�1 refers tothe C–H stretching of pyrrole. The characteristicpeaks for MMT are also shown in Fig. 1. The broadpeak observed at 3381 cm�1 is for O–H stretching ofMMT. The next peaks at 2951, 2460 and 1637 cm�1

are due to C–H vibration. The peak observed at910 cm�1 is due to Si–O stretching. The peaks foundat 596 and 531 cm�1 are due to Al–O stretching. Thepeak at 470 cm�1 is due to Si–O bending.

The FTIR spectrum for Ppy–MMT composite isalso shown in Fig. 1. The broad peak observed at3428 cm�1 is the characteristic for O–H vibration ofMMT and pyrrole. The peaks 1632 and 1482 cm�1

mg/

min

TGA

TGA

DTA

DTA

ppy

DTA

MMT

T and (c) Ppy–MMT composites.

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are due to C–H vibration of MMT. The peak at1482 cm�1 is referred to the CQC vibration ofpyrrole. The peak at 1454 cm�1 is referred to theC–N vibration of pyrrole. C–C vibration of pyrrolecan be observed at 1309 and 1182 cm�1. C–Hstretching and N–H stretching of pyrrole can beobserved at 1037 cm�1. The peak at 906 cm�1 refersto Si–O stretching of MMT. C–H stretching ofpyrrole can be observed at 780 and 681 cm�1.The peaks at 615 and 517 cm�1 are due to Al–Ostretching of MMT. The peak observed at 460 cm�1

is due to Si–O vibration of MMT.The characteristic functional groups of both Ppy and

MMT can be observed in the Ppy–MMT spectrum inFig. 1. Thus, the FTIR spectrum of Ppy–MMTconfirms the formation of the Ppy–MMT composite.

Fig. 3. SEM microg

Fig. 4. SEM micro

The thermal properties of the Ppy–MMT compo-site, Ppy and MMT are shown in Fig. 2. It shows thatthe degradation of pure Ppy starts at 156.31 1C andthe percentage weight loss is 10.35%. The seconddegradation starts at 413.33 1C and weight loss is13.86%. This degradation process is referred to theorganic compound in the pure Ppy. The degradationis completed at 612.04 1C. The TGA analysis for pureMMT shows the degradation at 199.68 1C with aweight loss of 2.87%. The second degradation stepfor MMT starts at 341.21 1C and the weight loss is2.10%. The degradation process is complete at612.87 1C, which is attributed to the thermal decom-position of the organic modifier of the clay.

TGA patterns obtained in the Ppy–MMT com-posite reveal a better thermal stability compared to

raph of MMT.

graph of Ppy.

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Fig. 5. SEM micrograph of Ppy–MMT composite.

A. Kassim et al. / Materials Science in Semiconductor Processing 10 (2007) 246–251250

the pure Ppy. The dehydration step for Ppy–MMTstarts at a temperature of 43.69 1C and the weightloss is 6.30%. The degradation of the compositebegins at the temperature of 177.21 1C with a 5.60%percent weight loss. The second degradation startsat a temperature of 379.55 1C and the weight lossis 4.12%. The degradation of the composite iscomplete at 524.24 1C. The reasons could beattributed to a more ordered and dense structureof the Ppy–MMT composite and the interaction ofPpy within the clay structure, resulting in a shieldingeffect by clay layers in the thermal analysis.

The scanning electron micrographs reveal someinteresting morphological differences among MMT,Ppy and Ppy–MMT composite. Particles of MMT(Fig. 3) have irregular plate-like shapes of hundredsof micrometers in two dimensions, length andwidth. The typical Ppy structure (Fig. 4) is micro-globules. As shown by SEM, the original structureof MMT changes with the rearrangement of theMMT structure caused by the presence of Ppy(Fig. 5). It shows that Ppy modified the flakystructure of MMT nanoparticles. The SEM mor-phology for the Ppy–MMT composite (Fig. 5) alsoexhibits that the particles appear to be more denselypacked, with the manifestation of relatively smallerparticles size. It also shows that the Ppy–MMTcomposite has a globular pattern with non-uniformparticle distribution.

5. Conclusions

Ppy–MMT conducting polymer composites havebeen successfully prepared by polymerizing pyrrole

in the presence of FeCl3 � 6H2O and MMT clay inaqueous medium. The results reveal a decreasingtrend of the conductivity from 2.25 to 0.35 S/cmwith the increase in percentage of MMT clay. TheFTIR result shows the successful incorporation ofMMT in the Ppy structure. The TGA analyses showthe thermal degradation of the Ppy–MMT compo-site, which reveals that Ppy–MMT is thermally lessstable than pure Ppy due to the composite forma-tion of Ppy–MMT. The surface morphology shownby SEM reveals the denser and more compactmorphology of the Ppy–MMT composite.

Acknowledgments

The authors thank the Department of Chemistry,Faculty of Science and Environmental Studies,Universiti Putra Malaysia, for FTIR and TGAanalysis. The authors are also grateful to theInstitute of Bio Science of UPM for SEM analysis.

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