ISSN 0965�545X, Polymer Science, Ser. A, 2012, Vol. 54, No. 5, pp. 401–406. © Pleiades Publishing, Ltd., 2012.

401

1 INTRODUCTION

Polymer/clay nanocomposites have attracted con�siderable attention for various engineering applica�tions such as enhanced mechanical properties andthermal stability, reduced gas permeability, and self�extinguishing flame retardant characteristics [1–6]. Inthe case of conducting polymer/inorganic clay nano�composites, they provide the new synergistic proper�ties, which cannot be attained from individual materi�als, such that the conductivity is more easily con�trolled, and the mechanical or thermal stability isimproved through the synthesis of the nanocomposites[7]. Different types of clays have been used by variousresearchers with varying degree of success in obtainingthe nanocomposites. Owing to the interesting proper�ties of pillared layered clays (PILCs) such as large sur�face area, high pore volume and tunable pore size,high thermal stability, strong surface acidity and cata�lytic active substrates/metal oxide pillars [8] whichmake them attractive materials in catalytic reactions,separation technology, environmental protection,electrochemistry, membranes and sensors, we have re�cently reported preparation, characterization andelectrical conductivities of polypyrrole/aluminiumpillared montmorillonite (PPy/Al�PMMT) claynanocomposites [9]. Apart from the general tailorabil�ity of their chemical and physical properties thesecomposites showed enhanced AC and DC conductiv�ities as compared to pristine PPy [9], which makethese composites interesting materials for device ap�

1 The article is published in the original.

plications. Here we report dielectric properties ofthese nanocomposites and analyse the data using elec�tric modulus formalism.

EXPERIMENTAL

Materials

Pyrrole was purchased from Fluka, Aluminum Pil�lared Montmorillonite clay and FeCl3 ⋅ 6H2O was pro�vided by Aldrich. Pyrrole was freshly distilled beforeuse. All materials were used as provided without anyfurther purification.

Preparation of Polypyrrole (PPy)

FeCl3 ⋅ 6H2O was dissolved in 10 mL of distilledwater and stirred for 1 hour, and then pyrrole wasdropped slowly in the suspension during stirring at 5oCin the absence of light. The molar ratio of monomer tooxidant was kept 1 : 2. The suspension was left 24 h forpolymerization. Finally, the suspension was filteredwashed with distilled water again and again to removeFeCl3 ⋅ 6H2O and other adhering substances. Greenishblack powder of PPy was obtained which was dried at90°C for 24 h in vacuum oven.

Preparation of PPy�Aluminum Pillared Clay Composites

For the synthesis of PPy�Al�PMMT clay compos�ites in aqueous medium, the Al�PMMT clay disper�sion in aqueous medium was first prepared by adding

Dielectric Properties of Polypyrrole/Pillared Clay Nanocomposites1

Abdul Shakoora, Tasneem Zahra Rizvib, and Muhammad Saeeda

a Polymer Physics Laboratory, Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan

b Department of Physics, Quaid�i�Azam University Islamabad, Pakistane�mail: shakoor_47@yahoo.com

Received September 12, 2011; Revised Manuscript Received December 20, 2011

Abstract—Dielectric constant ε' and loss factor ε'' were measured in intercalated polypyrrole/aluminum pil�lared montmorillonite (PPy/Al�PMMT) clay nanocomposites in the frequency range 100 Hz to 1 MHz. ThePPy/Al�PMMT nanocomposites were prepared by in situ polymerization of pyrrole in aqueous dispersion ofvarying amounts of (Al�PMMT) clay from 0.2 to 10%, using FeCl3 · 6H2O as an oxidant. Formation of thenanocomposite was studied by FTIR and intercalation of PPy in the clay galleries was confirmed by XRD.The nanocomposites exhibited very large values of ε' and ε'' at low frequency which decreased with frequencyand increased with the clay content in the samples. Electric modulus formalism exhibited a peak in the fre�quency dependence curves of imaginary part of the electric modulus due to conductivity relaxation process.The peak of conductivity relaxation shifted towards higher frequencies and the magnitude of relaxationdecreased with the increase of MMT content in the composites.

DOI: 10.1134/S0965545X12050094

COMPOSITES

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known weights of Al�PMMT clay into known volumeof distilled water under constant stirring. After 1 h,FeCl3 ⋅ 6 H2O was added to the dispersion in such away that it would attain the required concentration.Pyrrole was then added into the dispersion so as tomake 2 : 1 mole ratio of FeCl3 · 6H2O to pyrrole underconstant stirring at temperature 5°C in the absence oflight. The total volume of the reaction mixture waskept at 50 mL. The gradual change of color from lightblack to deep greenish�black indicated the formationof polypyrrole. The reaction mixture was then keptunder room temperature for 24 h. The resulting deepgreenish�black mass was filtered and then it was thor�oughly washed with distilled water until it was com�pletely free from FeCl3. This process was repeated sev�eral times to remove all adhering substances. Finally,the product was washed with water again and then itwas dried at 90°C for 24 h to yield a very fine deepgreenish�black powder of PPy/Al�PMMT clay com�posite [9]. All the samples were kept desiccated priorto measurements.

Measurements

The Fourier transform infra red spectra was record�ed on KBr pellet samples in the range of 4000–400 cm–1 by using a Perkin�Elmer Fourier transforminfra red spectrometer. The samples were heated at therate of 10°C/min in nitrogen atmosphere. Tanδ andcapacitance were measured using Wayne Kerr LCRmeter Model 4275 in the frequency range 100 Hz to1 MHz at room temperature (300 K) and the dielectricconstant and dielectric loss were calculated by usingthe Eqs. (1a) and (1b), respectively

ε' = Cd/Aε0 (1a)

= ε''/ε' (1b)δtan

Here ε' is relative permittivity, C is capacitance, d isthickness of the sample, A is area of cross section andε0 is permittivity of free space. ε'' is imaginary part ofdielectric constant known as dielectric loss, whereas ε'is real part of dielectric constant known as relative per�mittivity ε'.

RESULTS AND DISCUSSION

FTIR absorption spectra of PPy, Al-PMMT andtheir composites in KBr pellets are shown in Fig. 1.The characteristic absorption bands of PPy are ob�served at 1539 cm–1 (C=C stretching) and 1034 cm–1

(C–H vibration) along with all other characteristicpeaks of PPy at 772, 903, 1177 and 1286 cm–1 arefound in spectra of all composites but they are shift�ed to higher wave numbers as shown in Fig. 1(curves a–c). Strong peaks at 1042 cm–1 and thepeaks at 923 and 790 cm–1 being the characteristicvibration of Al�PMMT [8–10] (Fig. 1, curve d) areshifted to lower wave numbers and intensities. Peakscorresponding to PPy in the spectrum of PPy�Al�PM�MT nanocomposites become stronger as amount ofAl�PMMT increases in PPy, which is a strong evi�dence for the intercalation of PPy in Al�PMMT gal�leries. Shifts in the positions of characteristic peaks re�veal the formation of new bonds and confirmed the in�terfacial interaction between PPy and Al�PMMT clayin the nanocomposites.

Figures 2 and 3 display the frequency dependenceof the measured dielectric constants and loss factors inpure PPy and PPy/Al�PMMT nanocomposites in theform of log�log plots. Both dielectric constant and di�electric loss exhibit unusually high values at low fre�quencies which decrease with frequency and increasewith MMT content.

3500

a

3000 2500

Absorbance, a.u.

4000 1500 5002000 1000Wave number, cm−1

b

c

d

Fig. 1. FTIR of (a) PPy, (b) 2% Al�PMMT, (c) 5% Al�PMMT in PPy, and (d) Pure Al�PMMT.

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DIELECTRIC PROPERTIES OF POLYPYRROLE/PILLARED CLAY NANOCOMPOSITES 403

The large positive real values of permittivity depictthe large effective sizes of metallic islands and easycharge transport through well ordered polymer chainsbetween disordered regions as suggested by Joo et al.[11]. The observed increase in the real permittivity val�ues with the increase of MMT content shows signifi�cant increase in the size and number of the metallic is�lands between the insulating regions with the increaseof MMT loading in the nanocomposites. With the in�crease in MMT content, more and more polymerchains acquire ordered structure due to intercalationin the clay galleries. This is consistent with our XRDresults, which showed increased polymer intercala�tion, via increased d�spacing, with the increase ofMMT content in the composites.

As regards the possible sources of the observed di�electric dispersion, we know that for any heteroge�neous system containing mobile charge carriers likeconducting polymers and their composites, the imag�inary part of the total dielectric permittivity is the sumof individual components due to dipolar reorienta�tion, dc conductivity and interfacial polarization [12].

(2)

Here is dielectric loss due to dipolar reorienta�tion, which is characterized by a Debye type dipolarloss peak [13, 14]. The corresponding real part of thedielectric permittivity due to a dipolar relaxation pro�cess, , exhibits frequency dependence ω–n withlimiting value of n = 2. In case of a charge carriersdominated system, the dielectric relaxation is attribut�ed to hopping of charge carriers between two localizedsites. is dielectric loss due to DC conductivity in

TOTAL DIOLAR DC INTERFACIAL' '' '' '' ε = ε + ε + ε

DIOLAR''ε

DIOLAR'ε

εDC''

the specimen given by = σDC/ε0ω , which is alsocharacterized by frequency dependence ω–n with n = 1,without any characteristic peak in the dielectric losscurve. The corresponding real part of the permittivity

due to dc conductivity is frequency independent[13, 14]. is dielectric loss due to interfa�cial polarization. In systems containing mobile chargecarriers which can migrate over a distance through thematerial, when a low frequency electric field is ap�plied, interfacial or space charge polarization occurswhen the motion of these migrating charges is imped�ed. The charges can become trapped within the inter�faces of a material and motion may also be impededwhen charges cannot be freely discharged or replacedat the electrodes. The former is termed as the Max�well�Wagner effect [15, 16] whereas the later is knownas electrode polarization effect [17]. Possibility ofelectrode polarization to be a source of our observedlow frequency dispersion can be ruled out here, as thedielectric behavior did not change with the change ofelectrode material or with the thickness of the testsamples. The possible source of interfacial polariza�tion in our samples may therefore be only Maxwell�Wagner effect. Very high values and very strong fre�quency dependence of dielectric constant at low fre�quencies, as has been observed in our results, is gener�ally attributed to Maxwell�Wagner polarization in het�erogeneous systems [15, 16].

As regards the charge carriers, it is now well estab�lished that polarons and bipolarons are the charge car�riers in PPy [18], and the charge transport is broughtabout via phonon assisted hopping or tunneling be�

DC''ε

DC'ε

INTERFACIAL''ε

6

2 3 4

8

10

logε'

4

5 6log f ( f in Hz)

2

12a

b

c

d

e

f

6

2 3 4

8

10

logε''

4

5 6log f ( f in Hz)

12

a

b

c

d

e

f

2

14

16

Fig. 2. Log(dielectric constant) vs log(frequency) of(a) PPy, (b) 0.2%, (c) 1%, (d) 2%, (e) 5%, and (f) 10% AlPMMT clay in PPy.

Fig. 3. Log(loss factor) vs log(frequency) of (a) PPy,(b) 0.2%, (c) 1%, (d) 2%, (e) 5%, and ( f ) 10% Al PMMTclay in PPy.

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tween localized sites in the system. Our PPy claynanocomposites composed of electrically conductingregions of PPy separated by non�conducting regionsof MMT, exhibit the Maxwell�Wagner effect at lowfrequencies due to accumulation of charges at the bor�ders of the conducting regions causing ε' to increaseenormously. At higher frequencies the charges do nothave time to accumulate and the interfacial polariza�tion does not occur since the charge displacement issmall compared to the dimensions of the conductingregion and hence ε' decreases with the increase in fre�quency. Moreover, increase in MMT content of thecomposites results in an increase in the capacitancevalues due to increase in the interfacial area betweenthe conducting polymer phase and the insulating clayphase for accumulation of charges.

In order to suppress the two strong dispersions dueto dc conductivity and interfacial polarization and toget an insight into conductivity relaxation process inPPy/Al�PMMT composites, electric modulus formal�ism is very useful. This formalism was first introducedby McCrum et al [19] and has been successfully ap�plied to conducting polymeric systems by a number ofinvestigators [20, 21]. According to this formalismcomplex electric modulus M* is defined as following

M* = 1/ε∗ = M ' + iM '', (3)where

M ' = ε'/(ε'2 + ε''2) (4)M '' = ε''/(ε'2 + ε''2) (5)

M ' and M '' are plotted as a function of log( f ) inFigs. 4 and 5, respectively. Both the figures show a de�crease in relaxation strength with the increase ofMMT content in the composite samples. A conductiv�ity relaxation peak is observed in the M '' vs log( f )

curve which shifts to higher frequency with the in�crease of MMT content in the composites. The in�crease in the peak frequency means shorter relaxationtime (τ = 1/fmax) and increased polaronic hopping ratein the composites with higher MMT content. This re�flects the increased chain order in the polymer due toincreased intercalation in composites with higherMMT content which is in agreement with the XRDresults [9].

Normalized plots of vs log( f/fmax) forpure PPy and all the nanocomposites are shown inFigure 6. It is evident from the decreasing width of therelaxation peak of these normalized plots that distri�bution of relaxation time decreases with the increasein the MMT content of the composites. It is remark�able to observe that the full width at half height(FWHH) of the conductivity relaxation peak ap�proaches the value 1.14; the FWHH value of a singlerelaxation time Debye process [13] when MMT con�tent of the sample exceeds 1%.

CONCLUSION

Aluminum pillared clay based nanocomposites ofpolypyrrole exhibited very large values of both real andimaginary parts of dielectric permittivity which in�creased with the decrease in frequency and increase inthe MMT content. DC conductivity, Maxwell�Wagnerpolarization and conductivity relaxation were consid�ered to be the sources of dielectric dispersion. Electricmodulus formalism showed conductivity relaxationpeak in the imaginary part of the electric moduluswhich shifted to higher frequency with the increase ofMMT content in the samples showing an increase in

max''/ ''M M

0.002

2 3 4

0.004

0.006

M '

0

5 6log f

a

b

c

d

e

f

Fig. 4. Real electric modulus (M') vs log(frequency) of (a) PPy, (b) 0.2%, (c) 1%, (d) 2%, (e) 5%, and ( f ) 10% Al PMMT clayin PPy.

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the mobility of the charge carriers with the increase ofclay content. Distribution of relaxation time wasfound to decrease with the increase in MMT contentof the composites due to increased number of interca�lated polypyrrole chains in the available inter lamellarclay galleries and also due to increased order in the in�tercalated chains. Peak width of the conductivity re�laxation approached that of a single relaxation time

Debye process when MMT content of the nanocom�posites exceeded 1%.

ACKNOWLEDGMENTS

Authors are grateful to Peter Foot “Leader materialresearch group Kingston University” for his continu�ous interest and extending his laboratory facilities forthe synthesis of samples, Dr. Simon De Mars for run�

0.0005

2 3 4

0.0010

0.0020

M ''

0

5 6log f

a

b

c

d

e

f

0.0015

0.2

−4 −3 −2

0.4

0.8

M ''/Mmax''

0

−1 0log( f/fmax)

a

b

c

d

e

f0.6

1 2

1.0

Fig. 5. Imaginary electric modulus (M') vs log (frequency) of (a) PPy, (b) 0.2%, (c) 1%, (d) 2%, (e) 5%, and ( f ) 10% Al PMMTclay in PPy.

Fig. 6. Normalized plots of vs log ( f/fmax) for (a) PPy, (b) 0.2%, (c) 1%, (d) 2%, (e) 5%, and ( f ) 10% Al PMMT clayin PPy.

max''/ ''M M

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ning the thermal analysis and authors gratefully ac�knowledge the financial support from Higher Educa�tion Commission (HEC), Pakistan, International Re�search Support initiative Program (IRSIP).

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