polyanilme-montmorilltonite (panl-mmt) nanocomposites: mechanochemical synthesis, structure,...

ISSN 0965�545X, Polymer Science, Ser. A, 2010, Vol. 52, No. 10, pp. 1034–1043. © Pleiades Publishing, Ltd., 2010.

1034

1INTRODUCTION

Electrical conductivities and dielectric transport inpolymeric materials have become interesting area ofresearch [1–7]. One of the remarkable features of con�ducting polymers is that it is possible to control con�ductivity of these polymers over a wide range from in�sulating to metallic by proper doping, fillers and by in�tercalating into different inorganic host materials likeTaS2 [8], CdPS3 [9], V2O5 [10], MoO3 [11], FeOCl[12], Y2O3 [13], and MMT [14, 15] PANI�MMT hasbeen investigated by a number of researchers in the lastdecade using water�based system due to the concernon environmental and health issues. The host materialMMT has its many advantages because of its very lowtoxicity, low cost and galleried structure. It is very goodhost material. Many polymers nanocomposites havecommonly been prepared under solvent system in thisway but this system has limits, some solvents are toxicand compatible solvents are not available, so a novelsolvent free mechanical method for preparation of

1 The article is published in the original.

PANI�MMT nanocomposites has recently been re�ported [16, 17]. This method is a very simple and at�mosphere friendly which can easily be utilized for in�tercalation of polymer into the layers of MMT clay atan industrial level. For industrial utilization of mecha�nochemically synthesised PANI�MMT nanocompos�ites we need to study the electrical conductivity and di�electric behaviour of these nanocomposites in detail,which have not been reported in literature yet. In thispaper we report temperature dependent electricalconductivity studies of mechanochemically synthe�sized PANI�MMT nanocomposites and comparetheir charge transport mechanism with PANI�MMTcomposites synthesized using water�based system [14,18]. Further characterization studies of the nanocom�posites have been carried out using XRD, FTIR, di�electric measurements, TGA, SEM and UV�Vis spec�troscopy.

EXPERIMENTAL

Aniline was obtained from Aldrich and vacuumdistilled before use. Ammoniumpersulphate (APS)

Polyanilme�Montmorilltonite (PANl�MMT) Nanocomposites: Mechanochemical Synthesis, Structure,

Thermostability and Electrical Properties1

Abdul Shakoora, Tasneem Zahra Rizvib, and Ahmad Nawaz Sangrab

a Physics Department, B.Z.U. Multanb Quaid�i�Azam University Islamabad, Pakistan P.O#45320

e�mail: [email protected] August 21, 2009;

in final form, February 13, 2010

Abstract—Polyaniline (PANI)�montmorillonite (MMT) nanocomposites were prepared by direct intercala�tion of aniline molecules into MMT galleries, followed by in situ mechanochemical polymerization undersolvent free conditions. X�rays diffraction, Fourier Transform Infra Red analyses and UV�vis spectroscopyconfirmed the successful synthesis of polyaniline chains between the MMT nano�interlayers. On increasingthe amount of MMT basal spacing decreased gradually, suggesting less intercalation with decreasing amountof aniline. Scanning electron micrographs demonstrated strong differences between the morphologies ofPANI�MMT nanocomposites and those of pure MMT and PANI. DC conductivity was measured in thetemperature range from 145 K to 303 K using four probe methods. Temperature dependent DC conductivityof PANI and all the PANI�MMT composites followed 3 dimensional variable range hopping (3D VRH)model. Frequency dependent AC conductivity (σAC), dielectric constant (ε') and loss factor (ε'') have beenmeasured in the frequency range 102–106. All these measured quantities; σAC, ε' and ε'' decreased with theincrease in MMT content in the composites at all frequencies. The frequency dependence of σAC displayed alow frequency region below 104 Hz with almost constant conductivity, while above this frequency a rapid risein σAC was observed with a power law of frequency dependence with an exponent equal to 0.7. Both real andimaginary parts of the permittivity exhibited a low frequency dispersion which has been attributed to hoppingof polarons and bipolarons in PANI and its composites. The thermal stability was checked by thermogravi�metric analysis (TGA) and was found to be enhanced due to addition of MMT in the PANI.

DOI: 10.1134/S0965545X10100056

COMPOSITES

POLYMER SCIENCE Series A Vol. 52 No. 10 2010

POLYANILME�MONTMORILLTONITE (PANI�MMT) NANOCOMPOSITES 1035

was obtained from Fluka and Montmorillonite(MMT) was supplied by Aldrich.

Aniline was grounded with APS and p�TSA themolar ratio of APS oxidant to monomer aniline waskept 2 : 1 and the molar ratio of p�TSA to aniline waskept 1 : 1.

Insertion of PANI into the layers of MMT was per�formed by direct intercalation of aniline molecules in�to galleries of MMT through mechanical processing.An appropriate amount of MMT clay was placed inmortar and subsequently known amount of aniline wasadded to mortar and the mixture was ground with pes�tle at room temperature (25°C) for 10 min, theamount of MMT was varied from 1 to 20 wt %, aftergrinding for 10 min the known amount of APS andp�TSA was added to the mortar and polymerizationwas conducted by further grinding with pestle for15 min. The dark green powder was obtained after al�lowing the mixture to stand for 72 h. The samples werewashed with distilled water and centrifuged repeatedly[16]. The final product was vacuum dried at 60°Cfor 24 h.

Measurements

For the measurement of D.C conductivity the sam�ples were connected to a Keithley 617 programmableelectrometer and Keithley 224 programmable currentsource under computer control. The reproducibility ofthe results for all the samples were checked. X�rays

powder diffraction analysis was carried out using anautomated diffractometer, Bruker�AXS model D8, us�ing Cu�K

α radiations. The instrument was operated at

40 kV and 30 mA and diffraction patterns of PANI�MMT samples mounted on a standard holder were re�corded over the range of 3° to 40° counting time was3 s and the step size was 0.10. The Fourier TransformInfra Red spectra (FTIR) were recorded on KBr pelletsamples in the range of 4000–400 cm–1 by using a Per�kin�Elmer FTIR spectrometer. The thermo gravimet�ric analysis (TGA) was carried out on Mettler thermobalance STAR S.W. 8.10 from room temperature(25°C) to 900°C for all the samples. The samples wereheated at the rate of 10 K/min in nitrogen atmosphere.Scanning electron microscopy was carried out on anEVO50 Zeiss instrument. Capacitance and loss tan�gent (tanδ) were measured by using Wayne Kerr LCRmeter Model 4275 in the frequency range from 100 Hzto 1 M Hz. The dielectric constant was calculated byusing the formula 1

εr = Cd/Aε0 (1)

εr is relative permittivity, C is capacitance, d is thick�ness of the sample, A is area of cross section and ε0 ispermittivity of free space. The thickness and diameterof samples were measured by using digital micrometer.

Dielectric loss factor ε'' was obtained as

tanδ = ε''/ε' (2)

20

Relative intensity

102θ, deg

30 40

a

b

c

d

e

Fig. 1. XRD pattern of (a) PANI, (b) 1% MMT, (c) 10% MMT, (d) 20% MMT in PANI and (e) pure MMT.

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ABDUL SHAKOOR et al.

RESULTS AND DISCUSSION

XRD Pattern of PANI�MMT Composites

Figure 1 compares the XRD pattern of PANI�MMT composites. The major peak at 8.8° in MMTclay (Fig. 1e) is shifted to lower angles in PANI�MMTcomposites (Figs. 1b–1d). The pattern of amorphousbroad peak (18° to 28°) in PANI (Fig. 1a) is present inall PANI�MMT composites, which is good agreementwith the FTIR and conductivity results confirmingthat PANI is present in all composites according to itsproportion. This amorphous pattern is absent in pureMMT (Fig. 1e). The main peak at 27° of (Silicon) Siin MMT along with its shoulder peaks is presents in allPANI�MMT composites (Figs. 2b–2e). The intensityof this peak increases as the content of MMT clay inPANI is increased.

The crystalline peak at 2θ value of 8.8° in pureMMT sample corresponds to the periodicity d = 1.29 nmin the direction of (001) of the clay samples. Thed�spacing in the direction of (001) in PANI�MMTcomposites samples increased to a value of 2.00 nm,1.95 nm, 1.90 nm and 1.87 nm with 1%, 5%, 10%, and20% MMT, respectively. Only a 0.71 nm to 0.58 nm in�crease in the d�value of (001) direction of MMT claywas observed. It is obvious that the (001) diffractionpeaks of the products shifts to lower 2θ values as com�pared with pristine MMT, which indicates the success�ful intercalation of PANI into the nano�interlamellarspaces of MMT clay by grinding under solvent freeconditions. The (001) peak position depends on theamount of PANI loading. The basal spacing of PANI�MMT at about 99% PANI loading is 2.00 nm showingthe expansion of 0.71 nm approximately. This suggests

that aromatic planes of PANI intercalated into inter�lamellar spaces located perpendicular to the clay layers[17, 21]. The driving force for intercalation of PANIinto MMT would be attributed to absorption with hy�drated sodium ions or the formation of hydrogenbonding interaction between the N–H of the PANIand the surface oxygen atoms of the clay [17].

Fourier Transform Infra�Red Spectroscopy (FTIR)

FTIR spectrum of PANI MMT compositesFigs. 2b–2e revealed bands at 1579 cm–1, 1497 cm–1

are N–H bending, 1374 cm–1, 1292 cm–1 (C–Nstretching) and 3430 cm–1 (–N–H stretching) as�cribed to PANI chains [22]. These principal peaks arepresent in all PANI�MMT composites except pureMMT (Fig. 2e). 3478, 1051 cm–1799 cm–1, and669 cm–1 are typical major MMT peaks, which are al�so presents in all PANI�MMT composites, except inpure PANI. The peak at 1051 cm–1 can be associatedwith the Si–O stretching vibrations. Furthermore, twocharacteristic peaks near 1029 and 1037 cm–1 are ob�served, which correspond to stretching mode of SO3

[23, 24] also confirmed that PANI is present in proto�nated state in all PANI�MMT nanocomposites.

Scanning Electron Microscopy (SEM)

Figure 3 represents SEM micrographs of PANI�MMT composites with different MMT content, to�gether with pristine MMT clay. The SEM photographsof PANI (Fig. 3a) and MMT (Fig. 3e) revealed mor�phologically distinct features from each other at lowresolution. Image of MMT is in the form of blocks.

2000

Absorbance

3000Wavenumber, cm–1

1000

a

b

c

d

e

4000

Fig. 2. FTIR of (a) PANI, (b) 1% MMT, (c) 10% MMT, (d) 20% MMT in PANI and (e) MMT pure.



The SEM image of PANI is granular and the SEM ofPANI�MMT composites (Figs. 3c–3d) clearly indi�cated the formation of composites of PANI�MMT. Athigh resolution images of PANI�MMT composites aresimilar to each other, when the polymer reaches thehighest amount in the composite the morphology issimilar to PANI without MMT. As the MMT contentincreases in PANI�MMT composites the morphologyalso changes and surface effect of MMT can be easilyobserved in all PANI�MMT composites, which sup�port the conductivity and XRD data classically. Theparticles of MMT clay seem swollen by insertingPANI, which indicates the intercalation of PANI intothe galleries of MMT in PANI�MMT nanocomposites(Figs. 3b–3c).

DC Conductivity Measurements

DC conductivities of all the composite sampleswere found to be less than that of pure PANI at all tem�peratures. The measured DC conductivity values atroom temperature, along with d spacings in pureMMT and all the composites, are shown in Table. 1.

Silicate layers with insulating character screen theinterchain interaction between the PANI chains and

depress the conductivity. As the content of MMT is in�creased in PANI the conductivity goes on decreasingdue to progressively increased shielding effect of sili�cate layers which isolate the molecular PANI chainsand hence restrict the effective delocalization ofcharge carriers in the composites.

DC conductivity measured in pure PANI and itscomposites with 5%, 10%, and 20% MMT clay isshown as a function of T–1/4 in Fig. 4. As is seen, tem�perature dependent DC conductivity in pure PANI

2 μm2 μm

2 μm2 μm(a) (b)

(c) (d)

Fig. 3. SEM of (a) PANI, (b) MMT pure, (c) 5% MMT and (d) 20% MMT in PANI.

Table 1. d�spacing and conductivity of PANI�MMT com�posites

Sample d�spacing,nm ± 0.05 nm

Conductivity, S/cm

MMT 1.29 10–10

Pure PANI 0.25

1% MMT in PANI 2.00 0.199

5% MMT in PANI 1.95 0.147

10% MMT in PANI 1.90 0.107

20% MMT in PANI 1.87 0.078

1038



and all its composites exhibited a linear relationshipwith T–1/4. This implies that the charge carrier’s trans�port mechanism of these polymers follows the 3�di�mensional variable range hopping (3D VRH) model.

σ = σ0exp[–(T0/T)1/n + 1], (3)

where n = 3, σ0 is the infinite temperature conductiv�ity. T0 is the effective energy barrier for electron to hopbetween localized states and can be written as

T0 = 16α3/kN(Ef), (4)

where α is coefficient of exponential decay of the lo�calized states (cm–1), N(Ef) is the density of states inFermi level and k is the Boltzmann constant. T0 wascalculated from the slope of graph T–1/4 v/s log of con�ductivity and was found to be 7.4 × 104 K. σ0 was foundto be 104 S/cm, showing metallic behaviour at abso�lute high temperature. The density of states was calcu�lated to be 2.6 × 1018 eV–1 cm–3 for pure PANI and de�creases with MMT contents in PANI. Mott’s parame�

ters for pure PANI and all the nanocompositesas calculated by using equations 3 and 4 are shown inTable 2.

It may be of interest to mention here that in con�trast to the 3D variable range charge carrier hoppingmechanism observed here in mechanochemically syn�thesized PANI�MMT nanocomposites, PANI�MMTcomposites prepared by solution or emulsion methodsin water based systems exhibit quasi 1D variable rangehopping [14, 18]. This difference in charge transportmechanism of PANI�MMT composites prepared bymechanochemical solvent free method and chemicalmethods in water based systems depicts a higher effec�tive interchain hopping rate (comparable to the intra�chain hopping rate) in case of mechanochemicallysynthesised nanocomposites. This may be attributed toa higher level of interchain coupling caused by me�chanical stresses on the polymer chains during mech�anochemical synthesis.

AC Conductivity

Frequency dependence of AC conductivity in purePANI and its composites with 1%, 5%, 10% and 20%MMT in PANI is shown in Fig. 5. AC conductivity inPANI�MMT composites was considerably lower thanthat in pure PANI and there was a steady decrease inthe AC conductivity of PANI�MMT composites withthe increase of MMT in all composites. At low fre�quencies, the AC conductivity in pure PANI and in allPANI�MMT composites was found to be independentof the frequency. At some onset frequency (ω0) there

−1.8

−1.4

−1.0

0.26

logσ (σ in S/cm)

0.24 0.28 0.28T1/4, K–1/4

a

b

c

d

−0.6

Fig. 4. Temperature dependent conductivity of (a) PANI, (b) 1% MMT, (c) 5% MMT and (d) 10% MMT in PANI.

Table 2. Mott’s parameters of PANI MMT nanocomposites

Sample T0 (K)Density of states

N(Ef) × 10–18,

eV–1 cm–3

σo × 10–3,S/cm

PANI 7.4 × 104 2.60 10.9

5% 9.1 × 104 2.10 4.513

10% 1.1 × 105 1.07 4.79

20% MMT 1.6 × 106 0.12 9.79



was a rapid rise in the AC conductivity with the in�crease of frequency in pure PANI as well as its claycomposites. A large variety of disordered solids exhibita similar frequency dependence of conductivity σ(ω),which is characterized by a low�frequency region ofconstant conductivity followed by a gradual transitionat higher frequencies to a frequency�dependent con�ductivity. The strong dispersion of the conductivitycan be approximated by an empirical power law, whichhas been termed by Jonscher as “universal power law”.The class of materials that show similar power law be�haviour in the frequency dependence of AC conduc�tivity is large, including amorphous or organic semi�conductors, ionic conducting glasses, ceramics, ionicor electronic conducting polymers, metal cluster com�pounds, transition metal oxides etc. The phenomenonis common to both electron and ion conducting mate�rials. The (real part of the) dynamic conductivity σac

can be described by a power law:

σac = Aωs, (5)

where A is the pre�exponential factor and s the expo�nent. Fitting our high frequency data to the abovepower law gives s = 0.7. This value lies between 0and 1; that is the generally found range for the expo�nent of the “universal power law” [19, 20].

Dielectric Constant

Frequency dependencies of ε' and ε'' in the fre�quency range 102–106 Hz at room temperature areshown in Figs. 6 and 7, respectively, as log–log plots.Both ε' and ε'' exhibit a strong dispersion at low fre�quency. Dielectric constant of pure PANI in our sys�

tem agrees well with the earlier reported value [25].It is further observed that dielectric constant and di�electric loss values are lower for higher contents ofMMT in PANI as has been observed in a.c and d.cconductivities following the basic rules of physics, bywhich, complex permittivity and a.c conuctivities arerelated by the following equations:

(6)

ε' ω( ) ε∞

– σ'' ω( )

ε0ω��, ε'' ω( )

σ' ω( )

ε0ω�� εd''

σdc

ε0ω��,+= = =

εd'' ω( )σ' ω( ) σdc–

ε0ω��,=

−0.2

0.4

3

log of AC conductivity (σ) (σ in S/cm)

2 4 5 6log f (f in Hz)

−0.8

1.0abc

ed

Fig. 5. AC conductivity of (a) pure PANI, (b) 1% MMT,(c) 5% MMT, (d) 10% MMT and (e) 20% MMT in PANI.

4

5

6

3

logε'

2 4 5

a

b

c

d

8

6log f (f in Hz)

3

7

Fig. 6. Dielectric constant of (a) pure PANI, (b) 5% MMT, (c) 10% MMT and (d) 20% MMT in PANI.

1040



where ω is the angular frequency, ε∞

is the unrelaxeddielectric constant, ε0 is the vacuum permittivity and

is the imaginary part of the permittivity after de�ducting the conductivity contribution. In a disorderedsystem where the hopping mechanism dominates, theconductivity increases as the frequency of the electricfield is increased, because the contribution of chargecarriers moving along smaller and smaller distances,i.e., confined inside clusters of progressively decreas�ing sizes, increases [26].

εd''

The log�log plot of ε' vs frequency shows linearvariation and hence the frequency dependence of ε'can be expressed by fractional power law ωn – 1 similarto the “universal dielectric response” [27] where theexponent n lies between 0 and 1. The exponents n ofpower laws for frequency dependence for the realcomponent of dielectric permittivity, however, arefound to decrease with the increase in the MMT con�tents in the composites. For the frequency dependenceof ε', n decreases from 1.02 for pure PANI to 0.643 for

8

3

logε''

2 4 5

12

6log f (f in Hz)

4

a

b

c

d

Fig. 7. Dielectric loss factor of (a) pure PANI, (b) 5% MMT, (c) 10% MMT and (d) 20% MMT in PANI.

8

3

M ' × 104


2

0

4

6

a

b

c

d

Fig. 8. Real part of dielectric modulus of (a) pure PANI, (b) 5% MMT, (c) 10% MMT and (d) 20% MMT in PANI.



PANI containing 20% MMT. The log�log plot of ε'' vsfrequency shows a dielectric loss peak, which is shiftedto higher frequency with MMT loading in PANI(Fig. 7).

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. [28] and has been successfully ap�plied to conducting polymeric systems by a number ofinvestigators [29, 30]. According to this formalismcomplex dielectric modulus M* is defined as

(7)

(8)

(9)

M ' and M '' are plotted as a function of log( f ) in Figs. 8and 9, respectively.

Both figures show the increase in relaxationstrength with the increase of MMT content in thecomposite samples.

Thermal Gravitational Analysis (TGA)

TGA mainly serves as analytical technique to quan�tify the amount of organic matter. In the PANI�MMTcomposites the gradual weight loss was observed in thetemperature range from 25°C to 900°C in all samples.

M* 1/ε* M' iM''+= =

M' ε'/ ε'2

ε'2

+( )=

M'' ε''/ ε'2

ε'2

+( )=

TGA curves exhibited four major stages of weight lossas shown in Fig. 10. The first weight loss below 100°Cis due to the release of adsorbed water on PANI. Thesecond stage in the temperature range 200–400°C isperhaps due to decomposition of low molecularweights substances present in PANI [31]. At the thirdstage in the temperature range from 400°C to 600°Cthe PANI itself decomposes whereas at the last stage inthe temperature range from 600°C to 900°C in addi�tion to the decomposition of remaining polymer, thestructural water in MMT is also released from thecomposite samples [32, 33]. The decomposition tem�perature of pure PANI is 443°C and this decomposi�tion temperature is shifted to higher value even by add�ing 1% MMT in PANI�MMT composites. As we in�crease the amount of MMT, this temperature is shiftedto higher values, the decomposition temperature ofPANI�MMT composite with 10% MMT is 553°Cwhich shows that the composites are becoming ther�mally more stable as the amount of MMT in PANI�MMT is increasing.

UV�Vis Spectroscopy

The UV�Vis spectra of pure PANI and in purePANI all nanocomposites in this study are shown inFig. 11. Two major characteristics peaks can be ob�served at about 380 and 685 nm in pure PANI. The ab�sorption at 380 nm is due to polaron�π* transition and

16

3

M '' × 106


4

0

8

12

a

b

c

d

Fig. 9. Imaginary part of dielectric modulus of (a) pure PANI, (b) 5% MMT, (c) 10% MMT and (d) 20% MMT in PANI.

1042



the second band at 685 nm can be assigned to excitontransition between benzenoid and quinoid rings [34,35]. Both bands are slightly shifted to lower wave�length near 355 and 640 nm, respectively, in the nano�composites, that reflects the interaction betweenPANI and MMT.

CONCLUSIONS

PANI was intercalated into nano�interlamellar spacesof MMT clay by solvent free mechanochemical route.XRD and FTIR results confirmed that the PANI chains

were synthesized between clay interlamellar spaces.The decrease in basal spacing with decrease in PANIcontents indicates that the synthesized PANI would bearranged in a parallel conformation in the interlamel�lar region of clay. It was clear in SEM micrographs thatthe surface morphologies of PANI�MMT nanocom�posites differ substantially from PANI. Temperaturedependant conductivity shows 3D VRH in all PANI�MMT nanocomposites. AC conductivities of all PA�NI�MMT nanocomposites follow the universal powerlaw. TGA results show enhancement in thermal stabil�ities of PANI chains in the nanocomposites as com�pared to the pristine PANI. UV�vis spectra showed

600

Absorbance

400Wavelength, nm800

a

bc

d

Fig. 11. UV vis of (a) PANI, (b) 5% MMT, (c) 10% MMT and (d) 20% MMT in PANI�MMT composites.

60

80

300

% by weight loss

100 500 700

a bc

e

900Temperature, °C

40

100

d

20

Fig. 10. TGA of (a) pure PANI, (b) 1% MMT, (c) 5% MMT, (d) 10% MMT and (e) 20% MMT in PANI.



that PANI is present in all PANI�MMT nanocompos�ites in protonated form. Dispersion in dielectric lossand dielectric constant curves arises due to hopping ofpolarons and bipolarons in PANI and its composites.Dielectric modulus formalism showed conductivityrelaxation peak in the imaginary part of the electricmodulus.

ACKNOWLEDGMENTS

Authors are grateful to Prof. Dr. P.J.S. Foot Leader,Material Research Group, Kingston University UK,KT1 2EE for his guidance and provision of his labora�tory facilities. Authors gratefully acknowledge thefinancial support from Higher Education Commission(HEC), Pakistan, International Research SupportInitiative Program (IRSIP).

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