Electrical properties and EMI shielding behavior of highly thermally stable polyaniline/colloidal graphite composites

Research Article

Received: 11 May 2008, Accepted: 16 June 2008, Published online in Wiley InterScience: 9 January 2009

(www.interscience.wiley.com) DOI: 10.1002/pat.1230

Electrical properties and EMI shieldingbehavior of highly thermally stablepolyaniline/colloidal graphite composites

Parveen Sainia, Veena Choudharyb and S. K. Dhawana*

A hybrid approach has been adopted by using a comb

Polym. Adv

ination of colloidal graphite (CG) as a conducting filler, 5-lithiumsulfoisophthalic (LiSIPA) acid as a dopant, and polyaniline (PANI) as a matrix to prepare LiSIPA doped PANI–CGcomposites. The thermal stability (�300-C) and electrical conductivity (67.4 S/cm at 17.4% CG content) have beenimproved significantly as compared to PANI doped with conventional inorganic dopants like HCl or H2SO4 (130–150-C). The maximum shielding effectiveness value was found to beS39.7 dB. X-ray diffraction and infrared spectroscopyshowed a systematic shifting of the characteristic peaks and bands with increase in the amount of CG, which indicatessignificant interaction exists between CG and PANI. The UV–Vis spectra showed the characteristic bands of PANI, witha shift to shorter wavelength with increase in the CG content. The interactionmechanism between doped PANI and CGin the resultant composites has been proposed. Copyright � 2009 John Wiley & Sons, Ltd.

Supporting information may be found in the online versi
on of this paper.
Keywords: conducting polymers; colloidal graphite; EMI shielding; polyaniline; TGA

* Correspondence to: S. K. Dhawan, Polymeric and Soft Materials Section,

National Physical Laboratory, New Delhi 110012, India.

E-mail: [email protected]

a P. Saini, S. K. Dhawan

Polymeric and Soft Materials Section, National Physical Laboratory, New

Delhi 110 012, India

b V. Choudhary

Centre for Polymer Science and Engineering, Indian Institute of Technology,

New Delhi 110 016, India 3

INTRODUCTION

The discovery of conducting polymers in 1977 has revolutionizedthe field of material science due to their novel electronic andelectrical properties. Their gamut of applications covers a broadspectrum, including energy storage,[1,2] sensors,[3–5] anticorrosivematerials,[6–8] electromagnetic interference (EMI) shielding,[9–11]

electrostatic charge dissipation (ESD),[12–14] organic light emittingdiodes (OLEDs),[15–18] photovoltaic,[19,20] and supporting materialfor catalysis.[21,22] However among other conducting polymers,polyaniline (PANI) has been extensively studied, due to itsnon-redox doping, good environmental stability, and economicfeasibility, besides possessing acceptable electrical conductivityand thermal stability. In spite of these advantages, the majordrawbacks of PANIs are their poor processability and tendency toundergo undoping either under alkaline or elevated temperatureconditions, which in turn results in significant decrease inelectrical conductivity. The electronic properties of such syntheticmetals can be tailored by carrying out the polymerization underthe controlled conditions and in the presence of specific dopantsand/or fillers. One approach is the use of graphite as a conductingfiller material. Tchmutin et al. have prepared composites ofgraphite and conducting polymers having low percolationthresholds.[23] Du et al. have prepared composites based onexfoliated graphite.[24] Luo et al. have prepared core–shellcomposite particles of PANI and carbon black by electrochemicalroute.[25] Bourdo and Viswanathan have utilized colloidalgraphite (CG) for making composites having conductivity betterthan CG or conducting polymer alone.[26] Several groups havealso reported the use of bulky counter-ion as a dopant,[27,28]

leading to polymer with improved thermal stability(�230–2508C) but reduced electrical conductivity. However, tothe best of our knowledge no reports are available on the

. Technol. 2009, 20 355–361 Copyright � 200

conducting composites based on PANI having such a highthermal stability (�3008C).The bulky dopant increases the thermal stability and

processability of PANI and decreases its electrical conductivity.When CG is incorporated in the PANI matrix its electricalconductivity increases. Therefore we have used the hybrid routeby using the combination of above approaches, i.e. by using CGas a conducting filler and 5-lithium sulfoisophthalic (LiSIPA) as abulky dopant to increase both electrical conductivity and thermalstability of the PANI. The polymerization was carried out via thechemical oxidative polymerization route. The polymers sosynthesized were characterized by various techniques likethermogravimetric analysis (TGA), Fourier transform infrared(FTIR), UV–Vis spectroscopy, and X-ray diffraction (XRD). Theirelectronic conductivities were measured by the four-probetechnique and morphologies were observed by scanningelectron microscopy (SEM). Apart from the characterization, wehave also proposed possible interaction mechanism between thedoped PANI and CG.These thermally stable hybrid-conducting particles may find

applications in developing conducting composites usingthermoplastic resin such as polyamides (PA), polyethylene

9 John Wiley & Sons, Ltd.

55

Table

1.Characteristicsofdoped

form

sofcomposites

Material

Code

Conductivity

(S/cm)

Thermal

stab

ility

(8C)

CGam

ount

(wt%)

SE(dB)

UV–V

isban

d,l(nm)

FTIR

ban

ds(cm

�1)

PANI–LiSIPA

-doped

PANI-D

1.1

295

031.4

450

617,1033,1102,1219,1293,1374,1458,1474,1543,1557,1652,1686,

1697,1716

PANI–CG-doped

(43%)

CG1-D

7.3

300

5.31

32.1

445

612,1033,1101,1216,1289,1372,1457,1473,1542,1557,1650,1685,

1700,1736

PANI–CG-doped

(50%)

CG2-D

8.7

302

6.59

32.9

441

612,1032,1098,1216,1290,1372,1456,1473,1542,1557,1651,

1685,1736

PANI–CG-doped

(56%)

CG3-D

9.3

303

9.23

33.0

440

615,1031,1097,1216,1290,1373,1457,1473,1542,1557,1651,1685,

1700,1738

PANI–CG-doped

(67%)

CG4-D

16.92

306

9.62

34.3

437

614,1032,1100,1216,1287,1372,1456,1473,1540,1557,1650,1684,

1698,1737

PANI–CG-doped

(80%)

CG5-D

34.1

302

13.17

36.2

438

613,1032,1093,1216,1288,1367,1455,1473,1541,1557,1650,

1685,1739

PANI–CG-doped

(90%)

CG6-D

67.4

297

17.35

39.7

438

613,1032,1092,1216,1288,1366,1456,1473,1541,1557,1650,

1685,1739

Colloidal

graphite

CG

125

900

100

42.2

—1016,1183,1457,1541,1558,1651,1700,

www.interscience.wiley.com/journal/pat Copyright � 2009

P. SAINI, V. CHOUDHARY AND S. K. DHAWAN

356

John

terephthalate (PET), and polycarbonate (PC). It is expected thatthey may find application under extreme conditions, i.e. alkalineenvironment or high temperatures.

EXPERIMENTAL

Materials used

Aniline (Loba Chemie, India), HCl (35.4% sd fine-chem), LiSIPA(Eastman Chemical Company, India), CG (75% solid content) andammonium persulfate (APS, Merck, India), liquid ammonia(Liq. NH3, 30%, Loba Chemie, India), N-methyl pyrrolidinone(NMP, Merck, India), and dimethyl sulfoxide (DMSO, Loba Chemie,India) were used as received. Double distilled water having aspecific resistivity of 106 Ohmcm was used in the preparation.

Polymer preparation

The doped PANI was prepared by free radical chemical oxidativepolymerization through indirect routes. In a typical reaction,0.1mol aniline and 1.0mol HCl were mixed with 9.3 g of CGdispersion in 1 L of distilled water. The abovemixture was allowedto stir for 30min and the polymerization was initiated bydropwise addition of APS [0.1mol, (NH4)2S2O8, in 100mL distilledwater]. The temperature of the mixture was maintained at0� 1.08C throughout the course of reaction (6 hr). The polymerhas been produced directly in the doped state as a dark greenprecipitate, dispersed in the reaction mixture. The above mixturewas filtered to separate the polymeric precipitate. The precipitateso obtained was washed repeatedly with distilled water, till thepH of the filtrate became neutral. The final precipitate was driedunder dynamic vacuum till constant weight. The dried mass wasthen crushed to obtain the powder of the doped polymer. Thedoped powder was then treated with 1.0M aqueous ammoniasolution and stirred for 2 hr to convert it to the base form (CG1-B).The base form was then treated with 0.5M aqueous LiSIPAsolution and stirred for 2 hr to obtain the doped form of the same(CG1-D). The doped and base forms of other composite materialsviz. CG2, CG3, CG4, CG5, and CG6 were prepared in similar fashionby keeping the amount of both HCl and aniline same as in CG1and changing the CG phase to 12.4, 15.5, 24.8, 49.6, and 111.6 g,respectively. The graphite percentages based on the relativeamounts of aniline and CG in the reaction mixtures were found tobe 43, 50, 56, 67, 80, and 90% for CG1, CG2, CG3, CG4, CG5, andCG6, respectively.The above-synthesized polymers were given abbreviated

names on the basis of doping/undoping status and percentageof CG, and detailed descriptions of undoped and dopedmaterialsalong with the abbreviations are presented in Tables 1 and 2,respectively. These abbreviated names will be used in the rest ofthe paper.

Structural characterization

For the conductivity measurements, pellets of 13mm length,7mm width, and �2mm thickness were prepared and theresistivities were measured by the four-point probe techniqueusing Keithley 220 Programmable Current Source and 181Nanovoltmeter. TGA (Mettler Toledo TGA/SDTA 851e) was used toobserve the thermal behavior. Materials were heated from 25 to7008C under a constant heating rate of 108C/min, in the inertatmosphere of nitrogen. The samples were also studied by using

Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 355–361

Table 2. Characteristics of undoped forms of composites

Material AbbreviationConductivity

(S/cm)Peak I,

peak II l(nm)

Ratio of benzenoidto quinoid units(peak I/peak II)

Ratio of quinoidto benzenoid

units (I1575/I1485)Average SE inX-band (dB)

PANI-undoped PANI-B 10�9 329, 635 1.158 0.976 �1.4PANI–CG-undoped (43%) CG1-B 1.02� 10�5 324, 628 1.197 0.927 �9.6PANI–CG-undoped (50%) CG2-B 3.22� 10�5 323, 627 1.214 0.904 �10.9PANI–CG-undoped (56%) CG3-B 1.48� 10�4 323, 626 1.219 0.762 �11.5PANI–CG-undoped (67%) CG4-B 0.00314 322, 628 1.223 0.725 �13.4PANI–CG-undoped (80%) CG5-B 0.677 320, 630 1.238 0.701 �18.6PANI–CG-undoped (90%) CG6-B 5.33 316, 629 1.251 0.637 �27.4

EMI SHIELDING BEHAVIOR OF POLYANILINE/COLLOIDAL GRAPHITE COMPOSITES

UV–Vis spectrophotometer (Shimadzu UV-1601) after preparingsolutions of their base forms in NMP as solvent. FTIR (NICOLET5700) and XRD (D8 Advance Bruker AXS X-ray diffractometer)were used to observe the characteristic bands/peaks of thesecomposites. Micromorphology was observed using SEM (Leo 440,UK). EMI shielding effectiveness (SE) values were measured byplacing rectangular pellets (�2mm thick) inside X-bandwaveguides and by using a Vector Network Analyzer (VNAE8263BAgilent Technologies). The input power level was kept at�5.0 dBm and measurements were taken in the frequency of8.2–12.4 GHz (X-band).

RESULTS AND DISCUSSION

UV–Vis spectroscopy

UV–Vis spectra of the base form of composites are shown in Fig. 1.In undoped PANI, there are two peaks in the UV–Vis range. Thefirst around 320 nm (Peak I) is due to the p –>p� transition(band-gap) and is a measure of the effective conjugation of thematerial. The other peak at 630 nm (Peak II) is due to the transitionbetween highest occupied molecular orbital (HOMO) ofbenzenoid rings and lowest unoccupied molecular orbital(LUMO) of the quinoid rings.[29]

The characteristic UV–Vis bands of the base form of the purePANI (PANI-B) and composites are reported in Table 2, which

Figure 1. UV–Vis spectra of CG, PANI-B, CG2-B, CG4-B, and CG6-B in NMP.This figure is available in color online at www.interscience.wiley.com/

journal/pat

Polym. Adv. Technol. 2009, 20 355–361 Copyright � 2009 John Wiley

3

reveals that p –>p� band of the composite exhibits ahypsochromic shift relative to PANI-B. This may be due to thereduction in conjugation lengths with the increase in the CGcontent. The graphite particles are inserted between the PANIchains hindering the effective overlap between the electronicwave functions. This leads to the decrease in the chain couplingand consequently decreases the effective conjugation lengthwith increase in the CG phase. Therefore, PANI-B absorbs at329 nm, whereas CG1-B and CG6-B absorb at 324 and 316 nm,respectively. Further, the ratio of peak I to peak II gives anestimate of the oxidation state of the material.[29] As we movefrom CG1-B toward CG6-B this ratio increases slightly that reflectsthe decrease in the oxidation level with increase in the graphitecontent. This is quite reasonable because during the in situpolymerization, some amount of the oxidant was also consumedin oxidizing the CG. Thus with the increase in the graphite contentthe fraction of oxidant that was utilized by PANI decreases,leading to slightly more reduced polymer. The decrease in thequinoid units also affects the doping level as the doping takesplace on the quinoid nitrogen atoms. In order to further confirmour findings we have taken the UV–Vis spectra of the dopedmaterials (Supportive Document Fig. 1) in the DMSO solvent andthe characteristic bands are presented in Table 1. The polaronicpeak in PANI–Li at 450 nm is shifted to 438 nm in CG6-Liindicating charge localization and interaction at benzenoid units.

FTIR details

Figure 2 shows the FTIR spectra of CG, CG2-D, and PANI-D. Thecharacteristic bands of dopant and all the composites aretabulated in Table 1. The band around 798 cm�1 is due to the outof the plane C–H bending vibrations. Also in the region1650–1400 cm�1, bands due to aromatic ring breathing, N–Hdeformation, and C–N stretching are observed. The bands around1560 and 1480 cm�1 are characteristic stretching bands ofnitrogen quinoid (N––Q––N) and benzenoid (N–B–N) and are dueto the conducting state of the polymer. The bands around 1290and 1240 cm�1 are assigned to the bending vibrations of N–Hand asymmetric C–N stretching modes of benzenoid rings,respectively. The absorption band around 1120 cm�1 (C–Nstretching) is due to the charge delocalization over the polymericbackbone[30] and in the undoped polymer the intensity of thisband goes down significantly.In undoped materials, the intensity ratio of the bands, i.e. ratio

of 1580 and 1490 cm�1 was used to determine the oxidation state

& Sons, Ltd. www.interscience.wiley.com/journal/pat

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Figure 2. FTIR spectra of CG, PANI-D, and CG2-D. This figure is availablein color online at www.interscience.wiley. com/journal/pat


358

of the material.[31] This ratio was 0.976 for PANI-B and decreasesfrom 0.927 to 0.637 (Table 2) onmoving from CG1-B to CG6-B. Theabove findings suggest that the proportion of benzenoid unitsincreases with increase in the amount of CG.In the FTIR spectra of all the composites, an intense band at

1740 cm�1 was observed. This band could be due to carboxylgroup of the partially oxidized CG as it was also present in thecontrol sample. In the composite this band showed a continuousincrease in intensity with increase in the amount of CG. Thepresence of characteristic bands of both graphite and PANI

Figure 3. Proposed interaction mechanism between CG and doped PANI. Th

pat

www.interscience.wiley.com/journal/pat Copyright � 2009 John

confirmed the presence of both phases in the composite.However, all these bands showed systematic shifting thatindicates significant interactions between PANI and graphite.These interactions are thought to be initiated by the oxidantmoieties (leading to partial functionalization of the graphiteduring in situ polymerization under acidic conditions) andmediated by dopant molecules, whichmight form ternary systemwith the PANI and graphite phases. There are certain reports onthe interaction mechanism of PANI[32] and other conductingpolymers[33,34] with graphite, and some groups[35] have suggestedsuch interactions as consequence of charge transfer betweenquinoid moiety of PANI and graphite. However, we propose thatinteraction between graphite sheets (electron acceptor) anddoped PANI (donor or p-type material) may be attributed to theformation of charge transfer complex leading to partial transfer oflone pair of electrons of amine nitrogen atoms to the graphiteparticles. As a result, amine nitrogen acquires partial positivecharge, whereas graphite particles develop equivalent negativecharge. The formation of such a complex causes additionaldoping on amine nitrogen of PANI, leading to further enhance-ment in the conductivity. Such an interaction is depicted in Fig. 3.In order to prove this idea, we have taken FTIR spectra of thedoped materials (Supporting Document Fig. 2), which revealedthat with the increase in graphite content, the relative intensity ofabsorption bands at 1033, 1102, and 1293 cm�1 decreases,whereas that of 1219 and 1374 cm�1 increases. These resultssuggest that with the increase in the CG phase, doping level andcharge delocalization over PANI backbone decrease. However,charge localization due to hindrance of proton induced spinunpairing is possible only when benzenoid nitrogen atoms are

is figure is available in color online at www.interscience.wiley.com/journal/


Figure 4. XRD of CG, CG2-D, CG4-D, and CG6-D. This figure is available incolor online at www.interscience.wiley. com/journal/pat


involved in some kind of interactions with CG. The relativeintensity ratio of quinoid/benzenoid peaks increased withincrease in the CG content which further indicates increasedinteractions and charge localization. The same has also beencomplemented by UV–Vis data.

XRD studies

Figure 4 shows the XRD of the powdered samples CG, CG2-D,CG4-D, and CG6-D. The patterns show a sharp peak centeredon 2u value of 268 which corresponds to the CG (002 planes)[36]

and its intensity goes on increasing with increase in the graphitecontent.The composite also shows characteristic peaks of the doped

PANI around 2u values of 208 and 258 that were absent in the XRDspectra of the pure CG. The intensity of these peaks decreasesabruptly with the increase in the CG content and in both CG4-Dand CG6-D, the 258 peak appears as a weak shoulder on theintense 268 peak of the CG phase. The relatively well developed258 peak indicates that the achieved doping levels are slightlyhigher in CG2-D than in CG4-D and CG6-D. UV–Vis data suggestthat this may be attributed to the less number of available dopingsites or decrease in oxidation state.

Conductivity measurements

The room temperature conductivity of doped and undopedmaterials is reported in Tables 1 and 2, respectively. Theconductivity increases with increase in the CG content, in spite ofthe decrease in both conjugation length and achieved dopinglevels. This may be attributed to the highly conducting nature of

Figure 5. SEM of PAN


CG, which acts as a conducting bridge between the metallicislands of the PANI. As the number of conductive network pathsincreases with increase in the CG content, the conductivity alsoincreases.The most striking feature of the conductivity pattern is that

even undoped samples show acceptable conductivities thatcould be increased bymany orders just by doping with a protonicacid. This provides us a method to control and tune theconductivity of the resultant composites. Depending upon theCG content and degree of doping almost any value ofconductivity is possible between highly conducting CG andinsulating emeraldine base. These materials could be quite usefulparticularly for sensors, actuators, and electrochromic devices, asresponse time and reversibility could be improved dramatically.Furthermore, these composites also show less decrease in theconductivity due to undoping of the materials especially underharsh environmental conditions (slightly alkaline environment orrelatively elevated temperatures).

Morphological characterization

SEM micrographs of PANI-D, CG2-D, and CG are shown in Fig. 5.These micrographs clearly show the presence of flakes (havinglayered morphology characteristic of the graphite) in the case ofCG and highly agglomerated globular morphology in doped PANI(PANI-D). However in the case of composite (CG2-D), one canclearly see the graphite flakes coated with PANI particlesexhibiting coal-briquette morphology. This may be attributedto the high specific surface area of the graphite flakes, whichprovides large number of binding sites to PANI. These PANIcoated graphite flakes exist as globular agglomerates due to thesignificant interaction between the doped PANI chains.

Thermogravimetric analysis

Figure 6a shows the thermogravimetric (TG) traces of dopedmaterials PANI-D, CG2-D, CG4-D, CG6-D, and pure CG. The TG-traces show that weight loss occurs in five systematic steps eachcorresponds to the loss of particular species. All the steps havebeen clearly demarcated by vertical lines on the TG plots dividingit into five regions.First (region I) a minor loss occurs up to 1408C, which may be

attributed to the loss of adsorbed water molecules, volatilecompounds and traces of monomers, oligomers, and initiator, etc.Minor loss in region II (140–2808C)may be attributed to the loss of–COOH functional groups by intermolecular condensationreactions. Region III (280–3208C) involves the loss of –SO3Hfunctional groups of dopant, various intermolecular conden-sation reactions, and onset of degradation of PANI phase. The

I, CG2-D, and CG.


359

Figure 6. TG/DTG traces of CG and doped materials. This figure is

available in color online at www.interscience.wiley. com/journal/pat


360

losses in regions IV and V may be related to the breakage of thepolymeric backbone, side chains and groups (–OH), dopantmoieties, etc. The composites show little weight loss between 550and 7008C (region VI), and the residues remaining in this regionare mainly thermally stable inert materials like minerals andmetallic impurities, defunctionalized graphite, and the carbo-nized polymeric and dopant fragments. Since the amount ofresidue increases with the increase in CG fraction, it could be usedto determine the amount of actually incorporated CG content.However, such an estimation is based on the assumption thatexcept from the graphite phase the percentage of all other inertsremains same in all composites and the CG did not lose anyweight in 550–7008C. Therefore, the CG amounts actuallyincorporated into the system were determined (reported inTable 1) by taking the residue percentage of the pure doped PANIand subtracting it from the total residue of the respectivecomposites. The results indicate that actually incorporatedgraphite content is quite smaller than the corresponding weightratio of aniline: CG in the initial reactionmixture. Thus on the basisof char residue, the conductivity value of the doped material at17.4% CG content (CG6-D) has been found to be 67.4, which isconsistent with the findings of Bourdo and Viswanathan.[26]

However, significantly higher thermal stability of our materialsdistinguished them from other similar works. It is quite clear fromthe TG traces that with the increase in the CG content, initiallythermal stability increases up to CG4-D and then comes back to

www.interscience.wiley.com/journal/pat Copyright � 2009 John

the original value. This behavior may be attributed to the initialincrease in interactions (between doped PANI and CG) up toCG4-D followed by phase separation especially at higher CGcontent (CG5-D and CG6-D). Although the consolidated weightloss figure for the composites has been given by TG traces, themuch clearer picture was given by derivative thermogravimetric(DTG) traces (Fig. 6b). DTG traces revealed the presence ofsubsteps for all compositions in region II and only for PANI-D inregion IV. This bimodal loss step in region II shows two peaks (200and 2158C) in the DTG trace, which may be related to theinteraction between partially functionalized CG and doped PANI.As the amount of CG increases, both the sharpness and intensityof the 2008C peak decrease, whereas the peak around 2158Cbecame more prominent. However, in region IV only PANI-Dshows a bimodal loss step that indicates significant interactionbetween phases (doped PANI and weakly functionalized CG) inthe case of composites. These interactions may be mediated bydopant moieties leading to the formation of a ternary systemhaving a single thermal stability range. Formation of such aternary system also supports the better thermal stability of thecomposites.From the TG studies, it can be concluded that these composites

are thermally stable up to 3008C and can be a good candidate formelt blending not only with conventional thermoplastics likepolyethylene (PE), polypropylene (PP), polystyrene (PS), etc., butalso with engineering thermoplastics like PA, PET, or PC.

EMI shielding behavior

The average value of SE of the doped and undoped forms ofcomposites has been measured in the X-band of the microwaverange (8.2–12.4GHz) and the results are reported in Tables 1 and 2,respectively. The SE can be expressed by the following relations:

SE ¼ 10 log10ðPt=PiÞ

where Pi and Pt are the magnitudes of incident and transmittedpowers through the shield material. The results clarify that SEincreases with the CG content; however increase in SE was foundto be much faster than increase in conductivity. This indicatesthat some other loss mechanisms apart from reflection (functionof conductivity) play a crucial role. The acceptable values of SEindicate that these materials could be utilized effectively for theshielding purposes in the X-band region of the microwave range.

CONCLUSIONS

Highly conducting composites of PANI and CG have beenprepared by in situ polymerization. These materials showsignificant conductivities even in the undoped forms and areresistant to the conductivity degradation under relatively alkalineor elevated temperature conditions. This envisages the use ofthese composites for sensing and electrochromic applications.Higher electronic conductivity and faster charge transport makethem potential candidates as an electrode material for batteryapplications. Morphological details reveal the presence of alayered structure in pure CG and highly agglomerated globularparticles in composites. UV–Vis and FTIR data showed systematicshifting of the characteristic bands and peaks, indicating thatsignificant interactions exist between PANI and CG in thecomposite. These interactions between graphite sheets anddoped PANI were attributed to the formation of charge transfer



complex and proposed to be due to the partial transfer of lonepair of electrons of amine nitrogen atoms to the CG particles. XRDpatterns also show the characteristic peaks of both PANI and CG.Thermal data suggest that composites have good stability even inthe vicinity of 3008C and thus could be used for melt blendingwith engineering thermoplastics. The SE of composites changeswith increase in CG content from �1.4 dB (PANI-B) to �27.4 dB(CG6-B) in the case of base and from �31.4 dB (PANI-B) to�39.7 dB (CG6-B) in doped samples. The acceptable values of SEindicate that these materials could be utilized effectively for theEMI shielding purposes in the X-band region of the microwaverange. Our further studies will be concentrated on the meltblending of these composites with thermoplastics like PE, PP, PS,PA, PET, PC, etc.

Acknowledgements

Authors thank Professor Vikram Kumar for his keen interest in thework and for his valuable suggestions. They also thank Mr. K. N.Sood for recording the SEMmicrographs and Dr. R. P. Pant for XRDpatterns.

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Electrical properties and EMI shielding behavior of highly thermally stable polyaniline/colloidal graphite composites