Polymer International 42 (1997) 380È384

Investigation of Structure and ElectricalConductivity in Doped Polyaniline

H. K. Chaudhari & D. S. Kelkar*

Institute of Science, Department of Physics, 15 Madam Cama Road, Mumbai 400032, India

(Received 1 October 1996 ; accepted 26 October 1996)

Abstract : Polyaniline (PAn) was synthesized chemically and doped with variousdopants, such as HCl, HCOOH, and methylene blue S), by anI2 (C16 H18 ClN3immersion method. The structure of these samples was investigated by infrared(IR) spectroscopy and wide-angle X-ray di†raction (WAXD) analysis. Remark-able changes have been observed in the IR spectra of doped PAn, indicating thatdoping is a†ecting the chemical structure. The percentage crystallinity was alsofound to increase after doping. The electrical conductivity (p) of these sampleswas measured at various temperatures (T \ 308 K to 423 K). Plots of log pversus Ty, where y \ [1/2, [1/3, [1/4, were obtained and used to identity theconduction mechanism. Undoped PAn shows semiconducting behaviour, whiledoped samples show a variable range hopping mechanism. A primary cell wasconstructed with HCl-doped PAn as one of the electrodes and a copper plate asthe other electrode. It gave an open circuit voltage of 0É38 V and a short circuitcurrent of about 5É4 mA.

Key words : polyaniline, chemical doping, crystallinity, electrical conductivity,primary cell.

INTRODUCTION

Conducting polymers such as polyacetylene, poly-pyrrole, polyaniline (PAn) and polythiopene have beenwidely investigated over the past few years. PAn hasreceived great attention because of its good environ-mental stability and also its interesting electrochemicalproperties for the development of light-weight batteriesand electrochromic display devices.1

Polyaniline can be synthesized chemically as well aselectrochemically.2h5 Infrared spectroscopic analysis ofPAn has been widely carried out.3,6h8 IR spectra ofundoped PAn showed main absorption bands at about3237, 3040, 1587, 1510, 1300, 1250 and 850 cm~1.Changes in intensities of these bands were observedafter doping the samples. The bands appearing at about1587 cm~1 and 1510 cm~1 are assigned to non-symmetric ring stretching.6,7 It was reported thatC6with di†erent concentrations of HCl, the relative inten-sities of these two bands changed and simultaneously

* To whom all correspondence should be addressed.

the conductivity in PAn was a†ected. Ohsaka et al.8 dis-cussed the mechanism of polymerization of PAn interms of changes in the IR spectra.

According to Wang et al.9 crystallinity decreased(Xc)with increasing HCl concentration during synthesisover the range from 0É001 N to 7 N. The reason for thiswas that at high concentrations of HCl addition of Clatoms to the quinoid ring occurs. It was reported thatthe crystallinity of the sample depends on the size ofboth oxidant and dopant. Dopant and oxidant withsmaller sizes lead to higher crystallinity.9 It was alsoreported that emeraldine base powder was c. 50%(orthorhombic) crystalline.10,11 Chen and Lee12 foundthat the X-ray di†raction (XRD) pattern of PAn powderexhibited a broad amorphous peak at 2h \ 19¡ and twoweak crystalline peaks at 2h \ 15¡ and 2h \ 24¡, andbecame more crystalline after doping with HCl.

From a literature survey it was found that most ofthe studies have concentrated on HCl-doped polyanil-ine.13,14 It was reported13 that when undoped PAn wastreated with aqueous acid such as HCl, it undergoes aninsulator-to-metal transition involving an increase in

380Polymer International 0959-8103/97/$09.00 1997 SCI. Printed in Great Britain(

Investigation of doped PAn 381

Fig. 1. Infrared spectra of PAn powder : (a) HCl-doped ;(b) iodine-doped ; (c) formic acid-doped.

Fig. 2. Infrared spectra of PAn powder : (a) undoped ;(b) methylene blue-doped.

Fig. 3. WAXD scans of PAn in powder form: (a) undoped ;(b) HCl-doped ; (c) formic acid-doped.

Fig. 4. WAXD scans of PAn in powder form: (a) undoped ;(b) iodine-doped ; (c) methylene blue-doped.

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382 H. K. Chaudhari, D. S. Kelkar

conductivity of the order of 10. Hence it was thoughtinteresting to investigate the structure and electricalconduction in PAn when it is doped with variousdopants such as formic acid, iodine, methylene blue aswell as HCl. The structure of these samples has beeninvestigated using infrared (IR) spectroscopic analysisand wide-angle X-ray di†raction (WAXD) analysis.Electrical conductivity study of these samples at varioustemperatures has been undertaken. An attempt has alsobeen made to use HCl-doped PAn as one of the elec-trodes in a primary cell.

EXPERIMENTAL

Sample preparation

The PAn used was synthesized chemically using anilineas monomer and ammonium peroxodisulphate as anoxidant in aqueous media.5 The PAn thus obtained wasin the doped form, which was converted into the baseform by treatment with aqueous ammonia.

Doping

Undoped PAn powder was doped by immersion invarious solutions, such as 1 M aqueous HCl, 10%aqueous formic acid, 1% iodine solution in acetone and1% aqueous methylene blue solution, for 24 h, followedby drying under dynamic vacuum at room temperature.It was then pressed into pellets of about 1 cm diameterand 1 mm thickness.

IR spectroscopy

The IR absorption spectra were obtained for pristineand modiÐed PAn samples in the spectral range 4000È400 cm~1 using a Perkin Elmer FTIR model 1600spectrometer.

WAXD scans

The X-ray di†raction parameters were recorded using aJEOL JDX-8030 X-ray di†ractometer. From the radialscans of intensity versus 2h, the lateral order or the crys-tallinity index of these samples was calculated usingManjunathÏs formula.15

Electrical conductivity measurements

A two-probe method was used to measure the electricalconductivity of the PAn pellets in a sandwich conÐgu-ration.16

A very simple primary cell was constructed, whichconsisted of an aqueous solution as electrolyte.CuCl2One of the electrodes used was an HCl-doped PAn

pellet, while the other electrode was a copper plate ofabout the same area (1 cm2).

RESULTS AND DISCUSSION

Figure 1 shows the IR spectra of HCl-, iodine- andformic acid-doped PAn and Fig. 2 shows the IR spectraof undoped and methylene blue-doped PAn. Remark-able changes are observed after doping ; the mostimportant described below.

In the NwH stretching region, a band appears atabout 3237 cm~1. This band shifts towards higherwavenumbers in the case of all doped samples, exceptthat doped with formic acid where there is a slight shifttowards lower wavenumbers. The shift in this bandindicates a hydrogen amine (NH) group.6 In the CwHstretching region, the peak at about 3060 cm~1 forundoped PAn is reduced after doping, which indicatesreduction in the number of H atoms bonded to thebenzene ring.6 The CwN stretching band at about1300 cm~1 shifts slightly towards lower wavenumbersand becomes stronger than the key bands in the case ofHCl-, HCOOH- and samples. SimultaneousI2-dopedchanges are observed in the NwH and CwN stretchingbands, indicating the possibility of the dopant moleculesbecoming attached to the more reactive amine nitrogensites of PAn.

After doping, the key bands assigned to the quinoidand benzenoid units are at about 1582 cm~1 and1496 cm~1, respectively, showing slight shifts towardslower wavenumbers. The relative intensity of these twobands increases after doping.

In the case of methylene blue-doped samples, unlikethe other doped samples, almost all the peaks shift

Fig. 5. Plot of log p versus 1/T for undoped PAn powder.

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Investigation of doped PAn 383

Fig. 6. Plot of log p versus T ~1@4 for : (a) HCl-doped ; (b) HCOOH-doped ; (c) I2-doped.

towards higher wavenumbers and the bands becomebroad.

The WAXD patterns of all the PAN samples are wellresolved and show distinct peaks (Figs 3 and 4). Forcalculation of the crystallinity index, ManjunathÏsformula was used.15

For undoped samples, peaks appeared at about2h \ 9É5¡, 14É7¡, 20É4¡ and 25É1¡, and the percentage,crystallinity calculated using ManjunathÏs formula was45%. This indicates that undoped PAn is not fullyamorphous, but partially crystalline. It is also observedthat the crystalline peak at 2h \ 25¡ is more prominent

Fig. 7. Plot of log p versus T ~1@4 for methylene blue-doped PAn powder.

POLYMER INTERNATIONAL VOL. 42, NO. 4, 1997

384 H. K. Chaudhari, D. S. Kelkar

than the amorphous peak, which is reported at2h \ 19É5¡.

After doping with HCl and formic acid, PAn becomesmore crystalline. The percentage crystallinity increasesremarkably for HCl- and HCOOH-doped samples(Table 1). In the case of iodine-doped samples, peaks arenot as distinct and sharp as they are for undoped PAn.Furthermore, the amorphous peak is more intense thanthe crystalline one. The percentage crystallinity reducesto 35%, which is even less than that of the undopedmaterial. After doping with methylene blue, the crys-tallinity of the sample does not show as much improve-ment as is observed in HCl- and HCOOH-doped PAn.

From these observations it can be concluded that thedopant species a†ects the structure and also the crys-tallinity of PAn.

For undoped PAn, variation of conductivity (p) withtemperature (T ) was studied in the temperature range308 K to 423 K. A plot of log p versus 1/T for thissample shows a straight line Ðt (Fig. 5).

A straight line relationship between log p and 1/Tindicates that the behaviour is semiconducting. Similarplots of log p versus 1/T were obtained for the otherdoped samples, but good Ðts were not observed. ForHCl, HCOOH and dopants, a plot of log p versusI2T ~1@4 shows a good Ðt (Fig. 6), while for methyleneblue-doped PAn a plot of log p versus T ~1@3 shows agood Ðt (Fig. 7). These observations indicate that afterdoping the conduction mechanism does not remainsemiconducting, but changes to variable range hopping.

An attempt was made to construct a primary cellcontaining 40% solution as an electrolyte. ItCuCl2gave an open circuit voltage of 0É38 V and a shortcircuit current of about 5É4 mA.

It was observed that the PAn electrode becomes posi-tive when a voltage is developed. This shows that PAnloses electrons, which may then recombine with metal

TABLE 1. Percentage crystallinity, and electricalXc

conductivity (measured at room temperature) of

PAn in powdered form

PAn powder Xc

(%) Conductivity (S cmÉ1)

Undoped 45 5·27 Ã10É7

HCl-doped 74 6·0 Ã10É2

Formic acid-doped 65 2·0 Ã10É1

Iodine-doped 35 3·68 Ã10É4

Methylene blue-doped 52 7·18 Ã10É7

cations. The reaction may be as follows :

At the anode (polymer) : PAn] PAn`] e~

CuCl2 ] Cu2`] 2Cl~

At the cathode metal) : Cu2`] 2e~ ] Cu

Copper ions then deposit on the metal electrodemaking it negative.

During the course of this study, it was seen that thePAn electrode was not visibly a†ected and could bereused after washing.

CONCLUSIONS

From the IR spectra and WAXD scans it can be con-cluded that the dopant species a†ect the structure andalso the crystallinity of PAn. Pristine PAn has semicon-ducting behaviour, which changes to variable rangehopping after doping. PAn can be used as the anode forconstructing a primary cell.

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