Thermal stability of polyaniline

Thermal stability of polyaniline

Synthetic Metals, 25 (1988) 243 - 252 243 THERMAL STABILITY OF POLYANILINE T. HAGIWARA*, M. YAMAURA* and K. IWATA* The Research Association for Basi...

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Synthetic Metals, 25 (1988) 243 - 252

243

THERMAL STABILITY OF POLYANILINE T. HAGIWARA*, M. YAMAURA* and K. IWATA* The Research Association for Basic Polymer Technology, 2-5-21 Toranomon, Minato-ku, Tokyo 105 (Japan)

(Received May 6, 1987; in revised form March 24, 1988; accepted March 25, 1988)

Abstract The thermal stability of conductive polyaniline prepared by the chemical oxidation of aniline with a m m o n i u m persulfate in hydrochloric acid was investigated by a heat-aging test at 150 °C. The stability in terms of electrical conductivity was found to depend upon the sample form as well as on the environment. The extent of deterioration in electrical conductivity decreased in the order of the powder in air, the disk in air and the powder in vacuo. From the elemental analysis, infrared and Raman spectroscopies, X-ray photoelectron spectroscopy and electron probe microanalysis, the deterioration in the presence of air was assumed to be based on the chemical change involving the elimination of hydrochloric acid on the amino group and the simultaneous chlorination of the aromatic ring.

Introduction Considerable attention has been paid to polyaniline as one of the electrically conducting polymers [ 1 - 14]. This polymer is characterized not only by its high conductivity but also by its high stability on standing. Although the polymer has been well known as aniline black for nearly a century [15 - 1 8 ] , the chemical structure had not been well understood for very long. Recently, however, the semiquinone radical-cation has been proposed as the chemical species that is responsible for the high conductivity [10 - 14]. On the other hand, the thermal stability and thermal degradation have been relatively little investigated. During the course of our study on polyaniline, we found that the change in electrical conductivity of polyaniline [P(A)-HC1] upon heat aging varied with the sample form. This motivated us to analyse the change in the chemical structure in relation to the deterioration in electrical conductivity. In this paper, we report

*Mailing address; Tokyo Research Center of Teijin Ltd., 4-3-2 Asahigaoka, Hino-city, Tokyo 191, Japan. 0379-6779/88/$3.50

© Elsevier Sequoia/Printed in The Netherlands

244 experimental results on the thermal stability of polyaniline upon heat aging and the analysis of the chemical change due to thermal degradation.

Experimental

Materia! (a) Protonated polyaniline [P(A ). HCI] A solution of 34.2 g (0.15 mol) of a m m o n i u m persulfate in 100 ml of water was added to a solution of 9.3 g (0.1 mol) of aniline in 800 ml of 2.3 N hydrochloric acid with stirring at 0 - 5 °C over a period of 1 h. The reaction mixture was further stirred at this temperature for 48 h. The powder precipitated was collected by filtration, washed with water and subsequently with acetonitrile, and dried at 50 °C to obtain 10.5 g of P(A).HC1.

(b) Deprotonated polyaniline [P(A)] A mixture of 1.0 g of P(A).HC1 in 50 ml of 2 N aqueous sodium hydroxide was heated under reflux for 10 h. The powder precipitated was collected by filtration, washed repeatedly with hot water and dried at 50 °C to obtain P(A).

(c) Deprotonated poly(o-chloroaniline) [P(CIA )] A solution of 4.6 g (20 mmol) of a m m o n i u m persulfate in 80 ml of water was added to a solution of 8 g (6.3 mmol) of o-chloroaniline in 2 N hydrochloric acid with stirring over a period of 1 h at room temperature. The reaction mixture was stirred for 5 h. The resulting powder was collected by filtration, washed with water and subsequently with acetonitrile, and dried at 50 °C to obtain 1.0 g of P(C1A). HC1. The powder was heated under reflux in 2 N aqueous sodium hydroxide. The resulting powder was collected by filtration, washed with water and dried to obtain 1.0 g of P(C1A).

Heat-aging test The aging test was conducted at 150 °C in air with the powder and the compressed disks, and also in vacuo (<1 Torr) with the powder. The disks 10 mm in diameter and 0.3 - 0.4 mm thick were fabricated by a compression moulding technique under 3.7 t o n / c m 2 at room temperature. In the case of aging in vacuo, the test was carried out in evacuated ampoules. Before evacuation and sealing, the air in the ampoules was replaced with nitrogen gas.

Measurement Infrared (i.r.) spectra were observed with a Perkin-Elmer model 983 spectrometer. Raman spectra were observed with a Jobin Yvon U-1000 spectrometer attached to a microscope by excitation at 488.0 nm. X-ray photoelectron spectroscopy (XPS) was carried out with a JEOL JESCA-4

245

employing Mg K s radiation. Disk samples were pulverized before measurement. Electron probe microanalysis (EPMA) was carried o u t with a Shimazu EPMA 810 apparatus b y employing a 20 keV electron beam. Electrical conductivity below 10 -s S/cm was measured by the two-probe technique with a YHP 4140B picoammeter and that above 10 -s S/cm by the fourprobe technique with a HP 3456 A digital voltmeter.

Results and discussion Polyaniline can be prepared b y both chemical and electrochemical oxidation of aniline in an acidic medium. As-prepared polyaniline, a conducting form, is obtained in a protonated form with acid. The counter ion included depends u p o n the acid used for the preparation. The polyaniline becomes an insulating form when deprotonated with a base. For the heat-aging test in this experiment, we used protonated polyaniline [P(A).HC1] prepared chemically in hydrochloric acid. We employed d e p r o t o n a t e d polyaniline [P(A)] and poly(o-chloroaniline) [(P(C1A)] as model polymers for the comparison with the aged P(A).HC1. The aging test of P(A)-HC1 was carried o u t under three conditions, i.e., at 150 °C in air with the powder and the compressed disks, and also in v a c u o (<1 Torr) with the powder. Figure 1 shows the change in electrical conductivity of P(A).HC1 upon heat aging. In the case of the powder aged in air, the conductivity of the samples was also measured after acid treatment, in which the heataged samples were heated under reflux in 2 N hydrochloric acid for 10 h. Upon aging the disks in air, the decrease in conductivity was not significant: initially 1.7 × 101 S/cm, 4.8 X 10 -1 S/cm after aging for 100 h and 1.7 × 10 -2 S/cm after aging for 2000 h. On the other hand, the conductivity of

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Fig. 1. C h a n g e in c o n d u c t i v i t y o f p o l y a n i l i n e [ P ( A ) ' H C 1 ] u p o n h e a t aging a t 1 5 0 °C, ~: disk in air, ©: p o w d e r in air, o: p o w d e r in air + HC1 t r e a t m e n t , []: p o w d e r in v a c u o .

246

the powder decreased dramatically to 5.1 × 1 0 - 9 S/cm after aging for only 100 h. The conductivity was not restored to the initial level even by treatment with hydrochloric acid, which ruled out the idea of the deterioration in conductivity being due to the reversible elimination of hydrochloric acid on the amino group. The powder sample aged in v a c u o retained its electrical conductivity at a higher level than that aged in air, the conductivity being 4.2 × 10 -2 S/cm even after 2000 h of aging. In order to study the chemical change, the chemical composition of the aged samples was examined by elemental analysis {Table 1). The composition of the disks changed relatively little upon aging in air and that of the powder also changed little upon aging in v a c u o . Upon aging the powder in air for 2000 h, the composition changed considerably, as indicated by the change in C1/N and C/N ratios. One should notice that the composition changed little upon aging at 150 °C for 100 h, while the conductivity loss was approximately ten orders of magnitude. This suggests that the decrease in electrical conductivity is not simply due to the reversible elimination of hydrochloric acid as mentioned before. The chemical structure was further studied by infrared (i.r.) spectroscopy. Figure 2 shows spectra of the unaged and aged samples together with those of the unaged samples of deprotonated polyaniline [P(A)] and deprotonated poly(o-chloroaniline) [P(C1A)] employed as model TABLE 1 Electrical conductivity and elemental analysis o f P(A). HCI before and after heat aging Heat aging condition a Sample form

Atmosphere

Time (h)

Conductivity

Elemental analysis

(S/cm)

(composition) C(%)

Powder

m

H(%)

N(%)

C1(%)

0

17

53.66 4.83 10.42 (C6.01H6.49 N 1.00C10.63)

16.51

Disk

in air

100

4.8 × 10 -1

60.62 4.55 11.84 (Cs.97Hs.a4N 1.00Cl0.s8)

17.25

Disk

m air

2000

1.7 × 10 - 2

61.32 3.59 12.39 (C5.7sH4.oaN 1.ooClo.53)

16.60

Powder

m air

100

5.1 × 10 - 9

61.34 3.13 12.15 (C s.s9Ha.saN 1.ooClo.53)

16.12

Powder

m air

2000

< 1 0 -1°

58.04 2.55 13.18 (Cs.laH2.68N 1.ooClo.35)

11.81

Powder

m

100

3.1 × 10 -2

59.41 4.61 11.52 (C6.02Hs.sbN 1.00C10.s 3)

15.45

Powder

in vacuo b

2000

4.2 × 10 -2

61.48 4.59 11.83 (C 6.06I-I5.38N 1.00C10.46)

13.74

aAt 150 °C. b< 1 Tort.

vacuo b

247

%

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Disk in 0ir

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Time

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to p

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.g



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1 t 1 , 1 4 + , , I , , , I , , + 1 , 1 . 1

3000

2000

Wove

1600 1200

800

number{era -1}

400

z,O00

(b)

3000

2000

1600

1200

800

400

Wave number(cm-1)

Fig. 2. Change in infrared spectrum (KBr) of polyaniline [(P(A)'HCI] upon heat aging at 150 °C. (a) Disk in air and powder in vacuo; (b) Powder in air. Spectra of unaged samples of P(A) and P(CIA) are also shown in (b).

polymers. In the spectrum of the disk aged in air for 100 h, a strong peak assignable to the vibration due to the protonated structure was observed at 1140 cm -1 [2], although the spectrum was poorly defined because of difficulty in pulverization. On the other hand, the peak was small with the powder aged in air for 100 h and almost unobservable with that aged for 2000 h. The spectrum was rather similar to those of P(A) and P(C1A). Upon aging in v a c u a , the spectrum remained substantially unchanged. Similar behavior was also observed rather clearly in the Raman spectra. Figure 3 shows the spectra together with those of P(A) and P(C1A). The spectra of the inner part and the surface of the disk aged in air for 100 h were different. The former was substantially the same as that of the initial P(A). HC1, while the latter was quite different and rather similar to the spectra of P(A) and P(C1A). In the spectrum of the surface, sharp peaks at 1192 and 1622 cm -] characteristic of the protonated structure were missing and a broad band characteristic of the deprotonated structure was observed instead [13]. The spectrum of the inner part also completely changed upon prolonged aging for 2000 h. The spectrum of the powder changed quickly when aged in air, while it changed very slowly when aged in vacua. With the sample aged in vacua, b o t h peaks at 1192 and 1622 c m - ' still remained

248

Disk

in air

Powder in air

~m

2,000 h

5 d

k 5

~100h

I,

10o h

IL

t-

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x

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2o0 400 600 800 1000 1200 I~00 1600 1800 (a) W a v e n u m b e r C c r n q)

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800

200 4O0 600 800 1000 1200 1400 1600 1 W o v e n u m b e r ( c m -1 ) (b)

Powder in vacuo

2j 000 h

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I

i

a

,

i

,

!

a

I

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200 400 600 800 1000 1200 1400 1600 1800 (C) W a v e n u m b e r ( c m -I )

Fig. 3. Change in Raman spectrum with excitation o f 488.0 nm of polyaniline [P(A).HCI] upon heat aging at 150 °C. (a) Disk in air; (b) powder in air; (c) powder in t~lCUO,

249 after 2000 h, although a broad band was observable between the peaks. These experimental results suggest that a chemical change due to air oxidation occurs and that it proceeds gradually from the surface to the inner part in the case of the disk samples. Previously, we reported that P(A). HC1 consists of the repeating units of various oxidation levels formed via the following scheme [13]: NH2

_

H H ~N/

~

H

-H

I

H

+H

II

H

H

H +CI-

H III

IV

The amine (I) and semiquinone radical-cation (II) structures were found to be major components, and the latter structure was considered to be responsible for the high electrical conductivity [ 1 1 - 14]. In addition, we detected the amine structure (IV) with a chlorinated aromatic ring as a minor component, which was assumed to be derived b y further oxidation from II through the quinoidal structure (III). The same scheme would be applicable to the thermal degradation of P(A). HCI in the presence of air: the protonated structures [(I) and (II)] are oxidized in the presence of air to give III, which is further transformed into the deprotonated structure (IV) b y chlorine substitution. Here, oxygen in air plays the role of an oxidizing agent, like a m m o n i u m persulfate used in the synthesis of polyaniline. This scheme explains well the experimental results that the degradation was retarded in v a c u o and that the surface of the disk degraded faster than the inner part. This scheme also explains the result that the decrease in the chlorine content was small with the sample aged in air for 100 h, despite the fact that the protonated structure disappeared almost completely, as observed in the infrared and Raman spectra. X-ray photoelectron spectroscopy (XPS) is one of the powerful tools for the structural analysis of polyaniline. Previously, we reported that P(A).HC1 showed two bands due to C12p at 197 and 202 eV, which were assigned to ionically- and covalently-bonded chlorine, respectively [13]. We also found that the former originates from I and II, and the latter from IV. Based on this finding, we compared the XPS spectrum of the aged sample with that of the unaged one in order to make clear the nature of chlorine included in the samples. All measurements were carrried o u t with samples in powder form (the aged disks were pulverized before measurement). Figure 4 shows the spectra. The spectrum of the disk changed little u p o n aging in air for 100 h, b u t upon aging for 2000 h, it changed dramatically. The band due to the ionically-bonded chlorine disappeared almost

250 Covalent

Ionic Clzp

I

i

Unaged

DiSK In air 100h

'%+__

2. O00h

Powder in air 100h

2,000h

I

~P°wdeI

in vacuo lOOh

2.000h I

210

I

I

I

200

I

190

Binding energy(eV) Fig. 4. Change in X-ray photoelectron spectrum heat aging.

(Cl2p)of polyaniline [P(A).HCI]

upon

completely and only the band due to the covalently-bonded chlorine remained. With the powder, the band due to the ionically-bonded chlorine disappeared upon aging in air within 100 h. Upon aging in vacuo, on the other hand, the band due to the covalently-bonded chlorine disappeared, while that due to the ionically-bonded chlorine did not. These results suggest that the oxidation from II to IV is retarded in the absence of air, and the backward reaction from IV to II occurs instead'by intermolecular oxidationreduction. In the latter reaction, the role of reducing agent would be played by I, which itself is oxidized to II. These results obtained b y XPS closely correspond to those obtained b y i.r. and Raman spectroscopies, suggesting that chloride ions are consumed b y substitution with protons in the aromatic rings u p o n heat aging in the presence of air. The results from the Raman spectra suggest that the surface of the disk is more susceptible to air oxidation than the inner part. In order to examine the inhomogeneity, the distribution of chlorine was measured b y electron probe microanalysis (EPMA). Figure 5 shows the distribution of chlorine across the cross-section of the disk. The chlorine was found to be highly concentrated in the surface region. The concentrated region expanded inward upon prolonged aging. Despite the fact that the chlorine distribution became inhomogeneous, the total chlorine content decreased only a little, as was consistent with the results of elemental analysis. A possible explanation of the complicated chlorine distribution is that the chloride ion depleted faster in the surface region than in the inner part, which gave

251 10

-

Kcps

4.22~ lO0 micron 0h

_ ~

100 micron 100 h

.... 100 micron 2,000 h

Fig. 5. Distribution of CI measured by electron probe microanalysis across the disk of polyaniline [P(A).HC1] before and after heat aging at 150 °C in air.

rise to the migration of chloride ion from the inner part to the surface region.

Conclusion (1) Thermal stability of conductive polyaniline P(A).HC1 depends upon the sample form as well as on the environment. The extent of deterioration in electrical conductivity decreased in the order of the powder in air, the disk in air and the powder in v a c u o . (2) The deterioration was found to be based on a chemical change involving the elimination of hydrochloric acid on the amino group and the simultaneous chlorination of the aromatic ring.

Acknowledgements This work was performed under the management of the Research Association for Basic Polymer Technology for synthetic metals as part of a project on Basic Technology for Future Industries sponsored by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry. The authors wish to thank Mr. Hiroshi Itagaki, Director of the Departm e n t of Research and Development of Teijin Ltd., and Dr. Shuji Ozawa, Director of Central Research Laboratories of Teijin Ltd., for suggestions and encouragement.

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