Polymer International 39 (1996) 153-159

Synthesis and Properties of Oxalic Acid -doped Po I ya n i I i ne

Emine Erdem

Department of Chemistry, Faculty of Education, Hacettepe University, 06532 Beytepe, Ankara, Turkey

Mehmet SaCak* & Meral Karakiqla

Department of Chemistry, Faculty of Science, Ankara University, 06100 Tandogan, Ankara, Turkey

(Received 28 April 1995; accepted 5 October 1995)

Abstract: Conductive polyaniline was synthesized in aqueous 1.0 M oxalic acid containing 0.1 M aniline by electrochemical and chemical oxidation and charac- terized by conductivity, solubility, ultraviolet and infrared spectroscopy, and cyclic voltammetry. The solubility experiments showed that the solubility of oxalic acid-doped polyaniline in dimethylsulfoxide and dimethylformamide increased to a certain extent. The soluble part of the polyaniline was free from impurities such as quinones. Cyclic voltammetric studies in oxalic acid medium revealed that aniline exhibited a similar behaviour to that in H,SO, and the polymerization rate was much slower than that in H,SO, .

Key words: conductive polymers, polyaniline, oxalic acid.

INTRODUCTION

Studies on the synthesis, characterization and applica- bility of conductive polymers such as polypyrrole, poly- aniline, polythiophene, and polycarbazole have been intensified in recent years because of their great techno- logical utility.'-4 Polyaniline (PANI) especially has drawn the interest of many workers as a conductive polymer because of its high electrical conductivity, sta- bility in air, cheapness as a raw material, and easy syn- thesis.

Although PANI has been known as an oxidative polymer from aniline for a long time,5*6 the importance it gained as a conductive polymer was due to the studies of MacDiarmid et aL3

PANI can easily be synthesized by electrochemical or chemical methods. However, the electrochemical method has an advantage as the resulting polymer does

* To whom correspondence should be addressed.

not contain contaminants from the oxidative agents necessary for chemical synthesis. In addition, PANI can be obtained as a conductive film upon the electrode surface in the electrochemical method.

Initially, PANIs in doped form did not show solu- bility or melting, which made them highly unprocessable and gave them poor mechanical proper- ties. One of the methods employed to improve the pro- cessability of PANI was the preparation of composites or blends by the use of an insulating polymer as the main matrix. This aimed to combine the good mechani- cal properties of the insulating polymer with the electri- cal properties of PANI. The methods reported in the literature for the preparation of composites or blends of PANI are the electrochemical polymerization of PANI upon an insulating polymer precoated the exposure of the absorbed aniline polymer film to the vapour of an oxidant agent and an acid together,' the polymerization of aniline in an emulsion containing an insulating polymer," or solution processing."

153 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

154 E. Erdem, M . SaCak, M . Karakzqla

However, although the mechanical properties of these composites or blends were satisfactory to a certain extent, their electrical conductivity was relatively low and conventional percolation behaviour with a thresh- old about 10-15% has often been observed."

The processability of PANI can also be improved by increasing its solubility. The solubility of PANI has been reported to be better in solvents such as dimethyl- formamide, dimethylsulfoxide, m-cresol, N-methyl pyr- rolidinone, and water."-'

The methods employed to improve the solubility of PANI were the following: the pre-functionalization of

I ! : ! : : ! : : : ,

0 4 0 0 0 4 08 1 2 1 6 E ( V )

I : : : : : ! '

- 0 4 00 0 4 0 8 ' 1 ' 2 ' 1 ' 6

E ( v )

Y ' P -e=== I : : : ! : : : : : <

0 4 00 0 4 0 8 1 2 1 6

E ( V )

Fig. 1. Multisweep cyclic voltammograms in (a) 0 . 1 ~ aniline + 1 . 0 ~ H,SO,, (b) 1 . 0 ~ oxalic acid, and (c) 0 . 1 ~ aniline + 1 . 0 ~ oxalic acid, versus Ag/Ag+ at a scan rate of

50 mV/s. The numbers show the cycles.

t : : : : : , - 0 4 00 0 4 08

E ( V )

Fig. 2. Multisweep cyclic voltammograms between -0.4 V and 0.8V in (a) 0 . 1 ~ aniline + 1 . 0 ~ H,SO, and (b) 0 . 1 ~ aniline + 1 . 0 ~ oxalic acid, versus Ag/Ag+ at a scan rate of

50mV/s. The numbers show the cycles.

the aniline,'6 synthesis of PANI graft copolymer^,^' or the use of suitable anionic dopants.I3-l5

The type of anion dopant in PANI affects its electri- cal transport,'* stability," and morphology," as well as its solubility. Therefore the investigation of the effect of various dopant anions upon the polymerization and properties of PANI is quite important from the scienti- fic point of view.

This study deals with the electrochemical and chemi- cal synthesis of PANI in oxalic acid medium. Conduc- tive PANI synthesized in this way was characterized by techniques such as cyclic voltammetry, infrared and ultraviolet spectroscopy, and solubility and conductivity measurements.

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Oxalic acid-doped polyaniline 155

EXPERIMENTAL

Chemicals

Aniline (Merck) was double distilled in vacuum and freshly prepared before the experiments. Oxalic acid was recrystallized from glacial acetic acid. Other chemi- cals such as K,Cr,O, , H,SO,, dimethylformamide (DMF) and dimethylsulfoxide (DMSO) were chemically pure grades.

Cyclic voltammetry

The cyclic voltammograms were taken in a cell using 0.5mm Pt wire as working electrode, a 2cm coiled Pt wire counter electrode and a Ag/Ag+ reference elec- trode in the form of an isolated Luggin capillary. The system consisted of a Bank Elektronik ST 88 poten- tiostat, a Bank Elektronik VSG 2000 function gener- ator, and a Karl Kolb Servogor recorder.

Electrochemical polymerization

The electrochemical polymerization of aniline was carried out in a three electrode cell previously described.” The working, counter and reference elec- trodes were as in cyclic voltammetry. The cell was purged with nitrogen for 20min prior to each experi- ment and the system was kept under nitrogen atmo- sphere throughout the polymerization. The insoluble material which formed at the surface of the working electrode was scraped off, washed thoroughly with doubly distilled water and 1.0 M oxalic acid and dried in vacuum at 50°C.

Chemical polymerization

The chemical polymerization of aniline was carried out in aqueous medium using K,Cr20, as an oxidizing agent. The K,Cr,O, solution was added dropwise to 1 . 0 ~ oxalic acid solution containing 0 . 1 ~ aniline over 2 h at 0°C. The solution was stirred for 8 h and the dark green product was filtered off, washed with water and 1.0 M oxalic acid and dried in vacuum at 50°C.

Undoped PANI was obtained by stirring oxalic acid- doped PANI in 15% alkaline NH, solution for 24h, before filtering, washing with water and drying in vacuum.

Characterization

The electronic spectra were taken using a Shimadzu 160 A spectrophotometer between 300 and 800 nm. Oxalic acid solutions in DMSO at different pH values were prepared and soluble PANI was added to them. The reference solutions were oxalic acid solutions in DMSO

(without PANI) at the same pH values. The reference solution for undoped PANI was DMSO alone.

The IR spectra were recorded with a Matsonn 1000 FTIR spectrometer using KBr pellets.

The conductivity measurements were carried out using pelleted samples (1 cm diameter x 0.1 cm) and the standard four-probe method.

RESULTS AND DISCUSSION

Multisweep cyclic voltammetric studies

Figure l(a) shows the cyclic voltammograms of 0 . 1 ~ aniline in 1 . 0 ~ H2S04, Fig. l(b) of 1 . 0 ~ pure oxalic acid, and Fig l(c) of 0.1 M aniline in 1 . 0 ~ oxalic acid. In the first cycle, the single anodic peak observed at about 0.89V (peak A) in the aniline-H,SO, system (Fig. la) corresponds to the formation of a radical cation of aniline by electrochemical oxidation, whose location shifts depending upon the concentration of H2S04 .21

On the repetitive cycling, new oxidation peaks C and D appear indicating that these radical cations undergo further coupling to give polymer and the intensity of peak B corresponding to polymer formation starts to increase.22

The cyclic voltammogram taken in 1 . 0 ~ pure oxalic acid (Fig. lb) yielded a typical single oxidation peak at 1.20V corresponding to the oxidation of oxalic acid to carbon dioxide.,,

On the other hand, the cyclic voltammogram obtained in a medium containing aniline and oxalic acid gave two oxidation peaks (Fig. lc). These peaks were attributed to the oxidation of aniline and oxida- tion of oxalic acid upon the Pt electrode surface modi- fied with PANI, respectively.

The polymerization of aniline upon the Pt electrode surface may be considered to take place in two stages.

0 0 1 v

1 : : : : : a -0.4 0.0 0.4 0.8

Fig. 3. Cyclic voltammogram of PANI film on a Pt electrode in 1 . 0 ~ oxalic acid, versus Ag/Ag+ at a scan rate of 50mV/s.

E ( V )

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156 E. Erdem, M . SaCak, M . Karakijla

The first step takes place upon the bare Pt surface, or the Pt surface partially blocked by PANI. That is why the polymerization of PANI in the second stage is closely related to the morphology of the PANI film fully covering the electrode surface. The coverage of the elec- trode surface with a porous PANI layer facilitates ade- quate aniline diffusion towards the electrode surface and the oxidation rate of aniline increases to a certain extent.

The intensity of the peak at 0.86V attributed to the oxidation of aniline in Fig. l(c) shows a sudden decrease after the first scan and increases thereafter. On the other hand, this peak (peak A) in the aniline-H,SO, system vanishes after a few cycles as seen in Fig. l(a). Based upon these findings it can be concluded that the PANI film formed in oxalic acid medium is porous in nature in contrast to the film formed in H2S04, which covers the electrode in a much more compact manner. However, an increase in the number of cycles (> 20) in the aniline-oxalic acid system caused a gradual decrease in peak intensity due to the PANI layer becoming thicker and the depletion of the aniline concentration near the electrode surface.

A similar behaviour has been observed in the electro- chemical polymerization of aniline in H3P0,, H F and HBF, and it was found that polymerization takes place upon a surface covered with a porous PANI layer in HBF, in contrast to other acids.',

Another feature is the formation of some peaks in 1.0 M oxalic acid solution containing aniline at lower potentials after repetitive cycling. No such changes are observed in pure 1 . 0 ~ oxalic acid solution even with a high number of cycles (Fig. lb). The cyclic voltam- mograms of 0.1 M aniline in 1 . 0 ~ oxalic acid and 0.1 M aniline in 1 . 0 ~ H,SO, solution between -0-4 and 0.8V are given together in Fig. 2, at higher sensitivity. Peaks B, C and D are clearly observed in the cyclic voltammogram of 0 . 1 ~ aniline in 1 . 0 ~ oxalic acid (Fig. 2b), similar to those observed in H2S0, medium (Fig. 2a).

All these observations show that aniline polymerizes in oxalic acid in a similar way to that in H,SO,. However, there are some variations in peak potentials

I : : : : : I -0.4 0.0 0.4 0.8

Fig. 4. Cyclic voltammograms of (-) oxalic acid-doped PANI solution in DMSO containing 1 . 0 ~ oxalic acid and (. . . -) oxalic acid in DMSO, versus Ag/Ag+ at a scan rate of

50 mV/s.

W)

as seen in Table 1. Some oxidation peak potentials reported in the literature are also included in this table and they differ slightly according to the electrolytic medium used.

A comparison of the intensities of peak B in Fig. 2(a) and 2(b), a measure of polymer formation rate,,' reveals that the increase in intensity in oxalic acid is slower than in H,SO,. This shows that the rate of poly- merization of aniline is much slower in oxalic acid than in H,SO,. The Pt electrode was immediately covered with a green coloured layer when the electrolysis was carried out in H 2 S 0 4 . However this process took around 2 h in oxalic acid.

Solubility

The solubilities of chemically and electrochemically syn- thesized oxalic acid-doped PANI were examined in various solvents.

The solubility values of chemically synthesized PANI in DMSO and DMF were measured as 1.20g/100ml and 0-85 g/100 ml, while these values were found to be 0.52 g/100 ml and 0.30 g/100 ml for electrochemically synthesized PANI. The solubility and conductivity values of PANI doped with various acids in DMSO are

TABLE 1 . Oxidation peak potentials observed in different media containing aniline

Medium Potential (V) Reference

peak B peak C peak D

Oxaiic acid -0.06 0.22 0.40 this work H2S04 -0.02 0.25, 0.33 0.54 this work H2S04 -0.05 0.22, 0.28 0.55 25 Benzene

sulphonic acid 0.1 0 0.40 0.70 14 H2S04 0.09 0.36 0.62 14

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Oxalic acid-doped polyaniline 157

also given in Table 2 for comparison with PANI obtained in oxalic acid. Oxalic acid-doped PANI showed no solubility in acetone, acetonitrile, tetra- hydrofuran, benzene, chloroform, rn-cresol, dichloro- methane, or ethanol.

The most important factors which determine the solubility of PANI are the interaction between PANI chains and the interaction between PANI chains and solvent molecules. The close relation between solubility of PANI and the type of doped anion shows that dopants facilitate polymer-solvent interactions and overcome polymer-polymer interactions.

The dopant anions reported to increase the solubility of PANI to a certain extent are generally large and contain carboxyl s u b ~ t i t u e n t s . ' ~ - ' ~ * ~ ~ This shows that the carboxyl groups in dopants create polymer-solvent interactions stronger than the polymer-polymer inter- actions, which is similar to our system.

Although dopant anions of oxalic acid are not very large, the presence of two carboxylic acid groups might be the reason for the increased solubility.

The undoping of PANI obtained in oxalic acid medium induced an insulating character as would be expected. The undoped PANI can be re-doped with oxalic acid, or other acids such as HCl and H,SO,.

Cyclic voltammograms of polyaniline

The surface of the working electrode was covered with PANI film by constant potential electrolysis at 0.8V in 1 . 0 ~ oxalic acid containing 0.1 M aniline. This electrode was then transferred to a cell containing 1 . 0 ~ oxalic acid (without aniline) to measure its cyclic voltam- mogram (Fig. 3). Figure 4 shows the cyclic voltam- mograms of soluble PANI in DMSO containing 1 . 0 ~ oxalic acid.

When these cyclic voltammograms are compared, it can be seen that the oxidation peak observed at 0.26V

2.0

al V C m e

1.0 n m

0.0 I 1 300 500 700

A . nrn

Fig. 5. Electronic spectra of PANI solution in DMSO at dif- ferent pH: (a) pH = 7.1; (b) pH = 3.9; (c) pH = 2.7.

in PANI deposited upon Pt surface (Fig. 3) is missing in soluble PANI (Fig. 4) This middle peak was attributed to the formation of quinones as a result of hydrolysis reactions.26 Therefore we can claim that the soluble part of PANI obtained in oxalic acid medium does not contain any impurities such as quinones.

Electronic spectra

Figure 5 shows the electronic spectra of undoped PANI and solutions of PANI in DMSO having different pH produced by the addition of oxalic acid. Undoped PANI dissolved in DMSO (pH = 7.1) has two adsorp- tion bands at 633 and 324nm (curve a). The band at 324nm is caused by a n--x* transition of aniline and/or anilinium radicals, and the band at 633nm is due to a n-n* transition of quinone-imine groups.27 The colour and adsorption bands of oxalic acid-doped PANI in DMSO depend upon the pH of the solution.

The intensity of the band at 633nm decreases and new bands appear at about 800 and 435nm when the

TABLE 2. Solubility and conductivity values of PAN1 obtained in different media

Medium and Conductivity Solubility Reference method (S/cm) in DMSO

(silo0 mi)

Oxalic acid electrochemical chemical

electrochemical chemical

electrochemical

electrochemical chemical

Sulphamic acid

Benzene sulphonic acid

H,SO,

0.9 0.52 th i s work 0.5 1.20 this work

2.0 0.80 26 0.2 1 .I0 26

2.0 0.82 14

1 . 2 nil 27 0.7 nil 28

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158 E . Erdem, M . SaCak, M . Karakigla

4000 3000 2000 1000 400

w a v e n u rn be rs , c rn"

Fig. 6. FTIR spectra of (a) undoped PANI and (b) oxalic acid-doped PANI obtained by the chemical method.

pH of the blue coloured PANI solution at pH 7.1 is brought to pH 3.9 (curve b). The colour of the solution turns to green, when the pH changes from 4 to 3.

At low pH values (pH = 2.7, curve c) the adsorption band at 633nm completely vanishes and the band at 435nm, which corresponds to a n-n* transition of quinone-iminium ions, and the broad band at about 800nm which is due to trapped excitons centred on quinoid moieties,28 or delocalized free electron states,29 become visible.

Infrared spectra

Figure 6 shows the major FTIR bands of doped and undoped PANI. The bands observed at 1500 and 1583cm-' in undoped PANI shift to 1483 and 1567cm-' in doped PANI. These changes are indica- tive of the benzenoid-quinoid rings transition in PANI.I3 The increase in the intensity of the band at 1140 cm- was attributed to charge delocalization on the polymer ba~kbone.~' The peaks observed at 1650 and 1250cm-' in doped PANI are due to the carboxyl groups of the dopant and these peaks vanish on removal of dopant.

CONCLUSIONS

This study showed that conductive PANI can be chemi- cally and electrochemically synthesized in oxalic acid

medium. The electrochemical behaviour of aniline in oxalic acid is similar to that in H2S0, . However, the polymerization rate is slower and there are some varia- tions in oxidation peak potentials. The use of oxalic acid medium improved the solubility of PANI in DMSO and DMF to a certain extent. This was attrib- uted to the presence of the carboxyl groups in the dopant anion.

ACKNOWLEDGEMENT

We would like to thank Ankara University Research Fund for their financial support of this work.

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