INTRODUCTION:

Electrically conducting polymers recently received much interest due to their potential technological importance. (1,2,3) Their discovery and development led to the award of Nobel prize for Chemistry in 2000.(4,5) Polymers show the electrical and optical properties of metal or semiconductors and the same time retain the attractive mechanical and processing advantages of polymers.(5) The Properties of conducting polymers formed electrochemically depend on the nature of ions of supporting electrolyte and their size. Previously we reported the electrochemical polymerization of pyrrole and aniline on some commodity metals.(6,7) Herein is described the cyclic voltammetry and conductivity study of polyaniline prepared electrochemically in an aqueous solution of sulfosalicylic acid, naphthalene-2-sulfonic acid, anthraquinone-2-sulfonic acid and polyacrylic acid.

MATERIALS AND METHODS:

Aniline and water were purified by fractional distillation. Salts were of analytical grade and used without further purification. Cyclic voltammograms were recorded with a CV-27 BAS (Bio Analytical System U.S.A) equipped with BAS X-Y recorder.

All electrical measurements were performed in a single compartment, three electrode cells under N₂ atmosphere. The working electrode was a platinum microelectrode. The counter electrode was a platinum wire and the reference electrode was Saturated Calomel Electrode (SCE). The polymer films were grown galvenostatically on a platinum sheet of area 3.5 cm²at 0.85 V vs SCE. The resistivity in ohm-cm of the polymer films was measured using a high vacuum tube voltmeter.

RESULTS AND DISCUSSIONS:

The voltammetric behavior of the solution of aniline (0.2mole/L) in the aqueous solution of sulfosalicylic acid (1.0 mole/L) was studied.

Figure 1 shows multiple cycled voltammograms of the solution recorded in the potential range of -0.2V and 1.0V vs SCE. The redox peak currents gradually increased with the number of potential scan indicating that conducting film formed and its thickness increased with potential cycling. All oxidation peaks shifted positively and reduction peaks negatively with cycles.

Figure 2 and 3 exhibit cyclic voltammograms of polyanilineat the concentration of 0.70mole/L and 0.50 mole/L of sulfosalicylic acid in water respectively. Redox peak currents decreased with the decrease of salt concentration. The middle redox peaks did not appear when the potential scan did not exceed 0.85 V as shown in the Figure 4.

The middle peak current is attributed to the formation of a toxic cancer causing agent benzidine by partial degradation of the film. The formation of a toxic compound is avoided in the electrochemical synthesis of aniline if anodic reaction is restricted to 0.80 V. Figure 4 also shows that the redox peak current increases with increasing scan rates suggesting that redox reaction is reversible

Cyclic voltammograms of polyaniline film in the absence of aniline:

The green polyaniline film was deposited on the platinum micro electrode. The film was washed several times with distilled water and then immersed in an aniline free aqueous solution of sulfosalicylic acid (1.0 mole/L). The cyclic voltammograms of the film were recorded and are shown in Figure 5. The multiple cycled voltammograms confirm that the film is fairly stable without any severe film degradation

Cyclic voltammograms for napthalene-2-sulfonic acid and anthraquinone-2-sulfonic acid:

The cyclic voltammograms of naphthalene-2-sulfonic acid and anthraquinone-2-sulfonic acid were also recorded (not shown here). They were quite similar to those observed in an aqueous solution of sulfosalicylic acid (Figure 1). The electro activity of polyaniline film depends on the nature of the anion. Here the difference is not pronounced because anions are the same. The size effect may be related simply to the diffusion co-efficient and mobility. In case of a large anion the loss of electro activity suggests that an anion ingress in the film becomes much more difficult. In order to examine this view polyacrylate anion, a large anion, was used in the formation of conducting polyaniline

Polyacrylate anion:

An attempt has been made to form polyacrylate anion doped polyaniline. No other kinds of anions were present in the system.

Figure 6 shows cyclic voltammograms of an aqueous solution of aniline (0.5ml) containing polyacrylic acid (30 wt%).Unlike smaller anions as discussed earlier, only oxidation peak current is seen with polyacrylate anion. The peak current vanished on the second scan. It seems that diffusion co-efficient, mobility and pore size exclusion effect becomes important in case of a big anion. The big anion has difficultly to penetrate in to the smallest pores of the polymer film so that complete oxidation is not achieved. Therefore this work suggests that such large anion is not suitable for use in the formation of conducting polymers

Conductivity of polyaniline:

Polyaniline films were prepared at a constant potential electrolysis at +0.85V vs SCE on a platinum sheet of area 3.5 cm² in the aqueous solution of aniline containing different supporting electrolytes. After electrolysis the anode covered with polymer films was washed several times with distilled water and finally repeatedly rinsed with acetone. The film was dried and its resistivity (ohm-cm) was measured on the platinum substrate at ambient temperature with a high vacuum tube voltmeter and the average value was noted (8) and the results are summarized in Table-1. Resistances of the films indicate that film conductivities are dependent on the nature of supporting electrolytes.

Table -1Conductivity of polyaniline at 0.85 V vs SCE Using a fixed concentration of aniline (0.20 mole/L) and Salt (1-0 mole/L) at 1 hr electrolysis Time.

Sl No.	Salt	Surface Resistance (Ohm cm)
1	Sulfosalicylic acid	250.
2	Napthalene-2-sulfonic acid	345.
3	Anthraquinone-2-sulfonic acid	290.

Mechanism:

The electrochemical polymerization of aniline takes place via a cation radical (9, 10, 11)The anodic oxidation of aniline generates a monomeric radical cation that couples to yield p-amino-diphenyle amine (I) as illustrated below

(I) isoxidized at anode near 0.22V vs SCE.

This oxidation process gives rise to redox current peaks near 0.22V as appear in the voltammograms of Figures 1, 2 and 3. The quinine dication (II) suffers further expulsion of 2H⁺ to form quinine diimine (III) which is anodic ally oxidized at a potential near 0.80Vvs SCE as it is seen from Figures1,2and3.

The redox current peaks near 0.80V is owing to this oxidation steps. The process is repeated with 2e^- and 2H⁺involved in each addition step to finally produce polyaniline of the following structure ¹⁰.

With x=0 signifying fully reduced form of polymer, x=1/2, polyemeraldine and x=1, completely oxidised polyaniline. The acid concentration of polymerizing solution and the degree of polymer oxidation controll the relative ratio of benzenoid and quinoid forms in polymer chain.

CONCLUSION:

Electrochemically conducting polyaniline films were prepared and Characterized cyclic voltammetrically. The multiple cycled voltammograms confirm that the film is fairly stable.

REFERENCES:

1. Skothiem T. A. et al, Editors, Handbook of Conducting polymers, Volume I and II, Marcel Dekker Inc, New York. 1986 also its 2^nd revised Ed 1998.

2. Nalwa H. S., Editor, Hand Book of organic conductive materials and polymers, Wiley, New York. 1997.

3. Aldissi M. A., Editor, Inherently Conducting Polymers. Kluwer Academic Publisher, London. 1992.

4. MacDiarmid A.G., “Synthetic Metal : A Novel role for organic polymers(Novel Lecture). Angewandte Chemie. Int. Edition. 40; 2001: 2581-2590.

5. Heegar A. J. Semiconducting and Metallic Polymers :The Fourth Generation of Polymeric materials. American Chemical Society. 105 (36); 2001: 8475-8491.

6. Bhadani Reena, Kumari M., Baranwal P. P. and Bhadani S. N. Electrochemical Polymerization of Pyrrole and Aniline on some Commodity Metals. Journal of Polymer Materials. 19 ; 2002 : 93-102.

7. Bhadani Reena, Barnwal P. P. and Bhadani S. N. Electrically Conducting Nylon Fibers by InSitu Electrochemical Formation of Polypyrrole. Journal of Polymer Materials.19; 2002: 259-264.

8. Bhadani S. N., Sen Gupta S. K., Gupta M. K. and Prasad. J. Electro-synthesis of Conducting Poly(p-phenylene) Applied Polymer Science. 47 ; 1993 : 1215-1218.

9. Bhadani S. N., Gupta M. K. and Sen Gupta S. K. Cyclic Voltammetry and Conductivity Investigation of Polyaniline J. Appl. Polym. Sci. 49; 1993: 397-403.

10. BalCrazak E. S. Editor. Electro polymerization. Intech Europe. Croatia. 2011:77-96.

11. R. Catranescu R., Bobirnac I., Crizan M., Cojocaru A. and Maior I. Studies Regarding Electrochemical Polymerization of Aniline in ionic liquid and Polymer Properties. Sci. Bull. 74 ; 2012 : 49-58.

Received on 19.05.2013 Modified on 12.06.2013

Asian J. Research Chem. 6(7): July 2013; Page 637-640

Sulfosalicylic acid