**3.1 Electrochemical synthesize of polymers**

The electropolymerization of pyrrole was performed by cyclic voltammetry in the potential range of -100 to 900 *mV* through 15 scans. As shown in figure 1, at the first scan, there is an anodic peak at ca. 800 *mV*. By continuing electropolymerization through second scan, another anodic peak was observed at 550 *mV* indicating formation of polypyrrole. After the formation of black colored polymer film on the GC electrode surface, the electrode was taken out from electrochemical cell and was washed with acetonitrile. For the resulted polymer, the cyclic voltammograms at various scan rates were shown in figure 2 indicating a quasi-reversible behaviour.

Fig. 1. Cyclic voltammograms of 7.4×10-3 M pyrrole in 0.1 M LiClO4 /CH3CN electrolyte at scan rate 50 mV/s vs. Ag/AgCl

The cyclic voltammetry investigations of SNS were carried out in the potential range of -400 to 1500 *mV* (Fig. 3). At the first scan two anodic peaks at ca. 570 and 1300 *mV* were observed resulting from the oxidation of SNS. In the backward scan from 1500 to -400 *mV*, there is one peak indicating a quasi-reversible reaction. At the second scan, a new anodic peak current was observed indicating formation of the electroactive poly(SNS) with an ionic structure. As shown in figure 3, after 7 scans, the second anodic peak at 1300 mV was eliminated. The cyclic voltammogram of poly(SNS) in the potential ranges between -400 to 1000 *mV* at various scan rates was shown in figure 4.

Electrosynthesis and Characterization

scan rates

of Polypyrrole in the Presence of 2,5-di-(2-thienyl)-Pyrrole (SNS) 123

Fig. 4. Cyclic voltammograms of poly(SNS) in 0.1 M LiClO4 /CH3CN electrolyte at various

Fig. 5. Cyclic voltammograms of Py-SNS (100:1 mole ratio) in 0.1 M LiClO4 /CH3CN

electrolyte at scan rate 50 mV/s vs. Ag/AgCl

Fig. 2. Cyclic voltammograms of poly(Py)in 0.1 M LiClO4 /CH3CN electrolyteat various scan rates

Fig. 3. Cyclic voltammograms of 7.4×10-5 M SNS in 0.1 M LiClO4 /CH3CN electrolyte at scan rate 50 mV/s vs. Ag/AgCl

Fig. 2. Cyclic voltammograms of poly(Py)in 0.1 M LiClO4 /CH3CN electrolyteat various

Fig. 3. Cyclic voltammograms of 7.4×10-5 M SNS in 0.1 M LiClO4 /CH3CN electrolyte at scan

scan rates

rate 50 mV/s vs. Ag/AgCl

Fig. 4. Cyclic voltammograms of poly(SNS) in 0.1 M LiClO4 /CH3CN electrolyte at various scan rates

Fig. 5. Cyclic voltammograms of Py-SNS (100:1 mole ratio) in 0.1 M LiClO4 /CH3CN electrolyte at scan rate 50 mV/s vs. Ag/AgCl

Electrosynthesis and Characterization

that of poly(Py).

poly(Py-SNS)

6.

of Polypyrrole in the Presence of 2,5-di-(2-thienyl)-Pyrrole (SNS) 125

Figure 7 presents the plot of anodic peak currents vs. different scan rates for obtained polymers. These curves show that the slop for poly(Py) and poly(Py-SNS) increases from 2.83 to 12.37 *mAs*.*mV-1*. These results indicated considerable increase in the electroactivity and rate of electropolymerization of polypyrrole in the presence of a small amount of SNS compared to the those of polypyrrole and poly(SNS). According to extracted data from cyclic voltammetries of polymers (see Table1), it can be seen that at various scan rates, EPa for poly(Py-SNS) is less than those two for other polymers, but ipa for former polymer is more than those for two others. In other words, the conductivity of SNS included polypyrrole is better than polypyrrole alone and poly(SNS). Also, it is evident that at scan rates of less than 50 *mV.s-1*, ∆Ep for poly(Py-SNS) is lower those for than two others, indicating improvement of redoxability for poly(Py-SNS) in comparison with poly(Py) and poly(SNS). At scan rate of 50 mV.s-1 the redoxability of poly(Py-SNS) is relatively similar to

The overall scheme of electrosynthesis of polypyrrole in the presence of SNS as shown Fig.

Fig. 7. Plots of anodic peak currents vs. scan rates for (a) poly(SNS), (b) poly(Py), (c)

During the electropolymerization of pyrrole in the presence of the SNS (7.4 *mmole*: 0.074 *mmole*) two anodic peaks appeared at 570 and 800 *mV*. These peaks are due to the oxidation of SNS and pyrrole, respectively. In addition, the anodic peak at 570 *mV* was absent during electropolymerization of pyrrole without SNS (Fig. 5). Because of conjugated backbone of SNS, the oxidation potential of this monomer is less than pyrrole. The cyclic voltammogram for the resulted polymer in various scan rates showed a relatively reversible behaviour (Fig. 6).

Fig. 6. Cyclic voltammograms of poly(Py-SNS) in 0.1 M LiClO4 /CH3CN electrolyte at various scan rates

During the electropolymerization of pyrrole in the presence of the SNS (7.4 *mmole*: 0.074 *mmole*) two anodic peaks appeared at 570 and 800 *mV*. These peaks are due to the oxidation of SNS and pyrrole, respectively. In addition, the anodic peak at 570 *mV* was absent during electropolymerization of pyrrole without SNS (Fig. 5). Because of conjugated backbone of SNS, the oxidation potential of this monomer is less than pyrrole. The cyclic voltammogram for the resulted polymer in various scan rates showed a

N S

H \* <sup>n</sup> <sup>m</sup> <sup>N</sup>

N S

H

Fig. 6. Cyclic voltammograms of poly(Py-SNS) in 0.1 M LiClO4 /CH3CN electrolyte at

S

\* <sup>N</sup> <sup>S</sup>

H

n>>m

relatively reversible behaviour (Fig. 6).

SNS **:**

SNS


H

various scan rates

Figure 7 presents the plot of anodic peak currents vs. different scan rates for obtained polymers. These curves show that the slop for poly(Py) and poly(Py-SNS) increases from 2.83 to 12.37 *mAs*.*mV-1*. These results indicated considerable increase in the electroactivity and rate of electropolymerization of polypyrrole in the presence of a small amount of SNS compared to the those of polypyrrole and poly(SNS). According to extracted data from cyclic voltammetries of polymers (see Table1), it can be seen that at various scan rates, EPa for poly(Py-SNS) is less than those two for other polymers, but ipa for former polymer is more than those for two others. In other words, the conductivity of SNS included polypyrrole is better than polypyrrole alone and poly(SNS). Also, it is evident that at scan rates of less than 50 *mV.s-1*, ∆Ep for poly(Py-SNS) is lower those for than two others, indicating improvement of redoxability for poly(Py-SNS) in comparison with poly(Py) and poly(SNS). At scan rate of 50 mV.s-1 the redoxability of poly(Py-SNS) is relatively similar to that of poly(Py).

The overall scheme of electrosynthesis of polypyrrole in the presence of SNS as shown Fig. 6.

Fig. 7. Plots of anodic peak currents vs. scan rates for (a) poly(SNS), (b) poly(Py), (c) poly(Py-SNS)

Electrosynthesis and Characterization

**impedance spectroscopy (EIS)** 

SNS.

Table 2.

Y0 : CPE Admittance n : CPE exponent

R1(Ω.cm2 Sample Rs(Ω) )

Rs : uncompensated resistance of the solution

poly(Py-SNS) on the GC electrode in 3.5% NaCl

of Polypyrrole in the Presence of 2,5-di-(2-thienyl)-Pyrrole (SNS) 127

The CV experiments were performed to study the effect of SNS in the polypyrrole film in the electron transfer of ferro/ferricyanide redox system. Figure 8 shows the CV of electron transfer ferro/ferricyanide redox on different modified GC electrodes with poly(Py), poly(SNS), and poly(Py-SNS). This figure indicates that the electron transfer of ferro/ferricyanide on polypyrrole in the presence of the SNS is more feasible than that of polypyrrole alone, because the conductivity of polypyrrole increases in the presence of

**3.2 Evaluation of conductivity and electrochemical behaviour by electrochemical** 

Electrochemical impedance spectroscopy (EIS) is a measurement technique which allows for a wide variety of coating evaluations. EIS is an effective method to probe the interfacial properties of surface-modified electrodes. EIS has been used to characterize the electrical properties of the electropolymerized films [Kiani et al., 2008a, 2008b]. The electrochemical behaviour of polypyrrole changes in the presence of SNS .In order to choose a suitable electrical equivalent circuit for EIS experimental data fitting, one must take in consideration the physicochemical picture of the system under study. In other words, each element of the equivalent circuit should have a physicochemical aspect attributable to it. In the model circuit chosen, Rs presents the uncompensated resistance of the solution between working and reference electrode. CPE1 and R1 stand for the dielectric and resistive characteristics of the conductive polymer on the GC electrode, respectively. In this case R1 is a reverse measure of polymer conductivity. CPE2 and R2 show the capacitance and resistance of the polymer/GC interface. As is evident from the high values of R2 for all as well as the Nyquis plot of studied polymers, due to the fact that polymer layer is impermeable to the ionic charge carrier species, low frequency behaviour of the polymer/GC interface tends to be of capacitive nature. Again more evidence for this fact is reflected in the values of nCPE2 which for all studied samples is not very far than unity. Our main aim by EIS studies was to determine the polymer layer bulk resistivity (or its reciprocal i.e conductivity). In the selected equivalent circuit R1 corresponds to this parameter. The studied electrical parameters were calculated using Zview(II) software. All fitting results are presented in

CPE2

CPE2 n

CPE1 Y0

CPE1 n

R2(Ω.cm2) Y0

Poly(Py) 43.86 5.81 2.5E4 8.0E-4 0.64 8.3E-4 0.98 Poly(SNS) 38.94 2282 7.2E4 1.3E-4 0.72 1E-4 1.00 Poly(Py-SNS) 30.50 1.46 7.2E4 0.67 0.58 0.01 0.98

Table 2. Impedance parameters obtained by fitting the EIS data of poly(Py), poly(SNS) and

R1 and CPE1 : dielectric and resistive characteristics of the conductive polymer R2 and CPE2 : capacitance and resistance of the polymer/ GC interface


EPa: potential of anodic peak EPc: potential of catidic peak iPa: anodic peak currents

Table 1. Obtained date for poly(Py), poly(SNS) and poly(Py-SNS) at scan rates 10 and 50 mV/s

Fig. 8. Cyclic voltammograms of (a) poly(Py-SNS), (b) poly(SNS), and (c) poly(Py) on the GC electrode in 1 M H2SO4 and 1 mM Fe(CN)6 4-/ 3- redox system at 50 mV/s scan rate

Table 1. Obtained date for poly(Py), poly(SNS) and poly(Py-SNS) at scan rates 10 and 50

Fig. 8. Cyclic voltammograms of (a) poly(Py-SNS), (b) poly(SNS), and (c) poly(Py) on the GC

electrode in 1 M H2SO4 and 1 mM Fe(CN)6 4-/ 3- redox system at 50 mV/s scan rate

Epa(V)

Epc(V)

∆Ep(V)

ipa(μA)

mV/s

EPa: potential of anodic peak EPc: potential of catidic peak iPa: anodic peak currents

Scan rates Poly(Py) Poly(SNS) Poly(Py-SNS)

10 0.450 0.715 0.052 30 0.500 0.750 0.075 50 0.460 0.841 0132

10 -0.123 0.405 -0.240 30 -0.070 0.277 -0.329 50 -0.050 0.201 -0.431

10 0.573 0.310 0.292 30 0.570 0.473 0.404 50 0.510 0.640 0.563

10 32 19 120 30 94 33 380 50 103 47 620 The CV experiments were performed to study the effect of SNS in the polypyrrole film in the electron transfer of ferro/ferricyanide redox system. Figure 8 shows the CV of electron transfer ferro/ferricyanide redox on different modified GC electrodes with poly(Py), poly(SNS), and poly(Py-SNS). This figure indicates that the electron transfer of ferro/ferricyanide on polypyrrole in the presence of the SNS is more feasible than that of polypyrrole alone, because the conductivity of polypyrrole increases in the presence of SNS.
