Deposition of copolymer of aniline with o-chloro aniline by pulse potentiostatic method and characterization

Materials Chemistry and Physics 69 (2001) 62–71

Deposition of copolymer of aniline with o-chloro aniline by pulse potentiostatic method and characterization V. Rajendran a , S. Prakash a , A. Gopalan a,1 , T. Vasudevan a , Wei-Chih Chen b , Ten-Chin Wen b,∗ a

b

Department of Industrial Chemistry, Alagappa University, Karaikudi 630 001, India Chemical Engineering Department, National Cheng Kung University, Tainan, Taiwan 701, ROC Received 16 February 2000; received in revised form 15 May 2000; accepted 18 May 2000

Abstract Electrocopolymerization of aniline (ANI) with o-chloro aniline (OCA) was carried out using pulse potentiostatic method (PPSM). The polymeric films were grown on the surface of working electrode for various experimental conditions. Cyclic voltammograms of the deposited films were recorded under all the conditions. A growth equation was deduced as Growth = k[ANI][OCA]−1 , where k is the rate constant for copolymer deposition, correlating the charge associated with the redox processes of the polymeric films with the experimental conditions. Formation of the copolymer with the mixture of monomers in the feed was ascertained by a critical comparison of the results obtained with the polymerization of the individual monomers, ANI and OCA, independently. The surface parameters like surface excess and thickness of the films were evaluated. Further characterization of the copolymer, poly(ANI-co-OCA) was done through conductivity measurements, FTIR and UV–VIS spectroscopic techniques. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pulse potentiostaic; Deposition; Copolymer; Growth equation; Characterization

1. Introduction The increasing interest and use of conducting polymers as materials to have applications in rechargeable batteries. electromagnetic interference shielding, electrochromic display devices and possibility of extension in molecular engineering and space craft design in future necessitates several researches to work in this field [1–4]. Among these, polyaniline, PANI, receives greater attention owing to the combinational characteristics like electrochromic effects [5], simplicity for doping [6], well-behaved electrochemistry [7] and good stability in air and water [8]. The associated difficulty of PANI for the application due to restricted solubility in common organic solvents has been tried to overcome by several researchers by different approaches [9–11]. In the process of obtaining novel conducting polymers with improved mechanical strength and processability, copolymerization becomes an effective route to perform with judicious choice of suitable co-monomers [12]. In fact, copolymerization provides a simple way of preparation of new polymers with inbuilt tailor-made properties suitable for applications [13–15]. An extension of this idea makes sev∗

Corresponding author. Present address: Chemical Engineering Department, National Cheng Kung University, Tainan, Taiwan 701, PR China. 1

eral copolymerization studies with aniline as one monomer. Besides this, the polymers of substituted derivatives of aniline exhibit better solubility than aniline itself and hence incorporation of these units in the copolymer can have this additional effect. It is noticed that the presence of halogen atom in the ring of aniline units can produce polymers with better solubility in common organic solvents [16]. Copolymers of aniline with metanilic acid [17], o-aminobenzonitrile [17], o-anisidine [18,19] have been successfully synthesized and characterized. For the preparation of conducting polymers, electrochemical method is often preferred over chemical method due to its intrinsic possibility of simultaneous characterization. Electrochemical oxidative polymerization was tried by either potentiostatically [20], or galvanostatically [21] or potential cycling method [22]. Tsakova and Milchev [23,24] deposited PANI films by applying periodic, cathodic, and anodic rectangular pulses using a solution containing aniline. They reported [23,24] activated growth of PANI film deposition on platinum working electrode with the above method, namely, pulse potentiostatic method (PPSM). Interestingly, PPSM offers several operational parameters like switching potential, cathodic and anodic pulse width (Pw ), pulse number (Pn ) and electrolysis time (texp ) which can be properly utilized to deposit good films on the surface of the working electrode and whose electrochemical characteristics can subsequently be

0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 8 3 - 7

V. Rajendran et al. / Materials Chemistry and Physics 69 (2001) 62–71

identified. For the evaluation of electrochemical characteristics, cyclic voltammetry can be used. With this combined possibility it becomes possible to bring about the growth behavior of the deposited films relating the operational parameters. Such an attempt was not made in the previous works [23,24]. Besides that, PPSM deposited films can be best tested for copolymer formation when mixture of monomers are used for the polymerization. In our previous reports cyclic voltammetry was used as a diagnostic tool to identify the copolymer formation on IrO2 coated platinum electrode when mixture of para-phenylene diamine [25,26] or 2,5-diamino benzene sulphonic acid [27] and aniline were used. These copolymers were then successfully prepared and characterized by X-ray photoelecron spectroscopy [25,27]. This paper aims to use PPSM for the polymerization of aniline (ANI) with o-chloro aniline (OCA) and to establish the growth behavior of deposited film. The possibility of copolymer formation would be analyzed by depositing the polymeric films using PPSM on the surface of the working electrode with mixture of monomers in the initial feed, employing the various tunable parameters of PPSM and evaluating the characteristics by using cyclic voltammetric technique. Besides, the changes in the growth characteristics of the individual homopolymers are also monitored and compared to decide on the copolymer formation. Chemical synthesis of polymer/copolymer and consequent characterization are also done to substantiate the polymer/copolymer structure.

2. Experimental O-chloro aniline (Fluka, USA) was used as received. Aniline (E. Merck) was distilled and used. The other chemicals used were of E. Merck products. All solutions were prepared from doubly distilled water. 2.1. Electrochemical copolymerization by pulse potentiostatic method The electrochemical copolymerization was carried out by using BAS 100A-Electrochemical analyzer with a three-electrode cell assembly and employing the mixture of aniline (ANI) and o-chloro aniline (OCA) of varying feed ratios (0.2, 0.4, 0.5, 0.6, 0.8) while keeping the total molar concentration as 100 mM in 0.5 M H2 SO4 solution. A platinum and Ag/AgCl, as working and reference electrodes, respectively, were used. By applying specific cathodic (Ec ) and anodic (Ea ) potential pulses with cathodic time interval (tc ) and anodic time interval (ta ) for the total experimental time (texp ), polymeric films were deposited on the working electrode (Scheme 1). In the above scheme, Pn is the pulse number which represents the number of times the potential pulses are given between the switching potential limits Ea and Ec ·Pw is the pulse width which represents the static duration to reside in

63

Scheme 1. A schematic diagram for the pulse program.

the upper or lower limits of the potentials. The static duration for the anodic limit is the anodic pulse width which is designated as ta Similar definition is applicable for the tc . The experimental time which is represented by texp is Pn ta or Pa tc (since t a = t c in our experiment). 2.2. Characterization by cyclic voltammetry The polymer coated film electrode was then placed in a monomer free background electrolyte (0.5 M H2 S04 ) and equilibrated by cycling the potential between 0.0 and 0.8 V till a constant cyclic voltammogram (CV) pattern without any appreciable change in the peak potential and peak current was obtained. This equilibration was noticed within a few potential cycles and which is indicative of stable nature of the film. The CVs of the stabilized copolymeric films were then recorded. Electrochemical homopolymerizations of ANI and OCA were also performed independently under the pulse conditions and the CVs of the homopolymer coated film electrode was recorded as mentioned earlier. 2.3. Chemical polymerization Copolymer of ANI and OCA was prepared by oxidative polymerization of mixtures of monomers in a fixed feed ratio (1:1) with potassium peroxodisulphate (PDS) as an oxidant. A typical procedure for the preparation of poly(ANI-co-OCA) is outlined below. A mixture of OCA and ANI was prepared by dissolving 1.276 gm of OCA (50 mM) and 0.984 gm of ANI (25 mM) in 200 ml of 0.5 M H2 SO4 . It was cooled well below 273 K using freezing mixture. A pre-cooled solution of PDS (60 mM) containing 1.40 gm in 80 ml of 0.5 M H2 SO4 was then slowly added drop wise to the mixture with stirring. The mixture was further stirred for an hour in the freezing mixture by keeping the temperature as 5◦ C. An emerald green colored precipitate was obtained. The resulting precipitate was filtered through a sinteredglass crucible and washed with 0.5 M H2 SO4 till the filtrate became colorless. The acid doped copolymer was then dried under dynamic vacuum at room temperature. The homopolymer of ANI and OCA were also prepared adopting the similar procedure.

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2.4. Other characterizations The conductivities of the chemically synthesized homopolymer and copolymer samples of ANI and OCA were determined through four-probe resistivity set-up (Concord Instruments, India). FTIR spectra of the chemically synthesized homopolymer and copolymers were recorded using Bruker IFS 66v FTIR spectrophotometer in the region of 500–4000 cm−1 using KBr pellets. The homopolymers (poly(aniline) and poly(o-chloro aniline) were) and the copolymer was tested for their solubility. All the polymers were found to be soluble in acetone, N-methyl pyrrolidone and dimethylformamide (DMF). Comparatively, polyaniline was found to be a difficult soluble and all the polymers showed solubility of 0.05 mg/ml in DMF (without presence of any insoluble residue) which was sufficient to record the UV–VIS spectrum of these three polymers in comparative concentration. UV–VIS spectrum of the chemically synthesized homopolymers and copolymers were recorded in DMF solvent (solution concentration was 0.05 mg/ml) using Shimadzu UV–VIS spectrophotometer. 3. Results and discussion In the present investigation, PPSM was used for the copolymerization of mixture of the monomers ANI and OCA. This electropolymerization was done by keeping the fixed pulse parameters, Pn as 1000 and Pw as 500 ms. During the application of pulse potential, the color of the medium did not change and also a visible appearance of green colored deposit on the electrode surface was noticed. These observations in conjunction indicated the absence of dissolution of oligomeric products and deposition of good adherent films on the surface of working electrode on using PPSM. The possibility of copolymer formation was carefully examined by a close comparison of the results obtained through the electropolymerization of ANI and OCA independently and also with mixture of monomers, ANI and OCA. The results obtained on the electrochemical characteristics of the polymeric films deposited by PPSM by taking ANI and OCA in various feed ratios and also ANI and OCA with the respective concentrations while keeping the pulse parameters of Pn and Pw as 1000 and 500 ms, respectively, are used towards this purpose. 3.1. Dependence of peak potential and peak current on feed ratio of ANI PPSM was used to deposit the polymeric films by employing feed ratio of ANI as 0.2, 0.4, 0.5, 0.6, and 0.8 by taking the following ratios of monomers of ANI to OCA as 20:80, 40:60, 50:50, 60:40 and 80:20 mM, respectively, in the initial feed of the electropolymerization.

Fig. 1. Cyclic voltammograms of polymeric films deposited by PPSM with different feed ratios of aniline. PPSM: P n = 1000, P w = 500 ms, E c /E a = 0.0/1.0 V; CV of film: E c /E a = 0.0/1.0 V, scan rate = 100 mV/s. Feed ratio of ANI: 0.2 (a); 0.4 (b); 0.5 (c); 0.6 (d); 0.8 (e).

The polymeric films are characterized for the electrochemical properties by recording the stabilized cyclic voltammograms (CVs) of the deposited films (Fig. 1) using 0.5 M H2 SO4 as monomer-free background electrolyte. The results are carefully compared to obtain an insight about the differences in the electrochemical behavior of these films in comparison with the PANI and poly(o-chloro aniline), POCA films deposited by PPSM. An attempt was made to perform electrochemical polymerization of OCA by PPSM on platinum surface by keeping [OCA] = 50 mM, P n = 1000 and P w = 500 ms in the switching potential between 0.0 and 1.20 V versus Ag/AgCl. A potential window with a higher anodic limit was required, as the oxidation potential of OCA was far higher than DPA. One anodic and one cathodic peak appears with Epa (I) at 607 mV and Epc (I) at 562 mV. However, the peak current values were found to be very low. On increasing the value of Pn above 2000, no increase in the peak current was noted. Thus, a limited growth of insulating film of POCA was noticed. Further, if the potential window was kept as between 0.0 and 0.8 V as maintained for polymerization of mixture of monomers, no POCA deposition was observed. The peak potential and peak current values of the redox processes corresponding to the polymeric films deposited with different feed ratio of ANI are now taken for analysis (Fig. 1). The CVs showed three anodic peaks Epa (I), Epa (II) and Epa (III) in the ranges of 307–317, 662–678 and 891–906 mV, respectively. Also the three cathodic peaks, Epc (I), Epc (II) and Epc (III) were found to be in the ranges of 208–221, 611–627 and 858–874 mV, respectively (Table 1). The peak potentials of these three redox processes can now be compared with PANI films deposited under the same conditions of [ANI], Pn and Pw (Table 1). The peak potentials of the PANI films with [ANI] as 20 mM can now be compared with the peak potentials of the polymeric films deposited with the same concentration

V. Rajendran et al. / Materials Chemistry and Physics 69 (2001) 62–71

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Table 1 Electrochemical polymerization of ANI with OCA by PPSM electrochemical characteristics of polymeric films by CVa [ANI] (mM)

Feed ratio of ANI

Peak potential Ep (mV) Copolymer film

PANI film

Epa

20 40 60 80

0.2 0.4 0.6 0.8

Epc

Epa

Epc

I

II

III

I

II

III

I

II

III

I

II

III

317 312 304 301

678 673 665 662

906 902 893 891

221 218 211 208

627 623 614 611

874 869 861 858

292 287 281 276

654 651 646 643

896 894 891 887

153 151 147 144

598 597 593 591

846 843 838 834

a Peak potential and peak current values for various feed ratio of ANI. PPSM: P = 1000, P n w = 500 ms, E c /E c = 0.0/1.0 V; CV of film: E c /E a = 0.0/1.0 V, scan rate = 100 mV/s.

of ANI and in the presence of [OCA] = 80 mM. It is noteworthy to witness that all the three redox processes of the polymeric films deposited with the mixtures of monomers were found to be totally different and shifted probably than the deposited PANI films under identical conditions. The anodic peaks of the polymeric films were found to be shifted to the higher anodic potentials in comparison with PANI films. The polymeric films with the mixture of monomers did not show any resemblance with POCA films. Of course. no independent POCA deposition was noticed in the potential window in which copolymerization was performed. These differences were also noticed when the concentrations of aniline were kept as 40, 50, 60 and 80 mM in the homopolymerization and in the copolymerization (Table 1). This gives an information that in the presence of mixture of monomers, a new polymeric film, a copolymer, was deposited in contrast to simple PANI films. In the light of copolymer formation the peak current values of the films can also be viewed. For this purpose, ipa (I), ipa (II) and ipa (III) values of the polymeric films deposited here are taken into account for comparison to the corresponding values noted in the PANI films. These values were found to vary from each other. For example, the films deposited with [ANI] as 20 mM in the homopolymerization and same concentration of ANI in the feed of the copolymerization is given. ipa (I)

ipa (II)

ipa (III)

PANI films (␮A) 364

76

178

The copolymer film 82

16

32

Interestingly, ipa (I), ipa (II) and ipa (III) of the present polymeric film is very much lower than the ipa (I), ipa (II) and ipa (III) of PANI film. This observation also suggests a new polymer material namely, a copolymer of ANI and OCA with entirely new characteristics than PANI film formation. On increasing [ANI], the peak current values were found to increase steadily (Fig. 2). This can be observed from the

plots of ipa (I), ipa (II), ipa (III), ipc (I), ipc (II) and ipc (III) versus [ANI] (Fig. 2). The E1/2 values of these polymeric films deposited with different feed ratios of ANI are given (Table 2). E1/2 values of PANI films deposited in the absence of OCA are also found to be different (Table 2). 3.2. Electrochemical characterization of poly(ANI-co-OCA) films deposited by PPSM by cyclic voltammetry The charge associated with the anodic and cathodic portions of the CVs of the copolymeric films were used. These were determined by graphical integration and after suitable subtraction of the background charges. 3.3. Dependence of charge on in the feed ratio of ANI The total anodic charge Qa and total cathodic charge Qc of the polymeric films deposited in the presence of mixture of monomers are compared with the three redox processes of PANI films produced under identical conditions. The charge values in the copolymerization conditions are found to be much lower in comparison with the PANI films. The ratio of Qa to Qc was found to be close to unity and hence Qa values of the films are considered here for further discussion. The Qa values were found to increase with increase in [ANI]. The POCA film was reported to be good insulator by earlier workers [28] with a trend of decreasing peak current values with increase in the film thickness of the POCA. The incorporation of OCA units in the copolymeric structure may decrease the electoactivity causing lower Qa values in this study. 3.4. Deducing the growth equation A growth equation correlating the amount of polymer deposited and the experimental conditions maintained in PPSM studies for the deposition of the copolymeric film would be deduced by using the charge values Qa associated with deposition. The following equation may be thought initially.

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Fig. 2. Effect of peak current values on the concentration of ANI: Ipa (I) (A), Ipa (II) (B), Ipa (III) (C), Ipc (I) (D), Ipc (II) (E), Ipc (III) (F) vs. [ANI]. PPSM: P n = 1000, P w = 500 ms, E c /E a = 0.0/1.0 V; CV of film: E c /E a = 0.0/1.0 V, scan rate = 100 mV/s.

Growth = k 0 [ANI]x [OCA]y Pn · Pw .

(1)

In the copolymerization studies Pn and Pw were kept as constant and using this Eq. (1) can be rewritten as Growth = k[ANI]x [OCA]y

(1a)

where k = k 0 P n · P w , the rate constant for the deposition of the copolymer.

The values of x and y are to be determined experimentally. The procedure adopted for the polymerization of ANI by earlier workers [29] could not be directly applied for determining the exact dependence of [ANI] and [OCA] on growth, since both the concentration terms, [ANI] and [OCA] are varying simultaneously while maintaining the feed ratios. For PANI films deposited on platinum surface Stilwell and Park [29] deduced growth equation, which can not be used

Table 2 Electrochemical characteristics of poly(ANI-co-OCA) films deposited by PPSMa [ANI] (mM)

Feed ratio of ANI

Peak potential Ep (mV) Copolymer film

PANI film peak

a E1/2

20 40 60 80

0.2 0.4 0.6 0.8

b E1/2

a E1/2

b E1/2

I

II

III

I

II

III

I

II

III

I

II

III

182 179 173 171

607 604 596 591

813 809 801 796

376 374 369 367

738 736 728 736

954 952 943 942

143 140 l36 134

581 578 573 570

768 764 760 757

352 349 345 343

687 684 681 678

934 931 928 926

a Half wave potential values for various feed ratio of ANI. PPSM: P = 1000, P = 500 ms, E /E = 0.0/1.0 V; CV of film: E /E = 0.0/1.0 V, n w c a c a scan rate = 100 mV/s.

V. Rajendran et al. / Materials Chemistry and Physics 69 (2001) 62–71

here. Hence, a new method is evolved here to deduce the growth equation for copolymer deposition. By substituting any two of the experimentally determined anodic charge values Qa corresponding to two different feed ratios of the monomer at constant Pn and Pw , two equations were set up which were used to arrive at two simultaneous equations involving x and y terms. Similarly by choosing different sets of experimental values of Qa corresponding to different sets of feed ratio of monomer keeping Pn and Pw as 1000 and 100, respectively, several pairs of simultaneous equations in x and y were set up. Solving these pairs of simultaneous equations for x and y, the average values of x and y were arrived nearly as 1.0 and −1.0, respectively. In order to determine the exact dependence of [ANI] and [OCA], the logarithmic form of Eq. (1) was used. To find out the dependence of [ANI], the double logarithmic plot of log(Qa [OCA]) versus log[ANI] was drawn for fixed values of Pn and Pw (figure not given). This plot was found to be linear with unit slope. This confirms the first order dependence of the film growth. This was further ascertained by plotting the direct plot of Qa [OCA] versus [ANI] (Fig. 3) and this plot also showed linearity with negligible intercept. Similarly, the dependence of growth on [OCA] was deduced by drawing double logarithmic form plot of

67

log(Qa /[ANI]) versus log[OCA] (figure not given) and the plot was found to be linear with the slope of −1.0. This indicates the negative first order dependence of [OCA] on growth. This was further confirmed by drawing the direct plot of Qa /[ANI] versus [OCA]−1 (Fig. 3) and this was a linear one with negligible intercept. Taking into account of the values of x and y, the growth equation was deduced as Growth = charge(mC) = k[ANI][OCA]−1 .

(2)

Using slopes of the plots Qa [OCA] versus [ANI] (Fig. 5) and Qa /[ANI] versus [OCA]−1 (Fig. 6), k value was found to be 6.75 × 10−8 and 6.72 × 10−8 mC mM−1 , respectively. The closeness of the calculated k values justifies the process of deducing the growth equation as deduced as Eq. (2). 3.5. Determination of surface parameters Γ a and d of these deposited films were calculated by employing Eq. (3). Qa =nFAΓa = nFACd

(3)

where C is the film concentration of redox centers, Γ a represents surface coverage of redox centers, A the area of the electrode and d the thickness of the film.

Fig. 3. Effect of [ANI] and [OCA] on charge for poly(ANI-co-OCA) films deposited by PPSM. PPSM: P n = 1000, P w = 500 ms, E c /E a = 0.0/1.0 V; CV of film: E c /E a = 0.0/1.0 V, scan rate = 100 mV/s.

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Table 3 Electrochemical behavior of poly[ANI-co-OCA] films deposited by PPSMa Feed ratio of ANI

0.2 0.4 0.6 0.8

Charge, Qa (mC)

0.080 0.227 0.495 0.358

d (␮m)c Γ × 10−1 −2 b (mol cm )

0.330 0.936 2.042 5.602

0.194 0.522 1.074 2.743

Ca (M)

1.70 1.79 1.90 2.04

ra (M−1 )

C a ra d

FWHM (V) I

II

III

I

II

III

I

II

III

0.057 0.059 0.062 0.064

0.086 0.089 0.093 0.097

0.108 0.110 0.114 0.117

0.582 0.542 0.483 0.444

0.012 −0.046 −0.125 −0.203

−0.419 −0.458 −0.537 −0.596

0.342 0.303 0.254 0.218

0.007 −0.026 −0.066 −0.099

−0.246 −0.256 −0.283 −0.292

a Surface parameters for various feed ratio of ANI. PPSM: P = 1000, P = 500 ms, E /E = 0.0/1.0 V; CV of film: E /E = 0.0/1.0 V, scan n w c a c a rate = 100 mV/s. b Q /nFA. a c Q V/zrF. a d 1.7 − (0.47∆E p/2 nF/RT).

3.6. Determination of surface excess The Γ a was calculated for all the copolymer films using the values of Qa and this was found to increase with increase in the molar feed ratio of ANI ranging from 0.33 × 10−7 to 5.6 × 10−7 mol cm−2 for the various films deposited. Comparing these Γ a obtained for the corresponding PANI films, it is inferred that Γ a value is very low for copolymeric films. It is also found that especially films produced with high feed ratio of OCA, the Γ a values are very low. 3.7. Determination of film thickness Calculations were made using Eq. (3) to determine the d of the polymeric films [27]. Molar volume, V was calculated considering the weighed average of the molar masses of the two monomers, ANI and OCA. The evaluated d value of the copolymeric films varied from 0.19 to 2.74 ␮m. These values are also much lower than the d values of the PANI films deposited under otherwise identical conditions (Table 3). 3.8. Determination of interaction parameter Attempt was also made to evaluate the interaction parameter, ra from the value of full width at half maxima of redex peaks, FWHM [27]. Substituting the values of the Ca , ra was calculated and presented in Table 3. It is interesting to note the differences in the interaction parameters also infer change over of interaction due to the formation of different types of polymers in the homopolymerization and copolymerization, respectively. The wide variations in ra with the feed ratio of ANI indicates the random incorporation of the OCA units in the copolymer structure influencing the nature of interaction with electrodes.

The copolymers was synthesized by bulk electropolymerization at a mixture of solution containing the monomers in the feed ratio of 0.5–0.5 by keeping the potential as 0.8 V during electrolysis. An emerald green colored solid material was obtained in large quantities which was further processed for analysis. The copolymer was also synthesized using the oxidant potassium peroxodisulphate in acidic aqueous solution. 3.10. Characterization of poly(ANI-co-OCA) Using the suspension of the copolymer in acetone, the copolymer was cast as film on the surface of platinum electrode [30]. The copolymer film-coated electrode was then placed in monomer-free electrolyte (0.5 M H2 S04 ) and pre-conditioned. The CVs were recorded for the copolymeric film coated platinum electrode in 0.5 M H2 SO4 in the potential range of 0.0–l.0 V at a scan rate of l00 mV/s. The CVs obtained for electrode coated with chemically synthesized/bulk electro polymerized poly(ANI-co-OCA) were compared and which in turn was compared with electro deposited film with PPSM (Fig. 4). The CVs resemble each other possessing the characteristic pattern of three redox processes corresponding to the quasi-reversible processes at 0.3, 0.65 and 0.90 V versus Ag/AgCl, respectively. The parity observed amongst the CVs indicates that the samples of copolymers prepared in these different conditions are one and the same.

3.9. Bulk polymerization For the purpose of ascertaining the structure and characteristics of poly(ANI-co-OCA), copolymerization of ANI with OCA was carried out by potentiostaic and controlled chemical oxidation methods.

Fig. 4. Comparison of cyclic voltammograms of Pt/poly(ANI-co-OCA) modified electrodes: (A) chemically synthesized; (B) electrochemically deposited by PPSM.

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Fig. 5. FTIR spectrum of poly(ANI-co-OCA).

Fig. 6. UV–VIS spectra of polymers/copolymer in DMF (inset-visible region) (a) PANI; (b) poly(ANI-co-OCA); (c) POCA.

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3.11. Other characterization of poly(ANI-co-OCA) The conductivity of the copolymer sample was measured with four-probe technique. FTIR spectra and UV–VIS spectra have been recorded to get structural information of the copolymer.

and resulted good adherent film on the surface of platinum working electrode. The cyclic voltammograms of the films showed differences in contrast to poly aniline films. It was possible to establish a growth equation relating the polymerization condition. Chemical synthesis and subsequent characterization supported poly(anline-co-o-chloroaniline) formation.

3.12. Conductivity measurements The conductivity of the doped copolymer at room temperature was found to be 0.113 S cm−1 , which is low when compared to the doped PANI, 0.423 S cm−1 which reflects the decreasing order of conjugation in the copolymeric structure formed due to the incorporation of OCA units.

Acknowledgements The financial support of National Science Council of Republic of China (NSC 89-281l-E-006-0016) to one of the author (A. Gopalan) is gratefully acknowledged.

3.13. FTIR spectroscopy FTIR spectrum (Fig. 5) was recorded for the bulk electro polymerized sample of the copolymer. The strong absorption band at 3435 cm−1 is caused by N–H stretching mode of the secondary amine. The strong absorption band at 1609 cm−1 can be assigned to the bending mode of aromatic secondary amine [31–34]. The absorption band at 1495 cm−1 , is characteristic of the multiple bond stretching of the benzene ring [35]. The absorption bands at 848 and 880 cm−1 indicate that 1,2,4 tri substitution, head to head coupling is likely due to the para-orienting ability of the amino group [34,35]. Literature reveals that the C–Cl absorption is at 1053 cm−1 in the structure of 2-chloro-1,4-dihydrobenzoquinone [36]. 3.14. UV–VIS spectroscopy The UV–VIS spectra was recorded for the poly(ANI-coOCA) using a solution of the copolymer in DMF (Fig. 6). The spectra of the copolymer exhibits well defined peaks at 270, 340 nm and a broad band at 510 nm. These peaks may be assigned due to the ␲-␲* transition of the benzenoid rings in the polymeric backbone and polaronic excitation of the benzenoid to quininoid ring, respectively. A broad absorption band at around 780 nm which signifies the polaronic transition of the doped state for the copolymer [37]. The peaks were found to show shifts in terms of all the transitions corresponding to PANI and POCA (Fig. 9). The presence of peak at 270 nm indicates the incorporation of OCA moieties in the copolymer structure. The slight hypsochromic shifts of the peak from 282 nm of PANI to 270 nm of the copolymer perhaps reflect the decreasing order of conjugation due to steric hindrance of the Cl atom and this can also be attributed to the presence of OCA moieties in the polymeric backbone. This is in accordance with earlier reports [38,39]. 4. Conclusion Pulse potentiostatic method was used for the copolymerization mixture of monomers, aniline and o-chloroaniline

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