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In situ UV–visible spectroelectrochemical evidences for conducting copolymer formation between diphenylamine and m-methoxyaniline
Spectrochimica Acta Part A 59 (2003) 1937 /1950 www.elsevier.com/locate/saa
In situ UV visible spectroelectrochemical evidences for conducting copolymer formation between diphenylamine and m-methoxyaniline /
M. Thanneermalai a, T. Jeyaraman a, C. Sivakumar a, A. Gopalan a,*, T. Vasudevan a, T.C. Wen b b
a Department of Industrial Chemistry, Alagappa University, Karaikudi 630003, India Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Received 17 September 2002; accepted 8 November 2002
Abstract Electrochemical copolymerization of diphenylamine (DPA) with m -methoxy aniline (MA) was carried out in 4 M H2SO4 by cyclic voltammetry (CV). Cyclic voltammograms (CVs) of the copolymer films were recorded in monomerfree background electrolyte. In situ sepectroelectrochemical studies were carried out on an optically transparent electrode (Indium tin oxide (ITO) coated glass) in 4 M H2SO4 for different feed ratios of the comonomers. Constant potential and potential sweep methods were employed for performing polymerization. UV /visible absorption spectra were collected continuously and concurrently during the copolymerization in both the cases. The results from constant potential electropolymerisation indicated the formation of an intermediate with an absorption peak at 576 nm. Derivative cyclic voltabsorptogram (DCVA) was deduced from the results of cyclic spectrovoltammetry. The DCVA derived at 576 nm confirms the intermediates formed during the electrochemical copolymerization. The compositional changes of the two monomers in the copolymers with changes in feed composition of two monomers as predicted from in situ spectro electrochemical studies are evident from elemental analysis. A plausible copolymerization mechanism is suggested. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Electrocopolymerization; Cyclic voltammetry; Copoymer coposition; In situ spectroelectrochemistry; UV /visible spectroscopy
1. Introduction Polyaniline (PANI), has been regarded as one of the most versatile conducting polymers on the * Corresponding author. E-mail address: [email protected] (A. Gopalan).
basis of its environmental stability and its electrical, optical and electrochemical properties [1 /3]. However, the restricted processability arising from its low solubility in organic solvents is one of the drawbacks to be solved. In the process of obtaining soluble conducting polymers Yue and Epstein [4] have synthesized the self-doped PANI through
1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-1425(02)00441-9
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the reaction between an emeralddine base form of PANI and fuming sulfuric acid. Several PANI type aqueous soluble conducting polymers are then synthesized by introducing the sulphonic acid group into PANI backbone [5,6]. Likewise, treating the leuco-emeralddine base of PANI with 1,3propane-sultone [6] and 1,4-buta-sultone [7] in the presence of sodium hydride as the catalyst resulted in polymers, which are soluble in aqueous alkaline solutions. Several other studies involving homopolymerization of suitable aniline derivatives [8 / 10] and copolymerization of aniline with different kinds of ring or N -alkyl substituted derivatives [11 /13] have been employed to improve the solubility. Poly(o-methoxy aniline) (POMA) (doped with functionalized acids) was prepared by Macinnes et al. [14] and Depaoli et al. [15]. Electrochromic properties of POMA have been studied by Bulhoes et al. [16]. High coloumbic efficiency and intense chromic changes were noticed for pure films [16] and the electrochromic window could be observed for POMA in aqueous, non-aqueous and polymer electrolyte media. DPA, the N-substituted aniline derivative has been electro polymerized by Zotti et al. [17] in a mixture of 4 M H2SO4 and ethanol. Polymerization was performed in the presence of ethanol as cosolvent which caused dissolution of oligomeric products and hindered growth of poly(DPA), PDPA on electrode surface. Studies on the synthesis of poly(N -alkyl DPA), poly(3-methoxydiphenylamine) [18] and poly(3-chloro DPA) [19] have also been made and the polymer was found to have C /C phenyl /phenyl coupling in their backbone. Copolymerization is an easy method to prepare polymers with specifically tailored properties. Also, copolymerization could lead to the knowledge of reactivities of monomers and their relationship with chemical structure of monomers and, therefore, to a better understanding of mechanism of polymerization. Bagheri et al. [20] reported the electrochemical preparation and characterization of poly(diphenylamine-co-benzidine). The electroactivity of copolymer has been found to be altered by the variation of monomer concentrations in the feed. Nicolini et al. [21] reported electrochemical
synthesis of conducting poly(aniline-co-o-toluidine) and poly(aniline-co-o -anisidine). Copolymers prepared with /CH3 or /OCH3 groups in the phenyl rings of aniline are known to have better salvation effect with faster electro chromic responses [22]. Dao et al. [23 /25] have prepared a series of poly(aniline-co-N -butyl aniline) by chemical and electrochemical methods. Films of poly(aniline-co-o-toluidine) and poly(aniline-coo-anisidine) have also been deposited electrochemically on Indium tin oxide (ITO) coated glass and silicon electrodes, respectively. Several copolymers of aniline with phenyl substituted and Nsubstituted anilines show better solubility, disordered structure and decreased electrical conductivity [26 /30]. Electrochemical copolymerization of aniline derivatives with p -phenylenediamine in aqueous H2SO4 medium has been investigated using cyclic voltammetry (CV) [31]. Poly(diphenylamine-coaniline) [32] has been deposited by pulse potentiostatic method and a growth equation was deduced correlating the conditions of copolymerization with the charge associated with film deposition. Also, electrochemical copolymerization of DPA with anthranilic acid [33] was performed using CV. While studies on preparation of conducting PANI derivatives have been made aplenty, a limited attention has been made on following the intermediate species formed in the polymerization of aniline/aniline derivatives. Conventional techniques could not follow such intermediates as they are reactive. In situ spectro electro chemical studies can be specifically used for studying the early stages of polymerization of aniline and its derivatives. Such studies reveal that PANI formation proceeds via coupling of cation radicals [34 / 37]. Kemp et al. [38] have used fast scan UV / visible spectrophotometry in an attempt to detect the intermediates in the oxidation of various substituted anilines, but could not obtain sufficient evidences. Hanbitzer and Stassen [39,40] reported the detection of soluble products by electro oxidation of aniline by using electrochemical thermo spray mass spectroscopy. In the present study, copolymerization of DPA with m -methoxy aniline (MA) was performed using CV and the deposited films were character-
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Fig. 1. (A /C) CVs recorded during the polymerization of DPA with MA in 4 M H2SO4. Scan rate /100 mV/s. Molar feed ratio of [DPA] (a) 0.75, (b) 0.625, (c) 0.5, (d) 0.375, (e) 0.25. Total concentration of DPA and MA/40 mM. (A) First, (B) Second, (C) Twentieth scan of potential.
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Fig. 2. CVs of growth of PDPA film on a platinum electrode in 4 M H2SO4. Scan rate of 100 mV/s. [DPA]/40 mM. (a) First scan, (b) Second scan, (c) Twentieth scan of potential.
ized for electrochemical polymerization. Literature reveals that studies on following the early stages of electrochemical copolymerization by in situ spectro electrochemistry are scarce. In the present study, in situ UV /visible spectroelectrochemical studies were made to identity the intermediates in the copolymerization. The spectral changes asso-
ciated with the copolymer formation when prepared from mixture of DPA and MA in different feed ratios were followed. The presence of substituted aniline type units in the PANI type backbone in different extent is expected to cause changes in poloranic/bipoloranic excitation bands. The changes in UV /visible spectral characteristics
Fig. 3. (A) CVs of PDPA and copolymer films in monomer-free background electrolyte (4 M H2SO4) at the scan rate of 200 mV/s. (a) PDPA film; (b) /(f) copolymer film prepared by having molar feed ratio of [DPA] (b) 0.75, (c) 0.625, (d) 0.5, (e) 0.375, (f) 0.25. Total concentration of DPA and MA/40 mM. (B) Inset, effect of scan rate on peak current, film preparation: molar feed ratio of [DPA] (a) 0.75, (b) 0.625, (c) 0.5, (d) 0.375, (e) 0.25; Cycles/20, Scan rate /100 mV/s.
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Fig. 4. Absorbance-wavelength /potential profile recorded during the electrochemical copolymerization of DPA with MA. (The spectra collected for the anodic potential scans are used). [DPA]/8 mM; Ea/Ec /0.0/0.7 V; [MA]/2 mM; Scan rate /5 mV/s.
were suitably monitored by performing the copolymerization with different feed ratios of DPA/ MA. The influence of OCH3 groups on the oxidation state of monomer units in the backbone of copolymer could be understood using these studies. The ultimate products in the copolymerization, copolymers, were characterized by elemental analysis to obtain the composition of DPA and MA units in the copolymer and correlated with the results from in situ spectroelectrochemical studies.
2. Experimental MA (E-Merck) was doubly distilled and kept in dark at 5 8C. DPA (E-Merck) was used without any further purification. Sulfuric acid (E-Merck) was used as such. Electrolyte solution (4 M H2SO4) was prepared from doubly distilled water.
2.1. Electrochemical polymerization by cyclic voltammetry Electrochemical synthesis and cyclic voltametric studies were performed by using EG & G, PAR, Potentiostat/Galvanostat, Versastat Model II. A three-electrode cell assembly was used with SCE as the reference electrode. Platinum wire was used as counter electrode. A platinum foil of area 1 cm2 was used as the working electrode. A luggin capillary, whose tip was set at a distance of about 1 mm from the surface of the working electrode, was used to minimize errors due to iR drop in the electrolytes. Syntheses of polymers and measurements were performed under nitrogen atmosphere. For the electrochemical copolymerization studies, mixture of DPA and MA in different molar feed ratios were used in 4 M H2SO4. Electropolymerization was achieved by cycling the potential in the range 0.0 /1.0 V for 20 cycles with a sweep rate of 100 mV/s for the conditions of different molar feed ratios of DPA and MA. The total concentra-
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Table 1 Composition of poly (DPA-co-MA) by elemental analysis Feed ratio
Elemental composition (%)
Molar composition in the copolymer
DPA (mM)
MA (mM)
C
N
H
O
DPA
MA
10 20 30
30 20 10
58.77 66.5 73.80
5.45 5.95 5.955
16.5 17.24 17.37
19.28 10.31 2.87
0.43 0.51 0.63
0.57 0.49 0.37
tion of DPA and MA was kept as 40 mM. The cyclic voltammograms (CVs) were recorded simultaneously with the synthesis.
containing mixture of DPA and MA. This has been done to avoid hysterisis in potential /wavelength relation [41,42].
2.2. Electrochemical characterization of polymer films
2.4. Synthesis of copolymers and composition
The deposited films of polymers were placed in a monomer-free background electrolyte (4 M H2SO4) and CVs of the film-coated electrode were recorded after stabilization.
Copolymers were synthesized by applying constant potential (0.8 V) using BAS 100 B Electrochemical analyzer on a solution containing the two monomers (DPA and MA) in different feed ratios.
2.3. In situ spectroelectrochemistry In situ spectroelectrochemical studies on copolymerization of DPA with MA were carried out in 4 M H2SO4 using Shimadzu UVPC-2401/UV 2100 high speed multi spectrophotometer. Spectroelectrochemical experiments were done using a quartz cuvette of 1 cm path length assembled as an electrochemical cell with an optically transparent ITO coated glass working electrode, a platinum wire as a counter electrode and Ag/AgCl as reference electrode. ITO glass plate with specific surface conductivity of 10 V/cm2 was used. Before performing each spectroelectrochemical experiments, the ITO coated glass plate was cleaned repeatedly with acetone and double distilled water. Electropolymerization was performed by CV or constant potential (1.0 V vs. Ag/AgCl) electrolysis using computer controlled BAS 100 A electrochemical analyzer. UV /visible spectra were continuously collected while performing CV or constant electrolysis experiments. For recording dynamic spectroelectrochemical results, CV was performed by cycling the potential in the range of 0.0 /0.8 V with a slow scan rate (5 mV/s) on a solution
Fig. 5. UV /visible spectra recorded during the copolymerization of DPA with MA while sweeping the potentials between 0.0 and 0.8 V [DPA]/8 mM, [MA]/2 mM.
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Fig. 6. Absorbance-wavelength /potential profile recorded during the electrochemical copolymerization of DPA with MA. (The spectra collected for the anodic potential scans are used). [DPA]/5 mM; Ea/Ec /0.0/0.7 V; [MA]/10 mM; Scan rate/5 mV/s.
The composition of two monomers in the copolymers was determined using Hereaeus C H N /O rapid analyzer.
3. Result and discussion 3.1. Electrochemical copolymerization and homopolymerization Electrochemical copolymerization of DPA with MA was performed for different molar feed ratios of DPA (0.75, 0.625, 0.5, 0.375, 0.25) in 4 M H2SO4 by scanning the potentials in the limits 0.0 / 1.0 V for 20 cycles. Fig. 1(A /C) represents CVs recorded for the first, second and 20th potential scan while performing polymerization with different feed ratios of DPA or MA. In the first potential scan, a single anodic peak was noticed for polymerization with any of the feed composition of DPA. However, the peak potential was found to be significantly shifted upon changing the molar feed ratio of DPA or MA. The peak
position is shifted to more anodic side on increasing the feed ratio of MA. It is also important to note that the peak potential noticed for the mixture of DPA and MA is different from the peak potential noticed for the polymerization of DPA [32,33] or MA alone. The reason is envisaged as follows. In the same potential limits both DPA and MA gets oxidized to form the respective cation radicals (DPA cation radical, DPACR and m -methoxy anilium cation radical, MACR). Their consequent reactions can result oligomer with different proportions of DPA or MA in the backbone structure. The differences in peak potential may arise due to oxidation of such oligomers. Hence, it is envisioned that during polymerization with different feed ratios of DPA or MA oligomers having varying proportions of DPA or MA in the backbone may be formed. The results from in situ UV /visible spectroelectrochemical studies (discussed in the later part of discussion) clearly support this supposition. The differences in the peak positions noticed for the reduction
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processes in the CV of first cycle also favor the above proposal. Hence, tendency of copolymerization could be noticed even from the first cycle (Fig. 1A). The CVs recorded for the second (Fig. 1B) and 20th potential (Fig. 1C) scans during the polymerization of mixture of DPA and MA also distinctly differ from the CVs recorded during polymerization of DPA alone. The twin redox characteristics [32,33,42] that were noticed for polymerization of DPA (Fig. 2c) was virtually shrinked to a single redox process in the cases of polymerization with mixture of DPA and MA (Fig. 1C). The CVs of 20th cycle typically represents the respective polymeric films. To authenticate the differences in the redox characteristics between films of PDPA and the copolymers from DPA and MA, CVs of the respective polymeric films were recorded in monomer-free background electrolyte (Fig. 3). The CV pattern of the stabilized polymer films deposited under different conditions of copolymerization (Fig. 3) are similar to CVs of 20th potential scan during copolymerizaion (Fig. 1C). This confirms that redox characteristics observed during copolymrization are typically due to the respective copolymers deposited on the electrode surface. The stable and the surface bound nature of these copolymers film were evident from the linearity [32,43] obtained between peak current of redox process and scan rate (Fig. 3B inset). The copolymers were prepared by keeping the molar feed concentrations of DPA maintained during the CV studies. The compositions of DPA and MA units in the copolymer prepared with different feed ratios of DPA were determined by elemental analysis (Table 1). Increasing DPA concentration in the feed makes an increase in composition of DPA in the copolymer. Clearly, the copolymers prepared under different feed composition of DPA are having different molar composition of MA or DPA units in the copolymer. It is pertinent to note that the redox characteristics of polymer films (Fig. 3A) deposited under different feed composition of DPA are reflective of the differences in the composition of the monomer units in the copolymer.
3.2. In situ spectroelectrochemical studies In situ UV /visible spectro electrochemical studies were specifically performed under different molar feed composition of DPA to obtain further evidence for the copolymer formation. On close comparison of the spectroelectrochemical results of the polymerization of mixture of DPA and MA with different feed ratios of DPA and polymerization of DPA (or) MA, clear differences in spectral characteristics between them could be seen. A probable incorporation of both monomer units in the polymer during polymerization can cause such differences. In situ UV /visible spectro electrochemical results have been obtained by cycling the potential between 0.0 and 0.80 V with a slow scan rate of 5 mV/s for a solution containing mixture of DPA and MA and collecting the UV /
Fig. 7. UV /visible spectra recorded during the copolymerization of DPA with MA while sweeping the potentials from 0.0 to 0.8 V [DPA]/5 mM, [MA]/10 mM.
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Fig. 8. DCVA deduced at different wavelengths using UV /visible spectra collected from first potential cycle [DPA]/8 mM; Ec/Ea / 0.0/0.8 V; [MA]/2 mM; Scan rate/20 mV/s; (A) 426 nm, (B) 576 nm.
visible spectra concomitantly. Fig. 4 represent absorbence-wavelength/potential profile (3D profile) recorded during the electrochemical copolymerization with a mixture of DPA and MA having 0.75 as molar feed ratio of DPA. The UV /visible spectra are extracted from the 3D profile (Fig. 4) and given in Fig. 5. On scanning the potential beyond 0.65 V, the UV /visible spectra start exhibiting two prominent peaks at 426 and 576 nm, a shoulder around 476 nm and a broad bond beyond 700 nm (Fig. 5). Under these potential ranges oxidation of both monomers can occur (Fig. 1A) to result their
cation radicals, DPACR and MACR, with probable fast cross chemical reactions to result oligomer (or) polymer having different compositions of DPA. Hence, the UV /visible spectral characteristics noticed in the potential beyond 0.65 V for mixture of DPA and MA can be assigned for the spectral bands corresponding in to the oligomer/ polymer. Spectroelectrochemical experiments performed with the different feed composition of DPA (0.33) also show two peaks at 426 and 746 nm. The absorbance-wavelength /potential profile recorded during the electrochemical polymerization of DPA
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Scheme 1. Probable dimer/oligomer structures in DPA polymerization: (a) monomer, (b) dimer, (c) oligomer, (d) oligomer cation radical (consisting of DPB + and ANI + ).
with MA (Fig. 6) and the extracted spectra at specific potentials (Fig. 7) inform the above facts. It is worthwhile to compare the UV /visible spectral characteristics of the polymerization of DPA alone and polymerization of mixture of DPA and MA generated in the same potential range to deduce any conclusion about the spectral characteristics of oligomer/polymer. On switching the potential of the electrode beyond 0.50 V during the polymerization of DPA, a shoulder around 430 nm, a peak at 500 nm and a broad band beyond 600 nm was noticed [45]. The peak and band positions noticed in the UV /visible spectra recorded during the polymerization of DPA with MA (Fig. 5) are totally different from the band
and peak positions noticed for polymerization of DPA alone. A distinct peak was noticed around 576 nm in the case of copolymerization which was virtually absent in the case of polymerization of DPA. Hence, this peak can be assigned for the oligomer/polymer formed from the mixture DPA and MA. The presence of peak at 500 nm has earlier been assigned for the DPACR during polymerization of DPA [43] or polaronic transition arising from the diphenylbenzidine (DPB) type oligomer/oligomer (Scheme 1). This has been done based on the fact that upon the electrochemical oxidation of DPA, N ,N ?-diphenylbenzidine is formed as a dimeric intermediate (Scheme 1). However, the band corresponding to DPB type
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Scheme 2. Mechanism of copolymerization.
intermediate is observed at 476 nm in the case of copolymerization. Instead of a shoulder observed around 430 nm for DPA polymerization [44] a clear peak was noticed at 426 nm for polymerization with mixture of monomers. Previous workers [45,46] have observed an absorption band around 420 nm during aniline polymerization of aniline and assigned to aniline cation radical or polaronic transition. In considering this, the peak observed at 426 nm in the present study is assigned for the cation radical generated from the polaronic transi-
tion of aniline type moieties in the oligomer/ polymer. Hence, in the present study for the polymerization of DPA with MA considering the appearance new peak at 576 nm and a notable shift for DPB type intermediate, a mechanism for copolymerization consisting of cross reactions between cation radicals generated from DPACR and MACR (Scheme 2) is proposed. To support the assignment for the peak at 426 and 576 nm, DCVA were deduced at the respective wavelengths from the results obtained from dy-
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Fig. 9. UV /visible absorption spectra recorded during the constant potential electrochemical copolymerization of DPA with MA in 4 M H2SO4. [DPA]/5 mM; [MA]/5 mM; Potential/0.8 V vs. Ag/AgCl.
namic spectroelectrochemical studies (Fig. 8A and B). On superimposing the DCVA curves for 426 and 576 nm with the CVs (first cycle) recorded for the polymerization of mixture of DPA and MA, it was found that the species responsible for the peaks in the UV spectra are produced in the potential range given by the cyclicvoltammogram where oxidation of both monomers takes place. A specific drop in the time derivative of absorption with potential beyond 0.70 V can be noticed in the DCVA curves for both wavelengths beyond 0.70 V. This gives a clue that the DPACR and MACR immediately undergo chemical reaction to result a dimer consisting of both DPA and MA moieties. Further oxidation of the dimer also can occur at the same potential range with an ultimate formation of oligomer/polymer. To support the assignments made for the peaks noticed 426 and 576 nm, bulk potentiostatic
polymerization studies were made with different feed compositions of DPA by holding the potential at 0.80 V. The UV /visible spectra were collected continuously while performing the electropolymerization (Fig. 9 and Fig. 10) through constant potential electrolysis. Here again, two peaks (426 and 576 nm) could be noticed with progressive increase in the absorbance. It is also possible to have an idea about the changes in proportions of DPA and MA units in the copolymer on changing the feed composition of the two monomers certainly, there is variation in the ratio of absorbance corresponding to the peaks at 426 /576 nm upon changing the feed ratio of DPA. This supports that changes in the feed composition of DPA produce a copolymer with difference in DPA or MA content in it. Hence, the results of in situ spectro electrochemical studies (Fig. 9 and Fig. 10) correlates with the changes in
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Fig. 10. UV /visible absorption spectra recorded during the constant potential electrochemical copolymerization of DPA with MA in 4 M H2SO4. [DPA] /5 mM; [MA]/10 mM; Potential/0.8 V vs. Ag/AgCl.
the DPA/MA content in the copolymer as noticed by elemental analysis (Table 1).
4. Conclussion The CVs representing the growth of copolymers during the electrochemical copolymerization of DPA with MA on electrode surface are found to show variations based on compositional changes of the two monomers in the copolymers. The new peak observed at 576 nm in the in situ UV /visible spectrum reveals the formation of intermediate with an incorporation of both monomeric units. A clear matching between peaks in DCVA deduced at 576 nm and CV supports the formation of intermediates in the potential range beyond 0.70 V. Elemental analysis of the copolymers confirms the compositional changes of DPA/MA in the copolymer as inferred from in situ spectro electrochemical results.
Acknowledgements The financial assistance from Department of Science and Technology (DST) (SP/S1-H12/97), New Delhi, India is acknowledged.
References [1] E.M. Genis, A. Boyle, M. Lapkwoski, C. Tsintavis, Synth. Met. 36 (1990) 139. [2] A.G. MacDiarmid, A.J. Epstein, Faraday Discuss. Chem. Soc. 88 (1989) 317. [3] A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65 (1994) 103. [4] J. Yue, A.J. Epstein, J. Am. Chem. Soc. 112 (1990) 2800. [5] J.Y. Bergenon, J.W. Charvelier, L.H. Dao, J. Chem. Soc. Chem. Commn. 180 (1990). [6] M.T. Ngyune, P. Kasai, J.L. Miller, A.F. Diaz, Macromolecules 27 (1994) 3625. [7] P. Hany, E.M. Genies, C. Santer, Synth. Met. 31 (1989) 369.
1950
M. Thanneermalai et al. / Spectrochimica Acta Part A 59 (2003) 1937 /1950
[8] Y. Wei, G.E. Wnek, A.G. MacDiarmid, A. Ray, W.W. Foke, J. Phys. Chem. 93 (1989) 495. [9] A. Watanabe, A. Lwabuchi, Y. Lwasaki, O. Ito, K. Mori, Y. Nakamura, Macromolecules 22 (1989) 3521. [10] C. DeArmitt, S.P. Armes, J. Winter, F.A. Uribe, S. Gottesfeld, C. Mombourguette, Polymer 34 (1993) 158. [11] H.S.O. Chan, S.C. Ng, W.S. Sim, K.L. Tan, B.T.G. Tan, Macromolecules 25 (1992) 6029. [12] A.A. Karyakin, A.K. Strakhova, A.K. Yatsimirsky, J. Electroanal. Chem. 371 (1994) 259. [13] M.T. Nguyen, A.F. Diaz, Macromolecules 28 (1995) 3411. [14] D. Macinnes, B.L. Funt, Synth. Met. 69 (1995) 235. [15] W.A. Gazotti, Jr, M.J.D.M. Jannini, S.I. Cordoba de Torresi, M.A. Depaoli, J. Electroanal. Chem. 440 (1997) 193. [16] D. Concalves, B. Matrienko, L.O.S. Bulhoes, J. Electroanal. Chem. 371 (1994) 267. [17] N. Comisso, S. Daolio, G. Zotti, S. Zecchin, R. Salmaso, G. Mengoli, J. Electroanal. Chem. 255 (1998) 97. [18] M.T. Nguyen, L.H. Dao, J. Electroanal. Chem. Interfacial Electrochem. 28 (1990) 37. [19] M.T. Nguyen, R. Paynter, L.H. Dao, Polymer 33 (1992) 214. [20] A. Bagheri, M.R. Nateghi, A. Massoumi, Synth. Met. 97 (1998) 85. [21] M.K. Ram, E. Maccioni, C. Nicolini, Thin Solid Films 303 (1997) 27. [22] S.K. Dhawan, D.C. Trivedi, Synth. Met. 60 (1993) 63. [23] J.Y. Bergeron, L.H. Dao, Macromolecules 25 (1992) 3332. [24] S. Ye, S. Besner, L.H. Dao, A.K. Vijh, J. Electroanal. Chem. 381 (1995) 71. [25] S. Ye, N.T. Do, L.H. Dao, A.K. Vijh, Synth. Met. 88 (1997) 65. [26] M.E. Jozefowicz, A.J. Epstein, J.P. Pouger, J.G. Masters, A. Ray, A.G. MacDiarmid, Macrmolecules 24 (1991) 5863.
[27] Z.H. Wang, A. Ray, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B43 (1991) 4373. [28] J. Pouger, S.L. Zhao, Z.H. Wang, Z. Oldakowski, A.J. Epstein, S.K. Manohar, J.M. Weisinger, A.G. MacDiarmid, C.H. Hsu, Synth. Met. 55 /57 (1993) 341. [29] A. Ito, K. Ora, K. Tanaka, T. Yamabe, Macromolecules 28 (1995) 5618. [30] J.W. Chavalier, J.Y. Bergenon, L.H. Dao, Macromolecules 25 (1992) 3325. [31] C.H. Yang, T.C. Wen, J. Electrochem. Soc. 144 (1997) 2078. [32] V. Rajendran, A. Gopalan, T. Vasudevan, T.C. Wen, J. Electrochem. Soc. 147 (2000) 3014. [33] M.S. Wu, T.C. Wen, A. Gopalan, J. Electrochem. Soc. 148 (2001) D65. [34] J. Bacon, R.N. Adams, J. Am. Chem. Soc. 90 (1968) 6596. [35] R.L. Hand, R.F. Nelson, J. Am. Chem. Soc. 96 (1974) 850. [36] R.L. Hand, R.F. Nelson, J. Am. Chem. Soc. 125 (1978) 1058. [37] D.M. Mohilner, R.N. Adams, W.J. Argesinger, J. Am. Chem. Soc. 13 (1992) 680. [38] T.J. Kemp, P. Moore, G.R. Quick, J. Chem. Res. 301 (1981) 1981. [39] G. Hembitzer, P.P. Heinz, I. Stassen, J. Heitbaum, Synth. Met. 55 (1993) 1317. [40] G. Hembitzer, I. Stassen, Synth. Met. 55 /57 (1993) 1045. [41] A. Malinauskus, R. Holz, Synth. Met. 97 (1998) 31. [42] C.Y. Chung, T.C. Wen, A. Gopalan, Electrochim. Acta 47 (2001) 423. [43] W.C. Chen, T.C. Wen, A. Gopalan, J. Electrochem. Soc. 148 (2001) 11. [44] M.S. Wu, T.C. Wen, A. Gopalan, Mater. Chem. Phys. 74 (2002) 58. [45] T.C. Wen, L.M. Huang, A. Gopalan, Electrochim. Acta 46 (2001) 2463. [46] T.C. Wen, C. Sivakumar, A. Gopalan, Electrochim. Acta 46 (2001) 1071.
In situ UV visible spectroelectrochemical evidences for conducting copolymer formation between diphenylamine and m-methoxyaniline /
M. Thanneermalai a, T. Jeyaraman a, C. Sivakumar a, A. Gopalan a,*, T. Vasudevan a, T.C. Wen b b
a Department of Industrial Chemistry, Alagappa University, Karaikudi 630003, India Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Received 17 September 2002; accepted 8 November 2002
Abstract Electrochemical copolymerization of diphenylamine (DPA) with m -methoxy aniline (MA) was carried out in 4 M H2SO4 by cyclic voltammetry (CV). Cyclic voltammograms (CVs) of the copolymer films were recorded in monomerfree background electrolyte. In situ sepectroelectrochemical studies were carried out on an optically transparent electrode (Indium tin oxide (ITO) coated glass) in 4 M H2SO4 for different feed ratios of the comonomers. Constant potential and potential sweep methods were employed for performing polymerization. UV /visible absorption spectra were collected continuously and concurrently during the copolymerization in both the cases. The results from constant potential electropolymerisation indicated the formation of an intermediate with an absorption peak at 576 nm. Derivative cyclic voltabsorptogram (DCVA) was deduced from the results of cyclic spectrovoltammetry. The DCVA derived at 576 nm confirms the intermediates formed during the electrochemical copolymerization. The compositional changes of the two monomers in the copolymers with changes in feed composition of two monomers as predicted from in situ spectro electrochemical studies are evident from elemental analysis. A plausible copolymerization mechanism is suggested. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Electrocopolymerization; Cyclic voltammetry; Copoymer coposition; In situ spectroelectrochemistry; UV /visible spectroscopy
1. Introduction Polyaniline (PANI), has been regarded as one of the most versatile conducting polymers on the * Corresponding author. E-mail address: [email protected] (A. Gopalan).
basis of its environmental stability and its electrical, optical and electrochemical properties [1 /3]. However, the restricted processability arising from its low solubility in organic solvents is one of the drawbacks to be solved. In the process of obtaining soluble conducting polymers Yue and Epstein [4] have synthesized the self-doped PANI through
1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-1425(02)00441-9
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the reaction between an emeralddine base form of PANI and fuming sulfuric acid. Several PANI type aqueous soluble conducting polymers are then synthesized by introducing the sulphonic acid group into PANI backbone [5,6]. Likewise, treating the leuco-emeralddine base of PANI with 1,3propane-sultone [6] and 1,4-buta-sultone [7] in the presence of sodium hydride as the catalyst resulted in polymers, which are soluble in aqueous alkaline solutions. Several other studies involving homopolymerization of suitable aniline derivatives [8 / 10] and copolymerization of aniline with different kinds of ring or N -alkyl substituted derivatives [11 /13] have been employed to improve the solubility. Poly(o-methoxy aniline) (POMA) (doped with functionalized acids) was prepared by Macinnes et al. [14] and Depaoli et al. [15]. Electrochromic properties of POMA have been studied by Bulhoes et al. [16]. High coloumbic efficiency and intense chromic changes were noticed for pure films [16] and the electrochromic window could be observed for POMA in aqueous, non-aqueous and polymer electrolyte media. DPA, the N-substituted aniline derivative has been electro polymerized by Zotti et al. [17] in a mixture of 4 M H2SO4 and ethanol. Polymerization was performed in the presence of ethanol as cosolvent which caused dissolution of oligomeric products and hindered growth of poly(DPA), PDPA on electrode surface. Studies on the synthesis of poly(N -alkyl DPA), poly(3-methoxydiphenylamine) [18] and poly(3-chloro DPA) [19] have also been made and the polymer was found to have C /C phenyl /phenyl coupling in their backbone. Copolymerization is an easy method to prepare polymers with specifically tailored properties. Also, copolymerization could lead to the knowledge of reactivities of monomers and their relationship with chemical structure of monomers and, therefore, to a better understanding of mechanism of polymerization. Bagheri et al. [20] reported the electrochemical preparation and characterization of poly(diphenylamine-co-benzidine). The electroactivity of copolymer has been found to be altered by the variation of monomer concentrations in the feed. Nicolini et al. [21] reported electrochemical
synthesis of conducting poly(aniline-co-o-toluidine) and poly(aniline-co-o -anisidine). Copolymers prepared with /CH3 or /OCH3 groups in the phenyl rings of aniline are known to have better salvation effect with faster electro chromic responses [22]. Dao et al. [23 /25] have prepared a series of poly(aniline-co-N -butyl aniline) by chemical and electrochemical methods. Films of poly(aniline-co-o-toluidine) and poly(aniline-coo-anisidine) have also been deposited electrochemically on Indium tin oxide (ITO) coated glass and silicon electrodes, respectively. Several copolymers of aniline with phenyl substituted and Nsubstituted anilines show better solubility, disordered structure and decreased electrical conductivity [26 /30]. Electrochemical copolymerization of aniline derivatives with p -phenylenediamine in aqueous H2SO4 medium has been investigated using cyclic voltammetry (CV) [31]. Poly(diphenylamine-coaniline) [32] has been deposited by pulse potentiostatic method and a growth equation was deduced correlating the conditions of copolymerization with the charge associated with film deposition. Also, electrochemical copolymerization of DPA with anthranilic acid [33] was performed using CV. While studies on preparation of conducting PANI derivatives have been made aplenty, a limited attention has been made on following the intermediate species formed in the polymerization of aniline/aniline derivatives. Conventional techniques could not follow such intermediates as they are reactive. In situ spectro electro chemical studies can be specifically used for studying the early stages of polymerization of aniline and its derivatives. Such studies reveal that PANI formation proceeds via coupling of cation radicals [34 / 37]. Kemp et al. [38] have used fast scan UV / visible spectrophotometry in an attempt to detect the intermediates in the oxidation of various substituted anilines, but could not obtain sufficient evidences. Hanbitzer and Stassen [39,40] reported the detection of soluble products by electro oxidation of aniline by using electrochemical thermo spray mass spectroscopy. In the present study, copolymerization of DPA with m -methoxy aniline (MA) was performed using CV and the deposited films were character-
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Fig. 1. (A /C) CVs recorded during the polymerization of DPA with MA in 4 M H2SO4. Scan rate /100 mV/s. Molar feed ratio of [DPA] (a) 0.75, (b) 0.625, (c) 0.5, (d) 0.375, (e) 0.25. Total concentration of DPA and MA/40 mM. (A) First, (B) Second, (C) Twentieth scan of potential.
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Fig. 2. CVs of growth of PDPA film on a platinum electrode in 4 M H2SO4. Scan rate of 100 mV/s. [DPA]/40 mM. (a) First scan, (b) Second scan, (c) Twentieth scan of potential.
ized for electrochemical polymerization. Literature reveals that studies on following the early stages of electrochemical copolymerization by in situ spectro electrochemistry are scarce. In the present study, in situ UV /visible spectroelectrochemical studies were made to identity the intermediates in the copolymerization. The spectral changes asso-
ciated with the copolymer formation when prepared from mixture of DPA and MA in different feed ratios were followed. The presence of substituted aniline type units in the PANI type backbone in different extent is expected to cause changes in poloranic/bipoloranic excitation bands. The changes in UV /visible spectral characteristics
Fig. 3. (A) CVs of PDPA and copolymer films in monomer-free background electrolyte (4 M H2SO4) at the scan rate of 200 mV/s. (a) PDPA film; (b) /(f) copolymer film prepared by having molar feed ratio of [DPA] (b) 0.75, (c) 0.625, (d) 0.5, (e) 0.375, (f) 0.25. Total concentration of DPA and MA/40 mM. (B) Inset, effect of scan rate on peak current, film preparation: molar feed ratio of [DPA] (a) 0.75, (b) 0.625, (c) 0.5, (d) 0.375, (e) 0.25; Cycles/20, Scan rate /100 mV/s.
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Fig. 4. Absorbance-wavelength /potential profile recorded during the electrochemical copolymerization of DPA with MA. (The spectra collected for the anodic potential scans are used). [DPA]/8 mM; Ea/Ec /0.0/0.7 V; [MA]/2 mM; Scan rate /5 mV/s.
were suitably monitored by performing the copolymerization with different feed ratios of DPA/ MA. The influence of OCH3 groups on the oxidation state of monomer units in the backbone of copolymer could be understood using these studies. The ultimate products in the copolymerization, copolymers, were characterized by elemental analysis to obtain the composition of DPA and MA units in the copolymer and correlated with the results from in situ spectroelectrochemical studies.
2. Experimental MA (E-Merck) was doubly distilled and kept in dark at 5 8C. DPA (E-Merck) was used without any further purification. Sulfuric acid (E-Merck) was used as such. Electrolyte solution (4 M H2SO4) was prepared from doubly distilled water.
2.1. Electrochemical polymerization by cyclic voltammetry Electrochemical synthesis and cyclic voltametric studies were performed by using EG & G, PAR, Potentiostat/Galvanostat, Versastat Model II. A three-electrode cell assembly was used with SCE as the reference electrode. Platinum wire was used as counter electrode. A platinum foil of area 1 cm2 was used as the working electrode. A luggin capillary, whose tip was set at a distance of about 1 mm from the surface of the working electrode, was used to minimize errors due to iR drop in the electrolytes. Syntheses of polymers and measurements were performed under nitrogen atmosphere. For the electrochemical copolymerization studies, mixture of DPA and MA in different molar feed ratios were used in 4 M H2SO4. Electropolymerization was achieved by cycling the potential in the range 0.0 /1.0 V for 20 cycles with a sweep rate of 100 mV/s for the conditions of different molar feed ratios of DPA and MA. The total concentra-
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Table 1 Composition of poly (DPA-co-MA) by elemental analysis Feed ratio
Elemental composition (%)
Molar composition in the copolymer
DPA (mM)
MA (mM)
C
N
H
O
DPA
MA
10 20 30
30 20 10
58.77 66.5 73.80
5.45 5.95 5.955
16.5 17.24 17.37
19.28 10.31 2.87
0.43 0.51 0.63
0.57 0.49 0.37
tion of DPA and MA was kept as 40 mM. The cyclic voltammograms (CVs) were recorded simultaneously with the synthesis.
containing mixture of DPA and MA. This has been done to avoid hysterisis in potential /wavelength relation [41,42].
2.2. Electrochemical characterization of polymer films
2.4. Synthesis of copolymers and composition
The deposited films of polymers were placed in a monomer-free background electrolyte (4 M H2SO4) and CVs of the film-coated electrode were recorded after stabilization.
Copolymers were synthesized by applying constant potential (0.8 V) using BAS 100 B Electrochemical analyzer on a solution containing the two monomers (DPA and MA) in different feed ratios.
2.3. In situ spectroelectrochemistry In situ spectroelectrochemical studies on copolymerization of DPA with MA were carried out in 4 M H2SO4 using Shimadzu UVPC-2401/UV 2100 high speed multi spectrophotometer. Spectroelectrochemical experiments were done using a quartz cuvette of 1 cm path length assembled as an electrochemical cell with an optically transparent ITO coated glass working electrode, a platinum wire as a counter electrode and Ag/AgCl as reference electrode. ITO glass plate with specific surface conductivity of 10 V/cm2 was used. Before performing each spectroelectrochemical experiments, the ITO coated glass plate was cleaned repeatedly with acetone and double distilled water. Electropolymerization was performed by CV or constant potential (1.0 V vs. Ag/AgCl) electrolysis using computer controlled BAS 100 A electrochemical analyzer. UV /visible spectra were continuously collected while performing CV or constant electrolysis experiments. For recording dynamic spectroelectrochemical results, CV was performed by cycling the potential in the range of 0.0 /0.8 V with a slow scan rate (5 mV/s) on a solution
Fig. 5. UV /visible spectra recorded during the copolymerization of DPA with MA while sweeping the potentials between 0.0 and 0.8 V [DPA]/8 mM, [MA]/2 mM.
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Fig. 6. Absorbance-wavelength /potential profile recorded during the electrochemical copolymerization of DPA with MA. (The spectra collected for the anodic potential scans are used). [DPA]/5 mM; Ea/Ec /0.0/0.7 V; [MA]/10 mM; Scan rate/5 mV/s.
The composition of two monomers in the copolymers was determined using Hereaeus C H N /O rapid analyzer.
3. Result and discussion 3.1. Electrochemical copolymerization and homopolymerization Electrochemical copolymerization of DPA with MA was performed for different molar feed ratios of DPA (0.75, 0.625, 0.5, 0.375, 0.25) in 4 M H2SO4 by scanning the potentials in the limits 0.0 / 1.0 V for 20 cycles. Fig. 1(A /C) represents CVs recorded for the first, second and 20th potential scan while performing polymerization with different feed ratios of DPA or MA. In the first potential scan, a single anodic peak was noticed for polymerization with any of the feed composition of DPA. However, the peak potential was found to be significantly shifted upon changing the molar feed ratio of DPA or MA. The peak
position is shifted to more anodic side on increasing the feed ratio of MA. It is also important to note that the peak potential noticed for the mixture of DPA and MA is different from the peak potential noticed for the polymerization of DPA [32,33] or MA alone. The reason is envisaged as follows. In the same potential limits both DPA and MA gets oxidized to form the respective cation radicals (DPA cation radical, DPACR and m -methoxy anilium cation radical, MACR). Their consequent reactions can result oligomer with different proportions of DPA or MA in the backbone structure. The differences in peak potential may arise due to oxidation of such oligomers. Hence, it is envisioned that during polymerization with different feed ratios of DPA or MA oligomers having varying proportions of DPA or MA in the backbone may be formed. The results from in situ UV /visible spectroelectrochemical studies (discussed in the later part of discussion) clearly support this supposition. The differences in the peak positions noticed for the reduction
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processes in the CV of first cycle also favor the above proposal. Hence, tendency of copolymerization could be noticed even from the first cycle (Fig. 1A). The CVs recorded for the second (Fig. 1B) and 20th potential (Fig. 1C) scans during the polymerization of mixture of DPA and MA also distinctly differ from the CVs recorded during polymerization of DPA alone. The twin redox characteristics [32,33,42] that were noticed for polymerization of DPA (Fig. 2c) was virtually shrinked to a single redox process in the cases of polymerization with mixture of DPA and MA (Fig. 1C). The CVs of 20th cycle typically represents the respective polymeric films. To authenticate the differences in the redox characteristics between films of PDPA and the copolymers from DPA and MA, CVs of the respective polymeric films were recorded in monomer-free background electrolyte (Fig. 3). The CV pattern of the stabilized polymer films deposited under different conditions of copolymerization (Fig. 3) are similar to CVs of 20th potential scan during copolymerizaion (Fig. 1C). This confirms that redox characteristics observed during copolymrization are typically due to the respective copolymers deposited on the electrode surface. The stable and the surface bound nature of these copolymers film were evident from the linearity [32,43] obtained between peak current of redox process and scan rate (Fig. 3B inset). The copolymers were prepared by keeping the molar feed concentrations of DPA maintained during the CV studies. The compositions of DPA and MA units in the copolymer prepared with different feed ratios of DPA were determined by elemental analysis (Table 1). Increasing DPA concentration in the feed makes an increase in composition of DPA in the copolymer. Clearly, the copolymers prepared under different feed composition of DPA are having different molar composition of MA or DPA units in the copolymer. It is pertinent to note that the redox characteristics of polymer films (Fig. 3A) deposited under different feed composition of DPA are reflective of the differences in the composition of the monomer units in the copolymer.
3.2. In situ spectroelectrochemical studies In situ UV /visible spectro electrochemical studies were specifically performed under different molar feed composition of DPA to obtain further evidence for the copolymer formation. On close comparison of the spectroelectrochemical results of the polymerization of mixture of DPA and MA with different feed ratios of DPA and polymerization of DPA (or) MA, clear differences in spectral characteristics between them could be seen. A probable incorporation of both monomer units in the polymer during polymerization can cause such differences. In situ UV /visible spectro electrochemical results have been obtained by cycling the potential between 0.0 and 0.80 V with a slow scan rate of 5 mV/s for a solution containing mixture of DPA and MA and collecting the UV /
Fig. 7. UV /visible spectra recorded during the copolymerization of DPA with MA while sweeping the potentials from 0.0 to 0.8 V [DPA]/5 mM, [MA]/10 mM.
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Fig. 8. DCVA deduced at different wavelengths using UV /visible spectra collected from first potential cycle [DPA]/8 mM; Ec/Ea / 0.0/0.8 V; [MA]/2 mM; Scan rate/20 mV/s; (A) 426 nm, (B) 576 nm.
visible spectra concomitantly. Fig. 4 represent absorbence-wavelength/potential profile (3D profile) recorded during the electrochemical copolymerization with a mixture of DPA and MA having 0.75 as molar feed ratio of DPA. The UV /visible spectra are extracted from the 3D profile (Fig. 4) and given in Fig. 5. On scanning the potential beyond 0.65 V, the UV /visible spectra start exhibiting two prominent peaks at 426 and 576 nm, a shoulder around 476 nm and a broad bond beyond 700 nm (Fig. 5). Under these potential ranges oxidation of both monomers can occur (Fig. 1A) to result their
cation radicals, DPACR and MACR, with probable fast cross chemical reactions to result oligomer (or) polymer having different compositions of DPA. Hence, the UV /visible spectral characteristics noticed in the potential beyond 0.65 V for mixture of DPA and MA can be assigned for the spectral bands corresponding in to the oligomer/ polymer. Spectroelectrochemical experiments performed with the different feed composition of DPA (0.33) also show two peaks at 426 and 746 nm. The absorbance-wavelength /potential profile recorded during the electrochemical polymerization of DPA
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Scheme 1. Probable dimer/oligomer structures in DPA polymerization: (a) monomer, (b) dimer, (c) oligomer, (d) oligomer cation radical (consisting of DPB + and ANI + ).
with MA (Fig. 6) and the extracted spectra at specific potentials (Fig. 7) inform the above facts. It is worthwhile to compare the UV /visible spectral characteristics of the polymerization of DPA alone and polymerization of mixture of DPA and MA generated in the same potential range to deduce any conclusion about the spectral characteristics of oligomer/polymer. On switching the potential of the electrode beyond 0.50 V during the polymerization of DPA, a shoulder around 430 nm, a peak at 500 nm and a broad band beyond 600 nm was noticed [45]. The peak and band positions noticed in the UV /visible spectra recorded during the polymerization of DPA with MA (Fig. 5) are totally different from the band
and peak positions noticed for polymerization of DPA alone. A distinct peak was noticed around 576 nm in the case of copolymerization which was virtually absent in the case of polymerization of DPA. Hence, this peak can be assigned for the oligomer/polymer formed from the mixture DPA and MA. The presence of peak at 500 nm has earlier been assigned for the DPACR during polymerization of DPA [43] or polaronic transition arising from the diphenylbenzidine (DPB) type oligomer/oligomer (Scheme 1). This has been done based on the fact that upon the electrochemical oxidation of DPA, N ,N ?-diphenylbenzidine is formed as a dimeric intermediate (Scheme 1). However, the band corresponding to DPB type
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Scheme 2. Mechanism of copolymerization.
intermediate is observed at 476 nm in the case of copolymerization. Instead of a shoulder observed around 430 nm for DPA polymerization [44] a clear peak was noticed at 426 nm for polymerization with mixture of monomers. Previous workers [45,46] have observed an absorption band around 420 nm during aniline polymerization of aniline and assigned to aniline cation radical or polaronic transition. In considering this, the peak observed at 426 nm in the present study is assigned for the cation radical generated from the polaronic transi-
tion of aniline type moieties in the oligomer/ polymer. Hence, in the present study for the polymerization of DPA with MA considering the appearance new peak at 576 nm and a notable shift for DPB type intermediate, a mechanism for copolymerization consisting of cross reactions between cation radicals generated from DPACR and MACR (Scheme 2) is proposed. To support the assignment for the peak at 426 and 576 nm, DCVA were deduced at the respective wavelengths from the results obtained from dy-
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Fig. 9. UV /visible absorption spectra recorded during the constant potential electrochemical copolymerization of DPA with MA in 4 M H2SO4. [DPA]/5 mM; [MA]/5 mM; Potential/0.8 V vs. Ag/AgCl.
namic spectroelectrochemical studies (Fig. 8A and B). On superimposing the DCVA curves for 426 and 576 nm with the CVs (first cycle) recorded for the polymerization of mixture of DPA and MA, it was found that the species responsible for the peaks in the UV spectra are produced in the potential range given by the cyclicvoltammogram where oxidation of both monomers takes place. A specific drop in the time derivative of absorption with potential beyond 0.70 V can be noticed in the DCVA curves for both wavelengths beyond 0.70 V. This gives a clue that the DPACR and MACR immediately undergo chemical reaction to result a dimer consisting of both DPA and MA moieties. Further oxidation of the dimer also can occur at the same potential range with an ultimate formation of oligomer/polymer. To support the assignments made for the peaks noticed 426 and 576 nm, bulk potentiostatic
polymerization studies were made with different feed compositions of DPA by holding the potential at 0.80 V. The UV /visible spectra were collected continuously while performing the electropolymerization (Fig. 9 and Fig. 10) through constant potential electrolysis. Here again, two peaks (426 and 576 nm) could be noticed with progressive increase in the absorbance. It is also possible to have an idea about the changes in proportions of DPA and MA units in the copolymer on changing the feed composition of the two monomers certainly, there is variation in the ratio of absorbance corresponding to the peaks at 426 /576 nm upon changing the feed ratio of DPA. This supports that changes in the feed composition of DPA produce a copolymer with difference in DPA or MA content in it. Hence, the results of in situ spectro electrochemical studies (Fig. 9 and Fig. 10) correlates with the changes in
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Fig. 10. UV /visible absorption spectra recorded during the constant potential electrochemical copolymerization of DPA with MA in 4 M H2SO4. [DPA] /5 mM; [MA]/10 mM; Potential/0.8 V vs. Ag/AgCl.
the DPA/MA content in the copolymer as noticed by elemental analysis (Table 1).
4. Conclussion The CVs representing the growth of copolymers during the electrochemical copolymerization of DPA with MA on electrode surface are found to show variations based on compositional changes of the two monomers in the copolymers. The new peak observed at 576 nm in the in situ UV /visible spectrum reveals the formation of intermediate with an incorporation of both monomeric units. A clear matching between peaks in DCVA deduced at 576 nm and CV supports the formation of intermediates in the potential range beyond 0.70 V. Elemental analysis of the copolymers confirms the compositional changes of DPA/MA in the copolymer as inferred from in situ spectro electrochemical results.
Acknowledgements The financial assistance from Department of Science and Technology (DST) (SP/S1-H12/97), New Delhi, India is acknowledged.
References [1] E.M. Genis, A. Boyle, M. Lapkwoski, C. Tsintavis, Synth. Met. 36 (1990) 139. [2] A.G. MacDiarmid, A.J. Epstein, Faraday Discuss. Chem. Soc. 88 (1989) 317. [3] A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65 (1994) 103. [4] J. Yue, A.J. Epstein, J. Am. Chem. Soc. 112 (1990) 2800. [5] J.Y. Bergenon, J.W. Charvelier, L.H. Dao, J. Chem. Soc. Chem. Commn. 180 (1990). [6] M.T. Ngyune, P. Kasai, J.L. Miller, A.F. Diaz, Macromolecules 27 (1994) 3625. [7] P. Hany, E.M. Genies, C. Santer, Synth. Met. 31 (1989) 369.
1950
M. Thanneermalai et al. / Spectrochimica Acta Part A 59 (2003) 1937 /1950
[8] Y. Wei, G.E. Wnek, A.G. MacDiarmid, A. Ray, W.W. Foke, J. Phys. Chem. 93 (1989) 495. [9] A. Watanabe, A. Lwabuchi, Y. Lwasaki, O. Ito, K. Mori, Y. Nakamura, Macromolecules 22 (1989) 3521. [10] C. DeArmitt, S.P. Armes, J. Winter, F.A. Uribe, S. Gottesfeld, C. Mombourguette, Polymer 34 (1993) 158. [11] H.S.O. Chan, S.C. Ng, W.S. Sim, K.L. Tan, B.T.G. Tan, Macromolecules 25 (1992) 6029. [12] A.A. Karyakin, A.K. Strakhova, A.K. Yatsimirsky, J. Electroanal. Chem. 371 (1994) 259. [13] M.T. Nguyen, A.F. Diaz, Macromolecules 28 (1995) 3411. [14] D. Macinnes, B.L. Funt, Synth. Met. 69 (1995) 235. [15] W.A. Gazotti, Jr, M.J.D.M. Jannini, S.I. Cordoba de Torresi, M.A. Depaoli, J. Electroanal. Chem. 440 (1997) 193. [16] D. Concalves, B. Matrienko, L.O.S. Bulhoes, J. Electroanal. Chem. 371 (1994) 267. [17] N. Comisso, S. Daolio, G. Zotti, S. Zecchin, R. Salmaso, G. Mengoli, J. Electroanal. Chem. 255 (1998) 97. [18] M.T. Nguyen, L.H. Dao, J. Electroanal. Chem. Interfacial Electrochem. 28 (1990) 37. [19] M.T. Nguyen, R. Paynter, L.H. Dao, Polymer 33 (1992) 214. [20] A. Bagheri, M.R. Nateghi, A. Massoumi, Synth. Met. 97 (1998) 85. [21] M.K. Ram, E. Maccioni, C. Nicolini, Thin Solid Films 303 (1997) 27. [22] S.K. Dhawan, D.C. Trivedi, Synth. Met. 60 (1993) 63. [23] J.Y. Bergeron, L.H. Dao, Macromolecules 25 (1992) 3332. [24] S. Ye, S. Besner, L.H. Dao, A.K. Vijh, J. Electroanal. Chem. 381 (1995) 71. [25] S. Ye, N.T. Do, L.H. Dao, A.K. Vijh, Synth. Met. 88 (1997) 65. [26] M.E. Jozefowicz, A.J. Epstein, J.P. Pouger, J.G. Masters, A. Ray, A.G. MacDiarmid, Macrmolecules 24 (1991) 5863.
[27] Z.H. Wang, A. Ray, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B43 (1991) 4373. [28] J. Pouger, S.L. Zhao, Z.H. Wang, Z. Oldakowski, A.J. Epstein, S.K. Manohar, J.M. Weisinger, A.G. MacDiarmid, C.H. Hsu, Synth. Met. 55 /57 (1993) 341. [29] A. Ito, K. Ora, K. Tanaka, T. Yamabe, Macromolecules 28 (1995) 5618. [30] J.W. Chavalier, J.Y. Bergenon, L.H. Dao, Macromolecules 25 (1992) 3325. [31] C.H. Yang, T.C. Wen, J. Electrochem. Soc. 144 (1997) 2078. [32] V. Rajendran, A. Gopalan, T. Vasudevan, T.C. Wen, J. Electrochem. Soc. 147 (2000) 3014. [33] M.S. Wu, T.C. Wen, A. Gopalan, J. Electrochem. Soc. 148 (2001) D65. [34] J. Bacon, R.N. Adams, J. Am. Chem. Soc. 90 (1968) 6596. [35] R.L. Hand, R.F. Nelson, J. Am. Chem. Soc. 96 (1974) 850. [36] R.L. Hand, R.F. Nelson, J. Am. Chem. Soc. 125 (1978) 1058. [37] D.M. Mohilner, R.N. Adams, W.J. Argesinger, J. Am. Chem. Soc. 13 (1992) 680. [38] T.J. Kemp, P. Moore, G.R. Quick, J. Chem. Res. 301 (1981) 1981. [39] G. Hembitzer, P.P. Heinz, I. Stassen, J. Heitbaum, Synth. Met. 55 (1993) 1317. [40] G. Hembitzer, I. Stassen, Synth. Met. 55 /57 (1993) 1045. [41] A. Malinauskus, R. Holz, Synth. Met. 97 (1998) 31. [42] C.Y. Chung, T.C. Wen, A. Gopalan, Electrochim. Acta 47 (2001) 423. [43] W.C. Chen, T.C. Wen, A. Gopalan, J. Electrochem. Soc. 148 (2001) 11. [44] M.S. Wu, T.C. Wen, A. Gopalan, Mater. Chem. Phys. 74 (2002) 58. [45] T.C. Wen, L.M. Huang, A. Gopalan, Electrochim. Acta 46 (2001) 2463. [46] T.C. Wen, C. Sivakumar, A. Gopalan, Electrochim. Acta 46 (2001) 1071.