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Studies on monitoring the composition of the copolymer by cyclic voltammetry and in situ spectroelectrochemical analysis
EUROPEAN POLYMER JOURNAL
European Polymer Journal 41 (2005) 97–105
www.elsevier.com/locate/europolj
Studies on monitoring the composition of the copolymer by cyclic voltammetry and in situ spectroelectrochemical analysis P. Santhosh a, A. Gopalan a
a,*
, T. Vasudevan a, T.-C. Wen
b
Department of Industrial Chemistry, Alagappa University, Alagappapuram, Karaikudi 630 003, India b Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 5 June 2004; received in revised form 1 August 2004; accepted 1 August 2004 Available online 18 September 2004
Abstract Electrochemical copolymerization of diphenylamine (DPA) with ortho-toluidine (OT) was carried out in 4 M sulphuric acid medium by cyclic voltammetry. Cyclic voltammograms (CVs) of the copolymer films were recorded to deduce the electrochemical characteristics. In situ UV–visible spectroelectrochemical studies on copolymerization were carried out using indium tin oxide (ITO) coated glass plate as working electrode for different feed ratios of DPA and OT. UVvisible spectral characteristics show clear dependencies on the molar feed composition of DPA or OT used in electropolymerization. Derivative cyclic voltabsorptogram (DCVA) was deduced at the wavelength corresponding to the absorption by the intermediate species and used to identify the intermediates generated during the electropolymerization. The molar composition of DPA and OT units in the copolymer for the copolymers synthesized with different molar feed ratios of DPA and OT was determined by UV–visible spectroscopy. Reactivity ratios of DPA and OT were deduced by using Fineman–Ross and Kelen–Tudos methods and the observed differences in the composition of DPA/ OT in the copolymers were correlated with CV characteristics and results obtained from in situ spectroelectrochemical studies. 2004 Elsevier Ltd. All rights reserved. Keywords: Electrocopolymerization; Cyclic voltammetry; In situ spectroelectro studies; UV–visible spectroscopy; Copolymer composition; Reactivity ratios
1. Introduction Electrically conducting polymers have attracted a great deal of attention in the last few decades because
* Corresponding author. Tel.: +91 4565 225205; fax: +91 4565 225202. E-mail address: [email protected] (A. Gopalan).
of their unusual intrinsic properties. The possibility of synthesizing materials having combined properties of organic polymers and of semiconductors becomes the main interest for both the academician and industrial researchers [1–4]. Among conducting polymers, polyaniline (PANI) is the most frequently used one for commercial applications, probably due to its thermal and environmental stability [5–7]. The multi various applications for conducting polymer demand different properties and good processability. As far as PANI is
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.08.003
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P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
concerned, its insolubility in common organic solvents and infusibility restricts processability. Nevertheless, enhanced solubility has been noticed for alkyl, alkoxy and sulphonic acid substituted PANIs [8–10]. However, many substituted PANI showed lesser conductivity than PANI [11]. Furthermore, the conductivity of doped emeraldine form of PANI is mainly due to the formation of polarons, which confer metallic properties of the polymer [11,12]. The synthesis of poly(alkyl anilines) has enabled the stabilization of charge carriers, since the electron–– donating effect of these substituents increases the basicity of the imine units [13]. Recently, polymerization of poly(2,3-dimethoxy aniline) were carried out and used as humidity sensor [14]. Extensive studies on PANI and some of its substituted derivatives have been carried out in the past few years. Poly(ortho-toluidine) (POT) is a derivative of PANI, where –CH3 is substituted in the aromatic ring. It has been reported that the conductivity of POT at room temperature is one or two order of magnitude lower than PANI [15]. Meixiang Wan et al. [16] studied the structure and electrical properties of POT in aqueous HCl medium. Electrochromic properties of POT have been reported [17]. The N-substituted derivatives exhibit an additional property of having conductivity comparable to that observed for PANI. Poly(diphenylamine) (PDPA), a Naryl substituted PANI derivative, possesses properties intermediates between PANI and poly(p-phenylene). In comparison with PANI, the copolymers with phenyl– substituted and N-substituted anilines give better solubility, disordered structure and decreased conductivity with enhanced electrochemical stability [18–21]. Reports are also available on the polymerization of N-alkyldiphenylamine, 3-methoxydiphenylamine and 3-chlorodiphenylamine [22,23]. Poly(N-methyl aniline), an alkyl N-substituted aniline is one of the materials which has received relative attention and few reports discussing its electrochemical properties [24]. Copolymerization of aniline with some of its derivatives, which bear various functional groups, leads to modified copolymers, having some remaining functionalities and possessing interesting properties. The primary advantage of copolymerization is that it leads to a homogenous material, the properties of which can be regulated by adjusting the ratio of the concentration of monomers in the feed. Karyakin et al. [24,25] obtained self-doped PANI by copolymerization of aniline with some carboxyl and sulfonyl substituted derivatives. Yang and Wen [26,27] reported an acceleration of rate for electropolymerization of PANI with p-phenylenediamine or with 2,5-diaminobenzenesulfonic acid. Several reports are available on the copolymerization of aniline and aniline derivatives [24–27]. However, reports involving diphenylamine (DPA) as one of the monomer in copolymerization is scarce. Recently Wu et al. [28] re-
ported the electrochemical copolymerization of DPA with anthranilic acid and the copolymer formation were further confirmed by X-ray photon spectroscopy (XPS). Electrochemical copolymerization of DPA with aniline using pulse potentiostatic method have been reported [29]. Electrochemical copolymerization of DPA with benzidine [30] have been carried out. Electrochemical copolymerization of DPA with 2,5-diaminobenzene sulphonic acid (DABSA) were reported [31]. The intermediates formed during the electrochemical copolymerization between DPA and DABSA were followed through in situ UV–visible studies. XPS was used to characterize the copolymer formed between DPA and DABSA. Copolymerization of 2-methoxyaniline with substituted anilines have been recently reported [32]. The characterization of the conducting forms of polymers have been performed by in situ spectroelectrochemical measurements, including optical absorption, vibration spectroscopy etc. UV–visible spectroscopy appears to be a useful tool for studying the electropolymerization process. Highly reactive intermediates are formed during the electropolymerization, these species react subsequently with solution species, giving oligomers and polymers, which are deposited on the electrode surface as a compact phase. This kind of study has the advantage to follow the doping process step by step. Leger et al. [33] presented a detailed study on the electropolymerization of o-toluidine by fast scan UV–visible differential reflectance spectroelectrochemistry. They identified a few possible intermediates as precursors to polymer formation. Genies and Lapkowski [34] used UV–visible spectroscopy to follow the course of polymerization of aniline and reported that the formation of nitrenium cations as intermediates during the polymerization. Malinauskas et al. [35,36] has monitored the early stages of polymerization of aniline and its derivatives through UV–visible spectroscopy. In this present study, DPA and OT have been selected as monomers for performing electrochemical copolymerization in 4 M H2SO4 medium. Cyclic voltammetry was used to deposit polymeric films on platinum working electrode and subsequent characterization. UV–visible spectroelectrochemical studies were performed to identify the intermediates formed during copolymerization. Copolymers were synthesized and the composition of the two monomers in the copolymer and reactivity ratios of DPA and OT were determined. A copolymer model which explains the changes in electrochemical and spectroelectrochemical characteristics is deduced.
2. Experimental Diphenylamine (E-Merck) and ortho-toluidine (EMerck) were used without any further purification. All the reagents used are of analytical grade.
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
2.1. Electrochemical synthesis of homo/copolymers The electrochemical copolymerization was performed in a single compartment reaction cell fitted with platinum disk (area 0.0245 cm2) as working electrode, a Pt wire and Ag/AgCl as auxiliary electrode and reference electrode, respectively by using EG&G PAR-Model 173, Potentiostat/Galvanostat Versastat II. The copolymer films were deposited electrochemically on the Pt electrode surface from aqueous 4 M H2SO4 solution of mixture of DPA and OT by reversible cycling the potential between 0.0 V and 0.8 V at a constant scan rate of 100 mV/s. The cyclic voltammograms (CVs) of the growing film of copolymer were recorded continuously and coincidently with synthesis. Polymerization experiments were performed for various mole fractions (0.125, 0.375, 0.75 and 0.875) of DPA and CVÕs were recorded for 50 cycles. Electrochemical homopolymerization of DPA and OT were also performed by cyclic voltammetry in the same potential range. 2.2. In situ spectroelectochemical studies A Shimadzu UVPC-2401 UV–visible spectrophotometer was used to record the in situ UV–visible spectra during the polymerization using time course mode. The experiments were done in quartz cuvette of 1 cm path length by assembling cell with ITO (indium doped tinoxide glass electrode) as working electrode, Ag/AgCl as reference and a platinum wire as counter electrode. Spectra were recorded concomitantly with polymerization while applying constant potential of about 0.8 V and potential sweep from 0.0 to 0.8 V at a scan rate of 1 mV/s by using BAS 100 A Electrochemical Analyzer. 2.3. Synthesis of copolymers Copolymers were synthesized by chemical oxidative polymerization [37] for various molar feed ratios of DPA and o-toluidine by using potassium peroxydisulphate (PDS) as the oxidant. A solution of monomers (40 mM DPA and 40 mM OT) were prepared in 4 M H2SO4 was cooled below 273 K using freezing mixture. A pre-cooled solution of PDS (30 mM) was then added dropwise to the monomer solution with the constant stirring over a period of 20 min. The solution was further stirred for 1 h in the freezing mixture. A bright emerald green precipitate was formed. The precipitate was filtered through a sintered-glass crucible and washed with 4 M H2SO4 continuously till the filtrate was colourless. The acid-doped polymer was then dried under vacuum for 48 h at room temperature. The copolymers were also prepared by performing bulk electropolymerization (0.8 V vs. Ag/AgCl) for various feed ratios of the monomers. Proper polymerization time is given to ensure the percentage formation of polymer as less than 10%. UV–
99
visible spectra were recorded for the synthesized polymers with different molar feed compositions in dimethyl formamide, DMF for specific concentration and the molar extinction coefficients at specific wavelength were calculated to obtain composition of the copolymer.
3. Results and discussion 3.1. Electrochemical homo/copolymerization Cyclic voltammogram recorded during the electrochemical copolymerization of DPA with OT for different feed ratios of DPA as 0.125, 0.375, 0.75, 0.875 in 4 M H2SO4 (Fig. 1a–d) are compared with CVs of the electochemical polymerization of OT or DPA (Fig. 1e and f). There seem to be differences in growth and electrochemical characteristics between copolymerization and individual polymerization of DPA or OT. During the first anodic scan of potential on polymerization with a mixture of DPA and OT, an anodic peak was observed beyond 0.60 V with a gradual shift in peak position towards more anodic side on increasing the feed ratio of DPA (Fig. 1a–d). We envisage the formation of electro active species generated from the oxidation of both DPA and OT that can ultimately result oligomer/ polymer formation having different proportions of DPA/OT units. This is justifiable by the fact that at the anodic potential beyond 0.60 V, both DPA and OT could be simultaneously oxidized [38] to generate the respective cation radicals, diphenylamine cation radical (DPACR) and o-toluidine cation radical (OTCR). In the reverse scan, two distinct peaks were observed at around 0.60 V and 0.40 V respectively. These peaks were assigned to the reduction of the oligomer/polymer formed by the cross-reaction between the intermediate species, DPACR and OTCR. In the second and subsequent CVs recorded for the copolymerization also reflect variations in the anodic and cathodic peak positions (Fig. 1a–d). While twin anodic peaks could be seen for the feed ratios of DPA as 0.125 and 0.375 (Fig. 1a and b), a single (merged) peak could be noticed for the higher feed ratios of DPA (Fig. 1c and d). Probably, the first anodic peak representing the growth of copolymer gets shifted to more anodic side with increasing feed ratios of DPA in the polymerization and ultimately merged with anodic second peak to result a single wave. On comparing these CV characteristics with CVÕs recorded for individual polymerization of OT and DPA, distinct differences could be identified. Clearly, the CVÕs of the polymerization of OT (Fig. 1e) which represents the growth of poly(o-toluidine), shows two anodic peaks at 0.21 V and 0.43 V respectively, with corresponding reduction counter parts at 0.16 V and 0.36 V [39]. The redox characteristics of
100
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105 150.00
60.00
(a)
(b) 50th cycle
100.00 40th
Current (µA)
Current (µA)
40.00
20.00 50th cycle 40th 30th 20th
30th
50.00 20th 10th 1st
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1st
0.00
-50.00
-20.00
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Potential (mV)
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Potential (mV)
1200.00
1200.00
(c)
(d) 50th cycle
800.00
800.00
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40th 30th
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Current (µA)
400.00
Current (µA)
20th 10th 1st
0.00
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Potential (mV) 2000.00
600.00
800.00
1500.00
(e)
(f)
50th cycle 45th
50th cycle
1000.00
1000.00
40th 35th 30th 25th 20th
500.00
15th
40th 1st cycle
Currrent (mA)
30th
Current (µA)
400.00
Potential (mV)
20th 10th
0.00
10th 5th 1st
0.00
-1000.00 -500.00
-1000.00
-2000.00 0.00
200.00
400.00
600.00
800.00
Potential (mV)
200
400
600 Potential (mV)
800
1000
Fig. 1. CVs recorded during the electrochemical copolymerization of DPA and OT with feed ratios of DPA: 0.125 (a), 0.375 (b), 0.75 (c), 0.875 (d). Total concentration of DPA and OT = 40 mM (a–d), Homopolymerization of OT (e); [OT] = 40 mM, Homopolymerization of DPA (f); [DPA] = 40 mM. CVs of first cycle and every subsequent 10 cycles are given.
POT have not been reflected at all for the polymer films deposited during copolymerization. Also, the twin redox characteristics of copolymer showed variations in comparison with PDPA (Fig. 1f). The peak positions in the CVs of copolymer growth showed changes with feed
composition of the two monomers. Taking into account of all these facts in conjunction, it is envisaged that copolymers having different molar compositions of DPA and OT are deposited on electrode surface when electropolymerization was performed with different feed
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
composition of the monomers, DPA and OT (Fig. 1a–d). To support our proposal, CVs of the copolymer film were recorded in 4 M H2SO4 for the copolymer films deposited with different ratios of DPA/OT (Fig. 2). The peak position corresponding to redox transitions remains the same when compared during growth. However, shift in peak position could be noticed for the copolymeric films deposited with different molar feed ratios of DPA or OT. These observations are in accordance with deposition of surface bound polymer film on the platinum electrode surface having variations in redox characteristics as a result of incorporation of DPA or OT in different extent.
101
300.00
g f e
Current (µA)
200.00
d c
100.00
b
0.00
a
-100.00
3.2. In situ spectroelectrochemical studies
-200.00 0.00
200.00
400.00
600.00
800.00
Potential (mV)
In situ UV–visible spectroelectrochemical studies were performed with different feed ratios of DPA to explore the possibility of copolymer formation between DPA and OT. Fig. 3a–c represents the UV–visible spectra recorded during the electrocopolymerization of DPA
Fig. 2. CVs of the film of poly(DPA-co-OT) in 4 M H2SO4. The copolymer films were deposited by using different feed ratios of DPA: 0.125 (a), 0.025 (b), 0.375 (c), 0.50 (d), 0.625 (e), 0.75 (f) and 0.875 (g). Scan rate 20 mV/s.
0.80
0.40
(a)
(b) 10 9 0.60
0.30
8
Absorbance
Absorbance
10 6 5 4 3 2 1
0.20
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7 0.40
6
0.20
4 3 2 1
5
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Absorbance
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Absorbance
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Wavelength (nm)
0.20 480.00
580.00
680.00
Wavelength (nm)
780.00
400.00
500.00 600.00 Wavelength (nm)
700.00
800.00
Fig. 3. In situ UV–visible spectra collected during constant potential electropolymerization of DPA with OT; Potential = 0.80 V vs. Ag/AgCl. DPA:OT = 8:2 (a), 8:5 (b), 5:5 (c), Homopolymerization of OT; [OT] = 40 mM (d).
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with OT while keeping the feed ratio of DPA as 0.8, and 0.5. Typically, four transition bands around 420 nm, 460 nm, 570 nm and >700 nm were noticed with shifting in position by the changes in the feed ratios of the comonomers. Besides, these spectra show variations in relative absorbances at these bands when the feed ratio of DPA or OT is changed. Earlier work [40] on electropolymerization of DPA reports three bands around 430 nm, 500 nm and >600 nm. Based on the literature [41], the band around 430 nm is assigned for the aniline type cation radical or oxidized benzidine type dimer generated in the backbone structure. The other two bands around 500 and >600 are assigned for the generation of diphenyl benzidine type oligomer cation radical (DPB+.) and N–N 0 diphenyl benzidine type dication (DPB2+) of the oligomer respectively. Incidentally, the bands observed in simple polymerization of OT (Fig. 3d) around 430 nm and 700 nm are assigned for the polaronic and bipolaronic transitions respectively [38]. It is interesting to note that a new band was noticed around 570 nm in the copolymerization, which was virtually absent in the case of polymerization of DPA or OT (Fig. 3d) and assigned as follows. Under the applied potential (0.80 V), DPA and OT can be oxidized to produce their corresponding cation radicals (DPACR and OTCR), which undergo cross-reaction to result dimer/ oligomers (Scheme 1). The existence of clear peak around 570 nm in the copolymerization is attributed to the formation of intermediate (dimer/oligomer) as a result of cross-reaction between DPACR and OTCR (Scheme 1). Further, for the copolymerization, the band due to the formation of anilinum type cation radical (428 nm) did not show any change in peak position whilst the band around 570 nm which corresponds to the generation of diphenyl benzidine type oligomer cation radical showed shifts while increasing the feed ratio of DPA in the copolymerization. On comparing the relative increase in the absorbance for the band around 570 nm in comparison with band around 420 nm, it can be inferred that DPACR is predominantly formed and participate in the copolymerization while increasing the feed ratio of DPA in the copolymerization (Fig. 3a and b). Derivative cyclicvoltabsorptogram (DCVA) was deduced at 570 nm [42] by utilizing UV–visible spectra collected while sweeping the potential during electropolymerization of DPA with OT (Fig. 4). A good correlation between the anodic peak noticed in DCVA (around 700 mV) and the peak in the CV (Fig. 1d) of the first anodic potential scan could be noticed. The decrease in the time derivative of absorbance (dA/dt) beyond 700 mV inform that the dimer/oligomer generated around 700 mV undergo further chemical reaction to result polymer. The reduction peaks around 0.55 mV and 0.43 mV indicate the stepwise reduction of dication form of the oligomer/polymer to neutral form.
3.3. Copolymer composition and reactivity ratios The results from the electrochemical and spectroelectrochemical studies show the copolymer formation during the polymerization with mixture of DPA and OT. The changing spectral characteristics in the copolymerization with variations in the molar feed composition of DPA/OT also signifies that a copolymer of varying compositions of DPA/OT would be formed under these conditions. To ascertain this, the composition of the two monomer units (DPA or OT) in the copolymer was determined by UV–visible spectroscopy based on Ramelow and Baysal [43] and extended for conducting polymers in the recent report [44]. Copolymers were synthesized by chemical oxidative polymerization for various molar feed ratios of DPA and OT. Proper polymerization time is given so as to ensure the percentage formation of polymer as less than 10% and the initial charge applied for producing the copolymer through electropolymerization was chosen depending on the monomer feed ratio. UV–visible spectra were collected for a series of diluted solutions (1 · 102 g/l to 6 · 102 g/l) of PDPA. The kmax value was found to be 341 nm. Using the values of absorbance, plots were made for absorbance vs. concentration of PDPA. These plots were found to be linear with negligible intercepts. Using absorbance, A = eCl, the average value of specific extinction coefficient, e(314 nm) was calculated as 0.316 · 102 l/g. Similarly, for POT solutions of varying concentrations (1 · 102 g/l to 6 · 102 g/l), UV– visible spectra were recorded and the molar specific extinction coefficient, e(354 nm), was calculated at kmax (354 nm), as 0.077 · 102 l/g. Using the spectra recorded for the copolymers synthesized with different molar feed concentrations of DPA or OT, the molar extinction coefficient of the copolymer, e12, was calculated and presented in Table 1 and used to determine molar composition of DPA and OT units in the copolymer. The mole fraction X1 of the monomer 1 in the copolymer has been determined by X1 ¼
e12 e2 e1 e2
where e12, e1, and e2 are the specific extinction coefficient of the copolymer (1 and 2), homopolymer 1 and 2 respectively. A plot of molar composition of OT in the copolymer (FOT) was drawn with molar feed composition of OT (fOT), which gives an azeotropic composition (Fig. 5). The reactivity ratios of DPA and OT (r1 and r2) which characterize the tendency of cross-reactions as given in simple first order kinetic scheme for copolymerization are determined by employing Fineman–Ross [45] and Kelen–Tudos [46] procedures. The molar compositions of DPA or OT in the copolymer, determined for various feed composition of the monomer, were used to find the reactivity ratios of
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
..
.NH +
[
e-
NH
.
103
]
+
NH
H
500 nm
CH 3 e-
.NH .
2
CH3
[
CH3
.
.N+H
2
NH 2
+
H
]
H
..N
+
430 nm
CH3
-2H+
CH 3 e-
H2 N +
H
.N
..
NH +
H2 N
+
700 nm
570 nm
CH3
H
.N+H
..N
.
+
+
NH
H
-2H+
H
CH3 + N
H
.
.. N
.N.
H
570 nm e
-
H
CH3 + N
H N +
.N.
H 700 nm CH 3
..
..
NH
H2 N
POLY (DPA-co-OT)
Scheme 1. Copolymerization of diphenylamine with ortho-toluidine.
DPA and OT as 0.46 and 0.23 (by Fineman–Ross [45] method) and as 0.44, 0.22 for DPA and MT (Kelen– Tudos [46] method), respectively (Fig. 6). From these re-
sults, it is concluded that a change in the feed composition of DPA or OT could alter the compositions of the DPA or OT units in the copolymer.
104
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105 1.2
FOT
0.9
0.6
0.3
0 0
Fig. 4. Derivative cyclic voltabsoptogram for 570 nm deduced from UV–visible spectra collected while sweeping the potential during copolymerization of DPA with OT (DPA:OT = 8:2).
0.3
0.6
0.9
1.2
f OT
Fig. 5. A plot of composition of OT in the copolymer vs. feed composition of OT.
4. Conclusions The results from electrochemical and UV–visible spectroelectrochemical studies on the copolymerization of DPA with OT inform that the electrochemical and spectral characteristics are different from polymerization of DPA or OT alone. Further, both the electrochemical and spectral characteristics during copolymerization are found to depend on molar feed composition of DPA or OT. A clear presence of a band around 570 nm in electropolymerization of mixture of DPA and OT indicates the generation of intermediate produced as a result of cross-reaction between radical generated from DPA
and OT with neutral OT/DPA. The derivative cyclic voltabsorptogram deduced at the wavelength of 570 nm corresponding to intermediate generation correlates with the information from cyclic voltammetry and inform that such intermediates are generated beyond 0.70 V. The molar composition of DPA in the copolymer increases with feed composition of DPA. The difference in reactivity ratio between DPA (0.46) and OT (0.23) could be the cause for the variations in the UV–visible spectral and electrochemical characteristics, when electropolymerization was performed with different feed ratios of DPA or OT.
Table 1 Molar composition of DPA or OT in Poly(DPA-co-OT) by UV–visible spectroscopy Feed ratio
Concentration of copolymer (·102 g/l)
DPA (mM)
OT (mM)
5
35
4 2
15
25
20
Average e (·102 l/g)
Molar composition of the copolymer DPA
OT
0.297
0.08
0.92
4 2
0.266
0.21
0.79
20
4 2
0.196
0.49
0.52
25
15
4 2
0.125
0.80
0.21
35
5
4 2
0.081
0.98
0.02
eDPA = 0.316 · 102 l/g, eOT = 0.077 · 102 l/g, kmax(PDPA) = 341 nm, kmax(POT) = 354 nm, kmax(DPA-co-OT) = 339 nm.
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105 0.9
(a)
F
0.6
0.3
0 0
0.5
1
1.5
2
G
0.9
(b) η
0.6
0.3
0 0.3
0.35
0.4
0.45 ξ
0.5
0.55
0.6
Fig. 6. (a) Fineman–Ross and (b) Kelen–Tudos plots for poly(DPA-co-OT) of OT.
Acknowledgment The authors gratefully acknowledge the financial assistance from Department of Science and Technology (DST), New Delhi, India (SP/S1-H12/97).
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[12] Jozefowicz ME, Laversanne R, Javadi HHS, Epstein AJ, Pouget JP, Tang X, et al. Phys Rev B 1989;39:12958. [13] Aprano GD, Leclerc M, Zotti G. Macromolecules 1992; 25:2145. [14] Kulkarni MV, Athwale AA. J Appl Polym Sci 2001; 81:1382. [15] Jiang H, Geng Y, Li J, Jing X, Wang F. Synth Met 1997;54:125. [16] Wan M, Yang J. Synth Met 1995;73:201. [17] Kumar D. Eur Polym J 1999;35:1919. [18] Ito A, Ota K, Tanaka K, Yamabe T. Macromolecules 1995;28:5618. [19] Dhawan SK, Trivedi DC. Synth Met 1993;60:63. [20] Nguyen MT, Diaz AF. Macromolecules 1995;28:3411. [21] Bagheri A, Nateghi MR, Massoumi A. Synth Met 1998;97:85. [22] Nguyen MT, Dao LH. J Electroanal Chem 1990;37:289. [23] Nguyen MT, Paynter R, Dao LH. Polymer 1992;33:214. [24] Karyakin AA, Strakhova AK, Yatsimirsky AK. J Electroanal Chem 1994;371:259. [25] Karyakin AA, Maltsev IA, Lukachova LV. J Electroanal Chem 1996;402:217. [26] Yang CH, Wen TC. J Appl Electrochem 1994;24:166. [27] Yang CH, Wen TC. J Electrochem Soc 1994;141:2624. [28] Wu MS, Wen TC, Gopalan A. J Electrochem Soc 2001; D65:148. [29] Rajendran V, Gopalan A, Vasudevan T, Wen TC. J Electrochem Soc 2000;147:3014. [30] Bagheri A, Mateghi MR, Massoumi A. Synth Met 1998; 97:85. [31] Chang CF, Chen WC, Wen TC, Gopalan A. J Electrochem Soc 2002;149:E298. [32] Mav I, Zigon M. Synth Met 2001;119:145. [33] Leger JM, Beden B, Lamy C, Ocon P, Sieiro C. Synth Met 1994;62:9. [34] Genies EM, Lapkowski M. J Electroanal Chem 1987; 236. [35] Malinauskas A, Holze R. Electrochim Acta 1998;43: 2413. [36] Malinauskas A, Holze R. Electrochim Acta 1999;44: 2613. [37] Huang WS, Humphrey BD, MacDiarmid AG. J Chem Soc Faraday Trans 1986;82:2385. [38] Wen TC, Sivakumar C, Gopalan A. Spectrochim Acta 2002;58:167. [39] Prakash S, Sivakumar C, Rajendran V, Vasudevan T, Gopalan A, Wen TC. Mater Chem Phys 2002;74:74. [40] Wu M, Wen TC, Gopalan A. Mater Chem Phys 2002;74:58. [41] Johnson BJ, Park SM. J Electrochem Soc 1996;143:1277. [42] Wen TC, Sivakumar C, Gopalan A. Electrochim Acta 2001;46:1071. [43] Ramelow U, Baysal BM. J Appl Polym Sci 1986;32:5865. [44] Santhosh P, Gopalan A, Vasudevan T. Spectrochim Acta 2003;59:1427. [45] Fineman M, Ross SD. J Polym Sci 1950;5:259. [46] Kelen T, Tudos F. J Macromol Sci Chem A 1975;9:1.
European Polymer Journal 41 (2005) 97–105
www.elsevier.com/locate/europolj
Studies on monitoring the composition of the copolymer by cyclic voltammetry and in situ spectroelectrochemical analysis P. Santhosh a, A. Gopalan a
a,*
, T. Vasudevan a, T.-C. Wen
b
Department of Industrial Chemistry, Alagappa University, Alagappapuram, Karaikudi 630 003, India b Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 5 June 2004; received in revised form 1 August 2004; accepted 1 August 2004 Available online 18 September 2004
Abstract Electrochemical copolymerization of diphenylamine (DPA) with ortho-toluidine (OT) was carried out in 4 M sulphuric acid medium by cyclic voltammetry. Cyclic voltammograms (CVs) of the copolymer films were recorded to deduce the electrochemical characteristics. In situ UV–visible spectroelectrochemical studies on copolymerization were carried out using indium tin oxide (ITO) coated glass plate as working electrode for different feed ratios of DPA and OT. UVvisible spectral characteristics show clear dependencies on the molar feed composition of DPA or OT used in electropolymerization. Derivative cyclic voltabsorptogram (DCVA) was deduced at the wavelength corresponding to the absorption by the intermediate species and used to identify the intermediates generated during the electropolymerization. The molar composition of DPA and OT units in the copolymer for the copolymers synthesized with different molar feed ratios of DPA and OT was determined by UV–visible spectroscopy. Reactivity ratios of DPA and OT were deduced by using Fineman–Ross and Kelen–Tudos methods and the observed differences in the composition of DPA/ OT in the copolymers were correlated with CV characteristics and results obtained from in situ spectroelectrochemical studies. 2004 Elsevier Ltd. All rights reserved. Keywords: Electrocopolymerization; Cyclic voltammetry; In situ spectroelectro studies; UV–visible spectroscopy; Copolymer composition; Reactivity ratios
1. Introduction Electrically conducting polymers have attracted a great deal of attention in the last few decades because
* Corresponding author. Tel.: +91 4565 225205; fax: +91 4565 225202. E-mail address: [email protected] (A. Gopalan).
of their unusual intrinsic properties. The possibility of synthesizing materials having combined properties of organic polymers and of semiconductors becomes the main interest for both the academician and industrial researchers [1–4]. Among conducting polymers, polyaniline (PANI) is the most frequently used one for commercial applications, probably due to its thermal and environmental stability [5–7]. The multi various applications for conducting polymer demand different properties and good processability. As far as PANI is
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.08.003
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P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
concerned, its insolubility in common organic solvents and infusibility restricts processability. Nevertheless, enhanced solubility has been noticed for alkyl, alkoxy and sulphonic acid substituted PANIs [8–10]. However, many substituted PANI showed lesser conductivity than PANI [11]. Furthermore, the conductivity of doped emeraldine form of PANI is mainly due to the formation of polarons, which confer metallic properties of the polymer [11,12]. The synthesis of poly(alkyl anilines) has enabled the stabilization of charge carriers, since the electron–– donating effect of these substituents increases the basicity of the imine units [13]. Recently, polymerization of poly(2,3-dimethoxy aniline) were carried out and used as humidity sensor [14]. Extensive studies on PANI and some of its substituted derivatives have been carried out in the past few years. Poly(ortho-toluidine) (POT) is a derivative of PANI, where –CH3 is substituted in the aromatic ring. It has been reported that the conductivity of POT at room temperature is one or two order of magnitude lower than PANI [15]. Meixiang Wan et al. [16] studied the structure and electrical properties of POT in aqueous HCl medium. Electrochromic properties of POT have been reported [17]. The N-substituted derivatives exhibit an additional property of having conductivity comparable to that observed for PANI. Poly(diphenylamine) (PDPA), a Naryl substituted PANI derivative, possesses properties intermediates between PANI and poly(p-phenylene). In comparison with PANI, the copolymers with phenyl– substituted and N-substituted anilines give better solubility, disordered structure and decreased conductivity with enhanced electrochemical stability [18–21]. Reports are also available on the polymerization of N-alkyldiphenylamine, 3-methoxydiphenylamine and 3-chlorodiphenylamine [22,23]. Poly(N-methyl aniline), an alkyl N-substituted aniline is one of the materials which has received relative attention and few reports discussing its electrochemical properties [24]. Copolymerization of aniline with some of its derivatives, which bear various functional groups, leads to modified copolymers, having some remaining functionalities and possessing interesting properties. The primary advantage of copolymerization is that it leads to a homogenous material, the properties of which can be regulated by adjusting the ratio of the concentration of monomers in the feed. Karyakin et al. [24,25] obtained self-doped PANI by copolymerization of aniline with some carboxyl and sulfonyl substituted derivatives. Yang and Wen [26,27] reported an acceleration of rate for electropolymerization of PANI with p-phenylenediamine or with 2,5-diaminobenzenesulfonic acid. Several reports are available on the copolymerization of aniline and aniline derivatives [24–27]. However, reports involving diphenylamine (DPA) as one of the monomer in copolymerization is scarce. Recently Wu et al. [28] re-
ported the electrochemical copolymerization of DPA with anthranilic acid and the copolymer formation were further confirmed by X-ray photon spectroscopy (XPS). Electrochemical copolymerization of DPA with aniline using pulse potentiostatic method have been reported [29]. Electrochemical copolymerization of DPA with benzidine [30] have been carried out. Electrochemical copolymerization of DPA with 2,5-diaminobenzene sulphonic acid (DABSA) were reported [31]. The intermediates formed during the electrochemical copolymerization between DPA and DABSA were followed through in situ UV–visible studies. XPS was used to characterize the copolymer formed between DPA and DABSA. Copolymerization of 2-methoxyaniline with substituted anilines have been recently reported [32]. The characterization of the conducting forms of polymers have been performed by in situ spectroelectrochemical measurements, including optical absorption, vibration spectroscopy etc. UV–visible spectroscopy appears to be a useful tool for studying the electropolymerization process. Highly reactive intermediates are formed during the electropolymerization, these species react subsequently with solution species, giving oligomers and polymers, which are deposited on the electrode surface as a compact phase. This kind of study has the advantage to follow the doping process step by step. Leger et al. [33] presented a detailed study on the electropolymerization of o-toluidine by fast scan UV–visible differential reflectance spectroelectrochemistry. They identified a few possible intermediates as precursors to polymer formation. Genies and Lapkowski [34] used UV–visible spectroscopy to follow the course of polymerization of aniline and reported that the formation of nitrenium cations as intermediates during the polymerization. Malinauskas et al. [35,36] has monitored the early stages of polymerization of aniline and its derivatives through UV–visible spectroscopy. In this present study, DPA and OT have been selected as monomers for performing electrochemical copolymerization in 4 M H2SO4 medium. Cyclic voltammetry was used to deposit polymeric films on platinum working electrode and subsequent characterization. UV–visible spectroelectrochemical studies were performed to identify the intermediates formed during copolymerization. Copolymers were synthesized and the composition of the two monomers in the copolymer and reactivity ratios of DPA and OT were determined. A copolymer model which explains the changes in electrochemical and spectroelectrochemical characteristics is deduced.
2. Experimental Diphenylamine (E-Merck) and ortho-toluidine (EMerck) were used without any further purification. All the reagents used are of analytical grade.
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
2.1. Electrochemical synthesis of homo/copolymers The electrochemical copolymerization was performed in a single compartment reaction cell fitted with platinum disk (area 0.0245 cm2) as working electrode, a Pt wire and Ag/AgCl as auxiliary electrode and reference electrode, respectively by using EG&G PAR-Model 173, Potentiostat/Galvanostat Versastat II. The copolymer films were deposited electrochemically on the Pt electrode surface from aqueous 4 M H2SO4 solution of mixture of DPA and OT by reversible cycling the potential between 0.0 V and 0.8 V at a constant scan rate of 100 mV/s. The cyclic voltammograms (CVs) of the growing film of copolymer were recorded continuously and coincidently with synthesis. Polymerization experiments were performed for various mole fractions (0.125, 0.375, 0.75 and 0.875) of DPA and CVÕs were recorded for 50 cycles. Electrochemical homopolymerization of DPA and OT were also performed by cyclic voltammetry in the same potential range. 2.2. In situ spectroelectochemical studies A Shimadzu UVPC-2401 UV–visible spectrophotometer was used to record the in situ UV–visible spectra during the polymerization using time course mode. The experiments were done in quartz cuvette of 1 cm path length by assembling cell with ITO (indium doped tinoxide glass electrode) as working electrode, Ag/AgCl as reference and a platinum wire as counter electrode. Spectra were recorded concomitantly with polymerization while applying constant potential of about 0.8 V and potential sweep from 0.0 to 0.8 V at a scan rate of 1 mV/s by using BAS 100 A Electrochemical Analyzer. 2.3. Synthesis of copolymers Copolymers were synthesized by chemical oxidative polymerization [37] for various molar feed ratios of DPA and o-toluidine by using potassium peroxydisulphate (PDS) as the oxidant. A solution of monomers (40 mM DPA and 40 mM OT) were prepared in 4 M H2SO4 was cooled below 273 K using freezing mixture. A pre-cooled solution of PDS (30 mM) was then added dropwise to the monomer solution with the constant stirring over a period of 20 min. The solution was further stirred for 1 h in the freezing mixture. A bright emerald green precipitate was formed. The precipitate was filtered through a sintered-glass crucible and washed with 4 M H2SO4 continuously till the filtrate was colourless. The acid-doped polymer was then dried under vacuum for 48 h at room temperature. The copolymers were also prepared by performing bulk electropolymerization (0.8 V vs. Ag/AgCl) for various feed ratios of the monomers. Proper polymerization time is given to ensure the percentage formation of polymer as less than 10%. UV–
99
visible spectra were recorded for the synthesized polymers with different molar feed compositions in dimethyl formamide, DMF for specific concentration and the molar extinction coefficients at specific wavelength were calculated to obtain composition of the copolymer.
3. Results and discussion 3.1. Electrochemical homo/copolymerization Cyclic voltammogram recorded during the electrochemical copolymerization of DPA with OT for different feed ratios of DPA as 0.125, 0.375, 0.75, 0.875 in 4 M H2SO4 (Fig. 1a–d) are compared with CVs of the electochemical polymerization of OT or DPA (Fig. 1e and f). There seem to be differences in growth and electrochemical characteristics between copolymerization and individual polymerization of DPA or OT. During the first anodic scan of potential on polymerization with a mixture of DPA and OT, an anodic peak was observed beyond 0.60 V with a gradual shift in peak position towards more anodic side on increasing the feed ratio of DPA (Fig. 1a–d). We envisage the formation of electro active species generated from the oxidation of both DPA and OT that can ultimately result oligomer/ polymer formation having different proportions of DPA/OT units. This is justifiable by the fact that at the anodic potential beyond 0.60 V, both DPA and OT could be simultaneously oxidized [38] to generate the respective cation radicals, diphenylamine cation radical (DPACR) and o-toluidine cation radical (OTCR). In the reverse scan, two distinct peaks were observed at around 0.60 V and 0.40 V respectively. These peaks were assigned to the reduction of the oligomer/polymer formed by the cross-reaction between the intermediate species, DPACR and OTCR. In the second and subsequent CVs recorded for the copolymerization also reflect variations in the anodic and cathodic peak positions (Fig. 1a–d). While twin anodic peaks could be seen for the feed ratios of DPA as 0.125 and 0.375 (Fig. 1a and b), a single (merged) peak could be noticed for the higher feed ratios of DPA (Fig. 1c and d). Probably, the first anodic peak representing the growth of copolymer gets shifted to more anodic side with increasing feed ratios of DPA in the polymerization and ultimately merged with anodic second peak to result a single wave. On comparing these CV characteristics with CVÕs recorded for individual polymerization of OT and DPA, distinct differences could be identified. Clearly, the CVÕs of the polymerization of OT (Fig. 1e) which represents the growth of poly(o-toluidine), shows two anodic peaks at 0.21 V and 0.43 V respectively, with corresponding reduction counter parts at 0.16 V and 0.36 V [39]. The redox characteristics of
100
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105 150.00
60.00
(a)
(b) 50th cycle
100.00 40th
Current (µA)
Current (µA)
40.00
20.00 50th cycle 40th 30th 20th
30th
50.00 20th 10th 1st
0.00
10th
1st
0.00
-50.00
-20.00
-100.00 0.00
200.00
400.00
600.00
800.00
0.00
200.00
Potential (mV)
400.00
600.00
800.00
Potential (mV)
1200.00
1200.00
(c)
(d) 50th cycle
800.00
800.00
50th cycle
40th
40th 30th
30th
Current (µA)
400.00
Current (µA)
20th 10th 1st
0.00
-400.00
400.00
20th 10th 1st
0.00
-400.00
-800.00
-800.00
0.00
200.00
400.00
600.00
800.00
0.00
200.00
Potential (mV) 2000.00
600.00
800.00
1500.00
(e)
(f)
50th cycle 45th
50th cycle
1000.00
1000.00
40th 35th 30th 25th 20th
500.00
15th
40th 1st cycle
Currrent (mA)
30th
Current (µA)
400.00
Potential (mV)
20th 10th
0.00
10th 5th 1st
0.00
-1000.00 -500.00
-1000.00
-2000.00 0.00
200.00
400.00
600.00
800.00
Potential (mV)
200
400
600 Potential (mV)
800
1000
Fig. 1. CVs recorded during the electrochemical copolymerization of DPA and OT with feed ratios of DPA: 0.125 (a), 0.375 (b), 0.75 (c), 0.875 (d). Total concentration of DPA and OT = 40 mM (a–d), Homopolymerization of OT (e); [OT] = 40 mM, Homopolymerization of DPA (f); [DPA] = 40 mM. CVs of first cycle and every subsequent 10 cycles are given.
POT have not been reflected at all for the polymer films deposited during copolymerization. Also, the twin redox characteristics of copolymer showed variations in comparison with PDPA (Fig. 1f). The peak positions in the CVs of copolymer growth showed changes with feed
composition of the two monomers. Taking into account of all these facts in conjunction, it is envisaged that copolymers having different molar compositions of DPA and OT are deposited on electrode surface when electropolymerization was performed with different feed
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
composition of the monomers, DPA and OT (Fig. 1a–d). To support our proposal, CVs of the copolymer film were recorded in 4 M H2SO4 for the copolymer films deposited with different ratios of DPA/OT (Fig. 2). The peak position corresponding to redox transitions remains the same when compared during growth. However, shift in peak position could be noticed for the copolymeric films deposited with different molar feed ratios of DPA or OT. These observations are in accordance with deposition of surface bound polymer film on the platinum electrode surface having variations in redox characteristics as a result of incorporation of DPA or OT in different extent.
101
300.00
g f e
Current (µA)
200.00
d c
100.00
b
0.00
a
-100.00
3.2. In situ spectroelectrochemical studies
-200.00 0.00
200.00
400.00
600.00
800.00
Potential (mV)
In situ UV–visible spectroelectrochemical studies were performed with different feed ratios of DPA to explore the possibility of copolymer formation between DPA and OT. Fig. 3a–c represents the UV–visible spectra recorded during the electrocopolymerization of DPA
Fig. 2. CVs of the film of poly(DPA-co-OT) in 4 M H2SO4. The copolymer films were deposited by using different feed ratios of DPA: 0.125 (a), 0.025 (b), 0.375 (c), 0.50 (d), 0.625 (e), 0.75 (f) and 0.875 (g). Scan rate 20 mV/s.
0.80
0.40
(a)
(b) 10 9 0.60
0.30
8
Absorbance
Absorbance
10 6 5 4 3 2 1
0.20
0.10
0.00
7 0.40
6
0.20
4 3 2 1
5
0.00 400.00
500.00
600.00
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Wavelength (nm) 0.50
680.00
780.00
1.00
(c)
0.30
(d) 10 10 9 7 6 5
8
4 3 2
0.20 1
7
0.60
6 5 4 3 2 1
0.40
0.10
0.00 380.00
9
0.80
Absorbance
0.40
Absorbance
580.00
Wavelength (nm)
0.20 480.00
580.00
680.00
Wavelength (nm)
780.00
400.00
500.00 600.00 Wavelength (nm)
700.00
800.00
Fig. 3. In situ UV–visible spectra collected during constant potential electropolymerization of DPA with OT; Potential = 0.80 V vs. Ag/AgCl. DPA:OT = 8:2 (a), 8:5 (b), 5:5 (c), Homopolymerization of OT; [OT] = 40 mM (d).
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P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
with OT while keeping the feed ratio of DPA as 0.8, and 0.5. Typically, four transition bands around 420 nm, 460 nm, 570 nm and >700 nm were noticed with shifting in position by the changes in the feed ratios of the comonomers. Besides, these spectra show variations in relative absorbances at these bands when the feed ratio of DPA or OT is changed. Earlier work [40] on electropolymerization of DPA reports three bands around 430 nm, 500 nm and >600 nm. Based on the literature [41], the band around 430 nm is assigned for the aniline type cation radical or oxidized benzidine type dimer generated in the backbone structure. The other two bands around 500 and >600 are assigned for the generation of diphenyl benzidine type oligomer cation radical (DPB+.) and N–N 0 diphenyl benzidine type dication (DPB2+) of the oligomer respectively. Incidentally, the bands observed in simple polymerization of OT (Fig. 3d) around 430 nm and 700 nm are assigned for the polaronic and bipolaronic transitions respectively [38]. It is interesting to note that a new band was noticed around 570 nm in the copolymerization, which was virtually absent in the case of polymerization of DPA or OT (Fig. 3d) and assigned as follows. Under the applied potential (0.80 V), DPA and OT can be oxidized to produce their corresponding cation radicals (DPACR and OTCR), which undergo cross-reaction to result dimer/ oligomers (Scheme 1). The existence of clear peak around 570 nm in the copolymerization is attributed to the formation of intermediate (dimer/oligomer) as a result of cross-reaction between DPACR and OTCR (Scheme 1). Further, for the copolymerization, the band due to the formation of anilinum type cation radical (428 nm) did not show any change in peak position whilst the band around 570 nm which corresponds to the generation of diphenyl benzidine type oligomer cation radical showed shifts while increasing the feed ratio of DPA in the copolymerization. On comparing the relative increase in the absorbance for the band around 570 nm in comparison with band around 420 nm, it can be inferred that DPACR is predominantly formed and participate in the copolymerization while increasing the feed ratio of DPA in the copolymerization (Fig. 3a and b). Derivative cyclicvoltabsorptogram (DCVA) was deduced at 570 nm [42] by utilizing UV–visible spectra collected while sweeping the potential during electropolymerization of DPA with OT (Fig. 4). A good correlation between the anodic peak noticed in DCVA (around 700 mV) and the peak in the CV (Fig. 1d) of the first anodic potential scan could be noticed. The decrease in the time derivative of absorbance (dA/dt) beyond 700 mV inform that the dimer/oligomer generated around 700 mV undergo further chemical reaction to result polymer. The reduction peaks around 0.55 mV and 0.43 mV indicate the stepwise reduction of dication form of the oligomer/polymer to neutral form.
3.3. Copolymer composition and reactivity ratios The results from the electrochemical and spectroelectrochemical studies show the copolymer formation during the polymerization with mixture of DPA and OT. The changing spectral characteristics in the copolymerization with variations in the molar feed composition of DPA/OT also signifies that a copolymer of varying compositions of DPA/OT would be formed under these conditions. To ascertain this, the composition of the two monomer units (DPA or OT) in the copolymer was determined by UV–visible spectroscopy based on Ramelow and Baysal [43] and extended for conducting polymers in the recent report [44]. Copolymers were synthesized by chemical oxidative polymerization for various molar feed ratios of DPA and OT. Proper polymerization time is given so as to ensure the percentage formation of polymer as less than 10% and the initial charge applied for producing the copolymer through electropolymerization was chosen depending on the monomer feed ratio. UV–visible spectra were collected for a series of diluted solutions (1 · 102 g/l to 6 · 102 g/l) of PDPA. The kmax value was found to be 341 nm. Using the values of absorbance, plots were made for absorbance vs. concentration of PDPA. These plots were found to be linear with negligible intercepts. Using absorbance, A = eCl, the average value of specific extinction coefficient, e(314 nm) was calculated as 0.316 · 102 l/g. Similarly, for POT solutions of varying concentrations (1 · 102 g/l to 6 · 102 g/l), UV– visible spectra were recorded and the molar specific extinction coefficient, e(354 nm), was calculated at kmax (354 nm), as 0.077 · 102 l/g. Using the spectra recorded for the copolymers synthesized with different molar feed concentrations of DPA or OT, the molar extinction coefficient of the copolymer, e12, was calculated and presented in Table 1 and used to determine molar composition of DPA and OT units in the copolymer. The mole fraction X1 of the monomer 1 in the copolymer has been determined by X1 ¼
e12 e2 e1 e2
where e12, e1, and e2 are the specific extinction coefficient of the copolymer (1 and 2), homopolymer 1 and 2 respectively. A plot of molar composition of OT in the copolymer (FOT) was drawn with molar feed composition of OT (fOT), which gives an azeotropic composition (Fig. 5). The reactivity ratios of DPA and OT (r1 and r2) which characterize the tendency of cross-reactions as given in simple first order kinetic scheme for copolymerization are determined by employing Fineman–Ross [45] and Kelen–Tudos [46] procedures. The molar compositions of DPA or OT in the copolymer, determined for various feed composition of the monomer, were used to find the reactivity ratios of
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105
..
.NH +
[
e-
NH
.
103
]
+
NH
H
500 nm
CH 3 e-
.NH .
2
CH3
[
CH3
.
.N+H
2
NH 2
+
H
]
H
..N
+
430 nm
CH3
-2H+
CH 3 e-
H2 N +
H
.N
..
NH +
H2 N
+
700 nm
570 nm
CH3
H
.N+H
..N
.
+
+
NH
H
-2H+
H
CH3 + N
H
.
.. N
.N.
H
570 nm e
-
H
CH3 + N
H N +
.N.
H 700 nm CH 3
..
..
NH
H2 N
POLY (DPA-co-OT)
Scheme 1. Copolymerization of diphenylamine with ortho-toluidine.
DPA and OT as 0.46 and 0.23 (by Fineman–Ross [45] method) and as 0.44, 0.22 for DPA and MT (Kelen– Tudos [46] method), respectively (Fig. 6). From these re-
sults, it is concluded that a change in the feed composition of DPA or OT could alter the compositions of the DPA or OT units in the copolymer.
104
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105 1.2
FOT
0.9
0.6
0.3
0 0
Fig. 4. Derivative cyclic voltabsoptogram for 570 nm deduced from UV–visible spectra collected while sweeping the potential during copolymerization of DPA with OT (DPA:OT = 8:2).
0.3
0.6
0.9
1.2
f OT
Fig. 5. A plot of composition of OT in the copolymer vs. feed composition of OT.
4. Conclusions The results from electrochemical and UV–visible spectroelectrochemical studies on the copolymerization of DPA with OT inform that the electrochemical and spectral characteristics are different from polymerization of DPA or OT alone. Further, both the electrochemical and spectral characteristics during copolymerization are found to depend on molar feed composition of DPA or OT. A clear presence of a band around 570 nm in electropolymerization of mixture of DPA and OT indicates the generation of intermediate produced as a result of cross-reaction between radical generated from DPA
and OT with neutral OT/DPA. The derivative cyclic voltabsorptogram deduced at the wavelength of 570 nm corresponding to intermediate generation correlates with the information from cyclic voltammetry and inform that such intermediates are generated beyond 0.70 V. The molar composition of DPA in the copolymer increases with feed composition of DPA. The difference in reactivity ratio between DPA (0.46) and OT (0.23) could be the cause for the variations in the UV–visible spectral and electrochemical characteristics, when electropolymerization was performed with different feed ratios of DPA or OT.
Table 1 Molar composition of DPA or OT in Poly(DPA-co-OT) by UV–visible spectroscopy Feed ratio
Concentration of copolymer (·102 g/l)
DPA (mM)
OT (mM)
5
35
4 2
15
25
20
Average e (·102 l/g)
Molar composition of the copolymer DPA
OT
0.297
0.08
0.92
4 2
0.266
0.21
0.79
20
4 2
0.196
0.49
0.52
25
15
4 2
0.125
0.80
0.21
35
5
4 2
0.081
0.98
0.02
eDPA = 0.316 · 102 l/g, eOT = 0.077 · 102 l/g, kmax(PDPA) = 341 nm, kmax(POT) = 354 nm, kmax(DPA-co-OT) = 339 nm.
P. Santhosh et al. / European Polymer Journal 41 (2005) 97–105 0.9
(a)
F
0.6
0.3
0 0
0.5
1
1.5
2
G
0.9
(b) η
0.6
0.3
0 0.3
0.35
0.4
0.45 ξ
0.5
0.55
0.6
Fig. 6. (a) Fineman–Ross and (b) Kelen–Tudos plots for poly(DPA-co-OT) of OT.
Acknowledgment The authors gratefully acknowledge the financial assistance from Department of Science and Technology (DST), New Delhi, India (SP/S1-H12/97).
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