Using JEI format

Journal of The Electrochemical Society, 148 共5兲 D65-D73 共2001兲

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0013-4651/2001/148共5兲/D65/9/$7.00 © The Electrochemical Society, Inc.

Electrochemical Copolymerization of Diphenylamine and Anthranilic Acid with Various Feed Ratios Ming-Sieng Wu,a Ten-Chin Wen,a,*, z and A. Gopalana,b a

Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan

Electrochemical copolymerization of diphenylamine 共DPA兲 with anthranilic acid 共AA兲 was performed in aqueous 4 M H2SO4 solution for different feed ratios of DPA using cyclic voltammetry. The copolymer film was grown for different numbers of cycles, C n , and feed ratios of DPA. Electrochemical homopolymerization of DPA was also carried out under identical conditions and compared. A growth equation of the deposited films in terms of concentrations of DPA and AA and C n is deduced as Q G ⫽ k 关 DPA兴 2.0关 AA兴 ⫺0.2C 1.2 n by utilizing the charge associated for film deposition. The copolymer compositions were determined by X-ray photoelectron spectroscopy. Reactivity ratios of DPA and AA were computed by using Fineman-Ross and Kelen-Tu¨do¨s methods. © 2001 The Electrochemical Society. 关DOI: 10.1149/1.1366625兴 All rights reserved. Manuscript submitted October 13, 2000; revised manuscript received January 22, 2001.

In recent years, polyaniline 共PANI兲 has attracted much attention for its applications in electrode materials,1,2 microelectronics,3-5 antistatic materials,6 and electrochromic materials,3,7-9 due to its unique reversible dopability, good recyclability, chemical stability, variable electrical conductivity, low cost, and ease of preparation. But the intractable and nonprocessable nature of PANI, owing to the stiffness of its backbone,10 restricts its further applications for commercial products. This necessitates modification of the PANI structure to achieve better processability. Modifications of the structure of the PANI chain have been achieved by several methods: 共i兲 posttreatment of the parent polyaniline base,11-13 共ii兲 homopolymerization of aniline derivatives,14-16 and 共iii兲 copolymerization of aniline/ aniline derivatives with different kinds of aniline derivatives.17-23 Several aniline derivatives bearing substituents such as alkyl, aryl, sulfonic acid, and alkoxy groups in the ring24-29 and alkyl, benzyl, and aryl groups at the nitrogen atom30-33 have been proposed for the preparation of conducting polymers, which are soluble in common organic solvents. Diphenylamine 共DPA兲, an N-substituted derivative, has been electropolymerized by Zotti and co-workers27 in a mixture of 4 M H2SO4 and ethanol. They could not grow poly共diphenylamine兲共PDPA兲 film on the electrode surface. The presence of ethanol as cosolvent caused dissolution of oligomeric products and hindered the growth. PDPA has recently been prepared by a special method and characterized.34 Studies on the synthesis of poly共N-alkyldiphenylamine兲, poly共3-methoxy兲diphenylamine,35 and poly共3-chlorodiphenylamine兲36 have been reported. Recently, DPA has been copolymerized with aniline37 and benzidine.38 These studies revealed that the phenyl-ended DPA moieties can couple with the ⫺NH2 group of aniline-like monomers to produce the copolymer. However, copolymerization studies with DPA as one monomer and a substituted aniline as the other monomer have not been made so far. While poly共anthranilic acid兲共PAA兲 was nonconducting, poly共aniline-co-anthranilic acid兲 was reported to be conducting. The copolymer showed a decreasing trend of conductivity with increase in the anthranilic acid 共AA兲 proportion in the copolymer.23 From X-ray photoelectron spectroscopy 共XPS兲 analysis, the copolymer was reported to have 42-60% AA units. In the present study, electrochemical copolymerization of DPA with AA was carried out under suitable conditions to deposit the copolymer using various feed ratios of the comonomers. Cyclic voltammetry was used to deposit and concomitantly characterize the copolymer films. Stilwell and Park39 deduced a growth equation for PANI deposition using cyclic voltammetry. In the present work, for

* Electrochemical Society Active Member. b z

Permanent address: Department of Industrial Chemistry, Alagappa University, Karaikudi 630003, India. E-mail: [email protected]

the copolymer deposition, a growth equation correlating the charge associated for deposition and growth conditions is deduced. The electrochemically synthesized copolymer is also characterized by XPS for its composition. Experimental Chemicals.—Analytical grade DPA 共Merck兲 and AA 共Riedel-de Hae¨n兲 were used as received. Solutions of DPA and AA were prepared in 4 M H2SO4 共Merck兲. Electrochemical copolymerization/homopolymerization.—The electrochemical copolymerization was performed in a single compartment reaction cell fitted with a Pt disk 共area 2 cm2兲 working electrode, a Pt wire auxiliary electrode, and a Ag/AgCl reference electrode by using an Autolab with PGSTAT 30 共Eco Chemie B. V., Netherlands兲. The copolymer films were deposited electrochemically on the Pt electrode surface from aqueous 4 M H2SO4 solution of DPA and AA in a fixed feed ratio of DPA 共represented as mole fraction of DPA in the feed兲 by reversibly cycling the potential between 0.00 and 0.70 V for 50 cycles at a constant scan rate of 100 mV/s. The cyclic voltammograms 共CVs兲 of the growing film of the copolymer were recorded continuously and coincidently with synthesis. Similar experiments were conducted using various feed ratios 共0.2, 0.4, 0.5, 0.6, and 0.8兲 of DPA, and CVs of the growing films of the copolymers were recorded for 50 cycles. Electrochemical homopolymerization of DPA was also performed by cyclic voltammetry in the same potential range. Electrochemical behavior of copolymer/homopolymer films.—The copolymer/homopolymer film coated electrode was repeatedly washed with 4 M H2SO4. The electrode was then placed in a monomer-free background electrolyte 共4 M H2SO4) and equilibrated by repetitive cycling of the potential in the range of 0.00-0.70 V until a constant CV pattern without any appreciable change in the peak current was obtained. Equilibration was achieved with a few cycles of potential scanning. The CVs of the stabilized copolymer/ homopolymer film were then recorded in the same potential range for various scan rates. XPS.—The binding energy and atomic percentage were determined by XPS using an ESCA 210 and MICROLAB 310D 共VG Scientific Ltd., U.K.兲 spectrometer. The copolymer films for the XPS analysis were obtained by performing electropolymerization at constant potential 共0.70 V兲 by using Pt as the working electrode for 15 min. The spectra employed Mg K␣ (h␯ ⫽ 1253.6 eV兲 irradiation as the photon source with a primary tension of 12 kV and an emission current of 20 mA. The pressure of the analysis chamber during the scans was about 10⫺10 mbar. Low resolution survey scans were done at 50 eV pass energy by using a step of 1 eV to obtain the atomic percentage data. After the survey spectra were obtained,

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Figure 1. CVs recorded during the growth of PDPA 共a兲 copolymer films on Pt electrodes for various molar feed ratios of DPA: 共b兲 0.8, 共c兲 0.6, 共d兲 0.4, and 共e兲 0.2 in 4 M H2SO4 at a scan rate of 100 mV/s. Total concentration of DPA and AA ⫽ 40 mM.



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higher resolution survey scans were performed at a pass energy of 20 eV, with a step of 200 meV, for at least 10 scans. Deconvolution of the XPS spectra was achieved by analysis with Grams/32 software 共Galactic兲 to obtain the subpeaks. Results and Discussion Electrochemical copolymerization/homopolymerization.—Figure 1 represents the CVs recorded continuously for 50 cycles during the polymerization of DPA and also for the copolymerization of DPA with AA using different molar feed ratios of DPA 共0.8, 0.6, 0.4, and 0.2兲. The anodic potential limit was kept at 0.70 V for both homopolymerization and copolymerization to obtain adherent depositions of polymeric/copolymeric films by avoiding thinning of the films at higher anodic potentials. A slight shift of the peak potential and suppression of the peak current values were noticed with increasing feed ratios of AA in the CVs corresponding to the copolymerization in comparison with homopolymerization of DPA. During the homopolymerization, a sharp rise in peak current values was observed beyond 0.6 V in the first anodic scan of potential. No prominent change in peak current was noticed in the cathodic scan. The second and subsequent anodic scans of potential showed two distinct anodic peaks at ca. 0.6 V (E ap共I兲兲 and 0.70 V (E ap共II兲兲, respectively. The corresponding cathodic peaks appeared at ca. 0.42 V (E cp共I兲兲 and 0.65 V (E cp共II兲兲, respectively, in the reverse scans. This type of increasing trend of peak current values was also noticed by other workers during the electropolymerization of aniline and substituted anilines in cyclic voltammetric studies.40,41 Stilwell and Park42 have attributed the progressive increase in the peak current to the continuous growth of electroactive polymer film on the electrode surface during electropolymerization. A green-colored deposit on the surface of the working electrode appeared in the present study. Otherwise, the color of the medium did not change during polymerization. These facts implied an adherent deposition of green-colored polymer on the surface of the working electrode. The growth of electroactive PDPA on the electrode can be explained by the ECE mechanism43 shown in Scheme 1. The DPA cation radical (DPA•⫹) was formed by fast one-electron oxidation of DPA, followed by a deprotonation process to produce the DPA radical 共steps 1 and 2兲. Then, the combination of the two DPA radicals produced C-C para-coupled diphenoquinonediimine 共steps 3 and 4兲. Further reactions were proposed to generate PDPA. PDPA subsequently can have diphenosemiquinoneamineimine and diphenoquinonediimine sequences as shown in Scheme 2b,c. The CVs recorded for the copolymerization revealed that the peak potentials and the peak current values were slightly different from those of the homopolymerization of DPA alone. The first anodic peak potential was shifted to more positive values for copolymerization 共0.605-0.611 V兲 in comparison with that for polymerization of DPA 共0.57-0.59 V兲. This kind of shift was noticed in the copolymerization for all the feed ratios of DPA. Furthermore, the peak current showed a decreasing trend with increasing feed ratio of AA. Hence, a definite participation of AA in altering the growth characteristics could be envisioned. Earlier reports on the electropolymerization of aniline in the presence of aniline derivatives as additives revealed that the additives were incorporated into the structure of the polymer41 and altered the rate of polymerization. Additives such as pphenylenediamine, N,N ⬘ -diphenylhydrazine, benzidine, and 4-aminodiphenylamine accelerated the polymerization of aniline, whereas hydroquinone and DPA retarded PANI formation. However, no correlation has been obtained between the oxidation potential of these additives and the rate of alteration in PANI growth. DPA and hydroquinone have higher oxidation potentials than aminodiphenylamine 共0.5 V兲 and phenylenediamine, but do not give a higher rate for aniline 共ANI兲 polymerization. Hence, the possibility of producing radical cations and the presence of sterically accessible aromatic amino groups are essential for the additive to incorporate into the structure of PANI in order to influence the growth. In a

Scheme 1. Mechanism for electropolymerization of DPA.

recent report from our research group, copolymer formation between ANI and DPA has been reported during the electropolymerization of mixtures of DPA and ANI.37 In this study, we propose a mechanism to explain the incorporation of AA into the polymer. This involves the reaction between the oligomeric cation radical (M1* ) of DPA and AA to produce AA•⫹ (M2* ). 共See Scheme 3.兲 Subsequent reactions of M2* with ANI or AA would produce copolymer in line with the mechanism of copolymerization proposed by earlier workers.44,45 Note that the reactions of M* 2 with AA become less probable due to the absence of polymerization of AA under these conditions. A mechanism of copolymerization of DPA and AA is proposed here based on our experimental results. The upper anodic potential limit was only sufficient to oxidize DPA to produce DPA cation radical (DPA•⫹) during initial oxidation37 共Scheme 3兲. The consequent production of oligomeric cation radical in the diphenosemi-


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Scheme 4. Mediatory role of DPA2⫹. 共a兲 AA• ⫹ formation and 共b兲 DPA• ⫹ formation.

redox reactions. It is therefore envisioned here that the radical cation of DPA catalyzes AA•⫹ formation through DPA2⫹, which can easily be switched from reduced to oxidized form 共Scheme 4兲. Scheme 2. General structure for reduced and oxidized forms of PDPA 共a兲 Reduced form of PDPA, 共b兲 oxidized PDPA with diphenosemiquinoneamineimine units, and 共c兲 oxidized PDPA with diphenoquinonnedimine units.

quinoneamineimine 共Scheme 2b兲 form can catalytically oxidize AA to produce AA•⫹. Several substituted aniline derivatives,46,47 such as 4-aminobiphenyl, p-phenylenediamine, N,N,N ⬘ ,N ⬘ tetramethyl-p-phenylenediamine, and N-phenyl-p-phenylenediamine produced stable radical cations and showed a marked effect in catalyzing the chemical oxidation of aniline. The generation of cation radicals from para-substituted triphenylamine has been reported to catalyze the oxidative polymerization of triphenylamine.48,49 Experimental evidence clearly indicated that these radical cations are capable of acting as mediators for electron-transfer reactions in the electrochemical polymerization. These mediators were reported to decrease the redox potentials of the reactions and facilitated the

Deducing the growth equation.—To establish the copolymer formation between DPA and AA, a growth equation relating the charge associated with deposition (Q G), cycle number (C n), and the feed ratio of DPA was deduced. A systematic approach was used to obtain the growth equation. Growth of the copolymer film was monitored by measuring the charge under the cathodic portion of the CV for various conditions. The charge associated with polymer deposition was calculated by integrating the current between the potentials 0.00 and 0.70 V. The cathodic charge, Q G , increased with increasing C n for a fixed feed ratio of DPA and with increasing feed ratio of DPA for a fixed value of C n as well. A general equation relating Q G to C n , 关DPA兴, and 关AA兴 can be written as follows Growth ⫽ Q G ⫽ k 关 DPA兴 x 关 AA兴 y C zn

关1兴

where k is the reaction rate constant for the deposition of the copolymer, and x, y, and z are the exponents of 关DPA兴, 关AA兴, and C n , respectively. To obtain the exact dependences 共x, y, and z兲 of growth of copolymer film on C n , 关DPA兴, and 关AA兴, a nonlinear regression method was utilized. The growth equation was thus deduced as Q G ⫽ k 关 DPA兴 2.0关 AA兴 ⫺0.2C 1.2 n

关2兴

Equation 2 was derived by considering the charge associated with the polymer deposition in the potential limit between 0.00 and 0.70 V. Q G values obtained for the variation of 关DPA兴, 关AA兴, and C n were used to deduce the growth equation. The dependence of growth on C n was further verified by drawing direct plots of Q G vs. C 1.2 n for fixed ratios of DPA, as shown in Fig. 2. These were clear straight lines passing through the origin. Also, the plot of Q G vs. 关 DPA兴 2.0关 AA兴 ⫺0.2 共Fig. 3兲 for fixed C n showed straight lines passing through the origin. The nature of these plots 共Fig. 2 and 3兲 clearly pointed out that growth of copolymer deposition followed the expression as given in Eq. 2. The involvement of AA in the growth equation clearly reveals that AA is incorporated into the structure of the polymer. It is obvious from the exponents of 关DPA兴 and 关AA兴 that increasing 关DPA兴 accelerates the growth of the polymer. Increasing 关AA兴 retards the growth. The rate constant, k, was calculated using Eq. 2 under various feed ratios of DPA and cycle number. All the calculated values are listed in Table I. The closeness of the values obtained through different approaches clearly supported Eq. 2 as correctly representing the growth of the copolymer.

Scheme 3. Initiation of copolymerization.

Electrochemical behavior of the poly(DPA-co-AA) film.—The CV patterns of the stabilized copolymer films 共Fig. 4b-e兲 are similar to the CV patterns obtained during copolymerization. The obvious peak size changes are attributed to changes in the number of charge sites available within the film. The slight differences in the peak



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Table I. Evaluation of rate constant for copolymer deposition.

DPA ratio

k value (⫻ 106 ) 共C mM⫺1.8兲

0.8

1.14

0.6

1.02

0.4

1.27

0.2

1.31

Cycle number 5 10 15 20 25 30 35 40 45 50

k value (⫻ 106 ) 共C mM⫺1.8兲 1.14 1.15 1.20 1.21 1.21 1.21 1.21 1.20 1.20 1.18

conditions of increasing 关DPA兴 also suggests that the active surface area of the film increases with DPA content in the copolymer.

potentials which were observed between homopolymerization of DPA and copolymerization alone could be seen for the copolymer films in comparison with PDPA films. Figure 5 shows the dependence of the peak current on the square root of sweep rate ( 冑v ) for the polymer films deposited with different feed ratios of DPA. The linear relationship obtained for the copolymer films indicates diffusion control of the reaction. The slopes of the plots 共Fig. 5兲 can be considered as proportional to the effective area of the films. The increase in slope values for the

XPS analysis.—XPS scans were taken for PDPA and copolymer films prepared by potentiostatic 共0.70 V兲 polymerization in 4 M H2SO4. Copolymer films were deposited with various feed ratios of DPA 共0.2, 0.4, 0.5, 0.6, and 0.8兲 in copolymerization. The surveylevel XPS scan clearly reveals the presence of C, N, S, and O in all the samples investigated. The C1s, N1s, and O1s core-level spectra for PDPA film and copolymer film deposited with a feed ratio of DPA as 0.6 are given in Fig. 6 and 7, respectively. The deconvolution of core-level peaks reveals the differences in structure of the films. The relative concentrations of C, N, S, and O in PDPA and copolymer films are derived from the respective photoelectron peak areas and are presented in Table II. Sensitivity factor corrections are applied for C1s 共1.00兲, N1s 共1.77兲, and O1s 共2.85兲 signals when obtaining the atomic percentages. The C1s core level was deconvoluted into four components, C—C or C—H 共283.92 eV兲, C—N or CvN 共285.02 eV兲, C—N⫹ or CvN⫹ 共285.80 eV兲, and CvO 共286.59 eV兲 for the PDPA film. Interestingly, all the copolymer samples show an extra component at a higher binding energy level 共288.14-288.81 eV兲20 for the existence of O—CvO, which was not found in the PDPA sample. The existence of the fifth component (O—CvO) in

Figure 3. Plots of charge vs. 关 DPA兴 2.0关 AA兴 ⫺0.2 obtained at different cycles in cyclic voltammetry during the growth of copolymer films. Scan rate: 100 mV/s.

Figure 4. CVs of PDPA and copolymer films in monomer-free background electrolyte. Films were prepared by 共a兲关 DPA兴 ⫽ 40 mM, molar feed ratio of DPA ⫽ 共b兲 0.8, 共c兲 0.6, 共d兲 0.4, and 共e兲 0.2. Scan rate: 100 mV/s.

Figure 2. Effect of cycle number on charge for the copolymer deposition with various molar feed ratios of DPA: 共a兲 0.8, 共b兲 0.6, 共c兲 0.4, and 共d兲 0.2. Scan rate: 100 mV/s.


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Journal of The Electrochemical Society, 148 共5兲 D65-D73 共2001兲 the summation of C—N and C—N⫹ components 共through deconvolution of N1s兲. 共See Fig. 7兲. Since C—C⬘ para-linked coupling exists 共Scheme 2兲 while forming copolymers, it can be considered that the N to C ratio differs for the portion of DPA units and AA units in the copolymer. Thus, the ratio of DPA to AA in the copolymer was determined as 关 DPA兴共 A C—N ⫺ A O—CvO兲 ⫽ 2A O—CvO 关 AA兴

关3兴

where A C—N and A O—CvO are the subpeak areas corresponding to C—N and O—CvO signals. These ratios were further used to obtain the reactivity ratios of DPA and AA in the copolymer. A simple copolymerization model was proposed to deduce the copolymer composition equation and the reactivity ratios of the two monomers. In the simple copolymerization model, after the formation of M* 1 and M* 2 as described in Schemes 2 and 3, the chain growth occurs by the addition of monomer units to M1* and M* 2 , generating active sites with new terminal units capable of undergoing further reactions.44 The reactions of M1* and M2* generate four possible reactions as follows k 11

M* 1 ⫹ M1 ——→ M1* Figure 5. Effect of scan rate on peak current 共first anodic peak兲 for the PDPA/copolymer films. 共a兲 DPA ⫽ 40 mM, feed ratio of DPA ⫽ 共b兲 0.8, 共c兲 0.6, 共d兲 0.4, and 共e兲 0.2. Number of cycles used for deposition is 50 cycles.

M* 1 ⫹ M2 ——→ M* 2

Copolymer composition and reactivity ratios.—The ratios of DPA and AA in the copolymer were calculated through area ratios of O—CvO component through C1s deconvolution 共Table II兲 and

Rate ⫽ k 12关 M* 1 兴关 M2 兴

k 22

M* 2 ⫹ M2 ——→ M* 2 the copolymer samples clearly indicates the presence of —COOH groups in the copolymer as a result of incorporation of AA units in the copolymer. The area ratio of the uO—CvO peak 共Table II兲 increases with the feed ratio of AA used for the deposition of copolymer film. This area ratio was therefore used to estimate the copolymer composition and reaction ratios of the two monomers, DPA and AA. The N1s core level for PDPA and copolymers were decomposed into four components.20 These four components were assigned for CvN, C—N, and CvN⫹ and C—N⫹ based on the binding energy of core electrons. The doping level as measured from the percentage area of positively charged nitrogen 共in the range of 401.34-402.41 eV兲 shows changes between PDPA and copolymer. As the AA ratio in the feed of copolymerization increases, the doping level decreases. This can be explained by the reduction in conjugation length caused by steric repulsion of —COOH groups in AA units of the copolymer. The lowering of conjugation length in the copolymer by the incorporation of AA units23 reduces the number of NH units in the copolymer, which subsequently reduces the doping level. In the copolymerization of aniline with AA, such a localization effect was attributed to intramolecular or intermolecular chelate ring formation.20 All the O1s envelopes have four components for the copolymer samples in contrast to the three observed for PDPA. The O1s spectrum of PDPA was decomposed into three subpeaks with CvO at 531.68 eV, S—O at 532.63 eV, and O—H at 533.2 eV.20 Interestingly, for all copolymer samples, there exists a fourth component of C—O in the range 532.49-533.75 eV.20 This clearly reveals the incorporation of AA units into the copolymer structure. Besides that, a few general observations can also be noted from the deconvolution of O1s core-level spectra. All the samples contained water. The amount of CvO component is slightly higher than C—O. The existence of CvO component in PDPA is attributed to the degradation of polymer film through hydrolysis to produce benzoquinone.50

Rate ⫽ k 11关 M1* 兴关 M1 兴

k 12

Rate ⫽ k 22关 M* 2 兴关 M2 兴

k 21

M* 2 ⫹ M1 ——→ M1*

Rate ⫽ k 21关 M2* 兴关 M1 兴

where k 11 , k 12 , k 22 , and k 21 are propagation rate constants. M1* stands for the diphenoquinonediimine form of oligomer 共Scheme 2兲, and M 2* stands for the cation radical form of oligomer chain with AA unit at the end. In the above copolymer model, the monomer disappearance is considered mainly in the propagation step, neglecting the changes in the concentration of monomer in the initiation and termination reactions.51 The reactivity ratios of DPA and AA are defined as r 1 and r 2 , respectively, from the ratios of propagation rate constants r1 ⫽

k 11 , k 12

r2 ⫽

k 22 k 21

A differential form of the simple copolymerization equation is then represented as d 关 M1 兴关 M1 兴共 r 1 关 M1 兴 ⫹ 关 M2 兴兲 ⫽ d 关 M2 兴关 M2 兴共 r 2 关 M2 兴 ⫹ 关 M1 兴兲

关4兴

The monomer reactivity ratios 共r 1 and r 2 兲 were calculated using the linearization procedure of the above equation by the Fineman-Ross52 and Kelen-Tu¨do¨s53 methods. Toward this purpose, the copolymer composition equation is rearranged as y⫽x

冉

1 ⫹ r 1x r2 ⫹ x

冊

where x and y are the molar concentration ratios of the comonomers in the feed and copolymer, respectively. On rearrangement, the Fineman-Ross form is obtained as G ⫽ r 1F ⫺ r 2 where G ⫽ x(y ⫺ 1)/y and F ⫽ x 2 /y.



Figure 6. XPS narrow scans of PDPA film found on Pt electrode by potentiostatic 共0.70 V兲 polymerization of DPA 共40 mM兲 for 共a兲 C1s, 共b兲 N1s, and 共c兲 O1s. The raw data 共jagged curve兲 have been deconvoluted into Gaussian peaks.

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Figure 7. XPS C1s narrow scans of copolymer film of DPA and AA with a feed ratio of DPA as 0.6 deposited on Pt electrode by potentiostatic 共0.70 V兲 polymerization of DPA 共40 mM兲 for 共a兲 C1s, 共b兲 N1s, and 共c兲 O1s. The raw data 共jagged curve兲 have been deconvoluted into Gaussian peaks.



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Table II. Surface elemental makeup of PDPA and copolymers. Feed ratio of DPA 1.0 0.8 0.6 0.5 0.4 0.2

Atomic percentage 共%兲 C1s

N1s

O1s

S2p

35.0 38.3 37.9 40.2 41.1 38.6

5.3 5.7 5.0 5.0 6.0 5.9

58.2 54.4 55.5 53.2 50.7 54.0

1.5 1.6 1.6 1.5 2.2 1.4

C1s*

N1s* ⫹

⫹

C—C/C—H C—N/CvN C—N /CvN 75.2 70.2 64.0 64.5 67.3 75.2

16.6 18.7 25.1 28.1 18.5 17.4

5.1 7.4 5.9 4.2 5.0 3.2

O1s*

CvO O—CvO CvN C—N —N 3.1 3.4 4.1 1.7 7.7 1.6

— 0.3 0.9 1.5 1.5 2.6

3.7 0.5 3.6 1.7 0.8 9.2

15.8 18.6 16.5 22.3 25.9 21.9

⫹

80.5 80.9 79.9 76.0 73.3 68.9

CvO SO4 C—O H2O 54.5 38.8 38.6 38.4 37.1 40.9

10.5 9.3 9.4 9.3 13.7 8.4

— 39.9 36.7 38.4 36.7 39.4

35.0 12.0 15.3 13.9 12.5 11.3

* Area ratios of subpeaks

The plot of G vs. F 共Fig. 8a兲 gives r 1 and r 2 as 10.04 and 0.02, respectively. To check the certainty of these values, the results were also analyzed through the Kelen-Tu¨do¨s procedure.53 It is also known that the Kelen-Tu¨do¨s 共K-T兲 procedure can be successfully applied to copolymerization data obtained up to a higher conversion 共50%兲 without much loss of precision with respect to copolymerization parameters.54 Since the copolymer films analyzed for composition of DPA and AA were electrochemically deposited, it is reasonable to consider the percent conversion as low 共far less than 50%兲. Hence, the K-T procedure was also adopted to verify r 1 and r 2 values. Kelen and Tu¨do¨s53 refined the linearization form by introducing an arbitrary constant ␣ to spread the data more evenly so as to apply equal weight to all data points. The K-T equation can be expressed as

冋

␩ r1 ⫹

册

r2 r2 ␰⫺ ␣ ␣

where ␩ ⫽ G/(␣ ⫹ F), ␰ ⫽ F/(␣ ⫹ F), and ␣ ⫽ 冑F m ⫻ F M , (F m is the minimum F value; F M is the maximum F value兲, and is used for obtaining r 1 and r 2 . A plot of ␩ vs. ␰ 共Fig. 8b兲 gives r 1 and r 2 values of 9.93 and 0.011, respectively. The values obtained through the Fineman-Ross equation 共Fig. 8a兲 and the K-T equation differ from one another by only 1.1% for r 1 . The observed high value of r 1 in comparison with r 2 indicates that electrophilic attack of the oxidized growing end by the DPA or AA chain preferentially occurs with DPA. Earlier, a similar explanation was given for the copolymer formation between aniline and o-ethylaniline.18 Conclusion Copolymers of DPA and AA with various feed ratios can be deposited on Pt surfaces by cyclic voltammetry. A growth equation interrelating the cathodic charge of copolymer deposition and concentration of DPA and AA, as well as the cycle number (C n), is deduced as Q G ⫽ k 关 DPA兴 2.0关 AA兴 ⫺0.2C 1.2 n . The k values derived through different approaches agree with each other. XPS results clearly demonstrate an increased proportion of AA in the copolymer with an increase in the molar ratio of AA in the feed. The reactivity ratios of the two monomers were computed through Fineman-Ross 9.93 Kelen-Tu¨do¨s method as 10.02 and 0.02, 0.011 for DPA and AA, respectively. This indicates that electrophilic attack of the oxidized growing end preferentially occurs with DPA during copolymerization. The details of all possible propagation reactions are given in the copolymerization model proposed for electrochemical copolymerization. Acknowledgment Figure 8. 共a兲 Fineman-Ross and 共b兲 Kelen-Tu¨do¨s plots for the copolymerization of DPA with AA. ␣ ⫽ 0.084.

The authors are grateful to the National Science Council in Taiwan for its financial support through NSC 89-2214-E-006-048.


Journal of The Electrochemical Society, 148 共5兲 D65-D73 共2001兲 National Cheng Kung University assisted in meeting the publication costs of this article.

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