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Studies on electrochemical copolymerization of aniline with o-phenylenediamine and degradation of the resultant copolymers via electrochemical quartz crystal microbalance and scanning electrochemical microscope
Synthetic Metals 156 (2006) 444–453
Studies on electrochemical copolymerization of aniline with o-phenylenediamine and degradation of the resultant copolymers via electrochemical quartz crystal microbalance and scanning electrochemical microscope Canhui Xiang a , Qingji Xie a,b,∗ , Jiming Hu a , Shouzhuo Yao b a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China
b
Received 2 October 2005; received in revised form 10 January 2006; accepted 18 January 2006 Available online 23 March 2006
Abstract Electropolymerization of aniline in sulfuric acid solution in the presence of o-phenylenediamine (oPD) of various concentrations was investigated via the electrochemical quartz crystal microbalance (EQCM) technique. It was found that the polymerization occurred more favorably at high aniline-to-oPD molar ratios (F1 , 20 or above). The stabilities of the resultant copolymers against degradation were efficiently improved compared with that of polyaniline (PANI). The first-order kinetic constants for polymer degradation were estimated to be 2.07 × 10−3 s−1 for polyaniline, and 3.91 × 10−4 and 1.28 × 10−4 s−1 for copolymers with F1 values of 50 and 20, respectively. The degradation product, benzoquinone, was also detected at the tip electrode of a scanning electrochemical microscope (SECM). © 2006 Elsevier B.V. All rights reserved. Keywords: Copolymerization of aniline and o-phenylenediamine; Degradation; Electrochemical quartz crystal microbalance; Scanning electrochemical microscope
1. Introduction Polyaniline (PANI) has been extensively studied [1–7] due to its high conductivity, good redox reversibility and swift change of color with potential. But it is sometimes limited in practical applications by its intractable nature, such as its insolubility, high brittleness, the strong dependence of its conductivity and electrochemical activity on medium pH, acid-catalyzed oxidative degradation, and so on. In recent years, a lot of work has been done to overcome these problems by copolymerization of aniline with different aniline derivatives [8–18]. The stability and degradation of PANI is often a big concern for its enduring applications, and thus many techniques have been used to study this subject [19–33].
∗
Corresponding author. Tel.: +86 731 8865515; fax: +86 731 8865515. E-mail address: [email protected] (Q. Xie).
0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.01.010
In recent years, there were some reports on the interesting copolymers of aniline with aromatic diamines, p-, m-, or o-phenylenediamine (oPD). Tang et al. [34] reported that the presence of p-phenylenediamine exhibited a catalytic effect on the electropolymerization of aniline, and poly(aniline-cop-phenylenediamine) had a high polymerization degree and high conductivity. Lee et al. [35] studied the copolymerization of aniline with p-phenylenediamine for developing a cathode material of lithium secondary battery. The copolymerization of m-phenylenediamine with aniline and the memory effect of the polymers were also reported [36]. It was reported that the electropolymerization of o-phenylenediamine alone could interestingly form PANI-like linear chains and the ladder structure with phenazine units [37]. Malinauskas et al. [38] investigated the copolymerization of oPD and aniline by means of Raman spectroelectrochemistry. However, to our knowledge, information about the stability and degradation of poly(aniline-co-oPD) is rather limited, except that Sun et al. [33] studied this sub-
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ject by conventional cyclic voltammetry and UV–vis absorption spectrophotometric analyses of the degradation products. Since the pioneering work in 1985 by Bruckenstein and Shay [39], the electrochemical quartz crystal microbalance (EQCM) has been largely developed as a very powerful technique for dynamically detecting an electrode mass change down to the nanogram level in an electrochemical reaction/process [40–57]. The Sauerbrey equation has been widely used for measuring the electrode-mass change (m) from the change of EQCM resonant frequency shift (f0 ) [39–44]: m 2 2 m = −2.264 × 10−6 f0g f0 = −2f0g √ A ρq µ q A
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well as the consumption of dissolved oxygen at the corresponding cathodic locations was monitored. To our knowledge, the SECM has seldom been used in monitoring in situ the degradation process of conducting polymers. In this work that is motivated by the interesting nature of the copolymer of aniline with aromatic diamines, we investigate the copolymerization of aniline with oPD via EQCM, while the copolymers’ degradation processes are studied via both EQCM and SECM. The degradation kinetics constants, properties of the resultant copolymers and in situ SECM analysis of the degradation products are examined.
(1)
where f0g in hertz is the unperturbed resonant frequency of the fundamental mode of the crystal, A in cm2 is the piezoelectrically active area, ρq (=2.648 g cm−3 ) is the density of the crystal, and µq (=2.947 × 1011 g cm−1 s−2 ) is the shear modulus of quartz. The EQCM is also sensitive to density and viscosity of the local solution near the EQCM electrode surface [54], and a net Newtonian-liquid-loading effect for a piezoelectric quartz crystal (PQC) with one side contacting solution can be characterized by the following equation [45,53–57]: fµq R1L = −4πL1q f0L ≈ −4πL1q f0L (2) f0g c¯ 66 where f0L and R1L are changes in f0 (the resonant frequency, and f0 = 1/[2π(L1 C1 )1/2 ]) and R1 due to variations of solution density and viscosity, respectively, f0g is the resonant frequency in air, L1q is the motional inductance for the PQC in air, and c¯ 66 (2.957 × 1011 g cm−1 s−2 ) is the lossy piezoelectrically stiffened quartz elastic constant [45,47–49,53–57]. According to Eq. (2), the characteristic value of f0L /R1L for a net viscous effect on the 9 MHz PQC resonance is ca. −10 Hz −1 . Obviously, for an investigated system the larger the absolute values of f0 /R1 , the weaker the viscous effect and the stronger the mass effect reflected by the Sauerbrey equation. If the absolute value of f0 /R1 in a practical process is significantly larger than the characteristic value of |f0L /R1L | for a net viscous effect, the mass effect should govern the frequency response in this process, since a net mass effect changes the frequency as given in Eq. (1), but the simultaneous change in R1 is negligible. Scanning electrochemical microscope (SECM) usually works in the bipotentiostat mode, with a micrometer or submicrometer ultra-microelectrode (UME, working electrode no. 1) scanned along the x-, y-, or z-axis over a conducting (working electrode no. 2) or insulating substrate [58–62]. The SECM has been used to locate enzyme sites of surface immobilized with redox-active enzymes [63,64], to investigate the transport of species through films [65,66], to character the polymers/films at the substrate [67,68] and to investigate symmetric current oscillations at tip and substrate electrodes of SECM during silver deposition/stripping [69]. Very recently, Souto et al. [70] used SECM to investigate the corrosion processes occurring at defective coated metals (carbon steel plates). The release of Fe(II) ionic species into the solution phase from local anodic sites, as
2. Experimental 2.1. Instrumentation and reagents Experiments were carried out with a HP4395A impedance analyzer, a CHI660A electrochemical workstation (CH Instruments Co., USA) or a CHI900A SECM (CH Instruments Co., USA) and two personal computers (PC) [32]. Synchronous conductance (G) and susceptance (B) measurements were conducted on the HP4395A controlled by a user-written Visual Basic (VB) 5.0 program. Butterworth–Van Dyke (BVD) equivalent circuit parameters were dynamically obtained during experiments using the same VB program by fitting each group of G and B to the BVD model based on a Gauss–Newton non-linear leastsquares fitting algorithm and a selection of R1 , C0 , f0 , and 1/C1 as estimation parameters [45]. The simultaneous electrochemical experiments were conducted on a CHI660A electrochemical workstation or a CHI900A SECM. AT-cut 9 MHz piezoelectric quartz crystals (1.25 cm in diameter) were used here. An Au electrode of 0.65-cm diameter on one side of the PQC was in contact with the solution and served as the working electrode (WE), while a gold electrode on the other side of the PQC was located in air. The PQC Au electrode facing the solution was used as the working electrode. A glassy carbon plate was used as the counter electrode (CE). The reference electrode (RE) was a saturated KCl calomel electrode (SCE) with a salt bridge filled with supporting electrolyte, and all potentials are reported with respect to it. All chemicals were of analytical grade or better quality. Aniline was distilled to give a colorless liquid prior to use. Other chemicals were used as received. Doubly distilled water and freshly prepared solutions were used throughout. 2.2. Procedures To remove possible surface contamination, the electrode surface was treated with one drop of concentrated HNO3 for ca. 15 s, then washed with doubly distilled water and dried via blowing of clean air. The HNO3 treatment was repeated thrice. The HNO3 treated electrode was then subjected to potential cycling between 0.2 and 1.5 V (30 mV s−1 ) in 0.2 mol L−1 HClO4 to obtain reproducible cyclic voltammograms finally. The electrode was rinsed with doubly distilled water after completion of cycling. PANI, poly(o-phenylenediamine) (PoPD) and poly(anilineco-o-phenylenediamine) (PAPD) films were grown at Au
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electrodes by applying cyclic potential sweeps (30 mV s−1 ) between −0.1 and 0.9 V. The polymerization of aniline or oPD was carried out in 0.1 mol L−1 monomer 0.1 mol L−1 H2 SO4 + 0.1 mol L−1 Na2 SO4 aqueous solutions. In the case of copolymers, while the concentration of aniline was fixed at 0.1 mol L−1 , the molar ratio of aniline to oPD (F1 ) was varied in the following way, 100:2 (F1 = 50), 100:5 (F1 = 20), 100:10 (F1 = 10), 100:100 (F1 = 1). The amount of surface deposits were controlled by the frequency response, expect where specified, the films with frequency shifts of ca. −1.4 kHz were prepared. The cyclic voltammetric experiments of the polymers and their degradation experiments were performed in 0.5 mol L−1 aqueous H2 SO4 . The detection of degradation products of the polymers was conducted on the CHI900A SECM. The working electrode #1 (tip electrode, WE1) was either a platinum disk microelectrode sealed in glass (25 m in diameter, CH Instruments Co., USA) or a bare EQCM Au electrode. The working electrode #2 (substrate electrode, WE2) was the EQCM electrode modified with a specified polymer. In order to increase the detection current at WE1, the polymers were electro-synthesized at the WE2 with ca. −3 kHz frequency shift, and the polymer details will be given below. The potential of WE2 was set at 0.9 V for the overoxidation of polymers, while the WE1 was subjected to potential cycling between −0.2 and 0.7 V for the simultaneous detection of degradation products. The distance between WE1 and WE2 will be give below. 3. Results and discussion 3.1. The electropolymerization of aniline and/or o-phenylenediamine Fig. 1 depicts the in situ records for current, f0 , and R1 during the cyclic voltammetric polymerization of aniline (A), oPD (B) and a series of aniline/oPD mixtures (C: F1 = 50, D: F1 = 20, E: F1 = 10, F: F1 = 1). As is seen from Fig. 1A, after the onset of oxidation of aniline monomer at ca. 0.8 V in the first cycle, the EQCM frequency began to decrease, indicating that the polymerization of aniline resulted in polymer deposition on the EQCM electrode. After the first cycle, however, the frequency began to decrease at ca. 0.6 V, being due possibly to the self-catalysis of PANI. The net increase in R1 after each potential cycle, which represents the more energy dissipation of the quartz crystal resonance into the environment, should result mainly from an increase in the electrode surface roughness due to PANI deposition [32]. In the oPD case (Fig. 1B), however, the EQCM frequency decreased at potentials positive to ca. 0.6 V in the first cycle, indicating that the formation of oPD radical cations proceeded at less anodic potentials in comparison with aniline, and thus the PoPD deposited more easily on the electrode surface. The R1 value decreased during the polymerization, suggesting the PoPD deposits were smoother than the PANI. It is naturally expected that when the F1 value is relatively low, the PoPD deposition on the electrode surface should occur first, and thus the resultant copolymers should exhibit more sig-
nificant PoPD-like nature. As shown in Fig. 1, when F1 = 10 and 1, the cyclic voltammetric curves were much similar to that in the oPD case, and the aniline-oxidation current became inconspicuous, while the frequency started to decrease at ca. 0.6 V in both cases, suggesting that the polymerization of oPD dominated the reaction and the copolymers here were PoPD-dominated or even just thick layers of PoPD plus much thinner PANI films. However, when F1 = 50 and 20 (Fig. 1C and D), the potentials where the resonant frequency started to decrease were shifted to ca. 0.8 V, and the characteristic responses for redox currents of both aniline and oPD were obvious, implying that the deposits consisted of comparable PoPD and PANI moieties. Since the net increases of R1 during polymerization decreased with the decrease of the F1 , the copolymers with smaller F1 values were more rigid and smoother than the PANI film [18], which is supported by the scanning electronic microscope observation [33]. Fig. 1 also shows that the rates for copolymerization were slower than that for polymerization of aniline or oPD alone, suggesting the generation of a new material with lower conductance and electroactivity than both PANI and PoPD [18,71]. Our result that the copolymerization rate was obviously decreased even at a very low concentration of oPD is in agreement with the report by Malinauskas et al. [38]. However, the addition of p-phenylenediamine to the polymerization solution promoted markedly the copolymer growth [34]. These interesting phenomena may be explained by the structure difference of the copolymers, since the copolymers in the oPD case may easily develop branch chains to grow a more compact film, as shown in Scheme 1. The compact structure of the copolymer should lead to an effective increase in the resistance of the solution’s penetrating into the copolymer and thus slow down the copolymerization rate. 3.2. The simultaneous response of current, frequency and motional resistance of the prepared polymers during potential cycling Fig. 2 shows the current, f0 and R1 of PANI, PoPD, and PAPD with F1 values of 50, 20, 10 and 1 during potential cycling, respectively. As for the PANI in Fig. 2A, there are three redox peaks centering at ca. 0.2, 0.5 and 0.7 V, corresponding to the leucoemeraldine/emeraldine couple, benzoquinone–hydroquinone couple and emeraldine/pernigraniline couple, respectively, while the benzoquinone–hydroquinone couple came from the side reaction due to hydrolyzation of the oligomers [24]. In vivid contrast to PANI, two couples of redox current peaks were very obvious at negative potentials in the case of PoPD (Fig. 2B). When the oPD was added into the copolymerization bath of aniline at a more and more decreased F1 value, the potential of the leucoemeraldine/emeraldine redox couple shifted more positively, and that of the emeraldine/pernigraniline couple shifted more negatively, being similar to the cases of other substituted polyanilines [8]. In this changeover, the more anodic peak (ca. 0.7 V) was affected more obviously. For the copolymer (e.g. F1 = 20), this peak became a small shoulder to the large center peak, which was probably due to the short aniline segments in the polymer backbone. This phenomenon suggested that the restric-
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Fig. 1. Simultaneous records of current, f0 and R1 during the cyclic voltammetric polymerization of aniline (A), oPD (B) and their copolymerization (C: F1 = 50; D: F1 = 20; E: F1 = 10; F: F1 = 1) in 0.1 mol L−1 H2 SO4 + 0.1 mol L−1 NaSO4 solution; dE/dt = 30 mV s−1 . The concentrations of the monomers are depicted in the text.
tions of PANI-like -conjugation in the copolymer backbone had a greater effect on formation of the perniganiline structure [71]. The less anodic peak at ca. 0.2 V was also affected. It shifted anodically and gradually merged into a broad peak with the decrease of the F1 value. The different effects of copolymerization on the redox peaks of leucoemeraldine/emeraldine and emeraldine/pernigraniline couples suggest that the restrictions of PANI-like conjugation were less important for the redox couple of leucoemeraldine/emeraldine. These current responses should imply the formation of a new material with differentiated properties, rather than a simple linear addition of the nature of PANI and PoPD. The frequency changes of PANI suggested anion insertion into the film in the first oxidation process but expulsion in the second one. Changes in R1 should reflect the variations of film topology, solvent content and so on during potential cycling.
Larger values of R1 of the polymer film in its oxidation state should imply a greater non-rigidity compared with the reduced polymer film, and the frequency decrease after the oxidation also suggested anion insertion and thus probably more obvious swelling of the polymer. The mass increase during negative potential scan of PoPD was in agreement with the result reported by Martinusz et al. [72], probably as a result of the incorporation of protons and water molecules into the film during electrochemical reduction of the polymer in an acidic solution. The resistance change here was small, suggesting that the PoPD was more compact. The frequency-change values during potential cycling of the copolymers (Fig. 2C: F1 = 50, Fig. 2D: F1 = 20) were smaller than that of PANI, which may be attributed to the proton doping [72]. The R1 values almost did not vary with potentials, which reinforces the conclusion that the PoPD film was a pro-
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Scheme 1. Structures of PANI (A) and PoPD (B, C) as well as possible degradation pathway of poly(aniline-co-oPD) (D, E).
ton doping film and thus little change of film topology. From Fig. 2C and D, one can find that the frequency changes in the first PANI-like redox process for the copolymers, i.e. anion insertion in oxidation and expulsion in its reduction, were discriminably less than those of PANI, which was caused by partial proton doping in this film. However, the changes of the frequency corresponding to the second redox process even disappeared when F1 = 20. This fact further indicated that the more anodic peak was affected first, and the restriction of PANI-like conjugation in the copolymer backbone was more important for the redox couple of emeraldine/pernigraniline. The changes of R1 here were less than those of PANI, which implied that copolymer was more rigid than PANI. With the increase of the F1 value, the f0 and R1 responses of the copolymer (F1 = 50) became closer to those of the PANI in shape and amplitude, indicating the increase of PANI moieties in the copolymer chain. The properties of copolymers were getting increasingly close to those of the PANI film with the decrease in the amount of oPD in the feed solutions, however, the quantitative composition of the feed solution can be rather different from the true composition of the material
deposited, due to the differences in the relative reactivity of the two monomers/homopolymers [8], though a positive correlation between solution and film composition can be expected [73]. 3.3. The degradation of the polymers Studies on the stability of PANI, PoPD and copolymers supported on Au electrodes were performed by a potential-step method from −0.1 to 0.9 V versus SCE for sufficient time length. Fig. 3 shows typical EQCM responses to polymers’ degradation in 0.5 mol L−1 aqueous H2 SO4 . For PANI shown in Fig. 3A, the motional resistance, being a reversed change during its deposition, should represent the decrease in the electrodesurface roughness during PANI degradation. The frequency increased with time, suggesting the loss of film mass during PANI degradation. As for the PoPD, the frequency also increased with time, but the frequency increase was much smaller than that of PANI, so the PoPD should have a higher stability in 0.5 mol L−1 H2 SO4 solutions [74]. The motional resistance,
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Fig. 2. Simultaneous records of current, f0 and R1 during potential cycling of prepared PANI (A), PoPD (B), PAPD film with F1 value of 50 (C), 20 (D), 10 (E) or 1 (F) in 0.5 mol L−1 aqueous H2 SO4 ; dE/dt = 30 mV s−1 .
however, increased during the degradation, also being a reversed change during its deposition, which indicated the increase of the electrode-surface roughness. Figs. 3C and D shows that the stability of the copolymers was improved markedly after adding small quantity of oPD to the polymerization bath. The changes in the motional resistance for the copolymers were as small as within ±1 , indicating that the morphology of the copolymer changed little during the degradation experiments, which further confirmed the high stability of the copolymers. The degradation percentage decreased from 37.8% (PANI) to 9.84% (F1 = 50) and 3.13% (F1 = 20), respectively. It is worth pointing out that the degradation percentage for F1 = 20 was even lower than that of PoPD, and this phenomenon should be attributed to the compact structure of the copolymer as mentioned above, which largely prevented the electrolyte solution from penetrating into the copolymer and thus slowed down the degradation.
The relevant degradation kinetic parameters were evaluated from the dynamic frequency responses under the experimental conditions. The kinetic data obtained were linearized in the firstorder reaction coordinates, i.e. ln(f1 − f2 ) versus time. From the linearized data, first-order rate constants can be obtained according to the equation: ln(f1 − f2 ) = −k1 t + C
(3)
where f1 is the absolute value of the frequency decrease after the polymerization, f2 is the value of the frequency increase during the degradation, k1 is the first-order rate constant, and C is a constant. Note that the term of (f1 − f2 ) reflects the remaining film mass at any moment during the degradation. The kinetic constants of the polymers are thus calculated to be 2.07 × 10−3 , 3.91 × 10−4 and 1.28 × 10−4 s−1 for PANI, PAPD (F1 = 50 and 20), respectively. Thereinto, the rate constant of
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Fig. 3. EQCM responses to potentiostatic degradation of PANI (A), PoPD (B) and PAPD film with F1 value of 50 (C) or 20 (D) on Au electrodes at 0.9 V in 0.5 mol L−1 aqueous H2 SO4 .
PANI is in accordance with the reported values in order of magnitude [19,32]. The results showed that the degradation ratio and k1 values decreased with the decrease of F1 , implying that the anti-degradation stability of copolymer was reinforced due to joint of the oPD unit. 3.4. Cyclic voltammetric behavior of polymers after partial degradation Fig. 4 shows the cyclic voltmmetric responses, f0 and R1 of partially degraded PANI, PoPD and PAPD with identical mass in 0.5 mol L−1 aqueous H2 SO4 . Compared with the voltammetric responses given in Fig. 2, all the redox current peaks of PANI became smaller, and the frequency increase associating with the second PANI-like redox process even disappeared after the degradation. The characteristic peaks of the oPD units were still discernable, and the frequency responses over the negative potential region for the oPD units were still retained. All these findings indicated that the degradation of the copolymers might take place mainly via chains’ breakdown at the PANI units, as shown in Scheme 1. 3.5. In situ SECM analysis of the degradation products The final degradation product of PANI was reported to be benzoquinone [32,33]. It is interesting to study the degrada-
tion products for the copolymers involved here via the SECM in the generation-collection (GC) mode [58,75]. Fig. 5 shows that there was only one redox couple at ca. 0.5 V at the detection EQCM Au electrode during the degradation of PANI or PAPD (F1 = 50), whose peak potentials agreed well to those of the benzoquinone/hydroquinone couple, indicating that the breakpoint was most likely at the aniline unit. The −3 kHz films were used here. Here, the detection electrode was a bulk EQCM electrode that was located near the copolymer-modified electrode, with a closest distance of ∼0.5 mm from the polymer surface due to the protuberance on the PQC fringe that was resulted from adhesive sealing. The cathodic peaks around −0.1 V are assigned to the reduction of oxygen at the detection electrode, since a similar peak was also observed on a bare Au electrode. The piezoelectric responses observed at the detection EQCM electrode was negligibly small, indicating that (1) the degradation product here cannot be polymerized again to give a deposit and (2) the possible deposition of the well-known quinone–hydroquinone charge-transfer complex is also ruled out, as a result of the relatively low concentration of generated quinone. In order to detect the degradation product at a location as close as possible to the polymer surface to follow the surface concentration of the degradation product, a 25-m Pt microelectrode was used to detect the degradation products, and the results are shown in Fig. 6. Similarly, there was only one S-shaped curve representing the benzoquinone/hydroquinone redox cou-
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Fig. 4. Responses of current, f0 and R1 to potential cycling of PANI (A), PoPD (B) and PAPD film with F1 value of 50 (C) or 20 (D) after 500 s degradation in 0.5 mol L−1 aqueous H2 SO4 ; dE/dt = 30 mV s−1 .
Fig. 5. Cyclic voltammograms at the detection EQCM electrode during degradation of PANI (solid line) and PAPD film (F1 = 50, broken line) in 0.5 mol L−1 aqueous H2 SO4 . Dashed line is the cyclic voltammogram at a bare EQCM Au electrode in 0.5 mol L−1 aqueous H2 SO4 containing 0.1 mmol L−1 1,4benzoquinone; dE/dt = 30 mV s−1 . The detection EQCM electrode was located near the polymer-modified EQCM electrode held at 0.9 V vs. SCE (with ca. 0.5 mm gap).
Fig. 6. Cyclic voltammograms at the platinum tip electrode (WE1) during degradation of PANI (solid line) and PAPD film (F1 = 50: broken line, F1 = 20: dotted line) in 0.5 mol L−1 aqueous H2 SO4 . Dashed line is for the blank experiment without degradation; EWE2 = 0.9 V, dEWE1 /dt = 50 mV s−1 . The distance between WE1 and WE2 was 10 m.
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4. Conclusions In summary, we have found that copolymerization of aniline and oPD occurred much more favorably at high aniline to oPD molar ratios, and the presence of oPD even at a molar ratio of aniline to oPD of 50 effectively slowed down the electropolymerization of aniline in sulfuric acid solution. This phenomenon can be explained as being due to the compact copolymer films in the presence of oPD, as shown in Scheme 1D. The compact structure of the copolymer resulted in an effective increase in the resistance of the solution’s penetrating into the copolymer, and thus both the polymerization and degradation rates were obviously decreased. The kinetic constants of the polymers have been estimated from the first-order kinetic law. The polymers’ degradation product, benzoquinone, has been detected via SECM with satisfactory results. Acknowledgements
Fig. 7. Currents of the platinum tip electrode (WE1) at 0.1 V vs. SCE during degradation of PANI (solid circles) and PAPD (F1 = 50, solid triangles) film at the WE2 in 0.5 mol L−1 aqueous H2 SO4 . Other conditions are the same as in Fig. 6.
This work was supported by the National Science Foundation of China (20275010, 20335020), the Basic Research Special Program of the Ministry of Science and Technology of China (2003CCC00700), and the Foundation of the Ministry of Education of China (jiaorensi [2000] 26, jiaojisi [2000] 65). References
ple during the degradation of PANI and PAPD (F1 = 50). The wave heights for PANI and PAPD (F1 = 50) returned local benzoquinone concentrations of ∼15 and ∼4 mmol L−1 , respectively, as estimated from a working curve for a series of standard benzoquinone solutions, if the positive feedback effect of the SECM can be overlooked here. As for the copolymer of F1 = 20, little redox current was observed at the tip electrode, so the local concentration of the degradation products may be very low or the compact PAPD film acts as a significant barrier for the degradation product to diffuse to the tip vicinity for detection. The time-dependent reduction current at the tip (potentiostated at 0.1 V versus SCE) during the films’ degradation were also examined, as shown in Fig. 7, which virtually depicts the transient concentration of the degradation product captured by WE1 at a 10-m distance from the film surface. The maximum initial rate for film degradation led to an increase of the concentration of benzoquinone species at first, followed by its decrease due to a decreased degradation rate and the product’s diffusion away from WE1. Here the tip current during PANI degradation was obviously larger than that during PAPD degradation, indicating an enhanced stability of the copolymer. In addition, only very slight redox tip currents were observed during the degradation of PAPD (F1 = 20). These findings from SECM have also indicated that a little addition of oPD efficiently inhibited the degradation of the PANI film and improved the stability of PANI, being in accordance with the EQCM results. A future theoretical study on the SECM tip currents may also render the kinetics parameter for the polymer degradation, after considering the diffusion of the degradation product via digital simulation [54].
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Studies on electrochemical copolymerization of aniline with o-phenylenediamine and degradation of the resultant copolymers via electrochemical quartz crystal microbalance and scanning electrochemical microscope Canhui Xiang a , Qingji Xie a,b,∗ , Jiming Hu a , Shouzhuo Yao b a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China
b
Received 2 October 2005; received in revised form 10 January 2006; accepted 18 January 2006 Available online 23 March 2006
Abstract Electropolymerization of aniline in sulfuric acid solution in the presence of o-phenylenediamine (oPD) of various concentrations was investigated via the electrochemical quartz crystal microbalance (EQCM) technique. It was found that the polymerization occurred more favorably at high aniline-to-oPD molar ratios (F1 , 20 or above). The stabilities of the resultant copolymers against degradation were efficiently improved compared with that of polyaniline (PANI). The first-order kinetic constants for polymer degradation were estimated to be 2.07 × 10−3 s−1 for polyaniline, and 3.91 × 10−4 and 1.28 × 10−4 s−1 for copolymers with F1 values of 50 and 20, respectively. The degradation product, benzoquinone, was also detected at the tip electrode of a scanning electrochemical microscope (SECM). © 2006 Elsevier B.V. All rights reserved. Keywords: Copolymerization of aniline and o-phenylenediamine; Degradation; Electrochemical quartz crystal microbalance; Scanning electrochemical microscope
1. Introduction Polyaniline (PANI) has been extensively studied [1–7] due to its high conductivity, good redox reversibility and swift change of color with potential. But it is sometimes limited in practical applications by its intractable nature, such as its insolubility, high brittleness, the strong dependence of its conductivity and electrochemical activity on medium pH, acid-catalyzed oxidative degradation, and so on. In recent years, a lot of work has been done to overcome these problems by copolymerization of aniline with different aniline derivatives [8–18]. The stability and degradation of PANI is often a big concern for its enduring applications, and thus many techniques have been used to study this subject [19–33].
∗
Corresponding author. Tel.: +86 731 8865515; fax: +86 731 8865515. E-mail address: [email protected] (Q. Xie).
0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.01.010
In recent years, there were some reports on the interesting copolymers of aniline with aromatic diamines, p-, m-, or o-phenylenediamine (oPD). Tang et al. [34] reported that the presence of p-phenylenediamine exhibited a catalytic effect on the electropolymerization of aniline, and poly(aniline-cop-phenylenediamine) had a high polymerization degree and high conductivity. Lee et al. [35] studied the copolymerization of aniline with p-phenylenediamine for developing a cathode material of lithium secondary battery. The copolymerization of m-phenylenediamine with aniline and the memory effect of the polymers were also reported [36]. It was reported that the electropolymerization of o-phenylenediamine alone could interestingly form PANI-like linear chains and the ladder structure with phenazine units [37]. Malinauskas et al. [38] investigated the copolymerization of oPD and aniline by means of Raman spectroelectrochemistry. However, to our knowledge, information about the stability and degradation of poly(aniline-co-oPD) is rather limited, except that Sun et al. [33] studied this sub-
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ject by conventional cyclic voltammetry and UV–vis absorption spectrophotometric analyses of the degradation products. Since the pioneering work in 1985 by Bruckenstein and Shay [39], the electrochemical quartz crystal microbalance (EQCM) has been largely developed as a very powerful technique for dynamically detecting an electrode mass change down to the nanogram level in an electrochemical reaction/process [40–57]. The Sauerbrey equation has been widely used for measuring the electrode-mass change (m) from the change of EQCM resonant frequency shift (f0 ) [39–44]: m 2 2 m = −2.264 × 10−6 f0g f0 = −2f0g √ A ρq µ q A
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well as the consumption of dissolved oxygen at the corresponding cathodic locations was monitored. To our knowledge, the SECM has seldom been used in monitoring in situ the degradation process of conducting polymers. In this work that is motivated by the interesting nature of the copolymer of aniline with aromatic diamines, we investigate the copolymerization of aniline with oPD via EQCM, while the copolymers’ degradation processes are studied via both EQCM and SECM. The degradation kinetics constants, properties of the resultant copolymers and in situ SECM analysis of the degradation products are examined.
(1)
where f0g in hertz is the unperturbed resonant frequency of the fundamental mode of the crystal, A in cm2 is the piezoelectrically active area, ρq (=2.648 g cm−3 ) is the density of the crystal, and µq (=2.947 × 1011 g cm−1 s−2 ) is the shear modulus of quartz. The EQCM is also sensitive to density and viscosity of the local solution near the EQCM electrode surface [54], and a net Newtonian-liquid-loading effect for a piezoelectric quartz crystal (PQC) with one side contacting solution can be characterized by the following equation [45,53–57]: fµq R1L = −4πL1q f0L ≈ −4πL1q f0L (2) f0g c¯ 66 where f0L and R1L are changes in f0 (the resonant frequency, and f0 = 1/[2π(L1 C1 )1/2 ]) and R1 due to variations of solution density and viscosity, respectively, f0g is the resonant frequency in air, L1q is the motional inductance for the PQC in air, and c¯ 66 (2.957 × 1011 g cm−1 s−2 ) is the lossy piezoelectrically stiffened quartz elastic constant [45,47–49,53–57]. According to Eq. (2), the characteristic value of f0L /R1L for a net viscous effect on the 9 MHz PQC resonance is ca. −10 Hz −1 . Obviously, for an investigated system the larger the absolute values of f0 /R1 , the weaker the viscous effect and the stronger the mass effect reflected by the Sauerbrey equation. If the absolute value of f0 /R1 in a practical process is significantly larger than the characteristic value of |f0L /R1L | for a net viscous effect, the mass effect should govern the frequency response in this process, since a net mass effect changes the frequency as given in Eq. (1), but the simultaneous change in R1 is negligible. Scanning electrochemical microscope (SECM) usually works in the bipotentiostat mode, with a micrometer or submicrometer ultra-microelectrode (UME, working electrode no. 1) scanned along the x-, y-, or z-axis over a conducting (working electrode no. 2) or insulating substrate [58–62]. The SECM has been used to locate enzyme sites of surface immobilized with redox-active enzymes [63,64], to investigate the transport of species through films [65,66], to character the polymers/films at the substrate [67,68] and to investigate symmetric current oscillations at tip and substrate electrodes of SECM during silver deposition/stripping [69]. Very recently, Souto et al. [70] used SECM to investigate the corrosion processes occurring at defective coated metals (carbon steel plates). The release of Fe(II) ionic species into the solution phase from local anodic sites, as
2. Experimental 2.1. Instrumentation and reagents Experiments were carried out with a HP4395A impedance analyzer, a CHI660A electrochemical workstation (CH Instruments Co., USA) or a CHI900A SECM (CH Instruments Co., USA) and two personal computers (PC) [32]. Synchronous conductance (G) and susceptance (B) measurements were conducted on the HP4395A controlled by a user-written Visual Basic (VB) 5.0 program. Butterworth–Van Dyke (BVD) equivalent circuit parameters were dynamically obtained during experiments using the same VB program by fitting each group of G and B to the BVD model based on a Gauss–Newton non-linear leastsquares fitting algorithm and a selection of R1 , C0 , f0 , and 1/C1 as estimation parameters [45]. The simultaneous electrochemical experiments were conducted on a CHI660A electrochemical workstation or a CHI900A SECM. AT-cut 9 MHz piezoelectric quartz crystals (1.25 cm in diameter) were used here. An Au electrode of 0.65-cm diameter on one side of the PQC was in contact with the solution and served as the working electrode (WE), while a gold electrode on the other side of the PQC was located in air. The PQC Au electrode facing the solution was used as the working electrode. A glassy carbon plate was used as the counter electrode (CE). The reference electrode (RE) was a saturated KCl calomel electrode (SCE) with a salt bridge filled with supporting electrolyte, and all potentials are reported with respect to it. All chemicals were of analytical grade or better quality. Aniline was distilled to give a colorless liquid prior to use. Other chemicals were used as received. Doubly distilled water and freshly prepared solutions were used throughout. 2.2. Procedures To remove possible surface contamination, the electrode surface was treated with one drop of concentrated HNO3 for ca. 15 s, then washed with doubly distilled water and dried via blowing of clean air. The HNO3 treatment was repeated thrice. The HNO3 treated electrode was then subjected to potential cycling between 0.2 and 1.5 V (30 mV s−1 ) in 0.2 mol L−1 HClO4 to obtain reproducible cyclic voltammograms finally. The electrode was rinsed with doubly distilled water after completion of cycling. PANI, poly(o-phenylenediamine) (PoPD) and poly(anilineco-o-phenylenediamine) (PAPD) films were grown at Au
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electrodes by applying cyclic potential sweeps (30 mV s−1 ) between −0.1 and 0.9 V. The polymerization of aniline or oPD was carried out in 0.1 mol L−1 monomer 0.1 mol L−1 H2 SO4 + 0.1 mol L−1 Na2 SO4 aqueous solutions. In the case of copolymers, while the concentration of aniline was fixed at 0.1 mol L−1 , the molar ratio of aniline to oPD (F1 ) was varied in the following way, 100:2 (F1 = 50), 100:5 (F1 = 20), 100:10 (F1 = 10), 100:100 (F1 = 1). The amount of surface deposits were controlled by the frequency response, expect where specified, the films with frequency shifts of ca. −1.4 kHz were prepared. The cyclic voltammetric experiments of the polymers and their degradation experiments were performed in 0.5 mol L−1 aqueous H2 SO4 . The detection of degradation products of the polymers was conducted on the CHI900A SECM. The working electrode #1 (tip electrode, WE1) was either a platinum disk microelectrode sealed in glass (25 m in diameter, CH Instruments Co., USA) or a bare EQCM Au electrode. The working electrode #2 (substrate electrode, WE2) was the EQCM electrode modified with a specified polymer. In order to increase the detection current at WE1, the polymers were electro-synthesized at the WE2 with ca. −3 kHz frequency shift, and the polymer details will be given below. The potential of WE2 was set at 0.9 V for the overoxidation of polymers, while the WE1 was subjected to potential cycling between −0.2 and 0.7 V for the simultaneous detection of degradation products. The distance between WE1 and WE2 will be give below. 3. Results and discussion 3.1. The electropolymerization of aniline and/or o-phenylenediamine Fig. 1 depicts the in situ records for current, f0 , and R1 during the cyclic voltammetric polymerization of aniline (A), oPD (B) and a series of aniline/oPD mixtures (C: F1 = 50, D: F1 = 20, E: F1 = 10, F: F1 = 1). As is seen from Fig. 1A, after the onset of oxidation of aniline monomer at ca. 0.8 V in the first cycle, the EQCM frequency began to decrease, indicating that the polymerization of aniline resulted in polymer deposition on the EQCM electrode. After the first cycle, however, the frequency began to decrease at ca. 0.6 V, being due possibly to the self-catalysis of PANI. The net increase in R1 after each potential cycle, which represents the more energy dissipation of the quartz crystal resonance into the environment, should result mainly from an increase in the electrode surface roughness due to PANI deposition [32]. In the oPD case (Fig. 1B), however, the EQCM frequency decreased at potentials positive to ca. 0.6 V in the first cycle, indicating that the formation of oPD radical cations proceeded at less anodic potentials in comparison with aniline, and thus the PoPD deposited more easily on the electrode surface. The R1 value decreased during the polymerization, suggesting the PoPD deposits were smoother than the PANI. It is naturally expected that when the F1 value is relatively low, the PoPD deposition on the electrode surface should occur first, and thus the resultant copolymers should exhibit more sig-
nificant PoPD-like nature. As shown in Fig. 1, when F1 = 10 and 1, the cyclic voltammetric curves were much similar to that in the oPD case, and the aniline-oxidation current became inconspicuous, while the frequency started to decrease at ca. 0.6 V in both cases, suggesting that the polymerization of oPD dominated the reaction and the copolymers here were PoPD-dominated or even just thick layers of PoPD plus much thinner PANI films. However, when F1 = 50 and 20 (Fig. 1C and D), the potentials where the resonant frequency started to decrease were shifted to ca. 0.8 V, and the characteristic responses for redox currents of both aniline and oPD were obvious, implying that the deposits consisted of comparable PoPD and PANI moieties. Since the net increases of R1 during polymerization decreased with the decrease of the F1 , the copolymers with smaller F1 values were more rigid and smoother than the PANI film [18], which is supported by the scanning electronic microscope observation [33]. Fig. 1 also shows that the rates for copolymerization were slower than that for polymerization of aniline or oPD alone, suggesting the generation of a new material with lower conductance and electroactivity than both PANI and PoPD [18,71]. Our result that the copolymerization rate was obviously decreased even at a very low concentration of oPD is in agreement with the report by Malinauskas et al. [38]. However, the addition of p-phenylenediamine to the polymerization solution promoted markedly the copolymer growth [34]. These interesting phenomena may be explained by the structure difference of the copolymers, since the copolymers in the oPD case may easily develop branch chains to grow a more compact film, as shown in Scheme 1. The compact structure of the copolymer should lead to an effective increase in the resistance of the solution’s penetrating into the copolymer and thus slow down the copolymerization rate. 3.2. The simultaneous response of current, frequency and motional resistance of the prepared polymers during potential cycling Fig. 2 shows the current, f0 and R1 of PANI, PoPD, and PAPD with F1 values of 50, 20, 10 and 1 during potential cycling, respectively. As for the PANI in Fig. 2A, there are three redox peaks centering at ca. 0.2, 0.5 and 0.7 V, corresponding to the leucoemeraldine/emeraldine couple, benzoquinone–hydroquinone couple and emeraldine/pernigraniline couple, respectively, while the benzoquinone–hydroquinone couple came from the side reaction due to hydrolyzation of the oligomers [24]. In vivid contrast to PANI, two couples of redox current peaks were very obvious at negative potentials in the case of PoPD (Fig. 2B). When the oPD was added into the copolymerization bath of aniline at a more and more decreased F1 value, the potential of the leucoemeraldine/emeraldine redox couple shifted more positively, and that of the emeraldine/pernigraniline couple shifted more negatively, being similar to the cases of other substituted polyanilines [8]. In this changeover, the more anodic peak (ca. 0.7 V) was affected more obviously. For the copolymer (e.g. F1 = 20), this peak became a small shoulder to the large center peak, which was probably due to the short aniline segments in the polymer backbone. This phenomenon suggested that the restric-
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Fig. 1. Simultaneous records of current, f0 and R1 during the cyclic voltammetric polymerization of aniline (A), oPD (B) and their copolymerization (C: F1 = 50; D: F1 = 20; E: F1 = 10; F: F1 = 1) in 0.1 mol L−1 H2 SO4 + 0.1 mol L−1 NaSO4 solution; dE/dt = 30 mV s−1 . The concentrations of the monomers are depicted in the text.
tions of PANI-like -conjugation in the copolymer backbone had a greater effect on formation of the perniganiline structure [71]. The less anodic peak at ca. 0.2 V was also affected. It shifted anodically and gradually merged into a broad peak with the decrease of the F1 value. The different effects of copolymerization on the redox peaks of leucoemeraldine/emeraldine and emeraldine/pernigraniline couples suggest that the restrictions of PANI-like conjugation were less important for the redox couple of leucoemeraldine/emeraldine. These current responses should imply the formation of a new material with differentiated properties, rather than a simple linear addition of the nature of PANI and PoPD. The frequency changes of PANI suggested anion insertion into the film in the first oxidation process but expulsion in the second one. Changes in R1 should reflect the variations of film topology, solvent content and so on during potential cycling.
Larger values of R1 of the polymer film in its oxidation state should imply a greater non-rigidity compared with the reduced polymer film, and the frequency decrease after the oxidation also suggested anion insertion and thus probably more obvious swelling of the polymer. The mass increase during negative potential scan of PoPD was in agreement with the result reported by Martinusz et al. [72], probably as a result of the incorporation of protons and water molecules into the film during electrochemical reduction of the polymer in an acidic solution. The resistance change here was small, suggesting that the PoPD was more compact. The frequency-change values during potential cycling of the copolymers (Fig. 2C: F1 = 50, Fig. 2D: F1 = 20) were smaller than that of PANI, which may be attributed to the proton doping [72]. The R1 values almost did not vary with potentials, which reinforces the conclusion that the PoPD film was a pro-
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Scheme 1. Structures of PANI (A) and PoPD (B, C) as well as possible degradation pathway of poly(aniline-co-oPD) (D, E).
ton doping film and thus little change of film topology. From Fig. 2C and D, one can find that the frequency changes in the first PANI-like redox process for the copolymers, i.e. anion insertion in oxidation and expulsion in its reduction, were discriminably less than those of PANI, which was caused by partial proton doping in this film. However, the changes of the frequency corresponding to the second redox process even disappeared when F1 = 20. This fact further indicated that the more anodic peak was affected first, and the restriction of PANI-like conjugation in the copolymer backbone was more important for the redox couple of emeraldine/pernigraniline. The changes of R1 here were less than those of PANI, which implied that copolymer was more rigid than PANI. With the increase of the F1 value, the f0 and R1 responses of the copolymer (F1 = 50) became closer to those of the PANI in shape and amplitude, indicating the increase of PANI moieties in the copolymer chain. The properties of copolymers were getting increasingly close to those of the PANI film with the decrease in the amount of oPD in the feed solutions, however, the quantitative composition of the feed solution can be rather different from the true composition of the material
deposited, due to the differences in the relative reactivity of the two monomers/homopolymers [8], though a positive correlation between solution and film composition can be expected [73]. 3.3. The degradation of the polymers Studies on the stability of PANI, PoPD and copolymers supported on Au electrodes were performed by a potential-step method from −0.1 to 0.9 V versus SCE for sufficient time length. Fig. 3 shows typical EQCM responses to polymers’ degradation in 0.5 mol L−1 aqueous H2 SO4 . For PANI shown in Fig. 3A, the motional resistance, being a reversed change during its deposition, should represent the decrease in the electrodesurface roughness during PANI degradation. The frequency increased with time, suggesting the loss of film mass during PANI degradation. As for the PoPD, the frequency also increased with time, but the frequency increase was much smaller than that of PANI, so the PoPD should have a higher stability in 0.5 mol L−1 H2 SO4 solutions [74]. The motional resistance,
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Fig. 2. Simultaneous records of current, f0 and R1 during potential cycling of prepared PANI (A), PoPD (B), PAPD film with F1 value of 50 (C), 20 (D), 10 (E) or 1 (F) in 0.5 mol L−1 aqueous H2 SO4 ; dE/dt = 30 mV s−1 .
however, increased during the degradation, also being a reversed change during its deposition, which indicated the increase of the electrode-surface roughness. Figs. 3C and D shows that the stability of the copolymers was improved markedly after adding small quantity of oPD to the polymerization bath. The changes in the motional resistance for the copolymers were as small as within ±1 , indicating that the morphology of the copolymer changed little during the degradation experiments, which further confirmed the high stability of the copolymers. The degradation percentage decreased from 37.8% (PANI) to 9.84% (F1 = 50) and 3.13% (F1 = 20), respectively. It is worth pointing out that the degradation percentage for F1 = 20 was even lower than that of PoPD, and this phenomenon should be attributed to the compact structure of the copolymer as mentioned above, which largely prevented the electrolyte solution from penetrating into the copolymer and thus slowed down the degradation.
The relevant degradation kinetic parameters were evaluated from the dynamic frequency responses under the experimental conditions. The kinetic data obtained were linearized in the firstorder reaction coordinates, i.e. ln(f1 − f2 ) versus time. From the linearized data, first-order rate constants can be obtained according to the equation: ln(f1 − f2 ) = −k1 t + C
(3)
where f1 is the absolute value of the frequency decrease after the polymerization, f2 is the value of the frequency increase during the degradation, k1 is the first-order rate constant, and C is a constant. Note that the term of (f1 − f2 ) reflects the remaining film mass at any moment during the degradation. The kinetic constants of the polymers are thus calculated to be 2.07 × 10−3 , 3.91 × 10−4 and 1.28 × 10−4 s−1 for PANI, PAPD (F1 = 50 and 20), respectively. Thereinto, the rate constant of
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Fig. 3. EQCM responses to potentiostatic degradation of PANI (A), PoPD (B) and PAPD film with F1 value of 50 (C) or 20 (D) on Au electrodes at 0.9 V in 0.5 mol L−1 aqueous H2 SO4 .
PANI is in accordance with the reported values in order of magnitude [19,32]. The results showed that the degradation ratio and k1 values decreased with the decrease of F1 , implying that the anti-degradation stability of copolymer was reinforced due to joint of the oPD unit. 3.4. Cyclic voltammetric behavior of polymers after partial degradation Fig. 4 shows the cyclic voltmmetric responses, f0 and R1 of partially degraded PANI, PoPD and PAPD with identical mass in 0.5 mol L−1 aqueous H2 SO4 . Compared with the voltammetric responses given in Fig. 2, all the redox current peaks of PANI became smaller, and the frequency increase associating with the second PANI-like redox process even disappeared after the degradation. The characteristic peaks of the oPD units were still discernable, and the frequency responses over the negative potential region for the oPD units were still retained. All these findings indicated that the degradation of the copolymers might take place mainly via chains’ breakdown at the PANI units, as shown in Scheme 1. 3.5. In situ SECM analysis of the degradation products The final degradation product of PANI was reported to be benzoquinone [32,33]. It is interesting to study the degrada-
tion products for the copolymers involved here via the SECM in the generation-collection (GC) mode [58,75]. Fig. 5 shows that there was only one redox couple at ca. 0.5 V at the detection EQCM Au electrode during the degradation of PANI or PAPD (F1 = 50), whose peak potentials agreed well to those of the benzoquinone/hydroquinone couple, indicating that the breakpoint was most likely at the aniline unit. The −3 kHz films were used here. Here, the detection electrode was a bulk EQCM electrode that was located near the copolymer-modified electrode, with a closest distance of ∼0.5 mm from the polymer surface due to the protuberance on the PQC fringe that was resulted from adhesive sealing. The cathodic peaks around −0.1 V are assigned to the reduction of oxygen at the detection electrode, since a similar peak was also observed on a bare Au electrode. The piezoelectric responses observed at the detection EQCM electrode was negligibly small, indicating that (1) the degradation product here cannot be polymerized again to give a deposit and (2) the possible deposition of the well-known quinone–hydroquinone charge-transfer complex is also ruled out, as a result of the relatively low concentration of generated quinone. In order to detect the degradation product at a location as close as possible to the polymer surface to follow the surface concentration of the degradation product, a 25-m Pt microelectrode was used to detect the degradation products, and the results are shown in Fig. 6. Similarly, there was only one S-shaped curve representing the benzoquinone/hydroquinone redox cou-
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Fig. 4. Responses of current, f0 and R1 to potential cycling of PANI (A), PoPD (B) and PAPD film with F1 value of 50 (C) or 20 (D) after 500 s degradation in 0.5 mol L−1 aqueous H2 SO4 ; dE/dt = 30 mV s−1 .
Fig. 5. Cyclic voltammograms at the detection EQCM electrode during degradation of PANI (solid line) and PAPD film (F1 = 50, broken line) in 0.5 mol L−1 aqueous H2 SO4 . Dashed line is the cyclic voltammogram at a bare EQCM Au electrode in 0.5 mol L−1 aqueous H2 SO4 containing 0.1 mmol L−1 1,4benzoquinone; dE/dt = 30 mV s−1 . The detection EQCM electrode was located near the polymer-modified EQCM electrode held at 0.9 V vs. SCE (with ca. 0.5 mm gap).
Fig. 6. Cyclic voltammograms at the platinum tip electrode (WE1) during degradation of PANI (solid line) and PAPD film (F1 = 50: broken line, F1 = 20: dotted line) in 0.5 mol L−1 aqueous H2 SO4 . Dashed line is for the blank experiment without degradation; EWE2 = 0.9 V, dEWE1 /dt = 50 mV s−1 . The distance between WE1 and WE2 was 10 m.
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4. Conclusions In summary, we have found that copolymerization of aniline and oPD occurred much more favorably at high aniline to oPD molar ratios, and the presence of oPD even at a molar ratio of aniline to oPD of 50 effectively slowed down the electropolymerization of aniline in sulfuric acid solution. This phenomenon can be explained as being due to the compact copolymer films in the presence of oPD, as shown in Scheme 1D. The compact structure of the copolymer resulted in an effective increase in the resistance of the solution’s penetrating into the copolymer, and thus both the polymerization and degradation rates were obviously decreased. The kinetic constants of the polymers have been estimated from the first-order kinetic law. The polymers’ degradation product, benzoquinone, has been detected via SECM with satisfactory results. Acknowledgements
Fig. 7. Currents of the platinum tip electrode (WE1) at 0.1 V vs. SCE during degradation of PANI (solid circles) and PAPD (F1 = 50, solid triangles) film at the WE2 in 0.5 mol L−1 aqueous H2 SO4 . Other conditions are the same as in Fig. 6.
This work was supported by the National Science Foundation of China (20275010, 20335020), the Basic Research Special Program of the Ministry of Science and Technology of China (2003CCC00700), and the Foundation of the Ministry of Education of China (jiaorensi [2000] 26, jiaojisi [2000] 65). References
ple during the degradation of PANI and PAPD (F1 = 50). The wave heights for PANI and PAPD (F1 = 50) returned local benzoquinone concentrations of ∼15 and ∼4 mmol L−1 , respectively, as estimated from a working curve for a series of standard benzoquinone solutions, if the positive feedback effect of the SECM can be overlooked here. As for the copolymer of F1 = 20, little redox current was observed at the tip electrode, so the local concentration of the degradation products may be very low or the compact PAPD film acts as a significant barrier for the degradation product to diffuse to the tip vicinity for detection. The time-dependent reduction current at the tip (potentiostated at 0.1 V versus SCE) during the films’ degradation were also examined, as shown in Fig. 7, which virtually depicts the transient concentration of the degradation product captured by WE1 at a 10-m distance from the film surface. The maximum initial rate for film degradation led to an increase of the concentration of benzoquinone species at first, followed by its decrease due to a decreased degradation rate and the product’s diffusion away from WE1. Here the tip current during PANI degradation was obviously larger than that during PAPD degradation, indicating an enhanced stability of the copolymer. In addition, only very slight redox tip currents were observed during the degradation of PAPD (F1 = 20). These findings from SECM have also indicated that a little addition of oPD efficiently inhibited the degradation of the PANI film and improved the stability of PANI, being in accordance with the EQCM results. A future theoretical study on the SECM tip currents may also render the kinetics parameter for the polymer degradation, after considering the diffusion of the degradation product via digital simulation [54].
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