Russian Journal of Electrochemistry, Vol. 40, No. 3, 2004, pp. 280–285. From Elektrokhimiya, Vol. 40, No. 3, 2004, pp. 319–324. Original English Text Copyright © 2004 by Aoki, Mukoyama, Chen.
Competition between Polymerization and Dissolution of Poly(3-methylthiophene) Films* K. Aokiz, I. Mukoyama, and J. Chen Department of Applied Physics, Fukui University, 3-9-1, Bunkyo, Fukui-shi, 910-8507 Japan Received February 26, 2003
Abstract—Films of electrically conducting polymer, poly(3-methylthiophene), are dissolved in monomer-free solutions at positive potentials to become thin, whereas they are polymerized in monomer-rich solutions at the same potentials as for the dissolution. A question arises whether they are dissolved or polymerized in solutions including a given concentration of the monomer when a positive potential is applied to the film. Conditions of the competition between the polymerization and the dissolution were searched at potentiostatic experiments for poly(3-methylthiophene) films at various concentrations of monomers and potentials. The dissolution prevailed over the polymerization as the concentration decreased and the potential was less positive. Chronoamperometric currents exhibited oscillation under the competition conditions. The oscillation was explained in terms of the Lotka–Volterra model for a simple oscillation reaction, in which competitive species were the conducting polymer and the monomer. Key words: conducting polymers, poly(3-methylthiophene), oscillation reaction, chronoamperometry, polymerization, dissolution
1. INTRODUCTION Most of electrically conducting polymers are insoluble in their doped states [1]. A strategy of affording to dissolve conducting polymers is to introduce a functional group with solvent affinity to a monomer or a polymer backbone. For example, polyaniline films substituted with poly(o-toluidine), poly(o-methoxyaniline), poly(o-ethoxyaniline), and poly(N-methylaniline) enhanced an extent of dissolution and swelling, so that the film processability has been improved [2–6]. Polyaniline is partially soluble when it is functionalized with organic acids [7, 8]. Introduction of long alkane chains to a thiophene ring makes it possible to dissolve the polythiophene in organic solvents [9–11]. A recently reported technique of the dissolution is the electrochemical method by applying positive potentials to conducting polymers in a monomer-free solution [12]. This method is based on the overoxidative degradation of the polymers [11, 13–17], and then formation of dissolved species when a potential similar to the polymerization potential is applied to the film. The electrochemical dissolution can be employed for electrochemical machining of conducting polymers by scanning a thin tip electrode on the conducting polymer surface [12] during the potential application.
* This article was submitted by the authors in English. z Corresponding author, e-mail:
[email protected]
The polymerization increases the film thickness obviously in a solution of the highly concentrated monomer, whereas the electrochemical dissolution makes the film thin in a solution without the monomer at potentials similar to the polymerization potential. Thus, it is a concentration of the monomer that determines whether the film grows or not. The polymerization may compete the dissolution at the concentrations at which the film neither grows nor reduces. A strong competition in general causes extraordinary behavior such as oscillation or pattern formation. This paper is devoted to searching the conditions of the competition between the polymerization and the dissolution of poly(3-methylthiophene) films by means of chronoamperometry, as well as capacitance measurements. It also describes a new finding of the oscillation in chronoamperometric current under the competition conditions. 2. EXPERIMENTAL Chemicals. All the chemicals were of analytical grade. Nitrobenzene (NB) was purified by distillation and then was kept over active alumina of 300 mesh per day. 3-Methylthiophene (3MT) and tetra-n-butylammonium tetrafluoroborate (TBATFB) were used as received. Acetonitrile was treated with molecular sieves. Electrochemistry. A standard three-electrode cell was used for the electrochemical measurements. The working electrode for measurements of film thickness
1023-1935/04/4003-0280 © 2004 åÄIä “Nauka /Interperiodica”
COMPETITION BETWEEN POLYMERIZATION AND DISSOLUTION
281
L/L0 1.0✩ ✩
a
0.8 ✩ ✩
0.6
✩
0.4
b c
✩
0.2
0 Fig. 1. Photographs of the PMT-coated film polymerized on the rod platinum wire 0.1 mm in diameter, (left) 60 s and (right) 600 s after applying 2.4 V in monomer-free 0.1 M TBATFB + NB solution. The rod was mounted perpendicularly in the cell.
was a platinum wire 0.1 mm in diameter. The working electrode for voltammetry was a platinum disk 1.6 mm in diameter. The counterelectrode was a platinum coil, and the reference electrode was Ag/0.01 M (mol/dm3) AgNO3 in acetonitrile. The potential was controlled with a HQ-101B potentiostat (Hokuto Denko). Ac impedance measurements were carried out with a combination of the potentiostat and a lock-in amplifier at 40 Hz with 10 mV amplitude of the applied ac voltage. A delay of the potentiostat was confirmed to be negligible by the use of a dummy cell. Polymerization. Poly(3-methylthiophene) was synthesized electrochemically by applying potentials of more than 1.5 V in NB containing 0.1 M TBATFB and 0.1 M 3MT (monomer) at room temperature. Then, the polymer film was rinsed well with NB before electrochemical measurements. Thickness of the film was evaluated from an optical microscope.
2000
4000
6000 t, s
Fig. 2. Time variations of thickness, L, of the PMT-coated films responding to potential applications of (‡) 1.5, (b) 1.9, and (c) 2.0 V in the monomer-free 0.1 M TBATFB + NB solution. The thickness was normalized with the initial thickness, L0.
decreased with the time and then reached a steady-state thickness at the application of 1.5 V. The higher was the potential, the thinner became the film. Figure 3 shows chronoamperometric curves at the PMT-coated Pt disk electrode with several thicknesses of the film in the 0.1 M TBATFB + NB solution. The currents decreased and reached zero at the time of which the films disappeared (at 350 s in c). The thinner was the film, the smaller was the current. The dissolution was accelerated with stirring the solution, as is shown in Fig. 4. Chronoamperometric curves under the hydrodynamic conditions decreased monotonically I, µÄ 200
3. RESULTS AND DISCUSSION When potentials more than 1.5 V were applied to a poly(3-methylthiophene) (PMT) film in NB solution without the monomer, the film disappeared, exhibiting a faintly blue flow from the film [12]. The potentials are in the domain of degradation of the PMT. Therefore, the film may be degraded and then dissolved in nitrobenzene. Figure 1 shows photographs of the PMT rod polymerized on the platinum wire after 2.4 V was applied to the rod in nitrobenzene including 0.1 M TBATFB. The film (Fig. 1, left) got thin and then disappeared (Fig. 1, right) to expose the bare Pt wire. The top part of the rod was dissolved preferentially, probably because a flow from the top to the bottom was formed by natural convection. The time variation of the thickness of the PMT film is shown in Fig. 2. The thickness RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 40
100
‡ b c 200
0
400
600 t, s
Fig. 3. Chronoamperometric current of the PMT-coated electrode in 0.1 M TBATFB + NB quiescent solution at 1.80 V. Initial thicknesses of the films were (‡) 43, (b) 30, and (c) 12 µm. No. 3
2004
282
AOKI et al.
I, µÄ 100
‡ b
50
c d
0
20
40
60
80
100 t, s
Fig. 4. Chronoamperometric current of the PMT-coated electrode in 0.1 M TBATFB + NB stirred solution at 1.80 V. Initial thicknesses of the films were (a) 18, (b) 15, (c) 9, and (d) 4 µm. The solution was stirred with a magnetic stirrer.
(Fig. 4), whereas those in the quiescent solution included wave form superimposed on the decreasing curves. The oscillation will be discussed in the latter section. If the dissolution was to be ascribed to a surface reaction, the current should be time-independent, because the surface area of the film is independent of the time. The monotonic decrease in the current implies that the dissolution should occur over the whole film as a volume reaction rather than at the film surface. In a solution with a high concentration of the monomer, the polymerization obviously occurs, prevailing over the dissolution. Then, the polymerization current increases with the electrolysis time because the poly-
merization occurs even within the growing film. In contrast, the dissolution exhibits decreasing current, as is shown in Fig. 3. Thus, there may be a critical concentration at which the polymerization is compensated with the dissolution. In order to find this possibility, chronoamperometric curves of the PMT-coated electrode were obtained at several concentrations of 3MT and are shown in Fig. 5. The current at high concentrations increased with time. This behavior is actually the same as repolymerization of the PMT-coated electrode in a 3MT solution. For monomer concentrations close to 20 mM (Fig. 5, curve c), the current kept the steady state for a long time. The polymerization rate may be close to the dissolution rate at this concentration. An effect of concentration variation can generally be replaced by an effect of potential variation through the Nernst equation or the Butler–Volmer equation. It is predicted that the concentration dependence of chronoamperometric curves in Fig. 5 can also be obtained for a variation of potentials. Figure 6 shows chronoamperometric curves responding to several potentials at 15 mM monomer solution. The application of less positive potentials decreased the slope of the current up to a negative value (Fig. 6, curve c), as was observed at lower concentrations of the monomer (Fig. 5, curves c and d). Consequently, the dissolution is preferential at less positive potentials in lower concentrations. In contrast, the polymerization is predominant at more positive potentials in higher concentrations. Whether the polymerization rate or the dissolution rate takes precedence can be determined by a balance between monomer concentrations and applied potentials. We consider the amount of the redox charge of the conducting polymer at chronoamperometric time, t, as I, µÄ 200
I, µÄ
‡ 300 b 100
‡
200
b 100
c
c d
0
100
0 200
100
200 t, s
300 t, s
Fig. 5. Time variation of the current of the PMT-coated electrode in 0.1 M TBATFB + NB solution including (a) 60, (b) 40, (c) 20, and (d) 10 mM 3MT, responding to the potential application of 1.8 V. The solution was stirred with a magnetic stirrer.
Fig. 6. Time variation of the current of the PMT-coated electrode in 0.1 M TBATFB + NB solution including 15 mM 3MT, responding to the application of potentials of (a) 1.9, (b) 1.8, and (c) 1.7 V. The solution was stirred with a magnetic stirrer.
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 40
No. 3
2004
COMPETITION BETWEEN POLYMERIZATION AND DISSOLUTION
a measure of identifying which occurs predominantly the polymerization or the dissolution. If we neglect the capacitive charge by the potential step, the amount of the total redox charge, Q, is represented by the charge of the initial film, Qin, plus the charge by the polymerization, QP , and minus the charge by the dissolution, QD, i.e. Q = Q in + Q P – Q D .
∆I(I∆t)–1, s–1 0.03
(1)
The polymerization enhances obviously the redox charge with the electrolysis time and, hence, leads to the condition dQ/dt = I = IP + ID > 0, where IP is the polymerization current given by dQP/dt and ID is the dissolution current given by –dQD/dt. In contrast, the dissolution is predominant when dQ/dt = I = IP + ID < 0. However, I > 0 is self-evident for the oxidation current, while I < 0 is unreasonable for the dissolution. More reasonable conditions are required for distinguishing the dissolution from the polymerization. If condition ID > IP is kept for a long time, Q decreases to zero according to Eq. (1). In contrast, the condition ID < IP makes the polymerization predominant over the dissolution. When ID = IP or QD – QP = 0, the film thickness is maintained to be the initial value. Thus, this is the condition of balancing the polymerization and the dissolution. Unfortunately, this condition has two drawbacks, one being experimentally unavailable values of ID or IP and the other being a possibility of oscillating the total current maintaining ID = IP , depending on hydrodynamic conditions (Figs. 3, 4). We are concerned with the realistic competition conditions for a long-term variation. If the chronoamperometric current increases monotonically with the time for a long time, the film should become thick by the polymerization. If the current decreases monotonically with the time, the film should disappear by the dissolution. Therefore, the practically useful diagnoses are conditions of dI/dt > 0 for the polymerization and dI/dt < 0 for the dissolution. We obtained chronoamperometric curves of the PMT-coated electrode at various concentrations of 3MT (c) and at various potentials (E) and classified them into two groups for dI/dt < 0 and dI/dt > 0 except for the initial transient portion. This classification is shown in the three-dimensional (E, logc, ∆I/I∆t)-map in Fig. 7, where ∆t was taken to be 10 s and I is the current at 10 s. The (E, logc)-plain can be divided into the upper-right and the lower-left parts, corresponding, respectively, to the polymerization (dI/dt > 0) and the dissolution (dI/dt < 0). The intersection between two domains is at the coexistence of the polymerization and the dissolution and has a linear relationship. The slope of the coexistence line is about –0.26 V per log c , which has an opposite sign for the Nernst equation. It is of interest to examine in details whether both processes really coexist or not. We tried to estimate ID and IP separately from I. The separation requires one more variable than I. We selected the variable to be at first the film thickness. A well-known in situ tool of RUSSIAN JOURNAL OF ELECTROCHEMISTRY
283
Vol. 40
0
1.6
–0.03
1.0
1.9 E, 1.8 V 1.7
1.4 ] 1.2 mM [ c log
0.8
Fig. 7. Plot of ∆I/I∆t against E and log c for the chronoamperometric experiments at PMT-coated electrode.
evaluating film thickness is the electrochemical quartz crystal microbalance. Although this has an advantage of detecting very sensitively frequency changes, it suffers from ambiguity of weight of thick and soft films owing to viscoelastic properties. In order to avoid the ambiguity, we employed the capacitance measurement by the ac impedance. Figure 8 shows the time variations of the capacitance, C, simultaneously obtained at the chronoamperometry in Fig. 5. The time variations of the capacitance are similar to those of the chronoamperometric curves. According to Eq. (1), the observed capacitance is given by C = Cin + CP – CD, where the suffixes are common to those in Qin, QP, and QD. Since the capacitance is proportional to the integral of the current with a proportional constant k, we can rewrite C as
∫
C = k [ ( I in + I P – I D ) dt ].
(2)
The differentiation of Eq. (2) with respect to t yields IP – ID = dC/kdt – Iin. Combining this equation with IP + ID = I leads to I P = ( dC/kdt – I in + I )/2,
(3)
I D = ( – dC/kdt + I in + I )/2.
(4)
A value of Iin under the steady state is zero. A value of k was obtained from chronoamperometric curve under the conditions of the exhaustive dissolution without monomer. Then, we evaluated ID and IP for curves c of Figs. 5 and 8 and plotted them against t in Fig. 9. The polymerization and the dissolution contribute equally to the total current. The equal contribution (IP = ID) No. 3
2004
284
AOKI et al.
C, µF 15
ated with a heterogeneous concentration distribution of the monomer near the electrode. The polymerization rate is approximately proportional to both the thickness of the film and the monomer concentration, i.e. k1Γc, where Γ is the thickness of the PMT film and k1 is the rate constant. According to Figs. 3 and 4, the dissolution rate is approximately proportional to the thickness of the PMT film without the polymerization, i.e. k2Γ, where k2 is the rate constant. Then, the net rate of the film thickness is expressed by
‡ 10
b
d Γ /dt = k 1 Γ c – k 2 Γ .
5 c d 100
0
200
300 t, s
Fig. 8. Time variation of the capacitance of the PMT-coated electrode in 0.1 M TBATFB + NB solution including (a) 60, (b) 40, (c) 20, and (d) 10 mM 3MT, responding to the potential application at 1.8 V. The solution was stirred with a magnetic stirrer. These variations were obtained at the same time as those for in Fig. 5.
demonstrates that the competition condition can be represented by dI/dt = 0 (Fig. 5, curve c). When chronoamperometry was carried out in the quiescent monomer solution under competition conditions, the current was complicated by oscillation as is shown in Fig. 10. The oscillation can also be found in the curves of Fig. 3, which obtained under the quiescent solution. The oscillation disappeared when the solution was stirred. It was retrieved when the solution was quiescent. This fact indicates that the oscillation is associID, µA;
IP, µA
On the other hand, the concentration near the film is consumed with the polymerization. The consumption rate may be proportional to the concentration as well as the thickness of the film, i.e. k3Γc. The local consumption of the monomer causes diffusion of 3MT from the bulk toward the film–solution interface. The rate of the diffusional supply of the monomer is proportional to its local concentration, i.e. k4c. Then, the local mass balance of c is given by dc/dt = – k 3 Γ c + k 4 c.
(6)
The combination of Eqs. (5) and (6) is equivalent to the kinetic equation of the Lotka–Volterra type, which is known as a self-driven oscillation reaction [18]. Therefore, the current–time curve under the competition conditions is accompanied with the oscillation. Stirring the solution makes the diffusional concentration distribution uniform; that is, c in k4c is independent of the time. Therefore, the oscillation does not occur, as is in accordance with the experimental result. The oscillation can be explained as follows. When a concentration of the monomer is so low that k1c – k2 < 0 in Eq. (5), a value of dΓ/dt is negative and, hence, the thickness of the film decreases with the time. The I, µA 50
50 40
(5)
polymerization
40
30 20
30 dissolution
10
20 0
100
200
300 t, s
Fig. 9. Time variation of the partition of I into ID and IP under the conditions of Figs. 5 and 8 in 20 mM 3MT + 0.1 M TBATFB + NB solution.
0
500
1000
t, s
Fig. 10. Time variation of the current of the PMT-coated electrode in 0.1 M TBATFB + NB solution including 8 mM 3MT, responding to the potential application of 1.8 V. The solution was quiescent.
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 40
No. 3
2004
COMPETITION BETWEEN POLYMERIZATION AND DISSOLUTION
decrease in Γ leads to –k3Γ + k4 > 0 in Eq. (6). Then, dc/dt > 0 enhances a value of c with the time. The increase in c alters the sign of k1c – k2 (=dΓ/dt) from negative to positive. Then, a value of Γ increases with time. The increase in Γ changes the sign of −k3 + k4 from the positive to the negative value, which decreases c through dc/dt < 0. The decrease in c yields k1c – k2 < 0. These steps are iterated to oscillate the thickness as well as the current. 4. CONCLUSIONS PMT films dissolve in monomer-free solutions at the same potentials as the polymerization potentials. The polymerization and the dissolution occur simultaneously in the solution including the monomer. The former increases the thickness of the film, whereas the latter reduces it. Thus, these processes are competitive. Whether the films are dissolved or polymerized depends on concentrations of the monomer and potentials. When E < [–0.26logc (mM) + 2.06] (in V) obtained from Fig. 7, the dissolution is predominant over the polymerization. Under the competitive conditions, chronoamperometric curves contained oscillation. Therefore, the oscillating current is far from the equilibrium in the context of balancing the polymerization and the dissolution. The oscillation was explained in terms of the Lotka–Volterra model for a simple oscillation reaction, in which competitive species were the conducting polymer and the monomer. REFERENCES 1. Evans, G.P., Advances in Electrochemical Science and Engineering, Gerischer, H. and Tobias, C.W., Eds., New York: VCH, vol. 1, 1990.
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 40
285
2. Ram, M.K., Adami, M., Sartore, M., Paddeu, S., and Nicolini, C., Synth. Met., 1999, vol. 100, p. 249. 3. Pellerino, J., Radebaugh, R., and Mattes, B.R., Macromolecules, 1996, vol. 29, p. 4985. 4. Leclerc, M., Guay, J., and Dao, L.H., Macromolecules, 1989, vol. 22, p. 649. 5. Kilmartin, P.A. and Wright, G.A., Synth. Met., 1999, vol. 104, p. 145. 6. Mattoso, L.H.C., Paterno, L.G., Campana, S.P., and Oliveira, O.N., Jr., Synth. Met., 1997, vol. 84, p. 123. 7. Cao, Y., Smith, P., and Heeger, A.J., Synth. Met., 1992, vol. 48, p. 91. 8. Laska, J., Trzanadel, M., and Pron, A., Mater. Sci. Forum, 1993, vol. 122, p. 177. 9. Genies, E.M., Tsintavis, C., and Syed, A.A., Mol. Cryst. Liq. Cryst., 1985, vol. 121, p. 181. 10. Jen, K.Y., Miller, G.G., and Elsenbaumer, R.L., J. Chem. Soc. Chem. Commun., 1986, p. 1346. 11. Sugimoto, R., Takeda, S., Gu, H.B., and Yoshino, K., Chem. Express, 1986, vol. 1, p. 635. 12. Mukoyama, I., Aoki, K., and Chen, J., J. Electroanal. Chem., 2002, vol. 531, p. 133. 13. Marque, P., Roncali, J., and Garnier, F., J. Electroanal. Chem., 1987, vol. 218, p. 107. 14. Tsai, E.W., Basak, S., Ruiz, J.P., Reynolds, J.R., and Rajeshwar, K., J. Electrochem. Soc., 1989, vol. 136, p. 3683. 15. Kobayashi, T., Yoneyama, H., and Tamura, H., J. Electroanal. Chem., 1984, vol. 177, p. 293. 16. Refaey, S.A.M., Schwitzgebel, G., and Schneider, O., Synth. Met., 1999, vol. 98, p. 183. 17. Kobayashi, T., Yoneyama, H., and Tamura, H., J. Electroanal. Chem., 1984, vol. 161, p. 419. 18. Atkins, P.W., Physical Chemistry, Oxford: Oxford University Press, 1998, 6th ed., p. 809.
No. 3
2004
