Cation driven actuation for free standing PEDOT films prepared from propylene carbonate electrolytes containing TBACF3SO3

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Electrochimica Acta 53 (2008) 2593–2599

Cation driven actuation for free standing PEDOT films prepared from propylene carbonate electrolytes containing TBACF3SO3 Rudolf Kiefer, Graham A. Bowmaker, Ralph P. Cooney, Paul A. Kilmartin, Jadranka Travas-Sejdic ∗ Polymer Electronics Research Centre, Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 12 June 2007; received in revised form 17 October 2007; accepted 17 October 2007 Available online 23 October 2007

Abstract Free standing PEDOT [poly(3,4-ethylenedioxythiophene)] films (with surface conductivities of 200–400 S cm−1 ) were generated in tetrabutylammonium trifluromethanesulfonate (TBACF3 SO3 ) electrolytes by potentiostatic (EP 1.05 V vs. Ag wire) electropolymerisation in propylene carbonate (at −27 ◦ C) and methyl benzoate (at −4 ◦ C). Films obtained in the TBACF3 SO3 electrolytes showed a length increase of 2–3% during scans to negative potentials under isotonic (constant load 1.35 MPa) and stress of 0.3 MPa under isometric (constant length) conditions. Cation movement occurred due to immobilisation of CF3 SO3 − anions during electropolymerisation. The system showed good stability and low creep during square wave electrochemical cycling in the potential range from 0.0 to 1.0 V. The surface morphology (SEM) of the PEDOT films showed that the polymer structure is dependent upon the solvent used during the polymerisation process. © 2007 Elsevier Ltd. All rights reserved. Keywords: Free standing PEDOT films; Isotonic; Isometric; TBACF3 SO3 electrolyte; Cathodic actuation; Solvent effect

1. Introduction The conducting polymers PEDOT [poly-(3,4-ethylenedioxythiophene)] and PPy (polypyrrole) are of great interest for micro-actuator applications [1] such as micro-pumps and microvalves [2]. Work on conducting polymers for actuators, has focused mostly on PPy, which has the advantage that polymerisation can take place in aqueous solutions. In the case of PEDOT, it has been shown that the actuation of bilayers depends upon the polymerisation potential such that films obtained at a lower polymerisation potential showed a higher deflection [3,4]. Compared to PPy, little research has been carried out on PEDOT free standing films under isotonic (constant load) or isometric (constant length) conditions. One of the complications in the case of PEDOT films is their low stability which limits their performance as actuators, noted, for example, when polystyrenesulfonate (PSS) was used as the dopant [5]. Research on PEDOT films prepared in tetra-

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0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.10.033

butylammonium hexafluorophosphate (TBAPF6 ) in propylene carbonate (PC) and tested under isotonic conditions (1 MPa load) showed good performance and a maximum strain of about 2% [6]. When the ionic liquid 1,3-butylmethylimidazolium hexafluorophosphate (BMIMPF6 ), was used as the electrolyte, a strain of 4% (under 1 MPa load) was observed, and the movement of the imidazolium cation determined the actuation [7]. For PEDOT/PSS, as with PPy in sodium dodecyl benzene sulfonate (NaDBS) [5,8], mainly cation driven actuation was observed due to the entrapment of large anions within the polymer network during polymerisation. In the case of medium sized anions such as tetrabutylammonium triflouromethanesulfonate (TBACF3 SO3 ), mixed anion and cation driven actuation during charging and discharging was observed in polypyrrole [9], in which case anion movement dominated the actuation behaviour. We have shown that free standing PPy/TBACF3 SO3 films polymerised and tested in propylene carbonate (PC) showed mainly cation movement and cathodic actuation at higher scan rates [10]. In this paper, free standing PEDOT films were polymerised and the electrochemomechanical deformation (ECMD) was

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studied under isotonic and isometric conditions, similar to our previously published research on PPy [10].

the percentage length changes are referenced against that point.

2. Experimental

2.4. Conductivity measurements

2.1. Materials

The conductivity of dry, free standing polymer films was measured using a Jandel four-probe conductivity meter (Model RM2, Jandel 4-Point Probe Head). The conductivity was measured on several different areas on the front side of the polymer films and the mean values with standard deviations are reported.

EDOT (3,4-ethylenedioxythiophene) (Aldrich) was distilled and stored in the dark under nitrogen. The electrolyte tetrabutylammonium trifluromethanesulfonate (TBACF3 SO3 ) (Aldrich), propylene carbonate (PC) and methyl benzoate (MB) were used as supplied. 2.2. Film deposition PEDOT films were deposited potentiostatically in a three electrode cell at 1.05 V (vs. Ag wire; +0.15 V vs. Ag/AgCl (3 M KCl)) from propylene carbonate (PC) or methyl benzoate (MB) solutions of 0.1 M EDOT and 0.1 M TBACF3 SO3 on a stainless steel working electrode using a CH Instruments electrochemical workstation (model 440). The counter electrode was a Pt-net and the reference electrode was an Agwire. Before electropolymerisation the electrolyte solution was purged with nitrogen to remove oxygen. The temperature of the polymerisation cell was held constant at −27 ◦ C for PC and at −4 ◦ C for the MB solution. The electropolymerisation of PEDOT/PC/TBACF3 SO3 was stopped when a total charge of 12.48 C was passed (the stainless steel working electrode surface area was 4.16 cm2 ) and for PEDOT/MB/TBACF3 SO3 at a total charge of 17.28 C (stainless steel working electrode surface area of 4.32 cm2 ), which corresponds to a film thickness [11] of 15 ␮m for PEDOT/PC/TBACF3 SO3 and 20 ␮m for PEDOT/MB/TBACF3 SO3 . 2.3. Electrochemomechanical deformation (ECMD) measurements of free standing PEDOT films in PC/TBACF3 SO3 The PEDOT films were peeled off the stainless steel electrode and cut into 1.0 cm × 0.3 cm strips. For linear actuation measurements of the free standing PEDOT films a Dynamical Muscle Analyzer (Aurora Scientific Inc.) was used. One side of the PEDOT film was fixed to a polyvinyl chloride holder with a Pt wire as the contact at the bottom and the other side was connected to the servo actuator arm. For linear length changes a constant force of 61 mN (1.35 MPa) was applied and each film was preloaded overnight before the ECMD measurements were commenced. For stress measurements a constant length of 4 mm was maintained, with initial pre-stretching of the sample achieved by applying a force of 110 mN. The free standing PEDOT films were connected to the working electrode in a three electrode cell containing a reference electrode Ag/AgCl (3 M KCl) and a Pt-sheet counter electrode. The length between the clamps was 3 mm for PEDOT/PC/TBACF3 SO3 films and 4 mm for PEDOT/MB/TBACF3 SO3 films. In presenting results for the length change, the shortest length achieved within a particular experiment is given a value of zero, and

2.5. SEM images For scanning electron microscopy (SEM) experiments a Phillips XL30-FEG instrument was used. 3. Results and discussion Linear actuation (strain, l (%)) of the free standing PEDOT/PC/TBACF3 SO3 films was measured under isotonic conditions during cyclic voltammetry (CV) experiments at different scan rates (v = 2–50 mV s−1 ) in the potential range from −1.0 to +1.0 V. The results are presented in Fig. 1a–d. Upon cycling from +1.0 to −1.0 V, the strain increased until the polymer was fully reduced at −1.0 V. The strain curve shows a sharp change in the slope during reduction at −0.5 V, which occurs on all the strain curves at the different scan rates (Fig. 1a–d). We assume that the change in the slope indicates a decreased rate of cation ingress at more negative potentials. During the reverse scan the strain in the potential range from −1.0 to −0.36 V was nearly constant. Further oxidation led to a sharp decrease in strain up to a potential of +0.65 V, followed by a small increase in strain to the fully oxidized state at +1.0 V presumably due to ingress of CF3 SO3 − anions at higher potentials. At the lower scan rates (2–10 mV s−1 , Fig. 1a–c) the increase in strain at positive potentials is clearly visible but almost completely disappears at the higher scan rate of 50 mV s−1 (Fig. 1d). The hysteresis in ECMD curves presented in Fig. 1 indicates a difference in energy required for cation diffusion in and out of the polymer film, while the difference in the starting and finishing points of ECMD curves for each individual cycle reflects the creep often associated with actuation of conducting polymers (see below) [12–14]. Most of the strain at negative potentials can be explained as being due to solvated cation incorporation [10]. The triflate anion (CF3 SO3 − ) with an ionic radius of 0.270 nm [15] is larger than the PF6 − anion (radius 0.256 nm) [15] commonly used as the electrolyte for free standing PEDOT films [6] in PC. Cation movement during reduction of the films investigated here can be explained by CF3 SO3 − anion entrapment inside the polymer network during polymerisation. This could be due to the structural form of the triflate anion, which is non-spherical [15,16] (cylindrical) and carries a highly delocalised charge [17]. Upon reduction, some of these anions remain inside the polymer network and cations move inside to maintain electroneutrality.

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Fig. 1. Strain profile (l (%)) and cyclic voltammograms (2nd cycle) for PEDOT/PC/TBACF3 SO3 films polymerised in PC/TBACF3 SO3 at −27 ◦ C and cycled in 0.1 M PC/TBACF3 SO3 at scan rates of (a) 2 mV s−1 ; (b) 5 mV s−1 ; (c) 10 mV s−1 ; (d) 50 mV s−1 .

Based on quartz crystal microbalance measurements on poly(Nvinylcarbazole) films in acetonitrile, Carlier et al. [18] proposed that both anions and cations can be immobilised inside the polymer network. Dual ion movement during electrochemical redox processes was also found in PEDOT/TBAPF6 films in PC [6]. Under isotonic conditions (a constant load of 1.35 MPa) we estimated the volumetric work capacity [19] to be 37 kJ m−3 (upon reduction at −1.0 V) which compares favourably with PPy helix tubes where a value of 83 kJ m−3 was observed upon oxidation [20] and for PPy/NaDBS films where a value of 73 kJ m−3 was recorded [21]. The efficiency of PPy actuators reported in the literature is 0.2% [21] which is low compared to that of natural muscle which lies in the range of 45–75% [22]. The efficiency of the PEDOT/TBACF3 SO3 films under isotonic conditions was estimated [23,24] to be about 0.1%. It has been postulated that the efficiency of actuation can be increased by lowering the resistivity of the electrolyte and by using thinner films [24]. A closer view of the stress developing during one voltammetric cycle is shown in Fig. 2a. Starting from a potential of +1.0 V the stress decreased until the potential reached −1.0 V, while on the reverse scan the stress increased. At a potential of +0.7 V a small decrease in the stress was observed upon oxidation, which correlates with a strain increase in Fig. 1b under isotonic conditions that can be related to anion movement in the film.

Fig. 2b presents stress and potential time profiles during four CV cycles. A small negative creep of −0.14 MPa occurs during the four cycles. Some authors [12,13] attribute such a creep to anisotropic chain alignment of conducting polymer chains in the stretching direction. Madden et al. [14] studied the creep of

Fig. 2. (a) Stress and current response for a PEDOT/TBACF3 SO3 film versus potential during the second cycle and (b) stress and potential time profiles during cyclic voltammetry at v = 5 mV s−1 (4 cycles).

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Fig. 3. Strain (mean value of three measurements) () and stress () vs. scan rate v for PEDOT/TBACF3 SO3 films.

PPy films under isotonic conditions at different applied loads (2–120 MPa). As the film stiffness increased with time the creep decreased. They also found that higher temperatures and higher actuations also increased the creep [14]. Isotonic measurements on PEDOT/TBACF3 SO3 films were carried out during cyclic voltammetry at different scan rates (2–100 mV s−1 ) and the results are presented in Fig. 3. At a low scan rate of 2 mV s−1 a high strain (2.8%) and relatively high stress (0.34 MPa) were observed. The strain and stress decreased as the scan rate increased from 2 to 100 mV s−1 and at a scan rate of 100 mV s−1 a strain of 1.3% and stress of 0.9 MPa were measured. The strain:charge ratio for the scan rates in the range from 2 to 50 mV s−1 were similar with an average value of 16.5 ± 1.5% C−1 cm−2 suggesting a linear relationship between charge and the resulting strain. The stability upon cycling and the magnitude of the strain and stress for prolonged cycling are usually limiting factors in actuator applications. In order to investigate the stability of the actuator upon cycling, square wave potentials between −1.0 and +1.0 V were applied (50 cycles) with a frequency of 0.033 Hz and the response under isotonic (Fig. 4a) and isometric conditions (Fig. 4b) was recorded. Fig. 4a and b shows the stress (MPa) and strain time profiles, and Fig. 4c the stress and strain during 50 potential step cycles for the PEDOT film. The insets of Fig. 4a and b shows the strain and stress curve for the 30th cycle of applied square wave potential where at a negative potential an increase of strain and decrease of stress was seen, similar to that observed during cyclic voltammetry measurements. The stress values were close to the minimum value of zero at +1 V during the last 10 cycles. Notice that under isotonic conditions the strain increases slightly with the number of steps. On the contrary, the stress decreased more significantly (cycle 1: 0.147 MPa and cycle 50: 0.089 MPa). The calculated strain rate was found to be in the range ±0.1% s−1 which is similar to values calculated for the strain rate of PEDOT/TBAPF6 films cycled in PC/TBAPF6 (at 0.2%) [7]. A positive creep of 5.3% after 50 cycles can be observed during film testing under isotonic conditions and a negative creep of −0.13 MPa under isometric conditions (Fig. 4a and b). We have

noticed that when the films were cycled over a limited potential range, the creep can be minimised. Fig. 5a–c presents the results for square wave potential experiments when the potential steps were limited to 0.0 – +1.0 V. The strain obtained under isotonic conditions at a constant frequency of 0.033 Hz was found to be in the range of ±0.6% with a strain rate of +0.04% s−1 on reduction (+1.0–0.0 V) and −0.07% s−1 on oxidation (0.0 – +1.0 V). The stress change under isometric conditions was found to be about 8 kPa. The insets of Fig. 5a and b show the strain and stress profiles for cycle 30. The film shows, as before, contraction on oxidation and expansion on reduction. Notice that no creep was seen under isotonic conditions and only a small creep under isometric conditions (−8 kPa) over this limited potential range (Fig. 5c). One possible explanation for the smaller creep of the PEDOT films upon cycling (Fig. 5b) could be more balanced charge densities during oxidation (+70 mC cm−2 on stepping from 0.0 to +1.0 V) and reduction (−65 mC cm−2 on stepping from +1.0 to

Fig. 4. Square wave ECMD measurements (50 cycles, each for a 30 s potential step between −1.0 and +1.0 V) of a PEDOT film in PC/TBACF3 SO3 . (a) Stress (σ (MPa)); (b) strain (l (%)), with insets for the stress and strain profiles for the 30th cycle and (c) strain () and stress changes () obtained for potential steps between −1.0 and +1.0 V versus the number of cycles. The error bars correspond to the standard deviation obtained for three repeat measurements.

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Fig. 5. Square wave ECMD measurements (50 cycles, each for a 30 s potential step between 0.0 and +1.0 V) of a PEDOT film in PC/TBACF3 SO3 . (a) Stress (σ (kPa)); (b) strain (l (%)), with insets for the stress and strain profiles for the 30th cycle; (c) strain () and stress changes () obtained for potential steps between 0.0 and +1.0 V vs. the number of cycles. The error bars correspond to the standard deviation obtained for three repeat measurements.

0.0 V). This can be compared to the charge densities passed when the potential range of +1.0 to −1.0 V was examined, where the charge density upon reduction was −147 mC cm−2 and upon oxidation +99 mC cm−2 . We assume that the higher discharging values relate to the higher creep (Fig. 4 b) provoked by electrochemical processes such as reduction of the solvent or other processes, including the influence of osmotic pressure [25]. Spinks et al have shown a similar ‘instability’ during cycling of PPy actuators which could be overcome by current pulses [26]. We have also investigated the effect of the solvent during PEDOT polymerisation on the electrochemical properties and actuation under isotonic conditions. Kaneto and co-workers [27–33] reported that PPy/TBACF3 SO3 films prepared from methyl benzoate (MB) electrolyte showed higher actuation strains than films prepared from propylene carbonate. A comparison of the results obtained in this study is presented in Fig. 6a and b. The thickness of the polymer films was different, because the mechanical stability of the films polymerised in MB was poor, so thicker films were grown in this case. Therefore, in order to compare the results, the volume current density (IVP ,

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A cm−3 ) is shown in Fig. 6a rather than the raw current values. Fig. 6a shows that the oxidation wave of a PEDOT film polymerised from MB shifted to more positive potentials compared to films polymerised from PC (Eox (PC) = 0.0 V, Eox (MB) = +0.34 V) and the reduction peak is shifted to more negative potentials (Ered (PC) = −0.3 V, Ered (MB) = −0.66 V). The strain curves under isotonic conditions (constant load of 1.35 MPa) (Fig. 6b) showed a higher strain (∼2%) at negative potentials for PEDOT films polymerised from PC compared to films polymerised in MB (a strain of 1.3%). The ECMD profiles of both PEDOT films were similar and showed predominantly cathodic actuation upon reduction. However, a small anodic actuation can be observed during oxidation; more so for the film polymerised from MB. The smaller strain obtained for the PEDOT film polymerised from MB may be partly explained by slower ion diffusion into the film [34] as indicated also by a greater separation of redox peaks in cyclic voltammograms (Fig. 6a) in spite of the film’s higher conductivity (see later). Otero et al. [35] studied the solvent effect in PPy actuators and found that the best solvents are those having high dipole moments, low polarizability and a high capacity to donate electrons. The properties of PC (density, ρ, of 1.998 g cm−3 ; viscosity, η, of 2.513 mPa s; relative permittivity, εr , of 64.92) [36] are different from those of MB (ρ = 1.085 g cm−3 [37], η = 1.918 mPa s [37], εr = 6.74 [38]). The relative permittivity is thus 10 times higher for PC compared to MB. We assume that the lower permittivity of MB leads to a slower deposition of the polymer chains at the working electrode during polymerisation forming a denser polymer network. This would be in agreement with the higher oxidation potential of polypyrrole films obtained from MB which have a more compact morphology. The SEM images of both PEDOT films are shown in Fig. 7A and B. The morphology of the PEDOT film polymerised from PC (Fig. 7A) shows a very porous structure where the pores are connected with nano-fibres (15–20 nm). When a PEDOT film was polymerised from MB under the same conditions, the morphology of the film was different (Fig. 7B). The structure was

Fig. 6. Cyclic voltammetry of PEDOT films v = 5 mV s−1 prepared from PC (15 ␮m) (—) and MB (20 ␮m) ( – – – ), polymerised at EP = +1.05 V and cycled in PC/TBACF3 SO3 . (a) Current density (IVP , current/volume) and (b) ECMD profile versus potential E.

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ration affected the obtained strain, morphology and conductivity of the PEDOT films. Acknowledgement The authors gratefully acknowledge New Zealand Foundation for Science and Technology for financial support for this work (New Economy Research Fund, contract no. UOAX0408). References [1] [2] [3] [4] [5] [6]

[7] [8] [9] [10] Fig. 7. SEM images (scale bar 1 ␮m) of (A) PEDOT/PC and (B) PEDOT/MB.

less porous with a significant tubular morphology (80–100 nm). It has been shown before that the surface morphology of PEDOT films are influenced by the polymerisation potential [39] which affects also the conductivity and actuation [4]. High polymerisation potentials (1.0–1.2 V) produce films with a high surface roughness and low actuation [4]. The surface conductivities of the PEDOT films polymerised from PC were determined to be 202 ± 3 S cm−1 and the film polymerised from MB at 419 ± 19 S cm−1 . The higher conductivity obtained for PEDOT films polymerised from MB can be explained as being due to the effect of the solvent on the growth of more aligned polymer chains during polymerization [40]. 4. Conclusions The linear actuation behaviour of PEDOT/PC/TBACF3 SO3 films, investigated under isotonic conditions, showed a maximum strain of around 3%, and under isometric conditions a maximum stress of about 0.4 MPa. During electrochemomechanical measurements on PEDOT/TBACF3 SO3 films, mainly cation driven actuation was observed at higher scan rates (>10 mV s−1 ) and a small additional anionic actuation was seen at low scan rates (2 mV s−1 ). Longer term cycling revealed a significant creep in the stress and strain if a potential range of −1.0 to +1.0 V was employed in square wave potential experiments. However, the creep was minimized when the potential range was limited to between 0.0 and +1.0 V, where the charge densities during oxidation and reduction were more balanced. Using of MB instead of PC as the solvent during the film prepa-

[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

E.W.H. Jager, E. Smela, O. Ingan¨as, Science 290 (2000) 1540. E. Smela, Adv. Mater. 15 (2003) 481. X. Chen, K.-Z. Xing, O. Ingan¨as, Chem. Mater. 8 (1996) 2439. R. Kiefer, D.G. Weis, J. Travas-Sejdic, G. Urban, J. Heinze, Sens. Actuators B 123 (2007) 379. H. Yamato, K.-I. Kai, M. Ohwa, T. Asakura, T. Koshiba, W. Wernet, Synth. Met. 83 (1996) 125. N. Vandesteeg, P.G.A. Madden, J.D. Madden, P.A. Anquetil, I.W. Hunter, Proceedings of SPIE—The International Society for Optical Engineering, vol. 5051, 2003, p. 349. N.A. Vandesteeg, P.A. Anquetil, I.W. Hunter, Proceedings of SPIE—The International Society for Optical Engineering, vol. 5385, 2004, p. 182. J. Ouyang, C.-W. Chu, F.-C. Chen, Q. Xu, Y. Yang, Adv. Funct. Mater. 15 (2005) 203. Q. Pei, O. Inganaes, Solid State Ionics 60 (1993) 161. R. Kiefer, S.Y. Chu, P.A. Kilmartin, G. Bowmaker, R. Cooney, J. TravasSejdic, Electrochim. Acta 52 (2007) 2386. N. Lassalle, P. Mailley, E. Vieil, T. Livache, A. Roget, J.P. Correia, L.M. Abrantes, J. Electroanal. Chem. 509 (2001) 48. T. Herod, J.B. Schlenoff, J. Chem. Mater. (1993) 5. R. Pytel, E. Thomas, I. Hunter, Chem. Mater. 18 (2006) 861. J.D. Madden, D. Rinderknecht, P. Anquetil, I. Hunter, Sens. Actuators A 133 (2007) 210. M. Ue, J. Electrochem. Soc. 143 (1996) L270. M. Ue, M. Takeda, M. Takehara, S. Mori, J. Electrochem. Soc. 144 (1997) 2684. P. Marque, F. Roncali, F. Garnier, J. Electroanal. Chem. 218 (1987) 107. V. Carlier, M. Skompska, C. Buess-Hermann, J. Electroanal. Chem. 456 (1998) 139. G.M. Spinks, T.E. Campbell, G.G. Wallace, Smart Mater. Struct. 14 (2005) 406. G.M. Spinks, B. Xi, V.-T. Truong, G.G. Wallace, Synth. Met. 151 (2005) 85. E. Smela, M. Kallenbach, J. Holdenried, J. Microelectromech. Syst. 8 (1999) 373. R.H. Baughman, Synth. Met. 78 (1996) 339. K. Kaneto, F. Hisashi, K. Masakatsu, W. Takashima, Smart Mater. Struct. 16 (2007) S250. I. Hunter, P. Anquetil, J. D. W. Madden, P. G. A. Madden, MIT Home Automation and Healthcare Consortium, 2000. V. Tabard-Cossa, M. Godin, P. Gruetter, I. Burgess, R.B. Lennox, J. Phys. Chem. B 109 (2005) 17531. G.M. Spinks, B. Xi, D. Zhou, V.-T. Truong, G.G. Wallace, Synth. Met. FIELD 140 (2004) 273. S. Hara, T. Zama, S. Sewa, W. Takashima, K. Kaneto, Chem. Lett. 32 (2003) 576. S. Hara, T. Zama, W. Takashima, K. Kaneto, Polym. J. (Tokyo, Japan) 36 (2004) 151. S. Hara, T. Zama, W. Takashima, K. Kaneto, Synth. Met. 149 (2005) 199. S.S. Pandey, W. Takashima, M. Fuchiwaki, K. Kaneto, Synth. Met. 135/136 (2003) 59. W. Takashima, S.S. Pandey, K. Kaneto, Thin Solid Films 438/439 (2003) 339. T. Zama, S. Hara, W. Takashima, K. Kaneto, Bull. Chem. Soc. Japan 77 (2004) 1425.

R. Kiefer et al. / Electrochimica Acta 53 (2008) 2593–2599 [33] T. Zama, S. Hara, W. Takashima, K. Kaneto, Bull. Chem. Soc. Japan 78 (2005) 506. [34] A.S. Hutchison, T.W. Lewis, S.E. Moulton, G.M. Spinks, G.G. Wallace, Synth. Met. FIELD 113 (2000) 121. [35] T.F. Otero, I. Cantero, H. Grande, Electrochim. Acta 44 (1999) 2053. [36] M. Ue, J. Electrochem. Soc. 141 (1994) 3336. [37] P.S. Nikam, S.K. Kharat, J. Chem. Eng. Data 50 (2005) 455.

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[38] P.Winget, M.D. Dolney, D.J. Giesen, C.J. Cramer, D.G. Truhlar, Minnesota Solvent Descriptor Database, Minneapolis, 1999, p. MN 55455. [39] L. Niu, C. Kvarnstr¨om, A. Ivaska, NATO Advanced Research Workshop on Electrochemistry of Electroactive Polymer Filmsv WEEPF-2000, 2000, 21. p. [40] M.C. Morvant, J.R. Reynolds, Synth. Met. 92 (1998) 57.