J . Phys. Chem. 1988, 92, 833-840 long-range aggregation, even though not necessarily crystallinelike. Somehow, this structure is induced by a critical and very small water concentration. Actually, gels are formed with a stoichiometric amount of water (2-8 water molecules per lecithin molecule in most cases) and it is then likely that water molecules are tightly bound to the polar surfactant head. But aside from these general considerations, the structure of these organogels is not clear yet. The empirical observations made in this paper are, however, very useful to build a preliminary relation between the chemical structure of the solvent and the wo(gel) value. It is apparent, for example, that linear hydrocarbons are those that are characterized by the lowest wo(gel) values (in the range 1-3), whereas cyclic hydrocarbons have wovalues that are larger by a significant factor. The largest wo(gel) are displayed by rather bulky solvent molecules, such as cyclodecane, phenylcyclohexane, and cis-pinane. Light scattering data are in progress, but is is perhaps from small-angle X-ray scattering and from electron microscopy that the most useful information may come in. Also, the work by Furhop and co-workers for aqueous gels may offer a very good reference point in this regard.I1J2 (1 1) Furhop, J.-H.; Chem. Soc., in press.
Schnieder, P.; Rosenberg, J.; Boekema, E. J . Am.
833
Acknowledgment. W e acknowledge with gratitude Hoffmann-La Roche (Dr. Steffen) for the gift of soybean lecithin and the ETH fund as well as the Swiss National Fund (NF-19) for financial support. We are very grateful to Dr. Kramer at the EMPA (Dubendorf) for performing the dynamic shear viscosity measurements. Registry No. Ethyl laurate, 106-33-2; butyl laurate, 106-18-3; ethyl myristate, 124-06-1; isopropyl myristate, 110-27-0; isopropyl palmitate, 142-91-6; isooctane, 540-84-1; cyclopentane, 287-92-3; cyclohexane, 110-82-7; cycloheptane, 291-64-5; cyclooctane, 292-64-8; cyclodecane, 293-96-9; methylcyclohexane, 108-87-2; tert-butylcyclohexane, 3 17822-1; phenylcyclohexane, 827-52-1; bicyclohexyl, 92-5 1-3; 1,3,5-triisopropylbenzene, 717-74-8; octylbenzene, 2189-60-8; trans-decalin, 49302-7; (1R)-(+)-trans-pinane, 4863-59-6; (1R)-(+)-cis-pinane,4795-86-2; n-pentane, 109-66-0;n-hexane, 110-54-3;n-heptane, 142-82-5;n-octane, 11 1-65-9; n-nonane, 11 1-84-2; n-decane, 124-18-5; n-undecane, 112021-4; n-dodecane, 112-40-3; n-tridecane, 629-50-5; n-tetradecane, 62959-4; n-pentadecane, 629-62-9; n-hexadecane, 544-76-3; n-heptadecane, 629-78-7; 2,3-dimethylbutene, 27416-06-4; 1-hexene, 592-41-6; 1,7-octadiene, 37 10-30-3; tripropylamine, 102-69-2; tributylamine, 102-82-9; triisobutylamine, 1116-40-1; trioctylamine, 11 16-76-3; dibutyl ether, 142-96-1; 2-dodecen-1-ylsuccinic anhydride, 19780-11-1. (12) Furhop, J.-H.; Hermann, H.-H.; Mathieu, J.; Liman, U.; Winter, H.-J.; Boekema, E. J . Am. Chem. SOC.1986, 108, 1785-1791.
Electroactivity of Transparent Composite Films from Conducting Poly(thiophenes) J. Roncali* and F. Garnier Laboratoire de Photochimie Solaire, CNRS E R 241, 2 rue Henry Dunant. 94320 Thiais, France (Received: September 23, 1986; In Final Form: August 17, 1987)
Conducting composite films containing an electropolymerizable conducting polymer such as poly(3-methylthiophene) (PMeT) alloyed with poly(viny1 chloride) (PVC) have been prepared in a one-step process from synthesis media already containing dissolved PVC. This procedure based on the simultaneous electropolymerization and dip-coating processes allows a large control of the composition, morphology, optical transmittance, conductivity, and electroactivity of the composite films. The growth of PMeT in synthesis media containing dissolved PVC has been analyzed. Increasing the PVC concentration produces a slight decrease of the MeT electropolymerization rate with no apparent modification of the polymerization mechanism. The electrochemical properties of the composite films have been investigated in acetonitrile by using cyclic voltammetry and chronoamperometry. At low scan rate (10 mV/s), the electrochemical responses of the composite films are identical with that of bare PMeT films prepared under the same conditions. At higher scan rates, a dependence of the electroactivity of the films on their PVC content is observed and the electrochemical response turns progressively from an adsorptionlike behavior to a diffusion-controlled one. It is shown that the electrolyte concentration used for the synthesis of the composite films is the key factor controlling their electrochemical behavior. The incorporation of PMeT within the PVC matrix does not affect its spectroelectrochemical properties and furthermore leads to an improved electrochemical stability of the film under redox cycling.
Introduction Although many new organic conducting polymers have been synthesized in the last few years,' conducting polymers obtained by electrooxidation of five-membered heterocycles and particularly poly(pyrro1e) and poly(thiophenes) are still subject to intensive research efforts aiming toward the improvement of their synthesis2 and the development of their many possible applications for energy (1) (a) Waltman, R. J.; Diaz, A. F.; Bargon, J. J . Electrochem. SOC.1984, 132, 740. (b) Satoh, M.; Kaneto, K.; Yoshino, K. J . Chem. Soc., Chem. Commun. 1984, 1627. (c) Yang, N.-L.; Wang, S. S.; Hou, C. J.; Rodriguez, L.; Jolson, J.; Waggoner, J. J. Chem. SOC.,Chem. Commun.1985, 1632. (d) Mizogami, S.; Yoshimura, S. J . Chem. Soc., Chem. Commun. 1985, 427. (2) (a) Sato, M.; Tanaka, S.; Kaeriyama, K. J . Chem. SOC.,Chem. Commun. 1985, 713. (b) Satoh, M.; Kaneto, K.; Yoshino, K. Jpn. J . Appl. Phys. 1985, 24, L423. (c) Roncali, J.; Garnier, F. Nouu. J . Chim. 1986, 10, 238. (d) Roncali, J.; Garnier, F.; Lemaire, M.; Garreau, R. Synth. Met. 1986, 15, 323.
storage, electrochromic devices, and electrocatalysis with functionalized electrode^.^ However, the poor mechanical properties of these polyheterocycles often appear as a limit to the extension of their applications. In order to improve these mechanical properties, the preparation of composite matcrials containing an electrogenerated conducting polymer alloyed with a classical insulating polymer has been proposed. Such composite materials are of great interest not only because they extend the applications of conducting polymers in the fields of electronic devices or antistatic coatings but also because they represent a new route for the development of modified electrodes as demonstrated in recent (3) (a) Kaufman, J. H.; Chung, T. C.; Heeger, A. J.; Wudl, F. J . Elecrrochem. SOC. 1984, 131, 2092. (b) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J . Appl. Phys. 1983, 22, L567. (c) Garnier, F.; Tourillon, G.; Gazard, M.; Dubois, J. C. J . Electroanal. Chem. Interfacial Electrochem. 1983, 148, 299. (d) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J . Appl. Phys. 1983, 22, L412. (e) Tourillon, G.; Garnier, F. J . Phys. Chem. 1984, 88, 5281.
0022-365418812092-0833$01.50/0 0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
works4 The preparation of poly(pyrrole)/PVC composites by electropolymerization of pyrrole within a PVC matrix, previously deposited on an electrode surface by dip-coating, was reported simultaneously by two research group^.^ This method of preparation, which has been improved by the loading of the insulating matrix with an electrolyte,6 has been extended to the preparation of poly(pyrrole)/poly(vinyl alcoh01)~or poly(pyrrole)/polyester composites.8 More recently, the preparation of conducting composite films has been achieved by the chemical polymerization of pyrrole in either liquid or gaseous phases on an insulating polymer containing ferric c h l ~ r i d e . ~These J ~ two methods have in common the fact that they require as a preliminary step the preparation of a solid-state host polymer matrix prior to pyrrole polymerization. As a consequence, the spatial distribution of the conducting polymer phase in the host polymer matrix appears then to be largely dependent on the structure of the host matrix leading in some cases to poorly homogeneous materials.” In recent works, we reported a new simple and efficient one-step procedure for the preparation of homogeneous conducting composite films which consists in the direct electropolymerization of a five-membered heterocycle such as methyl-3-thiophene (MeT), in a reaction medium already containing a dissolved insulating polymer, such as poly(methy1 methacrylate) (PMMA) or PVC.’’ A similar procedure has been reported independently by Bard et al. for the preparation of poly(pyrrole)/Nafion composite^.^^ This technique allows a larger control of the characteristics of the resulting composites by means of the synthesis conditions. In this work, we describe the one-step preparation of homogeneous PMeT/PVC films and the analysis of their electrochemical properties. The effects of the dissolved PVC on the electropolymerization of MeT have been investigated. It is shown that the presence of the dissolved PVC exerts only a limited effect on the polymerization rate of MeT with no apparent modification of the electropolymerization mechanism. The effects of the concentrations of PVC and of electrolyte used for the synthesis of the composite films on their electrochemical properties have been analyzed by cyclic voltammetry and chronoamperometry. At low scan rates, these two parameters have no effect on the electroactivity of the composite films, which remains identical with that of PMeT films. However, increasing the PVC content of the composites leads to an increasing scan-rate dependence of the electrochemical response of the films. We show that the electrochemical response can be restored by increasing the electrolyte concentration used for the synthesis of the films. The spectroelectrochemical properties of these composite films have been briefly investigated. The incorporation of PMeT within the PVC matrix leads to an improved electrochemical stability under cycling, whereas the spectroelectrochemical properties of PMeT are completely preserved.
Experimental Section The purification of solvents and reagents has been already described.2c The composite films have been prepared in a single-compartment three-electrode cell, from a reaction medium (4) (a) Mizutani, F.; Iijima, S.; Tanabe, Y.; Tsuda, K. J . Chem. SOC., Chem. Commun. 1985, 1728. (b) Fan, F.-R.; Bard, A. J. J . Electrochem. Soc. 1986, 133, 301. (c) Penner, R. M.; Martin, C. R. J . Electrochem. Soc. 1986, 133, 310. (d) Iyoda, T.; Ohtani, A,; Shimidzu, T.; Honda, K. Chem. Letr. 1986, 687. (5) (a) De Paoli, M.; Waltman, R. J.; Diaz, A. F.; Bargon, J . J . Chem. SOC.,Chem. Commun.1984, 1015. (b) Niwa, 0.;Tamamura, T.J . Chem. Soc., Chem. Commun. 1984, 817. ( 6 ) Wang, T. T.; Tasaka, S.; Hutton, R. S.; Lu, P. Y . J. Chem. Soc., Chem. Commun. 1985, 1343. (7) Lindsey, S. E.; Street, G. B. Synth. Met. 1984185, 10, 67. (8) Lmdenberger, H.; Roth, S.; Hanack, M. Proceedings of an International Winter School, Kirchberg, Tirol, 1985, 2312-1 13; Springer-Verlag: Berlin, 1985. (9) Ojio, T.; Miyata, S . Polym. J . 1986, 18, 95. (10) Bocchi, V.; Gardini, G. P. J . Chem. Soc., Chem. Commun. 1986, 148. (1 1) Niwa, 0.; Hikita, M.; Tamamura, T. Makromol. Chem. Rapid. Commun. 1985, 6, 375. (12) (a) Roncali, J.; Gamier, F. J . Chem. SOC.,Chem. Commun.1986, 783. (b) Roncali, J.; Mastar, A,; Gamier, F. Synth. Met. 1987, 18, 857.
Roncali and Gamier
Figure 1. Cyclic voltammograms recorded in LiCIO,/CH,CN (scan rate, 20 mV/s) of PMeT films synthesized using 100 mC/cm2 on I T 0 in (A) methylene chloride, Qr = 3.2 mC/cm2; (B) nitrobenzene, Q, = 7.8 mC/cmz; (C) 1:l mixture, Q, = 6.3 mC/cm2.
involving 0.2 M MeT, tetrabutylammonium perchlorate (Fluka purum), and PVC in 1:l methylene chloride/nitrobenzene. Low molecular weight PVC (Aldrich 18, 958-8) was used for solubility reasons. The working electorde was either a platinum disk (S = 0.07 cm2) polished with 0.05-wm diamond paste before each experiment, or an indium-tin oxide (ITO) coated glass electrode. An aluminum foil was used as cathode and a saturated calomel electrode (SCE) as reference. The solutions were degassed by argon bubbling prior to electropolymerization, and the synthesis were performed at ambient temperature under galvanostatic conditions, by applying various current densities. After the electropolymerization, the anode was dried at 50 OC and rinsed with acetonitrile. The composite film was then either removed from the anode for conductivity measurements or left on the electrode for spectroscopic and electrochemical characterizations. The films thicknesses were determined with a Sylvac P 100 thickness monitor. Electrochemical analysis were carried out in distilled acetonitrile containing 0.1 M lithium percholate, using a PAR 173 potentiostat equipped with a PAR 175 universal programmer and a PAR 179 plug-in digital coulometer. Absorption spectra and optical densities have been recorded on a Cary 2 19 spectrometer. In situ spectroelectrochemical experiments have been performed in 0.5 M lithium perchlorate/acetonitrile, in a 1-cm quartz cell which was placed in the spectrometer by means of micrometric positioners. A Pt wire was used as cathode and an Ag wire as quasi-reference.
Results and Discussion Selection of the Solvent System. The direct one-step preparation of poly(thiophene) conducting composites from reaction media containing a dissolved insulating polymer puts some specific requirements concerning the nature of the solvent. Since the reaction medium is intended to act at the same time as a dipcoating solution and as an electropolymerization medium, the solvent must present an electrochemical stability sufficient to allow the electropolymerization of the monomer and must also dissolve efficiently the host polymer. It has been shown previously that the most conductive poly(thi0phenes) are obtained with solvents like propylene carbonatek or nitrobenzene.2csd However, the high boiling points and viscosities of these solvents are not the most convenient for dip-coating processes. On the other hand, methylene chloride, which is widely used in electrochemistry, appears preferable for film casting, as its low boiling point allows an easy removal. However, to our knowledge, the electrosynthesis of poly(thiophenes) in methylene chloride has not been reported yet. Therefore, in a preliminary set of experiments, the electropolymerization of MeT in methylene chloride has been investigated. Figure 1 shows the cyclic voltammograms of PMeT films synthesized on I T 0 in the same galvanostatic conditions (2 mA/cm2), using a deposition charge of 100 mC/cm2, in methylene chloride, nitrobenzene, and a 1:l mixture. The comparison of these curves and of the amounts of charge exchanged during the redox processes of the films (Q,) clearly shows that the most electroactive films are obtained in pure nitrobenzene. Furthermore, larger scale electropolymerizations show that PMeT films prepared in
Electroactivity of Poly(thiophene) Composite Films
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 835
Ibl
absorbdnce 633 om
dt
0.4
0.3
Figure 2. Successive voltammograms between -0.3 and +1.8 V/SCE corresponding to the polymerization of MeT (scan rate, 100 mV/s). (a) in 1:l methylene chloride/nitrobenzene; (b) same a s (a) plus 10 g / L PVC; (c) same as (a) plus 30 g / L PVC. 0.2
methylene chloride are much thinner, more brittle, and less conductive than the films synthesized in pure nitrobenzene. These results suggest a lower polymerization yield in methylene chloride. Nevertheless, it appears that the films prepared in 1:l nitrobenzene/methylene chloride retain a rather high electrochemical activity. Therefore, this solvent system has been selected for the preparation of the composite films. Electropolymerization of MeT in the Presence of Dissolved PVC. Prior to the preparation of composite films, the effects of the presence of dissolved PVC on the electropolymerization of MeT have been analyzed. Figure 2 shows the voltammograms obtained when the potential is continuously cycled between -0.3 and +1.8 V/SCE at 100 mV/s in synthesis media containing MeT (Figure 2a) and MeT plus 10 and 30 g/L PVC (Figure 2b,c). The comparison of the three series of curves shows that increasing the concentration of dissolved PVC in the reaction medium results in a smaller increment of the quantity of conducting polymer produced during each cycle. These results indicate that the MeT electropolymerization rate decreases when the PVC concentration in the synthesis medium is increased. Concurrently, a more pronounced shift of both the anodic and the cathodic current peaks toward respectively more positive and more negative potentials is also observed. This result could reflect the effect of the increasing viscosity of the medium on the diffusion of the dopant,13 but they could also be related to changes in the polarity of the medium or in the capacitive behavior of the deposits.14 Further experiments are still necessary to clarify this point. A further comparison of the three series of curves reveals the presence of a second anodic wave at 1.OO V/SCE on Figure 2b,c. Since this second wave is not observed on the curves recorded in the absence of dissolved PVC, this modification of the voltammograms could be related to the formation within the deposits of discrete microdomains with higher oxidation potential due to the combined effects of the dissolved neutral polymer and of successive potential scans. Despite these differences, the shapes of the three series of curves are essentially similar, indicating that the electropolymerization of MeT, which occurs in every case, is not drastically affected by the presence of the dissolved PVC. In order to confirm this conclusion, the rate of the PMeT growth in the presence of increasing PVC concentrations has been analyzed. For this purpose, we have recorded the variation vs time of the optical density using I T 0 electrodes and applying the same potential to the anode (3.5 V/SCE). Figure 3 shows that in every case the optical density increases linearly with time and that the slope decreases when the PVC concentration is increased. These results, which are in agreement with the trend expressed by cyclic voltammetry, confirm the decrease of the polymerization rate when the PVC concentration in the synthesis medium is increased. As shown previously by Diaz et al. for poly(pyrrole), the linear increase of the optical density versus time indicates that the polymer deposition is not diffusion limited, and these authors have proposed that the rate-limiting step during the electropolymerization is the coupling of the radical cations produced by the anodic oxidation
of the m0n0mer.l~ In our case, the linearity of the optical density versus time observed in the four systems suggests that the presence of the dissolved host polymer does not modify the mechanism of the electropolymerization. Preparation of the Composite Films. In previous works describing conducting composites from poly(pyrrole), the preparation of the films involved two The first step consisted in the dip-coating of the anode by a PVC film, and the second preparation step consisted in the electropolymerization of pyrrole within this solid-state matrix. In the case of the one-step procedure, the mechanism by which the composite film is formed is quite different. As the reaction medium already contains the dissolved host polymer, the polymerization of the conducting polymer occurs in an homogeneous liquid phase. When adequate potential is applied across the cell, the conducting polymer starts growing onto the anode surface and the conducting polymer chains grow surrounded by solvent and host polymer molecules. As a consequence, the existence of the composite film remains only virtual as long a s the anode is immersed in t h e synthesis solution. Removing t h e anode from the reaction medium and evaporating the solvent solidify the system and result in the trapping of the conducting polymer within the host polymer matrix. One of the most interesting features of this one-step procedure is that the morphology of the resuling composite film depends mainly on the mode of growth of the conducting polymer and not on the structure of a
(13)Zhang,X.; Yang, H.; Bard, A. J. J. Am. Chem. SOC.1987,109,1916. (!4) (a) Feldberg, S.W. J . Am. Chem. SOC.1984,106,4671. (b) Mermilliod, N.; Tanguy, J.; Petiot, F. J . Electrochem. Soc. 1986, 133, 1073.
(15)Genies, E.M.; Bidan, G.; Diaz, A. F. J . Electroanal. Chem. Znterfacial Electrochem. 1983,149, 101.
Figure 3. Variation of the optical density of I T 0 electrodes versus time during the electrodeposition of MeT: (a) in 1:l methylene chloride/nitrobenzene; (b) same as (a) plus 10 g / L PVC; (c) same as (a) plus 20 g / L PVC; (d) same as (a) plus 30 g / L PVC. TABLE I: Composition and Thicknesses of PMeT/PVC Films as a Function of Deposition Charge ([PVC], 25 g/L; Electrode Surface, 4 c d ; Current Density, 5 mA/cm2) weight, mg 1.025 1.030 1.012
0.954 1.236 1.426 1.909
Qd,m C 0 32 64 125 256 512 1024
% PMeT calcd
0 2
5 10 16 28 42
thickness," pm 1.5 1.6 1.4 1.6 1.9 2.4 3.0
(0.2) (0.2) (0.2) (0.2) (0.2) (0.3) (0.3)
Values in parentheses represent uncertainties.
836 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Roncali and Garnier
5
10
IS J mAlcm2
Figure 5. Variation of the electrode-side conductivity of PMeT/PVC films prepared on IT0 using 200 mC/cm2, versus current density applied for polymerization.
Figure 4. Optical micrographs of PMeT/PVC composite films prepared on IT0 electrodes at various current densities: (top) 0.5 mA/cm2; (middle) 2 mA/cm*; (bottom) 5 mA/cm2.
preexisting solid-state matrix as in the case of the previously described two-step techniques. The composition of the composite films can thus be easily controlled by means of two parameters, e.g., the concentration of the dissolved host polymer in the reaction medium, and the deposition charge used for the electropolymerization which determines the quantity of conducting polymer incorporated in the film. Table I summarizes the composition and thicknesses of PMeT/PVC composite films prepared with increasing deposition charges from a synthesis medium containing 25 g/L PVC. The compositions of the various films have been determined by the ratio.of the weight of PMeT, calculated from the deposition charge (assuming 2.2 F/mol), to the total mass of the film. These results show that, up to 10% PMeT, the thickness of the film depends essentially on the concentration of the host
polymer in the feed solution, as in the case of classical dipcoating whereas, for higher PMeT contents, the overall thickness of the composite film depends on both the host polymer concentration and the deposition charge. However, composite films containing more than 50% conducting polymer become brittle, which reduces considerably their potential interest as composite materials. As shown in previous works on PMeT/PMMA composites, the homogeneity and the electrical properties of the composite films depend strongly on the current density applied for the electropolymerization.l 2 Figure 4 shows optical micrographs of PMeT/PVC films prepared on I T 0 with current densities of 0.5, 2, and 5 mA/cm2 from a synthesis medium containing 25 g/L PVC. These pictures reveal that with low applied current density, PMeT grows on disconnected preferential sites within the PVC matrix and forms large aggregates of 50-100 pm diameter. The conductivity of the electrode side of the film lies in this case in the range of 10-3-10-2 S/cm. As it seems that the percolation threshold is not reached in such structures, it is intriguing that these materials exhibit these rather high conductivities. These observations are in agreement with an electron hopping mechanism which has already been proposed to account for the charge transport in electroactive polymers.16 Increasing the applied current density increases the number of initial nucleation sites and produces thus more homogeneous materials (Figure 4, bottom). As could be expected, the improvement of the homogeneity of the films leads to an important increase of the conductivity. Figure 5 shows the variation of the electrode-side conductivity vs the applied current density for PMeT/PVC films prepared with a constant deposition charge of 200 mC/cm2. This curve shows that the conductivity increases sharply with the applied current density and reaches values of 10-1 5 S/cm for 5-10 mA/cm2. As shown already in the case of PMeT/PMMA films, for a given .applied current density the conductivity of the films depends also on the deposition charge. Thus conductivities exceeding 30 S/cm can be obtained with deposition charges greater than 500 mC/cm2. On the other hand, resistance measurements on the solution side of the films show much lower than less reproducible values, this side of the films appearing often as almost insulating, especially for the films prepared with low deposition charges. In summary, these results demonstrate that homogeneous conducting PMeT/PVC composite films can be obtained from reaction media already containing the dissolved host polymer and they also show that the composition, the morphology, and the electrical properties of the films can be varied over a wide range by means of a few synthesis parameters. ~
(16) Kaufman, F. B.; Shroeder, A. H.; Engler, E. M.; Kramer, R. S.; Chambers, Q. J. J. Am. Chem. SOC.1980, 102,483.
Electroactivity of Poly(thiophene) Composite Films
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 831
200
Figure 6. Cyclic voltammograms recorded in 0.1 M LiC104/CH3CNat 10 mV/s: (a) PMeT film, 100 mC/cm2 on Pt (S = 0.07 c d ) ; (b) PMeT/PVC film (30 g/L PVC), 100 mC/cm2 on Pt. Ibl
100
IC1
Figure 7. Cyclic voltammograms recorded in 0.1 M LiC104/CH3CN: (a) PMeT film, 100 mC/cm2on Pt; (b) PMeT/PVC film prepared with 20 g/L PVC, 100 mC/cm2 on Pt; (c) PMeT/PVC film prepared with 30 g/L PVC, 100 mC/cm2 on Pt.
Figure 8. Plots of anodic peak current vs scan rate for PMeT and PMeT/PVC films prepared with 100 mC/cm2 and 2 mA/cmZon t'F from synthesis media containing increasing PVC concentrations: 0,PMeT film; *, PMeT/PVC film prepared with 10 g/L PVC; 0 , PMeT/PVC film prepared with 20 g/L PVC; 0, PMeT/PVC film prepared with 30 g/L PVC. TABLE II: Cyclic Voltammetric Data of PMeT/PVC Films Prepared with 100 mC/cm2 and 2 mA/cm2 on Pt (S = 0.07 cm2) as a Function of PVC Concentration Used for Their Svnthesis
Electrochemical Properties of the Composite Films. The electrochemical properties of poly(pyrrole)/Nafion composite u, mV/s E,, V/SCE Q ,mC/cm2 membranes were recently described by several g r o ~ p s ; ~how~ * ~ * ~ ~ [PVC], g/L 0 10 0.54 10.8 ever, despite numerous works reporting the preparation of con0.54 20 ducting composites derived from poly(pyrro1e) and insulating 50 0.55 polymers, the electrochemical properties of such materials have 100 0.57 not been reported yet. PVC has been employed in a great variety 200 0.60 of membrane applications,'* and PVC composites including 10 10 10.8 0.54 conducting polymers are thus of great interest for the design of 20 0.54 functionalized electrodes or charge controllable membranes. In 0.56 50 100 0.58 previous works, we have shown that the electrochemical activity 0.62 200 of PMeT/PMMA composites prepared by the one-step procedure 20 10 0.54 10.7 depends on the synthesis parameters that control the morphology 20 0.55 and the conductivity of the films, e.g., the host polymer concen50 0.57 tration, the deposition charge, and the current density used for 100 0.60 their preparation.I2 However, because of the solubility of PMMA 200 0.67 in acetonitrile, the electrochemical experiments were performed 30 10 0.54 10.6 in aqueous medium in which, due to their hydrophobic character, 20 0.64 poly(thiophenes) are much less electroactive than in organic 50 0.75 100 0.83 solvents. In contrast, the insolubility of PVC in acetonitrile allows 200 0.87 the analysis of the electrochemical behavior of PMeT/PVC films in this solvent. Furthermore, the swelling of PVC in acetonitrile curves and the data in Table I1 show that the anodic peak potential results in an enhanced electroactivity when compared to E,, exhibits an increasing dependence on scan rate as the PVC PMeT/PMMA films. As a consequence, and contrary to content of the film increases. Figure 8 represents the variations PMeT/PMMA films, no effect of the homogeneity of PMeT/PVC of the intensity of the anodic peak current Zp as a function of scan films on their electrochemical behavior has been observed, whereas, rate for the different films. In the case of the films prepared with as it will be shown, the electroactivity of PMeT/PVC films de10 g/L PVC, Zp scales linearly with scan rate, similarly to PMeT pends essentially on two synthesis parameters, the PVC concenfilms, and as expected for surface attached species. In contrast, tration, and the electrolyte concentration used for their preparation. for the films prepared with higher PVC concentrations, Zpa turns ( a ) Effect of PVC Concentration Used for Preparation of the progressively to a linear dependence on the square root of scan Films. Figure 6 shows the cyclic voltammograms recorded at 10 rate which is indicative of a diffusion-controlled behavior. These mV/s in 0.1 M LiC104/CH3CNof PMeT and PMeT/PVC films results suggest that increasing the thickness of the PVC membrane synthesized on Pt with 100 mC/cm2 deposition charge at 2 results in a decreasing diffusion rate of the doping anion through mA/cmZ. These curves and the electrochemical data reported the PVC membrane. Chronoamperometric experiments have been in Table I1 show that the amount of charge exchanged upon performed in order to confirm this conclusion. The various films reduction of the film is the same in every case, indicating that have been submitted to double potential steps between -0.3 and the PMeT part of the composite films remains fully electroactive +1 V/SCE, and the apparent diffusion coefficient D was deterwhatever the quantity of PVC in the film. Figure 7 compares mined by taking the slope of the linear part of the Z-t-'I2 plots the cyclic voltammograms obtained at higher scan rates on comgiven by posite films containing an increasing proportion of PVC. These ~
~~~~~
(17) Nagasurbramanian, G.; Di Stefano, S.; Moacanin, J. J . Phys. Chem. 1986, 90, 4447. (18) Meyerhoff, M. E.; Fraticelli, Y.Anal. Chem. 1982, 54, 27R and references cited therein.
S = nFAD'I2CdI2 where C is the concentration of electroactive sites in the film and A the electrode area. As C is difficult to determine accurately, the chronoamperometric results are presented in the form D'/2C.
838 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Roncali and Garnier
TABLE III: Chronoamperometric Data of PMeT/PVC Films Prepared with 100 mC/cm* and 2 mA/cm* on Pt ( S = 0.07 em2) as a Function of PVC Concentration Used for Their Synthesis
[PVCI
3
1O ~ D ~ / ~ C ( O X ) ,
n/L
mol / (cm-2.s-'/z)
0 10 20 30
6.4 4.5 1.35 0.39
107Dl/2C(red), mol/ (cm-2.s-'/2) 18.2 12.6 6 1.4
Figure 10. Cyclic voltammograms recorded in 0.1 M LiC1O4/CH3CN of PMeT/PVC films prepared with 30 g/L PVC and (a) 0.1 M
Bu4NC104; (b) 0.2 M Bu4NC104.
Figure 9. Cyclic voltammograms (10 mV/s) for 1.5 X M ferrocene in 0.1 M LiC104/CH3CN: (a) on PMeT film (100 mC/cm2 on Pt); (b) on PMeT/PVC film prepared with 10 g/L PVC; (c) on PMeT/PVC film prepared with 30 g/L PVC.
However, since the various films have been produced with the same deposition charge and as they exchange the same amount of charge upon reduction at low scan rate, the comparison of the D1l2Cvalues provides a good picture of the relative diffusion coefficients in the different films. The D1lzCvalues obtained for the different films are reported in Table 111. These results show as could be expected that the diffusion coefficient decreases steadily when the PVC concentration used for the synthesis of the films is increased. Interestingly, these results also show that the undoping process remains in every case faster than the doping one. It has been proposed by several groups that the undoping process in poly(pyrrole) and poly(thiophene) films is largely determined by cation t r a n ~ p 0 r t . l ~Our results appear consistent with such an interpretation, indicating that the lithium cations diffuse more rapidly through the PVC membrane than anions. A further illustration of the decreasing permeability of the composite films as their PVC content increases has been obtained by examining the electrochemical response of ferrocene on the different films. Figure 9 shows the cyclic voltammograms obtained on PMeT and PMeT/PVC films prepared with respectively 10 and 30 g/L PVC. In the case of PMeT, the electrochemical response of ferrocene appears clearly on the voltammogram and does not differ noticeably from that recorded on Pt. However, the comparison with the curves obtained on the composite films shows that whereas the PMeT response remains unchanged, the intensity of both the anodic and the cathodic peaks of ferrocene decreases markedly as the PVC content of the films increases, indicating that the permeability of the films decreases. In short, the PVC concentration used for the preparation of the composite films determines the thickness of the PVC membrane between the electroactive part of the film and the bulk solution. As a consequence, the transport of solution species to the electroactive component of the composite film becomes more difficult as the quantity of PVC in the film increases. However, the fact that at low scan rates the electrochemical response of the films appears rather independent of their PVC content suggests that the redox processes of PMeT could involve also the electrolyte trapped within the films during their synthesis. ( b ) Effect of Electrolyte Concentration Used for Synthesis of the Films. In order to confirm this latter hypothesis, we have analyzed the effect of the electrolyte concentration used for the synthesis on the electrochemical behavior of PMeT/PVC films prepared with 30 g/L PVC. Figure 10 shows the cyclic voltam(19) (a) Genies, E. M.; Pernaut, J. M. Synth. Mer. 1984/85, 10, 117. (b) Marque, P.; Roncali, J.; Garnier, F. J . Electroanal. Chem. Interfacial Electrochem. 1987, 107, 218.
IO 20
50
200
100 v mv/s
Figure 11. Plots of anodic peak currents vs scan rate for PMeT/PVC films (100 mC/cmz on Pt) prepared with 30 g/L PVC and various Bu4NC104concentrations: 0 , 0.04 M; 0, 0.10 M; *, 0.20 M. TABLE I V Cyclic Voltammetric Data of PMeT/PVC Films Prepared with 30 g/L PVC, 100 mC/cm2, and 2 mA/cm2 on Pt ( S = 0.07 em2) as a Function of B4NC104Concentration Used for Their Synthesis
[electrolyte],mol/L 0.04
0.100
0.200
u,
mV/s 10 20 50 100
200 10 20 50 100 200 10 20 50 100 200
En*. V/SCE 0.54 0.64 0.75 0.83 0.87 0.54 0.60 0.65 0.70 0.75 0.54 0.56 0.60 0.63 0.68
mograms of PMeT/PVC films prepared on Pt with 100 mC/cm2 deposition charge from reaction media containing 30 g/L PVC and respectively 0.1 and 0.2 M Bu,NC104. At low scan rate, the increase of the electrolyte concentration used for the synthesis of the films produces no modification of the electroactivity of the films, which is still similar to that of PMeT films prepared in the same conditions. However, cycling the films at higher scan rates shows that the dependence of E,, on scan rate decreases and becomes equivalent to that observed on the films prepared with lower PVC concentrations (Table IV). Concurrently, Zpa, which scales linearly with the square root of scan rate in the case of 0.04
Electroactivity of Poly(thiophene) Composite Films
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 839
TABLE V Chronoamperometric Data of PMeT/PVC Films Prepared with 30 g/L PVC, 100 mC/cm2, and 2 mA/cm2 on Pt (S = 0.07 cm2) as a Function of B4NCI04Concentration Used for Their Synthesis 1O ~ D ~ ~ ~ C ( O X ) , 107D1/2C(red), [electrolyte], mol/(cm-*d2) moI/(cm-2.s-1/2) mol / L
Absorbance
ov
n 0.2v
~
0.04 0.10 0.20
0.39 0.46 I .89
1.40 1.92
0.4V
7.15 0.5 0.6V
400
Figure 12. Cyclic voltammograms (10 mV/s) for 1.5 X lW3 M ferrocene in 0.1 M LiC1O4/CH3CNon PMeT/PVC films (100 mC/em2 on Pt) prepared with 30 g/L PVC and (a) 0.10 M Bu4NC104;(b) 0.20 M Bu~NCIO,.
M Bu4NC104,returns progressively to a linear dependence on scan rate, as the electrolyte concentration used for the synthesis of the films increases (Figure 11). The results of chronoamperometric experiments listed in Table V are in good agreement with the cyclic voltammetric data and show that the D'12C values increase markedly with the electrolyte concentration used for the synthesis of the films and reach values similar to those obtained with films prepared with lower PVC concentrations. Two different interpretations could account for these astonishing results. On the one hand, the redissolution in acetonitrile of the electrolyte trapped in the film during its preparation could induce the formation of micropores in the PVC membrane. Such a procedure has been described already for the preparation of PVC membranes.*O As mentioned above, another possible interpretation could involve the electrolyte trapped in the films during their preparation. In order to obtain information allowing distinguishing between these two possibilities, the electrochemical response of the composite films produced with high electrolyte concentrations has been analyzed in the presence of ferrocene. The voltammograms obtained in that case (Figure 12) are essentially identical with those recorded on the films prepared with low electrolyte concentration (Figure Sc), suggesting that the area of the PMeT electrode which is accessible to the redox couple in solution is relatively constant and that the permeability of the film has not been noticeably modified by the increase of the electrolyte concentration used for the synthesis of the film. These results suggest that the faster electrochemical responses observed on the films prepared with high electrolyte concentrations are not related to a modification of the permeability of the PVC membrane but result essentially from the higher concentration of electrolyte confined within the as-grown composite film. In summary, these results show that the electrochemical properties of PMeT/PVC films are controlled by two synthesis parameters, the PVC concentration which determines the thickness of the membrane and thus the accessibility of solution species tu the electroactive part of the film and the electrolyte concentration which controls the rate of charge transfer thanks to the trapping of the dopant species within the composite film. ( c ) Spectroelectrochemical Properties of PMeT/PVC Films. As already indicated, the optical density of the composite films increases linearly with the amount of charge used for the electropolymerization. Thus transparent films showing 85% transmission at 633 nm and conductivities of 10-2-10-1 S/cm can be (20) Blumberg, A. A. J. Chem. Edur. 1986, 63, 415.
600
A
8co nm
Figure 13. Visible absorption spectra at various potentials vs Ag of a PMeT/PVC film (40 mC/cmz on ITO) prepared with 30 g/L PVC and 0.04 M B~4NC104. produced with low deposition charges. Similary to PMeT films, the redox behavior of the composite films is accompanied with reversible changes of the absorption spectrum from blue (oxidized doped form) to red (neutral undoped form). Figure 13 shows the in situ absorption spectra at various potentials of a PMeT/PVC film prepared with 20 g/L PVC, 40 mC/cm2, and 5 mA/cm2 on ITO. These spectra are identical with those obtained on bare PMeT films prepared under the same conditions, and the electrochromic efficiency, defined as the ratio of the change of optical density to the consumed charge, was 0.16 mC/cm2, which compares favorably to the values of 0.105-0.12 obtained on viologen derivatives.21 These results indicate that the spectroelectrochemical behavior of PMeT is not affected by the presence of PVC. This latter result is of particular interest for applications in electrochromic devices since the incorporation of PMeT within the PVC matrix is expected to improve the chemical stability of the conducting polymer. Preliminary stability tests have been performed in order to confirm this point. For this purpose, a composite film prepared under the conditions of Figure 13 has been submitted to potential steps between -0.3 and +1 V. The pulse duration has been adjusted to 2 s in order to correspond to a full charge and discharge of the film. After lo3 cycles in 0.5 M LiC104/CH3CN, the optical density at 520 nm still represents 88% of its initial value and the charge exchanged during one voltammetric cycle at 10 mV/s still represents 85% of its initial value. These values are respectively 65% and 80% for a PMeT film prepared and analyzed under the same conditions. These results show that the spectroelectrochemical properties of PMeT are completely preserved when incorporated in the PVC matrix and that this process leads to an improvement of the electrochemical stability when compared to bare PMeT films.
Conclusion A simple and versatile one-step procedure for the preparation of conducting composites from electropolymerizable conducting polymers and classical insulating polymers has been described. This method based on the simultaneity of the electropolymerization and dip-coating processes presents several distinct advantages. The fact that the growth of the conducting polymer occurs in an homogeneous liquid phase allows an easy control of its distribution and concentration within the composite by means of the electrical parameters involved in the electropolymerization. This technique, which is applicable to other conducting/insulating polymer systems leads thus to a better interpenetration of the two polymer networks (21) Akahoshi, H.; Toshima, S . ; Itaya, K . J . P h p . Chem. 1981, 85, 818.
840
J. Phys. Chem. 1988,92, 840-843
than the two-step procedures, making available very homogeneous composite films. The analysis of the electrochemical behavior of these films shows that, in the same way as their electrical and optical properties, their electroactivity depends strongly on the synthesis conditions and can thus be controlled by this way. The large possibilities offered by the one-step technique could be applied to the realization of solid-state all-polymeric batteries based on composite materials containing electronically and ionically conducting polymers which would result in a solid-state interface22with enhanced contacting area. On the other hand,
the interesting electrochemical properties exhibited by the composite films make them attractive candidates for applications in the fields of electrochromic devices or modified electrodes. works devoted to these various aspects are now in progress in our laboratory. Registry No. PMeT, 84928-92-7; PVC, 9002-86-2; Bu4NC104, 1923-70-2. (22) Skotheim, T. A. Synth. Mer 1986, 14, 31
Dielectric Properties of Binary Systems. 7. Carbon Tetrachloride with Benzene, with Toluene, and with p-Xylene at 298.15 and 308.15 K Adriin H. Buep and Miximo Bar6n*+ Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, PabellBn 1 - C, Universitaria, Niiiiez, 1428-Buenos Aires. Argentina (Received: July 7 , 1987)
Excess dielectric properties for CC14 + benzene, CC14 + toluene, and CCl, + p-xylene were calculated from permittivity and refractive index measurements, over the whole range of concentrations, at 298.15 and 308.15 K. To describe the excess dielectric properties a simple model is used that is based on the additivity of electrical susceptibilities and on the formation of a complex between the components. Since the equilibrium constant for the complex formation is known, the dipole moment of these complexes in solution could be calculated.
Introduction Specific interactions that appear to exist in binary liquid systems of CCl, and aromatic compounds have been explained through the formation of the donor-acceptor-type complexes.’-* They would result from interactions between chlorine atoms, with empty 3d shells, and the 7r electron clouds. Previous work3-$ has suggested that the presence of molecules having different symmetries can associate to form a complex with a distinct polarity, even in the case of nonpolar constituents. As a result the polarity of a liquid binary system should be influenced by these interactions. The formation of complexes between CC14 and aromatic compounds has been further studied through a number of different properties. Melting point diagrams show the existence of a 1:l complex for CCl, with benzene, toluene, p-xylene, anisole, and pseudocumene.’-2 Heat of mixing in these mixtures showed a curious increasing when benzene, toluene, and pxylene are present while decreasing with mesitylene. The excess molar volume7~* of CCl, + benzene increases with temperature. Far-IR spectroscopyg indicates that an increase in the number of methyl groups on the benzene ring leads to a more stable complex. In view of the polarity changes in these systems, as a result of complex formation, we decided to study the dielectric properties of three of them (CCl, with benzene, toluene, and p-xylene), measuring permittivities and refractive indices on the whole range of concentrations and at two different temperatures. The series of three measurements and their excess functions can then be related to the mentioned charge-transfer complexes in the liquid state. To do so a formalism is developed using a simple model based on the additivity of mixture susceptibilities that does not require the use of molar volumes. These imply necessarily an approximation as that used in the model considered by Baur et a1.I0
’
Member of Carrera del Investigador, Consejo de Investigaciones Cientificas y Ttcnicas.
0022-3654/88/2092-0840$01.50/0
Experimental Section The reaction grade compounds were fractionally distilled twice and kept in dark bottles, under dry nitrogen. Purity was better than 99.9 mol % through GPC. All solutions were prepared by weighing on a Mettler balance with a precision of 5 X g. Temperature control during all measurements was within 0.01 K. Static permittivity was measured as described previously’* with an error less than 0.0003 for all concentrations. Refractive indices for sodium light were measured on an ABBE type Bausch & Lomb precision refractometer with an error less than 0.000 05. Density, permittivity, and refractive indices of the pure liquids at 298.15 and 308.15K are shown in Table I. Results and Discussion Excess values of permittivity cE and refractive index nE2 for all systems are listed in Table I1 at both temperatures and all concentrations. These excess values were calculated from 1 and 2, cE =
6s
-
6ACA
-
+B6B
ng2 = nS(D)2 - 6AnA(D)* - 6BnB(D)’ (1) 1387. (2) (3) (4) 333.
(1)
(2)
Bevan Ott, J.; Rex Goates, J.; Budge, H. J . Phys. Chem. 1962, 66, Rex Goates, J.; Sullivan, R. J.; Ott, J. B. J . Phys. Chem. 1959,63, 589. Pijpers, F. W. Thesis, University of Leiden, 1958. Ptrez, P.; Block, T. E.; Knobler, C. M . J . Chem. Eng. Data 1971, 16,
( 5 ) Barbn, M.; Buep, A. H. An. Asoc. Quim. Argent. 1982, 70, 571. (6) McGlashan, M. L.; Stubley, D.; Watts, H. J . Chem. SOC.A 1969, 673. (7) Marsh, K. N. Int. Data Ser., Select. Data Mixtures, Ser. A 1975, 2,
129. ( 8 ) Kiyohara, 0.; Benson, G. C. J . Chem. Thermodyn. 1977, 9, 691. (9) Anderson, R.; Prausnitz, J. M. J . Chem. Phys. 1963,39, 1225; 1964, 40, 3443. Rosseinsky, D. R.; Kellawi, A. J . Chem. SOC.A 1969, 1207. (10) Baur, M. E.; Horsma, D. A.; Knobler, C. M.; Pbrez, P. J . Phys. Chem. 1969, 73, 641. (11) Barbn, M. An. Asoc. Quim. Argenr. 1976, 64, 383.
0 1988 American Chemical Society
