The kinetics of electropolymerization

Synthetic Metals, 41--43 (1991) 2825-2830

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T H E KINETICS OF E L E C T R O P O L Y M E R I Z A T I O N

J.W. S C H U L T Z E , A. THYSSEN Institut fi~r Physikalische Chemic und Elektrochemie der Heinrich-Heine-Universit~lt Diisseldorf, Universit~tsstr. 1, D-4000 Diisseldorf 1 /F.R.G.

ABSTRACT During the potentiostatic polymerization of aniline, the periods of nucleation, growth of nuclei and stationary growth can be distinguished. The rate determining step is an oxidation reaction with a transfer coefficient ~ = 0.2 to 0.5 and E a = 57 kJ/mol. Evidence for branching and termination reactions is obtained using derivatives of aniline. Decomposition reactions (overoxidation) are proved in absence of the monomer.

1. I N T R O D U C T I O N Electrode reactions are usually analyzed in terms of a single or two step process of electron or ion transfer reactions. In case of fast diffusion, the Butler-Volmer equation can be applied. Special extensions have been discussed for metal electrodes with rate determining nucleation processes. In case of electropolymerization, very few investigations have been carried out using these m o d e l s / e . g . 1/. The molecular problems of polymerization such as nucleation, branching, termination (blocking of growth sites) and side reactions, however, are usually neglected. Hence, in this paper electrochemical and molecular aspects of electropolymerization discussed in recent papers / 2 - 5 / will be summarized. The formation of polyaniline (PANI) and its derivatives is used as example.

2. E L E C T R O P O L Y M E R I Z A T I O N U N D E R POTENTIOSTATIC CONDITIONS To investigate the kinetics of electropolymerization, gravimetric, electrochemical and microscopic techniques can be used. A first survey is obtained from potentiostatic transients as shown in Fig. 1. Starting with a blank gold electrode polarized in an 0.1 M solution of aniline in 0.5 M H2SO4, the overall current density i, and the mass of the polymer m can be determined simultaneously in dependence on the time t. Due to the large dynamics of i and t the current transient is represented in a double logarithmic scale while m is shown linearly in dependence on log t. 0379-6779/91/$3.50

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The following discussion is confined to aniline, the derivatives will be discussed in Sections 5 and 6. The current transient shows various periods of the reaction: the adsorption and formation of oligomers in the first seconds after the nucleation, the growth of nuclei after some h u n d r e d seconds, and finally after 2000 s the steady state of the film growth / 4 / . T h e n the mass m increases very fast and the green-black colour of the electrodes indicates the presence of thick films, Microscopic investigations show that green nuclei of polyaniline are f o r m e d which almost grow hemispherically with a rate o f 1 n m / s / 4 / . The n u m b e r of nuclei strongly depends on the prepolarization conditions. U n d e r potentiostatic conditions on smooth surfaces N ~, l0 s cm -2 was observed.

3. THE I N F L U E N C E OF P O T E N T I A L ~ A N D T E M P E R A T U R E T The polymerization reaction is m u c h slower than the fast diffusion within the solution. Therefore the reaction is transfer controlled. Due to the fast changing surface properties, however, kinetic measurements of the influence of potential ~ and T have to be carried out in very short periods of time where constant surface conditions can be maintained. Therefore PANI films were formed during a longer period o f time, and then the potential or the temperature T were c h a n g e d rapidly. The data were recorded in dependence on the overall polymerization charge q. Fig. 2a shows such measurements of i in dependence on T as example. The lines for cooling (full lines) and heating up (dashed lines) coincide. Therefore i could be evaluated in dependence on T at constant charge q and plotted in an Arrhenius diagram (Fig.2b). The slope of 57 k J/mole is almost independent of the film state. The pre-exponential factor is small due to the small coverage of growth sites. Fig. 3 shows the Tafel plot of polymerization measured by fast potentiodynamic sweeps at constant film state. Various ~/t-programs yielded an active surface (a) or an overoxidized surface (b). From the slope (a) we obtained a b-factor b = 120 to 300 m V corresponding to a transfer coefficient of about 0.2 to 0.5 for a 1 electron transfer reaction. C o m b i n i n g these results with those of section 2 we can describe the current density by a modified B u t l e r - V o l m e r - e q u a t i o n : ~. = k,.,~zFc]," c u . K ( t , A ¢ ) . A ( t . A ¢ ) " e x p [ - - ~ - - ) ' e x p

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2827

where A~ = ~ - 1 V refers arbitrarily to the state at 1 V. This equation takes into account that the polymerization takes place at the surface A (t, A~) of nuclei K (t, A~) which both increase with time t as well as with the potential ~. T h e reaction order of aniline t, is smaller than 1 due to an influence of adsorption. -3

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Influence of film thickness d on the rate of polymerization due to electronic insulation (PAB/10/) or termination ( 2 . 6 - D M A / 5 / ) .

60

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4. OLIGOMERIZATION (SIDE REACTIONS) AND POLYMERIZATION Oligomers are formed as intermediates of polymerization /6/. The overall current density i = ipoI + iol is consumed to oxidize the monomer to soluble oligomers and insoluble polymers. The oligomers may take part in the further polymerization or they can diffuse into the solution. These side reactions iol diminish the polymerization efficiency 7- There are three methods to check the side reactions. i) The comparison of the mass increase of the electrode with the overall polymerization charge q shows that only a part 7 q is used for polymerization, but another part (l - 7) q is consumed for the formation o f soluble products. In case of aniline 7 is about 0.5 to l slightly depending on the state of the polymer film. ii) Measurements at the ring disk electrode reveal the presence of soluble oligomers which can be reduced at the ring electrode, ioi can be estimated from iR~/N where N is the collection efficiency of the ring electrode. An uncertainty of such a determination, however, consists in the assumption of the same redox mechanism for oligomerization and reduction of the oligomers which could not be checked. Measurements show that about 35 % are used for oligomerization corresponding to a polymerization efficiency of about 0.65, iol is independent of the potential

/2/. iii) Special analytical techniques may be used to determine the oligomers. Various mass spectrometric techniques used by Heitbaum et al./7/ showed that benzidine and other by-products were formed which are not favourable for further polymerization. 5. LINEAR POLYMERIZATION AND BRANCHING In the elder literature linear polymerization in p-position is assumed always, and the formula of emeraldine is used to explain the polyaniline structure. In reality, however, branching reactions take place as well. This can be demonstrated by three types of experiments: i) As Fig. l shows potentiostatic transients can be described by i = k • t n after the nucleation. Since progressive nucleation yields n = l only, the experimental value of n ~ 3 >> l shows that the area of a single nucleus increases proportionally to t 2 which means that the nucleus has a surface of a hemisphere. This can be explained only by an increasing number of growth sites which requires the participation of branching reactions /4/. In case of 2,6-dimethylaniline, branching is impossible, and n becomes even negative (see Fig. I). ii) Experiments with p-toluidine which cannot polymerize in the p-position shows that the polymerization rate is much less than with aniline but not zero. Hence, the polymerization in o-position is possible, too, and was estimated to be about l0 % o f that in p-position / 3 / (see Fig. 1). iii) Recently Dunsch has shown by mass spectrometry that polyaniline contains phenazine units which can be explained only by branching reactions /8/. 6. TERMINATION REACTIONS AND THE INFLUENCE OF ELECTRONIC CONDUCTIVITY Termination reactions are well-known in polymer science, but have not been considered in electropolymerization. Chemical reasons for termination reactions can be oxidations of growth sites to phenole groups, e.g. splitting of nitrogen containing groups, or an unfavourable head-head or tail-tail coupling of the next monomer. i) Capacity measurements using the potentiostatic pulse technique show a capacity maximum at intermediate coverages, but after extended polymerization the electrode capacity decreases slowly indicating a lower surface activity of a very thick film.

2829 ii) Experiments with an ultra-microelectrode do not only show the nucleation and growth period of the hemisphere but a decrease after longer polymerization times and only new increase after extended polymerization. The observed minimum after 2000 to 5000 s can be explained by a decreased surface activity only which may be caused by termination reactions or decreasing electronic conductivity /9/. iii) The most convincing result is obtained by the polymerization of 2.6-dimethyl-aniline: the current density decreases slowly, but the film thickness reaches a maximum after about a hundred seconds. That means that an electronic current flows through the film, but the polymerization is stopped. Fig. 4 shows a plot of the log i vs. the film thickness d. The almost linear decrease suggests a rate-determining tunnelling process as it is observed in case of an insulating layer, e.g. the poly-benzimidazole-derivative PAB /10/. The application of the Gamov-formula to the slope of the line shows, however, that in case of 2.6-DMA the resulting height of the energy barrier of some meV is too low for a tunnelling process. Hence, we have to consider that a simple blocking or terminating reaction as discussed above takes place which limits the polymerization. 7. DECOMPOSITION REACTIONS It is well-known that polyaniline is overoxidized at high potentials. Then the polymer is slowly decomposed, presumably by a chemical oxidation at the C-N-bond. The film becomes electronically insulating but remains ionically conducting/I 1/. Experiments have to be carried out in absence of the monomer to exclude further polymerization. The evidence for the overoxidation is threefold: i) The current density of overoxidation measured at the disk electrode is constant for some hundred s, then it decreases rapidly. At the ring electrode reducable, soluble decomposition products can be detected simultaneously. Results are shown in Fig. 5a. Au/PANI/O S M H2SOL -35"

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ii) Gravimetric measurements using the quartz micro balance show a decrease of the polymer weight (Fig. 5b). The decomposition rate increases from IV to 1.2V strongly, a further increase to 1.4V has a small influence only. There is a film residue of 60 % (IV) decreasing to 20 % (I.4V) which seems to be stable. iii) XPS-measurements show a different composition of the overoxidized s u r f a c e / 1 2 / . 8. CONCLUSIONS The rate of polymerization strongly depends on the number of growth sites which can be changed by the choice of the ~(t)-program, the concentration of the monomer and derivatives and the substrate preparation. Due to theses changes, kinetic investigations are complicated and simple steady state experiments are meaningless. A kinetic concept taking into account nucleation, branching and termination reactions, on the other hand, may be very useful for designing new polymers with special properties. Refergnces: / 1 / M.L. Marcos, L. Rodriguez, J. Gonzales - Velasco: Electrochim. Acta 32, 1453 (1987) / 2 / A. Hochfeld, R. Kessel, J.W. Schultze, A. Thyssen: Ber. Bunsenges. Phys. Chem. 92, 1405 (1988) / 3 / A. Thyssen, A. Hochfeld, R. Kessel, A. Meyer, J.W. Schuitze: Synth. Met. 29, E357 (1989) / 4 / M.M. Lohrengel, J.W. Schuitze, A. Thyssen: in S. Roth, H.J. Mair, Carl Hanser Verlag / 5 / A. Thyssen, A. Hochfeld, J.W. Schultze: Dechema-Monographien 112, 441 (1988), and A. Thyssen, Thesis, 1988, Universifftt Diisseldorf / 6 / J. Heinze, K. Hinkelmann, M. Dietrich, J. Martensen: Dechema-Monographien 102, 209 (1985) / 7 / G. Hambitzer, J. Heitbaum: Anal. Che. 58, 1067 (1986) / 8 / L. Dunsch: J. f. prakt. Chem. 317, 409 (1975) / 9 / K. Bade, J.W. Schultze: in preparation /10/ J.W. Schultze, D. Rolle: Makromol. Chem., Macromol. Syrup. [, 335 (1987) /11/ B. Pfeiffer, A. Thyssen, J.W. Schultze: J. Electroanal. Chem. 260, 393 (1989) /12/ R. Kessel, G. Hansen, J.W. Schultze: Ber. Bunsenges. Phys. Chem. 92, 710 (1988)