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release kinetics study
Ektrochimica Ada. Vol. 37, No. 3, pp. W-522, 1992 Printed in Great Britain.
OOC46S6/92
$5.00 + 0.00
0 1991. Pergamon Press plc.
POLYPYRROLE ELECTROPOLYMERIZATION MEDIATED BY N-METHYLPHENOTHIAZINE: MECHANISM AND IMMOBILIZATION/RELEASE KINETICS STUDY M. F. MJXNDESVEGAS, E. M. GEN&, M. FOULETIERand E. VIEIL* Laboratoire d’Electrochimie Mol&mlaim, Service d’Etudes des Syst&nes Mol&tlaires, Wpartement de Recherche Fondamentale sur la Mat&e. Condens& Centre d’Etudes Nucleaires de Grenoble, 85X, 38041 Grenoble Cedex, France (Received 22 April 1991) Ahstraet-Polypyrrole films (PPy) were synthesized in the presence of an electroactive species, Nmethvlohenothiaxine MMP). Cvclic voltammetric characterixation of the monomer and the NMP both in sohtiion revealed the formation of an intermediate species, responsible for the observed catalytic effect on the film growth’proccss. A simple model for the potentiostatic electrosynthesis is proposed with two parallel faradaic processes: the oxidation of the free and of the complexed monomer. This last one is thermodynamically and kinetically favoured compared to the free monomer oxidation. A rotating ring-disc electrode was used to quantify the incorporation of the NMP in the PPy films. A synthesis mechanism is proposed, which takes into account the catalytic effect and an incorporation process which depends on the stability of the intermediate species. Key words: polypyrrole, N-methylphenothiaxine, release, ring-disckiectrode, voltammetry.
INTRODUCTION In the last few years, new interesting properties have been developed for conducting polymers, through the
incorporation of molecules possessing specific properties like electroactivity, acido-basicity, complexation, etc. In order to synthesize, chemically or electrochemically, these materials two main procedures are used: (i) by covalent link between the molecule and the monomer, in which case the polymerization takes place between the substituted monomers[l, 21, or (ii) by incorporation through ionic bonding (ie doping by electrically charged species) either during the synthesis or after it; in this last case the incorporation becomes possible due to the polymer ionic exchange needed by the redox state change[3-51. It would be interesting to know if there are other possible ways for obtaining a functionalized conducting polymer and particularly to study the inclusion of free., that is to say not previously linked to the monomer, neutral molecules because of the wide range of possibilities thus offered. We propose to study the required conditions for candidate molecules such as N-methylphenothiazine (NMP), which has the advantage of being electroactive allowing its detection by electrochemical methods. This molecule belongs to the family of the phenothiazines, which possesses some specific pharmacological and biological properties[6]. Its incorporation within a conducting polymeric matrix constitutes therefore an interesting model for studying the use of these materials whether as sensors or as drug-release devices. It has been shown[7j that it *Author to whom correspondence
should be addressed.
electropolymerization
mechanism,
immobilization,
is possible to incorporate some phenazine (a molecule with a similar structure to NMP) in polyaniline films during the electrosynthesis process. The insertion was attributed to interactions between the phenazine rings and the aniline nitrenium cation. However it can be thought that the NMP electroactivity in the potential range of the film synthesis constitutes a disadvantage for its incorporation in a film, since both the NMP and the monomer are under their radical cation forms, which normally repel each other. But the presence of electrolyte anions, which is compulsory for the formation of polymer films, is probably able to weaken these electrostatic repulsions. We have chosen to study the electrosynthesis mechanism of polypyrrole (PPy) films in the presence of NMP and to study the kinetics of the incorporation/release process. The electroactivity of NMP in the working potential range, has permitted us to employ a rotating ring-disc electrode for identifying and measuring the species exchange.
EXPERIMENTAL Products and apparatus
Pyrrole from Aldrich was distilled and kept under were performed. Nargon until experiments Methylphenothiazine from Kodak, lithium perchlorate from GFS and acetonitrile (BDH for HPLC) were used without further purification. The electrolyte used was an 0.5 M LiClO, acetontrile solution, which was always &gassed prior to use. All the experiments were carried out under argon atmosphere at room temperature. The working electrode was, in the case 513
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MENDESVIEOASet
of the cyclic voltammetric experiments, a Pt disc of 0.5 mm diameter and in the case of the ring-disc experiments, it was a rotating ring @%)-disc (Pt) from Tacussel, with the following dimensions: r, = 2.00 f 0.01 mm, r, = 2.20 _+0.01 mm and r, = 2.40 + 0.01 mm, where r, is the disc radius, r, and r, are the inner and outer ring radius respectively, which give a disc area AD = 0.12 cm-* and a ring area A, = 0.03 cm-*. The speed regulation of the electrode driving motor was provided by means of an electronic control unit Asservitex 10000 from Tacussel. The electrode collection efficiency N, calculated from the Albery and Hitchman equation[8], was 0.25 f 0.05. The N experimental value was found to be 0.18, and it was determined with the iRING/iDISC ratio, measured using a 4 x IO-’ M potassium hexacyanoferrate (II), 1.OM sodium nitrate aqueous solution. The reference electrode was Ag/Ag+ (IO-* M), 0.5 M LiC104 in acetonitrile, and the auxiliary electrode was a Pt wire. A bipotentiostat (BIPAD) from Tacussel, allowing an independent control of ring and disc electrodes potentials, was used and when necessary, it was connected with a potential wave generator EG&G, model PAR 175. For the solutions cyclic voltammetric experiments, an EG&G potentiostat, mode1 PAR 173 was connected to the same wave generator. Charge values were measured by an IGdN Integrator, from Tacussel. The recorder was a SEFRAM TGM 164 X-Y-t. The electrode cleaning procedure consisted of polishing with aqueous suspension of 3 pm and 1 pm grade alumina and careful rinsing.
al.
voltammetry in an electrolyte solution free of monomer and of NMP, and the respective redox charges were measured. The experiments with the ring-disc electrode consisted of recording the ring currents as a function of the disc potential for a constant rotation speed. For that purpose the ring electrode potential was held constant at either at a reduction (-0.1 V) or an oxidation (+0.7 V) value, that is to say before and after the NMP/NMP’+ redox potential (+0.45 V). At the same time the disc potential was swept between those two values in the anodic or cathodic directions, respectively. The ring currents were also followed as a function of time when a strong oxidation potential value was imposed on the disc (Ex = -0.1 V and E,, = +0.7V). For both experiments described above, the electrode was rotated at 1500 rpm. All the ring-disc results presented are the first curves obtained immediately after the fllm synthesis, in a solution free of pyrrole and NMP. The films were rinsed with electrolyte solution after the synthesis.
RESULTS
AND DISCUSSIONS
(a) Solution cyclic voltammetry
(i) Solutions cyclic voltammetry. Solutions were prepared with 5 x 10m3M of pyrrole or N-methylphenothiazine and the respective cyclic voltammograms were recorded at 50mV s-‘. To each
A first approach to the study of the NMP influence on the electrochemical growth process of PPy films consisted of cyclic voltammetry experiments of NMP and Py in solution. As it is well known, NMP is an electroactive species whose electrochemical activity in different media has been intensively studied in the past[6]. This species exhibits in unbuffered acetonitrile, two redox couples due to the oxidation of the NMP to its radical cation and from this last one, to the dication form:
starting solution, successive amounts of NMP or Py were respectively added and cyclic voltammograms were recorded after each addition. The two species solution concentration ratios were for both cases 0.25, 0.5, 1.0, 2.0 and 0.4. (ii) Ring-disc electrode experiments. The films were synthesized on the electrode disc surface (non-rotative mode) with a constant potential value (+0.8 V us Ag/Ag+) from solutions containing 4 x 10d3 M of pyrrole(Py)or4x 10m3Mand2x lo-‘MofPyand N-methylphenothiazine (NMP), respectively. The polymerization charges were 0.158 C cm-* for the polypyrrole (PPy) film, 0.163 Ccm-* and 0.323 C cm-* for the PPy/NMP films. Considering that these charge values correspond to 100% polymerization yield, they correspond to films thickness of 0.44 pm for the PPy and 0.45 and 0.90pm for the PPy/ NMP[9]. All the films were characterized by cyclic
The potential peak value of the first redox couple is f0.45 V (us Ag/Ag+) and this couple behaves reversibly in this media. On the contrary, the second redox couple is an irreversible one and its oxidation potential peak value is + 1.07 V. The expected electrochemical behaviour was observed for a NMP solution (Fig. la). Figure 1 corresponds to an initial NMP solution to which Py was successively added. It can be seen that the NMP first redox couple (NMP/NMP’+) is not disturbed by the increasing amounts of Py in solution. In fact, the redox couple potential and current peak values remain the same after the additions of Py. The appearance of a second oxidation peak is noticed when Py is added to the initial NMP solution. Its potential peak value is at +0.78 V and the peak current value increases linearly with the Py increasing concentration. This peak will be discussed later in
Procedures
Polypyrrole electroPolymerization
515
that the NMP/NMP’+ couple is not a&&d by the monomer. It can also be seen that the second oxidation peak, in Fig. 2b presents a negligible reduction step. On the oxidation scan, the third redox couple observed in Fig. 2b, that is the NMP’+/NMP+ + one, does not seem to be very much a&c&d by the presence of Py. On the contrary, this couple reduction process appears to be quite disturbed, since its reduction current decreases significantly compared to the one of NMP alone. We can therefore conclude that the NMP dioxidized form, NMP+ +, is involved in a reaction with the Py, from which probably results the formation of a complex, evidenced by the second oxidation peak seen in Figs 1 and 2. The electrochemical behaviour of the pyrrole involves the radical cation of that species, and the couple Py/py’+ presents a highly irreversible oxidation process:
Fig. 1. Cyclic voltammograms of an NMP initial solution to which Py was added (0.5 M LiClO, acentonitrile solution; u = 50 mV s-l): (a) [NMP] = 5 x lo-) M; [NMP]/ m] = (b) 5.00; (c) 2.q (d) 1.00,(e) 0.50; (f) 0.25. -: anodic scan; -: cathodic scan. more detail. The third anodic peak which’kan be seen in Fig. 1 at + 1.07 V (it is in fact a double peak), is due to the NMP second redox couple (NMP’+/ NMP++). Upon addition of Py, this peak current value remains constant whilst [4r] Q [NMP], and after increases, lineally with the increase of Py concentration as soon as [py] 2 WMP]. The differences in the NMP electrochemical behaviour due to the presence of Py can be more klearly seen in Fig. 2. As already mentioned, it is observed
The standard potential value of this redox couple was determined by Andrieux et aL[lO] by using very fast cyclic voltammetry: E” = + 1.3 V 08 see. An extremely irreversible oxidation process was observed on the Py cyclic voltammogram, which was expected, due to the slow scan rate value used. The Py oxidation potential peak value is + 0.90 V us Ag/Ag+, (Fig. 3a). The cyclic voltammograms of a Py solution to which successive amounts of NMP were added are shown in Fig. 3. As expected, the NMP reversible redox couple, NMP/NMP’+, presents the same linear variation of the peak current with the NMP solution
P I
Fig. 2. Cyclic voltammograms of solutions containing (0.5 M LiClO, acctonitrile solution; v = SOmV s-l): (a) [NMP] = 2.0 x lo-‘M (b) [NMP]m] ~4.0 -: anodic scan; --: cathodic scan.
Fig: 3. Cyclic voltammograms of a Py initial solution to which NMP was added (0.5 M LiClO, a&or&rile solution: u = 50 mv s-l): (a) m] = 5 x 1O-3M; [NMP]/m]: (b) 0.20; (c) 0.40, (d) 1.00; (e) 2.00; (f) 4.00 -: anodic scan; ---: cathodic scan.
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concentration as in the absence of Py. The Py redox behaviour is however very much affected by the presence of NMP. In fact, its oxidation peak potential value, initially about Ep = 0.9 V, decreases towards more negative values when the NMP solution concentration increases. The Py oxidation peak current value decreases after the first NMP addition, increasing with the next ones, (however less quickly than the NMP/NMP’+ peak current), reaching finally a stable value for the NMP highest concentrations. These results put into evidence the formation of a species between the monomer (Py) and the NMP dioxidized form (NMP+ +), that can be expressed by the mechanism NMP -eNMP’+ -e-
it NMP’+, E” = O.&V us AgJAg+
(I)
* NMP++, Ep = l.OOV us Ag/Ag+(II) NMP+ + + Py $ (PyNMP)+ +.
(III)
The symbol chosen for designating the species (PyNMP)+ + is a formal one which simply indicates the association between the two species without specifying the nature of their interaction and the localization of the particle charges. Several possibilities can be envisaged. For example a nucleophilic attack of the lone electron pair on the Py nitrogen atom on the electron deficient NMP++ nitrogen or sulphur atoms, with the establishment of a covalent bond could be one possibility. Another one is (Py’+NMP‘+), in which is assumed an exchange of one electron from the Py (donor) to the NMP++ (acceptor) giving rise to a charge transfer complex. In fact, the lack of experimental data other than electrochemical, does not allow us to formulate a precise structure for this entity that we shall continue to represent as (PyNMP)+ +. Reactions (II) and (III) constitute an EC mechanism, which controls the second anodic peak in Fig. 3, whose redox potential is inferior to the one of the electrochemical reaction (II) alone and that depends on the concentration values of the present reagents. The chemical reaction described in (III) is observed to be rather quantitative. In fact, as it can be seen from Fig. 3, there is a complete consumption of the added NMP, to form more (PyNMP)++, whilst [NMP] < [Py]. After that point, an increase of the NMP’+/NMP++ peak current between 0.9V and 1.2 V can be observed, since all Py has reacted with the NMP+ +. The same effect is observed in the case of Fig. 1, when the Py solution concentration varies. For both cases it is found that when [NMP] = [Py], the species formation is quantitative, meaning that its stoichiometry is 1: 1. In Fig. 4 the peak currents of the three anodic peaks observed in Fig. 1, are plotted against the Py solution concentration. Curves a and c show two already discussed effects: that the NMP/NMP’+ redox couple is not affected by the presence of Py (curve a) and that the amount of NMP’+/NMP++ which is not involved in the complexation reaction (III) increases only once the stoichiometric point is reached, 1: 1 (curve c). In curve b the linear dependence of the second anodic peak observed in Fig. 1 with the Py concentration is shown. As can be seen, this peak current does not reach a constant value for [py] 3 [NMP], as it would be expected by the limited
04
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:
2
:
4
:
6
:
8
:
10
:
12
:
14 pY]
:
:
16
18
I
20
xdw
Fig. 4. Variation of the oxidation peaks currents with the Py solution concentration, measured from Fig. 1: (a) it (first peak: NMP/NMP’+); (b) $x (second peak: complex); (c) ip (third peak: NMP’+/NMP++).
amount of NMP in solution. On the contrary, it increases continuously with the Py concentration increase. This observation leads to the conclusion that the species (PyNMP)’ + is electroactive and undergoes an oxidation reaction that produces NMP+ +, which explains the continuous increase of the species current peak, upon addition of Py: (PyNMP)++ -ee- # Py” +NMP++.
(IV)
The second oxidation peak observed in Figs 1, 2 and 3 strongly supports the existence of that species as a relatively stable species, eliminating the possibility of this species dissociation into Py” and NMP’+ followed by the NMP’+ oxidation to NMP+ +. In fact such a mechanism could not explain the existence of the extra oxidation peak in the NMP solution cyclic voltammograms which is dependent on the Py concentration. Reactions (III) and (IV) show that in fact the Py oxidation is catalysed by NMP+ +. Such a mechanism reinforces the hypothesis that the species (PyNMP)+ + results from an electron exchange between the Py and the NMP++, giving rise to a charge transfer complex. In fact, if a covalent bond was established between them, reaction (IV) could not take place. It is therefore expected that the presence of NMP will affect the electrosynthesis of PPy films as well as the film redox behaviour. The comparative study of the electrochemical preparation and characterization of PPy films, in the presence and in the absence on NMP was therefore performed. (b) Potentiostatic
filmr electrosynthesis
1. Synthesis under duration control. Time controlled syntheses of PPy films were performed with solutions containing either the monomer alone, or the monomer and the NMP. In the last case, an [NMP]/
Polypyrrole electropolymerization 5.59
,
1
i
517
I 422 mn crne2
._ 2.58
1.29
b
r
0
30
60
90
li0
E /V
vr A,$*.$
W2Y
t/s
Fig. 5. Chronoamperometric curves of time controlled 6lms synthesis: 2sY= 110 s; Esv = +0.8 V (us Ag/Ag+ 1O-2 M); solutions composition: 0.5 M LiClO, , acetonitrile; m] = 4.0 x 1O-3 M; (a) [NMP]/[Py] = 0.5; charge passed after 110 s: QsY= 0.323 C cme2, (b) m] = 4.0 x lo-’ M; charge passed after 110 s: Qsv = 0.158 C cmv2 (c) chronoamperometric curve of the NMP direct oxidation on the electrode [NMP] = surface, under the synthesis conditions; 2.0 x lo-‘M; charge passed after 110s: Qox =0.032 C cme2. [Py] solution ratio of 0.5 was chosen because it led to the formation of good films within reasonable synthesis currents values. The current-time transients during the films growth are shown in Fig. 5a and b. The chronoamperometric curve of a simple NMP solution was also recorded, with the same synthesis experimental conditions (Fig. SC). The charge values involved in all these processes are 0.158 C cm-*, for the (PPy) film without NMP, 0.323 C cm-* for the (PPyNMP) film and 0.032 Ccm-*, for the NMP direct oxidation on the electrode surface (for the same potential of 0.8 V). These results show that the charge value for the lilm synthesis in the presence of NMP is about twice the one obtained for the PPy alone. Also, it was found that the charge involved in the oxidation process of the NMP alone occurring at the synthesis patential value (0.8 V) due to ,reaction (I), represents only 20% of the charge excess used for the synthesis of (PPy/NMP) films. It means that 80% of this synthesis charge increase is due either to the formation of film or to the parallel faradaic processes. Each film was then characterized by cyclic voltammetry in order to compare their electroactivity. Films electrochemical characterization by cyclic voltammetry. The PPy films prepared as described above were cycled between -0.7V and +0.7V (us Ag/Ag+) in an electrolyte solution free from monomer and from NMP. The cyclic voltammograms obtained are shown in Fig. 6. The redox charge of the (PPy/NMP) film, 75mCcm-*, is considerably higher than the one of the PPy lilm without NMP, 33 mCcm-*. The fjlm electroactivity increase due to the presence of NMP
Fig. 6. Cyclic voltamperommetric curves of two PPy films synthesized under the same synthesis time (110 s); the films were cycled in a 0.5 M LiClO,, a&on&rile solution at a sweep rate of 50 mV s-r: (a) a tilm synthesized in the absence of NMP (QSY= 0.158 Ccm-*) Qrtsnex = 33 mC cmT2, (b) film synthesized in the presence of NMP (Qsv = 0.323 C cmm2), Qmx = 75 mC cm-*.
is about the same order of magnitude of the observed synthesis charge increase, which can be expressed by the equality of the two following ratios,
which equal 2.0, approximately. Therefore this difference of synthesis charge is totally due to the polymerization process, meaning that there is an increase of the PPy film synthesis efficiency due to the presence of NMP. These results agree relatively well with the ones obtained by cyclic voltammetry on the species in solution. In fact the existence during the tilms synthesis of (PyNMP)++ species, more easily oxidizable than the monomer [as proposed in equilibrium (III)], can explain most of the obtained results. Considering equilibria (I) and (II) the following reaction scheme is proposed for the initial steps of the synthesis:
pY&-/Y
-C
-W
+
kl
-11 @mm++ c++
w
___)
BUIX
-ODE suRFAc!E
w++=
++
M. F. MENDESVEGAS et al.
518
where fib is the mass transfer rate constant for the Py transport process from the bulk solution to the electrode surface; this rate constant can be defined for any kind of mass transport process, convection, diffusion, etc.[ll]; k, and k2 are the charge transfer rate constants for the described irreversible oxidation processes and K is the equilibrium constant for the formation of the species (PyNMP)++. This scheme is obviously a very simplified one, since the reaction and diffusion layers are not considered. In fact the formation of the species will occur at some distance from the electrode, and the species concentration profile should be taken into account for an accurate treatment of this mechanism. The discussion presented below is therefore only a preliminary kinetics approach. During PPy films synthesis in the presence of NMP, the monomer exists under two different forms, in front of the electrode surface: as free Py and as (PyNMP)++, which we will designate as the Py complexed form for the sake of simplicity. However, the quantity of monomer available to undergo the polymerization reaction remains the same. In fact it does not depend on the species chemical nature since both of them can be oxidized to Py’+, leading to polymer film. On the contrary, the kinetics of the charge transfer process is very much affected by the presence ofthe complex (PyNMP)+ + since two different faradaic processes are in this case involved, possessing probably different rate constant values. The oxidation process of the complex (PyNMP)+ + has been seen to be thermodynamically favoured compared to the free monomer since its oxidation potential is lower than this last one. Kinetically the global synthesis process is also seen to be favoured, but the relative values of each rate constant k, and k, are not known. These values can be evaluated considering the total flux, J, of the electroactive species through the electrochemical cell, under each experimental conditions, that is to say, under a constant potential value and in the absence or in the presence of NMP. In the first case, that is in the absence of NMP, the only electroactive species in solution is the Py and the expression for the flux conservation through the electrochemical cell will be
where Co and Cm stands generally respectively for the species concentration at the electrode surface and in the bulk solution. This gives for the flux expression as a function of the Py bulk concentration J, =7;,C& where It, = &/[l + km/k,], is the apparent rate constant for the global process. This means that the synthesis current flowing through the cell at each instant is directly proportional to the monomer bulk concentration, through a proportionality constant, E, , which is function of the rate constants of the steps that can kinetically control the global process. During the synthesis in the presence of NMP, two electroactive species are involved: the free and the complexed monomer. However only the free Py undergo the mass transport step, since the complex (PyNMP)+ + is formed in the front of the electrode
surface where the NMP+ + is generated (according to our simplified model). In this case the conservation flux expression will be J,=tFtm(C$-C&)=k,C&,+k,C~mti. For the chemical step there is a formal equilibrium constant, K’, that must be taken into account in the global process. This constant can be written as K’=C~~~,ex/C~,initial=KCNMP++/l
+KGs,r++,
where
The expression for the flux as a function of the monomer bulk concentration is in this case J2 =&C$ where I;,=ti,/[l+fipy/k,(l--K’)+k*K’]. Two limiting process. (1) fi, -+ k,
cases can be considered and
for the
fiti Q k,(l - K’) + k, K’,
that is to say that the process is limited by the mass transport so the fluxes ratio, J2/JI, should be equal to the unity. From the synthesis currents values (at t = th,,) that ratio is found to be 2.0, excluding this way the possibility of the synthesis process be controlled by the mass transport step. (2) fib % kl
and 5~~ % k, (1 - K’) + k2 K’,
that is to say that the process is limited by the charge transfer processes. The flux expressions are then simplified: J, =k,C&
and J2 = [k, (1 - K’) + k2 K’]C$.
As has been said before, the flux ratio J2/J, is equal to 2.0, and we obtain finally for the charge transfer constants ratio k2/k, the relationship kzlk, = 2 + (l/KC&.++). This result it is the most relevant one since it means that, independently of the kinetic importance of the complexation chemical step, the kinetics of the charge transfer process of the complex oxidation will be always favoured compared to the free monomer one. The occurrence of a second parallel faradaic process with a rate constant (k2) at least twice superior than the one of the free monomer direct oxidation (k,), explains why the synthesis charge value of a PPy film is increased to the double, in the presence of the NMP. Until now we have been able to understand some of the most important kinetics aspects of the PPy films synthesis process in the presence of the NMP. However, in what concerns the eventual NMP immobilization in the film, the preceding study does not bring some important informations and a complementary study must be carried on. 2. Synthesis under charge control. Synthesis of PPy films in the absence and in the presence of NMP (using in this last case the same ratio of species solution concentration 0.5) were performed until the same charge value was reached for both cases. The current-time transients obtained during the synthesis can be seen in Fig. 7. As expected from our preceding
Polypyrrole electropolymerixation
L
215
b
a43 QOO 0
20
40
60
1
80 tls
Fig. 7. Chronoamperometric curves of charge controlled films synthesis: limiting charge: Qsy = 0.163 C cm-*; Ew = +0.8 V (USAg/Ag+IO-* M); solutions composition: 0.5 M LiClO, in a&o&rile; (a) m] = 4.0 x lo-’ M; (b) [NMP]/m] = 0.5.
conclusions, the film prepared in the presence of NMP reaches the &sired charge value before the other one: at each instant of the synthesis, higher amounts of fihn are produced due to the quicker parallel oxidation of the complex (PyNMP)+ +, as explained in the above described reaction scheme. These films, possessing the same synthesis charge, allow a correct evaluation of the NMP influence on the PPy films redox behaviour, again by comparison of their electroactivity. Films electrochemical characterization by cyclic voltammetry. The Glms’ cyclic voltammograms are shown in Fig. 8. The PPy film synthesized in the presence of NMP, i
I
0.22mAcfW2
519
Figure 8b, shows a redox couple peak potential displacement towards less positive values (from -0.2 to -0.3 V us Ag/Ag+). The presence of a second mdox couple can also be noticed, in this lilm, with a slight peak at about 0.45 V. The 6lms’ electroactivity, measured by the charge involved in the redox process, do not differ very much: 36 and 40 mC cme2, for the films prepared in the absence and in the presence of NMP, respectively. The different redox behaviour, particularly the E,(PPy) displacement, could be attributed to morphological changes arriving from differences in the film growth whenever the NMP is present. In fact, other authors[lO] have found significant PPy films morphological changes due to the influence of dopants with different chemical nature. Such a possibility cannot be excluded in our case, however it does not explain the complete rcdox behaviour of the films, namely the presence (even discrete) of the second rcdox couple with Ep = +0.45V. According to our previous results, it seems rcasonabIe to think that the observed reactivity of the NMP in the presence of the monomer, already discussed, should be found again with the polymer. It is possible that during the film growth, some PPy is formed under a complexed form with the NMP, although the presence of some free NMP molecules in the film is not excluded. On this basis, the film electroactivity during the cyclic voltammetry can be described by two parallel processes. The first one
+(na + m)ClO; S
[H(p6+, SClO,-),,
@IMP’ +, ClO; )J + (na + m)ewhere P stands for the unit
T
\/ zr
H
H
is seen between -0.7 V and less than +0.45 V. This process is the oxidation of the polypyrrole chains, some of which being in their complexed form (with an Ep at about -0.3 V). This process occurs simultaneously with the incorporation of the negatively charged perchlorate ions in order to maintain the material electroneutrality. The second one is + 0.45 v
r(NMP) + zClOb #(NMP’+ClO,-),
Fig. 8. Cyclic voltamperommetric curves of two PPy films sy&.hes~ under the-same synthesis charge (0.163 C &nS2); the i&s were cycled in a 0.5 M LiClO.. scetoninile solution at a swe8p rat;: of SOmVs-i: (a) tiiL synthm@ in the
=36mCcm-2(bHllmsymh&ul absents of NMP, Qin the presence of NMP, Q- 40 mC aS2.
+ ze-. (VI)
This second redox process is seen after + 0.45 V up to +0.7 V, and corresponds to the usual electrochemical response of the free NMP in the film. The total amount of NMP in the 6lm cannot be evaluated from its electroactive response followed by cyclic voltammetry. However an estimate of the amount of free NMP in the tilm can be done by considering that this amount is expressed by the slight oxidation peak fotmd at about +0.45 V in the cyclic voltammogram. For a very thin lilm, the peak current is related to the
M. F. MENDESVIEOAS et al.
520
or as a function of the time. This last experiment allowed us to understand better the synthesis process of such a film, and consequently to propose a mechanism for it.
-2.4
-0.i
+0:7
+0.45
0
gv
vs Ag/Ag+10-2M
Fig. 9. Ring currents us disc potential curves of a PPy film synthesized on the disc electrode surface, in the presence of NMP: QsY= 0.163 C ctn2; f_NMP]/m] = 0.5; 0.5 M LiClO,, acetonitrile; disc sweep rate = 1OmV s-r; electrode rotation speed = 1500 rpm; (a) ring limiting reduction current, obtained at i&,o = -0.1 V and En,,, between -0.1 V to f0.7 V, (b) ring limiting oxidation current, obtained at EalNo = $0.7 V and Eo,sc between f0.7V to -0.1 v.
concentration pression
of the oxidized species by the exiP=n2F2vVC&/4RT,
where v is the scan rate, V is the film volume and Czx is the species concentration in the film. The concentration of free NMP in the PPy film is then estimated to be 0.4% w/w, assuming a PPy density value of 1.51 g cme3[12] and a film volume estimated by V = (2.8 x 10-4Qsu)_4[13]. In order to obtain more quantitative data about the NMP total films contents and about the NMP electrochemical influence on the synthesis process, we have performed ring-disc electrode studies with the polymer film deposited on the disc. The ring current was followed either as a function of the disc potential
(c) Ring-disc electrode studies 1. Ring currents vs disc potential. The ring currents obtained as the disc (film) potential was scanned either in the anodic or in the cathodic direction, are shown in Fig. 9. These currents constitute for the studied system a quantitative measure of the amount of NMP (under an oxidized or a neutral form) leaving the PPy film from the disc, since for a rotating ring-disc electrode their currents are related by the collection efficiency coefficient (N): N = iRING/iDIx. For films synthesized with the same charge, it was observed that the quantity of NMP that left the film under oxidized form was about twice the one under an reduced form (Fig. 9a and b). The half-wave potential value for this process, E,,, = +0.45 V, is a strong indication in favour of the involvement of the NMP first redox couple (NMP/NMP’+). During the oxidation scan some NMP radical cations can be formed and repelled from the film due to a positive charge excess; during the cathodic scan it is possible that some NMP under the reduced form leaves the film simply because of an activity gradient at the interface polymer/solution. These results are not however sufficient to conclude under which forms the NMP exists in the film, or even on possible film morphological changes as previously discussed. 2. Ring current vs time A film synthesized with the same synthesis charge as the ones above studied, was forced to undergo exhaustive oxidation by application of a constant potential. The ring limiting current was then monitored versus time, until it reached a constant value equivalent to the residual current. This curve is shown in Fig. 10 for PPy film synthesized in the presence of NMP with a synthesis charge value of 0.163 C cm-2. The IR,L at the initial instant of the process was 3.2 PA cmm2 and the charge involved in the whole process was found to be 0.342mCcn1-~. The number of moles of NMP that leaves the film (in an oxidized form NMP’+) during that time can be considered equal to the NMP film contents (since the oxidation process was quantitative), and it can be. calculated by n = [l/NFJQ&$j’o. t/s
00
9
z
50
100
150 300
-” =I
3?0
Fig. 10. Ring limiting reduction current us time curve of a PPy film synthesized in the presence of NMP: Qsu = 0.163 C cn-‘; FMP]/[Py] = 0.5; 0.5 M LiClO,, acetonitrile; electrode rotation speed = 1500 rpm; slNo = -0.1 V and Emsc = +0.7 V; QEo = 0.342 mC cm-*.
Polypyrrole electropolymerization The concentration of NMP in the PPy 6lm, estimated by considering a PPy density value of 1Sl g cm-‘[12], is 2% w/w. Zagorska[l4], has found a concentration value of 14% w/w, in the case of PPy films doped with ferrocyanide anions. Our value has to be discussed on the basis that the NMP cannot play the role of a dopant as the ferrocyanide anions can, since the immobilization process of the NMP in PPy films is not electrostatically favoured. A polymerization mechanism can be proposed. Radicals formation H-P-H
-B H-P’+-H
+ e-
(1)
H-P-H + NMP+ + # (H-P-H NMP)+ + (H-P-H NMP)++ * (H-P’+-H
(2) NMP++) + e- (3)
(H-P’+-H NMP+ +) P H-p’+-H
+ NMP+ + (4)
Polymerization Initiation: (H-P-H NMP)+ + + H-P’+-H +2(6 + l)A- --) [H(p6+, 6A-),H, (NMP++, 2A-)] + 2H+ + (1 + 26)e-
(5)
Propagation: [H(p6+, SA-),,H, (NMP++, 2A-),,,I + (1 - y)H-P’+-H +(6 +2y)A-
+ y(H-P’+-H
NMP+ +)
*[H(P6+,6A-),+,H,(NMP++,
2A-),+,I + 2H+ + (6 + l)eNMP+ + Release: [H(p6+, SA-),H,(NMP++, * [H(pd+, 6A-),H]
(6)
2A-),I
+ m NMP++ + 2m A-
(7)
Steps (1) to (4) are the initial ones, where the species needed to start the polymerization reaction, are formed. Steps (2) and (4) are chemical equilibria, while steps (1) and (3) are electrochemical reactions, and since they are related with each other, we can describe them by using the classic square scheme. HPH
e-
I
-NMP++
(HPH NMP)++ e-
HP’+H (HP’+H NMP++) -e-
w+
++ I I
Py
*
CNMP
PYm++
KPY%-l++-
521
The species (HP’+H NMP++) is unstable, decomposing into HP’+H and NMP++, in agreement with the already proposed catalytic effect. In step (5) the initiation of the polymerization process is described as due to the reaction between the relatively stable complex (HPH NMP)++ and the free monomer radical cation (HP’ +H). The resulting species is a pyrrole dimer complexed with the NMP++, in association with the necessary doping anions in order to maintain the species electroneutrality. The polymer growth process can be thought as the reaction between the already formed oligomers, complexed with NMP++, and the monomer radical cation that exists under two different forms: free HP’+H, and complexed (HPH NMP)+ +, as described in step (6). Of course this occurs in parallel with the PPy growth process through the free monomer oxidation. This gives rise to complexed polymeric chains, of variable length and so possessing different stability constants values. Depending on those constants relative values (at the synthesis potential value) and also on the NMP solution concentration, the complexed chains in the polymer undergo a dissociation equilibrium, as shown in step (7), that it is probably strongly displaced towards the right, releasing the NMP ++ from the chains to the solution. According to step (7) the NMP++ is regenerated, maintaining this way the catalytic efficiency during the synthesis process. Therefore it was not expected to find a significant quantity of NMP in the PPy films. The value found, 2%, is in good agreement with this statement and the fact that there is nevertheless some quantity of NMP in the films must be related with the stability of the complex Py chains in the polymer. A PPy film cycled in an electrolyte solution after having been forced to discharge quantitatively its NMP contents, showed the redox behaviour that is usually found for PPy films doped with perchlorate anions (although presenting some slights signs of degradation due to the strong oxidation conditions). This is in good agreement with the proposed mechanism and revealed that eventual morphological changes in the PPy films due to the presence of NMP, as suggested earlier are certainly not irreversible. The above described synthesis mechanism can be schematically represented as (1-Y)
-
-c-
c
me+&+)
Y
r@+hNMPl++-... -ropkm++-
tcpysx*m++
M. F. MENDEsVre~~s et al.
522
where the anions from the electrolyte have been deliberately omitted in order to simplify the scheme. This presentation outlines the catalytic effect of the NMP++ during the synthesis which proceeds via the intermediate (PyNMP)++ and shows that the incorporation of NMP during the film growth depends strongly on the stability of the intermediate oxidized species.
CONCLUSIONS This study has shown that the incorporation within a polypyrrole matrix of neutral molecules such as NMP was possible during the electrosynthesis but in small quantities. The nature of the immobilization mechanism has been identified as a complex formation between the monomer and an oxidized form of the NMP. The electron donor/acceptor properties of each species explains the possibility of such an association which plays the role of a reaction intermediate, together with the influence of the anions, it also explains why the immobilization may occur even in the presence of electrostatic repulsions. The study of the releasing process has shown that it was potential dependant and irreveresible. This can be an advantage compared to ionic dopants which can be unnecessarily readsorbed into the polymeric film upon reversal of the electrode potential. The PPy/NMP system allows an absence of potential control once the release is achieved. Further investigations on other members of the phenothiazine family possessing ionic charges for
studying the influence of the electrostatic interactions are in progress. Acknowledgement-Dr G. Bidan is gratefully acknowledged for fruitful discussions on this paper.
REFERENCES 1. G. Bidan and B. Divisia-Blohom, J. them. Sot., Chem. Commun. 123 (1988). 2. A. Deronxier, M. Essakalli and J.-C. Moutet, J. them. Sot., Chcm. Commun 773 (1987). 3. T. Shimidxu, A. Ohtani and K. Honda, Bull. Chcm. Sot. Jpn 61, 2885 (1988). 4. B. Zinger, Synth. Met. 30, 209 (1989). 5. A. Ha&mett, S. Higgins, P. R. Fisk and W. J. Albery, J. electroanal. Chem. 270. 479 (1989). 6. G. Dryhurst, K. M. Kadish‘, F. ’ Seheller and R. Renneberg, in Biological Electrochemistry, Vol. 1. Aeademic Press, New York (1982). 7. E. M. Genies, M. Lapkowski and J. F. Penneau, J. electroanal. Chem. 249, 91 (1988). 8. J. Albery and M. L. Hitchman, in Ring-Disc Electrodes. Clarendon Press, Oxford (1971). 9. R. A. Bull, F. R. Fan and A. J. Bard, J. electrochem. Sot. 129, 1009 (1982). 10. P. Andrieux, P. Audebert, P. Hapiot and J.-M. Saveant, J. Am. them. sot. 112, 2439 (1990). 11. E. Vieil, J. electroanal. Chem: 279; 61 (1991). 12. T. A. Skotheim (Ed.). Handbook of Conductinn Polvmers. Marcel Dekker,.New York (i986). ’ 13. E. M. Genies and J.-M. Pemaut, Synth. Met. 10, 117 (1984/Q 14. M. Zagorska, in Electronic Properties of Conjugated Polymers, (Edited by H. Kuzmany, M. Mehring and S. Roth), Springer, Heidelberg (1979).
OOC46S6/92
$5.00 + 0.00
0 1991. Pergamon Press plc.
POLYPYRROLE ELECTROPOLYMERIZATION MEDIATED BY N-METHYLPHENOTHIAZINE: MECHANISM AND IMMOBILIZATION/RELEASE KINETICS STUDY M. F. MJXNDESVEGAS, E. M. GEN&, M. FOULETIERand E. VIEIL* Laboratoire d’Electrochimie Mol&mlaim, Service d’Etudes des Syst&nes Mol&tlaires, Wpartement de Recherche Fondamentale sur la Mat&e. Condens& Centre d’Etudes Nucleaires de Grenoble, 85X, 38041 Grenoble Cedex, France (Received 22 April 1991) Ahstraet-Polypyrrole films (PPy) were synthesized in the presence of an electroactive species, Nmethvlohenothiaxine MMP). Cvclic voltammetric characterixation of the monomer and the NMP both in sohtiion revealed the formation of an intermediate species, responsible for the observed catalytic effect on the film growth’proccss. A simple model for the potentiostatic electrosynthesis is proposed with two parallel faradaic processes: the oxidation of the free and of the complexed monomer. This last one is thermodynamically and kinetically favoured compared to the free monomer oxidation. A rotating ring-disc electrode was used to quantify the incorporation of the NMP in the PPy films. A synthesis mechanism is proposed, which takes into account the catalytic effect and an incorporation process which depends on the stability of the intermediate species. Key words: polypyrrole, N-methylphenothiaxine, release, ring-disckiectrode, voltammetry.
INTRODUCTION In the last few years, new interesting properties have been developed for conducting polymers, through the
incorporation of molecules possessing specific properties like electroactivity, acido-basicity, complexation, etc. In order to synthesize, chemically or electrochemically, these materials two main procedures are used: (i) by covalent link between the molecule and the monomer, in which case the polymerization takes place between the substituted monomers[l, 21, or (ii) by incorporation through ionic bonding (ie doping by electrically charged species) either during the synthesis or after it; in this last case the incorporation becomes possible due to the polymer ionic exchange needed by the redox state change[3-51. It would be interesting to know if there are other possible ways for obtaining a functionalized conducting polymer and particularly to study the inclusion of free., that is to say not previously linked to the monomer, neutral molecules because of the wide range of possibilities thus offered. We propose to study the required conditions for candidate molecules such as N-methylphenothiazine (NMP), which has the advantage of being electroactive allowing its detection by electrochemical methods. This molecule belongs to the family of the phenothiazines, which possesses some specific pharmacological and biological properties[6]. Its incorporation within a conducting polymeric matrix constitutes therefore an interesting model for studying the use of these materials whether as sensors or as drug-release devices. It has been shown[7j that it *Author to whom correspondence
should be addressed.
electropolymerization
mechanism,
immobilization,
is possible to incorporate some phenazine (a molecule with a similar structure to NMP) in polyaniline films during the electrosynthesis process. The insertion was attributed to interactions between the phenazine rings and the aniline nitrenium cation. However it can be thought that the NMP electroactivity in the potential range of the film synthesis constitutes a disadvantage for its incorporation in a film, since both the NMP and the monomer are under their radical cation forms, which normally repel each other. But the presence of electrolyte anions, which is compulsory for the formation of polymer films, is probably able to weaken these electrostatic repulsions. We have chosen to study the electrosynthesis mechanism of polypyrrole (PPy) films in the presence of NMP and to study the kinetics of the incorporation/release process. The electroactivity of NMP in the working potential range, has permitted us to employ a rotating ring-disc electrode for identifying and measuring the species exchange.
EXPERIMENTAL Products and apparatus
Pyrrole from Aldrich was distilled and kept under were performed. Nargon until experiments Methylphenothiazine from Kodak, lithium perchlorate from GFS and acetonitrile (BDH for HPLC) were used without further purification. The electrolyte used was an 0.5 M LiClO, acetontrile solution, which was always &gassed prior to use. All the experiments were carried out under argon atmosphere at room temperature. The working electrode was, in the case 513
514
M. F.
MENDESVIEOASet
of the cyclic voltammetric experiments, a Pt disc of 0.5 mm diameter and in the case of the ring-disc experiments, it was a rotating ring @%)-disc (Pt) from Tacussel, with the following dimensions: r, = 2.00 f 0.01 mm, r, = 2.20 _+0.01 mm and r, = 2.40 + 0.01 mm, where r, is the disc radius, r, and r, are the inner and outer ring radius respectively, which give a disc area AD = 0.12 cm-* and a ring area A, = 0.03 cm-*. The speed regulation of the electrode driving motor was provided by means of an electronic control unit Asservitex 10000 from Tacussel. The electrode collection efficiency N, calculated from the Albery and Hitchman equation[8], was 0.25 f 0.05. The N experimental value was found to be 0.18, and it was determined with the iRING/iDISC ratio, measured using a 4 x IO-’ M potassium hexacyanoferrate (II), 1.OM sodium nitrate aqueous solution. The reference electrode was Ag/Ag+ (IO-* M), 0.5 M LiC104 in acetonitrile, and the auxiliary electrode was a Pt wire. A bipotentiostat (BIPAD) from Tacussel, allowing an independent control of ring and disc electrodes potentials, was used and when necessary, it was connected with a potential wave generator EG&G, model PAR 175. For the solutions cyclic voltammetric experiments, an EG&G potentiostat, mode1 PAR 173 was connected to the same wave generator. Charge values were measured by an IGdN Integrator, from Tacussel. The recorder was a SEFRAM TGM 164 X-Y-t. The electrode cleaning procedure consisted of polishing with aqueous suspension of 3 pm and 1 pm grade alumina and careful rinsing.
al.
voltammetry in an electrolyte solution free of monomer and of NMP, and the respective redox charges were measured. The experiments with the ring-disc electrode consisted of recording the ring currents as a function of the disc potential for a constant rotation speed. For that purpose the ring electrode potential was held constant at either at a reduction (-0.1 V) or an oxidation (+0.7 V) value, that is to say before and after the NMP/NMP’+ redox potential (+0.45 V). At the same time the disc potential was swept between those two values in the anodic or cathodic directions, respectively. The ring currents were also followed as a function of time when a strong oxidation potential value was imposed on the disc (Ex = -0.1 V and E,, = +0.7V). For both experiments described above, the electrode was rotated at 1500 rpm. All the ring-disc results presented are the first curves obtained immediately after the fllm synthesis, in a solution free of pyrrole and NMP. The films were rinsed with electrolyte solution after the synthesis.
RESULTS
AND DISCUSSIONS
(a) Solution cyclic voltammetry
(i) Solutions cyclic voltammetry. Solutions were prepared with 5 x 10m3M of pyrrole or N-methylphenothiazine and the respective cyclic voltammograms were recorded at 50mV s-‘. To each
A first approach to the study of the NMP influence on the electrochemical growth process of PPy films consisted of cyclic voltammetry experiments of NMP and Py in solution. As it is well known, NMP is an electroactive species whose electrochemical activity in different media has been intensively studied in the past[6]. This species exhibits in unbuffered acetonitrile, two redox couples due to the oxidation of the NMP to its radical cation and from this last one, to the dication form:
starting solution, successive amounts of NMP or Py were respectively added and cyclic voltammograms were recorded after each addition. The two species solution concentration ratios were for both cases 0.25, 0.5, 1.0, 2.0 and 0.4. (ii) Ring-disc electrode experiments. The films were synthesized on the electrode disc surface (non-rotative mode) with a constant potential value (+0.8 V us Ag/Ag+) from solutions containing 4 x 10d3 M of pyrrole(Py)or4x 10m3Mand2x lo-‘MofPyand N-methylphenothiazine (NMP), respectively. The polymerization charges were 0.158 C cm-* for the polypyrrole (PPy) film, 0.163 Ccm-* and 0.323 C cm-* for the PPy/NMP films. Considering that these charge values correspond to 100% polymerization yield, they correspond to films thickness of 0.44 pm for the PPy and 0.45 and 0.90pm for the PPy/ NMP[9]. All the films were characterized by cyclic
The potential peak value of the first redox couple is f0.45 V (us Ag/Ag+) and this couple behaves reversibly in this media. On the contrary, the second redox couple is an irreversible one and its oxidation potential peak value is + 1.07 V. The expected electrochemical behaviour was observed for a NMP solution (Fig. la). Figure 1 corresponds to an initial NMP solution to which Py was successively added. It can be seen that the NMP first redox couple (NMP/NMP’+) is not disturbed by the increasing amounts of Py in solution. In fact, the redox couple potential and current peak values remain the same after the additions of Py. The appearance of a second oxidation peak is noticed when Py is added to the initial NMP solution. Its potential peak value is at +0.78 V and the peak current value increases linearly with the Py increasing concentration. This peak will be discussed later in
Procedures
Polypyrrole electroPolymerization
515
that the NMP/NMP’+ couple is not a&&d by the monomer. It can also be seen that the second oxidation peak, in Fig. 2b presents a negligible reduction step. On the oxidation scan, the third redox couple observed in Fig. 2b, that is the NMP’+/NMP+ + one, does not seem to be very much a&c&d by the presence of Py. On the contrary, this couple reduction process appears to be quite disturbed, since its reduction current decreases significantly compared to the one of NMP alone. We can therefore conclude that the NMP dioxidized form, NMP+ +, is involved in a reaction with the Py, from which probably results the formation of a complex, evidenced by the second oxidation peak seen in Figs 1 and 2. The electrochemical behaviour of the pyrrole involves the radical cation of that species, and the couple Py/py’+ presents a highly irreversible oxidation process:
Fig. 1. Cyclic voltammograms of an NMP initial solution to which Py was added (0.5 M LiClO, acentonitrile solution; u = 50 mV s-l): (a) [NMP] = 5 x lo-) M; [NMP]/ m] = (b) 5.00; (c) 2.q (d) 1.00,(e) 0.50; (f) 0.25. -: anodic scan; -: cathodic scan. more detail. The third anodic peak which’kan be seen in Fig. 1 at + 1.07 V (it is in fact a double peak), is due to the NMP second redox couple (NMP’+/ NMP++). Upon addition of Py, this peak current value remains constant whilst [4r] Q [NMP], and after increases, lineally with the increase of Py concentration as soon as [py] 2 WMP]. The differences in the NMP electrochemical behaviour due to the presence of Py can be more klearly seen in Fig. 2. As already mentioned, it is observed
The standard potential value of this redox couple was determined by Andrieux et aL[lO] by using very fast cyclic voltammetry: E” = + 1.3 V 08 see. An extremely irreversible oxidation process was observed on the Py cyclic voltammogram, which was expected, due to the slow scan rate value used. The Py oxidation potential peak value is + 0.90 V us Ag/Ag+, (Fig. 3a). The cyclic voltammograms of a Py solution to which successive amounts of NMP were added are shown in Fig. 3. As expected, the NMP reversible redox couple, NMP/NMP’+, presents the same linear variation of the peak current with the NMP solution
P I
Fig. 2. Cyclic voltammograms of solutions containing (0.5 M LiClO, acctonitrile solution; v = SOmV s-l): (a) [NMP] = 2.0 x lo-‘M (b) [NMP]m] ~4.0 -: anodic scan; --: cathodic scan.
Fig: 3. Cyclic voltammograms of a Py initial solution to which NMP was added (0.5 M LiClO, a&or&rile solution: u = 50 mv s-l): (a) m] = 5 x 1O-3M; [NMP]/m]: (b) 0.20; (c) 0.40, (d) 1.00; (e) 2.00; (f) 4.00 -: anodic scan; ---: cathodic scan.
516
M. F. Mnv~es VEGAS et al.
concentration as in the absence of Py. The Py redox behaviour is however very much affected by the presence of NMP. In fact, its oxidation peak potential value, initially about Ep = 0.9 V, decreases towards more negative values when the NMP solution concentration increases. The Py oxidation peak current value decreases after the first NMP addition, increasing with the next ones, (however less quickly than the NMP/NMP’+ peak current), reaching finally a stable value for the NMP highest concentrations. These results put into evidence the formation of a species between the monomer (Py) and the NMP dioxidized form (NMP+ +), that can be expressed by the mechanism NMP -eNMP’+ -e-
it NMP’+, E” = O.&V us AgJAg+
(I)
* NMP++, Ep = l.OOV us Ag/Ag+(II) NMP+ + + Py $ (PyNMP)+ +.
(III)
The symbol chosen for designating the species (PyNMP)+ + is a formal one which simply indicates the association between the two species without specifying the nature of their interaction and the localization of the particle charges. Several possibilities can be envisaged. For example a nucleophilic attack of the lone electron pair on the Py nitrogen atom on the electron deficient NMP++ nitrogen or sulphur atoms, with the establishment of a covalent bond could be one possibility. Another one is (Py’+NMP‘+), in which is assumed an exchange of one electron from the Py (donor) to the NMP++ (acceptor) giving rise to a charge transfer complex. In fact, the lack of experimental data other than electrochemical, does not allow us to formulate a precise structure for this entity that we shall continue to represent as (PyNMP)+ +. Reactions (II) and (III) constitute an EC mechanism, which controls the second anodic peak in Fig. 3, whose redox potential is inferior to the one of the electrochemical reaction (II) alone and that depends on the concentration values of the present reagents. The chemical reaction described in (III) is observed to be rather quantitative. In fact, as it can be seen from Fig. 3, there is a complete consumption of the added NMP, to form more (PyNMP)++, whilst [NMP] < [Py]. After that point, an increase of the NMP’+/NMP++ peak current between 0.9V and 1.2 V can be observed, since all Py has reacted with the NMP+ +. The same effect is observed in the case of Fig. 1, when the Py solution concentration varies. For both cases it is found that when [NMP] = [Py], the species formation is quantitative, meaning that its stoichiometry is 1: 1. In Fig. 4 the peak currents of the three anodic peaks observed in Fig. 1, are plotted against the Py solution concentration. Curves a and c show two already discussed effects: that the NMP/NMP’+ redox couple is not affected by the presence of Py (curve a) and that the amount of NMP’+/NMP++ which is not involved in the complexation reaction (III) increases only once the stoichiometric point is reached, 1: 1 (curve c). In curve b the linear dependence of the second anodic peak observed in Fig. 1 with the Py concentration is shown. As can be seen, this peak current does not reach a constant value for [py] 3 [NMP], as it would be expected by the limited
04
0
:
2
:
4
:
6
:
8
:
10
:
12
:
14 pY]
:
:
16
18
I
20
xdw
Fig. 4. Variation of the oxidation peaks currents with the Py solution concentration, measured from Fig. 1: (a) it (first peak: NMP/NMP’+); (b) $x (second peak: complex); (c) ip (third peak: NMP’+/NMP++).
amount of NMP in solution. On the contrary, it increases continuously with the Py concentration increase. This observation leads to the conclusion that the species (PyNMP)’ + is electroactive and undergoes an oxidation reaction that produces NMP+ +, which explains the continuous increase of the species current peak, upon addition of Py: (PyNMP)++ -ee- # Py” +NMP++.
(IV)
The second oxidation peak observed in Figs 1, 2 and 3 strongly supports the existence of that species as a relatively stable species, eliminating the possibility of this species dissociation into Py” and NMP’+ followed by the NMP’+ oxidation to NMP+ +. In fact such a mechanism could not explain the existence of the extra oxidation peak in the NMP solution cyclic voltammograms which is dependent on the Py concentration. Reactions (III) and (IV) show that in fact the Py oxidation is catalysed by NMP+ +. Such a mechanism reinforces the hypothesis that the species (PyNMP)+ + results from an electron exchange between the Py and the NMP++, giving rise to a charge transfer complex. In fact, if a covalent bond was established between them, reaction (IV) could not take place. It is therefore expected that the presence of NMP will affect the electrosynthesis of PPy films as well as the film redox behaviour. The comparative study of the electrochemical preparation and characterization of PPy films, in the presence and in the absence on NMP was therefore performed. (b) Potentiostatic
filmr electrosynthesis
1. Synthesis under duration control. Time controlled syntheses of PPy films were performed with solutions containing either the monomer alone, or the monomer and the NMP. In the last case, an [NMP]/
Polypyrrole electropolymerization 5.59
,
1
i
517
I 422 mn crne2
._ 2.58
1.29
b
r
0
30
60
90
li0
E /V
vr A,$*.$
W2Y
t/s
Fig. 5. Chronoamperometric curves of time controlled 6lms synthesis: 2sY= 110 s; Esv = +0.8 V (us Ag/Ag+ 1O-2 M); solutions composition: 0.5 M LiClO, , acetonitrile; m] = 4.0 x 1O-3 M; (a) [NMP]/[Py] = 0.5; charge passed after 110 s: QsY= 0.323 C cme2, (b) m] = 4.0 x lo-’ M; charge passed after 110 s: Qsv = 0.158 C cmv2 (c) chronoamperometric curve of the NMP direct oxidation on the electrode [NMP] = surface, under the synthesis conditions; 2.0 x lo-‘M; charge passed after 110s: Qox =0.032 C cme2. [Py] solution ratio of 0.5 was chosen because it led to the formation of good films within reasonable synthesis currents values. The current-time transients during the films growth are shown in Fig. 5a and b. The chronoamperometric curve of a simple NMP solution was also recorded, with the same synthesis experimental conditions (Fig. SC). The charge values involved in all these processes are 0.158 C cm-*, for the (PPy) film without NMP, 0.323 C cm-* for the (PPyNMP) film and 0.032 Ccm-*, for the NMP direct oxidation on the electrode surface (for the same potential of 0.8 V). These results show that the charge value for the lilm synthesis in the presence of NMP is about twice the one obtained for the PPy alone. Also, it was found that the charge involved in the oxidation process of the NMP alone occurring at the synthesis patential value (0.8 V) due to ,reaction (I), represents only 20% of the charge excess used for the synthesis of (PPy/NMP) films. It means that 80% of this synthesis charge increase is due either to the formation of film or to the parallel faradaic processes. Each film was then characterized by cyclic voltammetry in order to compare their electroactivity. Films electrochemical characterization by cyclic voltammetry. The PPy films prepared as described above were cycled between -0.7V and +0.7V (us Ag/Ag+) in an electrolyte solution free from monomer and from NMP. The cyclic voltammograms obtained are shown in Fig. 6. The redox charge of the (PPy/NMP) film, 75mCcm-*, is considerably higher than the one of the PPy lilm without NMP, 33 mCcm-*. The fjlm electroactivity increase due to the presence of NMP
Fig. 6. Cyclic voltamperommetric curves of two PPy films synthesized under the same synthesis time (110 s); the films were cycled in a 0.5 M LiClO,, a&on&rile solution at a sweep rate of 50 mV s-r: (a) a tilm synthesized in the absence of NMP (QSY= 0.158 Ccm-*) Qrtsnex = 33 mC cmT2, (b) film synthesized in the presence of NMP (Qsv = 0.323 C cmm2), Qmx = 75 mC cm-*.
is about the same order of magnitude of the observed synthesis charge increase, which can be expressed by the equality of the two following ratios,
which equal 2.0, approximately. Therefore this difference of synthesis charge is totally due to the polymerization process, meaning that there is an increase of the PPy film synthesis efficiency due to the presence of NMP. These results agree relatively well with the ones obtained by cyclic voltammetry on the species in solution. In fact the existence during the tilms synthesis of (PyNMP)++ species, more easily oxidizable than the monomer [as proposed in equilibrium (III)], can explain most of the obtained results. Considering equilibria (I) and (II) the following reaction scheme is proposed for the initial steps of the synthesis:
pY&-/Y
-C
-W
+
kl
-11 @mm++ c++
w
___)
BUIX
-ODE suRFAc!E
w++=
++
M. F. MENDESVEGAS et al.
518
where fib is the mass transfer rate constant for the Py transport process from the bulk solution to the electrode surface; this rate constant can be defined for any kind of mass transport process, convection, diffusion, etc.[ll]; k, and k2 are the charge transfer rate constants for the described irreversible oxidation processes and K is the equilibrium constant for the formation of the species (PyNMP)++. This scheme is obviously a very simplified one, since the reaction and diffusion layers are not considered. In fact the formation of the species will occur at some distance from the electrode, and the species concentration profile should be taken into account for an accurate treatment of this mechanism. The discussion presented below is therefore only a preliminary kinetics approach. During PPy films synthesis in the presence of NMP, the monomer exists under two different forms, in front of the electrode surface: as free Py and as (PyNMP)++, which we will designate as the Py complexed form for the sake of simplicity. However, the quantity of monomer available to undergo the polymerization reaction remains the same. In fact it does not depend on the species chemical nature since both of them can be oxidized to Py’+, leading to polymer film. On the contrary, the kinetics of the charge transfer process is very much affected by the presence ofthe complex (PyNMP)+ + since two different faradaic processes are in this case involved, possessing probably different rate constant values. The oxidation process of the complex (PyNMP)+ + has been seen to be thermodynamically favoured compared to the free monomer since its oxidation potential is lower than this last one. Kinetically the global synthesis process is also seen to be favoured, but the relative values of each rate constant k, and k, are not known. These values can be evaluated considering the total flux, J, of the electroactive species through the electrochemical cell, under each experimental conditions, that is to say, under a constant potential value and in the absence or in the presence of NMP. In the first case, that is in the absence of NMP, the only electroactive species in solution is the Py and the expression for the flux conservation through the electrochemical cell will be
where Co and Cm stands generally respectively for the species concentration at the electrode surface and in the bulk solution. This gives for the flux expression as a function of the Py bulk concentration J, =7;,C& where It, = &/[l + km/k,], is the apparent rate constant for the global process. This means that the synthesis current flowing through the cell at each instant is directly proportional to the monomer bulk concentration, through a proportionality constant, E, , which is function of the rate constants of the steps that can kinetically control the global process. During the synthesis in the presence of NMP, two electroactive species are involved: the free and the complexed monomer. However only the free Py undergo the mass transport step, since the complex (PyNMP)+ + is formed in the front of the electrode
surface where the NMP+ + is generated (according to our simplified model). In this case the conservation flux expression will be J,=tFtm(C$-C&)=k,C&,+k,C~mti. For the chemical step there is a formal equilibrium constant, K’, that must be taken into account in the global process. This constant can be written as K’=C~~~,ex/C~,initial=KCNMP++/l
+KGs,r++,
where
The expression for the flux as a function of the monomer bulk concentration is in this case J2 =&C$ where I;,=ti,/[l+fipy/k,(l--K’)+k*K’]. Two limiting process. (1) fi, -+ k,
cases can be considered and
for the
fiti Q k,(l - K’) + k, K’,
that is to say that the process is limited by the mass transport so the fluxes ratio, J2/JI, should be equal to the unity. From the synthesis currents values (at t = th,,) that ratio is found to be 2.0, excluding this way the possibility of the synthesis process be controlled by the mass transport step. (2) fib % kl
and 5~~ % k, (1 - K’) + k2 K’,
that is to say that the process is limited by the charge transfer processes. The flux expressions are then simplified: J, =k,C&
and J2 = [k, (1 - K’) + k2 K’]C$.
As has been said before, the flux ratio J2/J, is equal to 2.0, and we obtain finally for the charge transfer constants ratio k2/k, the relationship kzlk, = 2 + (l/KC&.++). This result it is the most relevant one since it means that, independently of the kinetic importance of the complexation chemical step, the kinetics of the charge transfer process of the complex oxidation will be always favoured compared to the free monomer one. The occurrence of a second parallel faradaic process with a rate constant (k2) at least twice superior than the one of the free monomer direct oxidation (k,), explains why the synthesis charge value of a PPy film is increased to the double, in the presence of the NMP. Until now we have been able to understand some of the most important kinetics aspects of the PPy films synthesis process in the presence of the NMP. However, in what concerns the eventual NMP immobilization in the film, the preceding study does not bring some important informations and a complementary study must be carried on. 2. Synthesis under charge control. Synthesis of PPy films in the absence and in the presence of NMP (using in this last case the same ratio of species solution concentration 0.5) were performed until the same charge value was reached for both cases. The current-time transients obtained during the synthesis can be seen in Fig. 7. As expected from our preceding
Polypyrrole electropolymerixation
L
215
b
a43 QOO 0
20
40
60
1
80 tls
Fig. 7. Chronoamperometric curves of charge controlled films synthesis: limiting charge: Qsy = 0.163 C cm-*; Ew = +0.8 V (USAg/Ag+IO-* M); solutions composition: 0.5 M LiClO, in a&o&rile; (a) m] = 4.0 x lo-’ M; (b) [NMP]/m] = 0.5.
conclusions, the film prepared in the presence of NMP reaches the &sired charge value before the other one: at each instant of the synthesis, higher amounts of fihn are produced due to the quicker parallel oxidation of the complex (PyNMP)+ +, as explained in the above described reaction scheme. These films, possessing the same synthesis charge, allow a correct evaluation of the NMP influence on the PPy films redox behaviour, again by comparison of their electroactivity. Films electrochemical characterization by cyclic voltammetry. The Glms’ cyclic voltammograms are shown in Fig. 8. The PPy film synthesized in the presence of NMP, i
I
0.22mAcfW2
519
Figure 8b, shows a redox couple peak potential displacement towards less positive values (from -0.2 to -0.3 V us Ag/Ag+). The presence of a second mdox couple can also be noticed, in this lilm, with a slight peak at about 0.45 V. The 6lms’ electroactivity, measured by the charge involved in the redox process, do not differ very much: 36 and 40 mC cme2, for the films prepared in the absence and in the presence of NMP, respectively. The different redox behaviour, particularly the E,(PPy) displacement, could be attributed to morphological changes arriving from differences in the film growth whenever the NMP is present. In fact, other authors[lO] have found significant PPy films morphological changes due to the influence of dopants with different chemical nature. Such a possibility cannot be excluded in our case, however it does not explain the complete rcdox behaviour of the films, namely the presence (even discrete) of the second rcdox couple with Ep = +0.45V. According to our previous results, it seems rcasonabIe to think that the observed reactivity of the NMP in the presence of the monomer, already discussed, should be found again with the polymer. It is possible that during the film growth, some PPy is formed under a complexed form with the NMP, although the presence of some free NMP molecules in the film is not excluded. On this basis, the film electroactivity during the cyclic voltammetry can be described by two parallel processes. The first one
+(na + m)ClO; S
[H(p6+, SClO,-),,
@IMP’ +, ClO; )J + (na + m)ewhere P stands for the unit
T
\/ zr
H
H
is seen between -0.7 V and less than +0.45 V. This process is the oxidation of the polypyrrole chains, some of which being in their complexed form (with an Ep at about -0.3 V). This process occurs simultaneously with the incorporation of the negatively charged perchlorate ions in order to maintain the material electroneutrality. The second one is + 0.45 v
r(NMP) + zClOb #(NMP’+ClO,-),
Fig. 8. Cyclic voltamperommetric curves of two PPy films sy&.hes~ under the-same synthesis charge (0.163 C &nS2); the i&s were cycled in a 0.5 M LiClO.. scetoninile solution at a swe8p rat;: of SOmVs-i: (a) tiiL synthm@ in the
=36mCcm-2(bHllmsymh&ul absents of NMP, Qin the presence of NMP, Q- 40 mC aS2.
+ ze-. (VI)
This second redox process is seen after + 0.45 V up to +0.7 V, and corresponds to the usual electrochemical response of the free NMP in the film. The total amount of NMP in the 6lm cannot be evaluated from its electroactive response followed by cyclic voltammetry. However an estimate of the amount of free NMP in the tilm can be done by considering that this amount is expressed by the slight oxidation peak fotmd at about +0.45 V in the cyclic voltammogram. For a very thin lilm, the peak current is related to the
M. F. MENDESVIEOAS et al.
520
or as a function of the time. This last experiment allowed us to understand better the synthesis process of such a film, and consequently to propose a mechanism for it.
-2.4
-0.i
+0:7
+0.45
0
gv
vs Ag/Ag+10-2M
Fig. 9. Ring currents us disc potential curves of a PPy film synthesized on the disc electrode surface, in the presence of NMP: QsY= 0.163 C ctn2; f_NMP]/m] = 0.5; 0.5 M LiClO,, acetonitrile; disc sweep rate = 1OmV s-r; electrode rotation speed = 1500 rpm; (a) ring limiting reduction current, obtained at i&,o = -0.1 V and En,,, between -0.1 V to f0.7 V, (b) ring limiting oxidation current, obtained at EalNo = $0.7 V and Eo,sc between f0.7V to -0.1 v.
concentration pression
of the oxidized species by the exiP=n2F2vVC&/4RT,
where v is the scan rate, V is the film volume and Czx is the species concentration in the film. The concentration of free NMP in the PPy film is then estimated to be 0.4% w/w, assuming a PPy density value of 1.51 g cme3[12] and a film volume estimated by V = (2.8 x 10-4Qsu)_4[13]. In order to obtain more quantitative data about the NMP total films contents and about the NMP electrochemical influence on the synthesis process, we have performed ring-disc electrode studies with the polymer film deposited on the disc. The ring current was followed either as a function of the disc potential
(c) Ring-disc electrode studies 1. Ring currents vs disc potential. The ring currents obtained as the disc (film) potential was scanned either in the anodic or in the cathodic direction, are shown in Fig. 9. These currents constitute for the studied system a quantitative measure of the amount of NMP (under an oxidized or a neutral form) leaving the PPy film from the disc, since for a rotating ring-disc electrode their currents are related by the collection efficiency coefficient (N): N = iRING/iDIx. For films synthesized with the same charge, it was observed that the quantity of NMP that left the film under oxidized form was about twice the one under an reduced form (Fig. 9a and b). The half-wave potential value for this process, E,,, = +0.45 V, is a strong indication in favour of the involvement of the NMP first redox couple (NMP/NMP’+). During the oxidation scan some NMP radical cations can be formed and repelled from the film due to a positive charge excess; during the cathodic scan it is possible that some NMP under the reduced form leaves the film simply because of an activity gradient at the interface polymer/solution. These results are not however sufficient to conclude under which forms the NMP exists in the film, or even on possible film morphological changes as previously discussed. 2. Ring current vs time A film synthesized with the same synthesis charge as the ones above studied, was forced to undergo exhaustive oxidation by application of a constant potential. The ring limiting current was then monitored versus time, until it reached a constant value equivalent to the residual current. This curve is shown in Fig. 10 for PPy film synthesized in the presence of NMP with a synthesis charge value of 0.163 C cm-2. The IR,L at the initial instant of the process was 3.2 PA cmm2 and the charge involved in the whole process was found to be 0.342mCcn1-~. The number of moles of NMP that leaves the film (in an oxidized form NMP’+) during that time can be considered equal to the NMP film contents (since the oxidation process was quantitative), and it can be. calculated by n = [l/NFJQ&$j’o. t/s
00
9
z
50
100
150 300
-” =I
3?0
Fig. 10. Ring limiting reduction current us time curve of a PPy film synthesized in the presence of NMP: Qsu = 0.163 C cn-‘; FMP]/[Py] = 0.5; 0.5 M LiClO,, acetonitrile; electrode rotation speed = 1500 rpm; slNo = -0.1 V and Emsc = +0.7 V; QEo = 0.342 mC cm-*.
Polypyrrole electropolymerization The concentration of NMP in the PPy 6lm, estimated by considering a PPy density value of 1Sl g cm-‘[12], is 2% w/w. Zagorska[l4], has found a concentration value of 14% w/w, in the case of PPy films doped with ferrocyanide anions. Our value has to be discussed on the basis that the NMP cannot play the role of a dopant as the ferrocyanide anions can, since the immobilization process of the NMP in PPy films is not electrostatically favoured. A polymerization mechanism can be proposed. Radicals formation H-P-H
-B H-P’+-H
+ e-
(1)
H-P-H + NMP+ + # (H-P-H NMP)+ + (H-P-H NMP)++ * (H-P’+-H
(2) NMP++) + e- (3)
(H-P’+-H NMP+ +) P H-p’+-H
+ NMP+ + (4)
Polymerization Initiation: (H-P-H NMP)+ + + H-P’+-H +2(6 + l)A- --) [H(p6+, 6A-),H, (NMP++, 2A-)] + 2H+ + (1 + 26)e-
(5)
Propagation: [H(p6+, SA-),,H, (NMP++, 2A-),,,I + (1 - y)H-P’+-H +(6 +2y)A-
+ y(H-P’+-H
NMP+ +)
*[H(P6+,6A-),+,H,(NMP++,
2A-),+,I + 2H+ + (6 + l)eNMP+ + Release: [H(p6+, SA-),H,(NMP++, * [H(pd+, 6A-),H]
(6)
2A-),I
+ m NMP++ + 2m A-
(7)
Steps (1) to (4) are the initial ones, where the species needed to start the polymerization reaction, are formed. Steps (2) and (4) are chemical equilibria, while steps (1) and (3) are electrochemical reactions, and since they are related with each other, we can describe them by using the classic square scheme. HPH
e-
I
-NMP++
(HPH NMP)++ e-
HP’+H (HP’+H NMP++) -e-
w+
++ I I
Py
*
CNMP
PYm++
KPY%-l++-
521
The species (HP’+H NMP++) is unstable, decomposing into HP’+H and NMP++, in agreement with the already proposed catalytic effect. In step (5) the initiation of the polymerization process is described as due to the reaction between the relatively stable complex (HPH NMP)++ and the free monomer radical cation (HP’ +H). The resulting species is a pyrrole dimer complexed with the NMP++, in association with the necessary doping anions in order to maintain the species electroneutrality. The polymer growth process can be thought as the reaction between the already formed oligomers, complexed with NMP++, and the monomer radical cation that exists under two different forms: free HP’+H, and complexed (HPH NMP)+ +, as described in step (6). Of course this occurs in parallel with the PPy growth process through the free monomer oxidation. This gives rise to complexed polymeric chains, of variable length and so possessing different stability constants values. Depending on those constants relative values (at the synthesis potential value) and also on the NMP solution concentration, the complexed chains in the polymer undergo a dissociation equilibrium, as shown in step (7), that it is probably strongly displaced towards the right, releasing the NMP ++ from the chains to the solution. According to step (7) the NMP++ is regenerated, maintaining this way the catalytic efficiency during the synthesis process. Therefore it was not expected to find a significant quantity of NMP in the PPy films. The value found, 2%, is in good agreement with this statement and the fact that there is nevertheless some quantity of NMP in the films must be related with the stability of the complex Py chains in the polymer. A PPy film cycled in an electrolyte solution after having been forced to discharge quantitatively its NMP contents, showed the redox behaviour that is usually found for PPy films doped with perchlorate anions (although presenting some slights signs of degradation due to the strong oxidation conditions). This is in good agreement with the proposed mechanism and revealed that eventual morphological changes in the PPy films due to the presence of NMP, as suggested earlier are certainly not irreversible. The above described synthesis mechanism can be schematically represented as (1-Y)
-
-c-
c
me+&+)
Y
r@+hNMPl++-... -ropkm++-
tcpysx*m++
M. F. MENDEsVre~~s et al.
522
where the anions from the electrolyte have been deliberately omitted in order to simplify the scheme. This presentation outlines the catalytic effect of the NMP++ during the synthesis which proceeds via the intermediate (PyNMP)++ and shows that the incorporation of NMP during the film growth depends strongly on the stability of the intermediate oxidized species.
CONCLUSIONS This study has shown that the incorporation within a polypyrrole matrix of neutral molecules such as NMP was possible during the electrosynthesis but in small quantities. The nature of the immobilization mechanism has been identified as a complex formation between the monomer and an oxidized form of the NMP. The electron donor/acceptor properties of each species explains the possibility of such an association which plays the role of a reaction intermediate, together with the influence of the anions, it also explains why the immobilization may occur even in the presence of electrostatic repulsions. The study of the releasing process has shown that it was potential dependant and irreveresible. This can be an advantage compared to ionic dopants which can be unnecessarily readsorbed into the polymeric film upon reversal of the electrode potential. The PPy/NMP system allows an absence of potential control once the release is achieved. Further investigations on other members of the phenothiazine family possessing ionic charges for
studying the influence of the electrostatic interactions are in progress. Acknowledgement-Dr G. Bidan is gratefully acknowledged for fruitful discussions on this paper.
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