Influence of electrode pretreatment, counter anions and additives on the electropolymerization of pyrrole in aqueous solutions

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Synthetic Metals 74 (1995) 241-249

Influence of electrode pretreatment, counter anions and additives on the electropolymerization of pyrrole in aqueous solutions Eeva-Liisa Kupila *, Jouko Kankare Department of Chemistry, Universityof Turku, FIN-20500Turku, Finland

Received29 March 1995; revised 10 May 1995; accepted9 June 1995

Abstract

Polypyrrole behaves as essentially different materials when prepared under slightly different conditions. In this work the effect of pretreatment of the working electrode with 4-aminothiophenol, decanethiol, allyl alcohol and cysteamine is studied. The pretreatment enhances in some cases the early stages of polymerization, but the beneficial effects are small. Polymers synthesized in the presence of surfactants or organic dopants are superior to those polymerized in the presence of small inorganic anions. Additives such as 3- and 4-nitrophenol have minor effects on the polymerization of pyrrole, but 2,4,6-trinitrophenol improves significantly the quality of the polymers. Keywords: Electrodepretreatment;Additives;Electropolyrnerization;Pyrrole

1. I n t r o d u c t i o n

In recent years conducting polymers as sensor materials have been the subject of numerous investigations. However, there are fewer commercial applications on the market than predicted a decade ago. Developing these materials has proved to be much more difficult than originally anticipated. Polypyrrole (PPy) has been considered to be one of the most promising materials for sensor applications, as it can be easily synthesized in aqueous media with a wide range of counter anions, and, being usable in aqueous near-neutral solutions, it is compatible with biological compounds. Although PPy has been considered to be such an easy material to use, or maybe even for that reason, its basic properties, behavior and especially optimal preparative conditions have not been studied as systematically and as thoroughly as it deserves. Also, comparing the results of different research groups is difficult, as PPy behaves as essentially different materials when prepared under slightly different conditions. Due to the insolubility of the material, PPy films are invariably made by electropolymerization. In principle, the method is extremely attractive because of its seemingly easy control of regulating the polymerization current, potential and time. However, the process is far from being easily controllable. The electrode process creates highly reactive electrophilic radicals which react with each other, nucleophilic residues in * Corresponding author.

0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI0379-6779(95)O3369-U

the polymerization medium, with surface-confined oligomers, etc. The surface of the substrate has a profound influence on the polymerization yield by variation in the number of nucleation sites, which depends on the history of the surface. The material deposited on the surface is composed of polymer chains of variable length entangled together, incorporating short-length oligomer molecules which gradually diffuse away. Many anticipated applications demand contact of material with electrolyte solution whereupon the oxidation-reduction processes involve simultaneous incorporation or expulsion of ions. In the entangled disordered medium, the diffusion of ions is a slow process due to the tortuous pathways. There is increasing evidence that the interchain conduction process involves stacking of the conjugated systems through ~r electron interaction. In order to be efficient, this interaction requires the presence of large ordered domains, again showing that the disordered structure usually produced in the electropolymerization process is not favorable. However, the most common alternative, spin-coating, is not possible with PPy, and what remains is the optimization of the conditions under which the electropolymerization is carried out. The factors which have influence on the quality of conducting polymer films made by electropolymerization can be classified as follows: Solvent. The solvent power, nucleophilicity and inertness toward reactive radicals are important properties of the solvent used in electropolymerization.

242

E.-L. Kupila, J. Kankare/ Synthetic Metals 74 (1995) 241-249

Electrode surface. Electrode material, its adsorption and chemisorption properties, homogeneity and smoothness have a profound influence on the first layer of the deposited polymer. On the other hand, the regularity and the degree of coverage of the first layer largely determine the morphology of the layers deposited at later stages. The surface properties can be varied by chemical modification, i.e. by chemisorbing suitable compounds on the surface. Deposition rate. In principle, there are two fundamentally different deposition mechanisms of conducting polymers [ 1 ]. In the first mechanism oligomers are formed in solution in the close vicinity of the electrode surface and are deposited onto the surface as the solubility limit is exceeded. In the other mechanism the radicals formed in the electrode processes are chemically bonded one by one to the already surface-linked oligomer chains. Obviously slow deposition favors the latter mechanism, which would produce more regular material. The rate of deposition can be controlled by temperature, current density and stirring rate, Ionic constituents. The natural constituent of the solution in an electropolymerization experiment is supporting electrolyte. However, in addition to carrying electric current in solution, the ions of the electrolyte are incorporated in the polymer material, thus modifying its properties. Inert additives. The compounds in solution, added on purpose or inadvertently, are partially incorporated in the polymer film, the extent of incorporation depending on the compound, solvent and polymer. The molecular shape, size and electronic configuration of these 'inert' additives may have a profound influence on the orderly arrangement of the polymer chains. These additives may be compared with softening agents used with conventional polymers. There is certainly no single parameter which could be used for the assessment of the quality of polymer film. Different applications demand different characteristics. However, when comparing the same polymer materials made under different conditions, there is one single parameter which becomes closest to ideal. Variation in specific conductivity tells about the variation in the order-disorder characteristics of the polymer chains. Coplanarity of the conjugated monomer units has influence on the intrachain conductivity, whereas the facile interchain hopping of charge carriers is governed by the mutual arrangement of polymer chains. These microstructural requirements are often also reflected in the macrostructure of the polymer, i.e., in its morphology. In the present work we pay special attention to the influence of electrode surface, ionic constituents and inert additives on PPy films. The other factors have been kept as constant as possible. The temperature during electropolymerization affects the spatial order of the polymer network, and consequently has a profound effect on the electronic properties of the resulting material. As the aim of this work is to compare the influence of the chosen synthesis conditions, all measurements have been performed at exactly the same temperature. The method developed previously by us, in situ a.c. conductimetry during electropolymerization [2], has been used for

monitoring the growth process, quality and surface adhesion of the polymer film.

2. Experimental The electropolymerizations were carried out in aqueous solutions in a one-compartment cell at 25 °C, either at constant potential or current density with simultaneous recording of conductance. All polymerizations lasted for 1 h. The solutions were deaerated except for the surfactants, and all reagents were of analytical grade. The working electrode was a double-band platinum electrode and the counter electrode was a short piece of Pt wire. The reference electrode was a calomel electrode filled with saturated NaC1 (SSCE). The concentrations of pyrrole (Py) and the various electrolytes were 0.1 M unless otherwise stated. Prior to every polymerization the surface of the working electrode was carefully polished with successively finer diamond pastes and finally lapped with 0.30/zm alumina. The apparatus and the method of determining specific conductivities have been described in a previous paper [2]. In those cases where the electrical conductivities of the polymers are compared, the conductance is plotted versus In(time) or In(charge), as the slope of the linear part of these curves gives a fast and convenient way to compare the specific conductivities [ 2]. 2.1. Polymerizations on pretreated electrodes

Polymerizations were performed in the presence of sodium dodecylsulfate (SDS, Aldrich), sodium toluenesulfonate (NaTs, Merck), NaCIO4 (G. Frederick Smith Chemical Company) or NaNO 3 (Merck). The pretreatment was carried out by immersing the electrode either for 15 min in a 2% solution of 4-aminothiophenol (Aldrich), allyl alcohol (EgaChemie) or cysteamine (Fluka), or for 18 h in a 1 mM solution of 1-decanethiol (Aldrich). As decanethiol is usually adsorbed from rather dilute solutions [ 3,4 ], the pretreatment time was prolonged. When the pretreatment time with decanethiol was prolonged to 68 h, the subsequent polymerization behavior did not change significantly. All solutions were made in ethanol, and the electrode was rinsed with ethanol and water prior to every polymerization. 2.2. Polymerizations with different types of counter anions

The polymerizations were performed in the presence of the sodium salts of the following compounds: 1-decanesulfonic acid, benzenesulfonic acid, ( 1 R ) - ( - ) - 1 0 - c a m p h o r sulfonic acid, 1,5-naphthalenedisulfonic acid, vinylsulfonic acid and polyvinylsulfonic acid. They were all purchased from Aldrich, except the sodium benzenesulfonate, which was purchased from Fluka. Experiments with the surfactants, including sodium undecyl, dodecyl and tridecylsulfate (sodium undecyl and sodium tridecylsulfate from Lancaster), with critical micelle concentrations in respective order

E.-L. Kupila, J. Kankare / Synthetic Metals 74 (1995) 241-249

1.6 × 10- 2, 8.3 X 10- 3 and 5.0 X 10- 3 M, were performed in solutions with 0.05 M Py concentration and varying surfactant concentrations. When polymerizing in the presence of biological buffers, including MES (4-morpholine-ethane sulfonic acid), PIPES ( [ 1,4-piperazinebis(ethanesulfonic acid) ] ), MOPS (4-morpholinepropanesulfonic acid) and HEPPS ( [ 4- (2-hydroxyethyl) - 1-piperazinepropanesulfonic acid] ) (all from BDH), the Py concentration was 0.5 M and the buffer concentration 0.1 M. 2.3. Polymerizations with additives Polymerizations were performed in the presence of SDS, NaTs or NaC104 with additive concentrations of 0.01, 0.05 or 0.1 M. The additives used were phenol (Merck), 4-nitrophenol (4nf, Merck), 3-nitrophenol (3nf, Fluka), 2,4,6-trinitrophenol (picric acid, pic, Merck), 2-nitrobenzoic acid (nba, Fluka) and 3-hydroxybenzoic acid (hba, Fluka). When needed, the pH of these solutions was adjusted with NaOH.

3. Results and discussion

The method chosen in this work for monitoring the quality of polymer films, a.c. conductimetry with a double-band platinum electrode, provides conductance data during the electropolymerization process. As we have previously noted [2], the regular growth of the film, where the growth rate and specific conductivity stay constant during the process, is shown as a straight line when conductance is plotted versus the logarithm of time (galvanostatic polymerization) or consumed charge (potentiostatic polymerization). The growth of PPy film in the presence of dodecyl sulfate is a good example of the regular growth mechanism. Any deviation from the straight line is a sign of changing the deposition mechanism. In any case, the conductance should be monotonically increasing. The decrease in conductance is a sign of loss of conductivity of the lower layers, either by overoxidation or by sheer detachment from the electrode surface. 3.1. Electrode pretreatment The modification of electrode surfaces involves the attachment of chemical substances by, e.g., physical adsorption or chemical bonding. The modified electrode may exhibit properties related to those of the modifying substance. It is well known that electrodeposition of metallic monolayers is remarkably sensitive to the nature of the adsorbed layer at the substrate surface [5]. Also in connection with conducting polymers, understanding the factors governing the initial surface reactions is of fundamental importance as the nature of the electrode affects, e.g., adsorption of the monomers or oligomers on the electrode surface and the distribution of nucleation centers. The nature of the adsorbate exerts a powerful influence upon the kinetics of the polymerization, as well as upon the structure of the electrodeposition. Earlier we

243

studied the effect of silanization, which makes the electrode surface more hydrophobic [6]. The pretreatment was found to be advantageous when polymerizing Py in the presence of relatively small anions, such as C104 and CI-, on the platinum surface. The reason for the frequently poor adhesion of conducting polymer films to metal electrodes is the chemical dissimilarity of these two phases. Due to this dissimilarity the nucleation sites are sparsely populated on the surface, resulting in hemispherical and insular morphology. Recently it has been shown in our group [ 1 ] and elsewhere [7,8] that the deposition of oligomeric or polymeric species from the solution in the close vicinity of the electrode plays an important role in the film formation. This deposition can be enhanced by treating the surface with suitable reagents which either modify the adhesive properties or provide reactive sites for the chain growth [ 1 ]. In the present work the electrode pretreatment was made by immersing the electrode in a solution of 4-aminothiophenol, cysteamine, 1-decanethiol or allyl alcohol. These compounds were chosen according to their different influence on the character of the surface. 1-Decanethiol increases the hydrophobic character of the surface, i.e., it has a similar effect as octadecyltrichlorosilane [6], except that its influence is restricted to the platinum surface and does not extend to the surrounding epoxy surface. Both cysteamine and allyl alcohol make the platinum surface more polar, thus changing its adsorption properties. 4-Aminothiophenol is somewhere between, perhaps having a special effect due to its aromatic ring. Some earlier studies [9,10] have shown that, when polymerizing aniline on gold, the pretreatment of the electrode surface with a self-assembled layer of 4-aminothiophenol results in improved film-substrate interaction, and more well-ordered and dense polymer films. The beneficial effect on film morphology was thought to be caused by a more uniform nucleation-and-growth process, resulting in a film with improved space filling. It is known [ 11 ] that molecules containing, e.g., thiol moieties are among the most strongly chemisorbed molecular adsorbates on platinum, whereas alcohols are rather weakly adsorbed. It is likely that cysteamine (HzN-CHzCHz-SH) and 4-aminothiophenol (H2N---C6H4-SH) bond to the substrate through the sulfur with the loss of H, in the latter case the ring being perpendicular to the surface, resulting in the amino group in para position toward the solution and providing a natural bonding site with the growing polymer. Cysteamine may also form a chelate structure, where it is adsorbed from both the amino and the sulfur end. Allyl alcohol (HzC=CH-CHz-OH) is most likely adsorbed through the unsaturated center, which is parallel to the surface [9,11 ]. Fig. 1 (a) and (b) show the conductance curves obtained when polymerizing Py in the presence of SDS or NaTs. Pretreating the electrode with allyl alcohol seems to enhance the nucleation process, as the conductance curves start to rise earlier. The specific conductivities are not altered, as seen from the equal slopes [2]. When polymerizing in the pres-

244

E.-L. Kupila, J. Kankare / Synthetic Metals 74 (1995) 241-249

4-

o (,.)

2

1

1

0

(a)

ln't (s) ~

1 2-

2

5

4

v

.-~

e, 0

0

(b)

In t (s) 0.08

0.0060.04

0.005-

0.02 (

,.w,

0.004-

o

I

0.003-

I:: 0 (J

0.002

0.001 0

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4 In t'(s) ~

ence of SDS, the pretreatment with decanethiol weakens the adherence of the polymer film, which explains the shape of the conductance curve. When the electrode is pretreated with 4-aminothiophenol or cysteamine, the polymerization behavior is drastically different in Py/SDS and Py/NaTs solutions. Pretreatment with cysteamine decreases the adherence of PPy/DS films to the electrode. This is not the case when polymerizing in the presence of NaTs, which indicates that these two materials require different surface characteristics to obtain the best possible polymers. Dodecylsulfate, with its long hydrocarbon backbone, favors hydrophobic conditions. In 4-aminothiophenol the aromatic ring compensates the influence of the rather polar amino end reaching towards the solution. With compounds such as cysteamine, the results might be improved using a longer carbon chain between the sulfur and amino end. When polymerizing in the presence of NaC104, only the decanethiol pretreatment has a significant positive effect (Fig. l ( c ) ) . With N a N O 3 , n o n e of the pretreatments improves the quality of the polymer (Fig. 1 (d)). These results are especially interesting considering that perchlorate and nitrate are still quite often used as counter anions when polymerizing Py, despite the poor conductance of the resulting films. Modifying the electrode surface should primarily affect the early stages of polymerization, either enhancing or hindering them. So far, only pretreating the electrode with octadecyltrichlorosilane [6] and decanethiol has had some advantageous effects when polymerizing Py in the presence of smaller anions. When polymerizing in the presence of NaC104, decanethiol pretreatment produces a clearly higher conductance maximum than the pretreatment with octadecyltrichlorosilane. With larger anions, the results are not improved. On the basis of these studies, it seems that pretreating the working electrode surface may have a slight effect on the early stages of the electropolymerization, but generally does not improve the conductivity of the polymer film with the present inactive surface modifiers. This being the case, we pursued our studies by examining the effect of various types of counter anions and additives on Py polymers.

0.0025-

3.2. Organic sulfates and sulfonates as counter anions 0.0020. o

0.0015

0.0010

(d)

2

4 In t (s)

8

Fig. 1. Polymerization of Py in the presence of (a) SDS ( I mA cm-2), (b) NaTs ( 1 mA cm-~), (c) NaCIO4 (2 m A c m -2) and (d) NaNO3 (2 mA c m - 2 ) on an electrodewith differentpretreatments: (l) no pretreatment, and pretreatmentwith (2) allylalcohol, (3) 4-aminothiophenol,(4) decanethiol and (5) cysteamine.

The use of a double-band electrode provides a fast and easy way to determine the quality of the polymer in statu nascendi. There are a few systematic studies on the influence of different types of counter anions on PPy [ 12-14], but none, to our knowledge, reports in situ conductances. Because the best PPy films so far in our laboratory have been made with dodecylsulfate as a counter anion, it was of interest to study the influence of changing the length of the carbon chain in the surfactant. In addition to sodium dodecylsulfate, polymerizations were performed in the presence of sodium undecyl and tridecylsulfate. Because of the differing critical micelle concentrations (cmc values), the polymerizations were performed in various concentrations both above and below the

E.-L. Kupila, J. Kankare / Synthetic Metals 74 (1995) 241-249

3

/a

r,.) 1

0

i

-2

i

=

-1

i

0

I

1

/

=

3

4

In Q (mC) Fig. 2. Polymerization ( +500 mV vs. SSCE) of Py in the presence of 0.0045 M (a) sodium tridecylsulfate, (b) sodium dodecylsulfate and (c) sodium undecylsulfate.

,-, 4

ab

3 o

2

o r..)

d 1

e

0

II

2

r

I

4

6

8

In t (s) Fig. 3. Polymerization (2 mA cm -2) of Py in the presence of various electrolytes: (a) SDS; (b) NaTs; (c) sodium naphthalene-l,5-disulfonate; (d) sodium decanesulfonate; (e) sodium benzenesulfonate; (f) sodium vinylsulfonate; (g) sodium polyvinylsulfonate; (h) sodium camphorsulfonate.

cmc, the ratio C(surfactant)/cmc being fixed. In addition,

films were synthesized using the same surfactant concentration in all cases (Fig. 2). The polymerizations were carried out potentiostatically due to the less danger of overoxidation. The results are similar in all the three series of measurements. The specific conductivities do not differ significantly, being 13.2, 13.8 and 14.0 S cm-1, as the carbon number increases from 11 to 13, respectively. The longer the carbon chain, the earlier the conductance curve starts to rise, due to enhanced adsorption. The alternation in physical properties of longchain polar organic compounds for even- and odd-numbered chain lengths has often been observed [ 15 ], but in the present case no such effect can be observed. The polymerization of Py was also performed in the presence of sodium salts of the following compounds: 1-decanesulfonic acid, benzenesulfonic acid, (IR)-( - )10-camphorsulfonic acid, 1,5-naphthalenedisulfonic acid, vinylsulfonic acid and polyvinylsulfonic acid (Fig. 3). The polymerizations were performed galvanostatically, as Py is not significantly polymerized in the presence of these anions at reasonably low potentials.

245

As the structures of these anions differ markedly, we hope to clarify some of the requirements for good counter anions when polymerizing Py. Table 1 shows the specific conductivities of some of the materials synthesized in this work. Dodecylsulfate is by far the best counter anion. It seems that surfactants with a long hydrocarbon backbone are in general good counter anions. However, this is not the only important factor, as tosylate and naphthalenedisulfonate produce better results than decanesulfonate, which differs from dodecylsulfate, in addition to the sulfonate group, only by a slightly shorter carbon chain. Naphthalenedisulfonate with its two aromatic rings is approximately as good a counter anion as tosylate. The multi-anionic nature of naphthalenedisulfonate is probably not crucial [ 14,16]. Tosylate is a much better counter anion than benzenesulfonate, although their structures differ only by one methyl group. The probable reason for this is that the additional carbon in the methyl group increases the hydrophobic character of this anion. In addition, both the methyl and the sulfonate groups have a tetrahedral structure, and the more symmetrical counter anion may have a favorable interaction with the Py chains, producing more ordered and densely packed materials. The inferiority of poly mers synthesized in the presence of polyvinylsulfonate proves that merely a long carbon chain is not adequate. The adsorption properties and spatial structure of this anion and, e.g., dodecylsulfate, are very different. Initial adsorption of the counter anions on the electrode surface may affect the nucleation process more than previously thought. Polystyrenesulfonate, e.g., has been observed to line up due to the influence of an electrical field [ 17]. This may be one reason for the good electrical and mechanical qualities of PPy/PSS polymers, as polymerization in an organized field of anions can result in a more organized polymer. Camphorsulfonate proved to be a poor counter anion. This must be due to stereochemical reasons, the structure being stiff, nonplanar and poorly fitted between polymer chains. Still, it is a better counter anion than the spherical perchlorate. The spatial order in conducting polymers is determined by the order of the polymer backbone, dopant anion and their mutual interaction. Charge conduction processes in conducting polymers are governed by the intrachain conjugation length, interchain packing and limitations introduced by various disorders. The exact location of the counter anions with respect to the PPy chains is in most cases not known at present [ 18], but it is determined by their size and the conditions of the polymer preparation. In addition, there is also disagreement concerning the interaction between these anions and the PPy matrix. There is, for instance, evidence for coil structure for both PPy [ 17,19] and polythiophene [ 19,20]. Isolated polymer helical chains have been found when polymerizing Py in the presence of, e.g., tosylate and polystyrenesulfonate. Scanning tunneling microscopy has revealed that while small counter anions, such as perchlorate, are incorporated to these helices, they are overcoated by polymeric anions. It has been reported that, although film morphology and the separation of the polymer chains show wide variations with varying

246

E.-L. Kupila, J. Kankare / Synthetic Metals 74 (1995) 241-249 0.010

counter anions, the electronic excited states are all very similar [ 14]. Thus, structural changes affect conductivity by changing interchain and interparticle carrier hopping. PPy with small counter anions can exist in a variety of structural forms depending on the film thickness [ 17]. Though widely used, the symmetrical C104- anion seems to be an inferior counter anion for Py polymerization. It has proven to be difficult to obtain reproducible stoichiometries of films with perchlorate as the counter anion [ 21 ], and there is evidence of PPy/C104- films being unstable [21,22]. It has been suggested that while the presence of some aromatic counter anions, such as toluene sulfonate [23], and alkyl sulfates or sulfonates [ 24 ] may induce a short range order with a layered structure, smaller anions, such as perchlorate, produce amorphous or porous polymer layers. In general, chloride as a counter anion cannot be recommended, as it has been observed that polymers synthesized in chloride-containing electrolytes are less stable [ 25 ]. Biological buffers are zwitterionic sulfonates which are extensively used as general buffer compounds due to their inertness and inability to complex inorganic cations. Buffer solutions made from these compounds have been used as electropolymerization media for producing PPy, and their use has been suggested for the immobilization of glucose oxidase [26]. Because these compounds have the sulfonate group in common with the earlier compounds, it is of considerable interest in this context to study whether their use is beneficial for the quality of the polymer film. Altogether four biological

0.008

~

MOPS

0.006

.

0.002

0

j

1000

i

2000

/PIPES i

3000

i

4000

t (s) Fig. 4. Polymerization ( +500 mV vs. SSCE) of Py in the presence of various 0.1 M buffer solutions (pH of each solution equals to the corresponding pKa): IVIES,PIPES, MOPS and HEPPS. buffers were chosen for the study: MES (pKa 6.15), PIPES (pKa 6.80), MOPS (pKa 7.20) and HEPPS (pK~ 8.00). The pH was in each case adjusted with sodium hydroxide to the pK~ value of the buffer in question. The polymerization solutions contained 0.2 mg m l - ~ of SDS, as this was observed to improve the adhesion of the polymer to the electrode. The conductance of these materials is very low (Fig. 4), and when viewed through an optical microscope the polymers appear rough. With MOPS the polymerization was also performed in a solution where the pH was adjusted to one unit below the pK~ value, but this does not significantly affect the results. The buffers remove the H + ions expelled in the polymeri-

Table 1 Structures and specific conductivitiesof polymers synthesizedat 2 mA cm -2 Counter anion

Structure

Specific conductivity (S cm- t )

Sodium dodecylsulfate

CH3(CH2) i JOSO3Na

15.8

Sodium toluenesulfonate

Sodium naphthalene-l,5-disulfonate

H3C- ~ S O 3

~

Na

Na

10.5

9.46

Sodium decane-l-sulfonate

S%N~ CH3(CH2)gSO3Na

7.19

Sodium benzenesulfonate

(~SO3Na

5.09

Sodium vinylsulfonate Sodium polyvinylsuifonate

Sodium ( 1R) - ( - ) - 10-camphorsulfonate

H2C=CH-SO3Na [-CH2~H-(SOaNa)-] .

O~SO3Na

1.46 0.97

0.48

E.-L. Kupila, J. Kankare / Synthetic Metals 74 (1995) 241-249

zation reaction, and the local pH near the electrode does not drop as it does in unbuffered solutions, where the pH of the solution in the vicinity of the working electrode is different from that of the bulk solution. This may indicate that H + ions play an important role during polymerization, as polymerizing at a lower pH often produces better polymers, as already reported by Pei and Qian [27]. For instance, we have observed that polymerization of Py in the presence of perchlorate is enhanced when polymerizing at pH 2 instead of neutral solutions. Biological buffers do not work well in Py polymerization, probably because the rate of polymerization is limited by geometric and morphological arrangement, as indicated in an earlier study [26a]. No further conclusions can be drawn on the basis of these data. It seems that PPy can form at least two major types of structures. One of them is very disordered and is formed with small inorganic anions. There is evidence that with aromatic counter anions a more ordered sandwich-like structure is formed, where the planes formed by the Py rings, separated by the anions, are parallel to the substrate surface [23,28]. In this stratified structure the conduction process is probably at least partially two dimensional, confined to a single 'polypyrrole plane'. This kind of anisotropy has not been observed with spherical counter anions such as CIO4. In a recent study [ 29 ] perpendicularly oriented columnar structures have been observed for PPy films formed from micellar solutions of anionic surfactants. Predicting the effects of different counter anions on the quality' of the polymer is difficult. However, there seem to be some basic rules for making good PPy films. In general, we have observed that films of good conductivity have also good mechanical properties. Films of good quality are most often polymerized using large organic dopants with planar aromatic components, or using dopants containing a long carbon chain, such as surface-active sulfates or sulfonates. These counter anions induce structural regularity into the polymers. They can more easily adsorb at the electrode surface and thus enhance the first steps of the polymerization, and in addition interact beneficially with the Py polymer to produce a more ordered structure with better space filling. The presence of these structure-forming counter anions during the electropolymerization is essential for well-ordered morphology. Ion exchange, subsequently carried out, does not change the conductivity, as has been observed previously by Yamaura et al. [30].

247

Table 2 The acidity constants of the additives used in the electropolyrnerization of

Py Additive

pKa

2,4,6-Tfinitrophenol 2-Nitrobenzoic acid 3-Hydroxybenzoic acid 4-Nitrophenol 3-Nitrophenol Phenol

0.29 2.21 4.08 7.15 8.39 9.98

measurement. The additives used were phenol, 3- and 4nitrophenol, picric acid (2,4,6-trinitrophenol), 2-nitrobenzoic acid and 3-hydroxybenzoic acid (for pKa values see Table 2). The polymers were synthesized in solutions conmining Py, SDS, NaTs or NaC104, and the additive. As these additives alter the pH of the solution, polymerizations were performed both in a medium where the pH was adjusted to the value it would have without the additive, and in solutions where the pH was unadjusted. In this way the effect of both the pH and the structure of the various additives can be accounted for. As phenol seemed to have only a very small effect on the results, the corresponding curves have been omitted from the figures. Fig. 5 and 6 show the conductance curves obtained in these experiments with 0.01 M additive concentrations when polymerizing in the presence of SDS or NaTs in neutral solutions. When polymerizing in the presence of SDS, the results show no significant improvement in acidic or neutral solutions. These results further support the conclusion that tampering with the polymerization conditions hampers the polymerization with this counter anion. When polymerizing in the presence of NaTs, the influence of pH can be clearly seen. When the pH is unadjusted, the conductance curve rises higher the lower the pKa value of the additive, because the salt reverts to its acid form. This can be seen in Fig. 6, which also shows the conductance curve obtained when polymerizing in the presence of toluenesulfonic acid at pH 1.3. The difference in the results obtained in the presence of NaTs,

~ o

3ad nf d.

4

3.3. Additives It has been reported earlier [ 31 ] that some seemingly inert additives in the polymerization solution improve the quality of PPy. The presence of various mono-substituted phenols in the polymerization solution has been reported to enhance the conductivity, thermal stability and tensile strength of PPy/ tripropylnaphthalenesulfonate films. The reason for this was believed to be increased structural regularity. In order to get reliable, easily comparable data, the effect of similar additives was studied in this work by using the in situ conductivity

1 o 3

4

5

6

7

8

In t (s) Fig. 5. Polymerization ( 1 mA cm -2) of Py in the presence of SDS and various 0.01 M additives in neutral solutions: 4 n f = 4-nitrophenol, 3 n f = 3nitrophenol, pic = 2,4,6-trinitrophenol, nba = 2-nitrobenzoic acid and hba = 3-hydroxybenzoic acid.

E.-L. Kupila, J. Kankare / Synthetic Metals 74 (1995) 241-249

248

6

~

HTs

51

E,

3

--''"" 5 6

4

4nf

7

In t (s)

8

Fig. 6. Polymerization (1 mA cm -2) of Py in the presence of NaTs and various 0.01 M additives in neutral solutions: 4 n f = 4-nitrophenol, 3 n f = 3nitrophenol, pic = 2,4,6-trinitrophenol, nba = 2-nitrobenzoic acid and h b a = 3-hydroxybenzoic acid. The curve labeled 'HTs' shows the results obtained when polymerizing Py in the presence ofp-toluenesulfonic acid at pH 1.3.

0.030. (a) hba

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°°2°I i 0.015 0.010 L~

0.005 0.000

In t (s)

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~

pie

o,o15

0.010 o

0.005 o.ooo

solution is in this case buffered, which prevents the local pH near the electrode from decreasing. This in turn lowers the conductivity of the resulting polymer. The results obtained when polymerizing in the presence of trinitrophenol are interesting, as it is evident that pH cannot in this case be the crucial factor. The influence of the additives is greatest when polymerizing in the presence of NaC104 (Fig. 7). With most additives, such as 3-hydroxybenzoic acid, pH is the critical factor. However, this is not the case with trinitrophenol and 2-nitrobenzoic acid. With trinitrophenol as additive the polymerization behavior is unexpected, resembling that observed in the presence of NaTs, giving good specific conductivities (Table 3). This effect is not caused entirely by pH, which is seen in Fig. 7(b). The quality of the polymers is markedly improved, being now very smooth compared to the bumpy and uneven films obtained without the additive. In addition the polymer spreads over the Pt bands, as it also does with NaTs and this additive. For comparison, Py was also polymerized in a solution containing only 0.01 M trinitrophenol (Fig. 7(a)). When Py is added to this acidic solution, the traces of red precipitate observed before stirring indicate either weak chemical polymerization or formation of a molecular complex. In some cases also higher additive concentrations, 0.05 and 0.1 M, were tested. In general, this does not improve the quality of the polymers. Although the influence of these additives is in many cases beneficial, the influence of trinitrophenol is exceptional and most interesting. The existence of charge transfer complexes between polynitro compounds and polycyclic compounds has been known for a long time (see e.g. [32] ). The structure of these complexes consists of alternate layers of two components, which have a loose electronic intermolecular interaction, as the two compounds share an electron pair. The planes of the molecules are parallel, although not necessarily face to face. Polynitro compounds, such as trinitrophenol, are the most common types of electron-accepting components. Stack conductivity is well recognized in charge transfer complexes and is presently a subject of intense research due to the high conductivities observed in certain molecular crystals. In conducting polymers, interactions of this kind would have major structural consequences and would provide a mechanism for

,

In t (s) Fig. 7. Polymerization (2 mA crn-2) of Py in the presence of NaC1Oa and various 0.01 M additives (a) with pH unadjusted and (b) in neutral solution: 4 n f = 4-nitrophenol, 3nf = 3-nitrophenol, pic = 2,4,6-trinitrophenol, nba = 2-nitrobenzoic acid and h b a = 3-hydroxybenzoic acid. The inset in (a) shows both the out-of-scale conductance curves and the curve obtained when polymerizing in a solution containing only Py with 0.01 M picric acid at pH 2.1 (PPy/pic). The inset in (b) shows the out-of-scale conductance curves.

with no additive, is great. The deposition region of the polymer on the electrode face also spreads wide over the platinum bands to the epoxy surface. As the initial polymerization pH 7 is close to the pKa value of 4-nitrophenol, the

Table 3 Specific conductivities of PPy obtained in different electrolyte solutions. All polymerizations at 1 mA cm -2 except for NaCIO4 at 2 m A c m -2. All concentrations are 0.1 M except for picric acid 0.01 M Electrolyte

Conductivity (S cm -~)

Toluenesulfonic acid Picric acid NaTs + picric acid/neutral pH NaCIO4 + picric acid/neutral pH NaC104 + picric acid/acidic pH NaTs SDS

21.1 13.7 14.0 11.8 15.0 10.0 16.3

E.-L. Kupila, J. Kankare / Synthetic Metals 74 (1995) 241-249

interchain charge transfer. In connection with thiophene oligomers [ 33,34], the existence of so-called "rr-stacks has been suggested. These w-stacks or rr-dimers were thought to be diamagnetic entities that are not bipolarons and which have favorable interchain interactions. It was explained that in such a structure one or more cationic thiophene rings form a "rrcomplex in which the spins are paired. The formation of structures analogous to charge transfer complexes provides an attractive mechanism for explaining the results obtained when polymerizing Py in the presence of trinitrophenol. It is possible, although presently highly speculative without further evidence, that trinitrophenol molecules could intercalate between the PPy chains, providing an alternative path for the charge conduction. The exact mechanism of how trinitrophenol improves the properties of PPy is worthy of further study, but at this stage it is clear that this a very promising novel approach for improving the quality of PPy.

4. Conclusions Present knowledge of the properties of PPy is far from complete. While some general trends have been found, much of the detail still remains to be understood. The working electrode pretreatments attempted in this work had only small beneficial effects on the early stages of polymerization. It seems that the nascent polymer has a more ordered morphology which then develops into a more disordered structure, and that PPy forms various, totally different types of structures depending on the counter anion. On the basis of this study it seems that with thick films only certain types of counter anions can induce considerable structural regularity. Finding out the exact interaction between the polymer and these counter anions is paramount when developing conducting polymers. In addition, the quality of the polymers can be improved with some inert additives in the polymerization solution, but the amount of data is still so scarce that further studies are needed before any rules on favorable structures of additives can be devised.

Acknowledgements Financial aid from the Academy of Finland is gratefully acknowledged.

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