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Chemical oxidative polymerization of pyrrole in the presence of m-hydroxybenzoic acid- and m-hydroxycinnamic acid-related compounds
Synthetic Metals 126 (2002) 111±116
Chemical oxidative polymerization of pyrrole in the presence of m-hydroxybenzoic acid- and m-hydroxycinnamic acid-related compounds P.A. Calvo, J. RodrõÂguez, H. Grande, D. Mecerreyes, J.A. Pomposo* CIDETEC, Center for Electrochemical Research and Development, Paseo Mikeletegi 61.1, E-20009 San SebastiaÂn, Spain Received 9 April 2001; received in revised form 10 September 2001; accepted 26 September 2001
Abstract Chemical oxidative polymerization of pyrrole was performed in the presence of two families of aromatic compounds with electronwithdrawing substituents. The ®rst one includes 3-hydroxybenzoic acid (HBA) and two HBA-related compounds, such as 6hydroxypicolinic acid (HPA) and citrazinic acid (CA). The second one includes trans-3-hydroxycinnamic acid (HCA) and two HCA-related compounds, such as trans-4-hydroxy-3-methoxycinnamic acid (HMA) and urocanic acid (UA). It was found that HBA, HPA and HCA enhance both the conductivity and stability of the resulting PPy. UA improves only the PPy stability whereas CA and HMA lead to low conducting PPy. Maximum PPy conductivity values of 81, 74 and 123 S/cm were obtained by using [HBA 0:25 M, [HPA 0:025 M and [HCA 0:025 M, respectively. Elemental analysis data and FTIR results showed no incorporation of the additives to the obtained PPy, except for HMA. The formation of additive-iron(III) charge±transfer complexes was identi®ed by means of UV spectroscopy of the starting solutions containing the organic additive and FeCl36H2O. The redox potential of these solutions was affected by the presence of the HBA- and HCA-related compounds. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemical polymerization; Polypyrrole; Conductivity; Stability; Additives
1. Introduction Chemical oxidative polymerization of pyrrole is the most attractive technique to obtain highly electrically conducting polypyrrole (PPy) in a powdery form for commercial applications. However, the powdery PPy obtained by chemical methods does nor reach the desired conductivity values shown by high-quality electrochemically prepared PPy thin ®lms (s > 300 S/cm) [1]. Another critical parameter both for chemically and electrochemically synthesized PPy is the lack of stability for the conductivity under environmental conditions (heat, humidity, light). This fact is of crucial importance for the future commercial use of PPy in electromagnetic interference (EMI) shielding of electronic equipment as well as in antistatic formulations [2±4]. It is well known that the morphology, the electrical conductivity and the stability of PPy synthesized by chemical oxidative polymerization of pyrrole depend strongly upon the speci®c reaction conditions. As expected, many typical parameters such as solvent, temperature, nature of * Corresponding author. Tel.: 34-943-309-022; fax: 34-943-309-136. E-mail address: [email protected] (J.A. Pomposo).
the oxidant, concentration, reactant stoichiometry and reaction time are important. But subtle experimental details such as order of addition of reactants, stirring rate, work-up conditions, solvent used for cleaning the powders, inert or atmospheric conditions and purity of solvents appeared to have serious effects on the results. Because of these subtle experimental conditions many problems have been found for reproducing some of the results claimed in the literature as evidenced by several groups [2,5,6]. Hence, original work by Meyers [7] reported PPy having a conductivity of 45 S/cm obtained by reacting anhydrous ferric chloride with pyrrole in ether at 22 8C for 1 h. By using hydrated ferric chloride under exactly the same reaction conditions PPy having a conductivity of 1:3 10 1 S/ cm was obtained in lower yield. This author points to reactant stoichiometry, temperature and reaction medium as critical reaction parameters giving no indication of environmental stability of the PPy. Machida et al. [8] stated that also reaction time and control of the oxidation potential determine PPy conductivity. Highly-conducting PPy samples (s > 100 S/cm) were claimed by optimizing the reactant stoichiometry, temperature, reaction time, oxidation potential and organic solvent used. However, samples
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P.A. Calvo et al. / Synthetic Metals 126 (2002) 111±116
obtained by the above procedure shown a marked decrease in conductivity upon aging at room temperature [9]. Rapi et al. [10] have reported that short-term polymerization and low temperatures yield the highest values of electrical conductivity of PPy synthesized in water although with lower yields. Interestingly, these authors observed positive effects on both conductivity and stability to aging when chemical oxidative polymerization of pyrrole is carried out in the presence of selected organic compounds. The best results (s 83 S/cm) were obtained by using substituted phenols carrying electron-withdrawing substituents such as 3-hydroxybenzoic acid (HBA) or 4-nitrophenol (NPh). Recently, Kudo et al. [11] have con®rmed the positive effect of NPh in both the conductivity and thermal stability of PPy synthesized in aqueous solutions containing Fe2(SO4)3 as oxidant and sulfonic surfactants as dopants (s 60 S/cm using sodium alkyl sulfonate). The enhanced conductivity and stability is attributed by these authors to the suppression of undesirable side reactions as well as to a certain improvement of the regularity of the resulting PPy. More recently, Kang and Geckeler [6] have demonstrated that also the puri®cation treatments (washing and drying steps) affect the ®nal PPy conductivity. Interestingly, they found that the presence of a polymer additive such as poly(ethylene glycol) (PEG) at relatively low amounts (0.006 M) increases the conductivity of the resulting PPy from 50 to 90 S/cm. In this paper we have investigated the chemical oxidative polymerization of pyrrole in the presence of several aromatic compounds with electron-withdrawing substituents as an extension of the pioneering work by Rapi et al. [10]. Two families of compounds have been selected to complete the original work by these authors. The ®rst group includes the HBA as reference additive and two HBA-related compounds such as 6-hydroxypicolinic acid (HPA) and citrazinic acid (CA). The second one includes trans-3-hydroxycinnamic acid (HCA) and two HCA-related compounds such as trans4-hydroxy-3-methoxycinnamic acid (HMA) and urocanic acid (UA).
Table 1 Chemical structures of the HBA- and HCA-related compounds used in this work
2. Experimental
2.3. Characterization
2.1. Materials
Characterization of the PPy samples was performed by elemental analysis in a LECO CHNS-932 apparatus and by FTIR spectroscopy in a MAGNA-IR 560 spectrometer. The starting solutions containing FeCl36H2O and the HBA- or HCA-related compounds were investigated by ultraviolet (UV) spectroscopy in a SHIMADZU UV-1603 spectrophotometer and by redox potential measurements in a CRISON GLP 22 apparatus using a platinum redox electrode (versus Ag/AgCl as reference electrode). The conductivity of the PPy samples was measured at room temperature by the standard four-point probe method using discs of 19.8 mm in diameter prepared by pressing the PPy powder at 300 bar for 3 min. The standard deviation of the conductivity data reported in this work is estimated to be 5 S/cm.
Pyrrole (98%), 2,5-dimethylpyrrole (98%), methanol (99.8%), HPA (95%), CA (97%), HCA (99%), HMA (99%) and UA (99%) were purchased from Aldrich. HBA (98%), ferric chloride hexahydrate (99%) from Fluka and acetone (99.5%) from Panreac, respectively. All the chemicals were used as received. The chemical structure of several of the above compounds is shown in Table 1. 2.2. Chemical oxidative polymerization of pyrrole In a typical reaction, the organic additive (HCA 0.381 g, 2.3 mmol) was ®rst dissolved at room temperature in 50 ml
Name
Abbreviation
3-Hydroxybenzoic acid
HBA
6-Hydroxypicolinic acid
HPA
Citrazinic acid
CA
Trans-3-hydroxycinnamic acid
HCA
Trans-4-hydroxy-3methoxycinnamic acid
HMA
Urocanic acid
UA
Chemical structure
of methanol. To this mixture, 50 ml of deionized water were added. Then, FeCl36H2O (13.52 g, 50 mmol) was added batchwise and the resulting solution was cooled to 0 8C. Finally, pyrrole (3.5 ml, 50 mmol) was added in a single portion under mild magnetic stirring. After a reaction time of 30 min, the resulting black PPy precipitate was ®ltered and repeatedly washed with deionized water and acetone and ®nally dried under vacuum until constant weight (0.362 g, yield 11%).
P.A. Calvo et al. / Synthetic Metals 126 (2002) 111±116
Aging experiments were performed by placing the PPy discs in an open oven at 150 8C. After the thermal treatment for different periods of time, the conductivity was determined at room temperature. For all the samples, after the thermal treatment the conductivity reaches a steady value and does not come back to the starting value before heating. 3. Results and discussion 3.1. Chemical oxidative polymerization of pyrrole in the presence of HBA-related compounds Conductivity data of PPy synthesized in the presence of HBA, HPA and CA as additives are shown in Table 2. HPA and CA have been selected to investigate the effect of subtle changes of the chemical structure of the additive over the properties of the obtained PPy (see Table 1). As it can be seen in Table 2, chemical oxidative polymerization of pyrrole without additives gives PPy with a conductivity value of 58 S/cm. In the presence of HBA in the reaction mixture, the conductivity increased up to a maximum value of 81 S/cm at [HBA 0:25 M corresponding to a HBA/ Table 2 Characteristics of the PPy synthesized in the presence of HBA- and HCA-related compoundsa Additive
Concentration (M)
±
±
11
58
HBA
0.025 0.125 0.25 0.5
12 11 10 10
76 74 81 50
HPA
0.01 0.025 0.125 0.25
11 11 14 19
71 74 63 20
CA
0.01 0.025 0.125 0.25
22 19 56 110
HCA
0.004 0.025 0.125 0.25 0.5
11 11 10 13 13
0.025 0.125 0.25
12 22 26
0.01 0.025 0.125 0.25
10 14 9 6
HMA
UA
a
Reaction yield (%)b
Conductivity (S/cm)
9 0.9 0.9 0.02 72 123 87 59 58 0.1 0.1 0.01 54 43 42 42
[FeCl3H2O 0:5 M; [pyrrole 0:5 M; temperature 0 8C; reaction time 30 min. b Calculated as (polymer weight/monomer weight 100.
113
FeCl36H2O ratio of 1/2 (Table 2). Rapi et al. [10] reported that PPy synthesized in water using FeCl36H2O and [HBA 0:025 M shows a conductivity value of 70 S/cm. This value is similar to that found in this work (s 76 S/cm at [HBA 0:025 M), although we use slightly different reaction solvent (water/methanol mixtures) to improve the solubility of the organic additive. For HBA contents above 0.25 M, a decrease of PPy conductivity is observed. In the case of HPA which is a nitrogen-containing HBA-analog, a similar behavior is observed as illustrated in Table 2. In this particular case, the maximum PPy conductivity (s 74 S/ cm) is obtained at [HPA 0:025 M. However, at [HPA > 0:125 M, the obtained PPy has a conductivity value lower than that prepared without additive. Finally, when CA is employed during the PPy synthesis as a HPA-related compound carrying an additional hydroxyl group, low conducting PPy is obtained (Table 2). On the other hand, the reaction yield remains low (10±20%) without additives as well as in the presence of HBA and HPA. However, in the case of CA the reaction yield increases in parallel to the reduction of PPy conductivity, pointing to a certain incorporation of the additive onto the polypyrrole at high CA contents. Aging experiments were performed at 150 8C to determine the thermal stability of PPy synthesized in the presence of HBA and HPA. As shown in Fig. 1 (entries C2 and C3), both compounds provide enhanced thermal stability when compared to PPy synthesized without additives (Fig. 1, entry C1). Furthermore, for PPy synthesized with [HPA 0:025 M, a conductivity overshoot is observed during the ®rst heating stages that could be attributed to annealing effects that enhance the orientation of the PPy chains [12]. To determine whether the stabilizing effect of both HBA and HPA is due to the incorporation of the additives to the conducting polymer, elemental analysis were performed for different PPy samples. According to the data given in Table 3, no signi®cant differences in chemical composition were observed by this technique. Additionally, the FTIR bands of the additives (i.e. carbonyl stretching band) were not found in the FTIR spectra of the PPy synthesized in the presence of HBA and HPA. These results indicate that these organic additives play their role during the PPy synthesis and they are not present in the ®nal PPy products. Table 3 Elemental analysisa data for PPy synthesized in the presence of HBA- and HCA-related compounds Additive
Concentration (M)
C
H
N
± HBA HPA CA HCA HMA UA
± 0.250 0.025 0.010 0.025 0.025 0.010
4.0 4.5 4.0 4.1 4.9 7.9 4.0
3.7 3.7 3.1 3.0 3.9 7.7 3.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0
a Oxygen and chloride cannot be determined in the employed equipment.
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Fig. 1. Evolution of the conductivity upon aging for different periods of time at 150 8C for PPy samples synthesized without additive (C1); in the presence of [HBA 0:25 M (C2); [HPA 0:025 M (C3); [UA 0:01 M (C4); and [HCA 0:025 M (C5).
Fig. 2. UV absorption spectra of the additive/FeCl36H2O/water/methanol (1/1) solution containing 0.25 M HBA (continuous line) and 0.025 M HPA (broken line) after subtraction of the UV absorption spectra of the HBA/ water/methanol (1/1) solution and the HPA/water/methanol (1/1) solution, respectively. The concentration of FeCl36H2O employed was 0.5 M.
In this sense, the starting FeCl36H2O/organic additive solutions were investigated by UV spectroscopy. HBA and HPA dissolved in water/methanol (1/1) mixtures shown sharp UV absorption bands centered at 296 and 311 nm, respectively. Upon addition of FeCl36H2O to the solution containing HBA, a new broad band with a maximum at 509 nm (see Fig. 2) is clearly seen in the difference UV spectrum obtained after subtraction of the UV spectrum of the HBA/water/methanol solution.1 The presence of such a 1
Alternatively, the UV spectrum of the FeCl36H2O/water/methanol solution was subtracted from the UV spectra of the additive/FeCl36H2O/ water/methanol solution obtaining near identical results (data not shown).
new non-symmetrical band suggests the formation of a distribution of different charge±transfer complexes between HBA as electron donor and iron(III) as electron acceptor, the most prominent species absorbing at 509 nm. It is well known that iron(III) has a marked preference for O-donor ligands, leading to a rich variety Fe±O charge±transfer complexes [13]. For the HPA/FeCl36H2O/water/methanol solution, two new UV bands at 535 and 563 nm are observed in the difference UV spectrum resulting from the subtraction of the UV spectrum of the HPA/water/methanol solution (Fig. 2) (see footnote No. 1). The presence of two UV absorption bands can be rationalized by taking into account that HPA has an additional binding site compared to HBA and also that keto-enol tautomerism is involved in this compound [14]. If pyrrole is added to the HBA (or HPA)/FeCl36H2O/ water/methanol solution, the rapid growing of the broad PPy UV absorption band centered at 880 nm is observed. Due to the strong absorption of the resulting black PPy, no clear information about the evolution of the HBA-iron(III) UV band can be ascertained. When 2,5-dimethylpyrrole (a reported non-polymerizable pyrrole [7]) was added to the HBA/FeCl36H2O/water/methanol solution, a progressive displacement of the band to higher wavenumbers was found (from 509 to 565 nm). This could be attributed to a certain interaction of the pyrrole derivative with the HBA-iron(III) charge±transfer complexes. A similar interaction is expected to be involved also for neat pyrrole during chemical oxidative polymerization. The formation of charge±transfer complexes between the organic additives and iron(III) affects also the redox potential of the FeCl36H2O-containing solutions even at very low additive concentration. Hence, the redox potential of the FeCl36H2O/water/methanol solution without additives was
P.A. Calvo et al. / Synthetic Metals 126 (2002) 111±116
115
657 mV at 0 8C. The redox potential decreased upon addition of HBA (0.025 M), HPA (0.025 M) and CA (0.010 M) to 655, 628 and 532 mV, respectively. PPy conductivity values of 58, 76, 74 and 9 S/cm were obtained by using solutions without additive, with HBA (0.025 M), HPA (0.025 M) and CA (0.010 M), respectively, showing no linear effect of the redox potential in the PPy conductivity. 3.2. Chemical oxidative polymerization of pyrrole in the presence of HCA-related compounds To improve the charge±transfer interaction between the organic additive and iron(III) during the chemical oxidative polymerization of pyrrole, HCA and two HCA-related derivatives (HMA and UA) were investigated (see Table 1). HCA incorporates a conjugated double bond next to the carboxylic moiety that could improve the electron donor effect of the compound when compared to HBA. On the other hand, HMA is a HCA-related compound carrying an additional methoxy group whereas in UA the phenolic moiety is formally replaced by the imidazole moiety. Conductivity data of PPy synthesized in the presence of HCA, HMA and UA as additives are shown in Table 2. As it can be seen, a maximum PPy conductivity of 123 S/cm is observed at a concentration of HCA 0.025 M corresponding to a HCA/FeCl36H2O ratio of 1/20. Contrary to HBArelated compounds, at high HCA contents the PPy conductivity obtained is not below that of the PPy control. As it can be seen in Table 2, HMA is not a bene®cial additive to improve the PPy conductivity. Even very low amounts of this compound in the reaction media leads to very low conducting PPy. When UA is employed (Table 2), a small decrease in PPy conductivity (about 27% at [UA 0:25 M) is observed. Elemental analysis data show that incorporation of the additive to the PPy is taking place only for HMA (Table 3). Reaction yields were 10±14% using HCA and UA, whereas higher yields were obtained in the case of HMA. Concerning the stability of the PPy synthesized in the presence of HCA and UA, a clear enhancement is found for both additives as illustrated in Fig. 1. In particular, HCA provides enhanced PPy stability even employing very low contents during the conducting polymer synthesis ([HCA 0:004 M). The best results are obtained for a concentration of HCA 0.125 M. It is worth noticing that UA improves PPy stability (Fig. 1) but does not PPy conductivity (Table 2). Ultraviolet absorption bands of HCA-iron(III) charge± transfer complexes were also clearly identi®ed in the difference UV spectra of the FeCl36H2O/HCA/water/methanol starting solutions, as illustrated in Fig. 3. In this case, the UV absorption maximum is located at 501 nm with a tail up to 750 nm suggesting a distribution of species. Conversely, UA shows only a broad UV band of lower intensity, as illustrated in Fig. 3. The formation of additive-iron(III) charge±transfer complexes is also re¯ected in the redox potential data of the
Fig. 3. UV absorption spectra of the additive/FeCl36H2O/water/methanol (1/1) solution containing 0.025 M HCA (continuous line) and 0.01 M UA (broken line) after subtraction of the UV absorption spectra of the HCA/ water/methanol (1/1) solution and the UA/water/methanol (1/1) solution, respectively. The concentration of FeCl36H2O employed was 0.5 M.
additive-containing starting solutions. Thus, redox potential values at 0 8C of 640, 528 and 643 mV were obtained for FeCl36H2O/water/methanol solutions containing HCA (0.025 M), HMA (0.025 M) and UA (0.010 M), respectively. PPy conductivity values of 123, 0.1 and 54 S/cm were obtained by using the above HCA-, HMA- and UA-containing solutions. 4. Conclusions The presence of HBA- and HCA-related compounds during the chemical oxidative polymerization of pyrrole using ferric chloride solutions in water/methanol mixtures have profound effects on the properties of the obtained PPy. Thus, HBA, HPA and HCA improve both the electrical conductivity and the thermal stability of the resulting PPy. On the other hand, UA, CA and HMA lead to PPy of lower conductivity values although PPy obtained in the presence of UA shows improved thermal stability. It is worth to remark that the PPy obtained in the presence of HCA shows a conductivity value of 123 S/cm. This is one of the ®rst examples where conductivities higher than 100 S/cm have been obtained for PPy by chemical oxidative polymerization. Furthermore, the PPy obtained in the presence of HCA shows an improved thermal stability which is a critical parameter for many applications. Future work will focus on the blending and processing of these products with thermoplastic materials. According to elemental analysis data and FTIR spectroscopy results on the obtained PPy, the additives are not present to a detectable level into the PPy products except for the HMA compound, suggesting that the effect of the organic additive takes place during the polymerization. Thus, the presence of HBA- and HCA-related compounds
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P.A. Calvo et al. / Synthetic Metals 126 (2002) 111±116
modify the redox potential of the starting ferric chloride solutions. It is worth noticing that for redox potentials bellow 550 mV (i.e. for CA and HMA additives) low conducting PPy is obtained. For redox potentials above 550 mV, no clear correlation is found between redox potential and PPy conductivity since starting solutions with similar redox potential values lead to PPy of different conductivity. Furthermore, important changes were observed in the UV spectra of the oxidant solutions after the addition of the organic additives. These data shows that the additives have an effect in the active species for the polymerization affecting their redox potential and chemical structure. This suggest that the changes on the active species leads to a PPy of less structural defects and/or longer conjugation length, hence, enhancing both PPy conductivity and stability. A more complete characterization of the PPy and the starting additive-iron(III) solutions by X-ray spectroscopy [15] would help to understand the involved kinetic mechanisms in these reactions and will be the subject of a future publication. Acknowledgements The authors gratefully acknowledge Yolanda Alesanco for kindly experimental assistance. Also, P.A.C. would like to thanks the Spanish Government for the support of this
work through a MIT-F2 Grant and Juan I. Guerra for his help with the computer treatment of graphics. References [1] C.O. Yoon, H.K. Sung, J.H. Kim, E. Barsoukov, J.H. Kim, H. Lee, Synth. Metals 99 (1999) 201. [2] P. Chandrasekhar, Conducting Polymers, Fundamental and Applications. A Practical Approach, Kluwer Academic Publishers, Norwell, MA, 1999, p. 381. [3] L. Rupprecht (Ed.), Conductive Polymers and Plastics in Industrial Applications, Noreich, New York, 1999, p. 69. [4] J.A. Pomposo, J. RodrõÂguez, H. Grande, Synth. Metals 104 (1999) 107. [5] E.T. Kang, K.G. Neoh, Y.K. Ong, K.L. Tan, B.T.G. Tan, Macromolecules 24 (1991) 2822. [6] H.C. Kang, K.E. Geckeler, Polymer 41 (2000) 6931. [7] R.E. Meyers, J. Electron. Mater. 15 (1986) 61. [8] S. Machida, S. Miyata, A. Techagumpuch, Synth. Metals 31 (1989) 311. [9] T. Yoshikawa, S. Machida, T. Ikegami, A. Techagumpuch, S. Miyata, Polym. J. 22 (1990) 1. [10] S. Rapi, V. Bocchi, G.P. Gardini, Synth. Metals 24 (1988) 217. [11] Y. Kudo, K. Akami, Y. Matsuya, Synth. Metals 95 (1998) 191. [12] R. Turku, C. Neamtu, M. Brie, Synth. Metals 53 (1993) 325. [13] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1984, p. 1265. [14] J. March, Advanced Organic Chemistry. Reactions, Mechanisms, and Structure, Wiley, New York, 1992, p. 72. [15] M. Magini, J. Chem. Phys. 76 (1982) 1111.
Chemical oxidative polymerization of pyrrole in the presence of m-hydroxybenzoic acid- and m-hydroxycinnamic acid-related compounds P.A. Calvo, J. RodrõÂguez, H. Grande, D. Mecerreyes, J.A. Pomposo* CIDETEC, Center for Electrochemical Research and Development, Paseo Mikeletegi 61.1, E-20009 San SebastiaÂn, Spain Received 9 April 2001; received in revised form 10 September 2001; accepted 26 September 2001
Abstract Chemical oxidative polymerization of pyrrole was performed in the presence of two families of aromatic compounds with electronwithdrawing substituents. The ®rst one includes 3-hydroxybenzoic acid (HBA) and two HBA-related compounds, such as 6hydroxypicolinic acid (HPA) and citrazinic acid (CA). The second one includes trans-3-hydroxycinnamic acid (HCA) and two HCA-related compounds, such as trans-4-hydroxy-3-methoxycinnamic acid (HMA) and urocanic acid (UA). It was found that HBA, HPA and HCA enhance both the conductivity and stability of the resulting PPy. UA improves only the PPy stability whereas CA and HMA lead to low conducting PPy. Maximum PPy conductivity values of 81, 74 and 123 S/cm were obtained by using [HBA 0:25 M, [HPA 0:025 M and [HCA 0:025 M, respectively. Elemental analysis data and FTIR results showed no incorporation of the additives to the obtained PPy, except for HMA. The formation of additive-iron(III) charge±transfer complexes was identi®ed by means of UV spectroscopy of the starting solutions containing the organic additive and FeCl36H2O. The redox potential of these solutions was affected by the presence of the HBA- and HCA-related compounds. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemical polymerization; Polypyrrole; Conductivity; Stability; Additives
1. Introduction Chemical oxidative polymerization of pyrrole is the most attractive technique to obtain highly electrically conducting polypyrrole (PPy) in a powdery form for commercial applications. However, the powdery PPy obtained by chemical methods does nor reach the desired conductivity values shown by high-quality electrochemically prepared PPy thin ®lms (s > 300 S/cm) [1]. Another critical parameter both for chemically and electrochemically synthesized PPy is the lack of stability for the conductivity under environmental conditions (heat, humidity, light). This fact is of crucial importance for the future commercial use of PPy in electromagnetic interference (EMI) shielding of electronic equipment as well as in antistatic formulations [2±4]. It is well known that the morphology, the electrical conductivity and the stability of PPy synthesized by chemical oxidative polymerization of pyrrole depend strongly upon the speci®c reaction conditions. As expected, many typical parameters such as solvent, temperature, nature of * Corresponding author. Tel.: 34-943-309-022; fax: 34-943-309-136. E-mail address: [email protected] (J.A. Pomposo).
the oxidant, concentration, reactant stoichiometry and reaction time are important. But subtle experimental details such as order of addition of reactants, stirring rate, work-up conditions, solvent used for cleaning the powders, inert or atmospheric conditions and purity of solvents appeared to have serious effects on the results. Because of these subtle experimental conditions many problems have been found for reproducing some of the results claimed in the literature as evidenced by several groups [2,5,6]. Hence, original work by Meyers [7] reported PPy having a conductivity of 45 S/cm obtained by reacting anhydrous ferric chloride with pyrrole in ether at 22 8C for 1 h. By using hydrated ferric chloride under exactly the same reaction conditions PPy having a conductivity of 1:3 10 1 S/ cm was obtained in lower yield. This author points to reactant stoichiometry, temperature and reaction medium as critical reaction parameters giving no indication of environmental stability of the PPy. Machida et al. [8] stated that also reaction time and control of the oxidation potential determine PPy conductivity. Highly-conducting PPy samples (s > 100 S/cm) were claimed by optimizing the reactant stoichiometry, temperature, reaction time, oxidation potential and organic solvent used. However, samples
0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 5 6 0 - 4
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P.A. Calvo et al. / Synthetic Metals 126 (2002) 111±116
obtained by the above procedure shown a marked decrease in conductivity upon aging at room temperature [9]. Rapi et al. [10] have reported that short-term polymerization and low temperatures yield the highest values of electrical conductivity of PPy synthesized in water although with lower yields. Interestingly, these authors observed positive effects on both conductivity and stability to aging when chemical oxidative polymerization of pyrrole is carried out in the presence of selected organic compounds. The best results (s 83 S/cm) were obtained by using substituted phenols carrying electron-withdrawing substituents such as 3-hydroxybenzoic acid (HBA) or 4-nitrophenol (NPh). Recently, Kudo et al. [11] have con®rmed the positive effect of NPh in both the conductivity and thermal stability of PPy synthesized in aqueous solutions containing Fe2(SO4)3 as oxidant and sulfonic surfactants as dopants (s 60 S/cm using sodium alkyl sulfonate). The enhanced conductivity and stability is attributed by these authors to the suppression of undesirable side reactions as well as to a certain improvement of the regularity of the resulting PPy. More recently, Kang and Geckeler [6] have demonstrated that also the puri®cation treatments (washing and drying steps) affect the ®nal PPy conductivity. Interestingly, they found that the presence of a polymer additive such as poly(ethylene glycol) (PEG) at relatively low amounts (0.006 M) increases the conductivity of the resulting PPy from 50 to 90 S/cm. In this paper we have investigated the chemical oxidative polymerization of pyrrole in the presence of several aromatic compounds with electron-withdrawing substituents as an extension of the pioneering work by Rapi et al. [10]. Two families of compounds have been selected to complete the original work by these authors. The ®rst group includes the HBA as reference additive and two HBA-related compounds such as 6-hydroxypicolinic acid (HPA) and citrazinic acid (CA). The second one includes trans-3-hydroxycinnamic acid (HCA) and two HCA-related compounds such as trans4-hydroxy-3-methoxycinnamic acid (HMA) and urocanic acid (UA).
Table 1 Chemical structures of the HBA- and HCA-related compounds used in this work
2. Experimental
2.3. Characterization
2.1. Materials
Characterization of the PPy samples was performed by elemental analysis in a LECO CHNS-932 apparatus and by FTIR spectroscopy in a MAGNA-IR 560 spectrometer. The starting solutions containing FeCl36H2O and the HBA- or HCA-related compounds were investigated by ultraviolet (UV) spectroscopy in a SHIMADZU UV-1603 spectrophotometer and by redox potential measurements in a CRISON GLP 22 apparatus using a platinum redox electrode (versus Ag/AgCl as reference electrode). The conductivity of the PPy samples was measured at room temperature by the standard four-point probe method using discs of 19.8 mm in diameter prepared by pressing the PPy powder at 300 bar for 3 min. The standard deviation of the conductivity data reported in this work is estimated to be 5 S/cm.
Pyrrole (98%), 2,5-dimethylpyrrole (98%), methanol (99.8%), HPA (95%), CA (97%), HCA (99%), HMA (99%) and UA (99%) were purchased from Aldrich. HBA (98%), ferric chloride hexahydrate (99%) from Fluka and acetone (99.5%) from Panreac, respectively. All the chemicals were used as received. The chemical structure of several of the above compounds is shown in Table 1. 2.2. Chemical oxidative polymerization of pyrrole In a typical reaction, the organic additive (HCA 0.381 g, 2.3 mmol) was ®rst dissolved at room temperature in 50 ml
Name
Abbreviation
3-Hydroxybenzoic acid
HBA
6-Hydroxypicolinic acid
HPA
Citrazinic acid
CA
Trans-3-hydroxycinnamic acid
HCA
Trans-4-hydroxy-3methoxycinnamic acid
HMA
Urocanic acid
UA
Chemical structure
of methanol. To this mixture, 50 ml of deionized water were added. Then, FeCl36H2O (13.52 g, 50 mmol) was added batchwise and the resulting solution was cooled to 0 8C. Finally, pyrrole (3.5 ml, 50 mmol) was added in a single portion under mild magnetic stirring. After a reaction time of 30 min, the resulting black PPy precipitate was ®ltered and repeatedly washed with deionized water and acetone and ®nally dried under vacuum until constant weight (0.362 g, yield 11%).
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Aging experiments were performed by placing the PPy discs in an open oven at 150 8C. After the thermal treatment for different periods of time, the conductivity was determined at room temperature. For all the samples, after the thermal treatment the conductivity reaches a steady value and does not come back to the starting value before heating. 3. Results and discussion 3.1. Chemical oxidative polymerization of pyrrole in the presence of HBA-related compounds Conductivity data of PPy synthesized in the presence of HBA, HPA and CA as additives are shown in Table 2. HPA and CA have been selected to investigate the effect of subtle changes of the chemical structure of the additive over the properties of the obtained PPy (see Table 1). As it can be seen in Table 2, chemical oxidative polymerization of pyrrole without additives gives PPy with a conductivity value of 58 S/cm. In the presence of HBA in the reaction mixture, the conductivity increased up to a maximum value of 81 S/cm at [HBA 0:25 M corresponding to a HBA/ Table 2 Characteristics of the PPy synthesized in the presence of HBA- and HCA-related compoundsa Additive
Concentration (M)
±
±
11
58
HBA
0.025 0.125 0.25 0.5
12 11 10 10
76 74 81 50
HPA
0.01 0.025 0.125 0.25
11 11 14 19
71 74 63 20
CA
0.01 0.025 0.125 0.25
22 19 56 110
HCA
0.004 0.025 0.125 0.25 0.5
11 11 10 13 13
0.025 0.125 0.25
12 22 26
0.01 0.025 0.125 0.25
10 14 9 6
HMA
UA
a
Reaction yield (%)b
Conductivity (S/cm)
9 0.9 0.9 0.02 72 123 87 59 58 0.1 0.1 0.01 54 43 42 42
[FeCl3H2O 0:5 M; [pyrrole 0:5 M; temperature 0 8C; reaction time 30 min. b Calculated as (polymer weight/monomer weight 100.
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FeCl36H2O ratio of 1/2 (Table 2). Rapi et al. [10] reported that PPy synthesized in water using FeCl36H2O and [HBA 0:025 M shows a conductivity value of 70 S/cm. This value is similar to that found in this work (s 76 S/cm at [HBA 0:025 M), although we use slightly different reaction solvent (water/methanol mixtures) to improve the solubility of the organic additive. For HBA contents above 0.25 M, a decrease of PPy conductivity is observed. In the case of HPA which is a nitrogen-containing HBA-analog, a similar behavior is observed as illustrated in Table 2. In this particular case, the maximum PPy conductivity (s 74 S/ cm) is obtained at [HPA 0:025 M. However, at [HPA > 0:125 M, the obtained PPy has a conductivity value lower than that prepared without additive. Finally, when CA is employed during the PPy synthesis as a HPA-related compound carrying an additional hydroxyl group, low conducting PPy is obtained (Table 2). On the other hand, the reaction yield remains low (10±20%) without additives as well as in the presence of HBA and HPA. However, in the case of CA the reaction yield increases in parallel to the reduction of PPy conductivity, pointing to a certain incorporation of the additive onto the polypyrrole at high CA contents. Aging experiments were performed at 150 8C to determine the thermal stability of PPy synthesized in the presence of HBA and HPA. As shown in Fig. 1 (entries C2 and C3), both compounds provide enhanced thermal stability when compared to PPy synthesized without additives (Fig. 1, entry C1). Furthermore, for PPy synthesized with [HPA 0:025 M, a conductivity overshoot is observed during the ®rst heating stages that could be attributed to annealing effects that enhance the orientation of the PPy chains [12]. To determine whether the stabilizing effect of both HBA and HPA is due to the incorporation of the additives to the conducting polymer, elemental analysis were performed for different PPy samples. According to the data given in Table 3, no signi®cant differences in chemical composition were observed by this technique. Additionally, the FTIR bands of the additives (i.e. carbonyl stretching band) were not found in the FTIR spectra of the PPy synthesized in the presence of HBA and HPA. These results indicate that these organic additives play their role during the PPy synthesis and they are not present in the ®nal PPy products. Table 3 Elemental analysisa data for PPy synthesized in the presence of HBA- and HCA-related compounds Additive
Concentration (M)
C
H
N
± HBA HPA CA HCA HMA UA
± 0.250 0.025 0.010 0.025 0.025 0.010
4.0 4.5 4.0 4.1 4.9 7.9 4.0
3.7 3.7 3.1 3.0 3.9 7.7 3.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0
a Oxygen and chloride cannot be determined in the employed equipment.
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Fig. 1. Evolution of the conductivity upon aging for different periods of time at 150 8C for PPy samples synthesized without additive (C1); in the presence of [HBA 0:25 M (C2); [HPA 0:025 M (C3); [UA 0:01 M (C4); and [HCA 0:025 M (C5).
Fig. 2. UV absorption spectra of the additive/FeCl36H2O/water/methanol (1/1) solution containing 0.25 M HBA (continuous line) and 0.025 M HPA (broken line) after subtraction of the UV absorption spectra of the HBA/ water/methanol (1/1) solution and the HPA/water/methanol (1/1) solution, respectively. The concentration of FeCl36H2O employed was 0.5 M.
In this sense, the starting FeCl36H2O/organic additive solutions were investigated by UV spectroscopy. HBA and HPA dissolved in water/methanol (1/1) mixtures shown sharp UV absorption bands centered at 296 and 311 nm, respectively. Upon addition of FeCl36H2O to the solution containing HBA, a new broad band with a maximum at 509 nm (see Fig. 2) is clearly seen in the difference UV spectrum obtained after subtraction of the UV spectrum of the HBA/water/methanol solution.1 The presence of such a 1
Alternatively, the UV spectrum of the FeCl36H2O/water/methanol solution was subtracted from the UV spectra of the additive/FeCl36H2O/ water/methanol solution obtaining near identical results (data not shown).
new non-symmetrical band suggests the formation of a distribution of different charge±transfer complexes between HBA as electron donor and iron(III) as electron acceptor, the most prominent species absorbing at 509 nm. It is well known that iron(III) has a marked preference for O-donor ligands, leading to a rich variety Fe±O charge±transfer complexes [13]. For the HPA/FeCl36H2O/water/methanol solution, two new UV bands at 535 and 563 nm are observed in the difference UV spectrum resulting from the subtraction of the UV spectrum of the HPA/water/methanol solution (Fig. 2) (see footnote No. 1). The presence of two UV absorption bands can be rationalized by taking into account that HPA has an additional binding site compared to HBA and also that keto-enol tautomerism is involved in this compound [14]. If pyrrole is added to the HBA (or HPA)/FeCl36H2O/ water/methanol solution, the rapid growing of the broad PPy UV absorption band centered at 880 nm is observed. Due to the strong absorption of the resulting black PPy, no clear information about the evolution of the HBA-iron(III) UV band can be ascertained. When 2,5-dimethylpyrrole (a reported non-polymerizable pyrrole [7]) was added to the HBA/FeCl36H2O/water/methanol solution, a progressive displacement of the band to higher wavenumbers was found (from 509 to 565 nm). This could be attributed to a certain interaction of the pyrrole derivative with the HBA-iron(III) charge±transfer complexes. A similar interaction is expected to be involved also for neat pyrrole during chemical oxidative polymerization. The formation of charge±transfer complexes between the organic additives and iron(III) affects also the redox potential of the FeCl36H2O-containing solutions even at very low additive concentration. Hence, the redox potential of the FeCl36H2O/water/methanol solution without additives was
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657 mV at 0 8C. The redox potential decreased upon addition of HBA (0.025 M), HPA (0.025 M) and CA (0.010 M) to 655, 628 and 532 mV, respectively. PPy conductivity values of 58, 76, 74 and 9 S/cm were obtained by using solutions without additive, with HBA (0.025 M), HPA (0.025 M) and CA (0.010 M), respectively, showing no linear effect of the redox potential in the PPy conductivity. 3.2. Chemical oxidative polymerization of pyrrole in the presence of HCA-related compounds To improve the charge±transfer interaction between the organic additive and iron(III) during the chemical oxidative polymerization of pyrrole, HCA and two HCA-related derivatives (HMA and UA) were investigated (see Table 1). HCA incorporates a conjugated double bond next to the carboxylic moiety that could improve the electron donor effect of the compound when compared to HBA. On the other hand, HMA is a HCA-related compound carrying an additional methoxy group whereas in UA the phenolic moiety is formally replaced by the imidazole moiety. Conductivity data of PPy synthesized in the presence of HCA, HMA and UA as additives are shown in Table 2. As it can be seen, a maximum PPy conductivity of 123 S/cm is observed at a concentration of HCA 0.025 M corresponding to a HCA/FeCl36H2O ratio of 1/20. Contrary to HBArelated compounds, at high HCA contents the PPy conductivity obtained is not below that of the PPy control. As it can be seen in Table 2, HMA is not a bene®cial additive to improve the PPy conductivity. Even very low amounts of this compound in the reaction media leads to very low conducting PPy. When UA is employed (Table 2), a small decrease in PPy conductivity (about 27% at [UA 0:25 M) is observed. Elemental analysis data show that incorporation of the additive to the PPy is taking place only for HMA (Table 3). Reaction yields were 10±14% using HCA and UA, whereas higher yields were obtained in the case of HMA. Concerning the stability of the PPy synthesized in the presence of HCA and UA, a clear enhancement is found for both additives as illustrated in Fig. 1. In particular, HCA provides enhanced PPy stability even employing very low contents during the conducting polymer synthesis ([HCA 0:004 M). The best results are obtained for a concentration of HCA 0.125 M. It is worth noticing that UA improves PPy stability (Fig. 1) but does not PPy conductivity (Table 2). Ultraviolet absorption bands of HCA-iron(III) charge± transfer complexes were also clearly identi®ed in the difference UV spectra of the FeCl36H2O/HCA/water/methanol starting solutions, as illustrated in Fig. 3. In this case, the UV absorption maximum is located at 501 nm with a tail up to 750 nm suggesting a distribution of species. Conversely, UA shows only a broad UV band of lower intensity, as illustrated in Fig. 3. The formation of additive-iron(III) charge±transfer complexes is also re¯ected in the redox potential data of the
Fig. 3. UV absorption spectra of the additive/FeCl36H2O/water/methanol (1/1) solution containing 0.025 M HCA (continuous line) and 0.01 M UA (broken line) after subtraction of the UV absorption spectra of the HCA/ water/methanol (1/1) solution and the UA/water/methanol (1/1) solution, respectively. The concentration of FeCl36H2O employed was 0.5 M.
additive-containing starting solutions. Thus, redox potential values at 0 8C of 640, 528 and 643 mV were obtained for FeCl36H2O/water/methanol solutions containing HCA (0.025 M), HMA (0.025 M) and UA (0.010 M), respectively. PPy conductivity values of 123, 0.1 and 54 S/cm were obtained by using the above HCA-, HMA- and UA-containing solutions. 4. Conclusions The presence of HBA- and HCA-related compounds during the chemical oxidative polymerization of pyrrole using ferric chloride solutions in water/methanol mixtures have profound effects on the properties of the obtained PPy. Thus, HBA, HPA and HCA improve both the electrical conductivity and the thermal stability of the resulting PPy. On the other hand, UA, CA and HMA lead to PPy of lower conductivity values although PPy obtained in the presence of UA shows improved thermal stability. It is worth to remark that the PPy obtained in the presence of HCA shows a conductivity value of 123 S/cm. This is one of the ®rst examples where conductivities higher than 100 S/cm have been obtained for PPy by chemical oxidative polymerization. Furthermore, the PPy obtained in the presence of HCA shows an improved thermal stability which is a critical parameter for many applications. Future work will focus on the blending and processing of these products with thermoplastic materials. According to elemental analysis data and FTIR spectroscopy results on the obtained PPy, the additives are not present to a detectable level into the PPy products except for the HMA compound, suggesting that the effect of the organic additive takes place during the polymerization. Thus, the presence of HBA- and HCA-related compounds
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modify the redox potential of the starting ferric chloride solutions. It is worth noticing that for redox potentials bellow 550 mV (i.e. for CA and HMA additives) low conducting PPy is obtained. For redox potentials above 550 mV, no clear correlation is found between redox potential and PPy conductivity since starting solutions with similar redox potential values lead to PPy of different conductivity. Furthermore, important changes were observed in the UV spectra of the oxidant solutions after the addition of the organic additives. These data shows that the additives have an effect in the active species for the polymerization affecting their redox potential and chemical structure. This suggest that the changes on the active species leads to a PPy of less structural defects and/or longer conjugation length, hence, enhancing both PPy conductivity and stability. A more complete characterization of the PPy and the starting additive-iron(III) solutions by X-ray spectroscopy [15] would help to understand the involved kinetic mechanisms in these reactions and will be the subject of a future publication. Acknowledgements The authors gratefully acknowledge Yolanda Alesanco for kindly experimental assistance. Also, P.A.C. would like to thanks the Spanish Government for the support of this
work through a MIT-F2 Grant and Juan I. Guerra for his help with the computer treatment of graphics. References [1] C.O. Yoon, H.K. Sung, J.H. Kim, E. Barsoukov, J.H. Kim, H. Lee, Synth. Metals 99 (1999) 201. [2] P. Chandrasekhar, Conducting Polymers, Fundamental and Applications. A Practical Approach, Kluwer Academic Publishers, Norwell, MA, 1999, p. 381. [3] L. Rupprecht (Ed.), Conductive Polymers and Plastics in Industrial Applications, Noreich, New York, 1999, p. 69. [4] J.A. Pomposo, J. RodrõÂguez, H. Grande, Synth. Metals 104 (1999) 107. [5] E.T. Kang, K.G. Neoh, Y.K. Ong, K.L. Tan, B.T.G. Tan, Macromolecules 24 (1991) 2822. [6] H.C. Kang, K.E. Geckeler, Polymer 41 (2000) 6931. [7] R.E. Meyers, J. Electron. Mater. 15 (1986) 61. [8] S. Machida, S. Miyata, A. Techagumpuch, Synth. Metals 31 (1989) 311. [9] T. Yoshikawa, S. Machida, T. Ikegami, A. Techagumpuch, S. Miyata, Polym. J. 22 (1990) 1. [10] S. Rapi, V. Bocchi, G.P. Gardini, Synth. Metals 24 (1988) 217. [11] Y. Kudo, K. Akami, Y. Matsuya, Synth. Metals 95 (1998) 191. [12] R. Turku, C. Neamtu, M. Brie, Synth. Metals 53 (1993) 325. [13] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1984, p. 1265. [14] J. March, Advanced Organic Chemistry. Reactions, Mechanisms, and Structure, Wiley, New York, 1992, p. 72. [15] M. Magini, J. Chem. Phys. 76 (1982) 1111.