«1. Introduction From its revolutionary discovery, up to day electroconducting polymers attracts attention of researchers along scientific community. ...»
Electrochemical Polymerization of Aniline
Milica M. Gvozdenović1,*, Branimir Z. Jugović2, Jasmina S. Stevanović3,
Tomislav Lj. Trišović2 and Branimir N. Grgur1
1Department of Physical Chemistry and Electrochemistry,
Faculty of Technology and Metallurgy, University of Belgrade, Belgrade,
2Institute of Technical Sciences, Serbian Academy of Science and Arts,
3Center for Electrochemistry, Institute of Technology and Metallurgy, Belgrade
From its revolutionary discovery, up to day electroconducting polymers attracts attention of researchers along scientific community. Among numerous known electroconducting polymers, polyaniline and its derivates are probably the most investigated. Such popularity, in both theoretical and practical aspects, is a consequence of its unique properties: existence of various oxidation states, electrical and optical activity, low cost monomer, red/ox reversibility, environmental stability etc. (Inzelt, 2008; Horvat-Radošević & Kvastek, 2006; Gospodinova & Terlemezyan, 1998). These diverse and important features seem to be promising in vide area of practical applications in: rechargeable power sources (Gvozdenović et al., 2011; Jugović et al., 2006; Jugović et al., 2009), sensors (Bezbradica et al., 2011; Dhaoui, 2008; Grummt et al. 1997, Gvozdenović et al., 2011; Mu & Xue, 1996), magnetic shielding, electrochemical capacitors (Xu et al., 2009), electrochromic devices (Kobayashi et al. as cited in Mu, 2004; Wallace et al., 2009), corrosion protection etc. (Biallozor & Kupniewska, 2005; Camalet et al., 1996, 1998, 2000a, 2000b; Gvozdenović & Grgur, 2009; Gvozdenović et al., 2011; Kraljić et al., 2003; Popović & Grgur, 2004; Popović et al. 2005, Grgur et al., 2006; Özylimaz et al., 2006). Polyaniline is commonly obtained by chemical or electrochemical oxidative polymerization of aniline (Elkais et al., 2011; Jugović et al., 2009; Lapkowski, 1990; Stejskal & Gilbert, 2002; Steiskal at al., 2010) although photochemically initiated polymerization (Kim et al., 2001 as cited in Wallace et al., 2009; Teshima et al., 1998) and enzyme catalyzed polymerization were also reported (Bhadra et al., 2009; Nagarajan et al., 2000).
Polyanilne obtained by electrochemical polymerization is usually deposited on the electrode, however electrochydrodinamic route was also developed resulting in polyaniline colloids of specific functionalities (Wallace et al., 2009). Electrochemical polymerization of aniline is routinely carried out in strongly acidic aqueous electrolytes, through generally accepted mechanism which involves formation of anilinium radical cation by aniline oxidation on the electrode (Hussain & Kumar, 2003). Electrochemical polymerization of aniline is proved to be auto-catalyzed (Mu & Kan, 1995; Mu et al., 1997). The experimental conditions, such as: electrode material, electrolyte composition, dopant anions, pH of the Corresponding Author *
electrolyte etc., all have strong influence on the nature of the polymerization process (Camalet et al., 2000; Ćirić Marjanović et al., 2006; Córdova et al., 1994; Duić & Mandić, 1992;
Giz et al., 2000; Gvozdenović & Grgur, 2009; Inzlet, 2008; Lippe & Holze, 1992; Mu & Kan, 1998; Mun & Kan, 1998; Nunziante & Pistoia, 1989; Okamoto & Kotaka, 1998a, 1998b, 1999;
Popović & Grgur, 2004; Pron et al., 1993; Pron & Rannou, 2010; Wallace et al., 2009). The low pH is almost always needed for preparation of the conductive polyaniline in the form of emeraldine salt, since it is evidenced that at higher pH, the deposited film is consisted of low chain oligomeric material (Stejskal et al., 2010). The doping anion incorporated into polymer usually determines the morphology, conductivity, rate of the polyaniline growth during electrochemical polymerization, and has influence on degradation process (Córdova et al., 1994; Lippe & Holze, 1992; Mandić et al., 1997; Okamoto & Kotaka, 1999, Pron & Rannou, 2010). The electrochemical polymerization of aniline is practically always carried out in aqueous electrolytes, although polymerization in organic solvents such is acetonitrile was also reported. Recently it was observed that ionic liquids electrolytes might be used for successful preparation of conductive polyaniline (Heinze et al., 2010; Wallace et al., 2009).
Finally, electrochemical polymerization and co-polymerization of numerous substituted aniline derivates, resulted in polymer materials with properties different from the parent polymer, were also investigated (Karyakin et al. 1994; Kumar, 2000; Mattoso & Bulhões, 1992; Mu, 2011; Zhang, 2006) Since there is a still growing interest for the research in the field of conducting polymers with polyaniline as the most representative, the aim of this paper is to review the extremely rich literature attempting to describe all important aspects of electrochemical polymerization of aniline.
2. Polyaniline Polyaniline is probably the eldest known electroconducting polymer, since it was used for textile coloring one century ago (Sayed & Dinesan, 1991; Wallace et al., 2009).The great interest in research of polyaniline is connected to discovery of its conductivity in the form of emeraldine salt and existence of different oxidation forms (Inzelt, 2008; MacDiarmid et al., as cited in Wallace et al., 2009; Syed & Dinesan, 1991).
2.1 Different oxidation states of polyaniline Unlike other know electroconducting polymers, polyaniline can exist, depending on degree of oxidation, in different forms, known as: leuoemeraldine, emeraldine and perningraniline.
Leucoemeraldine, eg. leucoemeraldine base, refers to fully reduced form; emeraldine, eg.
emeraldine base, is half-oxidized, while perningraniline, eg. perningraniline base, is completely oxidized form of polyaniline. The only conducting form of polyaniline is emeraldin salt, obtained by doping or protonation of emeraldine base (Fedorko et al., 2010;
MacDiarmid et al., 1987, Pron & Rannou, 2002).
The unique feature of mentioned polyaniline forms is ease of its mutual conversions by both chemical and electrochemical reactions as it can be seen in Fig.1. (Gospodinova & Terlemezyan, 1998; Kang et al., 1998; Stejskal et al.,1996).
Apart from the changes in oxidation levels, all the transitions among polyaniline forms are manifested by color and conductivity changes (Stejskal et al., 1996). The conducting protonated emeraldine in the form of green emeraldine salt, obtained as a product of
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electrochemical polymerization of aniline in acidic electrolytes, can be easily transformed by further oxidation to fully oxidized dark blue perningraniline salt, which can be treated by alkali to form violet perningraniline. Emeraldine salt can also be reduced to transparent leucoemeraldine, or can be transformed by alkali to blue non conducting emeraldine. The two blue forms of polyanilne, perningranilne salt and emeraldine have different shades of blue (Stejskal et al., 1996). Both reduction of emeraldine salt to leucoemeraldine and oxidation to perningraniline states are followed by decrease in conductivity (Stejskal et al., 2010).
Fig. 1. Different forms of polyaniline
2.2 Polyaniline conductivity The mechanism of polyaniline conductivity differs from other electroconducting polymers, owing to the fact that nitrogen atom is involved in the formation of radical cation, unlike most of the electroconducting polymers whose radical cation is formed at carbon. On the other hand, nitrogen is also involved in the conjugated double bonds system. Therefore, electrical conductivity of polyaniline is dependent both on the oxidation and protonation degrees (Fedorko et al., 2010; Genies at al., 1990; Pron & Rannou, 2002; Wallace et al., 2009).
As mentioned before, polyaniline is characterized by existence of various oxidation forms.
Polyaniline in the form of emeraldine base can be doped (protonated) to conducting form of emeraldine salt. Emeraldine base, half oxidized form, is consisted of equal amount of amine (-NH-) and imine (=NH-) sites. Imine sites are subjected to protonation to form bipolarone or dicatione (emeraldine salt form). Bipolarone is further dissociated by injection of two electrons both from electron pairs of two imine nitrogen, into quinodiimine ring, and the third double bond of benzenoid ring is formed (Stejskal, 2010).
Unpared electrons at nitrogen atoms are cation radicals, but essentially they represent polarons. The polaron lattice, responible for high conductivity of polyaniline in the form of emeraldine salt is formed by redistribution of polarons along polymer chain, according to shematic representation given in Fig. 2. (Wallace et al., 2009) Although both bipolaron and polaron theoretical models of emeraldine salt conductivity were proposed (Angelopulus et al., as cited in Wallace et al., 2009; Tanaka et al., 1990), it was lately confirmed that, beside from the fact that few of spineless bipolarons exist in www.intechopen.com 80 Electropolymerization polyaniline, formation of polarons as charge carriers explained high conductivity of polyaniline (Mu et al., 1998; Patil et al., 2002). As mentioned, unique property of polyaniline is conductivity dependence on the doping (proton) level (Chiang et al., 1986; Wallace et al., 2009). The maximal conductivity of polyaniline is achieved at doping degree of 50%, which corresponds to polyaniline in the form of emeraldine salt (Tanaka et al., 1989). For higher doping degrees some of the amine sites are protonated, while lower doping degrees means that some of the imine sites were left unprotonated (Wallace et al., 2009), explaining why, in the light of the polaron conductivity model, reduction of emeraldine salt to leucoemeraldine and oxidation to perningraniline states decrease the conductivity. The order of magnitude for conductivity varies from 10-2 S cm-1, for undoped emeraldine, up to 103 S cm-1 for doped emeraldine salt (Inzelt, 2008; Wallace et al., 2009).
Fig. 2. Schematic presentation of polyaniline conductivity Beside the fact that doping degree has the pronounced effect on the conductivity, various other factors such as: moisture amount (Kahol et al., 1997; Patil et al., 2002), morphology (Monkman & Adams, 1991; Zhou et al., 2007), temperature (Probst & Holze, 1995) etc. were also found to had influence on the polyaniline conductivity.
3. Mechanism and kinetics of the electrochemical polymerization of aniline Generally, electroconducting polymers are obtained by either chemical or electrochemical oxidative polymerization, although reductive polymerization was also reported (Heinze et al., 2010; Inzlet et al., 2000; Yamamoto & Okida, 1999;Yamamoto, 2003, as cited in Inzelt, 2008). Chemical polymerization is used when large quantity of polymer is requested.
Electrochemical polymerization is favorable, since in the most cases the polymer is directly deposited on the electrode facilitating analysis. On the other hand, electrochemical polymerization is especially useful if polymer film electrode is needed. By proper design of
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the electrochemical experiment, polymer thickness and conductivity can be easily controlled. It is believed that electrochemical polymerization is consisted of three different steps, in first, oxidation of the monomer at anode lead to formation of soluble oligomers in the diffusion layer, in the second, deposition of oligomers occurs through nucleation and growth process, and finally, the third step is responsible for chain propagation by solid state polymerization (Heinze et al., 2010). Unfortunately, a general mechanism of electrochemical polymerization could not be established, since it was evidenced that various factors had influence. However, it was observed that first step of the electrochemical polymerization was formation of reactive cation radicals (Heinze et al., 2010; Kankare, 1998).
The next step, strongly dependent on the experimental conditions, is believed to be essential for the polymer growth (Inzelt, 2008). The knowledge on the kinetics of the nucleation and growth process during electrochemical synthesis of electroconducting polymers is also of great interest, since it would be useful in control of the morphology, density, crystallinity etc. of the desired polymer.
3.1 Mechanism Both the mechanism and the kinetics of the electrochemical polymerization of aniline were extensively investigated (Andrade et al. 1998; Arsov et al. 1998; Carlos et al. 1997; Hussain & Kumar, 2003; Inzelt et al., 2000, Bade et al. 1992; Lapkowski, 1990; Malinauskas et Holze, 1998;
Mandić et al., 1997; Mu & Kan, 1995; Mu et al., 1997). Electrochemical, similarly to chemical, polymerization of aniline is carried out only in acidic electrolyte, since higher pH leads to formation of short conjugation oligomeric material, with different nature (Wallace et al., 2009).
As stated before, it is generally accepted that the first step of the polymerization process of aniline involves formation of aniline cation radicals, by anodic oxidation on the electrode surface, which is considered to be the rate-determining step (Zotti et al., 1987, 1988). The existence of aniline radical cation was experimentally confirmed, by introducing molecules, (resorcinol, hydroquinone, benzoquinine etc.), capable of retarding or even stopping the reaction, which evidenced a radical mechanism (Mu et al., 1997). The oxidation of the aniline monomer is an irreversible process, occurring at higher positive potentials than redox potential of the polyaniline (Inzelt, 2008).