Polypyrrole conditions

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

The stoichiometry of the redox reactions of conducting polymers (n and m in reactions 1 and 2) is quite variable. Under the most widely used conditions, polypyrroles and polythiophenes can be reversibly oxidized to a level of one hole per ca. 3 monomer units (i.e., a degree of oxidation, n, of ca. 0.3).7 However, this limit is dictated by the stability of the oxidized film under the conditions employed (Section V). With particularly dry and unreactive solvents, degrees of oxidation of 0.5 can be reversibly attained,37 and for poly-(4,4 -dimethoxybithiophene), a value of n = 1 has been reported.38 Although much fewer data are available for n-doping, it appears to involve similar stoichiometries [i.e., m in Eq. (2) is typically ca. 0.3].34,39"41 Polyanilines can in principle be reversibly p-doped to one... 

In connection with this problem it should be mentioned that 02-formation was found at CdS electrodes coated with polypyrrole and RUO2 under anodic polarization whereby the anodic decomposition could be considerably reduced. Under open circuit conditions only H2-evolution was observed, whereas O2 could obviously not be detected. This result is not in contradiction to the first experiment because the Fermi level can pass the electrochemical potential of H2O/O2 under bias. Very recently it was reported on photocleavage of H2O at catalyst loaded CdS-particels in the... 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

A large number of different pyrrole-based polymers have now been electrochemically synthesised, using a variety of conditions, and these are summarised in Table 2, although it should be noted that the size of this field and its rate of growth mean that it is impossible to make such a table completely comprehensive, and that reports of related new materials, particularly of copolymers incorporating pyrrole are continually appearing in the literature. Water-soluble polypyrroles have also recently been reported [246],... 

Figure 11 The reaction scheme for the polypyrrole precursor (11) synthesis (left top) with reagents and conditions (left hottom). Representative pyrrole (12) and thiophene (13 and 14)-based redox-active polypyrroles. (Adapted from refs. 32 and 33.)...

It should be emphasised here that very different growth mechanisms are found according to the conditions of the experiment and that the study of the growth of conducting polymers such as polypyrrole represents a major area of investigation in its own right. The work of Christensen and Hamnett (1991) on the growth of polypyrrole should not, therefore, be taken as representative, it is specific to the conditions they used. 

Polypyrrole readily forms acceptable films under a wide variety of conditions [86] though there are subtle distinctions in behaviour as a result of exact preparation procedure [87]. Ultrasound at 20 kHz at sufficient intensity impedes polypyrrole formation and removes the polymer coating from the electrode [88]. At higher ultrasonic frequencies (e.g. 800 kHz) a free-standing film is produced which can be peeled from the electrode. This film has the interesting feature that the imprint of the wave-... 

In general, polypyrrole can be prepared via electrochemical or chemical oxidative polymerization of pyrrole involving different highly reactive intermediates. Also, the pyrrole/cyclodextrin complex can be polymerized in aqueous solution under oxidative conditions by adding potassium peroxodisulfate as an ox-... 

In potentiometric sensors, an electrical potential between the working electrode and a reference electrode is measured at zero current conditions in a solution containing ions that exchange with the surface. The first potentiometric MIP sensor was prepared in 1992 by Vinokurov (1992). The substrate-selective polyaniline electrode was electrosynthesized with polypyrrole, polyaniline, and aniline-p-aminophenol copolymers. The development of an MIP-based potentiometric sensor was reported in 1995 by Hutchins and Bachas (1995). This potentiometric sensor has high selectivity for nitrite with a low detection limit of (2 + l)x 10 M (Fig. 15.10). 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

In order to get high-quality polymer films, it is essential to select best conditions of the electrolytic solution, electrode potential, temperature, etc. For synthesizing the films of polypyrrole, polythiophene, polyazulene, and their derivatives,... 

A nitrate-selective potentiometric MIP chemosensor has been devised [197, 198]. For preparation of this chemosensor, a polypyrrole film was deposited by pyrrole electropolymerization on a glassy carbon electrode (GCE) in aqueous solution of the nitrate template. Potentiostatic conditions of electropolymerization used were optimized for enhanced affinity of the resulting MIP film towards this template. In effect, selectivity of the chemosensor towards nitrate was much higher than that to the interfering perchlorate ( o3 cio4 = 5.7 x 10-2) or iodide ( N03, r = x 10 2) anion. Moreover, with the use of this MIP chemosensor the selectivity of the nitrate detection has been improved, as compared to those of commercial ISEs, by four orders of magnitude at the linear concentration range of 50 pM to 0.5 M and LOD for nitrate of (20 10) pM [197]. 

A more favourable approach is the incorporation of the active species in an electrically conducting polymer layer which then acts as an (electrical) intermediate between the electrode surface and the catalyst. Polypyrrole is considered to be especially suitable because it is acceptably stable under ambient conditions (2), has a high conductivity and can be easily prepared electrochemically from a great variety of solvent systems, including aqueous solutions (3-5). The catalytic species that have been applied in such polypyrrole-based systems comprise metal particles (6-9), metal chelates (10-13) (with anionic side groups) and enzymes (14-18). 

Several conjugated polymers such as polyaniline, polypyrrole and poly thiophene have been prepared by chemical methods using doping. The electrical conductivity of these polymers has been controlled by (i) the type of dopant, (ii) the concentration of doping, (iii) the conditions of doping (the current density, temperature of reaction, etc.)... 

TABLE 1. Effect of experimental conditions on the film thickness of polypyrrole. 

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