Neutral polypyrrole

Figure 3.70 Room temperature optical absorption spectra of a 45(X)A-lhick film of neutral polypyrrole doped with at0.03 torr. (a) before exposure to I2, conductivity <10 6fl 1cm"1 (b) after 2 minutes I2 exposure, conductivity 4.8 ft em 1 (c) after 7 minutes exposure, conductivity 6,7 ft em 1 (d) after 22 minutes I2 exposure, conductivity 32ft cm The three structures seen on the low-energy side of (a) (c) arc possibly artifacts due to interference effects in the films. From Pfluger et at. (1983).

Figure 12.1. Structures of neutral polypyrrole. The idealized structure appears in (a). The various defects believed to be present are illustrated in (b). Reprinted with permission from ref 1, copyright 1995, American Chemical Society.

The oxidation of a neutral polypyrrole film with chemical oxidizing agents increases the conductivity relative to that of the electrochemical oxidized materials [85]. 2,2 -Bipyrrole has been used as monomer, but the properties of the polymer obtained are similar to those of the parent polymer obtained from pyrrole itself [86]. 

Yellow-green films of neutral polypyrrole can be prepared by the electrochemical reduction of the perchlorate films. This is an insulator with a -10" S cm". ... 

Moreover, doped polypyrrole is relatively stable in air at open circuit [126]. However, when undoped polypyrrole is exposed to oxygen in air or to an electrolytic solution containing dissolved O2, it is easily oxidized [127]. Oxidation of neutral polypyrrole is thermodynamically favored because the standard reduction potential of Oj is more positive than that of polypyrrole. Possible reactions are [128] ... 

There are two major differences between conjugated and redox polymers. These differences are of a practical rather than a fundamental nature. First, the native conjugated polymers can be brought into a state in which they do not contain charged sites (neutral polypyrrole, polythiophene, etc.). In this state the polymer is rather hydrophobic oxidation/reduction to an ionized state involves a significant structural change. Second, the polymeric redox sites (polarons, etc.) are less clearly defined than redox polymer sites, in particular with respect to their electronic energy levels. 

Fig. 11-39 ESR intensity arid conductivity as a function of oxygen doping of neutral polypyrrole. After Reference [32], reproduced with permission.

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. 

Conducting Polymers Electronically conducting polymers (such as polypyrrole, polythiophene, and polyaniline) have attracted considerable attention due to their ability to switch reversibly between the positively charged conductive state and a neutral, essentially insulating, form and to incorporate and expel anionic species (from and to the surrounding solution), upon oxidation or reduction ... 

The value of the EQCM is exemplified by the data shown in Fig. 17.177 The first reduction of the polypyrrole film was initially accompanied by a mass decrease, as expected for anion expulsion according to Eq. (1). However, after the reduction was ca. 75% complete, the mass began to increase, indicating a switch of the charge neutralization mechanism to cation insertion [Eq. (5)]. 

Simultaneous ESR and electrochemical measurements on a polypyrrole film give convincing evidence that the charging process in this film involves the generation of paramagnetic species which are obviously intermediates in the process of switching from the neutral to the oxidized state In any case, independent of all other findings,... 

As might be expected, the properties of polythiophene show many similarities with those of polypyrrole. As with polypyrrole, polythiophene can be prepared via other routes than electrochemical oxidation both as the neutral material [390-392] or in the p-doped form [393]. This material is produced as an infusible black powder which is insoluble in common solvents (and stable in air up to 360°C), with conductivities ranging from approximately 10 11 Scm-1 in the neutral form [390] to 102 Scm-1 when doped [19, 393, 394]. Early work on thiophene polymers showed that the p-doped material is air-sensitive in that the conductivity decreases on exposure to the atmosphere [20, 395] although no evidence of oxygen-containing species was seen in XPS measurements [19],... 

Elemental analysis shows that the polymer generally contains four monomer units per dopant ion [20], and that there is also more hydrogen than would be expected (cf. polypyrrole) [20, 395], although this may vary depending on the starting material [409,410,414], Even in the neutral form, the polymer contains a small quantity of anions (0.5-1%) [19], although Waltman et al. [400] found that the extent to which the counter ion is incorporated into the polymer on polymerisation depends strongly on the nature of the /J-substituent (if present). 

Polythiophene films can be electrochemically cycled from the neutral to the conducting state with coulombic efficiencies in excess of 95% [443], with little evidence of decomposition of the material up to + 1.4 V vs. SCE in acetonitrile [37, 54, 56, 396,400] (the 3-methyl derivative being particularly stable [396]), but unlike polypyrrole, polythiophene can be both p- and n-doped, although the n-doped material has a lower maximum conductivity [444], Cyclic voltammetry shows two sets of peaks corresponding to the p- and n-doping reactions, with E° values at approximately + 1.1 V and — 1.4 V respectively (vs. an Ag+/Ag reference electrode)... 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

The neutral form of polypyrrole is weakly coloured while the oxidised form is a deep blue/black so that switching the state of the film not only changes its conductivity but is also accompanied by a marked colour change, a phenomenon termed electrochromism. 

The broad nature of the current peaks in the voltammogram of conducting polymers such as poly pyrrole has been interpreted in a number of w one of which was to attribute it to the movement of anions across the polymei, electrolyte interface, a vital process if the overall charge neutrality of the film is to be maintained. The participation of the electrolyte in the electrochemistry of the polymer film is easily seen by comparing the response of polypyrrole in a variety of different electrolytes (see Figure 3.74). 

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. 

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