Doping counter ion

Cao et al. [133] studied the air stability of re-doped polythiophene which was prepared electrochemically and then compensated by aqueous ammonia as detailed in Table 16.8. Ammonia-compensated polythiophene was found to be quite stable when stored in an ambient atmosphere for 3 months, as neither any weight gain nor any change in infra-red spectmm was observed. Both the chemically re-doped and electrochemically prepared polythiophene showed much better stability as compared to polyacetylene and the air stability of the polymer was found to be dependent on the doping counter-ion as well as the solvent used in electrochemical polymerization. Electrochemically prepared polythiophene from a mixture of CH3CN and CH3NO2 (1 1 by volume) maintained their electrical conductivity, whereas the polymer re-doped chemically by FeCls was observed to be most stable in ambient air. 

Palaniappan and Narayana [168] studied the thermal stabilities of chemically prepared polyanilines in different acid media by isothermal heat treatments at 150°, 200°, 275° and 375°C in combination with EPR, electronic absorption and infra-red spectroscopic measurements. They also observed a three-step weight loss, the first step is attributed to the loss of water from the polymer salt starting at 110°C, the second, minor step is due to volatilization of acid ranging from 110°C to 275°C, and the polymer undergoes thermo-oxidative degradation of the polymer backbone in the third step above 275°C. The thermal stability was observed to be dependent on the doping counter-ion and no structural... 

When the substituent is an ionic chain [Fig. 13(b)] with the anion on the organic side, some of the lateral anions act as counter-ions during electrochemical oxidation. The cation of the salt is expelled from, or included in, the material during oxidation or reduction, respectively. These are self-compensating or self-doping (chemical or physical terminology, respectively) materials.76... 

These processes can be made to occur in a number of ways, for example with gas phase reagents such as AsF5 and I2, solution species such as FeCl3 or using electrochemical oxidation and reduction, but regardless of the method used the basic process is the same. If the material is to maintain overall electrical neutrality during and after doping, a counter ion is required, i.e., for p-doped materials,... 

It is a convenient aspect of electrochemical doping of the polymer that the electrolyte can provide the counter ion, although a similar result can be achieved via chemical doping under certain circumstances, e.g., Using a radical ion containing species such as sodium naphthalide, Na+ npthT, i.e.,... 

NB. Where only a current density was given in the original Ref. this is quoted in place of the polymerisation potential. c Counter ions are as incorporated during the electrochemical polymerisation process or by subsequent electrochemical doping unless suffixed with (chem.) which indicates the use of chemical doping. d Conductivities f-film, p-pressed pellet. 

Because their oxidation potentials are similar, substituted pyrroles can be copolymerized with pyrrole, allowing the limiting conductivity of the fully-doped polymer to be varied 194,19S). The oxidation potentials of the monomers, and hence their reactivity ratios, are sensitive to the substituent196). Inganas et al.197) reported the synthesis of pyrrole-thiophene copolymers starting from terthiophene, whose oxidation potential is similar to that of pyrrole. Sundaresan et al.198) copolymerized pyrrole with 3-(pyrrol-l-yl)propanesulphonate to give a polymer in which the sulphonate counter-ion is a part of the polymer structure. 

In their studies of effects of oxidation of polyacetylene on its dopability, Pochan et al.545 reported that iodine-doped polymer loses its conductivity in vacuum and concluded that the I3 counter-ions are able to react with the polymer chain, leading to iodination. Huq and Farrington 5561 found that bromine- and iodine-doped polyacetylenes lose conductivity rapidly at temperatures below 60 °C, whereas samples doped with AsFs are very much more stable. 

Druy et al.562) showed that iodine- and perchlorate-doped samples lose conductivity quite rapidly in vacuum, due to reaction of the polymer with the counter-ion. Yang and Chien 56l) also observed the instability of these doped polymers and showed that the reaction of polyacetylene with perchlorate counter-ions can be explosive. They showed that doped samples are much more stable to oxygen than is the undoped material. Muller et al.565) also observed that the stability of polyacetylene in air depends on the extent of doping, as did Ohtsuka et al.566). Aldissi 5671 has suggested that iodine doped polyacetylene can be stabilized by phenolic antioxidants, although the effect was modest. 

In our own work on Durham polyacetylene 568) wfe find that the stability of doped polymers depends upon the extent of doping. Thus when AsF6 is the counter-ion, a polymer doped to low levels (< 1 mol %) shows very little change in conductivity over a period of days at room temperature in vacuum or dry air, whereas saturation doping (to about 17 mol%) produces a polymer whose conductivity decays rapidly, with ir evidence for the formation of C—F bonds in the polymer. 

Chien 6) has pointed out that doping not only stabilizes polyacetylene towards oxidation but also stabilizes the dopant. The most obvious example is AsF5, which reacts violently with atmospheric moisture but is stabilized in polyacetylene as the AsF6" counter-ion. 

There is no doubt that doped polypyrrole is very much more stable than is polyacetylene, but reports are variable of exactly how stable it is. Street 393) reported that the polymer loses less than 20% of its initial conductivity after one year in air at ambient temperature and Diaz and Kanazawa 589) claimed that the polymer is stable at 100-200 °C, depending on the counter-ion the latter authors also reported that polypyrrole is undoped reversibly by ammonia treatment. Munstedt 590) found that the conductivity of doped polypyrrole was unchanged after 200 days at 80 °C in... 

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