Conducting polymers anion release

As mentioned above in this section, PTh is oxidized with two oxidation peaks at around 0.28 and 0.58 V vs. Ag/Ag" in acetonitrile [1641. When oxidized to the second oxidation peak, however, the oxidation product is not stable and loses its electroactivity with protons released during the reverse scan [175]. When the potential is scanned to the first oxidation peak, the electrochemical conversion is chemically reversible with its oxidation product stable. This process is accompanied by the ion transport to maintain the electroneutrality as for oxidation of other conducting polymers. This process w.is found to be significantly dependent on the electrolyte for its shape as well as kinetics [21c]. Cations were also found to affect the redox processes of PTh [175]. These results indicate that both anions and cations can affect the redox chemistry of PThs as for PPy, depending on relative sizes/diffusion coefficients of anions or cations [176,177]. 

Figure 11.42 Mechanism of corrosion protection by conducting polymers and anion (DOP ) release. The mechanism was suggested by Kinlen et...

The QCM has recently been introduced to monitor the mass associated with the ion insertion process in conducting polymers. Most of the polymers insert and release anions upon redox cycling, and the QCM has been used to determine whether this is the only process which occurs. A typical experimental arrangement for such studies is shown in Figure 2.6. 

The redox properties of conducting heterocyclic polymers like polypyrrole are central to many applications of these materials. For this reason the electrochemistry of thin films of these polymers have received a lot of attention. For use in electrically controlled ion binding and delivery, the general idea is that the heterocyclic conducting polymers have cationic backbones and will incorporate counter anions. Upon reduction of the backbone, the anions will be flushed out. Thus, in principle one can develop devices to absorb anions of interest or to release them in response to an electric current. Our work has been spurred by the possibility of delivering drugs with a rate controlled by the current. 

Since suitable conducting polymers with anionic backbones are not available, we have resorted to a more complex approach for cation binding. Following Martin s workio on terpolymer redox films of polyvinylferrocene, we produced a new type of composite polymer, of cationic poly-(N-methylpyrrole) (PMP" ) with anionic poly-styrenesulfonate (PSS ). Upon reduction of a film of this material, cations are taken up. Thus, the entangled polymeric anion is not flushed out by reduction of the pyrrole units. Instead, cations are incorporated to balance the sulfonate charges. This has been shown for a variety of cations including protonated dopamine. The scheme below shows how this polymer works to bind protonated dimethyldopamine (DH ) cathodically and to release it anodically. 

Similar studies were carried out using the polyelectrolyte Kodak-AQ poly(ester sulfonic acid) and polypyrrole. The polyelectrolyte is also incorporated as a counteranion during the anodic polymerization of pyrrole onto a glassy carbon electrode [135]. The resulting film was tested as an ion-exchange membrane. The uptake of the Ru(II) tris-bipyridil complex by the AQ" anion, immobilized in the conductive polymer, was facilitated by reduction in the polypyrrole chains. Similarly, the release of loaded cations can be controlled by oxidation of polypyrrole. 

Several applications of conducting polymers have been investigated in the medical field, particularly for the controlled release of various anions such as glutamate and ferrocyanide (10). Biosensors have been produced in which enzymes, antibodies, and even whole living cells have been incorporated into the polymer structure. 

The self-healing effect provided by dopant ions has been observed in the woric of Dominis etal (2003) for emeraldine salt primers loaded with different eorrosion inhibitors. Also, in the work of Kinlen et al (2002), PANi formulations with phosphonic acid derivatives as dopant anions showed better corrosion protection performances than formulations with sulfonic acids and derivatives. Kendig et al (2003) also reported the development of smart coatings based on conducting polymers doped with different corrosion inhibitors, showing that the release of inhibitors was triggered by the electrochemical activity at defects. In the work of Paliwoda-Porebska et al (2005, 2006) the cathodic delamination rate in iron coated with PPy doped with phosphomolybdates was fonnd to decrease in comparison with PPy doped with molybdates. 

When oxalic acid is used to form the polypyrrole coating, the result is a film that is very stable in time without a significant change in the corrosion behaviour properties after 72 h of immersion to sodium chloride solution. The net corrosion current naturally increases after 72 h of immersion and is followed by a decrease in the polarisation resistance, but overall, it is a very stable system, with a safe passivation range (Fig. 15.9a,b. Table 15.5). Over-oxidation of a part of polypyrrole films would have occurred as discussed, but the amount of degraded polypyrrole should be small as the release of doping anions, a property of well formed conducting polymer film, is clearly seen. 

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