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Figure 2. UV-visible spectrum of a PPy HQS film grown galvanostati-cally onto ITO glass from a solution containing 0.2 M pyrrole and 0.1 M HQS. Reprinted with permission from [188], V. Misoska, Ph.D. Thesis, University of Wollongong, 2002.

ubilizing polypyrrole in organic solvents [31, 32]. The presence of alkyl and alkoxy substituents on the backbone of polypyrrole also increases their solubility in organic solvents. In contrast, polymerization of pyrrole monomers bearing sulfonated substituents has provided water soluble polypyrroles [33]. A very useful recent advance has been the development of a facile route for the modification of preformed polypyrroles containing good leaving group such as N-hydroxysuccinamide [34].

An important feature of these ICP structures is that they are amenable to facile oxidation/reduction processes that can be initiated at moderate potentials. For polypyrrole the oxidation state can be reversibly switched, as shown in Eq. (2). The doped oxidized forms exhibit good electrical conductivity (a = 1-100 S cm-1), while the reduced forms have very low conductivity (a ~ 10-8 S cm-1). The dynamic character of these polymer systems, with chemical, physical, and mechanical properties being a function of applied potential, is intriguing and is the basis of their proposed use in intelligent material systems [19].

If the dopant anion (A-) is small and mobile (e.g., Cl-), then upon reduction the anion will be efficiently ejected from the polymer [Eq. (2)]. However, extensive studies with polypyrroles have shown that if the dopant is large and immobile (e.g., if A- is a polyelectrolyte such as polystyrene sulfonate), an electrically induced cation exchange process occurs, according to

where the cation (X+ ) is incorporated from the supporting electrolyte solution. The effect of the original dopant incorporated as well as the other ions in the electrolyte on the electrochemical switching characteristics of polypyrroles has been clearly demonstrated [30].

Table 1. Effect of the counterion on the tensile strength and conductivity of polypyrrole membranes [30].

Membrane

PPy/BSA

PPy/PTS

PPy/EBS

PPy/MS

PPy/BS

PPy/NPS

Counterions

Tensile strength (MPa) Conductivity (S/cm)

SO3Na

0 0

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