Polypyrroles, properties switching

Following the discovery of the unique electronic properties of polypyrrole, numerous polymers of pyrrole have been crafted. A copolymer of pyrrole and pyrrole-3-carboxylic acid is used in a glucose biosensor, and a copolymer of pyrrole and A-methylpyrrole operates as a redox switching device. Self-doping, low-band gap, and photorefractive pyrrole polymers have been synthesized, and some examples are illustrated [1,5]. 

Kapui et al. prepared a novel type of polypyrrole films [168]. The film was impregnated by spherical styrene-methacrylic acid block copolymer micelles with a hydrophobic core of 18 nm and a hydrophilic corona of 100 nm. The properties of the micelle-doped polypyrrole films were investigated by cyclic voltammetry and SECM. It was found that the self-assembled block copolymer micelles in polypyrrole behave as polyanions and the charge compensation by cations has been identified during electrochemical switching of the polymer films. 

Various para substituted poly-N-arylpyrrole polymer films were prepared and their electrochemical properties were measured. Of particular interest are the poly-N-p-nitrophenylpyrrole films which can be oxidized to produce the polypyrrole cation and reduced to produce the nitrophenyl anion. The polymer films can be repeatedly switched between the neutral, cationic and anionic forms with coulombic reversibility and with little ir-interaction between the pyrrole and the aryl ring. 

Recognizing that the conducting polypyrrole films can be chemically modified (.1,2), the phenyl substituent assumes a particularly important role because it provides a means of introducing a wide selection of functional groups into the polymer. With this objective in mind, we have prepared a series of N-arylpyrrole polymers and find the thin poly-N-(p-nitrophenyl)pyrrole films of particular interest because they combine the electroactive properties of nitrobenzene and polypyrrole. With this combination, the polymer can be switched electrochemically between the cationic, neutral, and anionic form. 

Furthermore, porous CPs (e.g., polypyrrole, polyanUine) films have been used as host matrices for polyelectrolyte capsules developed from composite material, which can combine electric conductivity of the polymer with controlled permeability of polyelectrolyte shell to form controllable micro- and nanocontainers. A recent example was reported by D.G. Schchukin and his co-workers [21]. They introduced a novel application of polyelectrolyte microcapsules as microcontainers with a electrochemically reversible flux of redox-active materials into and out of the capsule volume. Incorporation of the capsules inside a polypyrrole (PPy) film resulted in a new composite electrode. This electrode combined the electrocatalytic and conducting properties of the PPy with the storage and release properties of the capsules, and if loaded with electrochemical fuels, this film possessed electrochemically controlled switching between open and closed states of the capsule shell. This approach could also be of practical interest for chemically rechargeable batteries or fuel cells operating on an absolutely new concept. However, in this case, PPy was just utilized as support for the polyelectrolyte microcapsules. 

PROPERTIES OF SPECIAL INTEREST presence of an extended 7r-bonding system, which imparts electrical properties to the polymer. Doping either p or m can enhance these properties. Polypyrrole is stable in air at room temperature as well as at temperatures as high as 250°C in its doped state. Also, polypyrrole can be synthesized in a doped state. It changes color when switched from its conducting to insulating state. 

Poly(pyrrole), known for over 50 years, is one of the most studied poly-mers. First synthesized by electrochemical polymerization in 1968, poly(pyrrole) is electrically conductive, stable, and insoluble when doped. The mechanism of its electrosynthesis was studied by Genies et Thin films of electropolymerized poly(pyrrole) (< 0.1 fim) are electroactive, and they can be switched between the neutral and the oxidized state at +0.1 V versus SCE. This particular property of polypyrrole can be applied in constructing conductivity... 

All the research devoted to the study of the different chemical and physical aspects of polypyrrole electrogeneration and switching is largely justified by the many technological applications of this material. Most applications require films with improved structural and/or electrochemical properties. The last section of the chapter will focus on the relationship between film properties and requirements for applications. 

Electrochemical and structural variables, such as anodic and cathodic potentials, temperature, composition of the electrolytic solution, potential scan rate (in potentiodinamic experiments), substituents on the pyrrole rings, etc. significantly affect both reversible (switching) and irreversible (degradation) oxidation of polypyrrole films. The influence of all those parameters on electrochemical properties of polypyrrole will be analyzed in this section. 

---