Conductive polymers with immobilised dopants: Ionomer composites and auto-doped polymers-a review and recent advances

The electrodeposition of poly(pyrrole) in the presence of a polyanionic electrolyte (poly(styrenesulphonate), poly(vinylsulphate) or Nafion®) leads to the formation of a film of a composite polymer at the surface of the two polymeric structures prevents the anionic dopants from being expelled during the dedoping, consequently cations are inserted. This cation-exchange behaviour is the origin of applications in the field of charge-controllable membranes such as water deionisation and the polymer battery, reported by Shimidzu and co-workers (1987). The immobilisation of the anionic dopant has also been achieved by their direct covalent binding to the conductive polymer backbone, leading to the concept of internal doping or ‘auto-doping’. The poly(3-alkylsulphonate-thiophenes) (1987) reported by Wudl, Heeger and co-workers (187) are revealed to be the first water-soluble conductive polymers. A study of the poly(3-methylpyrrole-4-carboxylic acid) by Pickup (1987) shows that the formal potential of this polymer is sensitive to pH. The copoly(pyrrole+N-alkylsulphonate-pyrrole) presented by the authors in more detail exhibits a remarkable mixed behaviour as a result of the coupling of cationic membrane with conductive polymer properties. In particular, the electrochemical behaviour of this co-polymer is very sensitive to the water content of the solvent.