Electronically conducting polymer doping processes

Gas separation is a very large potential field of application in the domain of chemical separation. In gas transport processes through membranes, electronically conductive polymers show a great selectivity, which can be varied by changing the doping level of the polymer, because the performance (selectivity, durability, flux) of a membrane is dependent on its microstructure. Preliminary work has shown that membranes that are very promising for gas separation can be made with polypyrrole [437]. Membranes for separation of oxygen from air can be also produced. Permeability is due to holes or to fissures of a few A in the polymer matrix. 

Polymers. Electronically conductive polymers may also be used as cathode materials in rechargeable lithium batteries. The most popular polymers are polyacetylene, polypyrrole, polyaniline, and polythiophene, which are made conductive by doping with suitable anions. The discharge-charge process is a redox reaction in the polymer. The low specific energy, high cost, and their instability, however, make these polymers less attractive. They have been used in small coin-type batteries with a lithium-aluminum alloy as the anode. 

However, it remains to be understood how this type of doping process can induce enhancement in the electronic transport of the polymers. Indeed, the clarification of the doping mechanism of conducting polymers and of the associated electronic band evolution is of fundamental importance for the comprehension of the operational behaviour of these compounds as novel electrode materials. 

In an attempt to illustrate in a simple way the general concept of the doping process in polymers, let us consider the p-doping (oxidation) process of polypyrrole. In the undoped state, polypyrrole is a poor electronic conductor with an energy gap of 3.2 eV between the conduction band (CB) and the valence band (VB) ... 

Also the case of polyaniline is somewhat different from that of heterocyclic polymers. It has been proposed (MacDiarmid and Maxfield, 1987) that the doping process does not induce changes in the number of electrons associated with the polymer chain but that the high conductivity of the emeraldine salt polymers is related to a highly symmetrical 7r-delocalized structure. 

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