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Conducting Polymers |
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Why can some polymers conduct? It is well known that graphite is a good conductor, previously it was thought that polymers which substitute a carbon (e.g. adding hydrogen's to make hydrocarbons) for another atom could not conduct, however our greater knowledge of conjugated systems has enabled the discovery of conducting polymers. As in a conjugated system the electrons are only loosely bound, electron flow may be possible. However as the polymers are covalently bonded the material needs to be doped for electron flow to occur. Doping is either the addition of electrons (reduction reaction) or the removal of electrons (oxidation reaction) from the polymer. Once doping has occurred, the electrons in the pi-bonds are able to "jump" around the polymer chain. As the electrons are moving along the molecule a electric current occurs. However the conductivity of the material is limited as the electrons have to "jump" across molecules so for better conductivity the molecules must be well ordered and closely packed to limit the distance "jumped" by the electrons. This occurs better in trans undoped polyactelyene. By doping, the conductivity of the polymer increases from 10-3 S m-1 to 3000 S m-1. An oxidation doping (removal of electrons) can be done using iodine. The iodine attracts an electron from the polymer from one of the pi bonds. Thus the remaining electron can move along the chain. |