Making polymers conduct |
Although polymers are semiconductors and will conduct electricity at high temperatures, it is possible to increase their efficiencies at low temperatures by encouraging electron transfer. This is called doping and was discovered when the optical properties of polyacetylene were being investigated. |
The flow of electric charge relies on the electrons not being tightly bound to their parent atoms. This occurs when electrons are delocalised over alternating double and single bonds (conjugation). This occurs when the p orbitals of the atoms are all orientated in the same plane for the π bond electrons to spread over. To increase the effective current, the polymer needs to be doped (turned into a salt).
Oxidative Iodine Doping9
This method of increasing the electric potential along the molecule was first descovered in 1977. To start with the polymer chain looses an electron to an iodine molecule, leaving I3- and the polymer positive. This has effectively left a double bond with a missing electron and so when a potential is applied the electrons else where in the molecule move towards the deficit of charge: a current flows.
Reductive doping1
Instead of removing an electron to produce a salt, an electron can be added in an electrolytic cell where poly(acetylene is the cathode and an organo-metalic reagent is the anode. And as a result, Li+ ions are released into the electrolyte to counter balance the charged polymer.
The electron transport chain can then conduct electricity in the same mechanism as shown above. (To see an animation of this process go to the nobel prize website)
| HOME | How to make a polymer | The discovery of conducting polymers | What is conduction? |
| What is conjugation? | How to make a polymer conduct | What can conducting polymers be used for? | References |
| Laura White: 24/05/04 |