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Conducting polymers (Read 3212 times)
Gerrit-Jan Linker
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Conducting polymers
26.02.10 at 18:54:41
Conducting polymers
In 1977 it was discovered that polyacetyle films oxidated with chlorine, bromine or iodine vapour werw 109 times more conductive than they were originally. Treatment with halogen was called “doping” by analogy with the doping of semiconductors. The “doped” form of polyacetylene had a conductivity of 105 Siemens per meter, which was higher than that of any previously known polymer. As a comparison, teflon has a conductivity of 10–16 S m–1and silver and copper 108 S m–1.  
A key property of a conductive polymer is the presence of conjugated double bonds along the backbone of the  
polymer. In conjugation, the bonds between the carbon atoms are alternately single and double. Conjugation is not enough to  
make the polymer material conductive. In addition – and this is what the dopant does – charge carriers in the  
form of extra electrons or ”holes” have to be injected into the material. When such a hole is filled by an electron jumping in from a neighbouring position, a new hole is created and so on, allowing charge to migrate a long distance.  
Bond alternation
The existence or non-existence of energy gaps is related to the concept of bond alternation. In a first approximation, the presence of high conductivity could mean some equalization of the bonds: upon doping, the charge transfer process would perturb the electronic distribution and make the chains more regular.
The concept of solitons in polyacetylene was explicitly introduced by Pople and Walmsley in 1962. The neutral soliton is a radical misfit which exists in the middle of a long polyene chain containing an odd number of conjugated carbons and which consists of several successive bonds of similar lengths near which the unpaired electron is localized. Pople and Walmsley suggested that such a defect could be mobile and, if charged, could be responsible of an high electrical conductivity. This idea is largely extended in the Su, Schrieffer, and Heeger (SSH) theory.
The Nobel Prize in Chemistry, 2000: Conductive polymers
See also:
Electrical Conductivity
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« Last Edit: 26.02.10 at 20:52:27 by Gerrit-Jan Linker »  

Gerrit-Jan Linker
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