Conductivity, electronic polyacetylene

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

The relatively high electronic conductivity of conducting polymers is connected with the presence in polymers of internal system of poly-7t-conjugated bonds. This is easy to understand on the example of a simplest type of conducting polymer (polyacetylene) in the main and activated states (Figure 6). 

The conductivity of polyacetylene is also increased by dopants that are electron donors. For example, the polymer can be doped with alkali metals to give, for example, [Li5 (CH) ln. The wide range of conductivities produced by these two forms of doping is illustrated in Figure 6.3. 

POLYACETYLENE. A linear polymer of acetylene having alternate single and double bonds, developed in 1978. It is electrically conductive, but this property can be varied in either direction by appropriate doping either with electron acceptors (arsenic pentaflnoride or a halogen) or with electron donors (lithium, sodium). Thus, it can be made to have a wide range of conductivity from insulators to n- or >-type semiconductors to strongly conductive forms, Polyacetylene can be made in both cis and trans modifications in the form of fibers and thin films, the conductivity... 

Doped polyacetylene conducts electrons via an intrinsic mechanism rather than by an extrinsic one. That is to say, conductivity of the polymer is due directly to electronic conductivity rather than to charge carrier motion. 

Note that the observation of the Overhauser enhancement does not necessarily lead us to the presence of the conduction electrons. As mentioned in section 4.1, the Overhauser effect is known to be observed widely in coupled spin systems, even in the semiconducting pristine trans-polyacetylene, with only the... 

In order to understand the physical properties of polyacetylene doped with divalent ions, it is important to consider the theory of conductivity of polyacetylene doped with monovalent ions. One of the most unusual characteristics of polyacetylene is that small amounts of dopant ions give rise to enormous increases in electrical conductivity without causing any increase in the number of unpaired electrons. In fact, the small level of paramagnetism observed in pristine polyacetylene actually decreases on doping (8). This is in contrast to what occurs in traditional semiconductors, such as silicon, where dopants increase both conductivity and paramagnetism. An explanation has been offered by the soliton theory of conductivity (9,10). 

Baeriswyl, G. Harbeke, H. Kiess, and W. Meyer, Conducting polymers Polyacetylene, "Electronic Properties of Polymers," J. Mort and G. Pfister, eds., Wiley-Interscience, New York (1982). 

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