Electrically Conductive Polymers
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Polymeric materials are typically considered as insulators, and in fact important applications do rely on their poor conductivity— e.g., electrical cable insulation and charged dielectric films (electrets, electrical analogs of magnets), the latter finding use in microphones. Research in the last decade, however, has lead to the discovery of polymeric materials with extremely high conductivity, approaching that of copper. This brief article will highlight recent work in the synthesis, processing and applications of these novel materials. Structure and Properties
Typical polymers, the oxidant (or "dopant") used to create carriers, and room temperature conductivities are given in Table I. A key feature shared by these materials is delocalized (at least over a few repeat units) 77-electron density. Such unsaturated polymers facilitate carrier generation because of the ability for resonance delocalization of the resulting radical ions, which can also offer good intramolecular carrier mobility. In addition, the geometry of 7r-orbitals allows for good orbital overlap and encourages /Mfermolecular carrier transport. That the polymer chains are considerably shorter than typical sample dimensions indicates that intermolecular transport is dominant, especially in view of the disorder observed in most conducting polymer systems. However, as the number of defects (crosslinks, "twists" which inhibit conjugation) decreases, it might be anticipated that carriers can travel greater distances along a chain before a (presumably) higher activation energy, intermolecular electron transfer becomes necessary, thus affording higher conductivity. Indeed, it has been recently reported that samples of very high quality, oriented polyacetylene, when treated with iodine,
intractability is the result of low entropies of dissolution and/or melting, with strong IT-TT intermolecular interactions rendering the free energy of dissolution and/or melting even more unfavorable through the enthalpic contribution. Fortunately, if most conducting polymers are, in fact, not crosslinked but are intractable for the reasons noted above, it should be possible to mitigate this problem by modifying the polymer backbones. Considerable progress has been made toward developing theoretical approaches to determine important parameters such as oxidation potentials, band gaps and band widths, with good agreement found in many cases between theory and experiment.4 Such work has important implications for the design of new conductive polymers with specific electronic properties. The elucidation of conduction mechanisms has been given much attention as well. To a first approximation, carrier transport may be viewed as a combination of intrachain resonance delocalization and interchain redox processes involving electron transfer between (mixed valent) seg-
ments of neutral and oxidized (or reduced) polymer.5 The latter process is reminiscent of intrastack conduction in small-molecule, organic, radical ion salts. These ideas are illustrated in Figure 1. Note that conduc
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