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Polymer Science and Engineering: The Shifting Research Frontiers
known as a "synthetic metal." The properties of these materials are intrinsic to a "doped" form of the polymer.
The concept of "doping" is the unique, central, underlying, and unifying theme that distinguishes conducting polymers from all other types of polymers. In the doped form the polymer has a conjugated backbone in which the π-system is delocalized. During the doping process, a weakly conducting organic polymer is converted to a polymer that is in the "metallic" conducting regime (up to 104 siemens per centimeter [S/cm]). The addition of small (usually <10 percent) and nonstoichiometric quantities of chemical species results in dramatic changes in the properties of the polymer. Increases in conductivity of up to 10 orders of magnitude can be readily obtained by doping. Doped polyacetylene approaches the conductivity of copper on a weight basis at room temperature. Doping is reversible. The original polymer can be recovered with little or no damage to the backbone chain. The doping and undoping processes, involving dopant counter ions that stabilize the doped state, may be carried out chemically or electrochemically. By controllably adjusting the doping level, a conductivity anywhere between that of the undoped (insulating or semiconducting) and that of the fully doped (metallic) form of the polymer may be obtained. Conducting blends with nonconducting polymers can be made. This permits the optimization of the best properties of each type of polymer.
All conducting polymers (and most of their derivatives), including polyacetylene, polyparaphenylene, poly(phenylene vinylene), polypyrrole, polythiophene, polyfuran, polyaniline, and the polyheteroaromatic vinylenes, undergo either p-and/or n-redox doping by chemical and/or electrochemical processes during which the number of electrons associated with the polymer backbone changes. P-doping involves partial oxidation of the π-system, whereas n-doping involves partial reduction of the π-system. Polyaniline, the best-known and most fully investigated example, also undergoes doping by a large number of protonic acids, during which the number of electrons associated with the polymer backbone remains unchanged.
Appropriate forms and derivatives of many conducting polymers, especially those involving polyaniline and polythiophene, are readily solution processible into freestanding films or can be spun into fibers that even at this relatively early stage of development have tensile strengths approaching those of the aliphatic polyamides. Blends of a few weight percent of conducting polymers with aromatic polyamides or polyethylene can exhibit conductivities equal to, or even exceeding, the conductivity of the pure conducting polymer while retaining mechanical properties similar to those of the host polymer. In addition, pure conducting polymers and their blends can be oriented by stretching to produce highly anisotropic electrical and optical properties.
The thermal, hydrolytic, and oxidative stability of doped forms of pure conducting polymers varies enormously from the n-doped form of polyacetylene, which undergoes instant decomposition in air, to polyaniline, which has sufficient