Polyaniline: an Old Polymer with New Physics
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POLYANILINE: AN OLD POLYMER WiTlH NEW PHYSICS
A.J. EPSTEIN* and A.G. MACDIARMID** *The Ohio State University, Department of Physics and Department of Chemistry, Columbus, OH 43210-1106 "**Universityof Pennsylvania, Department of Chemistry, Philadelphia, PA 19104
ABSTRACT Polyanilines have been known for over one hundred years. Recent studies of this chemically flexible polymer have demonstrated unusual electronic phenomena in both the insulating forms and the conducting forms. Studies of both forms show that the origins of the electronic phenomena are substantially different than those observed in polyacetylene and related earlier studied conjugated carbon backbone polymers. Unusual aspects include the formation of massive polarons upon photoexcitation in the insulating forms. These polarons have unusual time dynamics associated with the roles of ring-flipping and ring-conformation in the polymer system. A new model for the effects of electron lattice coupling via ring rotation has been introduced. The "metallic" form of the polymer shows that the metallic state is associated with the ordered regions of the doped polymer. The roles of localization are important in leading to the formation of a textured, granular metal. At low temperatures for emeraldine salt, and at higher temperatures for derivatized polyanilines, localization is important. Potential new technologies based on the polyanilines, including optical information storage, controlled microwave absorption, and use of a self-protonating derivative that is soluble in aqueous media are noted.
INTROI)UCTION The polyanilines differ substantially from earlier studied polyacetylene, polythiophene, polypyrrole, polydiacetylene and other recently studied electronic polymers [1, 21 in that their electronic structure is based on the overlap of alternating nitrogen atoms and C6 rings [3-6], Fig. 1. The presence of the nitrogen atoms provides a degree of chemical flexibility absent in most other electronic polymers, the ability to add or remove protons and pseudoprotons at the nitrogen sites, in addition to the ability to oxidize or reduce the backbone polymer [7]. A schematic illustration of the idealized structure of the well-characterized limiting states of the polyaniline system are summarized in Fig. 1. Leucoemeraldine base (LEB), emeraldine base (EB), and pernigraniline base (PNB) are insulators while the emeraldine salt (ES) polymer is conducting. The conducting form can be achieved by oxidation of LEB, protonation of EB, or protonation and reduction of PNB. The ability to prepare these materials in oriented films [8] and fibers [9] and of known crystallinity and crystal structures [10] has advanced the quantitative studies of this field. Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
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