Novel concepts in electronic polymers: Polyaniline and its derivatives

Polyanilines have been known for over one hundred years. Recent studies of this chemically flexible polymer have demonstrated unusual chemical, electrical, and optical phenomena, both in insulating forms and conducting forms. The polyanilines differ substantially from earlier studied polyacetylene, polythiophene, polypyrrole, polydiacetylene, and other polymers in that their electronic structure is based on the overlap of alternating nitrogen atoms and C₆ rings. Two classes of the emeraldine form of polyaniline are distinguished by preparation route and crystal structure. For both classes, the Pauli susceptibility indicative of metallic state is correlated with the formation of a three-dimensionally crystalline region. Charge conduction studies of oriented films and fibers demonstrate that three-dimensional order between chains is critical for high conductivities. Within the crystalline regions the conduction electrons are substantially delocalized in three-dimensions while the dc transport is dominated by quasi-one-dimensional variable range hopping in the disordered regions. This delocalized charge leads to high absorbance (loss) at microwave frequencies, important for a broad range of applications. The ability to derivatize polyaniline at ring and nitrogen positions allows one to test concepts for the control of conductivity as well as improved processing. The monomethyl derivative, poly(orthotoluidine), has nearly identical electronic structure and crystal structure, yet its conductivity is three orders of magnitude lower than that of emeraldine hydrochloride. This is attributed to increased interchain separation and decreased interchain correlation causing 1-D localization. Similar 1-D localization is observed in the self-doped sulfonated polyaniline system. Extensive photoinduced optical studies of the insulating base forms of polyaniline show the importance of ring rotation in stabilizing photoexcited positive polarons. Long-lived photoinduced absorptions are observed which are suggested to be a possible basis for erasable optical information storage technology.