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The emeraldine base (EB) form of polyaniline can be varied from insulating (o < 10-1°ohm-tcm l) to conducting (o-~ 10 + lohm-lcm -1) through protonation. That is, the number of electrons on the polymer backbone is constant while the number of protons is increased. We present here extensive magnetic, optical and transport data that demonstrate that the resulting emeraldine salt (ES) is metallic with axfinite density of states at the Fermi energy. The results are consistent with segregation into fully protonated emeraldine salt and unprotonated emeraldine base polymer regions. It is proposed that the observed transition is an isolated bipolaron-to-polaron lattice transition. The correspondence of this concept to the disproportionation between protonated imine plus amine to form two semiquinones is shown.

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