A.F. Richter

Polyaniline: a new concept in conducting polymers

Abstract The analytically pure base form of polyaniline, , corresponding to the emeraldine oxidation state is converted from an insulator (σ 10 −10 S/cm) to a metal (σ 5 S/cm) by treatment with 1M aqueous HCl to form the corresponding salt, emeraldine hydrochloride, . This involves a new type of doping of a conducting polymer in that the number of electrons associated with the polymer undergoes no change during the protonic acid doping process. The metallic emeraldine hydrochloride is believed to be a delocalized poly(semiquinone radical cation) having a polaron conduction band, with most of the positive charge residing on the nitrogen atoms. It exhibits a finite density of states at the Fermi energy.

Insulator-to-metal transition in polyaniline

Abstract The emeraldine base (EB) form of polyaniline can be varied from insulating ( σ −10 ohm −1 cm −1 ) to conducting ( σ ≈ 10 +1 ohm − cm − ) 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 a finite 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.

Polyaniline: Doping, structure and derivatives

Abstract Redox titration results and electronic spectral evidence show that the oxidation state of the “emeraldine” base form of polyaniline can vary depending on whether its synthesis is performed in the presence or absence of air. Chemical doping of leucoemeraldine, the completely reduced form of polyaniline, to selected oxidation states can be accomplished by a variety of oxidizing agents such as Cl 2 , (NO) + (PF 6 ) − , FeCl 3 , SnCl 4 and TCNQ.

Insulator-to-metal transition in polyaniline: Effect of protonation in emeraldine

Abstract The emeraldine base form of the polymer can be varied from insulating ( σ ∼ 10 −10 ohm −1 cm −1 ) to conducting ( σ ∼ 10 0 ohm −1 cm −1 ) states through protonation. Based upon extensive magnetic, optical and transport data, we demonstrate that the resulting emeraldine salt is metallic with a finite density of states at the Fermi energy. The roles of a novel bipolaron-to-polaron lattice transition and phase segregation into conducting and non-conducting regions are discussed.

Polyaniline: Synthesis and Characterization of the Emeraldine Oxidation State by Elemental Analysis

Detailed experimental procedures are given for the chemical synthesis from aniline of analytically pure emeraldine hydrochloride, a highly conducting polymer derived from the emeraldine oxidation state of polyaniline, which contains equal numbers of oxidized and reduced repeat units, the non-protonated base form of which has the composition,. In the as-synthesized polymer, ∼ 42% of the nitrogen atoms are protonated i.e. “doped”. Treatment of this material with 1.0M aqueous HCl gives by elemental analysis, the most highly conducting (metallic) form of the emeraldine oxidation state of polyaniline in which 50% of the nitrogen atoms are protonated. Experimental details are given for converting the as-formed emeraldine hydrochloride to analytically pure emeraldine base. The conductivities of samples of emeraldine base protonated by aqueous HCl to various extents as determined by elemental analysis are reported. Electrochemical studies involving the emeraldine base are consistent with its having a composition very close to the proposed composition involving equal numbers of oxidized and reduced repeat units.

Polyaniline: protonation/deprotonation of amine and imine sites

Abstract A model describing the dependence of the protonation level and the oxidation potential of polyaniline as a function of pH of equilibration and oxidation state has been derived. It is assumed that amine and imine sites do not function independently. Predictions are compared with previous experimental data.

Electronic structure of some polyanilines

Abstract We present some photoelectron spectroscopy data relevant to the chemical and electronic structure of certain members of the polyaniline family of conducting polymers.

A solid-state NMR investigation of the structure and dynamics of polyanilines

Abstract Solid-state nuclear magnetic resonance 13 C and 2 H measurements have been applied to characterize the structure and dynamics of three forms of polyaniline: emeraldine base, leucoemeraldine, and emeraldine hydrochloride. By comparison with the 13 C NMR spectra of pristine and partially oxidized leucoemeraldine, emeraldine base is shown to be an alternating copolymer of reduced [-(C 6 H 4 )-N(H)-(C 6 H 4 )-N(H)-] and oxidized [-(C 6 H 4 )-N=(C 6 H 4 )=N-] repeat units. Deuterium spectra show that, for all three polymers, the primary chain motion from 300 K to over 400 K is 180° flips of rings about their 1,4-axes. Although there is a very broad distribution of flip rates, it is possible to separate the deuterium spectra into two lineshape components, attributable to fast and slow flippers, and the ratio of these components is a convenient indicator of the polymer dynamics. For the conducting salt form, emeraldine hydrochloride, approximately one third as many rings can be classified as fast flippers as for the emeraldine base. The decrease in the number of fast flippers is caused by partial π-character of the ring-nitrogen bonds and/or steric interactions with adjacent chloride counterions. Possible structures for the salt form consistent with this observation are considered.

A covalent bond to bromine in HBr-treated polyaniline from X-ray diffraction

Abstract When the emeraldine base form of polyaniline is doped by aqueous HBr, the conductivity is increased to 2 (ohm cm)−1 for a compressed pellet. X-ray diffraction results on the conducting polymer together with elemental analysis indicate that a significant portion of the bromine is covalently bound.

15N NMR of polyaniline

Abstract 15 N NMR spectroscopy has been utilized as a structural probe for the leuco- and “emeraldine” base forms of polyaniline. Preliminary results suggest that the emeraldine base polymer exists as an alternating copolymer of oxidized and reduced units. End-groups are not detected at the experimental level of sensitivity.