Arthur J. Epstein

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.

The concept of secondary doping as applied to polyaniline

Phenomenologically, a primary dopant for a conducting polymer is a substance, a relatively small quantity of which drastically changes the electronic, optical, magnetic and/or structural properties of the polymer and is accompanied by a large increase in conductivity. De-doping results in a reversal of the newly induced properties. Phenomenologically, a secondary dopant is an apparently “inert” substance which, when applied to a primary-doped polymer, induces still further changes in the above properties including a further increase in conductivity. It differs froma primary dopant in that the newly enhanced properties may persist even upon complete removal of the secondary dopant. The concept will be illustrated using polyaniline and its derivatives. It will be shown that the effects of secondary doping are based primarily on a change in molecular conformation of the polyaniline from ‘compact coil’ to ‘expanded coil’ during the secondary doping process.

'Synthetic metals': a novel role for organic polymers

Summary form only given. The basic chemistry and physics of polyacetylene, (CH)/sub x/, the prototype conducting polymer which is characteristic of most other conducting polymers, were described together with a description of the polyanilines, a large new class of conducting polymers. Potential technological applications were discussed. Polyacetylene, (CH)/sub x/ is obtained in the form of silvery, flexible films by the polymerization of gaseous acetylene. >

Polyanilines: a novel class of conducting polymers

The polyanilines are a class of polymers the base form of which has the general formula [graphic omitted] containing y reduced and (1 –y) oxidized repeat groups. y can in principle be varied continuously from one, the completely reduced material, to zero to give the completely oxidized polymer. The emeraldine oxidation state (y= 0.5) consists of alternating reduced and oxidized groups. It can be protonated, i.e. doped, by aqueous acids with a concomitant increase in conductivity of almost 10 orders of magnitude (to a maximum conductivity of 101–102 S cm–1), forming a polysemiquinone radical cation such as [graphic omitted] which contains a delocalized half-filled broad polaron energy band. The polymer is readily solution-processed into films and fibres which can be mechanically aligned, the doped forms of which have a conductivity parallel to the direction of alignment significantly greater than that of non-aligned material. X-Ray studies show that the doped and undoped polymer exist in several different crystalline forms. A wide variety of derivatives can be synthesized by substitution on the ring or on the nitrogen.

A Room-Temperature Molecular/Organic-Based Magnet

The reaction of bis(benzene)vanadium with tetracyanoethylene, TCNE, affords an insoluble amorphous black solid that exhibits field-dependent magnetization and hysteresis at room temperature. The critical temperature could not be estimated as it exceeds 350 kelvin, the thermal decomposition temperature of the sample. The empirical composition of the reported material is V(TCNE)x�Y(CH2Cl2) with x ∼ 2 and Y ∼ 1/2. On the basis of the available magnetic and infrared data, threedimensional antiferromagnetic exchange of the donor and acceptor spins resulting in ferrimagnetic behavior appears to be the mode of magnetic coupling.

Secondary doping in polyaniline

Abstract Phenomenologically, a primary dopant for a conducting polymer is a substance, a relatively small quantity of which drastically changes the electronic, optical, magnetic, and/or structural properties of the polymer and it is accompanied by a large increase in conductivity. De-doping results in a reversal of the newly-induced properties. Phenomenologically, a secondary dopant is an apparently “inert” substance which, when applied to a primary-doped polymer, induces still further changes in the above properties including a further increase in conductivity. It differs from a primary dopant, in that the newly-enhanced properties may persist even upon complete removal of the secondary dopant. The concept will be illustrated using polyaniline and its derivatives. It will be shown that the effects of secondary doping are based primarily on a change in molecualar conformation of the polyaniline from “compact coil” to “expanded coil” during the secondary doping process.

Electrical conductivity and electromagnetic interference shielding of multiwalled carbon nanotube composites containing Fe catalyst

Thin and flexible composite films of raw or purified multiwalled carbon nanotube (MWCNT) with various mass fractions and poly(methylmethacrylate) (PMMA) were synthesized for electromagnetic interference (EMI) shielding material. From scanning electron microscopy and high-resolution transmission electron microscopy photographs, we observed the formation of a conducting network through MWCNTs in an insulating PMMA matrix and the existence of an Fe catalyst in MWCNTs. The dc conductivity (σdc) of the systems increased with increasing MWCNT mass fraction, showing typical percolation behavior. The measured EMI shielding efficiency (SE) of MWCNT–PMMA composites by using the extended ASTM D4935-99 method (50 MHz–13.5 GHz) increased with increasing MWCNT mass fraction as σdc. The highest EMI SE for raw MWCNT–PMMA composites was ∼27 dB, indicating commercial use for far-field EMI shielding. The contribution of absorption to total EMI SE of the systems is larger than that of reflection. Based on magnetic permeabili...

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.

Growth and alignment of polyaniline nanofibres with superhydrophobic, superhydrophilic and other properties

Polyaniline nanofibres can be prepared by a number of methods based on chemical oxidative polymerization and in situ adsorption polymerization. However, the lack of alignment in these nanostructures makes them unsuitable for many applications. Here, we report a simple approach to chemical oxidative polymerization that can control the growth and simultaneous alignment of polyaniline nanofibres grown on a range of conducting and non-conducting substrates in a wide variety of sizes. The diameters of the tips of the nanofibres can be controlled within the range 10-40 nm, and the average length can be controlled within the range 70-360 nm. Moreover, the coatings display a range of properties including superhydrophilicity and superhydrophobicity. Such nanostructured coatings may be useful for applications such as anti-fog coatings, self-cleaning surfaces, DNA manipulation, transparent electrodes for low-voltage electronics, and chemical and biological sensors.

Polyaniline: Solutions, Films and Oxidation State

Abstract The emeraldine oxidation state of polyaniline in its base form, “emeraldine base” can be solution-processed to yield large flexible films which can be doped to the metallic conducting regime (σ ∼ 1-5 S/cm). The approximate solubility of emeraldine base at room temperature in 80% aqueous acetic acid, 60% and 88% aqueous formic acid, dimethyl sulfoxide (DMSO), dimethylformamide (DMF) and N-methylpyrrolidinone (NMP) have been determined. Both emeraldine base and emeraldine hydrochloride can be sublimed onto a variety of substrates to produce high quality, ∼2000 A thick films which are similar but not identical to emeraldine base. Emeraldine base, which is slowly oxidized by air, can be reconverted to the emeraldine oxidation state by treatment with dilute aqueous acids.