Conducting Polymers

The new field of the electrochemistry of electronically conducting polymers is reviewed. A brief historical account traces the beginning of organic electrodes to Kallmann and Pope, who, in 1960, observed charge injection and conductance in anthracene electrodes.The progress in the discovery and use of new polymer electrodes is briefly discussed. Some of the possible applications of these new electrodes are suggested. As important background information for studying organic polymer electrochemistry, knowledge of the conduction mechanism is needed. The theory of bipolaron formation, as proposed by Bredas, et al., is presented. It is important to study the electrode-solution interface. Double layer models for metal, semiconductor, and insulator electrodes are probed. Recent work and applications of these electrodes are then briefly reviewed. This includes initiatives in the fields of electrode generated reactions, photoelectrochemistry, batteries, and molecular electronics. Finally, the needed areas of research, from an electrochemical point of view, are presented.

The use of conducting organic polymers as electrode materials in rechargeable batteries has been demonstrated in the past few years and in particular, polyacetylene, the prototype polymer, can be reversibly oxidized or reduced by electrochemical procedures. These doping processes lead to the incorporation of counterions within the polymer with a concomitant increase in the electrical conductivity into the metallic regime. This technique is now widely applied for a number of organic polymers used as electrodes since it offers a more precise control of the doping level as well as a much better dopant homogeneity than for a chemical doping process.

In situ Raman experiments are presented for an n-type doped polyacetylene film as a function of the doping level and compared to p-type doped systems. The modification occuring on the film during a charge-discharge cycle can be analyzed in details and in particular, the appearance of new features on the Raman spectra can be interpreted in terms of vibrational modes induced by doping as already evidenced in infrared spectroscopy. The possibility of using the Raman spectroscopy to study the morphology of the sample is described and the information gained from in situ experiments during the electrochemical process on electrodes composed of conducting organic polymers is emphasized.

Charge transfer complexes of porphyrins, phthalocyanines and phenothiazines have desirable electrical conductive properties. The parent compounds can also function as polymer precursors which in the past often suffered from poor solubility characteristics, limiting their utility in further chemical reactions. Recently, the synthesis of highly soluble metallotetrabenzporphyrin and phenothiazine homologs has made it possible to study the charge transfer characteristics of those compounds and their complexes in a variety of solvents. We wish to report on alternating and direct current measurements of some metal mesophenyltetrabenzporphyrins, phenothiazinyls, their iodine and TCNQ complexes in the solid and solution state, and ternary complexes of these compounds with polyvinylpyridine. An iodine:phenothiazine-ionene dimer complex exhibited an unusual stoichiometry, and has the lowest resistivity so far detected in iodine-ionene polymers. Well-defined ternary complexes of metallo-organics, I₂ and polyvinylpyridines have markedly lower resistivities and activation energies than the parent compounds or the dual component complexes. The structure/conductivity relationship of several of the complexes and ternary polymeric models are discussed.

An acetylene polymerization reaction catalyzed by an aged mixture of Ti(OC₄H₉)₄/Al(C₂H₅)₃ in a silicone oil reaction medium yields a homogeneous, defect-free polyacetylene film that can be stretched mechanically by up to 600% corresponding to stretching rates of 7. After we had doped with iodine, the film displayed electrical conductivity of ca. 11 000 S/cm. Both washed (catalyst-free) and unwashed (catalyst-containing) film was stretched.

Polyacetylene prepared by the Durham precursor route can be obtained in a highly-oriented and crystalline form if a free-standing precursor film is stretched during the transformation reaction Films produced in this way show similar “long-chain” characteristics to Shirakawa polyacetylene in which it is considered that spin and charge defects have considerable mobility along the polyene chains. These films, being nonfibrous offer the possibility of studying the intrinsic anisotropic properties of the material. Here, the results of polarisation-dependent measurements of optical absorption, reflectivity, photoinduced absorption (PA) and photoconductivity (PC) are discussed. These measurements demonstrate that optical absorption of band-gap light polarised with E vector perpendicular to the chains is due to inter-chain excitation from π to π* states. It is found that the long-lived photoexcited states which give rise to PA and PC are preferentially excited by these inter-chain transitions.

Poly(ethylene oxide) (PEO) forms solid solutions with MgCl₂, PbI₂, PbBr₂, and various other salts of divalent cations. DSC analysis and a study of the temperature dependence of conductivity indicate that these materials consist of several crystalline phases, corresponding to pure PEO and salt-rich complexes, and a coexisting elastomeric phase. MgCl₂.(PEO) has an ionic conductivity comparable to that of LiCF₃ SO₃.(PEO) from 10⁻⁵¹⁶ (ohm cm)⁻¹ at 80°C, and the conductivity of PbBr₂.(PEO)₈ is 10⁹⁶ − 10⁻⁷ (ohm cm)⁻¹ at 180°C and about 10⁻⁵ (ohm-cm)⁻¹ at 250°C. The lead halide complexes conduct both Pb²⁺ and halide anions. All of the divalent halide complexes are stable to nearly 300°C.

Rb Ag₄I₅ and KAg₄I₅ in a freshly prepared state have unusually high ionic conductivity at room temperature. Unfortunately these salts are unstable against disproportionation. Stable complexes of the form poly(ethylene oxide) — M Ag₄I₅ (M = Li, K, Rb) have been prepared and their ionic conductivities measured for various M:0 ratios. For example for K:0 = 1:1 the conductivity is 2 × 10⁻³ ohm⁻¹ cm⁻¹ at room temperature and the activation energy is 0.16 eV. Studies using differential scanning calorimetry, NMR and wide angle x-rays have been made and show that these complexes are polycrystalline but with no evidence of crystalline poly(ethylene-oxide). We suggest that the long polymeric chains inhibit the attainment of the positive activation volume required for disproportionation.

Electronically conducting polymers have recently been considered as electrode materials for various electrochemical devices, primarily batteries and electronic displays. However, in the case of the polymers yet studied, e.g. polyacetylene and polypyrrole, little regard has been paid to the ionic conductivity which, according to ambipolar diffusion theory, is required for mass transport of the electroactive species from the surface to the bulk of the electrode. In fact, experiments on polyacetylene in conjunction with solid polymer electrolytes have indicated that the charge-discharge rate will be limited by the ionic conductivity.

A model of electrode operation involving electronic and ionic resistance lines coupled by the charge storage capacitors can be used to replace the more conventional diffusion treatment. This has the advantage of facilitating the understanding of composite electrodes in which the rather low redox storage capacitance of conducting polymers may be supplemented by the addition of an inorganic insertion electrode. Present formulations of composite electrodes, e.g. carbon black, PEO, and V₆O₁₃ composite, suffer from an increased tortuosity of the electronic conduction path due to the prescence of the ionic conductor, and similarly that of the ionic path by the electronic conductor. Therefore a mixed conductor with acceptable conductivities of both carriers would be an advantage.

Some preliminary experiments were described in which the polymer design incorporates the structural elements required each for type of conductivity, and the mass transport parameters are evaluated with reference to the discharge rate of the polymer electrode in a solid polymer electrolyte cell.

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.

The general concepts of quasi-one-dimensional conducting polymers are introduced including the role of band theory, electron-phonon interactions, the Peierls ground state, and commensurability. The dominant defect states present upon doping, solitons, polarons and bipolarons, are discussed. Application of these concepts to polyaniline is made with emphasis on the mechanism for the insulator-to-metal transition.

Of prime importance and interest in the continuing search for new electronically conducting polymers is the achievement of small or vanishing values of the semiconductor band gap (E_g) which governs the intrinsic electronic properties of materials. This paper will report on experimental studies of the effects of molecular structure on the intrinsic electronic properties of polymers. First, the structure and band gap of hetero-aromatic semiconducting polymers, including polythiophenes and polypyrroles, are discussed. It is suggested that planarity of polymer backbone is essential to small band gaps. From the results on “model compounds” for poly(2,5-thiophenediyl) it is suggested that this polymer has a non-planar backbone and an S-cis chain structure which largely accounts for about half of its 2.2eV band gap. Novel polybithiophenes¹ and polyterthiophenes² which have band gaps as small as 1.20 eV are described. Second, the synthesis and intrinsic electronic properties of a novel class of conjugated polymers containing alternating aromatic and quinonoid sequences is described³. These polymers exhibit band gaps as small as 0.75 eV, the smallest known value of band gap for organic polymers⁴. The quinonoid character of the polymer chains which is related to molecular parameters amenable to synthetic manipulation only partially explains why the band gap of this class of polymers is generally small. It is suggested that quantum well and superlattice effects may exist in one-dimensional conjugated polymer chains containing “mixed polymer repeating units” having different band gaps similar to inorganic semiconductor superlattice and quantum well heterostructures^5–7.

A new polymer electrolyte is described which was prepared by the reaction of poly(methylhydrosiloxane), PMHS, with poly(ethylene glycol), PEG, and poly(ethylene glycol methyl ether), MePEG. The glass transition temperature of the polymer host is very low (207 K) and the polymer forms complexes with the Li, Na, and K salts of trifluoromethanesulfonate, all of which exhibit high ionic conductivities of up to 1 × 10⁻⁴ Ω⁻¹cm⁻¹ at 40°C. ²⁹Si NMR, AC complex impedance studies, DSC, and X-ray powder diffraction have been used to characterize the electrolyte. The temperature dependance of the conductivity has been modeled by the Vogel-Tammann-Fulcher equation.

Polyacetylene can be oxidized (“p-doped”) to the metallic regime by O₂ or H₂O₂ in the presence of an aqueous solution of a non-oxidizing acid such as HBF₄ or by an aqueous solution of an oxidizing acid such as HClO₄. The [(CH)^+y (A)^−y]_x (A⁻ = ClO ₄ ⁻ or BF ₄ ⁻ ) formed above may be used as a cathode by connecting it to a lead anode in the electrolyte whereupon it is reduced to (CH)_x. If the oxidizing agent is present during this process, the (CH)_x is continuously electrochemically reduced by the lead as rapidly as it is oxidized chemically by the oxidizing agent. Polyacetylene is therefore acting as a catalytic electrode.

Electrochemical characteristics of polythiophene and poly-3-methylthiophene have been investigated in liquid Li(SO₂)₃AlCl₄ electrolyte using cyclic voltammetry. These polymers were electropolymerized on smooth vitreous carbon rods and cycled between 2.0 and 4.0 volts relative to lithium. Although both polymers showed a 2.9 volt reduction wave (due to SO₂ reduction), only the poly-3-methylthiophene showed significant oxidation at 3.9 volts. Use of poly-3-methylthiophene as a catalytic electrode is therefore of interest to rechargeable Li/SO₂ cell technology.

Bridged macrocyclic transition metal complexes using phthalocyanine and naphthalocyanine as macrocycles e.g. [PcML]_n with M = Fe, Ru Co and L = dib, tz, CN show good semi-conducting properties without external doping. The mechanism of the formation of the bridged macrocyclic metal complexes is studied by ¹H-NMR spectroscopy using the monomer PcFe(Me₄dib)₂.

Phthalocyaninatoiron and ruthenium (PcFe, PcRu) as well as 2,3-naphthalocyaninatoiron (2,3-NcFe) form s-tetrazine bridged compounds e.g. [PcRu(tz)]_n, which exhibit high semi-conducting properties without additional doping. Their conductivities are comparable to cyanide bridged metallomacrocycles [PcMCN]_n.

Electrochemical doping of bridged phthalocyaninato transition metal complexes [PcML]_n is reported.

Langmuir-Blodgett techniques provide one means of preparing thin films ordered in two dimensions. An alternative approach uses self organisation of reactive molecules which can chemisorb onto surfaces. Each technique has advantages and limitations, especially in the context of preparation of functionalised surfaces such as conducting films. Recent work has produced a group of amphipathic azobenzene derivatives which are designed to allow either chemisorption or physisorption onto surfaces, and which should allow hybrid film fabrication, combining advantages of each technique. The synthesis and surface properties of these molecules is discussed, and application areas explored, particularly in the context of surface functionalisation. Successful exploitation of thin film technology in molecular electronics requires that organisation be controllable in the plane of a given monolayer, as well as from one monolayer to another, and this is discussed in relation to the azobenzene derivatives mentioned.

It has been shown that devices can be fabricated using electronically conducting organic polymers for the ambient temperature detection of several industrially important gases. In particular the resistance of thin films of polypyrrole has been shown to increase in the presence of 0.1% ammonia in air and to decrease in the presence of 0.1% nitrogen dioxide and 0.1% hydrogen sulphide. Devices based on conducting polymers may thus offer advantages in environmental monitoring over presently available semiconductor sensors which generally operate at elevated temperatures.

Orientation effects play an extremely important role in macromolecular science. orientation can determine whether a polymeric material is a useless powder or a high strength fiber, a film easily split or tough, optically polarizing or isotropic. Orientation is also important in determining the properties of electronically conducting polymers. In this conference, considerable attention was given to methods of achieving orientation and resulting properties. After a brief introduction, the discussion of orientation is presented in the following order: (1) methods of achieving orientation, (2) methods for obtaining single crystals.

Chain molecules, including those which contain conjugated unsaturated structures, may be highly oriented due to the manner of synthesis or processing. It is not surprising that electronic motion is found dependent on macromolecular orientation, as high electronic conductivity in organic polymeric solids requires the presence of a conduction band formed by the overlap of pi-orbitais. Orientation in polymers is of great interest, as it may be used to improve polymer properties or to assess mechanisms of electronic transport phenomena.

Unfortunately, few conducting polymers are processable by these conventional techniques which generally require the existence of an accessable glass transition or melting temperature. Ionic forces in the solid state tend to make partially oxidized, highly conducting polymers intractable.

One method of achieving orientation in a conducting polymer is the preparation of a prepolymer which is tractable. P.D. Towsand presented results on “Durham” polyacetylene (J. Feast) which is prepared by the thermal decomposition of a percursor polymer. A 20:1 ratio of ℓ to ℓ₀ may be obtained for the polyacetylene prepared in this way.

The resulting (CH)_x exhibits a well defined fiber pattern and provides the opportunity of determining whether intrachain or interchain processes are most efficient for generating photoinduced carriers. Towsand’s work on photoexcitation and charge transport in highly oriented Durham polyacetylene showed that there is a factor of 4 favoring interchain vs. intrachain motion.

H. Shirakawa gave an overview of methods for obtaining highly oriented polyacetylene. He described a new method using a nematic liquid crystalline host as a substrate for film growth. The films were grown in the presence of a magnetic field (H₀ = 10 Kgauss) to give well oriented fibrils. The conductivity parallel to the fiber axis was 1.2 × 10⁴S cm⁻¹, while that perpendicular to this direction was 4.8 × 10³ S cm⁻¹.

H. Naarman reported a new variation on the Zeigler-Natta route to polyacetylene. Naarman utilized a standard Ziegler-Natta catalyst in silicone oil at ambient temperature. This method gives a product which can be highly oriented (to 540%). The conductivity along the stretch direction is the highest reported thus far (1.7 × 10⁴ S cm⁻¹). Furthermore, a study of conductivity vs. time showed that iodine doped, highly oriented polyacetylene retained high conductivity in air for much longer periods of time than either unoriented or “Shirakawa” polyacetylene.

Other conducting polymeric materials exhibit anisotropic behavior. Conducting metal phtalocyanine/Kevlar^R molecular/macromolecular blends (MMB’s) have an anisotropic structure (Inabe, Marks, Wynne), but the conductivity appears isotropic or nearly so (peak conductivity is 5 S cm⁻¹). Fibers of doped MPcI_x/Kevlar^R may be spun out of strong acid solvents reflecting a degree of processability. The solid consist of microcrystallites of MPcI_x oriented along the fiber axis in an oriented crystalline Kevlar^R matrix. At MPcI_x loading levels of 40–50% by weight, where conductivity is nearly Maximum, a high modulus is retained.

Methods for Obtaining Polymer Single Crystals. R. Baughman pointed out that it is not possible to assess orientation definitively on the basis of a measurement such as conductivity, which is sensitive to effective conjugation length. He noted that it is best to combine measurement techniques, e.g., x-ray and spectroscopic methods. For the ultimate in detailed structural (x-ray) and spectroscopic (vibrational, optical) characterization single crystals required. H. Shirakawa noted that (SN)_x and polydiacetylene were the only polymer single crystals with unsaturated electronic structures.

To set the problem of obtaining single crystal conducting polymers in perspective, R. Baughman presented a general outline of methods for obtaining three dimensional order in organic polymeric solids. Three approaches were mentioned in the discussion:

In the context of this discussion, A. Epstein commented that one must always question the structure of a highly conducting material after doping relative to the structure of the starting polymer. There may be clustering (i.e., structural inhomogeneity) associated with dopant species (cations or anions). Generally, long range order is not observed. With the “new” structure after partial oxidation the question arises as to whether there will be a corresponding “new” magnetic behavior.

Electrochemical growth of charge transfer complexes was noted and A. MacDiarmid speculated that electrochemical growth of a conducting polymer might lead to a single crystal. The consensus was that this would be a most interesting and promising route which seems to have been overlooked.

Single crystals can give definitive physical information and oriented polymers may be useful for composite structures. However, B. Scrosatti pointed out that oriented materials generally have poorer kinetics with regard to electrode processes due to slower ion migration.

G. Farrington opened the panel discussion with some introductory remarks on the relevance of the amorphous phase of polymeric electrolytes for assuring high conductivity and on the possible routes which may be followed to lower the temperature range of stabilization of this phase.

At the beginning of this panel discussion, two subjects were introduced for open discussion. The first subject dealt with investigation of the lithium/solid polymer electrolyte interface. Based on A.C. complex plane impedance measuments, passivating layers are believed to be formed on the lithium when in contact with solid polymer electrolytes. During charging, this passivating layer may be responsible for non--uniform lithium deposition which can ultimately lead to dendrite formation. However, since these cells are usually fabricated in quite thin configurations in order to minimize IR drops across the solid polymer electrolyte, the large effective surface areas and resulting low current densities reduce the likelihood of dendrite formation. Several hundred cell cycles are possible. The second subject dealt with investigation of the interface between a polymer electrode and an organic liquid. Based on a number of studies, these polymer electrodes undergo a spontaneous self-discharge process upon standing. An understanding of factors contributing to loss of charge upon standing is crucial to the further development of polymer electrodes.

The panel discussion was actively animated by J. Bockris, Discussion Leader, who stressed from the beginning the need to correctly define the theoretical energy content of a battery (Wh/Kg or Wh/l) using the mean voltage of the theoretical discharge plateau and including the weight, or volume, of all chemical reactants involved in the electrode processes. A factor of 1/4 to 1/6 is usually observed between practical and theoretical energy content of current batteries because of dead weight: packaging, collectors, electrolyte... and inefficient electrochemical processes. It was recalled that conducting polymer batteries, like any other electrochemical generator, are inevitably subject to electrochemical phenomena that tend to reduce their energy efficiency; it is the case with IR drops or with activation and diffusion over-potentials present in the electrodes, electrolyte and at the interfaces. The members of the panel concluded to the necessity of defining the practical energy content of conducting polymer batteries in association with the power at which this energy is delivered, preferably in the form of conventional Ragone Plots: (Wh/Kg vs W/Kg) or (Wh/l vs W/l). Self--discharge and Cycle life were also identified as important characteristics that must be presented rigorously: it was suggested, for example, that deep-discharge cycle results (over 60%) be reported as they are much more demanding than superficial cycling.

As long as conducting polymers remained nonprocessible or thermally or environmentally unstable, there was little hope for commercial applications other than in specialty areas. However, major improvements to the properties of conducting polymers have been recently obtained. For example, it is now known that the presence of flexible substituents can provide solution or melt processibility without dramatically decreasing conductivities in the doped state. Also, a variety of conducting polymers is now known which are air stable and have reasonably high thermal stabilities.

Ionically conductive polymers are starting to be recognized as a unique class of electrolytes of great technological potential for applications in many fields. They are presently successfully applied to rechargeable lithium batteries as illustrated by recent published results. The major advantages of film solid state batteries result from the apparent compatibility of polyether complexes with all chemical reactants of the battery. Mechanical strength associated with flexibility of the electrolyte allow good cycling properties. Good energy and power densities are projected and partially demonstrated for warm batteries; at room temperature low to moderate power appear possible.