Polymer, conducting

Ordinary polymers are good insulators, and they are widely used in this capacity, as insulating covering 

The discrepancy is resolved in the following way. A linear chain of equispaced atoms in a metal, such as a chain of sodium atoms, is found to be energetically unstable. Instead, the spacing between the atoms will adjust itself so that an energy gap opens 

Polyacetylene is transformed into a metallic conductor by doping. This involves oxidation or reduction of the polymer. Electron acceptors such as halogens (chlorine, iodine, etc.) oxidise the polymer. In this process, electrons are taken from the filled lower band and used to form halide ions, leaving holes, which result in a p-type material. A typical reaction is  

The iodine enters the polymer between the molecular chains. 

Doping with alkali metals (lithium, sodium, etc.) reduces the polymer. In this process, the alkali metal 

Conducting polymers constitute another category of promising pseudo-capacitive materials. The most common ones include polypyrrole (PPy), polyaniline (PANI), and poly-(3,4)-ethylenedioxythiophene (PEDOT). This group is of particular interest due to low cost and ease of synthesis. These compounds can be polymerized directly onto a collector material via EPD. Alternatively, the polymerization can be done within surfactant emulsions 

Specific capacitance for vanadium nitride nanocrystals synthesized at 400°C and tested at varying scan rates and mass loadings onto collector in 1M KOH. Source Choi, D. G. E. Blomgren, and P. N. Kumta. 2006. Advanced Materials, 18,1178-1182. With permission.) 

Work by Yan et al. on PANI and graphene composites showed an electrode capacitance as high as 1000 F.gy in an aqueous electrolyte [101]. The same research also showed that incorporation of CNT additives helped enhance percolation and mechanical strength during the doping process, enabling the electrode to retain 94% of the original capacitance after 1000 cycles compared to retention below 50% without the CNT additive [101]. 

Current collector Undoped plate conducting polymer film 

The p-doping (a) and n-doping (b) of polymers as they undergo charging and discharging. (Source Rudge, A. et al. 1994. Conducting polymers as active materials in electrochemical capacitors, Journal of power sources, 47,89-107.) 

6 Conducting Polymers Electronically conducting polymers (such as polypyrrole, polythiophene, and polyaniline) have attracted considerable attention due to their ability to switch reversibly between the positively charged conductive state and a neutral, essentially insulating, form and to incorporate and expel anionic species (from and to the surrounding solution), upon oxidation or reduction  

FIGURE 4-18 Permselective coatings flow injection response of a poly(l,2-diaminoben-zene)-coated electrode to the following a, hydrogen peroxide (1 mM) b, ascorbic acid (1 mM) c, uric acid (1 mM) d, L-cysteine (1 mM) and e, control human serum. (Reproduced with permission from reference 63.) 

FIGURE 4-19 Scanning electron micrograph of a polyaniline-coated electrode. 

These polymers are readily prepared by in-situ electropolymerization (from the monomer solution). The oxidation of the monomer proceeds according to 

FIGURE 4-21 Conductivity range of common conducting polymers, along with their chemical structure. (Reproduced with permission from reference 72.) 

Often the first step in the electropolymerization process is the electrooxida-tive formation of a radical cation from the starting monomer. This step is commonly followed by a dimerization process, followed by further oxidation and coupling reactions. Well-adhered films can thus be formed on the surface in galvanostatic, potentiostatic, or multiscan experiments. The behavior of elec- 

Electropolymerization can also be used for the design of molecularly imprinted polymers (MIPs), capable of interacting with the analyte (template) molecule with high affinity and specificity (103,104). This is accomplished by electropolymerizing polypyrrole, polyaniline, or poly(o-phenylenediamine) in the presence of the analyte (template) molecule. At the end of the polymer- 

FIGURE 4-20 Use of negatively charged polymeric films for excluding anionic inteifer- 

The application of conducting polymers such as polyaniline, polypyrrole, and polythiophene for immobilizing capture antibodies in immunoassay systems is widespread. 

Electrochemical Sensors, Biosensors and Their Biomedical Applications 

A class of polymers that is the subject of active research are the conducting polymers. These are conjugated polymers that become electrically conducting when suitably doped with either electron donors such as alkali metals or electron acceptors such as iodine. 

Polyacetylene, (CH) , is the archetype of conducting polymers. It consists of alternating double and single bonds. As shown in Fig. 10.16 it exists as cis or trans forms, the latter is the most important. Recent work has moved away from polyacetylene itself, because it is difficult to process and is air sensitive. However, it remains important as a test-bed for understanding the conduction mechanism in this type of polymer. 

Early work on polyacetylene was only able to cover part of the spectral range and apparently suffers fi om a calibration error [28]. Fig. 10.17a shows the INS spectrum of polyacetylene [29] recorded on TFXA. The spectrum in the region below 700 cm is remarkable in that it consists of a series of terraces, each terminating in a bandhead. This is reminiscent of the Vs mode of polyethylene and suggests that the modes are strongly dispersed. This is confirmed by the dispersion curves [30], and the resulting INS spectrum calculated from them. Fig. 10.17b and c. 

The infrared and Raman spectra of polyacetylene undergo very large changes when the polymer is doped. This is because the electronic structure of the polymer has changed dramatically and infrared and Raman spectral intensities depend on the electronic properties. The metallic nature of the resulting materials also makes measuring the spectra difficult. All of these effects are irrelevant to the neutron so it 

In Fig. 10.18 we compare the INS spectra of oriented films of pristine /ra -polyacetylene and in the sodimn and potassium doped states. Even with poor resolution, differences are apparent between the spectra. The most striking is the gap at 80 cm that appears in the doped polymers. Molecular dynamics simulations show that the translational mode that peaks at 80 cm in the pristine sample shifts down to 64 cm in the doped samples and the mode at 160 cm shifts up to 230 cm.  

The majority of polymeric materials do not conduct electricity to any appreciable extent indeed, their frequently excellent insulating behaviour accounts for most of the applications of synthetic organic polymers in the electronics and electrical industries. Nevertheless there are a significant number of polymeric materials which do conduct electricity and these are the focus of our attention here. Other polymeric materials are already of importance in the sensors field and are discussed at various points throughout the book. 

In practice, the term conducting polymers includes a range of materials which display a wide variety of properties and consequently fit many diverse areas of established or potential application. The range of conductivity of interest is enclosed at one end of the scale by values associated with a typical good insulator, such as polyethylene, c. 10 (Ocm) and at the other end by those associated with a typical metallic conductor, such as copper, c. 10 (Qcm) Conductivity may be an intrinsic property of the material, for example poly(sulphurnitride) (SN), the first non-metallic metal , has a room 

The electrical properties of an infinite polyene have intrigued theoretical chemists for more than fifty years. Shortly after Hiickel introduced his 7i-electron theory for unsaturated systems in 1931, speculations began to appear about the carbon-carbon bond lengths to be expected in an infinite polyene. The question was interesting because, if the bonds were of equal length, 7i-electron theory predicted that the 7i-molecular orbitals would form a continuous band which would be half-filled, whereas alternating carbon- 

Carbon-carbon bond alternation Carbon-carbon bond equality — CH=CH — CH = CH —CH = CH— -CH-CH-CH-CH-CH-CH- 

To the naked eye the material produced by the Shirakawa technique appears to be a film, but electron microscopy reveals a fairly complex 

In 1975, a polymeric superconductor was found, (SN)x with Tc = 0.26 K. Apparently, this discovery had a greater impact on the physics community than the earlier efforts by the chemists in the field of conducting polymers. 

Although the motion of protons does not lead to electrical conduction in the case of benzoic acid, electronic and even ionic conductivity can be found in other molecular crystals. A well-studied example of ionic conduction is a film of polyethylene oxide (PEO) which forms complex structures if one adds alkaline halides (AX). Its ionic conductivity compares with that of normal inorganic ionic conductors (log [cr (Q cm)] -2.5). Other polymers with EO-units show a similar behavior when they are doped with salts. Lithium batteries have been built with this type of 

Furthermore, recent radiotracer experiments have shown that metals such as Cu, Ag, and Au can diffuse into various polymers including polyimides and polycarbonates at elevated temperatures. Arrhenius type temperature dependences are not always found. This is not unexpected considering the distribution of saddle-point energies in amorphous polymers [F. Faupel, R. Willecke (1994)]. 

Thin polymer films have many possible technical applications. Transistors and light-emitting diodes are the obvious ones. In ultra-thin films, one may even approach an electronics of molecular dimension. Molecular electronics will be a future challenge for basic and applied science. Nature applies it on a large scale in the reaction centers of the photosynthetic process, where photoinduced mobile charges are separated in some analogy to the separation of the photo-(p-n)-pair in the junction zone of a semiconductor (see Section 13.3.1). 

Ahlefeld, G., VOlkl, J. (1978) Hydrogen in Metals, Topics in Applied Physics, 28, 29, Springer, Berlin 

Colomban, P. (1992) Proton Conductors, Cambridge University Press, Cambridge 

Jennifer M. Pringle, Maria Forsyth, and Douglas R. MacFariane 

The utilization of ionic liquids for the synthesis and use of conducting polymers brings together two of the most exciting and promising areas of research from recent years. 

Research into conducting polymers has been increasingly intense for the last 25 years, since MacDiarmid, Heeger and Shirakawa published their seminal work on polyacetylene, which demonstrated that the conductivity of these materials can be increased by several orders of magnitude by doping with anions [6, 7]. The importance of these materials and the progress made in this field is reflected in the award of the Nobel Prize for Chemistry in 2000 to these founding researchers in this area. 

However, to allow the widespread use of conducting polymers, more research is needed to improve their general performance, and one of their present limitations is the rapid degradation of key properties such as conductivity and electrochemical cydability. This limitation is primarily a result of the electrolyte used in the 

Electrodeposition from Ionic Liquids. Edited by F. Endres, D. MacFariane, A. Abbott Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31565-9 

Maria Judith Percino and Victor Manuel Chapela 

A polymer (material containing a long chain of similar molecular structures) is the first and foremost electrical and heat insulator. The idea that polymers or plastics could conduct electricity had been considered absurd. Their wide application as an insulating material is the reason they are studied and developed in the first place. In fact, these materials are commonly used for surrounding copper wires and manufacturing the outer structures of electrical appliances that prevent humans from coming in direct contact with electricity. 

The polyene can be transformed into a conducting form when electrons are removed from the backbone resulting in cations or added to the backbone resulting in anions. Anions and cations act as charge carriers, hopping from one site to another under the influence of an electrical 

Handbook of Polymer Synthesis, Characterization, and Processing, First Edition. Edited by Enrique Saldivar-Guerra and Eduardo Vivaldo-Lima. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

Alkoxy-subtituted Poly(2,5-dial koxy)paraphenylenevinylene MEH-PPV 

There are three principal mechanisms whereby a polymer may exhibit electrical conductivity  

To produce conduction by the first method in an otherwise non-conducting polymer, the required concentration of metallic particles is likely to be greater than about 20% by volume, or 70% by weight. This high particle concentration is necessary in order to achieve continuity of conducting material from one side of the polymer sample to the other. Such high concentrations tend to destroy some of the desirable mechanical properties of the polymer, but composites of this kind are used in conducting paints and in anti-static applications. Conductivities of order 6 x 10 m  

30% by weight can be used to achieve conductivities of about 100 2 m and has the advantages of not changing the density of the polymer much and being cheap. In such composite materials the polymer itself takes no part in the conduction process and acts merely as a support for the conducting material. In the rest of section 9.3 these materials are not considered further instead, attention is directed towards the mechanisms (ii) and (iii) by which conduction takes place through the polymer itself 

The following section introduces some of the more important materials, and discusses their chemistry, preparation, conductivity and other important aspects relating to their potential for exploitation. 

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Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

In addition to conventional applications in conducting polymers and electrooptical devices, a number of recent novel applications have emerged. Switching of DNA electron transfer upon single-strand/double-strand hybridization fonns the basis for a new medical biosensor teclmology. Since the number of base pairs of length 20... 

Skotheim T A 1986 Handbook of Conducting Polymers vois 1 and 2 (New York Dekker)... 

Reversible oxidation and reduction of polymers is commonly used to increase conductivity in these systems. Ions from the electrolyte are usually incorporated into the polymer as part of this process (see Electrically conducting polymers). 

The conducting polymer poly(sulfur nitride) is unusual in that it is crystalline, consisting of chains of sulfur and nitrogen packed in parallel. 

Although its use as a transparent electrode in diodes has made it one of the most useful of the conductive polymers, the intractable nature of the material made the stmcture of polyaniline difficult to determine. Recent studies of polyphenyleneamineimines have conclusively shown that the stmcture of PANI is an exclusively para-linked system (21). 

Polymers. The Tt-conjugated polymers used in semiconducting appHcations are usually insulating, with semiconducting or metallic properties induced by doping (see Flectrically conductive polymers). Most of the polymers of this type can be prepared by standard methods. The increasing use of polymers in devices in the last decade has led to a great deal of study to improve the processabiUty of thin films of commonly used polymers. 

More recently, Raman spectroscopy has been used to investigate the vibrational spectroscopy of polymer Hquid crystals (46) (see Liquid crystalline materials), the kinetics of polymerization (47) (see Kinetic measurements), synthetic polymers and mbbers (48), and stress and strain in fibers and composites (49) (see Composite materials). The relationship between Raman spectra and the stmcture of conjugated and conducting polymers has been reviewed (50,51). In addition, a general review of ft-Raman studies of polymers has been pubUshed (52). 

A low (<0.4 W / (m-K)) thermal conductivity polymer, fabricated iato alow density foam consisting of a multitude of tiny closed ceUs, provides good thermal performance. CeUular plastic thermal insulation can be used in the 4—350 K temperature range. CeUular plastic materials have been developed in... 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Fig. 7. Schematic representation of enzyme covalently bound to a functionalized conductive polymer where ( ) represents the functional group on the polymer and (B) the active site on the enzyme (42). Courtesy of the American Chemical Society.

Polypyrroles. Highly stable, flexible films of polypyrrole ate obtained by electrolytic oxidation of the appropriate pyrrole monomers (46). The films are not affected by air and can be heated to 250°C with Htde effect. It is beheved that the pyrrole units remain intact and that linking is by the a-carbons. Copolymerization of pyrrole with /V-methy1pyrro1e yields compositions of varying electrical conductivity, depending on the monomer ratio. Conductivities as high as 10 /(n-m) have been reported (47) (see Electrically conductive polymers). 

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