Other Conducting Polymers

Many other types of ECP have been described. These include PS-block-PPO [61], boron containing PVA [62], polyethylene dioxythiophene/polystyrene sulfonate [65], PC-ABS composites [64], PEO composites [64], PEO complexes with sodium lanthanum tetrafluoride [63], chlorine substituted PANI [70], PVP-PVA coupled with potassium bromate [57], PANI-PA 6,6 composite films [72], talc-PPY composites [54], epoxy resin alpha-haematite nanorod composites [56], PP-montmorillonite composites [69], magnetite containing polymers [105], LDPE [27], PC-acrylonitrile-butadiene composites [106], sodium ion conducting PEO complexed with sodium lanthanum tetrafluoride [63], PVDE [107], PANI composites [108], PP novolac resins [109], dendrimers containing light switchable azobenzene [110],PVP/PVA [107] and PPY[111]. 

There is no doubt that doped polypyrrole is very much more stable than is polyacetylene, but reports are variable of exactly how stable it is. Street 393) reported that the polymer loses less than 20% of its initial conductivity after one year in air at ambient temperature and Diaz and Kanazawa 589) claimed that the polymer is stable at 100-200 °C, depending on the counter-ion the latter authors also reported that polypyrrole is undoped reversibly by ammonia treatment. Munstedt 590) found that the conductivity of doped polypyrrole was unchanged after 200 days at 80 °C in 

although polypyrrole appears superficially to be a stable material, it apparently undergoes quite complex chemical changes on storage even at room temperature. The contrast with polyacetylene is that the conjugated backbone of the polypyrrole chain is less reactive so that there is less destruction of conjugation and less decline in conductivity. 

Takenaka et al. 595) used XPS to study deterioration of a thin polythiophene film on an ITO electrode, such as might be used in a display device. After 105 dopingundoping cycles at 0.5 Hz with BF4 counter-ions there was evidence of extensive fluorination of the polymer, probably due to decomposition of the counter-ion or its hydrolysis by traces of water in the electrolyte. Corradini et al. 596) carried out a similar study of polypyrrole, polythiophene and their analogues with C104 counterions and concluded that all of the polymers are unstable to cycling or to standing in contact with the electrolyte, although polypyrrole performed best. 

The ionization potential of polyphenylene is around 8 eV and it is not surprising that oxidative degradation is not a problem the undoped polymer can withstand long periods at high temperatures in air with no change in its conductivity or its ability to dope 386). However, the high oxidation potential creates two problems. Firstly, the range of dopants with sufficient electron affinity to oxidize the polymer is limited, and there are few solvents in which the oxidation can take place without destruction of the solvent. Secondly, the doped polymer is expected to be reactive towards water and this is indeed the case 386). 

Elsenbaumer et aL 598 recently made a comparative study of stability of doped conducting polymers in air. They concluded that a combination of a stable polymer with a non-reactive dopant, such as butylthiophene-methylthiophene copolymer doped with FeClg, could give almost indefinite stability in air with an effective ceiling of 50 °C. 

FIGURE 18.16 (See color insert) Hole state in oxidized PPY. 

PPE and other conducting polymers sometimes do not have the desired physical properties. One may then combine them with other nonconducting polymers with the desired properties. This is referred to as a blend, or a polyblend. 

PS is widely used as a hard plastic. In its pure form, it is colorless. PS is not conducting, contrary to all the other polymers we have studied so far, and one may wonder why. The simplest reason is that the polymer chain is saturated. Electrons or holes due to doping would appear in the jt-system of the phenyl groups. The orbital overlap is too small to provide sufficient coupling between the nearest phenyl groups. X/A 1, which means that the electron is almost immobile on its phenyl group. 

Another question we may ask in connection with PS is why saturated chains are insulating (if holes can be created in a o chain). Doping has to be carried out in any case, so what is the difference between carriers in a jt-system and carriers in a saturated system Very likely, the difference lies in the delocalization of the carriers that 

FIGURE 18.19 Octomer of polystyrene (PS) containing a -(CH2) backbone with phenyl subunits. 

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These results illustrate that electrochemical techniques can be employed to synthesize a vast range of [Si(Pc)0]n-based molecular metals/conductive polymers with wide tunability in optical, magnetic, and electrical properties. Moreover, the structurally well-defined and well-ordered character of the polymer crystal structure offers the opportunity to explore structure/electro-chemical/collective properties and relationships to a depth not possible for most other conductive polymer systems. On a practical note, the present study helps to define those parameters crucial to the fabrication, from cheap, robust phthalocyanines, of efficient energy storage devices. 

The electrochemistry of polyaniline is more complex than that of other conducting polymers and given the large number of possible structures for the material, it is not surprising that many possible reaction schemes have been suggested [181,182, 195-197,205], Many of the properties of the material are pH-dependent [173,174, 206], including the open circuit potential [207] which is most positive at pH 0, and this is further complicated by the fact that not all the polymer chains are necessarily in exactly the same state at any given time [197]. Above pH 3 polyaniline films do not show any electroactivity, but are not electroactive even at low pH with... 

Solitons are considered to be important defect states in these conjugated polymers (see Fig. 6.48). It has however been shown that correlation energy is the more important factor in giving rise to the energy gap in (CH) (Soos Ramasesha, 1983). Other polymers related to polyacetylene are polythiophene, polypyrrole, poly-phenylenesulphide, and polyparaphenylene (Section 3.3). Extensive measurements on doped polyacetylenes have been reported in the last five years and these materials, unlike other conducting polymers such as (SN), seem to have good technological potential. 

Polypyrrole was the first conducting polymer used as ion-to-electron transducer in solid-state ISEs [43], and is still one of the most frequently used [45-68]. Other conducting polymers that have been applied as ion-to-electron transducers in solid-state ISEs include poly(l-hexyl-3,4-dimethylpyrrole) [69,70], poly(3-octylthiophene) [44,70-74], poly(3,4-ethylenedioxythiophene) [75-86], poly(3-methylthiophene) [87], polyaniline [44,67,73,88-99], polyindole [100,101], poly(a-naphthylamine) [102], poly(o-anisidine) [67] and poly(o-aminophenol) [103], The monomer structures are shown in Fig. 4.1. 

The cation radicals of terthiophene 18 reversibly dimerize even at low concentration (92JA2728). As a consequence, 71-dimers and 7r-stacks deserve attention as entities responsible for the properties of oxidized polythiophene and other conducting polymers. 

Unsubstituted polyacetylene, like many other conductive polymers, is an intractable material and thus its processing into useful shapes and morphologies is limited. One solution to the processing problems has been the use of soluble precursor polymers that can be transformed into conductive polymers. Application of ROMP in the formation of soluble polyacetylene precursors was elegantly pioneered by Feast and coworkers [61]. Using this approach, a precursor polymer is synthesized by the ROMP of a cyclobutene derivative. Once synthesized, the precursor polymer can undergo a thermally promoted, retro-Diels Alder reaction to split off an aromatic fragment and produce polyacetylene, Eq. (42). 

Thus a plot of E x versus l/n is linear for oligomeric substrands of known conducting polymers and should extrapolate to Emax = 0 for the perfectly degenerate, conjugated, infinite, linear, and "metallic" polymer. For instance, Ej, = 0 for graphite and for (SN)Y. For all other conducting polymers, this zero is not reached, because the polymer has finite strand length or because of conformational distortions or other defects. 

The origin of the conduction mechanism has been a source of controversy ever since conducting polymers were first discovered. At first, doping was assumed to simply remove electrons from the top of the valence band (oxidation) or add electrons to the bottom of the conduction band (reduction). This model associates charge carriers with free spins (unpaired electrons). However, the measured conductivity in doped polyacetylene (and other conducting polymers such as polyphenylene and polypyrrole) is r greater than what can be accounted for on the basis of free spin alone. 

Other conducting polymers can be investigated as supports for dispersing catalytic metallic particles. Shan and Pickup [21] used a composite of poly(3,4-ethylenedioxythiophene) and poly(styrene-4-sulfonate) (PEDOT/PSS) to disperse Pt particles. They compared the performances of such electrodes with carbon-supported Pt for the electroreduction of oxygen, and they found similar exchange... 

Electrochemical doping is reversible and thus polymers which can be successfully cycled between two dopant levels can serve as rechargeable electrodes. Only polyacetylene, poly(p-phenylene) and poly(p-phenylene sulfide) are both oxidizable and reducible. Other conducting polymers can only be either p- or n-doped. 

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