Explaining conductivity polyaniline

The monomer of the conducting polymer polyaniline is the compound aniline (aminobenzene). (a) Draw the structural formula of the aniline monomer. What is the hybridization of the N atom in (b) aniline (c) polyaniline (d) Indicate the locations of lone pairs, if any, in polyaniline, (e) Do the N atoms help to carry the current Explain your reasoning. See Box 19.1. 

The formation of -aminodiphenylamine is supposed to be the key intermediate in the formation of a dark green precipitate at the electrode surface during continued electrolysis of acidic aniline solutions. This has been characterized as an oligomer of aniline, for example, as the octamer emeraldine formed by a cascade of head-to-tail condensations [38,39]. Nelson, however, explained it as a mixture of mainly quinhydrone with a small amount of benzidine salt [37]. Today the electropolymerization of aniline under strongly acidic conditions is intensively studied as an important way to form the conducting polymer polyaniline [40] (see Chapters 31 and 32). 

In this section we will present the models used to describe relaxation phenomena in polymers or other materials. Then, we will present the special case of conducting polymers and the physical origin of the relaxation will be outlined. We will then propose a model able to explain the microwave properties of polyaniline samples. Experimental results concerning the effect of structural parameters of conducting polymers will be given at the end of this section. 

We have shown that the hexahydrous ferric chloride completely changes the microstructure of the blend. We observe a phase segregation creation of PVC nodules similar to the one formed by re-precipitation of PVC in the same conditions. A recent study [172] shows that in the case of polyaniline, the imine structure can be protonated with an acid HCl (by-product of the synthesis). The free water in the medium dissociates the acid, allowing the protonation of the chain, and explaining the increase in conductivity. This conductivity rise is about two decades when six water molecules are added for each molecule of FeClj, while the conductivity of the blend produced with FeCb, 6 H2O is lower than the one produced by anhydrous FeClj [127],... 

One application brings the conducting polymer as a dispersion coating to the surface [66]. A powder of polyaniline (with a particle size in the range of some tens of nanometers) is dispersed in a conventional paint, and this is painted onto the metal surface. The authors use a very low powder concentration of 4% and explain the corrosion-protection properties with a percolation model. 

Acrylic acid doped polyaniline exhibits sensor response in terms of the dc electric resistance on exposure to ammonia. The change in resistance is found to decrease linearly with NH3 concentration up to 58 ppm and saturate thereafter [33]. The decrease in resistance is explained on the basis of removal of a proton from the acrylic acid dopant by the ammonia molecules, thereby rendering free conduction sites in the polymer matrix. 

The conductivity of the conducting polymers in the oxidized state is not as easy to explain. The main feature of the conducting polymers is the chain structure of alternating double bonds. The structure can change from a benzoic to quinoidic form. For the most important conducting polymers like polythiophene, polypyrrole, and polyaniline, the two structures... 

PANI/silica hybrid nanofiber webs were prepared by using two different methods. The first one was in situ polymerization of aniline-doped silica nanofibers in an ammonium persulfate solution and the second one was immersing the silica nanofibers into a PANl solution. Effects of different pol5mierization parameters on conductivity were investigated. The PANI/silica hybrid nanofiber web electric conductivity increased with increasing monomer concentration. The electrical conductivities of the hybrid web were 5 X 10-5,17 X 10-3, 4 5 X 10-3, 3 2 x 10 3, and 1.07 S/cm for 0.2, 0.5, 0.7, and 1.0 M aniline solutions and pure polyaniline, respectively. The electrical conductivity showed the maximum at 1.0 of the molar ratio of the oxidant and aniline and decreased with an increase of the oxidant concentration. This result was explained by the prevention of polymerization by excess oxidant. The molar ratio of oxidant and aniline is generally about 1.0 for s5mthesis of PANI. 

This explains why there is no principal difference between the raw (doped) polyaniline and its dispersions, in whatever medium. The differences to be observed were only of quantitative, and not of qualitative nature, at least not in the direction, which was expected by most of those who still favor the fibril hypothesis. They believe that the chain is the primary active unit, which could also be dissolved and is believed to have conductive properties, even as a single chain. If that were the case, a dispersed (i.e., mechanically separated, in case of assumed fibrillar morphology, even destroyed) conductive polymer would not have the same conductivity and especially not the same transport properties in the dispersion medium above the critical volume concentration or after deposition from a low molecular weight liquid medium and drying. 

Conducting polymers have shown very promising results for application in gas sensors [107] and are currently used in electronic nose systems [108]. Because of the insolubility of these materials, chemoresistors are, in general, prepared by electrodeposition [107aj. Especially considering the limited reproducibility of this method, there remains a need for alternative methods for the preparation of chemoresistors [108bj. The authors of the paper presented a simple, wafer-scale fabrication method based on a commercial polyaniline product (experimental details explained in Ref. [106]). 

However, the production of all-organic fibers from ICPs is still complicated and expensive. Several attempts have been made to obtain conductive fibers from ICPs such as polyaniline, PEDOT PSS, and pure PEDOT with conductivity values from 150 to 250 S/cm [22,23]. However, due to poor mechanical strength, microscale size, a low production rate, brittleness, and difficult processing, useful commercial applications are stiU limited. On the other hand, combination of other textile materials with ICPs could enhance their application areas. It could be done in two ways, either by mixing ICPs with insulating polymers such as polypropylene, polyethylene, and polystyrene, or by coating conventional textiles with ICPs. The coating method is explained in the next section. 

To explain the unusually high conductivity of these polyblends of polyaniline Min et al. [112,113] have suggested secondary doping, which has been defined as an inert substance (vapour or liquid) which promotes the conductivity of an already doped form of the conducting polymers. These may also induce molecular conformational changes to reduce conjugation defects resulting in increased intramolecular conductivity and may also enhance crystallinity. It has been observed that in the absence of w-cresol the conductivity of camphor sulphonic acid-doped polyaniline film is... 

Leclerc et al. [78] have made a comparative study of the electropolymerized POT with those of alkoxy-substituted polyanilines. While POT exhibits an absorption maximum (310 nm) blue-shifted from PANI (315 nm), poly(2-methoxyaniline) shows a slightly red-shifted absorption peak (325 nm). The redox potential of poly(2-methoxyaniline) (0.08 V) is lower than that of POT (0.20 V). The electronic effect of the methoxy substituent alone cannot explain these observations and therefore steric as well as electronic effects of the substituents must be taken into consideration. It has been found that POT is less planar than alkoxy substituted polyanilines. However, the alkoxy mono-substituted polyanilines exhibit lower conductivity than that of POT, since the selectivity of the head-to-tail couplings is reduced by the incorporation of a methoxy group. Nevertheless, more regular and processable materials have been obtained with poly(2,5 -dialkoxyanilines). 

---