Neutral polyaniline

Polyanilines have been processed fi om solutions of neutral polyaniline in NMP, DMPU and others for some time[l,2]. Significant strides have been made in making fibers form these solutions. None eless, the technique suffers fi om the disadvantage that processed articles are non-conductive and need to be doped in a secondary step. The technique is not suited for preparation of coatings on a commercial scale. Processability of polyaniline in the doped form is more attractive as it removes the subsequent doping step. Functionalized protonic acids such as camphor sulfonic acid, preferably in the presence of m-cresol and dodecyl... 

Thompson and coworkers (83,90] found a reduction in corrosion in samples of steel that had been coated with neutral polyaniline (EB) and exposed to a salt solution (3.5% NaCI) and an acid environment (0.1 N HCl) after doping" and applying a top coat. They found, however, that they were able to achieve good corrosion protection with only certain dopants moreover, adhesion problems prevented them from obtaining reproducible and acceptable results. 

The measurements and the results shown in Figs. 19.66 and Figs. 19.67 make it clear that there is a fundamental reduction in the corrosion rate in the specimens with doped polyaniline in HCl and for the specimens with neutral polyaniline in an NaCI environment. 

D,e/np/s Neutral polyaniline coated on steel then topcoated with a layer of epoxy and drilled with a 1.2 mm diameter pinhole to the steel surface... 

The potentiodynamic scans shown in Fig. 31.19 indicate that there is strong passivation by the polypyrrole/ polyurethane composite on mild steel in 3.5% NaCl. There was fast and stable passivation on the exposed steel area. In the case of neutral polyaniline on mild steel, no passive appearance was noted but a decrease of corrosion current was observed. This result is also very consistent with the measured corrosion rates discussed earlier where some of the conducting polymers have better corrosion protection (D,e/py/s and D,e/np/ s) to exposed steel areas compared to others (D,e/dp/s and D,e/zk/s) in this corrosion environment after a long exposure time. 

The IBM group led by Brusic et al. [57,58] also studied the use of polyaniline derivatives for corrosion protection of copper as well as silver. The unsubstituted polyaniline, in neutral base form, provided good corrosion protection both at open-circuit potential and at high anodic potentials. The dissolution of metal (both Cu and Ag) was decreased by a factor of 100 when the metal surface was completely covered by the neutral polyaniline. However, polyaniline doped with dodecylbenzene-sulfonic acid (the conductive form of the polymer) increased the corrosion rate of Cu and Ag in water. The doped polymer in contact with the metal is spontaneously reduced at a rate faster than the oxygen reduction rate. The faster cathodic process in turn increases the overall rate of the anodic reaction, which is the dissolution of Cu and Ag, as opposed to the formation of a passive oxide layer. 

Fig. 11-25 DMTA from cast films of (a) as-made neutral polyaniline (b) THF soluble fraction (c) NMP soluble fraction. After Reference [429], reproduced with permission.

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

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 ... 

Polyaniline (PANI) was investigated as electrocatalyst for the oxygen reduction reaction in the acidic and neutral solutions. Galvanostatic discharge tests and cyclic voltammetry of catalytic electrodes based on polyaniline in oxygen-saturated electrolytes indicate that polyaniline catalyzes two-electron reduction of molecular oxygen to H2O2 and HO2". 

Other Substrates Deposition of cadmium was also studied on Bi, Sn and Pb [303], Ni [304], reticulated vitreous carbon [305], Ti [306], and indium tin oxide [307]. UPD of Cd on tellurium results in CdTe formation [270, 308]. Electrodes coated with conducting polymers were also used to deposit cadmium electrochemi-cally. In the case of polyaniline, the metal reduction potential corresponds to the neutral (nonconducting) state of the polymer, therefore cadmium was found to deposit on the substrate-glassy carbon electrode surface, in the open pores of the polymer film [309, 310]. 

Osteryoung and coworkers have also investigated the use of chloroaluminate ionic liquids for the synthesis of polyaniline [62, 63]. Unlike pyrrole and thiophene, aniline was successfully polymerized in acidic, neutral and basic chloroaluminate melts, although the best results were obtained using the neutral composition. 

Figure 67 Mechanism of polyaniline formation involving the coupling of a dication or cation with neutral monomer. (From Ref. 288.)...

Alternative oxidants such as potassium iodate were also explored for the intrazeolite polymerization of aniline in NaY and acidic forms of Y zeolite. With peroxydisulfate, the polymerization proceeded only if a sufficient supply of intrazeolite protons was available. No polymer formed in either NaY or in acid zeolites with neutral iodate solution, but at low pH polyaniline was obtained in all hosts. The open nature of the zeolite host, even when partially filled with polymer, permits the introduction of base (such as ammonia). On admission of ammonia into the emeraldine salt-containing zeolite, the protonated polymer was converted into the neutral emeraldine base form. 

Finally, the oxidation of D-glucose at Pt-based electrocatalysts incorporated in polypyrrole [55,56] or in polyaniline [57] was also considered. The first work [55] was carried out in Pt-doped polypyrrole films in a neutral medium (phosphate buffer) in view of biosensor applications. Then the use of Pt-Pd catalysts dispersed in PPy led to higher current densities of glucose oxidation than on pure metal dispersed in PPy. This may be related to the decrease of catalytic poisoning (by adsorbed CO as shown by infrared reflectance spectroscopy [58]), due to the presence of Pd. 

Polyanilines have been shown to be compatible with biomolecules in neutral and near neutral aqueous solutions - a medium of preference for most biomolecules [97, 105],... 

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