Blue emeraldine base

Under inert conditions, the conductive emeraldine salt of polyaniline does not react with copper. In air, it is transformed fast (within a minute) to the nonconductive blue emeraldine base and copper gets oxidized to Cu(I). In a slower process (within several hours), Cu(I) is then transformed to Cu(II). It was demonstrated that the presence of polyaniline changes the oxidation behavior of the copper. Copper evaporated onto polyaniline does not chemically react with the polyaniline or the tosylate anion and forms a dense layer of copper islands. 

The blue emeraldine base obtained after filtration is dried under dynamic vacuum. 

There are, however, some problems with this method. First, high molecular weight fractions are not soluble in the strongest solvent, A-methyl pyrroli-done, for emeraldine base. Second, Wessling has shown by membrane filtration and photon correlation spectroscopy that these transparent, clear blue liquids are in fact dispersions and not solutions. 

It can be conveniently prepared as an analytically pure, off-white powder, by the reduction of emeraldine base,28,29 the most commonly used reducing agents being phenyl hydrazine or hydrazine.7 Similarly to emeraldine base, free-standing films can be cast from NMP solutions.7 The surfaces of the particles in the powdei or of the film, are oxidized relatively slowly by air, turning blue however 13c NMR studies show that the bulk of the polymer is still in the leucoemeraldine oxidation state. 

The emeraldine base form of PANI (compound 1) was prepared by chemical oxidation method described by MacDiarmid and co-workers [33]. The PANI was extracted with THF until the extract was colorless. The dried PANI (0.25 g) reacted with 3-chIoropro-pyl thiol (0.5 ml) and ferric chloride (0.01 g) in anhydrous dimethyl sulfoide at 81°C for 24 hr. The black solution (propyl thiol substituted PANI) was precipitated with IM HCl, a dark green to black precipitate was formed the filtrate was greenish blue the precipitate was washed with IM HCl, then with acetonitrile. The precipitate (propyl thiol substituted PANI) was dried under continuous vacuum for 48 hr. 

Kinetic studies by Shim and Park [132] determined the autocatalytic rate for polyaniline growth. Other reaction kinetic studies and comparison of the properties of the chemically synthesized emeraldine to the electrochemical product led Mohilner et al. [131] to conclude that the oxidation of aniline occurs via a free radical mechanism that produces the emeraldine octamer as the primary product. Emeraldine is a blue-violet base form of polyaniline that forms dark green salts as described by Green and Woodhead [133]. Emeraldine loses two protons upon ox-... 

Self-doping was eonfirmed by the similarity between absorption speetra of the sulfonated polyaniline and the emeraldine hydrochloride form (Figure 20.44). The effect of the sulfonate group on steric interactions between adjacent rings is evident from the blue shift in the absorption spectra of the sodium salt of the non-protonated sulfonated derivative compared to the emeraldine base (Figure 20.45). 

The intermediate oxidation state has relatively complicated spectra, dependent upon the pH and dominated by the presence of charge carriers (polarons) in the polymer. Two forms of the polymer exist in this state, each having a proposed 3 1 benzenoid quinoid conqx)sition, the emeraldine base ( B) and the protonated emeraldine salt (ES). The visible spectra of the EB form closely matches that of the pemigraniline form since friey both contain similar functional units, the only difference observed in the band at 2.2 eV, which is red shifted to 2.0 eV (620 nm) (18). This red shift accounts for the blue color of this form. The ES form has somewhat more conq>licated spectra, which is a direct ramification of the presence of polarons in the polymer since the population of polaronic sites is dependent upon the concentration of acid in the solution. Generally, though the ES form possesses the following uv-vis bands, 1.5 (830 nm), 2.75-3.1 (450 nm), and 1.0 (1240 nm). The band at 1.0 eV is assigned to an intrachain free-carrier excitation, vdiile the variable band at 2.75-3.1 eV is due to polaron band and the band at 1.5 eV is the excitonic transition observed for EB (17,19). 

Using special dispersion techniques it is possible to make dispersions of PAni in several mainly polar solvents and, more recently, in water. The most efficient solvents for dispersing PAni are NMP and DMSO. Such solvents are used by many other groups for dissolving neutral PAni (emeraldine base, EB). We have shown by membrane filtration and by photon correlation spectroscopy (see Section IV.K) that the EB in NMP or DMSO, which are transparent clear blue liquids, are in fact also dispersions and not solutions. 

Polyaniline in the form of conductive emeraldine salt can be easily synthesized by chemical oxidation of aniline in aqueous acidic media [12,13]. Both inorganic acids (hydrochloric, sulfuric, etc.) and organic acids (methanesulfonic, toluenesulfonic, etc.,) can be used. Thus synthesized polyaniline salt is a dark blue-green powder with a typical bulk conductivity of 1-10 S/cm. In this form it is infusible and insoluble. The emeraldine salt can be converted to insulating emeraldine base on treatment with an alkaline solution. Ammonium hydroxide is most commonly employed to effect deprotonation. Repeated treatments are often necessary for complete deprotonation. The emeraldine base is a coppery bronze powder with a conductivity of less than I0 S/cm. Figure 38.1 shows the structure and property changes associated with the emeraldine base-emeraldine salt interconversion that forms the basis of a widely practiced route for processing polyanilines and is the subject of numerous publications. 

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