Polyaniline deprotonated base

Polyaniline (Scheme 3), is not strictly a heteroaromatic monomer, hut has a number of similarities to polypyrrole. There are differences, however, one of which is that are three redox levels each of which may be protonated or deprotonated. Of these, the pH-driven switch between the emeraldine (17) salt and emeraldine base (18) is attracting interest. 

The spectrum shown in Fig. 5.5a is for the doped BaO-polyaniline sample, which exhibits absorption bands at 420 nm and 780 nm, characteristic of Bronsted acid-doped polyaniline. The spectrum shown in Fig. 5.5b is for the same sample after treatment with a NH4OH 1.0 mol dm solution. As can be seen, the 420 nm and 780 nm absorption bands remain at the same positions, even after treatment in basic solution. This is not observed for the free polymer, for which a reaction with base promotes deprotonation, which causes the appearance of an absorption band at 625 nm. 

A second method of introducing sulfonate groups is accomplished by deprotonating polyaniline base and reacting with a sultone, i.e., 1,3-propanesultone [22]. This gives rise to an N-substituted polyaniline derivative that is water soluble. Another route involves the polymerization of sulfonated aniline monomer such as sodium salt of diphenylaminesulfonic acid [23]. 

The formation of electrically conductive fibers of polyaniline blended with poly(p-phenylene terephthalamide) processed from sulfuric acid was also reported by Hsu et al. [72]. A thoroughly mixed solution of 0.2 wt% emeraldine base and 17.6 wt% poly(p-phenylene terephthalamide) was dissolved in sulfuric acid and this dope solution was extruded at 80°C through a 20-hole spinneret into a 1°C water coagulation bath. The continuous 20 filament yarns that were collected on the bobbin were thoroughly washed with water, which probably led to partial deprotonation of the emeraldine salt. However, the... 

In addition to acids, polyaniline also responds well to bases. Figure 2b shows the response of the emeraldine salt form of polyaniline to anunonia. The resistance increases because polyaniline undergoes a transition from the emeraldine salt (conducting form) to the emeraldine base (insulating form). This conversion leads to a resistance increase of about two orders of magnitude for the nanofiber film. This response is much smaller and slower than the response to acid because of a different mechanism associated with each reaction. Upon exposure to base, a deprotonation step occurs removing the acid. This step takes longer than protonation so the response is slower. The deprotonation step is likely slower because, upon exposure of the emeraldine salt form to... 

Chiang and MacDiarmid [139] proposed four idealized representations of polyaniline as illustrated in Fig. 4. The polymers on the right (IS and 2S) are derived from the ammonium salts the polymers on the left (lA and 2A) are derived from the amine forms. These forms are easily interconverted through the use of either chemical or electrochemical techniques. For example, treatment of A with any protonic acid, such as hydrochloric acid, will convert form A to form S. The treatment of S with a base such as potassium hydroxide will deprotonate the polymer and regenerate the amine form. 

Oxidation State Studies. For the oxidation state studies polyaniline was chemically synthesized by addition of an ammonium persulfate solution (in 1.0 M HCl) to a stirred aniline solution (in 1.0 M HCl) over a two and one half-hour period. The temperature of the reaction mixture was controlled at -30 C. LiCl (6.0 M) was present to prohibit freezing. For this reaction, the aniline oxidant molar ratio was 2 1. The total reaction time was 24 hours after which the resultant emeraldine salt was washed with 6.0 L of deionized water. The emeraldine salt was deprotonated by stirring in 3 wt% NH40H for 25 hours and washed with 6.0 L deionized water. Oligomer was removed by stirring the emeraldine base in methanol for 45 minutes followed by rinsing with 6.0 L methanol. Gel permeation chromatography (polystyrene standards, N-methyl-2-pyrrolidinone/0.05 M LiBr as eluent, 80 C) was... 

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. 

Polyaniline can be "switched" by the addition of acids and bases that protonate and deprotonate the base sites within the polymer. This leads to the dependence of the polymer states, and thus the reactions, upon the pH of the solutions. IN solutions of pH greater than 4, polyaniline loses it electroactivity entirely because the emeraldine salt, the only conducting form of the polymer, is dedoped to form the insulator emeraldine base (EB). 

Polyaniline (PANl) was also utilized to develop a conductometric pH sensor. The response mechanism of the polymer film originates from the deprotonation of the polymer backbone nitrogen atoms when exposed to aqueous envirMiments. In acidic solution, emeraldine polyaniline exists in the form of a salt (ES), and in alkaline solution it is converted to the base form (EB). The function of the sensor depends on the change in different forms of polyaniline which arises due to the deprotonation of nitrogen atoms. Thus, the conductivity of the sensitive layer is changed when PANl interacts with the hydronium or hydroxyl ions present in the solution. PANl is an amorphous film, and the electrolyte penetrates into the film and interacts with individual particles of the polymer. The charge carriers can pass... 

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