Polyemeraldine base

More recently, polymers soluble in the doped (i.e. conducting) state have been prepared by the use of appropriately functionalized dopants. For example, it was demonstrated that acid-base doping (protonation) of polyemeraldine base with sulphonic acids [9] or phosphoric acid diesters [10] results in the fabrication of soluble conducting polyaniline. 

Alternatively, a polyemeraldine base can be prepared by electrochemical oxidation of aniline in electrolytes consisting of aqueous acidic solutions [48,49]. Good quality polyaniline him can be obtained if an NH4F 2.3HF eutectic mixture is used as the electrolyte [50]. Electrochemically prepared polyemeraldine can then be reduced electrochemically to give polyleucoemeraldine. 

It should be noted here that a polyemeraldine base is stable in air and no special precautions must be undertaken during its spectroscopic investigations. On the contrary, polyleucoemeraldine is air sensitive and undergoes partial oxidation even with minute amounts of air. Therefore all operations in which polyleucoemeraldine is involved must be carried out in an inert gas atmosphere. 

The solubility of both polyleucoemeraldine and polyemeraldine bases in N-methyl-2-pyirolidone allows for the registration of C NMR spectra of these compounds [51]. 

A much more complicated spectrum is expected for the polyemeraldine base. The co-existence of the oxidized and reduced units in 1 1 ratio will produce eight groups of non-equivalent carbons (Scheme 4.4). 

Figure 4.6 shows the solution C NMR spectrum of the polyemeraldine base. 

Due to significantly enhanced chain stiffness in the polyemeraldine base, as compared to polyleucoemer-aldine, the contributions of possible chain conformations may not average out and may give rise to distinctly different NMR peaks, A similar effect is expected for chain ends and for all types of coupling defects which can be introduced in the course of the chain growth, All these effects will of course increase the number of the observed NMR lines. 

Kenwright et al. [51] and Ni el al. [53] have proposed the assignment of principal NMR lines of polyemeraldine base by comparison with low molecular model compounds. The use of the d,e.p,t,-90 technique (distortionless enhancement by polarization transfer) allowed for the differentiation between proton-bonded carbons and those carbon atoms which did not form bonds will hydrogen. The proposed assignment is presented in Figure 4.6. 

Figure 4.6. C NMR spectrum of polyemeraldine base. (Reproduced from Polymer 33, 4292 (1992), A.M. Kenwright, et al. Copyright 1992, with kind permission from Elsevier Science Ltd, Kidlington 0X5 1GB, UK.)...

We found that when fully reduced leucoemeraldine film is exposed to air, the 2 eV peak appeared quickly, within the first several hours. The intensity continued to increase for the first two days, saturated during the fourth day, and remained almost constant during several more days of exposure. The relative intensity of the 2 eV to 4 eV absorption peaks of this auto-oxidized and air-stabilized polyaniline was much smaller than that of the polyemeraldine base. Comparing with the spectra of the various oxidation states of octaaniline, we conclude that the degree of oxidation for this sample is near to y=0.25 (corresponding protoemeraldine). 

It has been observed that the surface of the conducting polymer plays an important role in the effective immobilisation of the desired enzyme. The Langmuir-Blodgett (LB) technique can be successfully applied to deposit a desired monolayer with the desired orientation of the biomolecules/enzymes [142-145]. Ramanathan and co-workers [146] have utilised the polyemeraldine base LB films for the immobilisation of GOD. These films have been shown to function as amperometric glucose biosensors and have a linear range from 5 to 50 mM. LB films of PT immobilised with GOD and urease have also been prepared for application to respective biosensors [147, 148]. 

L-B films of polyemeraldine base have been deposited on ITO glass substrates by injecting a solution of 60% CHCI3 in N-methyl phenazine containing 100 pi of GOD. The activity of GOD immobilised in these polyemeraldine base films determined by the o-dianisidine procedure has been found to be 5 lU cm [146]. 

Pandey, S.S., M. Gerard, A.L. Sharma, and B.D. Malhotra. 2000. Thermal analysis of chemically synthesized polyemeraldine base. Appl Polym Sci 75 (1) 149—155. 

In this procedure polyemeraldine base was prepared first by polymerization of aniline with (NH4)2S20g in hydrochloric acid followed by its deprotonation with aqueous ammonia as recommended in [17]. 

As it can be seen from Table 1 the method of preparation of polymer-supported HPA strongly influences the doping level that can be achieved in this process. Thus the polymers that are doped in a two-step procedure (PAc and PANI) contain smaller amount of the dopant than the polymers that were doped in a one-step reaction (PPy and PANI). This may be due to the fact that during the preparation by the former method HPA are dispersed mainly on the surface of the polymer while in the latter one they are introduced also into the bulk of the polymer. Indeed we have observed differences in the doping level in the case of PANI/HPA when polyemeraldine base of various surface area was used for protonation. 

---