Self-doped PANI

The various ring-substituted PANI systems include one that embodies the concept of self-doping sulphonated polyaniline (SPAN). In SPAN, about half the number of phenyl rings of emeraldine salt arc monosubstituted with an — SOj group (Yue et al. [327]). This polymer is x-ray amorphous, as is its de-protonated, non-conductive salt form obtained upon dissolution of 

SPAN in NaOH or KOH solution [303]. Even more than in the case of POT, the introduction of substituents has introduced disorder, in particular larger and more strongly fluctuating dihedral angles between successive phenyl rings. 

4 Emeraldine PANl doped with sulphonic acids 

From the diffraction data of emeraldine base fibres obtained by compensation of the PANI-DBSA fibres in 10% NH4OH the authors conclude that the earlier results by Pouget et al. [302] concerning EB-Il, obtained on poorly oriented films, should be corrected in the sense that the values for the a and c-axes should be interchanged. This implies a considerably shorter chain repeat period, 7.56 A instead of 10.05 A, i.e. a 

Films cast from various solutions have been investigated by the same authors [331], They find essentially two different x-ray diffraction patterns. In the case of PANI-CSA, these correspond to films cast from M-cresol and DMSO, respectively, and they are also found for distinct processing procedures of PANI-DBSA. Each of these diffraction patterns corresponds to a characteristic UV-vis absorption spectrum of the solid, which is also very similar to the spectrum of the solution used to prepare the material. The emergence of the different crystal phases is therefore attributed to different chain conformations in the solution from which the films were cast. A helical main chain conformation is proposed for the crystal phase obtained 

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Self-doped PANI are very interesting due to their unique electrochemical behavior unlike PANI, the self-doped polymer remains in its doped state in near neutral or alkaline media [28]. Fully self-doped PANIs are not easy to synthesize due to the lower reactivity of acid-functionalized anilines. Kim et al. [29, 30] introduced an alternative approach in the template-assisted enzymatic polymerization of aniline. Previously, only polyanionic templates had been used for PANI synthesis. However, acid-functionalized anilines bear a net anionic charge in aqueous solution, and attempts to use SPS as template with carboxyl-functionalized aniline resulted in red-brown colored polymers with no polaron transitions, regardless of the synthetic conditions. The use of polycationic templates, such as those shown in Figure 8.2 allowed the synthesis of linear and electrically conductive PANIs with self-doping ability due to the doping effect of the carboxyl groups present in the polymer backbone. 

Figure 8.2 Polycationic templates used during enzymatic synthesis of self-doped PANI. (Reproduced with permission from Kim et al. [29], Copyright (2007) Taylor Francis).

These novel materials exhibit many properties that are different from those of the parent PANI and show promising applications in various fields. For instance, self-doped PANI is highly soluble in alkaline aqueous solutions and many nonaqueous media unlike PANI. Good solubility is essential for a polymer in order to facilitate postsynthetic processing. The solubility of PANI is greatly improved by the presence of the substituted acidic group. ... 

It is also possible to polymerize the 3-aminobenzenesulfonic acid to produce polymetanilic acid which is also soluble. Thus, preparation of a solid polymer of this type is not possible in aqueous acidic solutions, but it may be possible in a neutral solution of aqueous-organic mixed medium. However, to exhibit the electrical and electrochemical properties, protonation of the imine nitrogen of the PANI backbone in poly(metanilic acid) is necessary, which requires an acidic solution. An acid group-substituted, self-doped PANI has better electrical and electrochemical properties over a wider pH range. 

In addition, self-doped PANI (Fig. 6.17) has an extended pH range of electrical conductivity and electrochemical activity, covering that of many biocatalysts and sensors. " ... 

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