Self-doped PANIs

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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