Self-doped conducting polymers processability

In the case of polypyrrole, both of these situations have been reported. Obviously, then, the transport is optimized if it is dominated by only one diffusing species with a fast rate. In an attempt to ensure just this, a novel method was devised by the synthesis of a self-doped conducting polymer [60]. The key feature in this structure is the chemical anchoring of a dopant (anion) in the polymer chain itself, thus ensuring that cations are the mobile species during the redox process. Figure 4 shows the structure of the pyrrole-based polymer. 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibility, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with filming sulfuric acid (31). A variety of other soluble substituted N-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

Polyaniline is the conducting polymer most commonly used as an electrocatalyst and immobilizer for biomolecules [258-260]. However, for biosensor applications, a nearly neutral pH environment is required, since most biocatalysts (enzymes) operate only in neutral or slightly acidic or alkaline solutions. Therefore, it has been difficult or impossible to couple enzyme catalyzed electron transfer processes involving solution species with electron transport or electrochemical redox reactions of mostly polyaniline and its derivatives. Polyaniline is conducting and electroactive only in its protonated (proton doped) form i.e., at low pH valnes. At pH values above 3 or 4, polyaniline is insulating and electrochemically inactive. Self-doped polyaniline exhibits redox activity and electronic conductivity over an extended pH range, which greatly expands its applicability toward biosensors [209, 210, 261]. Therefore, the use of self-doped polyaniline and its derivatives could, in principle. 

Structural modifications of polyaniline have mainly been exploited to achieve improved processability and environmental stability. In general, the substituted polyanilines can be obtained via oxidative polymerization of the corresponding monomer. However, inductive and steric effects can make such monomers difficult to polymerize [42]. Several substituted polyanilines have been prepared by varying the nature (alkyl, alkoxy, halogen, etc.) and the position (2- vs 3-, 5-positions) of the substituent [24, 27-32, 34, 37, 43, 44]. These studies have shown that regardless of the nature and position of the substituent group, there is an adverse effect on polymerization and the properties of the polymer such as conductivity and electroactivity. To overcome these limitations, various synthetic methods have been developed to prepare self-doped sulfonated polyanilines. These methods involve controlled postpolymerization modifications by synthetic reactions on the whole polymer and copolymerization of less reactive monomers with aniline as described below. 

The conductivity of self-doped PABA without heat treatment was observed to be around 0.96 S/cm. This conductivity value is consistent with the 21 % doping suggested by NMR based on the conductivities of other forms of self-doped polyanilines [110, 113]. However, the conductivity was lower than HCl doped polyaniline, probably due to distortion of the polymer backbone by the presence of the boronic acid substituent [116-118]. After heat treatment at 100 and 500°C, a decrease in conductivity from 0.96 S/cm (without heat treatment) to 0.094 and 0.009 S/cm, respectively was observed. However, in the case of polyaniline, the conductivities of the heat treated polyaniline declined significantly compared with that of the self-doped PABA. The relative decrease in conductivity of heat treated PABA was less than that of HCl-doped polyaniline, probably due to the formation of a thermally stable boronic acid anhydride crosslink. In the case of polyaniline, the dramatic decrease in conductivity was a result of the decomposition of the backbone above 420 °C, as seen in the thermograms. In contrast, the process of crosslinking in the PABA polymer above 100 °C may make... 

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