Synthesis of nanostructured polyaniline

The organization of my thesis is as follows: (a) Chapter III describes the synthesis of bulk quantities of polyaniline nanofibers in one step using a simple and versatile high ionic strength aqueous system (HCl/NaCl) that permits the use of pure H2O2 as a mild oxidant without any added metal or enzyme catalyst. Polyaniline nanofibers obtained are highly conducting, sigma&sim1--5 S/cm, and spectroscopically similar to conventional polyaniline synthesized using stronger oxidants. The synthesis method is further extended to the synthesis of oligoanilines of controlled molecular weight, e.g., aniline tetramer, octamer, and hexadecamer. Microns long tetramer nanofibers are synthesized using this method. (b) Chapter IV describes the mechanism of nanofiber formation in polyaniline. It is proposed that the surfaces such as the walls of the reaction vessel and/or intentionally added surfaces play a dramatic role in the evolution of nanofibrillar morphology. Nucleation sites on surfaces promote the accumulation of aniline dimer that reacts further to yield aniline tetramer, which (surprisingly) is entirely in form of nanofibers and whose morphology is transcribed to the bulk by a double heterogeneous nucleation mechanism. This unexpected phenomenon could form the basis of nanofiber formation in all classes of precipitation polymerization systems. (c) Chapter V is the mechanistic study on the formation of oligoanilines during the chemical oxidation of aniline in weakly acidic, neutral or basic media using peroxydisulfate oxidant. It is proposed that the reaction proceeds via the intermediacy of benzoquinone monoimine that is formed as a result of a Boyland-Sims rearrangement of aniline. The initial role of peroxydisulfate is to provide a pathway for the formation of benzoquinone monoimine intermediate that is followed by a conjugate Michael-type addition reaction with aniline or sulfated anilines. The products isolated in pH 2.5--10.0 buffers are intermediate species at various stages of hydrolysis. Confirmatory evidence is obtained when a spectroscopically similar product is formed when solid 1,4-benzoquinone is added to an aqueous solution of aniline at room temperature in the absence of peroxydisulfate. These findings also offer a rationale for the residual sulfur and the high C/N ratios frequently observed in the product. (d) Chapter VI describes a flexible, lightweight, reversible NO2 vapor sensor based on resistance changes of a thin film of a doped aniline oligomers and poly-o-toluidine deposited on plastic substrates. Unlike the irreversible signals typically observed in organic films, signal reversibility in the concentration range 100--5 ppm is readily achieved using a short burst of UV irradiation at room temperature in ambient air without the aid of heating or pump down cycles. In case of poly-o-toluidine based sensor, it is observed that the signal response is dependent on the organic solvent used to cast the polymer film with dipolar aprotic solvents yielding films showing a strong response. There are also striking differences in electrical and electronic properties including morphology of films cast from different solvents. This phenomenon is traced to polymer chain conformation in solution that is preserved in the solid state (cast film) which opens a new structural vector to tailor sensor response and selectivity. This study also expands the use of conducting polymers for the detection of vapors that were previously believed to be chemically too aggressive. (Abstract shortened by UMI.)...