Study of the morphological structure of polyaniline base derivatives and impact on electronic properties of the doped form

Abstract

Extensive gel permeation chromatography coupled with surface structure and mechanical property measurements clearly indicate that polyaniline (pani) in the base form has a tendency to aggregate as a result of intermolecular hydrogen-bonding. The aggregation is present in the solid state powder. The degree of aggregation is found to be dependent on the prior history of the base powder material. In solution, the degree of aggregation is found to be dependent on the solvent, concentration, and temperature. As the solvent becomes a better solvent for the base material, the less aggregated is the structure. LiCl complexes the polymer via a `pseudo-doping' process, thereby disrupting the internal pani hydrogen-bonding and as a result, deaggregates the structure. As the polymer is deaggregated to different levels by a solvent or by LiCl, the individual chains can better be solvated and thus a conformational change occurs. The chains adapt a more expanded coil type of conformation. The degree of expansion depends on the solvation power of the solvent. These morphological changes have a dramatic effect on the surface structure properties of pani as well as on bulk properties such as mechanical properties and UV/visible/ near IR absorbance. As the level of deaggregation and subsequent chain expansion increases, the surface structure becomes smoother, the glass transition temperature decreases and a significant red shift is observed in the λmaximum of the absorbance. It is found that the LiCl induced morphological changes results in increased conductivity upon doping pani base with a protonic acid. In addition, a ring substituted hydroxyethyl pani is found to exhibit higher conductivity than the unsubstituted pani in NMP.