An electron-spin-resonance study of polymer interactions with moisture in polyaniline and its derivatives

Both experimental and theoretical studies reveal that the electronic behavior of polyaniline is sensitively dependent on moisture. For example, pumping under vacuum leads to a decrease in conductivity by an order of magnitude, decreased dielectric constant and increased Curie susceptibility. An understanding of the mechanism of conductivity enhancement and changes in the other properties due to absorbed moisture thus requires better insight into the nature of the absorbed water. It has been known for some time that molecular oxygen, through its magnetic interactions with polarons, broadens the electron-spin-resonance (ESR) signal in polyaniline. Using this property as an ESR probe, we have investigated the role played by water molecules in modifying the electronic properties of polyaniline and related polymers. A number of experiments has been performed which lead us to propose a ‘variable-size metallic island’ model. That is, protonated and highly ordered regions (metallic islands) are separated from unprotonated and amorphous regions by a less ordered region, whose width depends on the amount of moisture present in the sample. The water reduces the width of the boundary region by introducing order and thereby transforming partly localized polarons into either delocalized polarons or bipolarons. It is proposed that the conductivity enhancement is due to increased size of the metallic islands in the presence of moisture. We also show that water molecules are adsorbed at two sites: the N-site and the X-site. Furthermore, the observed ESR behaviors for polyaniline and its derivatives are interpreted as arising from the delocalized versus localized nature of the spins; water plays a crucial role in determining this behavior.