AN ELECTRON-SPIN-RESONANCE STUDY OF POLYMER INTERACTIONS WITH MOISTURE IN POLYANILINE AND ITS DERIVATIVES

Both experimental and theoretical studies reveal that the electronic b ehavior of polyaniline is sensitively dependent on moisture. For examp le, pumping under vacuum leads to a decrease in conductivity by an ord er of magnitude, decreased dielectric constant and increased Curie sus ceptibility. An understanding of the mechanism of conductivity enhance ment 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 i nvestigated the role played by water molecules in modifying the electr onic properties of polyaniline and related polymers. A number of exper iments has been performed which lead us to propose a 'variable-size me tallic island' model. That is, protonated and highly ordered regions ( metallic islands) are separated from unprotonated and amorphous region s by a less ordered region whose width depends on the amount of moistu re 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 propose d that the conductivity enhancement is due to increased size of the me tallic islands in the presence of moisture. We also show that water mo lecules are adsorbed at two sites: the N-site and the X-site. Furtherm ore, the observed ESR behaviors for polyaniline and its derivatives ar e interpreted as arising from the delocalized versus localized nature of the spins; water plays a crucial role in determining this behavior.