MECHANISTIC STUDIES ON THE INTERACTIONS BETWEEN POLY(PYRROLE) AND ORGANIC VAPORS

Results relating to the application of conducting polymers as chemiresistors for the sensing of vapors are reported. In particular, we describe the use of a piezoelectric microbalance with gold electrodes on which we have electrochemically deposited a thin layer of poly(pyrrole). Uptake of a selection of common neutral organic vapors, covering a range of physical properties, is then measured at the same time as measurement of the resistance of a similar thickness of poly(pyrrole) deposited on a narrow-gap resistance probe. Mass uptake and resistance change are considered together to avoid masking the real trends caused by differences in polymer-solvent compatibilities. Analysis of the mass measurements shows that the BET adsorption isotherm is obeyed, and the number of bound layers is approximately four for methanol and water and eight for dichloromethane. The fractional change in resistance varies linearly with fractional mass uptake below about 5% mass change, and the number and type of adsorption sites are the same for the three solvents. Analysis of the specific resistance change, i.e. resistance divided by mass, for a range of solvents reveals a positive correlation with the dielectric constant of the solvent. This can be explained by treating the variable range electron hopping process responsible for the conductivity as an electron-transfer mechanism, for which the electrochemical theory predicts a relationship between the rate constant and the dielectric constant of the medium.