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

Results relating to the application of conducting polymers as chemires istors for the sensing of vapors are reported. In particular, we descr ibe the use of a piezoelectric microbalance with gold electrodes on wh ich we have electrochemically deposited a thin layer of poly(pyrrole). Uptake of a selection of common neutral organic vapors, covering a ra nge of physical properties, is then measured at the same time as measu rement of the resistance of a similar thickness of poly(pyrrole) depos ited on a narrow-gap resistance probe. Mass uptake and resistance chan ge are considered together to avoid masking the real trends caused by differences in polymer-solvent compatibilities. Analysis of the mass m easurements shows that the BET adsorption isotherm is obeyed, and the number of bound layers is approximately four for methanol and water an d eight for dichloromethane. The fractional change in resistance varie s linearly with fractional mass uptake below about 5% mass change, and the number and type of adsorption sites are the same for the three so lvents. Analysis of the specific resistance change, i.e. resistance di vided 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 t he conductivity as an electron-transfer mechanism, for which the elect rochemical theory predicts a relationship between the rate constant an d the dielectric constant of the medium.