Water-instigated changes in elastic modulus of polypyrrole studied by thin rod acoustic wave sensor

A thin metal fiber (the thin rod) has been configured as an acoustical delay line in either flexural or extensional mode for operation as a chemical sensor. Polypyrrole was deposited by an electrochemical procedure on the fiber and subject to cycles of water sorption and desorption. The responses of the device in terms of changes in phase angle was measured for experiments involving fibers with two radii, and for different thicknesses of polymer film. The highest sensitivity was found when the device was operated in the flexural mode for the fiber of smallest radius. Comparison of experimental and theoretical phase changes indicates that the major effect contributing to sensor response is the change of elastic modulus of the polymer film.