Electroactive polymer film dynamics

The crystal impedance technique was used to study, in situ, electroactive polymer film dynamics. An equivalent circuit transmission line model was applied to extract physical characteristics of polymer films in solution in terms of their shear storage (G') and loss (G'') moduli. This model comprises three components that describe the surface roughness features of the quartz oscillator, the overlaying viscoelastic polymer film and the contacting electrolyte solution. Crystal impedance measurements were made on poly(3-hexylthiophene) (PHT) films, exposed to propylene carbonate electrolyte solutions, that were electrochemically maintained at different fixed potentials (corresponding to a range of film oxidation states) and at different temperatures. The p-doped film is substantially softer than the undoped film and G' and G" can show maxima at partial p-doping. Spectra recorded at a range of frequencies, corresponding to the fundamental (10 MHz) and higher harmonics (30 MHz to 110 MHz), indicate that PHT shear moduli are frequency dependent substantial increases in shear modulus are found upon increasing perturbation frequency. Film shear modulus values also exhibited temperature dependence, increased temperature corresponding to a decrease in shear modulus value. The role of solvent upon electroactive polymer film dynamics was explored by exposing PHT films to polar (propylene carbonate) and non- polar (dichloromethane) media. p-Doped films were highly swollen in the polar solvent and relatively compact in the non-polar medium, these trends being reversed for undoped films. Thin PHT films were also found to exhibit mechanical resonance effects. This is a special situation in which the mechanical shear deformation across the polymer film corresponds to one quarter of the acoustic wavelength in that medium. The new phenomenon of peak splitting was also observed at film resonance. This data indicates spatial variation of the PHT film shear modulus.