Manipulation of electroactive polymer film viscoelasticity: the roles of applied potential and frequency

We describe quartz crystal impedance measurements on thin films of poly(3-h exylthiophene) (PHT) electrochemically maintained at different potentials a nd exposed to propylene carbonate electrolyte solutions. Film shear modulus values, obtained at fixed potentials corresponding to a range of film oxid ation states ("doping levels"), show a marked variation of storage and loss moduli (G' and G ", respectively). The p-doped film is substantially softe r than the undoped film, and G' and G " can show maxima at partial p-doping . Even in nominally "equilibrium" experiments (at fixed potential) there is dramatic hysteresis in shear modulus values determined during stepwise dop ing and undoping. This general pattern of behaviour is observed at a range of frequencies, corresponding to the fundamental frequency (10 MHz) and hig her harmonics (30 MHz to 110 MHz). There are substantial increases in shear modulus with increasing frequency for all doping levels, and the loss tang ent (G "/G') is frequency dependent. A Voigt model is qualitatively incompa tible with these observations, and a Maxwell model can qualitatively explai n some features; more sophisticated models are required to provide quantita tive explanations. We discuss these observations in terms of potential-driv en film ion and solvent population changes. The data are consistent with no n-equilibrium film solvent populations for intermediate doping levels, even though the equilibrium ion populations (charge states) may be established. Together, selection of operating frequency, applied potential and time sca le offer the prospect of manipulating film viscoelastic parameters in a con trollable manner over several orders of magnitude, from "rubbery" to near " glassy" behaviour.