Dynamics of regioregular conducting polymer electrodes in response to electrochemical stimuli

We report thickness shear mode resonator acoustic wave data for films of regioregular poly(3-hexylthiophene) films exposed to acetonitrile solutions of LiClO4. Both the current- and acoustic wave-responses to constant (or slowly swept) potential show different regimes, whose characteristics, which are quite different to those of regiorandom electropolymerized films, can be interpreted in terms of crystalline and non-crystalline material. Oxidation of crystalline material, which occurs at low potentials, is dominated by anion transfer; as a result of the associated electrostatic effects, the film stiffens. Oxidation of non-crystalline material, which occurs at more positive potentials, involves significant solvent transfer; the solvent softens the film. We have been able to determine film thickness changes and shear modulus components (based on a homogeneous film model) (i) as functions of potential (ie. charge state) and of temperature at fixed potential, and (ii) as a function of time following a potential step. In each case, we have been able to rationalise the electrochemical and viscoelastic phenomena in terms of electrochemically controlled film composition, of which the most significant component is solvation state.