Evolution from gravimetric to viscoelastic response of poly(3-methylthiophene)-loaded acoustic wave resonators

Abstract

Acoustic wave resonators are used to explore the relationship between ion and solvent populations (composition) and shear moduli (dynamics) for poly(3-methylthiophene) films. The films were electropolymerized from monomer solutions in acetonitrile–LiClO₄ and subsequently characterized in a monomer-free electrolyte. Crystal impedance spectra provide a diagnostic for the rigid s. viscoelastic behaviour. Very thin films contain two types of zone, with different electrochemical responses and solvation characteristics. The more compact type of zone significantly influences the characteristics of very thin films. The more open, solvated type of zone dominates the characteristics of thicker films. Gravimetric interpretation of resonator frequency response shows that redox driven ion and solvent transfers exhibit compensatory motion: anions driven into the film to maintain electroneutrality displace some of the solvent present in undoped films. These transfers can be temporally resolved by variation of the potential scan rate in cyclic voltammetric experiments: solvent transfers more slowly. Resonator responses for thicker films are interpreted using a model in which a finite viscoelastic film contacts a semi-infinite Newtonian fluid. Compensatory motion of anions and solvent is again manifested through redox state independent film thickness; replacement of some solvent by denser dopant anions results in increased film density. Film shear moduli at 10 MHz are characteristic of lossy, viscoelastic materials. The storage shear modulus increases significantly (almost by a factor of two) upon p-doping of the film. The loss shear modulus, which is the larger component of the two, increases only marginally upon film doping.