Viscoelastic characterization of electroactive polymer films at the electrode/solution interface

TLDR

The study introduces a general transmission line model that captures the viscoelastic behavior of thin polymer films via their shear moduli. The model incorporates the lumped-element approximation as a limiting case and is applied to analyze crystal impedance spectra obtained during electrodeposition of poly(2,2′-bithiophene) films. During early electrodeposition the PBT film behaves as a rigidly coupled mass, then transitions to a Maxwell fluid with storage and loss moduli rising to ≈5 × 10⁶ dyn cm⁻², corresponding to an acoustic decay length of ~0.3 µm; films thicker than this act as bulk material and exhibit resonance effects.

Abstract

We describe and explore a new general transmission line model that describes the viscoelastic characteristics of thin films in terms of their shear moduli. The model contains within it the commonly used lumped-elementmodel as a limiting case, and we delineate the conditions under which thesimpler lumped-element model can be applied without significant error. We apply our analysis to the interpretation of crystal impedance spectra acquired dynamically during the electrodeposition of poly(2,2′-bithiophene) (PBT) conducting polymer films. In the very early stages of deposition, PBTentrapped within surface features behaves as a rigidly coupled mass. Beyondthis, it is a viscoelastic material behaving as a Maxwell fluid. PBT storage and loss moduli are initially polymer coverage dependent, rising to limiting values G′≈G″≈5×106 dyn cm-2. These values imply an acoustic decay length of ca. 0.3 µm. Accordingly, films significantly thicker than this behave as bulk material. For thick films there is also evidence for resonance effects.