Coating of electrode surfaces with thin films of electroactive polymers offers versatile and powerful means of manipulating interfacial composition and properties. Selective use of polymers and coating conditions allows one to produce electrodes with chemical, optical or electronic properties suitable for use in applications as diverse as electrocatalysis, sensors, electrochromic devices, batteries and supercapacitors. It also offers a means to generate and characterize new materials with novel characteristics.
The long term goal is rational interfacial design, delivering specified functions for the given application. A pre-requisite for achieving this is a detailed understanding of the relationships between interfacial composition, structure and properties. Recent progress has been enhanced by the simultaneous application of non-electrochemical probes to electrochemically controlled interfaces. For example, spectroscopic techniques have provided the identities of species, the nature of the local environment, and the distribution of electronic charge.
This presentation describes how acoustic wave techniques yield insight into redox driven changes in composition of electroactive polymer films. For acoustically thin ("rigid") films, thickness shear mode resonators function as gravimetric sensors ("microbalances"). Crystal resonant frequency changes are directly related to changes in the mass of the surface-immobilised film; sensitivity is at the sub-monolayer level. Long time scale responses yield equilibrium data for the overall compositional change of the film (ion and solvent populations). Short time scale frequency responses are kinetically controlled: the rate of change of the resonant frequency yields ion and solvent fluxes, i.e. polymer/solution interfacial transfer dynamics.
For acoustically thick ("non-rigid") films, resonator response is predominantly determined by film viscoelastic properties. Detailed analysis of the crystal resonance (notably frequency shift and damping) yields film shear modulus components. Quite generally, one would expect solvent penetration - as detected gravimetrically for acoustically thin films - should lead to softening of the polymer. This prediction of a correlation between solvent population (film composition) and polymer viscoelasticity (film dynamics) will be explored for redox and conducting polymer films. A common presumption in the field of polymer modified electrodes is that the film is homogeneous, both in terms of composition and properties. However, general experience of polymer adsorption studies suggests this is not the most likely scenario. We explore this question using neutron reflectivity measurements. This technique is analogous to optical reflection (ellipsometry), except that the reflectivity is sensitive not only to the elemental composition but also to the isotopic composition. This "contrast variation" method provides a powerful and selective means of identifying the spatial locations of component species within the system. We use a range of deuterated solvents to determine the spatial distribution of solvent within redox and conducting polymer films. The general result is that film composition is spatially variant, with significantly greater local solvent volume fractions at the polymer/solution interface than within the bulk of the film. The implications of this for film dynamics will be discussed.
