Viscoelastic characterization of benzo-crown ether functionalized electroactive films

Abstract

Electrochemical, acoustic wave and imaging (AFM) methods were used to characterize electroactive poly[Ni(3-Mesalophen-b15-c5)] films. The distinct physical principles of these techniques generate different responses to film characteristics, such that their simplistic correlation in the context of a single uniform film model does not adequately describe the data. Film behaviour can only be rationalised using a two layer model comprising a dense (poorly solvated) inner layer and an outer layer comprising islands of diffuse (highly solvated) polymer. The two layers can be observed directly by AFM imaging. The balance of their contributions shifts with polymer coverage: the inner layer dominates thin films and the outer layer dominates thick films. The inner layer is acoustically more rigid and, even if ion transport is slow, is sufficiently thin as to pose no major barrier to electrochemical communication with the underlying electrode. For very thin films, gravimetric interpretation of the acoustic wave response correlates simply with coulometric data; thicker films require viscoelastic phenomena to be considered. The low polymer segment density of the outer layer facilitates ion transport but results in weak acoustic coupling, such that the acoustic and coulometric assays differ. By combining the AFM, acoustic wave and electrochemical responses one can establish a quantitative picture of component layer populations and characteristics.