Modulation of electroactive polymer film dynamics by metal ion complexation and redox switching

Abstract

The viscoelastic properties of [Ni(salen)(crown ether)] conducting polymer films were characterized using high frequency acoustic wave resonators. Shear moduli values respond to film redox state and barium ion complexation. The variations in storage and loss moduli are significant, but always such that the films are viscoelastic, i.e. a soft material. Both components are increased by polymer oxidation, the loss modulus to a greater extent. In the absence of Ba²⁺, the similarity of shear moduli- and nanogravimetric-potential signatures (for acoustically thick and thin films, respectively) provides a compositional explanation for changes in film dynamics. Ba²⁺ uptake restricts the extent of film redox switching. In the presence of Ba²⁺ the pattern of redox driven shear modulus changes is unaltered, but the values of both components are systematically higher. These changes are interpreted in terms of (i) space limited accommodation of counter ions (ii) crown-Ba²⁺-crown crosslinking between polymer chains, and (iii) decreased free volume for plasticizing solvent. We suggest that the first of these is the origin of the restricted charge injection (controlled by electroneutrality), the second of the increased storage moduli, and the third of the increased loss moduli. Observations at different time scales, via acoustic wave resonator harmonics, reveal increases in both shear moduli components with increasing frequency; these are considered in the light of simple mechanical models for polymer dynamics.