Morphology-dependent mass transport model for mechanoelectrochemistry of polypyrrole

Abstract

Conducting polymers undergo volumetric deformations due to ingress/egress of counter-ions and solvent molecules during electrochemical reduction/oxidation. A precise measurement of volumetric deformations (as extensional strain) has been elusive due to lack of experimental methods and hence, different mathematical models make simplifying assumptions in attributing the strain to various transported species. Recently, an investigation into static and dynamic mechanoelectrochemistry of polypyrrole doped with dodecylbenzenesulfonate (PPy(DBS)) has provided morphology-dependent strain data for PPy(DBS) and can be used to precisely attribute volumetric strain of PPy(DBS) to various components that move into and out of the polymer. To address the longstanding need for a mass transport based mechanics model, we present a morphology-dependent mathematical framework that attributes the strain during electrochemical reduction/oxidation of PPy(DBS) to cations and solvent (water) molecules. This model builds upon a chemomechanical constitutive model for conducting polymers and combines it with measured strain to derive fundamental structure-property-function relationships in PPy(DBS). The polymer’s morphology is represented by macroscopic porosity in the oxidized state and it is observed that the porosity of the polymer in the oxidized state decreases linearly with electrochemical deposition charge density (C cm−2). The osmotic pressure due to ion and water transport is found to be inversely proportional to porosity of the polymer, and number of water molecules transported appear to be governed by the interplay between osmotic pressure and compliance of the film. These results are anticipated to serve as guidelines in the design of robust biomedical devices and energy storage materials using PPy(DBS) and other conducting polymers in aqueous/organic solvents.