Electromechanical Characterization of Polyelectrolyte Gels by Indentation

Abstract

We report an indentation method to quantify the electromechanical coupling in polyelectrolyte gels (PGs). PGs produce electric fields in response to mechanical stress and are therefore promising for mechanical sensor applications. The method exposes thin gel samples to well-defined pressure distributions through a spherical indentor, while the electrical response is measured with an array of platinum electrodes embedded in the support. A series of copolymer gels of acrylamide and acrylic acid were synthesized and equilibrated at a fixed pH, leading to samples with systematically varying spatial densities of both charged groups and cross-links. They were characterized by measuring the potential difference between the gel and the equilibrating solution (Donnan potential) as well as their electromechanical coupling through the indentation method. The electromechanical coupling was found to be proportional to the Donnan potential, while the latter is a universal function of the spatial density of ionizable groups in the gel, irrespective of the cross-link density.