Actuation of ionic polymer-metal composites with alkali metal counterions - A molecular dynamics study

Ionic polymer-metal composites (IPMCs) are soft, electroactive materials with unique actuation properties that have attracted the interest of physicists, chemists, and engineers for over two decades. Despite significant progress in our understanding of the phenomenology of their actuation, we are yet to fully elucidate the physics at the nanoscale that underlies their macroscopic actuation. Previous experiments have shown that IPMC actuation depends on the type of counterions that is used to neutralize the acidic polymeric backbone. Some continuum theories have attempted to explain such a modulatory effect, but a complete understanding of the physics at the nanoscale level is lacking. Here, we employ classical molecular dynamics to fill this gap in knowledge. Building upon recent developments in the field, we investigate the response of three IPMC membranes with different metallic counterions that have been considered in earlier experimental research. While we do not detect variations in the axial stress, the examination of the spatial distribution of the through-the-thickness stress components in the three membranes reveals important differences. We show that these differences are well explained in terms of variations in water content as it relates to the type of counterions, challenging existing continuum models of IPMCs that mostly overlooked these factors. Overall, our work brings to light new physics within active materials, inspiring new efforts in material design and engineering, as well as multiscale modeling of soft matter.