Molecular dynamics of ionic polymer-metal composites

Ionic polymer-metal composites (IPMCs) constitute a promising class of soft, active materials with potentially ubiquitous use in science and engineering. Realizing the full potential of IPMCs calls for a deeper understanding of the mechanisms underpinning their most intriguing characteristics: the ability to deform under an electric field and the generation of a voltage upon mechanical deformation. These behaviours are tightly linked to physical phenomena at the level of atoms, including rearrangements of ions and molecules, along with the formation of sub-nanometre thick double layers on the surface of the metal electrodes. Several continuum theories have been developed to describe these phenomena, but their experimental and theoretical validation remains incomplete. IPMC modelling at the atomistic scale could beget valuable support for these efforts, by affording granular analysis of individual atoms. Here, we present a simplified atomistic model of IPMCs based on classical molecular dynamics. The three-dimensional IPMC membrane is constrained by two smooth walls, a simplified analogue of metal electrodes, impermeable only to counterions. The electric field is applied as an additional force acting on all the atoms. We demonstrate the feasibility of simulating counterions' migration and pile-up upon the application of an electric field, similar to experimental observations. By analysing the spatial configuration of atoms and stress distribution, we identify two mechanisms for stress generation. The presented model offers new insight into the physical underpinnings of actuation and sensing in IPMCs. This article is part of the theme issue 'Progress in mesoscale methods for fluid dynamics simulation'.