Coarse-grained molecular dynamics simulations of interfacial structure and mixing in ionomer melts

Ionomers are polymers with modest fractions of charged groups. We perform coarse-grained molecular dynamics (MD) simulations to determine how synthetically controllable parameters (e.g., average ion spacing and level of neutralization) impact the interfacial adhesion of these materials. Specifically, we simulate freestanding ionomer films at different ion densities and subsequently bring them into contact. We use a Kremer-Grest-based model for the ionomers that includes uncharged backbone monomers, charged pendant groups, pendant stickers representing unneutralized acid groups, and unbound counterions. Composition profiles exhibit a layering of neutral and charged groups near the vacuum interfaces that becomes more pronounced with increasing ion density. Backbone bond orientation parameter results show a slight perpendicular orientation of backbones at the surfaces that becomes more pronounced with increasing ion content. We also track the mixing of ions and backbone monomers across adhering interfaces. We find that increasing ion content decreases the mixing of backbone monomers but increases the mixing of ions. We suggest that, at short time scales, an intermediate ion content yields the strongest interfacial adhesion.