Effect of Ion Content on Adhesion of Ionomers from Molecular Dynamics Simulations

Ionomers are polymers that contain a small amount of charged groups attached to a non-polar backbone, used in packaging and other applications. We consider a dense melt of ionomers and their counterions without solvent, in which aggregation of ions significantly impacts material properties. We use a coarse-grained model that includes uncharged backbone monomers, charged pendants and counterions, and sticker groups that represent unneutralized acid groups (similar to uncharged monomers but placed pendant to the backbone and with additional cohesive interactions with themselves and ionic groups). To understand interfacial behavior of ionic aggregates, we simulate a freestanding ionomer film thick enough to reach approximately bulk behavior in the center of the film. We find that aggregates are depleted near the interface, in agreement with prior experimental results. Density profiles indicate a pronounced layering of ionic aggregates close to the interface. To study adhesion, we analyze ionic aggregate and polymer backbone behavior after two surfaces are brought together; effects of pendant group mole fraction and neutralization level will be discussed.