Counterion Size and Polarity Effects on Ionomer Dynamics

Numerous studies have shown that clustering of ionizable polymers controls the structure and dynamics of these macromolecules and consequently affects their properties. The clustering process is impacted by several factors, among them polymer topology and electrostatic interactions. With the goal of understanding the interrelation between clustering and polymer dynamics, the current study uses atomistic molecular dynamics (MD) simulations to probe the effects of counterion size and polarity on the dynamics of polymer melts using polystyrene sulfonate in the ionomer regime with sulfonation fraction f=0.09, below its entanglement length. The counterion chosen is N R₄⁺ with R=H or CH₃, varying the ratio of CH₃:H affecting the size and the polarity of the cation. The systems were built in Biovia, Molecular Studio, and ran in LAMMPS and GROMACS. The static and dynamic structure factors were calculated and correlated with the characteristics of the ionic clusters. We find that this series of counterions significantly modifies the cluster size and shape in comparison to small inorganic cations such as Na⁺, impacting the structure and dynamics of the melts. The structure of the melts as the counterion is varied will be first discussed followed by introducing the dynamic structure factor S(q,t) and mean square displacement studies that together capture the motion of the polymers on multiple length scales.