Ionomer Self-Assembly in Dilute Solution: A Molecular Dynamics Study

Abstract

Self-assembly of ionomer chains in dilute solution is studied by classical molecular dynamics. Based on a bead-spring ionomer model, the simulation approach captures formation of cylindrical, bundle-like [1] aggregates with a hydrophobic core region, a surface layer of charged anionic headgroups and a halo of counterions (protons) in the surrounding aqueous phase. Ionomer aggregation enforces the stretching and stiffening of ionomer chains, which is caused mainly by the repulsion of anionic headgroups. The strengths of hydrophobic and electrostatic interactions are varied to explore their effect on the aggregation process. Strong hydrophobicity of the ionomer main chain results in greater aggregate sizes. Figure 1a shows the result of the changes in the aggregate size, <k>, over the range of explored hydrophobicity. Strong electrostatic interaction of ionic headgroups decreases the aggregate size, results are shown in figure 1b. Simulations also show the spontaneous formation of networks of ionomer bundles. Results of simulations will be discussed in the context of experimental studies on the formation of rodlike structures in ionomer solutions [2-3] that form the mechanically stable skeleton of polymer electrolyte membranes such as Nafion [4]. Moreover, implications of simulation results for water sorption phenomena [5], proton conductivity [6], membrane degradation [7] and membrane failure will be discussed. Figure caption: Figure 1. Changes of the average aggregate size, <k>, as a function of variation in strengths of hydrophobic interactions (a) and electrostatic interactions (b). Strength of hydrophobic interactions are controlled through variation of strength of excluded volume interaction of hydrophobic monomeric beads while the electrostatic strength is varied through changes in the Bjerrum length.