Clustering Effects on the Structure of Ionomer Solutions: A Combined SANS and Simulations Study

Abstract

Ionic assemblies, or clusters, determine the structure and dynamics of ionizable polymers and enable their many applications. Fundamental to attaining well-defined materials is controlling the balance between the van der Waals interactions that govern the backbone behavior and the forces that drive the formation of ionic clusters. Here, using small-angle neutron scattering and fully atomistic molecular dynamics simulations, the structure of a model ionomer, sulfonated polystyrene in toluene solutions, was investigated as the cluster cohesion was tweaked by the addition of ethanol. The static structure factor was measured by both techniques and correlated with the size of the ionic clusters as the polymer concentration was varied. The conjunction of SANS results and molecular insight from MD simulations enabled the determination of the structure of these inhomogeneous networks on multiple length scales. We find that across the entire concentration range studied, a network driven by the formation of ionic clusters was formed, where the size of the clusters drives the inhomogeneity of these systems. Tweaking the ionic clusters through the addition of ethanol impacts the packing of the sulfonated groups, their shape, and their size distribution, which, in turn, affects the structure of these networks.

From Molecular Constraints to Macroscopic Dynamics in Associative Networks Formed by Ionizable Polymers: A Neutron Spin Echo and Molecular Dynamics Simulations Study

“…In the f = 0.09 solutions, well-defined clusters are observed, and the polymer rich domains become denser and often segregated from the toluene. This significant phase segregation between the polymer-rich and solvent domains as has been previously reported by SANS . This inherent inhomogeneous in these associative systems drives the complex dynamics observed in S ( q,t ) .…”

“…SPS in toluene above the overlap concentration for polystyrene forms a network driven by concentration fluctuations and formation of clusters. The structure of SPS in melts and solutions have been studied by numerous groups. ,, An example of the small angle scattering (SANS) pattern of SPS with f = 0.03 in toluene is given in Figure , where the very low q range depicts the signature of an instantaneous network and the intermediate q range captures the signature of an interionic correlations, often referred to as the “ionic peak”. Note that in contrast of SANS patterns of melts, for these low sulfonation levels, the interionic correlations are expressed in the line shape of the pattern.…”

“…The number of distinctive, unique chains that participate in each of the clusters normalized to the number of sulfur atoms in a cluster N c , is shown in Figure a. Sulfur atoms residing within 6 Å radius of each other are defined to be in the same cluster . The average cluster size for f = 0.03 is 2.8 and for f = 0.09, it is 3.8.…”

Structure and Dynamics of THF Swollen Polystyrene Ionomer Melts

“…On the premise that the polar environment impacts the clusters, Weiss and co-workers have shown direct correlations between the nature of the counterion and the structure and rheology of these polymers. We have recently shown that the addition of small amounts of ethanol to SPS ionomers in toluene solutions decreases the average cluster size and increases the dynamics of the system. , However, further addition of ethanol collapses the chains, and the dynamics of the system decreases. These studies suggest that polar solvents with a relatively high solubility index, which will prevent the collapse of the polymer backbone but with a dielectric constant high enough to tweak the cohesion of the ionic clusters, would enhance the dynamics of ionomer melts.…”