Ions Form Thread-like Structures in Glassy Polymers for Desalination Membranes

Significance 

The application of ionic polysulfones in desalination membranes depends on numerous factors, including polymer hydration, exclusion of the coions, and the mobility of counterions. Unfortunately, the limited information on the nanostructure of the ionic domains in ionic polysulfones compromises their practical application in polymeric membranes. In addition, the bulk properties of glassy ionic polymers can only be scaled over a limited range with their ionic contents, and the underlying mechanism behind the transitions between the scaling regimes are poorly understood. Recent research revealed insights at the molecular level, which lay a strong foundation for a deeper understanding of the ionic polysulfone’s bulk properties.

Several theoretical frameworks have been developed to investigate the ionic interactions in polyelectrolyte solutions. Among them is Manning’s counterion condensation theory developed to study rod-like polyelectrolytes’ behaviour in a dilute solution. The theory has also been expanded to study other charged polymers in the solid state owing to its relevance to membrane selectivity. Using computational techniques, this research addresses one limitation of Manning’s theory, which is the requirement for an accurate distance between the fixed ions. The application of molecular dynamics to ionic polymers is a relatively new area, and obtaining good agreement between simulations and experimental methods, remains a challenge. However, simulations at the atomic and molecular scale can offer a deeper understanding of the underlying physiochemical mechanisms in solid ionic polymers. This understanding could facilitate the application of systematic design principles to improve the performance of ionic polymer membranes and widen their applications.

Herein, researchers from the Virginia Polytechnic Institute and State University: Dr. Britannia Vondrasek, Dr. Chengyuan Wen, Professor Shengfeng Cheng, Professor Judy S. Riffle, and Professor Jack Lesko investigated the interrelated effects of fixed ion spacing, hydration and cation-anion interaction in two different sulfonated polysulfones using molecular dynamics simulations. The sulfonated polysulfones exhibited different spatial ion distributions along the polymer backbone. The ionic interactions in the polysulfones were examined using the Manning’s counterion condensation theoretical framework. The main objective was to gain a deeper understating of the molecular-scale structures underpinning the ionic polymer’s bulk properties. The original research article is published in the journal, Macromolecules.

The authors used the radial distribution function for ions in the simulation to estimate the average distances between the ions. The resulting distance was larger for polymers with sulfonate ions that are more evenly distributed along the backbone. Furthermore, the shapes of the sulfonate-sulfonate radial distribution function (RDF) for disulfonated and monosulfonated polymers were similar at low water content and different at high water content. The difference in RDFs indicates different spatial distributions of the ions in the two polymers. The hydration level of the ions at equilibrium water content was similar to that in saturated aqueous solutions. Many ionic polymers, such as Nafion, exhibit micro-scale phase-separation between the ionic and non-ionic parts of the polymer. However, the authors did not observe any distinct ionic phases in the polysulfone simulation, which is consistent with experimental evidence. Instead, sodium counterions and sulfonate ions formed fibrillar aggerates at lower water contents, which began to disperse as the water content increased. It was worth noting that such thread-like aggregates could aid the transportation of ions in sulfonated polysulfone membranes without the need for long-range motion of the polymer backbone.

In summary, a molecular dynamics simulation was used to probe the cation-anion interaction, ion distribution, and hydration of sulfonated polysulfones. Results demonstrate therole of the spacing of the sulfonate ions along the polymer backbone in determining their spatial distribution in the hydrated polymer. The simulation was used to estimate the interionic distance, which is a necessary parameter in Manning’s counterion condensation theory. The number of condensed ions in the simulation agreed well with those predicted by Manning’s theory. In a statement to Advances in Engineering, first author Dr. Britannia Vondrasek said the results provided a better understanding of the spatial distribution of ions within glassy ionic polymers and facilitate further studies of properties of ionic polymer membranes.

Reference

Vondrasek, B., Wen, C., Cheng, S., Riffle, J., & Lesko, J. (2020). Hydration, Ion Distribution, and Ionic Network Formation in Sulfonated Poly(arylene ether sulfones)Macromolecules, 54(1), 302-315.

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