(637a) Effect of Polymer Interface on Gas and Ion Transport Through Polymer Membranes: A Molecular Dynamics Simulations Study

Polymers are attractive membrane materials due to their mechanical robustness and cost-effective fabrication. In our research, we investigate two distinct membrane applications: gas separation and ion exchange. In both cases, the transport of penetrants, whether gases or ions, plays a crucial role in overall process efficiency. Utilizing all-atom molecular dynamics simulations, we examine the impact of the polymer-gas interface in gas separation membranes and the polymer-water-ion interface in anion exchange membranes on penetrant transport.

An important feature central to gas separation membrane performance is the distribution of connected void spaces created by inefficient packing of bulky groups on the polymer backbone, known as free volume elements (FVEs). We study three polymers with different backbone flexibilities, across three different temperatures. Our analysis shows that chain segments near FVEs have higher mobility compared to the atoms in the bulk, like polymer segments near a free interface. The extent of this difference increases with chain flexibility. We find that rigid polymers show the most robust FVE distribution and are not significantly affected by temperature change. We relate these changes in FVE distribution to hydrogen transport.

In anion exchange membranes, the distribution of water channels dictates hydroxide transport through the material. We characterize hydroxide diffusion across eight different polyolefin-based anion exchange membranes and demonstrate how the water-polymer interface mediates the transport of hydroxide ions.