Engineering Water/Polymer Interactions to Control the Transport Properties of Hydrated Polymers for Desalination

Polymers are useful as membrane materials that can be used in desalination separation processes that address water shortage issues. To achieve a given desalination separation, the polymers in these processes must selectively control the rates of water and salt transport, which often requires precise control of polymer chemical features via molecular engineering strategies, that can, to a first approximation, be guided using fundamental theory. In this dissertation, we present a combination of experimental and theoretical techniques to characterize and rationalize molecular engineering strategies that influence the small molecule (i.e., salt and water) transport properties of hydrated polymers for desalination applications.
Understanding the extent and nature of interactions between water molecules and the solvated polymer are key to controlling the transport rates of ions in the solvated polymer. For example, engineering interactions between the water and polymer that restrict the dipolar motions of water molecules reduce the extent that salt can dissociate in the polymer via a so-called dielectric exclusion mechanism. We quantify the extent that interactions with the solvated polymer influence the molecular motions of water molecules using microwave-frequency dielectric relaxation spectroscopy. We demonstrate that in many polymers, reducing the dielectric constant suppresses their salt sorption coefficient in a manner that is qualitatively consistent with applications of the classic Born model that describe theoretically the interactions between ions and their induced polarization charges.
We show that, using theoretical modifications of the classic Born model to describe specifically ion solvation interactions in the hydrated polymer, the influence of the polymer dielectric constant on polymer salt sorption properties can be modeled quantitatively. These results may guide strategies that leverage dielectric exclusion to engineer desalination polymers. A key result of our findings is that generally, favorable ion association interactions (e.g., ion pairing or counter-ion condensation) in low dielectric constant media can minimize the extent of dielectric exclusion, and engineering out these favorable interactions would be desirable in desalination separation materials.
In addition, we quantify the extent that water/polymer interactions influence the kinetic factors that contribute to salt transport in hydrated polymers. We find that polymers with glassy hydrated chain dynamics suppress ion diffusion, and these results are explained using a physical picture where rigid polymer chains suppress the formation of large water clusters and shift the polymer free volume distribution towards smaller average free volume element sizes. This phenomenon effectively increases the entropic barrier for ion diffusion. This entropic barrier can also be engineered by controlling the characteristic free volume size required for diffusion, which is influenced by the length scale and magnitude of electrostatic interactions in the solvated polymer.
Finally, we report how combinations of these thermodynamic and kinetic factors guide the design of a series of N-ethylmaleimide polymers for desalination processes. These results contribute to demonstrating the power of using theoretical approaches to motivate rational design strategies for promising desalination polymer membrane materials. Ultimately, the results and analysis presented in this dissertation improve the fundamental understanding of water and salt transport processes in hydrated polymers.