On the Nature of Freezing/Melting Water in Ionic Polysulfones

TL;DR

This study combines experimental and simulation techniques to understand how ionic group spacing in hydrated sulfonated polysulfones influences the amount of melting water, revealing that ion dispersion affects water states and membrane properties.

Contribution

It provides new insights into the relationship between ion spacing and water states in ionic polymers, supported by combined DSC, FTIR, and MD simulation data.

Findings

More aggregated ions lead to higher melting water content.

Dispersed ions result in less bulk-like melting water.

Physical model correlates water states with ion distribution.

Abstract

We investigate the behavior of hydrated sulfonated polysulfones over a range of ion contents through differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and molecular dynamics (MD) simulations. Experimental evidence shows that at comparable ion contents, the spacing between the ionic groups along the polymer backbone can significantly impact the amount of melting water present in the polymer. When we only consider water molecules that can hydrogen bond to four neighboring water molecules as the melting water, the MD simulation results are found to agree with the experimental data. The states of water measured by DSC can therefore be described as "aggregated" (or bulk-like) for the melting component, and "isolated" for the nonmelting part. Using this physical picture, a polymer with more aggregated ions has a higher content of melting water, while a polymer at the same ion content but with more dispersed ions has a lower content of melting water. Therefore, ions should be well dispersed to minimize the amount of bulk-like water in ionic polymer membranes.