Ionizable Polymer Thin Films at the Water Interface

Using molecular dynamics simulations, we investigate the effects of water at the interface of thin films of ionizable polymers. These polymers consist of flexible chains substituted with ionizable groups, whose structure is dominated by the assembly of the ionizable groups, resulting in interfaces that consist of both hydrophilic and hydrophobic domains. These polymers serve as key components in clean energy generation, storage devices and biotechnology, where water is inherent to their function. Understanding the effects of water at structured interfaces of ionizable polymers remains a fundamental challenge with an immense technological impact. Here, the results of a fully atomistic MD simulation of films of a model polymer, sulfonated polystyrene (SPS), with sulfonation fractions f = 0.13, 0.20, 0.35 and 0.55 neutralized with Na⁺ counterions, exposed to water, are discussed. MD simulations were run using LAMMPS at 400 and 500 K. The results show that increasing the sulfonation level enhances the ionic domain connectivity, facilitating water uptake. Increasing the temperature to 500K accelerates both the water penetration and the dynamics of the polymer. The structure and dynamics of the polymeric interfaces will be discussed. The results offer guidelines for designing new polymers that can function effectively in an aqueous environment.