Molecular Dynamics Insights into Water Transport Mechanisms in Polyamide Membranes: Influence of Cross-Linking Degree.

Polyamide (PA) membranes are widely utilized in desalination and water treatment applications, yet the mechanisms underlying water transport within these amorphous polymer materials remain insufficiently understood. To gain more insight into these problems on a microscopic scale, we employ molecular dynamics (MD) simulations to analyze the relationship between the structural properties and the water permeation behavior of PA membranes. Two distinct atomistic models of PA membranes are developed by controlling their degrees of cross-linking (DC). We then conducted a comparative analysis on their microscopic structural properties and configurations of water inside the membranes and investigated how these differences lead to different water diffusion coefficients. Our results reveal that the membrane with a lower DC exhibits higher polymer mobility and a more orderly microscopic structure, allowing the formation of pores that can hold larger water clusters as well as more transient passages between pores, both contributing to an increased water diffusion coefficient. From these observations, we can conclude that water permeability within PA membranes is governed by both the morphology of semirigid pores and the oscillatory movements of the polymer chains. Overall, these findings contribute to a deeper understanding of the intricate mechanisms governing water permeation in PA membranes and may inform the design of more efficient membranes for reverse osmosis and other water treatment technologies.