Water exchangeability between pore volumes: a key mechanism governing water diffusion in highly cross-linked polymer coatings

Understanding the behavior of water in highly cross-linked coatings, such as melamine-curable systems, is essential for material design, particularly for outdoor applications where water absorption and desorption significantly affect performance. This study demonstrates that the ability of water molecules to navigate interconnected water clusters plays a more crucial role in water diffusion than previously anticipated factors like network polarity or the free volume of the network. Using a multi-scale simulation methodology validated by experimental data, we investigate the interactions and hydrogen bonding between water and various network components, as well as with other water molecules. A detailed analysis of the shapes and distributions of water clusters, along with the pathways of water molecules within the polymer network, reveals a direct relationship between water diffusion and the exchangeability of water between pore volumes. We show that water exchangeability can be tuned through molecular design to control water absorption, trapping, and desorption, all of which impact the material's functionality in applications.