Mechanism Study of Water Diffusion in Cross-Linked Epoxy Networks via Molecular Dynamics

Water diffusion in highly cross-linked epoxy networks was investigated through a combined experimental and modeling approach. Transport behavior was analyzed using ideal Fickian, dual-Fickian, and Berens–Hopfenberg (BH) models, with the BH model providing the most accurate description of both the rapid initial Fickian uptake and the delayed, relaxation-controlled component. While the magnitude of relaxation-induced water uptake showed limited sensitivity to cross-link density, its characteristic time scale increased markedly with network rigidity. Evolution of the diffusion front was examined using FTIR-ATR, enabling classification of water species according to their hydrogen-bonding states. These results reveal that the early stages of diffusion are dominated by network polarity rather than concentration gradients. Complementary molecular dynamics simulations further demonstrate that polarity is the primary factor governing water mobility, whereas segmental dynamics and free volume have secondary roles. Together, these findings deepen the understanding of water transport in cross-linked epoxy polymers and provide guiding principles for designing more durable epoxy-based barrier materials.