Evaluation of Water Diffusion Confined in Epoxies: Role of Atomic Local Mobility of Polymer Chains

The diffusion of water in epoxy resins is a fundamental property that characterizes their applications. While various factors influencing molecular diffusion in epoxies, such as voids and hydrogen bonding, have been widely investigated, the impact of chain mobility remains unclear. The dynamics of internal water molecules in two epoxy compositions with varying ratios were examined by molecular dynamics simulations, along with the relationship to the epoxy polymer chain mobility. The relationship between the characteristic hopping behavior of water molecules and epoxy polymer chain local mobility was explored by evaluating the atomic displacement of chain atoms over a short period, which is a distinctly smaller structural unit compared to segmental chain motion. This analysis showed a trend of high diffusivity of water molecules with high chain local mobility, with stoichiometric epoxy exhibiting higher values compared to epoxide-excess nonstoichiometric epoxy, which agrees with the experimental observations in QENS measurements. Atomic displacement, as the metric of chain local motion, illustrated that the discontinuous hopping behavior of water molecules occurs at locations of high chain local mobility, which leads to the transient interconnection of isolated voids in epoxy. This work suggests that water diffusivity is strongly related to chain local mobility in cross-linked resins because of the restriction of large-scale or segmental chain motion. This contributes to a better understanding of water-related properties in epoxy materials and facilitates the optimization of epoxy chemical structures.