Classification of interfacial water governed by water–polymer interactions in hydrated polymers: A molecular dynamics simulation study of ethylene-based and acrylate polymers

We perform molecular dynamics simulations to investigate hydration structures and dynamics in seven water-containing polymers: poly(vinyl alcohol) (PVA), poly(2-hydroxyethyl acrylate) (PHEA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(butyl acrylate) (PBA), poly(2-methoxyethyl methacrylate) (PMEMA), poly(ethylene glycol) (PEG), and poly(2-methoxyethyl acrylate) (PMEA). The analysis integrates four perspectives: the water-content dependence of the glass transition temperature Tg, polymer chain fluctuations characterized by dihedral angle distributions, hydrogen-bond lifetimes τHB between water and polymer functional groups, and the localization and exchange dynamics of confined water quantified by the distinct part of the van Hove correlation function. Hydroxyl-containing polymers (PVA, PHEA, and PHEMA) exhibit relatively high dry-state Tg values and a pronounced depression upon hydration. Chain fluctuations are limited, and τHB follows Arrhenius behavior, forming localized hydration shells. In contrast, PMEMA and PBA show low equilibrium water contents and hydrophobic character; although their dry-state Tg values are moderately lower and less sensitive to water content, chain fluctuations remain small, and τHB also obeys Arrhenius behavior, with hydrophobic aggregation promoting water localization. PEG and PMEA display low dry-state Tg values and weak water-content dependence. Greater rotational freedom around ether or methoxy oxygen atoms leads to larger chain fluctuations and loosely bound water. Below Tg, τHB between water and ether or methoxy oxygen atoms exhibits super-Arrhenius behavior. These results clarify three hydration types: highly hydrated (PVA, PHEA, and PHEMA), hydrophobic (PMEMA and PBA), and flexibly hydrated (PEG and PMEA), and provide a molecular-level framework for interpreting interfacial water governed by water-polymer interactions.