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

Abstract

We perform molecular dynamics simulations to investigate hydration structures and dynamics in seven water-containing polymers: PVA, PHEA, PHEMA, PBA, PMEMA, PEG, and PMEA. The analysis integrates four perspectives: the water-content dependence of the glass transition temperature $T_g$, polymer chain fluctuations characterized by dihedral angle distributions, hydrogen-bond lifetimes $τ_{\mathrm{HB}}$ between water and polymer functional groups, and the localization and exchange dynamics of confined water quantified by the distinct part of van Hove correlation function. Hydroxyl-containing polymers (PVA, PHEA, and PHEMA) exhibit relatively high dry-state $T_g$ values and its pronounced depression upon hydration. Chain fluctuations are limited, and $τ_{\mathrm{HB}}$ follows Arrhenius behavior, forming localized hydration shells. In contrast, PMEMA and PBA show low equilibrium water contents and hydrophobic character; although their dry-state $T_g$ values are moderately lower and less sensitive to water content, chain fluctuations remain small, and $τ_{\mathrm{HB}}$ also obeys Arrhenius behavior, with hydrophobic aggregation promoting water localization. PEG and PMEA display low dry-state $T_g$ 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 $T_g$, $τ_{\mathrm{HB}}$ 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.

Figures (7)

• Figure 1: Structure and classification of polymers studied in this paper.
• Figure 2: Water content dependence of the glass transition temperature $T_g$. $T_g^\mathrm{w}\approx 212$ K denotes the glass transition temperature of the TIP4P/2005 water model, as previously reported. kreck2014Characterization The dashed line represents the Fox equation [Eq. \ref{['eq:Fox']}], which estimates $T_g$ for the water-polymer mixture system based on the glass transition temperatures of the dry polymer and pure water.
• Figure 3: Proportions of H-bonding partners for hydroxyl oxygen atoms in PVA, PHEA, and PHEMA at 300 K. Magenta denotes intermolecular H-bonds to oxygen acceptors on other polymer chains; yellow denotes intramolecular H-bonds to oxygen atoms on the same polymer chain; blue denotes H-bonds to oxygen atoms of water molecules; gray denotes hydroxyl oxygen atoms that do not participate in H-bonding.
• Figure 4: Temperature dependence of $\sigma_\mathrm{E}$, which quantifies the spread of the Gaussian mixture model distribution, at a water content of 25 wt%. The left panel shows the main-chain carbon dihedral angle (C-C-C-C), and the right panel shows the carbon skeleton dihedral angle associated with the oxygen substituent (O-C-C-O). For PBA, the latter corresponds to the O-C-C-C dihedral angle that includes the butyl group. The arrows indicate the glass transition temperature $T_g$ for for PEG and PMEA in the panel for O-C-C-O and for the others in the panel for C-C-C-C.
• Figure 5: Arrhenius plots of $\tau_{\mathrm{HB}}$ between water molecules and acceptor oxygens of polymer functional groups at a water content of 25 wt% (260-420 K in 20 K intervals). Red symbols denote H-bonds with ether or methoxy oxygens, red symbols correspond to hydroxyl oxygens, and black symbols represent carbonyl oxygens in acrylate groups as acceptors. The solid lines indicate linear fits in the high-temperature range (340-420 K). The purple dashed line represents the estimated glass transition temperature $T_g$.