Hydrogen bonding and anti-plasticization in epoxy coatings: Molecular insights into water uptake and network stability

Water absorption behavior and hydrogen bonding interactions of absorbed water were investigated in a model epoxy-amine coating system at varying stoichiometric ratios. The polarity and free volume of the coatings were determined by water contact angle measurements and dynamic mechanical thermal analysis. Water uptake was found to depend on the polymeric network's polarity, while free volume exhibited no significant influence on water absorption. Absorbed water showed anti-plasticization effects; this phenomenon was further explored at the molecular level through molecular dynamics (MD) simulations. Using a two-dimensional potential of mean force (PMF) approach, hydrogen bond interactions between water molecules and the polymeric network were systematically classified and quantified. Hydroxyl groups emerged as the dominant hydrogen bonding sites due to their stronger bonding strength compared to other polar groups. Residual water molecules formed stable hydrogen bond bridges with hydroxyl groups, reinforcing the polymer network and contributing to anti-plasticization, particularly at higher epoxy-to-amine ratios. This study highlights the effectiveness of the PMF approach in characterizing hydrogen bonding interactions and provides a comprehensive framework for understanding water-induced modifications in epoxy coatings, offering valuable insights for optimizing coating formulations.