Weak Water–Polymer Interactions Govern Water Recrystallization in Nonwater-Soluble Polymers with High Glass Transition Temperature

The recrystallization behavior of water molecules sorbed into polystyrene (PS) and four PS analogs was systematically investigated using variable-temperature mid-infrared (VT-mid-IR) spectroscopy and molecular dynamics (MD) simulations. In contrast to poly(meth)acrylates, where the recrystallization temperature of sorbed water (<i>T</i>_RC) strongly correlates with the polymer glass transition temperature (<i>T</i>_g), PS and its analogs exhibited no such correlation. Instead, <i>T</i>_RC was found to correlate with the electrostatic characteristics of the polymer's aromatic rings, specifically the summed partial charges on their carbon and hydrogen atoms. In these polymers, water molecules are primarily trapped via π-H interactions on the aromatic rings. At low temperatures, the sorbed water exists as monomolecular species. Upon heating, recrystallization occurs when the thermal kinetic energy of the water molecules exceeds the relatively weak polymer-water interaction energy, allowing diffusion and subsequent ice formation to proceed independently of polymer chain mobility. This behavior contrasts with that in poly(meth)acrylates, where water diffusion is governed by polymer segmental motion above <i>T</i>_g. The onset of water recrystallization was well predicted by the temperature at which water diffusion becomes thermally activated, demonstrating that water diffusion is the key triggering process in both systems. These findings reveal a unified, diffusion-driven mechanism for water recrystallization in polymers, wherein the initiating factor, that is, thermal activation or polymer chain dynamics, is determined by the relative strength of water-polymer interactions.