Molecular Origins of Nonfrozen Water in Polyelectrolyte Brushes.

Polyelectrolyte brushes (PEBs) are promising coatings for reducing ice adhesion and regulating water freezing at interfaces, yet direct measurements of nonfrozen water retention at subzero temperatures remain scarce. Here, we investigate the freezing behavior of water confined in poly([2-(methacryloyloxy)ethyl]trimethylammonium) (PMETA) brushes with chloride, iodide, and sulfate counterions using a custom-built low-temperature attenuated total reflectance infrared spectroscopy system. Furthermore, we quantify the fraction of water that was present within the brush that does not freeze as well as the changes in polymer volume fraction within the brush as a function of temperature. Spectroscopic analysis of water vibrational modes reveals that PMETA brushes retain 25-35 vol. % water even at -60 °C, providing direct evidence of substantial water confinement in charged polymer networks. These findings advance the fundamental understanding of interfacial water behavior in PEBs and suggest molecular design strategies for engineering anti-icing and cryo-lubricating surface coatings.