AbstractsChemistry

Water between two solid surfaces plays an important role in interfacial adhesion, catalysis, corrosion, microelectronic industry and biomaterials. However, research on molecular-level structure of confined water between two solid surfaces and the disruption of interactions between these two surfaces caused by water are lacking. Infrared-visible sum frequency generation spectroscopy (SFG) is used to directly probe confined water between polyurethane (PU) and the sapphire substrate after exposing the polyurethane films to liquid water and water vapor. In liquid water condition, the observation of SFG peaks associated with H2O bands (3000-3400 cm-1) and D2O bands (2300-2600 cm-1) indicate water molecules ingress to the buried interface and exist in the form of hydrogen-bonded water network. The water layer disrupts interactions between polyurethane and hydroxyl groups on the substrate. When PU films were exposed to water vapor, the SFG signal corresponding to PU hydrocarbon groups significantly increase, while the SFG signal of sapphire hydroxylgroups decrease, which indicates water molecules reach the interface. However, no hydrogen-bonded water network was observed, instead, H2O peak at 3555 cm-1 and D2O peaks (2600-2700 cm-1) show up which can be assigned to low-coordination water. An alternate assignment could be the hydroxyl groups hydrogen bonded with carboxyl groups of PU. Water molecules cannot form a uniform monolayer at the interface and as a consequence cannot completely disrupt the PU-sapphire bonds. These results provide two distinct states of water at the polymer-solid interface, which could influence the interfacial bonding state differently and have important implications in understanding interfacial adhesion, coatings and corrosion.