Role of Oxygen Species on Water Uptake of a Heat-Treated Cyclo-olefin Polymer

Introduction of surface oxygen species is a widely employed technique to enhance the hydrophilicity of hydrophobic polymers. This study investigates the relationship between surface oxidation and water interaction on a cyclo-olefin polymer treated by heating. Surface characterization using X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure revealed three types of oxygen-containing functional groups: hydroxyl-type (C-O-C/C-OH), carbonyl-type (C═O/CHO), and carboxyl-type (COOC/COOH). Water adsorption behavior was analyzed by using X-ray fluorescence spectroscopy with <i>in situ</i> humidification. Results revealed that oxygen species provide a stable scaffold for surface hydration, whereby highly oxidized surfaces exhibited enhanced water retention capabilities even under low-humidity conditions. The "equilibrium point"─the maximum humidity level at which water is retained just before desorption of water clusters─shifted to higher humidity levels in slightly oxidized samples but reversed at higher oxidation levels. Carboxyl-type groups, which increased significantly in oxygen-rich samples, demonstrated superior water adsorption capacity and played a crucial role in stabilizing interfacial water under low-humidity conditions.