Fourier transform infrared study on the sorption of water to various kinds of polymer thin films

The state of sorbed water and the sorbing processes of water to various polymer thin films were studied with Fourier transform infrared (FTIR) spectroscopy. To prepare the polymer films, we used poly(ethylene glycol)s of different molecular weights and various kinds of vinyl polymers, such as poly(2-methoxyethyl acrylate). The O-H stretching band of water sorbed in the films increased gradually on contact with water vapor at 50% relative humidity and leveled off. When O-H stretching bands of water sorbed to polymer films were compared, the peak positions and profiles of water sorbed to the polymeric materials with the same hydrogen-bonding site were similar. A hybrid density-functional method supported the assignment of the peaks. Furthermore, the diffusion coefficient (D) of water vapor in the polymer films was estimated by time-resolved measurements of the sorbed water at the very initial stage (0-830 s). It was clearly shown that the D values of water vapor in the polymer materials with a strong hydrogen-bonding site were smaller than those in hydrophobic polymers. The usefulness of the FTIR technique to investigate water sorption to polymer materials was definitely demonstrated.