Fourier Transform Infrared Study on the State of Water Sorbed to Poly(ethylene glycol) Films

The state of sorbed water and sorption processes of water in poly(ethylene glycol) (PEG) films were studied by Fourier transform infrared. From the assignment and time evolution of the components of the O−H stretching band, the adsorption and penetration of water into the films are considered to proceed as follows: (1) At the polymer surface, a water molecule binds to the oxygen atoms of PEG with one of its hydrogen atoms (“binding water”). The same phenomenon more gradually occurs in the polymer matrix too. (2) The water binding to the oxygen atoms of PEG molecules with both of its hydrogen atoms (“bridging water”) is formed from the binding water, while a dimeric water is gradually formed by the association of the free water molecule with the binding water. (3) The amount of monomeric species (both binding water and bridging water) is equilibrated. (4) The water dimer is further formed by the association of the free water molecule with the binding water, and finally the amount of the dimer is equilibrated. When the molecular weight of PEG was small, a new band at 3400 cm-1 gradually appeared. This band could be attributed to the O−H stretching of water molecules attaching to hydroxyl groups at the end of PEG molecules. An ab initio molecular orbital calculation and hybrid density functional method supported the assignment of the peaks.