Investigation of water diffusion in hydrogel pore-filled membrane via 2D correlation time-dependent ATR-FTIR spectroscopy

Hydrogel pore-filled technique has become a very promising method to modify water separation membranes. The hydrogels inside the pore of the membrane are supposed to stay in their confined state. In this paper, the water diffusion behavior inside polyethersulfone (PES) membranes, whose pores are filled with poly (acrylamide) (PAAm) hydrogel, is investigated via time-dependent ATR-FTIR spectroscopy. Based on the 2DCOS analysis of OH stretching band, three states of water, including strong bound water, weak bound water and bulk water, were found during the diffusion process. Based on the Noda's role, it is suggested that the water molecules diffuse into the pore-filled membrane as weak bound water associated with PES substrate or free water in the pore of the membrane in the beginning. After then, the water molecules start to form strong bond with the PAAm, and the water in the membrane will exist as the strong bound water and bulk water inside the membrane. According to our knowledge, it is the first report about water diffusion in the hydrogel in a confined space.