Highly Asymmetric Water Permeation in Dense Laminated Membranes

Directional permeation through dense laminated membranes is relevant for applications in various fields, including separation processes, wound care, and packaging. While theoretical models have been used to describe the asymmetric permeation in heterogeneous dense membranes, only a few systems have been experimentally explored. Here, we report dense asymmetric laminated membranes based on hydrophilic poly-(vinyl alcohol) (PVA) and hydrophobic glycol-modified poly-(ethylene terephtalate) (PETG). Modeling the system allowed us to optimize the thickness of the PVA and PETG layers. While bilayer membranes made from the two components suffered from poor interfacial adhesion and delamination, this problem is overcome by using a thin poly-sty-rene-<i>block</i>-poly-(ethylene-<i>ran</i>-butylene)-<i>block</i>-polystyrene-<i>graft</i>-maleic anhydride (SEBS-MA) adhesive layer. The maleic anhydride groups (MA) react with the hydroxyl groups present in the two polymers, and this greatly improves the adhesion between the hydrophilic and the hydrophobic layers. Membranes with optimized geometry display an asymmetry factor of up to 6.7, one of the highest values ever reported. The directional water transport is caused by the moisture-induced plasticization of the PVA layer at high relative humidity (RH), which occurs only when the PVA side of the membrane is exposed to moisture.