Directional Moisture Transport in Compositionally Graded Multilayer Membranes

Directional water or moisture transport, which is vital for many living species, can be achieved by several operating principles and material designs, including dense membranes with a polarity gradient in the transverse direction. Here, we present a straightforward approach to create artificial membranes that mimic this design and offer highly directional moisture transport. The reported membranes consist of a hydrophobic poly-(styrene-<i>b</i>-butadiene-<i>b</i>-styrene) (SBS) layer and a hydrophilic layer made from a terpolymer (TP) of 2-hydroxyethyl methacrylate (HEMA), 2-hydroxyethyl acrylate (HEA), and 2-ethylhexyl methacrylate (EHMA) or blends of this terpolymer and SBS. When the hydrophilic TP-rich side of the membranes is exposed to moisture, the TP or TP blend swells and becomes plasticized, resulting in increased water permeability. Such plasticization is limited or absent when the SBS side is exposed to moisture. The mechanical robustness and asymmetric moisture-transport characteristics of such membranes were improved by employing an SBS-TP blend as an interfacial adhesive layer between the SBS and TP layers. Such trilayer membranes do not exhibit any signs of delamination even after extended exposure to moisture. They display highly directional transport with an asymmetry factor of 4.0 ± 1.1, outperforming many of the previously investigated graded membranes.