Combining fabrication approaches may help create polymer membrane desalination filters

Selective polymer membranes, which separate and filter out ions or molecules sharing similar properties, have practical uses in industrial processes such as water desalination and purification. However, it is difficult to maintain an appropriate level of permeability and selectivity in such membranes, especially for large-scale production. In a perspective on polymer membrane fabrication techniques, Zhao et al. discuss how to address this balance.

“To break the long-standing trade-off between permeability and selectivity, we need to fabricate thin membranes with a high density of uniformly-sized nanopores with pore size at around 1 nanometer,” said author Feng Liu. “Top-down and bottom-up fabrication approaches can complement each other to achieve this goal.”

In top-down fabrication, nanopores are created on the membrane by track-etching, which often results in nanopores that are highly uniform but too large and too far apart. The group has previously developed a track-UV top-down technique that can reduce the size of the nanopores, in which the membrane is subject to an electric field after being exposed to sufficient UV radiation. However, the pore density remains low.

On the other hand, with bottom-up approaches – in which the nanoporous membranes are self-assembled on a molecular level – the pores are about the right size, but lack uniformity or scalability.

The group believes the future of selective polymer membranes lies in combining these approaches to create ideal nanopore distributions.

“With the further development of new fabrication techniques, I would expect that we can reach the ultimate goal in precisely controlling the geometry, charging and chemical properties of polymer nanochannels by combining the molecular design-based synthesis from the bottom-up approach and molecular-level modification from the top-down approach,” Liu said.

Source: “Design and fabrication of highly selective and permeable polymer membranes,” by Shuang Zhao, Gehui Zhang, Wenhao Shen, Xue Wang, and Feng Liu, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0015975 .