Why is the PES membrane called an asymmetric membrane, what kind of molecular structure and applicat

Asymmetric PES membranes are prepared by a phase inversion process and are widely used in the precision filtration industry due to their unique structural characteristics. It is only called asymmetric polyethersulfone (PES), which refers to a membrane made of polyethersulfone material with an asymmetric structure. In this case, the term "asymmetric" describes the structure of the membrane, which has a pore size gradient across its thickness. Typically, the pore size decreases from the outer surface to the inner surface of the membrane. This pore size distribution gradient can improve filtration performance because it can combine high permeability with good selectivity.

This asymmetric structure is achieved in a manufacturing process in which a polymer solution is cast onto a support (also known as a substrate) and then immersed in a coagulation bath, leaving a thin layer of PES on the substrate, The resulting phase inversion process creates an asymmetric structure with varying pore sizes across the membrane. This unique structure gives asymmetric polyethersulfone membranes their name and distinguishes them from symmetric membranes, which have a uniform pore size distribution across their thickness.

The molecular structure of PES consists of a repeating unit containing an aromatic ring, a sulfone group and two ether groups. The aromatic rings provide rigidity to the polymer chain, while the sulfone and ether groups contribute to the high thermal stability and chemical resistance of the polymer.

The structural features of asymmetric PES membranes are determined by the morphology of the polymer layer consisting of a thin, dense skin layer on top of a porous support layer. The surface layer is formed by the rapid precipitation of PES near the membrane surface during phase inversion, while the support layer is formed by the slower precipitation of PES deep in the membrane. The surface layer is highly permeable to small molecules but resistant to larger molecules and particles, while the support layer provides mechanical support to the membrane and increases its overall filtration efficiency.

With its excellent filtration performance, durability and chemical resistance, asymmetric PES membrane is widely used in microelectronics, pharmaceuticals, biotechnology, ultrapure water, pharmaceutical solutions and biological liquid filtration, etc., such as its application in the microelectronics industry, Used to filter photoresists, developers, and other chemicals used in the photolithography process to remove contaminants and particles that can cause defects in the microelectronic devices being manufactured; also used to filter slurries used in the CMP process, This is critical to obtain a smooth and uniform surface on the wafer.