New route to thin porous membranes

May 09, 2020

(Nanowerk News) Scientists have discovered a new method of producing ultra-thin porous membranes. These new membranes have precisely controlled pore dimensions and surface chemistry (J. Am. Chem. Soc., “Self-Assembly of Metal–Organic Framework (MOF) Nanoparticle Monolayers and Free-Standing Multilayers”). The key to the new method is growth of a polymer “corona”—an ultrathin layer of polymer surrounding highly porous metal-organic-framework (MOF) nanoparticles. MOFs are porous materials made of metal atoms linked by rigid organic molecules that can trap and hold small molecules or act as a sieve or filter that can separate mixtures of different molecules. Scientists created ultrathin porous materials by growing a “corona”-like polymer layer on porous metal-organic framework nanoparticles. Films one particle thick (top) self-assembled into multiple layers that were formed from these nanoparticles. (Image: Seth Cohen, U.C. San Diego) The polymer corona-coated, MOF nanoparticles self-assemble into layers one particle thick and into multilayer, porous films that do not require external support. The new films have high MOF particle content—87 percent by weight. The ability to form flexible films with a very high content of MOF nanoparticles is a much sought-after goal. The high particle content is important because it advances efforts to produce highly functional porous membranes and coatings. These film properties are important for the use of MOFs in devices. For example, the new method could be useful in making uniform porous membranes with properties designed for specific applications such as batteries and fuel cells. Inorganic nanoparticles have many useful properties for a wide range of applications in energy storage and production. However, these particles are brittle, and films formed from porous nanoparticles alone or as additives in polymer films tend to be relatively thick, crumbly, and weak and have lower than desired nanoparticle content. These attributes make them hard to use for structures that are suitable for use in devices. Researchers have sought a way to produce flexible, thin films of these nanoparticles to address these challenges. They have achieved some success, but the films that have all the desired features are still rare. In this new study, scientists identified a novel approach that makes possible what the researchers believe is the first self-assembly of porous single-layer and multilayer films that do not require external support, both made up of MOF nanoparticles. The research produced these self-assembled MOF monolayers (SAMMs) in films with a high MOF content—87 percent by weight and 89 percent by volume. High MOF content makes these films operate more efficiently when used in specific applications. The SAMMs then could be stacked to make MOF multilayers that did not require external support, paving the way to new applications.