Scientists from King Abdullah University of Science and Technology (KAUST) have developed a cheaper and improved nanoporous film production process.

As reported in a recent issue of Macromolecules, the team from the KAUST Advanced Membranes and Porous Materials Center, along with researchers from the University’s Nanofabrication, Imaging and Characterization Lab, worked together in developing the method for producing nanoporous films, which contain extremely high pore density and regularity.

Their work advances the capability for mass production of high quality membranes for water purification, medical and pharmaceutical applications and also provides improvements to templates within the electronics industry.

Drs Suzana Nunes and Klaus-Viktor Peinemann chose this project knowing they had the full support of the KAUST Core Labs. With their vast experience in membranes and polymer solutions, the two identified a problem that was scientifically fascinating for membrane technology and could be tackled with the abundance of resources on offer at KAUST.

Membranes have been in practical use for over 50 years now and have evolved over time, driven by end-user needs for improved performance. Separation techniques have continuously improved, making the membranes more selective and able to remove finer particles. New breakthroughs have enabled lower energy consumption, reduced costs, increased membrane life span and, most importantly, significant benefits to end-users.

For example, patients with renal failure can now spend up to eight hours daily undergoing hemodialysis in order to remove waste products from their blood. This process involves diffusion of solutes across a semi-permeable membrane.

Due to their great output rate enhanced and precise filtration capability, high flux membranes with improved control of the pore size and distribution have the potential to vastly reduce the time a patient needs to be on a dialysis machine.

Human health is impacted worldwide constantly because of microbial and chemical contamination in water resources with new threats, such as pharmaceuticals in drinking water, continuously emerging.

Commercial membranes or water purification, for the most part, still resemble those developed in the 1960s. The required increase in the effectiveness of decontamination requires much higher fluxes and a membrane with uniform pore size distribution in the nano range, i.e. ultrafiltration.

Although simple to build into a membrane, the challenge with higher fluxes is to ensure that the pores are evenly sized and distributed. The KAUST researchers addressed this problem by taking a unique approach to the development of nanoporous films inspired by block copolymers, metal-directed supramolecular chemistry and nonsolvent induced phase separation.

Block copolymers are composed of at least two long sequences of monomer units, which are quite different from each other but are covalently bound. By diluting with a selective solvent, one of the polymers tends to avoid solvent contact, assembling into complex morphologies or forms. This offers numerous possibilities for tailoring nano-structures.

Dr Nunes and Dr Peinemann used metal–polymer complexation as a method for directing self-assembly, stabilizing micelles (a submicroscopic aggregation of molecules) in solution and providing inter-micellar cross-linking. Using a series of advanced microscopy methods and the expertise of Core Lab scientists, they experimented with a number of different solvents. This helped to determine the self-assembly in solution and its influence on the mechanism of pore formation, ultimately to find the one that formed the most orderly and even distribution of micelles.

However, micelles and vesicles that are formed in solution are dynamic, and evaporation immediately transforms the morphology. The challenge for the researchers was to control and stabilize the morphologies quickly to enable bottom-up nanostructure fabrication.

Film morphology is dependent upon not only the thermodynamic interaction between copolymers, but also upon other factors such as viscosity and the presence of impurities which could dramatically influence the kinetics of phase separation and self-assembly.

Dr. Peinemann, a veteran of the membrane technology industry, realized that “phase inversion”, ¬a well-defined process in the manufacture of commercial membranes, could be the key to getting their findings to market. Using a very simple and easily scalable casting procedure, followed by an immediate water bath quenching (for “phase inversion”), the pore size and distribution in the membrane can be stabilized and set.

With a patent application in process and a small-scale machine to manufacture small quantities of the film currently under construction, the KAUST researchers hope to form a new company to manufacture specialized membranes. Meanwhile, back in the lab, they will continue to refine the science and techniques of ultraporous films, moving toward the possibility of creating membranes with “responsive pores.”