This photo shows a free-standing polymer nanofilm membrane (200nm thick), supported on porous alumina. Photo: Qilei Song, Department of Chemical Engineering, Imperial College London.
This photo shows a free-standing polymer nanofilm membrane (200nm thick), supported on porous alumina. Photo: Qilei Song, Department of Chemical Engineering, Imperial College London.

The chemical separation processes used by industry are typically quite costly, with thermal separation processes such as distillation and evaporation currently accounting for 10–15% of the world's annual energy use. Synthetic polymer membranes offer a more efficient, non-thermal way to separate gases and chemicals, and so have the potential to reduce energy consumption significantly, as well as reducing pollution and cutting carbon dioxide (CO2) emissions.

Unfortunately, conventional membranes exhibit a relatively low permeance for gases and liquids, meaning the degree to which the membranes allow these substances to flow through them, which limits their use in large-scale separation processes. In addition, it has proved challenging to develop membranes that are resistant to the organic solvents used in petrochemical refining and chemical separation processes. Ongoing research efforts are being devoted to developing more cost effective, better performing membranes for gas and liquid separations, with the aim of achieving high permeance, high molecular selectivity and high stability in practical applications.

Now, researchers from Imperial College London, led by Andrew Livingston in the Department of Chemical Engineering, have developed a new synthetic approach for generating microporous polymer membranes with just these kinds of abilities. The membranes, which are described in a paper in Nature Materials, could find use in a wide range of industrial applications, including the purification of oil and natural gas, desalination, solvent nanofiltration and CO2 capture.

The researchers developed a novel approach for producing polymer membranes by linking twisted monomers to form crosslinked network polymers, known as 'polymers of intrinsic microporosity (PIMs)', which have an increased volume of internal cavities. These cavities allow the membrane to be very permeable, while the network polymer acts as a scaffold that ensures it remains rigid and stable. Combining this approach with a technique known as interfacial polymerization, the researchers were able to control the thickness of these microporous polymer membranes down to 20nm.

"This work reports new methods of fabricating polymer membranes using a molecular design approach," said Livingston. "We are able to design the free volume, which acts as pores in the membrane, by choosing the monomers used to make the membrane separating layer. So we have managed for the first time to create interconnected 3D polymer network membranes in which we can control the size of pores and their connectivity. This means we can make a more accurate separation between molecules and at a higher processing rate, making more efficient separations with less consumption of energy."

"We demonstrated a simple approach to preparing microporous thin polymer membranes using the aromatic polyester chemistry as an example" said Maria Jimenez-Solomon, co-lead author of the paper and a postdoctoral research associate in Livingston's group at Imperial College London. "However, the approach is not limited to synthesizing polyesters, it has opened up new ways of synthesizing membrane materials using a range of contorted molecules".

"To optimize and scale up the synthetic approach, we performed extensive characterizations to understand the structure and properties of these polymer membranes, however there are still many interesting scientific questions to study in the future," added Qilei Song, the other co-lead author of the paper and a junior research fellow in the Department of Chemical Engineering. "We expect that by tuning the molecular structure of the polymers in combination with nanoscale control of the membrane, the performance of polymer membranes can be enhanced even further".

The porous structure of these polymer membranes was also confirmed by molecular simulations performed by Kim Jelfs, a research fellow in the Department of Chemistry at Imperial College London, and a co-author of the paper. "The computational approaches allow us to elucidate the nature of the materials; for example, we can predict the polymer structure and porosity based on the large scale computational screening of precursor libraries," she explained.

In the paper, the researchers demonstrated several applications of their membranes in gas and organic solvent separations. They now plan to extend this approach to produce a wide range of porous polymers for various industrial applications, from water purification and desalination, to purification of pharmaceuticals, to hydrocarbon separation.

"If we are able to use membranes to accurately separate molecules which are in organic solvents, we can work towards replacing distillation and evaporation processes with more energy-efficient membrane separation technologies," said Livingston.

This story is adapted from material from Imperial College London, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.