Polymers of Intrinsic Microporosity (PIMs) are a class of polymers known for their unique microporous structure at the molecular level. They possess a rigid, contorted backbone with intramolecular free volume spaces, which gives them the ability to selectively adsorb or permeate small molecules.
The term “intrinsic microporosity” refers to the inherent microporous nature of these polymers, meaning that the microporosity is an integral property of the polymer structure itself, rather than being introduced through additional modifications or additives. The micropores in PIMs typically have dimensions less than two nanometers, allowing for the efficient separation and transport of gases or other small molecules.
PIMs have garnered significant attention in membrane research and applications, particularly in gas separation and purification processes. Their inherent microporosity provides them with high selectivity and permeability for specific gases, making them promising candidates for various industrial separations, such as carbon capture, hydrogen purification, and gas separation in petrochemical processes.
Dr Andrew Foster is an expert in PIMs and one of the SynHiSel team. He works in the University of Manchester and took some time to tell us more about his work in this area, why membranes are interesting to industry and how he came to work in the area.
Can you summarise your current research interest?
Understanding PIM topology and network formation to give better membrane performance.
I’m really interested in topology and network formation the process of forming chemical bonds between polymer chains to create a network structure in polymeric systems. This applies to cross linking in different forms and what effects can be achieved on the bulk behaviour of materials, so it’s more general than just for PIMs in membrane applications. Topology relates to the configuration of the polymer chains: linear, cyclic, branched, loop structures etc, some of which may ultimately also contribute to network formation in polymers. Both polymer topology and network formation can often be tuned to give improvements in material performance which surpass adding an extra additive such as a filler. These improvements are more readily observed in thin film applications.
You can see illustrations of topology in the image above which is reproduced from A. Foster et al. Macromolecules 2020, 53, 2, 569–583.
Earlier in life, did you envision yourself in your current position?
Probably not, no. At school I was always good at maths, but I didn’t want to pursue a pure maths degree because I didn’t think that it would be particularly interesting or fulfilling. I was also interested in chemistry and if you move into physical chemistry and materials, they have both mathematical and practical aspects to keep you stimulated. That obviously also applies to the study of chemical engineering. My undergraduate chemistry degree covered a huge number of areas from biochemistry to materials chemistry, such as polymers, and even towards chemical engineering to some extent as well. I have no regrets of my path to my current position as the disciplines of chemistry and chemical engineering have given me an opportunity to work on some of the challenges facing the world in terms of carbon capture and sustainability.
Why is membranes research an interesting area to be involved with?
The field of membrane research continues to witness novel discoveries and advancements. Incorporating 2D materials, optimizing polymers, and striving for higher selectivity and permeability are driving forces behind current research initiatives. As industries and funding agencies recognize the potential benefits of improved membranes, researchers are pushing the boundaries to develop membranes that offer significantly enhanced performance. By bridging the gap between selectivity and permeability, future advancements in membrane technology hold promise for a wide range of applications in various industries.
You were part of the research grant that the collaboration received before the SynHiSel award, can you tell us more?
Our part in the previous EPSRC project focused on improving the balance in polymeric membrane performance, which typically results in reduced permeance and increased selectivity over time. Attempts were primarily made to address the physical aging and maintain high permeance, particularly with PIMs, which initially have high free volume but experience a large decrease in permeance due to aging and densification. The new programme aims to now concentrate on boosting selectivity whilst maintaining the gains in permeance possible for high free volume polymers. By achieving highly selective membranes with significantly higher flux compared to current membranes, industries benefit from improved material separation and reduced surface area requirements. This scalability advantage could be exemplified by membrane applications such as in fuel tank inerting systems which maintain a non-combustible atmosphere (N2 rich) in the air space in an emptying aircraft fuel tank. Housing the O2/N2 membrane separation equipment in a smaller compact area on board a plane would provide an energy saving.
Will an increased focus on sustainability in the future be compatible with your research?
That’s where polymer topology comes in. We might have to consider whether more sustainable monomers can be used to make polymers with similar topology. The current polymerisation system involves the use of quite toxic solvents, which are still generally in use industrially, but are not environmentally sustainable in the longer term. This is why the topology is quite important – if a strong understanding of topology is developed, one might question whether there are any natural materials that feature the same types of structure. The monomer structures that we use are similar in structure, for example, to tannic acid. It will be difficult probably to completely move away from organic solvents, but it is important to explore better ways of polymer synthesis. The polymer purification process is also quite intensive in terms of organic solvents. In an ideal situation, the polymerisation would proceed in water or minimal amount of organic solvent, using sustainable monomers, with the polymeric structures generated, solution processable into films from water. Currently, the thin film composite solution casting process is from either chloroform or tetrahydrofuran, which also would needed to be addressed before further scale up.
When a company has an issue that could be solved with membranes, are membranes usually the primary choice, or are there other technologies that offer similar benefits?
Membranes often offer large energy saving benefits. The only alternative in many processes would be large scale distillation apparatus, with its huge associated infrastructure and energy cost. Membrane units provide a large heat energy saving, because there is much less heat flow associated with the process. There will be a pressurised gas flow, facilitated with compressors, which is often the main energy cost of a membrane process. Membranes are sometimes favoured over distillation in systems where the boiling points of the separating liquids are very close together. It can be visually striking to see a compact installed membrane unit replacing a huge steel distillation column in a chemical plant. However, companies will usually use an established method, such as distillation, rather than risking a process that they are not familiar with. We therefore have work to do in order to prove the technology.
This blog is condensed, with some help from ChatGTP from the transcript of an interview made back in summer 2022. SynHiSel hosted a Year 12 school student for a 2-week Nuffield Research Placement. This scheme supports talented students who have experienced disadvantage. It gives them the opportunity to complete an engaging real-life research project within a host organisation at the point that they are starting to consider options for undergraduate study. Part of our project in SynHiSel was for the student to interview Andrew and find out more about what it’s like researching in this area.
Thanks to both for sharing their conversation.