Mixed-method approach characterizes nanovoids in polymer films

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. Highly accurate characterization of the void space in porous media is a challenge given the tortuous, convoluted nature of some materials. A novel application of graph theory combined with electron tomography and molecular dynamics simulations has characterized the morphology of nanovoids in polyamide membranes.

FALON KALUTANTIRIGE: In this case we had this very beautiful but irregular, complex structure so we wanted to quantify it as a whole. So we used graph theory as method. So graph theory is a very mathematical concept and this is the first time we used graph theory on polymer membranes. Conventionally graph theory is used in fields like database management, social network analysis and even the pandemic spread models. All these networks I talked about are large-scale but irregular. So we thought of using that into a nanoscale, irregular membrane network.

LAURA LEAY: That was Dr. Falon Kalutantirige who led the research during her PhD at the University of Illinois. Polyamide membranes are widely used as separation media in the water industry, for molecular separation, and for organic solvent filtration. Despite this widespread use, there has been some uncertainty surrounding the exact mechanisms of separation. The key to understanding permeance of the membranes lay in understanding the void space which was carefully mapped using electron tomography. Using their mixed-method approach, the team was able to relate the nanoscale morphology to membrane function. The thickness of the membrane around the nanovoids and the surface area of the nanovoids both affect the permeance properties of the membrane, which was determined experimentally using methanol. A thinner membrane and more open voids led to better permeance. The nanostructure can also govern the mechanical response of the membranes. To determine the mechanical properties of the membranes, graph theory was used to convert the topographical data into a skeleton graph of nodes and branches which allowed the skeleton graph rigidity to be calculated. This correlated with the nanoscale modulus of the membrane. Using coarse-grained molecular dynamics simulations and a significant amount of computational power, the team was able to determine the growth mechanism of the nanovoids which, using traditional experimental techniques, are often obscured by crumples – tiny mountain ranges which form in the material. Professor Ying Li from the University of Wisconsin-Madison explains.

YING Li: The key idea is you have two phases: one is the water phase and one is the oil phase. So one of the monomers – the MPD monomer – is soluble in the water phase while the other monomer, TMC, is in the oil phase. These two monomers gradually diffuse towards the interfacial region between the water and oil and then they start having polymerization reactions. In particular we observed a very interesting phenomenon that is a so-called monomer coalescence growth process. A lot of nanovoids have been observed by Falon through the electron microscope process is happening on tens or even hundred nanometer scale. This is beyond the typical molecular simulation scale people can do, so that’s the reason why we adopted these highly coarse-grained molecular models. The whole polymerization process involves the diffusion reaction. That’s an extremely long diffusion reaction process simulation.

LAURA LEAY: For Dr. Kalutantirige, the impact of the work is exciting.

FALON KALUTANTIRIGE: Obviously there is a direct impact on understanding the separation and mechanical behavior of polymer membranes, but I was very surprised to understand that our work is not limited to polymer membranes only. So, the beauty of this multidisciplinary toolkit is the versatility; soft materials containing voids such as aerogels, metalorganic frameworks, nanotube sponges, and even other porous membranes are widely used in materials applications. The actual nanoscale aspect is very underexplored. We can capture the regular-ness of irregular structures.

LAURA LEAY: For Professor Li, the synergy between experiment and simulation is key, and demonstrates that some advances are only possible with collaboration, opening the door for further work.

YING Li: From a computational point of view, we do see a beautiful correlation between our computer simulation results with the electron tomography Falon has done, so it’s a really amazing synergy, right, between experiments and computational study. To my knowledge this is the very first, very detailed electron tomography with such a high resolution of the internal structure of the polyamide membranes. This material is the golden standard used in the industry over 40 years but people still don’t understand why it is working so well. I strongly believe this study lays down the foundation to understand better regarding the internal microstructure of the membrane and eventually how it relates to the high separation performance. 

LAURA LEAY: This work was published in a recent issue of Nature Communications. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.