I am currently a Postdoctoral Associate at the University of Maryland, working on battery materials. I am focused on researching new polymers and various methods of scientific communication.
Nanoporous polymer networks can help advance the next generation of catalytic technologies. In a paper authored by Dr. Keira Culley, in which I assisted with X-ray scattering experiments, we see the realm of possibilities offered by highly ordered lyotropic networks. Cubic networks made of lyotropic materials, as evidenced by previous work, can exhibit size-discrimination. If the catalytic site is within the membrane, you can selectively allow some reactions to take place while having some species selectively removed.
This work is the first example that we can find of a successful size discrimination-based heterogeneous catalysis in a nanoporous lyotropic polymer, allowing some alcohol degradations to occur, but not others. As seen in the figure below, the biggest alcohol (in blue) does not convert at all, while the alcohols below the size limit of the pores in the resin used can nearly complete the reaction.
I think that this is a worthy future direction, though the priorities for future research will be in reducing cost of materials and improving the mechanical properties. The elution rate of the membrane materials can be quite slow, so they need to be able to handle highly pressurized system or to be spuncoat in very thin layers.
The next generation of batteries and energy generation devices require both precise, size-controlled engineering and renewable materials sources. My good colleague Dr. Ruiqi Dong and I aimed to accomplish both of these goals in a study published in ACS Materials Letters. The article, linked here, showed that we can rapidly conduct potassium through a polymer electrolyte, when taking advantage of the solvent imbibed within the self-assembled nanopores.
This work shows that lyotropic liquid crystals have a lot of promise for energy storage and generation technologies. We get to take advantage of the higher internal surface areas and favorable solvents to rapidly move species of interest. I hope that this avenue is explored in the future – I am to do so in collaboration with the Osuji group and within my own time.
The Vagelos Institute for Energy Science and Technology at the University of Pennsylvania gave me the opportunity to meet some of the most talented scientists I have had the pleasure of working with. Myself, Dr. Ranadeb Ball, and Lizhu Zhang explored the fundamentals of ion transport in nanopores, using the lyotropic mesophase system that had been the focus of my Ph.D.
The first of these papers was published in ACS Nano, and explored the differences in morphology and hydration between different nanoporous morphologies made of the same chemical units. We went in with the hypothesis that the gyroids (GYR), with a larger pore size and minimal increase to tortuosity, would exhibit a higher relative conductivity. As shown by the image below, that was not the case. The highly constrictive hexagonally packed cylinders (HEX) were 3-4 times higher in conductivity at the same relative humidity, which was surprising.
The key phrase there is “relative humidity.” The hydration behavior of each mesophase is different, and largely determines the conductivity. The HEX mesophase is able to fit in more water at higher relative humidities, and is able to conduct much more readily at similar conditions. If we correct by the number of waters available per charge site (represented as a lambda), we get the following result:
From the article: (a) Vapor sorption data show water content in the form of hydration number as a function of RH at 25 °C (open symbols) and 70 °C (closed symbols). (b) Relative change in pore size dlimit/dlimit,0 as a function of hydration number from humidity-dependent (in situ) SAXS measurements. (c, d) Concentration normalized conductivity for bromide and hydroxide anions as a function of hydration number at 25 °C (open symbols) and 70 °C (closed symbols). (e) Activation energy in HEX and GYR for bromide and hydroxide ions determined at a fixed hydration number λ = 3.6.
Shown in this image, we see that the differences between our mesophases collapse for bromide conductivity! Arguably, for hydroxide conductivity, we could see that our initial hypothesis had the potential to be correct at high temperatures, but we can’t make statistically significant claims about that. What this tells us about the design of nanoporous polymers for ion separations is that if you are including a morphology in your material, don’t worry about the geometric parameters – focus on getting as much solvent as possible into the material. For lyotropic materials, the interactions of the ion at the nanopore interfaces will be very similar regardless of curvature, tortuousity, or pore size. For some mesophases, the amount of water available is much greater.
I am excited in the next year or so to show the next set of results from these materials, where we explore ion-identity and pore-size specific phenomena.
The Osuji group (where I did my Ph.D.) had a very strong and fruitful relationship with the former Gin group of the University of Colorado-Boulder. One collaboration that was particularly fun was a mechanical analysis study performed with Dr. Lauren Bodkin. The paper, entitled “Effect of localized control of cross-link density on mechanical properties of bicontinuous cubic lyotropic networks via copolymerization with different singly-polymerizable monomers,” investigated several different chemical moieties that were similar in their general chemistries, but had different crosslinking modes. One of the investigated monomers could only crosslink at its polar head, one in the nonpolar tail, and one in both the head and tail regions. We made gyroid networks (what a wonderful structure, the gyroid) and investigated the tensile strength of each.
We found that by increasing our nonpolar crosslinking, we strengthened the tensile properties of the material, which helps support the assertion that nonpolar crosslinking groups in aqueous lyotropic species are necessary for overall stability. There was a turning point, though, as when we only incorporated the two crosslinking-mode species, we found that the films became more brittle due to a depletion of reacted crosslinking groups at the polar heads. The nonpolar groups interacted so fast that it harmed the overall tensile strength of the material. Incorporating only tail-based groups also upset the phase stability. We see in this paper the balancing act that all lyotropic liquid crystal films have to undergo when optimizing properties and structure.
During the early days of my Ph.D., I worked in tandem with Dr. Keira Culley at the University of Colorado-Boulder, as part of her thesis work in the Gin group. She was interested in creating complex polymer structures that could aid in catalysis. I was quite skilled in identifying the order those polymer structures contained, so we worked together on a couple of occasions. The paper entitled “Sulfonic-acid-based lyotropic bicontinuous cubic polymer network for molecular-size-selective heterogeneous catalysis” was the first such example of us working together. Dr. Culley is an excellent scientist, and I was very lucky to have done my Ph.D. work around the same time as her.
The following is the abstract of work published in Advanced Materials Interfaces, which I performed as a co-first author with Dr. Patrick Li. This was my first scientific paper, and I had a lot of fun during it. We made films that were resilient to acid, and were the first example to our knowledge of a complex lyotropic structure that could be actively changed via acid concentration.
The last few weeks have been a whirlwind of excitement. I’ve passed so many milestones where I’ve thought “okay, now I’m really a doctor.” At this point, it is all done, and I’m happy to say that I have successfully completed a Ph.D. in Chemical and Biomolecular Engineering at the University of Pennsylvania. I defended my thesis, entitled “Structure and transport properties of nanoporous polymers derived from lyotropic mesophases.” I submitted a bunch of documents to my school and to journals. I walked across a stage! I got an envelope that said I walked across that stage! Amazing!
I thank a lot of people in my thesis, but the highlights here are my advisor Prof. Chinedum Osuji, my committee members Profs. Karen I. Winey, Daeyeon Lee, and Talid Sinno, my close labmates (Yvonne Zagzag and Ravisara Wattana especially), and my wife Sienna Pyle. A big thank you to everyone who has helped me or been a friend at some point in this massive undertaking. It means the world.
Now if you’ll excuse me, I am going to go take a nap.
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