2:30 p.m. Friday, July 8, 2016
405 Ferris Hall
“Multi-scale Materials Modeling of Polyethylene Glycol as an Additive to Proton Exchange Membranes”
Polyethylene glycol (PEG) has a wide variety of applications such as drug delivery via modification of therapeutic molecules (known as PEGylation) and electrochemical energy conversion as a proton exchange membrane (PEM) additive. The properties of PEG that give it such a wide application range are mainly hydrophilicity and pH-dependent behavior in aqueous environments. PEG has shown to enhance the proton conductivity of a class of PEMs based on the polymer crosslinked, sulfonated polycyclohexadiene, or xsPCHD. Yet, the exact mechanism for the enhancement of conductivity is unknown. Atomistic details of charge transport in the presence of PEG can give fundamental insight into this increase in conductivity and give aid in future PEM synthesis and development.
Materials modeling serves to develop fundamental relationships between the structure of a material and its thermodynamic and transport properties. Theory, modeling, and simulation can be tools that compliment experiment by helping unravel experimental characterization by potentially giving atomic-level details of a material and can also help guide the future synthesis of new materials. Many of the most challenging materials modeling problems facing researchers today display behavior or exploit phenomena that span more than one single length or time scale. Thus, we employ a multi-scale modeling effort to study the atomic-level effect of PEG on charge transport in aqueous environments.
We show the results of ab initio and reactive molecular dynamics (MD) for triethylene glycol (TEG) and PEG in aqueous environments. Specifically, we give details about the protonic defect structures found, the local solvation structures, the hydrogen-bond networks formed around the protonic defects, the mechanistic details, and transport properties observed from the ab initio MD study of TEG in an aqueous environment. We then multi-scale our effort by developing a reactive force field that can be used in reactive MD simulations, capable of simulating larger temporal and spatial scales. The results show that enhanced proton conductivity compared to bulk water is possible when an optimal, elongated PEG chain is present. We observe that there is an attraction of the amphiphilic hydronium ion to methylene groups. This is in line with previous literature results that show enhanced proton conductivity at oil-water interfaces. These results give the fundamental view of the effect of PEG on charge transport in an aqueous environment and hope to impact future membrane designs that incorporate PEG as an additive for a variety of applications, mainly to help reduce time-to-discovery for their development.
Mr. Marshall McDonnell is a PhD candidate in the Department of Chemical and Biomolecular Engineering at the University of Tennessee, Knoxville. He earned his BS in chemistry and mathematics at Lincoln Memorial University in 2010. He joined Dr. David Keffer’s group in 2012 where his work has focused mainly on multiscale molecular modeling of transport in hydrated polymeric membranes. Specifically, he studies proton transport in proton exchange membranes for fuel cell applications and oxygen permeability in chitosan food packaging films.
07/08/2016 - 2:30 pm - 4:00 pm
3:00 p.m. Friday, July 11, 2016
220-E Hodges Library
“Lignin-based Li-Ion Anode Materials Synthesized from Low-Cost Renewable Resources”
In today’s world, the demand for novel methods of energy storage is increasing quickly, particularly with the rise of portable electronic devices, electric vehicles, and the personal consumption and storage of solar energy. While other technologies have improved rapidly, battery technology has lagged behind largely due to the difficulty in devising new electric storage systems that are simultaneously high performing, inexpensive, and safe.
In order to tackle these challenges, novel Li-ion battery anodes have been developed at Oak Ridge National Laboratory that are made from lignin, a low-cost, renewable resource that is obtained from an abundant supply of biomass. The anodes that result from the lignin manufacturing process exhibit performance comparable to that of conventional graphitic anodes for a fraction of the cost. However, these materials are unusual in that they consist solely of a mixture of amorphous and crystalline carbon, and this complex, hierarchical material is not well understood. This work reveals the mechanism behind the structural composition and the performance of these carbon composite anodes.
The anodes are investigated using two distinct approaches: 1) a computational approach, whereby atomistic models of the composite systems are created and simulated using reactive molecular dynamics, and 2) an experimental approach, whereby the small scale structure of the material is elucidated using neutron diffraction.
Mr. Nicholas McNutt is a PhD candidate in the Department of Chemical and Biomolecular Engineering at the University of Tennessee. He earned his B.S. in Chemical and Biomolecular Engineering at the Georgia Institute of Technology in 2012. He joined Dr. David Keffer’s group in 2012 where his work has focused mainly on molecular modeling of Li-ion battery anodes. Specifically, he studies the structure of low-cost, lignin-derived carbon composite anodes, and the storage and energetics of the Li-ions contained within them.
07/11/2016 - 3:00 pm - 4:00 pm
1:00 p.m., Thursday, July 25, 2019
422 Dougherty Engineering Building
“Detergent Solubilization of Mixed-Lipid Liposomes and Mixed Liposomes for Incorporation of Photosynthetic Membrane Proteins”
This dissertation examines two phenomena related to the use of photosystem proteins and hydrogenase enzymes in liposomes to form light-catalyzed fusion complexes for producing molecular hydrogen.
First, the method to best form fusion proteins with photosystem I (PSI) using sortase-mediated ligation was examined. In this work, a surrogate for hydrogenase, green fluorescent protein, was used and the concentrations of the ligation reaction compounds were varied to most effectively achieve fusion complexes.
Second, the process of detergent solubilization of liposomes needed for PSI incorporation was examined. In this work, mixtures of lipids were used to more closely imitate natural thylakoid membranes than single lipid systems. I looked at factors that affect the ability to drive insertion of amphiphilic detergent molecules into lipid bilayers by observing changes in lipid vesicle morphology as the concentration of detergent is increased. Detergent molecules interact with liposomes by inserting their hydrophobic portion into the bilayer, thereby partitioning the lipids and causing swelling of the vesicle. As the vesicles are saturated with detergent, mixed lipid- detergent micelles bud off until there is complete dissociation of liposomes. By studying the detergent concentration these stages occur, I gained understanding about lipid-lipid interactions and factors that affect the incorporation of PSI into liposomes. In this case, I found that lipid shape, conical vs. cylindrical respectively, is a major influence in the incorporation of detergent, Triton X-100 (TX).
The conical nature of ePA leads to increased mismatching within the bilayer and promotes increased uptake of TX before dissociation. Additionally, the interaction of detergent with multiple membrane types present was examined with two populations of liposomes made from egg phosphatidic acid (ePA) and egg phosphatidylcholine (ePC). An equilibrium model was derived that predicts the point of saturation for mixtures of two liposomes types and aligns with experimental observations. Furthermore, the model estimates the number of detergent molecules incorporated into each of the liposome populations.
Samantha Tyane Clark comes from Mobile, Alabama. She received her Bachelors of Science degree in Nutrition and Food Science in December of 2010 from Auburn University in Auburn, Alabama. She studied chemical engineering at the bachelors level in 2011 at the University of South Alabama, and joined Dr. Paul D. Frymier’s research group at the University of Tennessee, Knoxville in August of 2012. She received her Masters of Science in August of 2015.
07/25/2019 - 1:00 pm - 2:30 pm