Date(s) - 07/08/2016
2:30 pm - 4:00 pm
405 Ferris Hall
Department of Chemical & Biomolecular Engineering
University of Tennessee, Knoxville
Advisor: Dr. David J. Keffer
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.