Joshua Sangoro, University of Tennessee
Polymers exhibit deviations from their bulk physical properties in the vicinity of solid interfaces. For close to three decades, there has been a concerted effort to understand the fundamental mechanisms determining the macroscopic physical properties of amorphous polymers under confinement because of the many possible practical applications, such as protective and lubricating coatings, smart window layers, adhesives, microelectronic encapsulants, polymer electrolytes, and dielectrics. Because interfacial interactions and free surfaces are expected to play a significant role in determining the overall properties of confined polymers, our recent focus has been directed toward designing experiments that give direct access to the polymer dynamics at the interface, rather than those that probe the global characteristics of the polymers. In this talk, I will present a recently developed direct experimental approach that employs lithographically prepared nanostructured electrodes to perform broadband dielectric spectroscopy studies of dynamics in ultrathin polymer films. Using model systems of non-ionic and ionic polymers, we take advantage of access to the distribution of relaxation times in an extended temperature range above the glass transition temperature, Tg, and find that while the mean rates of segmental and ionic relaxations remain bulk-like down to 7 nm film thickness, the molecular mobilities at the interfacial zones are significantly altered. Combining the experimental insights with results from multiscale molecular dynamics simulations, we find that both the slow dynamic modes arising from adsorbed polymer segments and the faster relaxations attributed to segments in the vicinity of the free interface have non-Arrhenius temperature activation. These interfacial regions span thicknesses of ~1.5 nm each just above the calorimetric Tg independent of molecular weight and film thickness. These deviations at interfaces are relevant for applications of polymers in adhesion, coatings, polymer electrolytes, and polymer nano-composites.
Joshua Sangoro is an Associate Professor and Associate Department Head in the Department of Chemical and Biomolecular Engineering, The University of Tennessee, Knoxville (UT). He received his doctorate in Experimental Physics in 2010 from the University of Leipzig (Germany) with Prof. Friedrich Kremer. His dissertation research focused on studies of ionic liquids by broadband dielectric spectroscopy. Sangoro worked as a Research Scientist at the University of Leipzig until early 2012 when he joined the Chemical Sciences Division of the Oak Ridge National Laboratory as a Postdoctoral Research Associate. In 2012, he was awarded the Feodor-Lynen Research Fellowship by the Alexander von Humboldt Foundation. Sangoro joined the Department of Chemical and Biomolecular Engineering at the UT in the fall of 2013 as an Assistant Professor and was promoted to Associate Professor in 2019. He has authored or co-authored over 70 peer-reviewed articles and book chapters, and he also has contributed to over 70 (28 invited) technical presentations at national and international meetings. He is a recipient of the ARO Young Investigator Program as well as the NSF CAREER awards. His current research focuses on understanding how the dynamics across multiple length-scales in the condensed phase emerge from complex correlations of atomic and molecular constituents and how we can employ this knowledge to engineer soft materials for current and future energy and sustainable technologies. His current research projects are supported through grants from the National Science Foundation, Department of Energy Office of Science, and Army Research Office.
Frans A. Spaepen