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Wednesday, March 2, 2022 12pm to 1pm

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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.

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