Probing the Metrology and Chemistry Dependences of the Onset Condition of Strong "Nanoconfinement" Effects on Dynamics
DD Vela and A Ghanekarade and DS Simmons, MACROMOLECULES, 53, 4158-4171 (2020).
Polymers in the nanoscale vicinity of interfaces exhibit a broad range of alterations in their dynamics and glass-formation behavior. A major goal in the study of these effects is to understand their strong apparent dependence on chemistry, measurement time scale, and metrology. Here we employ molecular dynamics simulations of thin freestanding polymer films over a range of thicknesses and polymer backbone stiffnesses to probe these dependences. Results suggest that a chemistry- and metrology-dependent onset of strong nanoconlinement may play an important role in chemical and metrological variations in the apparent strength of nanoconfinement effects. Beyond this onset, we find that the activation barrier for relaxation is subject to a simple temperature-insensitive rescaling near a surface at low temperatures, leading to a fractional power law decoupling relationship between thin film and bulk dynamics. We show that a generic two-barrier model of the glass transition can parsimoniously describe much of this phenomenology, with the "onset" of strong interface effects on dynamics related to a crossover in dominance from a high-temperature barrier to a low- temperature barrier. We suggest that variation of this onset time scale and temperature may play an important role in system-to-system and measurement-to-measurement variations in the observed strength of interfacial effects on dynamics and glass formation. These results may also explain why simulations at relatively short time scales commonly report effects of a magnitude comparable to experiments at much larger time scales.
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