Probing the Metrology and Chemistry Dependences of the Onset Condition of Strong “Nanoconfinement” Effects on Dynamics

Abstract

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 nanoconfinement 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 lowtemperature barrier. We suggest that variation of this onset time scale and temperature may play an important role in system-tosystem 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.

Near-Substrate Gradients in Chain Relaxation and Viscosity in a Model Low-Molecular Weight Polymer

“…Both Table and Figure b are consistent with an interfacial gradient of exponential decay range around 2. By contrast, simulations at free surfaces have indicated that the decay range here is about twice as large. ,,, This is consistent with prior work suggesting an asymmetry in the range of dynamical gradients at free surfaces versus buried substrates . Recent experimental work has also observed this asymmetry between the ranges of the gradient at buried interfaces for which T g is decreased (longer range) versus increased (shorter ranged), − suggesting that interfacial T g enhancements may generally propagate further into materials than T g suppressions.…”

“…The onset behavior of eq is nonuniform across the five relaxation functions, with viscosity exhibiting a more pronounced onset than the other four. In prior work, we explored this onset behavior for segmental translation and reorientation at a free surface across a matrix of bead–spring polymers and found that the timescale of the onset can vary with chemistry and even with the relaxation function at fixed chemistry . Similarly, we found variation in the “sharpness” of the onset, with bulk-like behavior recovered very abruptly at high temperature in some relaxation functions and more gradually in others.…”

“…As in our prior work, , we constrain our fit to eq to low temperatures, only, in an effort to avoid contamination by data below or within this onset. The fit range associated with each quantity is shown in the rightmost column of Table .…”

“…This may provide a useful design rule for obtaining relative uniform viscosity near interfaces. This tendency may also be overlaid with effects emanating from variations in the onset timescale of interfacial alterations between distinct relaxation functions, which may become more important as chain stiffness is increased . At the same time, evidence also suggests that chain relaxation exhibits less polymer-to-polymer variation in the fragility of glass formation than does segmental relaxation; this may suggest that polymer interfacial gradients in viscosity may be less sensitive to chemistry than are gradients in segmental dynamics.…”

“…This fact leads to a fractional power law relationship between local or confined dynamics at bulk dynamics at low temperatures ,, where τ* is a chemistry- and relaxation function-dependent bulk relaxation timescale of onset of strong interfacial effects on dynamics, such that dynamics are nearly bulk-like at temperatures for which the bulk relaxation time is less than this value. , …”