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Understanding the underlying nature of dynamical correlations believed to drive the bulk glass transition is a long-standing problem. Here we show that the form of spatial gradients of the glass transition temperature and structural relaxation time near an interface indeed provide signatures of the nature of relaxation in bulk glass forming liquids. We report results of long-time, large-system molecular dynamics simulations of thick glass-forming polymer films with one vapor interface, supported on a dynamically neutral substrate. We find that gradients in the glass transition temperature and logarithm of the structural relaxation time nucleated at a vapor interface exhibit two distinct regimes: a medium-ranged, large amplitude exponential gradient, followed by a long-range slowly decaying tail that can be described by an inverse power law. This behavior disagrees with multiple proposed theories of glassy dynamics but is predicted by the Elastically Collective Nonlinear Langevin Equation theory as a consequence of two coupled mechanisms: a medium-ranged interface-nucleated gradient of surface modified local caging constraints, and an interfacial truncation of a long-ranged collective elastic field. These findings support a coupled spatially local-nonlocal mechanism of activated glassy relaxation and kinetic vitrification.in both the isotropic bulk and in broken symmetry films.
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Glass formation and dynamics of model polymer films with one versus two active interfaces
Polymers and other glass-forming liquids can exhibit profound alterations in dynamics in the nanoscale vicinity of interfaces, over a range appreciably exceeding that of typical interfacial thermodynamic gradients. The understanding of these dynamical gradients is particularly complicated in systems with internal or external nanoscale dimensions, where a gradient nucleated at one interface can impinge on a second, potentially distinct, interface. To better understand the interactions that govern system dynamics and glass formation in these cases, here we simulate the baseline case of a glass-forming polymer film, over a wide range of thickness, supported on a dynamically neutral substrate that has little effect on nearby dynamics. We compare these results to our prior simulations of freestanding films. Results indicate that dynamical gradients in our simulated systems, as measured based upon translational relaxation, are simply truncated when they impinge on a secondary surface that is locally dynamically neutral. Altered film behavior can be described almost entirely by gradient effects down to the thinnest films probed, with no evidence for finite-size effects sometimes posited to play a role in these systems. Finally, our simulations predict that linear gradient overlap effects in the presence of symmetric dynamically active interfaces yield a non-monotonic variation of the whole free standing film stretching exponent (relaxation time distribution breadth). The maximum relaxation time distribution breadth in simulation is found at a film thickness of 4–5 times the interfacial gradient range. Observation of this maximum in experiment would provide an important validation that the gradient behavior observed in simulation persists to experimental timescales. If validated, observation of this maximum would potentially also enable determination of the dynamic gradient range from experimental mean-film measurements of film dynamics.
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- Award ID(s):
- 2208238
- PAR ID:
- 10525451
- Publisher / Repository:
- RSC
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 19
- Issue:
- 43
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 8413 to 8422
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D3SM00719G
