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.
more »
« less
Nature of dynamic gradients, glass formation, and collective effects in ultrathin freestanding films
Molecular, polymeric, colloidal, and other classes of liquids can exhibit very large, spatially heterogeneous alterations of their dynamics and glass transition temperature when confined to nanoscale domains. Considerable progress has been made in understanding the related problem of near-interface relaxation and diffusion in thick films. However, the origin of “nanoconfinement effects” on the glassy dynamics of thin films, where gradients from different interfaces interact and genuine collective finite size effects may emerge, remains a longstanding open question. Here, we combine molecular dynamics simulations, probing 5 decades of relaxation, and the Elastically Cooperative Nonlinear Langevin Equation (ECNLE) theory, addressing 14 decades in timescale, to establish a microscopic and mechanistic understanding of the key features of altered dynamics in freestanding films spanning the full range from ultrathin to thick films. Simulations and theory are in qualitative and near-quantitative agreement without use of any adjustable parameters. For films of intermediate thickness, the dynamical behavior is well predicted to leading order using a simple linear superposition of thick-film exponential barrier gradients, including a remarkable suppression and flattening of various dynamical gradients in thin films. However, in sufficiently thin films the superposition approximation breaks down due to the emergence of genuine finite size confinement effects. ECNLE theory extended to treat thin films captures the phenomenology found in simulation, without invocation of any critical-like phenomena, on the basis of interface-nucleated gradients of local caging constraints, combined with interfacial and finite size-induced alterations of the collective elastic component of the structural relaxation process.
more » « less- Award ID(s):
- 1854308
- NSF-PAR ID:
- 10281564
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 118
- Issue:
- 31
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- Article No. e2104398118
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Polymers are increasingly being used in applications with nanostructured morphologies where almost all polymer molecules are within a few tens to hundreds of nanometers from some interface. From nearly three decades of study on polymers in simplified nanoconfined systems such as thin films, we have come to understand property changes in these systems as arising from interfacial effects where local dynamical perturbations are propagated deeper into the material. This review provides a summary of local glass transition temperature T g changes near interfaces, comparing across different types of interfaces: free surface, substrate, liquid, and polymer–polymer. Local versus film-average properties in thin films are discussed, making connections to other related property changes, while highlighting several historically important studies. By experimental necessity, most studies are on high enough molecule weight chains to be well entangled, although aspects that connect to lower molecule weight materials are described. Emphasis is made to identify observations and open questions that have yet to be fully understood such as the evidence of long-ranged interfacial effects, finite domain size, interfacial breadth, and chain connectivity.more » « less
-
more » « less
ABSTRACT The physical aging behavior, time‐dependent densification, of thin polystyrene (PS) films supported on silicon are investigated using ellipsometry for a large range of molecular weights (MWs) from
M w = 97 to 10,100 kg mol−1. We report an unexpected MW dependence to the physical aging rate ofh < 80‐nm thick films not present in bulk films, where samples made from ultra‐high MWs ≥ 6500 kg mol−1exhibit on average a 45% faster aging response at an aging temperature of 40 °C compared with equivalent films made from (merely) high MWs ≤ 3500 kg mol−1. This MW‐dependent difference in physical aging response indicates that the breadth of the gradient in dynamics originating from the free surface in these thin films is diminished for films of ultra‐high MW PS. In contrast, measures of the film‐average glass transition temperatureT g (h ) and effective average film density (molecular packing) show no corresponding change for the same range of film thicknesses, suggesting physical aging may be more sensitive to differences in dynamical gradients. These results contribute to growing literature reports signaling that chain connectivity and entropy play a subtle, but important role in how glassy dynamics are propagated from interfaces. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019 ,57 , 1224–1238 -
more » « less
Abstract The secular evolution of disk galaxies is largely driven by resonances between the orbits of “particles” (stars or dark matter) and the rotation of non-axisymmetric features (spiral arms or a bar). Such resonances may also explain kinematic and photometric features observed in the Milky Way and external galaxies. In simplified cases, these resonant interactions are well understood: for instance, the dynamics of a test particle trapped near a resonance of a steadily rotating bar is easily analyzed using the angle-action tools pioneered by Binney, Monari, and others. However, such treatments do not address the stochasticity and messiness inherent to real galaxies—effects that have, with few exceptions, been previously explored only with complex
N -body simulations. In this paper, we propose a simple kinetic equation describing the distribution function of particles near an orbital resonance with a rigidly rotating bar, allowing for diffusion of the particles’ slow actions. We solve this equation for various values of the dimensionless diffusion strength Δ, and then apply our theory to the calculation of bar–halo dynamical friction. For Δ = 0, we recover the classic result of Tremaine and Weinberg that friction ultimately vanishes, owing to the phase mixing of resonant orbits. However, for Δ > 0, we find that diffusion suppresses phase mixing, leading to a finite torque. Our results suggest that stochasticity—be it physical or numerical—tends to increase bar–halo friction, and that bars in cosmological simulations might experience significant artificial slowdown, even if the numerical two-body relaxation time is much longer than a Hubble time. -
more » « less
Abstract Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer that can be used to make hydrogels for biomedical applications as well as biodegradable bags and films; however, compared to other plastics currently used for containers, it lacks mechanical strength, thermal stability, and can easily absorb water from humid environments. Although mechanical improvement has been observed by blending PVA with collagen in a hybrid hydrogel, there is a lack of fundamental understanding of the molecular mechanism, and it is not clear whether the improvement is limited to a hydrated state. Here, using classical molecular dynamics simulations based on fully atomistic models, we develop the equilibrated molecular structure of PVA with collagen and characterize its mechanics. We show that by interacting with a collagen molecule, PVA is equilibrated to a more ordered structure with each residue interacting with the near neighbors by forming more hydrogen bonds locally, making the structure stiffer than pure PVA. The structure shows higher thermal stability before melting, as well as higher rigidity in water. Our results provide the mechanism of the mechanical advantages of hybrid PVA-collagen polymer. The study demonstrates that the structure and mechanics of a synthetic polymer can be tuned by a tiny amount of a natural polymer at the molecular interface. Moreover, it may shed light on identifying a way to improve the mechanics of biodegradable polymer materials without adding much cost, which is crucial for environmental safety.
Impact statement Blending natural and synthetic polymers (e.g., polyvinyl alcohol [PVA] and collagen in a hybrid hydrogel) has shown advantages in polymer mechanics, but there is a lack of fundamental understanding. Using molecular dynamics (MD) simulations based on fully atomistic models, we develop the equilibrated structure of the PVA with collagen and characterize its mechanics. We show that by interacting with a collagen molecule, PVA is equilibrated to a more ordered structure with each residue interacting with the near neighbors by forming more H-bonds locally and the structure is stiffer than pure PVA. Moreover, the structure shows a higher thermal stability before the melting point of PVA, as well as higher rigidity in water. Our results demonstrate that the structure and mechanics of a synthetic polymer can be tuned by a tiny amount of a natural polymer at the molecular interface. It provides the mechanism of the mechanical advantages as experimentally observed. This study paves the way for the multiscale modeling and mechanical design of the hybrid polymer material. It sheds light on identifying a way to improve the mechanics of biodegradable materials without adding much cost for both material functionality and environmental safety.
Graphical abstract
