Supercooled Liquid Dynamics: Advances and Challenges

Richert, Ranko

doi:10.1002/9781118202470.ch1

Cited by 18 publications

(10 citation statements)

References 219 publications

(294 reference statements)

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“…These results are also consistent with large-scale simulations of OTP by Eastwood et al [236]. Now, the temperature dependence of β in many substances seems to deviate significantly [237] from the steady dependence seen in Fig. 32(b).…”

Section: A Correlation Between Non-exponentiality Of Liquid Relaxatio...supporting

confidence: 90%

Theory of the Structural Glass Transition: A Pedagogical Review

Lubchenko

2015

Preprint

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The random first-order transition (RFOT) theory of the structural glass transition is reviewed in a pedagogical fashion. The rigidity that emerges in crystals and glassy liquids is of the same fundamental origin. In both cases, it corresponds with a breaking of the translational symmetry; analogies with freezing transitions in spin systems can also be made. The common aspect of these seemingly distinct phenomena is a spontaneous emergence of the molecular field, a venerable and well-understood concept. In crucial distinction from periodic crystallisation, the free energy landscape of a glassy liquid is vastly degenerate, which gives rise to new length and time scales while rendering the emergence of rigidity gradual. We obviate the standard notion that to be mechanically stable a structure must be essentially unique; instead, we show that bulk degeneracy is perfectly allowed but should not exceed a certain value. The present microscopic description thus explains both crystallisation and the emergence of the landscape regime followed by vitrification in a unified, thermodynamics-rooted fashion. The article contains a self-contained exposition of the basics of the classical density functional theory and liquid theory, which are subsequently used to quantitatively estimate, without using adjustable parameters, the key attributes of glassy liquids, viz., the relaxation barriers, glass transition temperature, and cooperativity size. These results are then used to quantitatively discuss many diverse glassy phenomena, including: the intrinsic connection between the excess liquid entropy and relaxation rates, the non-Arrhenius temperature dependence of α-relaxation, the dynamic heterogeneity, violations of the fluctuation-dissipation theorem, glass ageing and rejuvenation, rheological and mechanical anomalies, super-stable glasses, enhanced crystallisation near the glass transition, the excess heat capacity and phonon scattering at cryogenic temperatures, the Boson peak and plateau in thermal conductivity, and the puzzling midgap electronic states in amorphous chalcogenides.

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Section: A Correlation Between Non-exponentiality Of Liquid Relaxatio...supporting

confidence: 90%

Theory of the Structural Glass Transition: A Pedagogical Review

Lubchenko

2015

Preprint

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“…Polymers confined at the nanoscale can exhibit profound changes in properties, including cooperative segmental dynamics and glass transition temperature ( T g ), among others. − For example, relative to bulk polymer T g , the average T g s in some nanoconfined, supported polymer films or model nanocomposites (films sandwiched between two substrates) have been reported to shift up or down by 40 °C or more. − Although the mechanisms underlying the average property perturbations in glass-forming, nanoconfined polymers (whether films, model nanocomposites (NCs), or real NCs with nanofiller dispersed in polymers) remain incompletely understood, the interfacial layer is recognized to be a key factor. In 2019, Schweizer and Simmons stated, “The origin of large near-interface alterations in dynamics and glass formation behavior has been a major open question over the past 25 years. −…”

Section: Introductionmentioning

confidence: 99%

Molecular Weight Dependence of the Glass Transition Temperature (T_g)-Confinement Effect in Well-Dispersed Poly(2-vinyl pyridine)–Silica Nanocomposites: Comparison of Interfacial Layer T_g and Matrix T_g

Wei

Torkelson

2020

Macromolecules

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Nanocomposites (NCs) of poly(2-vinyl pyridine) (P2VP) and nanosilica were prepared by spin-coating and characterized by fluorescence to assess the alterations in matrix glass transition temperature (T g,matrix ) and interfacial layer T g (T g,inter ) as functions of P2VP molecular weight (MW) and silica content. Relative to neat P2VP T g (T g,neat ), major increases in T g,matrix were observed with increasing silica content in low-MW, 2.1 kg/mol P2VP NCs; T g,matrix − T g,neat = 15 and 35 °C at 1.0 and 10 vol % silica, respectively. High-MW, 100 kg/mol P2VP NCs exhibited much smaller increases in T g,matrix . To understand better the T g,matrix enhancements, trace levels of dye were labeled to silica to characterize the nearinterface (≤4 nm from the surface) T g (T g,inter ). Profound increases in T g,inter were observed in low-MW P2VP NCs: T g,inter − T g,neat = 25 and 40 °C at 1.0 and 10 vol % silica, respectively. In contrast, in high-MW P2VP NCs, T g,inter − T g,neat = 8 and 16 °C at 1.0 and 10 vol % silica, respectively. Because interfacial T g perturbations propagate tens of nanometers into a matrix, the larger increases in T g,inter in low-MW P2VP and at increasing silica content lead to larger increases in T g,matrix . At a given silica content, the greater alteration in T g,inter in low-MW P2VP NCs is explained by differences in interfacial conformations of low-and high-MW P2VPs and greater interfacial hydrogen bonding in low-MW P2VP. The stronger attractive interfacial interactions in low-MW P2VP NCs lead to greater alterations in T g,inter , which result in greater changes in T g,matrix . In contrast, the increase in T g,inter with increasing filler is due to T g,inter perturbations propagating into the matrix. At sufficient silica loading, perturbations to T g from one interface modify T g s at other interfaces. At a constant silica content in the range of 3.0−10 vol %, spatial T g gradients in the NCs and near-interface gradients have little MW dependence. At 0.5 and 1.0 vol % silica, our results suggest that near-interface gradients are larger in low-MW P2VP NCs.

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“…One of the major questions in the understanding of polymer dynamics over the last 20 years has been the nature of apparent alterations in the dynamics, mechanics, and glass formation behavior of polymers and other glass-forming liquids in the nanoscale vicinity of interfaces. − Evidence suggests that these effects, commonly referred to as “nanoconfinement effects on the glass transition”, are in most (but perhaps not all) cases driven by interfacial gradients . The origins of these shifts and even their precise phenomenology remain unresolved questions, which are relevant both to the design of diverse nanostructured materials and to the understanding of putative correlation length scales associated with glass formation itself. − Here we analyze long-time molecular dynamics simulations of thick bead–spring polymer films to show that nanoconfinement effects on segmental-scale translational dynamics take the form of a position-dependent fractional power–law decoupling relationship with bulk dynamics.…”

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confidence: 99%

Temperature-Independent Rescaling of the Local Activation Barrier Drives Free Surface Nanoconfinement Effects on Segmental-Scale Translational Dynamics near T_g

2018

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Near-interface alterations in dynamics and glass formation behavior have been the subject of extensive study for the past two decades, both because of their practical importance and in the hope of revealing underlying correlation lengths underpinning glass transition more generally. Here we employ molecular dynamics simulations of thick films to demonstrate that these effects emerge, for segmental-scale translational dynamics at low temperature, from a temperature-independent rescaling of the local activation barrier. This rescaling manifests as a fractional power law decoupling relationship of local dynamics relative to the bulk, with a transition from a regime of weak decoupling at high temperatures to a regime of strong decoupling at low temperatures. The range of this effect saturates at low temperatures, with 90% of the surface perturbation in the barrier lost over a range of 12 segmental diameters. These findings reduce the phenomenology of T g nanoconfinement effects to two propertiesa position-dependent, temperature independent, barrier rescaling factor and an onset time scalewhile substantially constraining the predictions required from any theoretical explanation of this phenomenon.

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Supercooled Liquid Dynamics: Advances and Challenges

Cited by 18 publications

References 219 publications

Theory of the Structural Glass Transition: A Pedagogical Review

Theory of the Structural Glass Transition: A Pedagogical Review

Molecular Weight Dependence of the Glass Transition Temperature (T_g)-Confinement Effect in Well-Dispersed Poly(2-vinyl pyridine)–Silica Nanocomposites: Comparison of Interfacial Layer T_g and Matrix T_g

Temperature-Independent Rescaling of the Local Activation Barrier Drives Free Surface Nanoconfinement Effects on Segmental-Scale Translational Dynamics near T_g

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Supercooled Liquid Dynamics: Advances and Challenges

Cited by 18 publications

References 219 publications

Theory of the Structural Glass Transition: A Pedagogical Review

Theory of the Structural Glass Transition: A Pedagogical Review

Molecular Weight Dependence of the Glass Transition Temperature (Tg)-Confinement Effect in Well-Dispersed Poly(2-vinyl pyridine)–Silica Nanocomposites: Comparison of Interfacial Layer Tg and Matrix Tg

Temperature-Independent Rescaling of the Local Activation Barrier Drives Free Surface Nanoconfinement Effects on Segmental-Scale Translational Dynamics near Tg

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Molecular Weight Dependence of the Glass Transition Temperature (T_g)-Confinement Effect in Well-Dispersed Poly(2-vinyl pyridine)–Silica Nanocomposites: Comparison of Interfacial Layer T_g and Matrix T_g

Temperature-Independent Rescaling of the Local Activation Barrier Drives Free Surface Nanoconfinement Effects on Segmental-Scale Translational Dynamics near T_g