Approach to hyperuniformity in a metallic glass-forming material exhibiting a fragile to strong glass transition

Zhang, Hao; Wang, Xinyi; Zhang, Jiarui; Huang, Yu; Douglas, Jack F.

doi:10.1140/epje/s10189-023-00308-4

Cited by 5 publications

(2 citation statements)

References 126 publications

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“…This material state is defined through the hyperuniformity index H , − i.e., the ratio of S (0) to the value of S ( q ) at its primary peak S p . This quantity has been proposed to quantify the degree of particle localization at a molecular scale of the interparticle distance, and when this quantity reaches a critical value, particles become “delocalized,” a critical condition closely analogous to the empirical Lindeman condition for crystal melting . In particular, when H is less than or comparable to the critical value of H ∼ scriptO ( 1 0 − 3 ) , an effectively hyperuniform state is defined …”

Section: Resultsmentioning

confidence: 99%

“…This quantity has been proposed to quantify the degree of particle localization at a molecular scale of the interparticle distance, and when this quantity reaches a critical value, particles become "delocalized," a critical condition closely analogous to the empirical Lindeman condition for crystal melting. 164 In particular, when H is less than or comparable to the critical value of H (10 ) 3 , an effectively hyperuniform state is defined. 163 Figure 9 shows H as a function of m c and M, where we find that H is reduced in ring polymer melts in comparison to linear polymer melts.…”

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Glass Formation in Ring Polymer Melts Having Variable Knot Complexity and Molecular Mass

Dong,

Song,

et al. 2024

Macromolecules

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The variation of polymer topology provides an alternative way to tune the properties of polymer materials without varying the chemistry of the monomers composing the polymer and provides a powerful approach to creating new functional materials and testing ideas about molecular transport in polymer fluids having broad theoretical significance in polymer science. Here, we focus on flexible ring polymer melts by coarse-grained molecular dynamics simulations, where the rings have a fixed knot complexity defined by the minimal crossing number m c , and explore how basic thermodynamic and segmental dynamic properties are impacted by varying m c and polymer molecular mass M. We find that increasing m c has a similar influence on the density, thermal expansion coefficient, radius of gyration, and overall average polymer shape, along with the characteristic temperatures of glass formation and fragility, as we recently found upon increasing the number of star arms f in star polymer melts where f defines the corresponding measure of topological complexity. The common trend in the thermodynamic and segmental dynamic properties of these distinct topological classes of polymers can be understood from the generalized entropy theory, based on how increasing m c and f alters molecular packing frustration, as quantified by the dimensionless thermal expansion coefficient and isothermal compressibility. These findings confirm in a striking way that molecular packing frustration is a key factor in determining the characteristic properties of glass-forming liquids. Notably, the effect of ring knotting was hardly considered in previous simulation and experimental studies of ring polymers, which is akin to ignoring f in the case of star polymers, so we suggest that knotting might be a source of some variability in measurements on ring polymers, in addition to linear polymer contaminants.

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Section: Resultsmentioning

confidence: 99%

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confidence: 99%

Glass Formation in Ring Polymer Melts Having Variable Knot Complexity and Molecular Mass

Dong,

Song,

et al. 2024

Macromolecules

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Confinement effect of inter-arm interactions on glass formation in star polymer melts

Yang,

Xu,

Douglas

et al. 2024

The Journal of Chemical Physics

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We utilized molecular dynamic simulation to investigate the glass formation of star polymer melts in which the topological complexity is varied by altering the number of star arms (f). Emphasis was placed on how the “confinement effect” of repulsive inter-arm interactions within star polymers influences the thermodynamics and dynamics of star polymer melts. All the characteristic temperatures of glass formation were found to progressively increase with increasing f, but unexpectedly the fragility parameter KVFT was found to decrease with increasing f. As previously observed, stars having more than 5 or 6 arms adopt an average particle-like structure that is more contracted relative to the linear polymer size having the same mass and exhibit a strong tendency for intermolecular and intramolecular segregation. We systematically analyzed how varying f alters collective particle motion, dynamic heterogeneity, the decoupling exponent ζ phenomenologically linking the slow β- and α-relaxation times, and the thermodynamic scaling index γt. Consistent with our hypothesis that the segmental dynamics of many-arm star melts and thin supported polymer films should exhibit similar trends arising from the common feature of high local segmental confinement, we found that ζ increases considerably with increasing f, as found in supported polymer films with decreasing thickness. Furthermore, increasing f led to greatly enhanced elastic heterogeneity, and this phenomenon correlates strongly with changes in ζ and γt. Our observations should be helpful in building a more rational theoretical framework for understanding how molecular topology and geometrical confinement influence the dynamics of glass-forming materials more broadly.

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A closer examination of the nature of atomic motion in the interfacial region of crystals upon approaching melting

Zhang,

Douglas

2024

The Journal of Chemical Physics

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Although crystalline materials are often conceptualized as involving a static lattice configuration of particles, it has recently become appreciated that string-like collective particle exchange motion is a ubiquitous and physically important phenomenon in both the melting and interfacial dynamics of crystals. This type of collective motion has been evidenced in melting since early simulations of hard disc melting by Alder et al. [Phys. Rev. Lett. 11(6), 241–243 (1963)], but a general understanding of its origin, along with its impact on melting and the dynamics of crystalline materials, has been rather slow to develop. We explore this phenomenon further by focusing on the interfacial dynamics of a model crystalline Cu material using molecular dynamics simulations where we emphasize the geometrical nature and spatial extent of the atomic trajectories over the timescale that they are caged, and we also quantify string-like collective motion on the timescale of the fast β-relaxation time, τf, i.e., “stringlets.” Direct visualization of the atomic trajectories in their cages over the timescale over which the cage persists indicates that they become progressively more anisotropic upon approaching the melting temperature Tm. The stringlets, dominating the large amplitude atomic motion in the fast dynamics regime, are largely localized to the crystal interfacial region and correspond to “excess” modes in the density of states that give rise to a “boson peak.” Moreover, interstitial point defects occur in direct association with the stringlets, demonstrating a link between classical defect models of melting and more recent studies of melting emphasizing the role of this kind of collective motion.

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Approach to hyperuniformity in a metallic glass-forming material exhibiting a fragile to strong glass transition

Cited by 5 publications

References 126 publications

Glass Formation in Ring Polymer Melts Having Variable Knot Complexity and Molecular Mass

Glass Formation in Ring Polymer Melts Having Variable Knot Complexity and Molecular Mass

Confinement effect of inter-arm interactions on glass formation in star polymer melts

A closer examination of the nature of atomic motion in the interfacial region of crystals upon approaching melting

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