1,720,995 research outputs found
Boson Peak Decouples from Elasticity in Glasses with Low Connectivity
Full text linkWe perform molecular-dynamics simulations of the vibrational and elastoplastic properties of polymeric glasses and crystals and the corresponding atomic systems. We evidence that the elastic scaling of the density of states in the low-frequency boson peak (BP) region is different in crystals and glasses. Also, we see that the BP of the polymeric glass is nearly coincident with the one of the atomic glasses, thus revealing that the former - unlike the elasticity - is controlled by nonbonding interactions only. Our results suggest that the interpretation of the BP in terms of the macroscopic elasticity, discussed in highly connected systems, does not hold for systems with low connectivity
Metallic glass-formers in 2D exhibit the same scaling as in 3D between vibrational dynamics and structural relaxation
Full text linkGlass-forming systems approaching their glass transition exhibit universal correlations between picosecond vibrational dynamics and long-time structural relaxation, which can be described by the same master curve in the bulk or confined conditions. In this work, we study at a fundamental level the effects of the reduction of spatial dimensionality on this phenomenon. We perform molecular dynamics simulations of a metallic glass-formers in two dimensions (2D). We show that in the supercooled regime particle localization in the cage and structural relaxation are blurred by long-wavelength fluctuations specific to low-dimensional systems. Once these effects are properly removed, we demonstrate that the fast dynamics and slow relaxation comply, without any adjustment, with same scaling between the structural relaxation time and the Debye-Waller factor, originally observed in three-dimensions (3D)
Elastic modulus and yield strength of semicrystalline polymers with bond disorder are higher than in atomic crystals
Full text linkWe perform thorough molecular-dynamics simulations to compare elasticity and yielding of atomic crystals and model semicrystalline polymers, the latter characterized by very similar positional ordering with respect to atomic crystals and considerable bond disorder. We find that the elastic modulus G, the shear yield strength, τY , and the critical yield strain εc of semicrystalline polymers are higher than (G, τY ), or comparable to (εc ), the corresponding ones of atomic crystals. The findings suggest that the bond disorder suppresses dislocation-mediated plasticity in polymeric solids with positional order
Non-local cooperative atomic motions that govern dissipation in amorphous tantala unveiled by dynamical mechanical spectroscopy
Full text linkThe mechanisms governing mechanical dissipation in amorphous tantala are studied at microscopic scale via Molecular Dynamics simulations, namely by mechanical spectroscopy in a wide range of temperature and frequency. We find that dissipation is associated with irreversible atomic rearrangements with a sharp cooperative character, involving tens to hundreds of atoms arranged in spatially extended clusters of polyhedra. Remarkably, at low temperature we observe an excess of plastically rearranging oxygen atoms which correlates with the experimental peak in the macroscopic mechanical losses. A detailed structural analysis reveals preferential connections of the irreversibly rearranging polyhedra, corresponding to edge and face sharing. These results might lead to microscopically informed design rules for reducing mechanical losses in relevant materials for structural, optical, and sensing applications
Coincident correlation between vibrational dynamics and primary relaxation of polymers with strong or weak johari-goldstein relaxation
Full text linkThe correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known
Open and anisotropic soft regions in a model polymer glass
No full textThe vibrational dynamics of a model polymer glass is studied by Molecular Dynamics simulations. The focus is on the “soft” monomers with high participation to the lower-frequency vibrational modes contributing to the thermodynamic anomalies of glasses. To better evidence their role, the threshold to qualify monomers as soft is made severe, allowing for the use of systems with limited size. A marked tendency of soft monomers to form quasi-local clusters involving up to 15 monomers is evidenced. Each chain contributes to a cluster up to about three monomers and a single cluster involves a monomer belonging to about 2–3 chains. Clusters with monomers belonging to a single chain are rare. The open and tenuous character of the clusters is revealed by their fractal dimension df < 2. The inertia tensor of the soft clusters evidences their strong anisotropy in shape and remarkable linear correlation of the two largest eigenvalues. Owing to the limited size of the system, finite-size effects, as well as dependence of the results on the adopted polymer length, cannot be ruled out
Mutual information does not detect growing correlations in the propensity of a model molecular liquid
Full text linkThe dynamical spatial correlations detected by the mutual information (MI) in the isoconfigurational particle displacements of a monodisperse molecular viscous liquid are studied via molecular-dynamics simulations by changing considerably both the molecular mobility and the degree of dynamical heterogeneity. Different from atomic liquids, the MI correlation length does not grow on approaching the glass transition by considering the liquid both in full detail as a collection of monomers and as a coarse-grained ensemble of molecular centers of mass. In the detailed picture, it is found that: (i) the MI correlations between monomers are largely due to inter-molecular correlations, (ii) the MI length scale is numerically identical, within the errors, to the correlation length scale of the displacement direction, as drawn by conventional correlation functions. The time evolution of the MI spatial correlations complies with the scaling between the fast vibrational dynamics and the long-time relaxation. Our findings suggest that the characteristics of the MI length scale are markedly system-dependent and not obviously related to dynamical heterogeneity
Evidence of negative thermal expansion in supercooled tantala
No full textA density anomaly, i.e. a temperature region with negative thermal expansion (NTE) bounded by a density maximum and a density minimum at lower temperatures, is revealed and characterised in tantala for the first time by Molecular Dynamics simulations. The NTE region is evidenced in the metastable supercooled liquid and rather close to the glass transition. Since NTE is suppressed by poor structural equilibration, highlighting these phenomena is highly challenging due to the need for fulfilling competing constraints of slow cooling and avoidance of the crystallization. We find that the density anomaly is signalled by a decrease of the partial coordination numbers nTa,Ta and nO,O when lowering the temperature. The NTE magnitude is comparable to the ones of both stable water and solid-state materials with giant NTE
Nanoscale elastoplastic wrinkling of ultrathin molecular films
Full text linkUltrathin molecular films deposited on a substrate are ubiquitously used in electronics, photonics, and additive manufacturing methods. The nanoscale surface instability of these systems under uniaxial compression is investigated here by molecular dynamics simulations. We focus on deviations from the homogeneous macroscopic behavior due to the discrete, disordered nature of the deformed system, which might have critical importance for applications. The instability, which develops in the elastoplastic regime above a finite critical strain, leads to the growth of unidimensional wrinkling up to strains as large as 0.5. We highlight both the dominant wavelength and the amplitude of the wavy structure. The wavelength is found to scale geometrically with the film length, λ ∝ L, up to a compressive strain of ε ≃ 0.4 at least, depending on the film length. The onset and growth of the wrinkling under small compression are quite well described by an extended version of the familiar square-root law in the strain ε observed in macroscopic systems. Under large compression (ε > 0.25), we find that the wrinkling amplitude increases while leaving the cross section nearly constant, offering a novel interpretation of the instability with a large amplitude. The contour length of the film topography is not constant under compression, which is in disagreement with the simple accordion model. These findings might be highly relevant for the design of novel and effective wrinkling and buckling patterns and architectures in flexible platforms for electronics and photonics
Johari-Goldstein Heterogeneous Dynamics in a Model Polymer
No full textThe heterogeneous character of the Johari-Goldstein (JG) relaxation is evidenced by molecular-dynamics simulation of a model polymer system. A double-peaked evolution of dynamic heterogeneity, with maxima located at JG and structural relaxation time scales, is observed and mechanistically explained. The short-time single-particle displacement during JG relaxation weakly correlates with the long-time one observed during structural relaxation
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