We show that thermodynamic scaling can be derived by combining the Murnaghan equation of state (EOS) with the generalized entropy theory (GET) of glass formation. In our theory, thermodynamic scaling arises in the non-Arrhenius relaxation regime as a scaling property of the fluid configurational entropy density $s_c$, normalized by its value $s_c^*$ at the onset temperature $T_A$ of glass formation, $s_c / s_c^*$, so that a constant value of $TV^γ$ corresponds to a \textit{reduced isoentropic} fluid condition. Molecular dynamics simulations on a coarse-grained polymer melt are utilized to confirm that the predicted thermodynamic scaling of $τ_α$ by the GET holds both above and below $T_A$ and to test whether the extent $L$ of stringlike collective motion, normalized its value $L_A$ at $T_A$, also obeys thermodynamic scaling, as required for consistency with thermodynamic scaling. While the predicted thermodynamic scaling of both $τ_α$ and $L/ L_A$ is confirmed by simulation, we find that the isothermal compressibility $κ_T$ and the long wavelength limit $S(0)$ of the static structure factor do not exhibit thermodynamic scaling, an observation that would appear to eliminate some proposed models of glass formation emphasizing fluid `structure' over configurational entropy. It is found, however, that by defining a low temperature hyperuniform reference state, we may define a compressibility relative to this condition, $δκ_T$, a transformed dimensionless variable that exhibits thermodynamic scaling and which can be directly related to $s_c / s_c^*$. Further, the Murnaghan EOS allows us to interpret $γ$ as a measure of intrinsic anharmonicity of intermolecular interactions that may be directly determined from the pressure derivative of the material bulk modulus.
In this work we use molecular simulations to examine methods of controlling mechanical properties of polymeric glass materials such as elastic moduli, mechanical heterogeneity as well as their glass transition temperature. We study filled and unfilled polymers and examine the effect of particle size, volume fraction and polymer-particle interactions. We identify a relationship between mobility and dynamic heterogeneity with elastic moduli and glass transition temperature.
We apply our recently-developed mean-field 'SL-TS2' (two-state Sanchez-Lacombe) model to simultaneously describe dielectric alpha-relaxation time and pressure-volume-temperature (PVT) data in four polymers (polystyrene, poly(methylmethacrylate), poly(vinyl acetate) and poly(cyclohexane methyl acrylate)) and four organic molecular glass formers (ortho-terphenyl, glycerol, PCB-62, and PDE). Previously, it has been shown that for all eight materials, the Casalini-Roland thermodynamical scaling, /tau_{/alpha} = f(TV_{sp}^{/gamma}) (where T is temperature and V_{sp} is specific volume), (Casalini, …
Glassy dynamics and charge transport are studied for the polymeric Ionic Liquid (PIL) poly(tris(2-(2-methoxyethoxy)ethyl)ammonium acryloxypropylsulfonate) (PAAPS) with varying molecular weight (9700, 44200, 51600 and 99500 g per mol) by Broadband Dielectric Spectroscopy (BDS) in a wide frequency (0.01 - 10E7 Hz) and temperature range (100 - 400 K) and by DSC- and AC-chip calorimetry. The dielectric spectra are characterized by a superposition of (i) relaxation processes, (ii) charge transport and (iii) electrode polarization. The relaxation processes (i) are assigned to the dynamic glass trans…
We provide a perspective on polymer glass formation, with an emphasis on models in which the fluid entropy and collective particle motion dominate the theoretical description and data analysis. We first discuss the dynamics of liquids in the high temperature Arrhenius regime, where transition state theory is formally applicable. We then summarize the evolution of the entropy theory from a qualitative framework for organizing and interpreting temperature-dependent viscosity data by Kauzmann to the formulation of a hypothetical `ideal thermodynamic glass transition' by Gibbs and DiMarzio, follow…
The glass-forming ability is an important material property for manufacturing glasses and understanding the long-standing glass transition problem. Because of the nonequilibrium nature, it is difficult to develop the theory for it. Here we report that the glass-forming ability of binary mixtures of soft particles is related to the equilibrium melting temperatures. Due to the distinction in particle size or stiffness, the two components in a mixture effectively feel different melting temperatures, leading to a melting temperature gap. By varying the particle size, stiffness, and composition ove…
We describe a method to determine whether a material has isomorphs in its thermodynamic phase diagram. Isomorphs are state points for which various properties are invariant in reduced units. Such materials are commonly identified from strong correlation between thermal fluctuations of the potential energy, U, and the virial W, but this identification is not generally applicable to real materials. We show from molecular dynamic simulations of atomic, molecular, and polymeric materials that systems with strong U-W correlation cannot be pressure densified; that is, the density obtained on cooling…
Glass forming ability (GFA) is a property of utmost importance in glass science and technology. In this paper, we used a statistical methodology---involving bootstrap sampling and the Wilcoxon test---to find out which glass stability parameters can better predict the glass forming ability. We collected or measured the necessary data for twelve stoichiometric oxide glasses that underwent predominant heterogeneous nucleation (the most common case). We found that some GS parameters could predict the GFA of these oxide glasses quite well, whereas others perform poorly. Parameter $K_w$ was the top …
Rod-shaped colloids with attractive tips can form linear aggregates that may subsequently undergo hierarchical self-assembly into nematic fluids. Inspired by recent modelling efforts on chromonic liquid crystals, composed of discotic building blocks, we formulate a second-virial theory for reversible supramolecular rods. Unlike chromonics, these systems are capable of forming stable nematic phases in the high-temperature, monomeric limit in the absence of polymerization. Changing the tip potential from attractive to repulsive thus enables a smooth crossover from a monomeric to a polymeric nema…
On the possible existence of crystallites in glass-forming liquids
我们推测，玻璃形成液体可能包含相当大的和明确的微晶。这是基于认为，缓慢放松的“冻结”的应力特征的超粘性液体增加了成核的障碍，从而允许更大的不稳定的微晶比其他可能的。冻结应力也使微晶变形，使其难以观察;特别是有人认为，1/N的分子形成N X N X N微晶的情况将难以通过标准X射线或中子散射实验检测到。
The temperature dependence of the high-frequency shear modulus measured in the kHz range is compared to the mean-squared displacement measured in the nanosecond range for the two van der Waals bonded glass-forming liquids cumene and 5PPE. This provides an experimental test for the assumption connecting two versions of the shoving model for the non-Arrhenius temperature dependence of the relaxation time in glass formers. The two versions of the model are also tested directly and both are shown to work well for these liquids.
Rigidity plays an important role on the relaxation properties of glass forming melts, yet it is usually determined from the average co-ordination number through the chemical composition. A discussion is presented on how viscoelasticity can be used as an alternative way to determine glass rigidity and to give clues about the relaxation processes. In particular, it is shown that the transverse current dynamical structure factor of dense glass and crystal forming fluids contain rich information about rigidity that can be related with the presence of a dynamical-gap for transversal vibrational mod…
Extensive molecular dynamics (MD) computer simulations of an equimolar glass-forming AB mixture with large size ratio are presented. While the large A particles show a glass transition around the critical density of mode coupling theory $ρ_c$, the small B particles remain mobile with a relatively weak decrease of their self diffusion coefficient $D_{\rm B}$ with increasing density. Surprisingly, around $ρ_c$, the self-diffusion coefficient of the A particles, $D_{\rm A}$, also starts to follow a rather weak dependence on density. We show that this is due to finite-size effects that can be unde…
Despite the use of glasses for thousands of years, the nature of the glass transition is still mysterious. On approaching the glass transition, the growth of dynamic heterogeneity has long been thought to play a key role in explaining the abrupt slowdown of structural relaxation. However, it still remains elusive whether there is an underlying link between structural relaxation and dynamic heterogeneity. Here we unravel the link by introducing a characteristic time scale hiding behind an identical dynamic heterogeneity for various model glass-forming liquids. We find that the time scale corres…
In this letter, we analyze the dipole-dipole correlations obtained from the molecular dynamics simulations for strongly- and weakly-polar model liquids. As a result, we found that cross-correlations contribution to the systems total dipole moment correlation function, which is directly measured in the dielectric spectroscopy experiment, is negligible for weakly-polar liquids. In contrast, the cross-correlations term dominates over the self-correlations one for examined strongly polar-liquid. Consequently, our studies strongly support the interpretation of the dielectric spectra nature of the g…
We investigate the heterogeneity of dynamics, the breakdown of the Stokes-Einstein relation and fragility in a model glass forming liquid, a binary mixture of soft spheres with a harmonic interaction potential, for spatial dimensions from 3 to 8. Dynamical heterogeneity is quantified through the dynamical susceptibility $χ_4$, and the non-Gaussian parameter $α_2$. We find that the fragility, the degree of breakdown of the Stokes-Einstein relation, as well as heterogeneity of dynamics, decrease with increasing spatial dimensionality. We briefly describe the dependence of fragility on density, a…
Glass is a liquid that has lost its ability to flow. Why this particular substance undergoes its dramatic slowing down in kinetics while remaining barely distinguishable in structure from the fluid state upon cooling constitutes the central question of glass transition physics. Here, we experimentally tested the pathway of kinetic slowing down in glass$\textrm{-}$forming liquids that consisted of ellipsoidal or binary spherical colloids. In contrast to rotational motion, the exponential scaling between diffusion coefficient and excess entropy in translational motion was revealed to break down …
The relationship between short and long time relaxation dynamics is obtained for a simple solvable two-level energy landscape model of a glass. This is done through means of the Kramers transition theory, which arises in very natural manner to calculate transition rates between wells. Then the corresponding stochastic master equation is analytically solved to find the population of metastable states. A relation between the cooling rate, the characteristic relaxation time and the population of metastable states is found from the solution of such equation. From this, a relationship between the r…
Using extensive numerical simulations, we investigate the flow behaviour of a model glass-forming binary mixture whose constituent particles have a large size ratio. The rheological response to applied shear is studied in the regime where the larger species are spatially predominant. We demonstrate that the macroscopic rigidity that emerges with increasing density occurs in the regime where the larger species undergo a glass transition while the smaller species continue to be highly diffusive. We analyse the interplay between the timescale imposed by the shear and the quiescent relaxation dyna…
Glass-forming liquids are broadly classified as being fragile or strong, depending on the deviation from Arrhenius behavior of their relaxation times. A fragile to strong crossover is observed or inferred in liquids like water and silica, and more recently also in metallic glasses and phase change alloys, leading to the expectation that such a crossover is more widely realised among glass formers. We investigate computationally the well-studied Kob-Andersen model, accessing temperatures well below the mode coupling temperature $T_{MCT}$. We find that relaxation times exhibit a crossover in dyn…
Recent theories predict that when a supercooled liquid approaches the glass transition, particle clusters with a special "amorphous order" nucleate within the liquid, which lead to static correlations dictating the dramatic slowdown of liquid relaxation. The prediction, however, has yet to be verified in 3D experiments. Here, we design a colloidal system, where particles are confined inside spherical cavities with an amorphous layer of particles pinned at the boundary. Using this novel system, we capture the amorphous-order particle clusters and demonstrate the development of a static correlat…
This report proposes the solution of the long standing problem regarding the key property governing the pressure related dynamics in glass forming and complex systems: the determination of the apparent activation volume Va and its pressure dependence. Results are tested in the previtreous domain, up to P ~ 1.2 GPa, for glass forming diisobutyl phthalate (DIIP, low molecular weight liquid), isoctyloxycyanobiphenyl (8*OCB, liquid crystal), bisphenol A/epichlorohydrin (EPON 828, epoxy resin).
Enormous enhancement in the viscosity of a liquid near its glass transition is generally connected to the growing many-body static correlations near the transition, often coined as `amorphous ordering'. Estimating the length scales of such correlations in different glass-forming liquids is highly important to unravel the physics of glass formation. Experiments on molecular glass-forming liquids become pivotal in this scenario as the viscosity grows several folds ($\sim 10^{14}$), simulations or colloidal glass experiments fail to access the long-time scales required. Here we design an experime…
The term "fragility" describes the rate at which viscosity grows when a supercooled liquid approaches its putative glass transition temperature. The field of glassy materials is actively searching for a structural origin that governs this dynamical slowing down in the supercooled liquid, which occurs without any discernible change in structure. Our work shows clear evidence that growing many-body static amorphous order is intimately correlated with the kinetic fragility of glass-forming liquids. It confirms that the system's dynamical response to temperature is concealed in its micro-structure…
In a recent article, Schmelzer and Tropin [Entropy 20 [2] 103 (2018)] presented an unfounded, confusing critique of several aspects of modern glass science. Relying on pre-Socratic Greek philosophy and state-of-the-art scientific understanding from the 1920s-1930s, Schmelzer and Tropin propagate an antiquated view of glass physics that is at odds with well-accepted knowledge in the field from both theory and experiments conducted in the post-World War II era. The objective of this short letter is to elucidate and extinguish their critique. This rebuttal is directed to our colleagues and especi…
The fragility, that controls the temperature-dependent viscous properties of liquids as the glass transition is approached, in various glass-forming liquids with different atomic interactions and densities is investigated by molecular dynamic simulations. We show the landscape of fragility in purely repulsive systems can be separated into three regions with qualitatively disparate dynamic behaviors, suggesting that the density plays an unexpected role for understanding the repulsive steepness dependence of fragility. Furthermore, the vastly dissimilar influences of attractive interaction on fr…
Polarization dependence of opto-mechanical behavior of monodomain photochromic glassy liquid crystal (LC) polymers under polarized ultraviolet light (PUV) is studied. Trans-cis photo-isomerization is generally known to be most intense at 'parallel illumination' (polarization parallel to LC director), as light-medium interactions are active when polarization aligns with trainsition dipole moment. We show that at parallel illumination though cis isomers are converted from trans the most near surface, they can be the least below certain light propagation depth. Membrane force, an average effect o…
Melting temperatures at ambient pressure of systems of isosceles Lennard-Jones trimers with angles ranging from 70 degrees to 100 degrees are determined. Two crystal structures are considered: a distorted body centered cubic structure and a distorted face centered cubic structure with preferred angles of 77 and 96 degrees, respectively. Liquid dynamics are slowed down when the angle is increased. A trimer angle of 83 degrees yields the largest distance between isochrones and the melting temperature, suggesting that this value gives the optimal glass-forming ability. It is conjectured that bett…
A key property of glass forming alloys, the anomalously small volume difference with respect to the crystal, is shown to arise as a direct consequence of the soft repulsive potentials between metals. This feature of the inter-atomic potential is demonstrated to be responsible for a significant component of the glass forming ability of alloys due to the decrease in the enthalpy of fusion and the associated depression of the freezing point.
In this Comment, we argue that the behavior of the overlap functions reported in the commented paper can be fully understood in terms of the physics of simple liquids in contact with disordered substrates, without appealing to any particular glassy phenomenology. This suggestion is further supported by an analytic study of the one-dimensional Ising model provided as Supplementary Material.
In the last few decades, theoretical and experimental studies of glass-forming liquids have revealed presence of universal regularities in the viscosity-temperature data. In the present work, we propose a viscosity model for scaling description of experimental viscosity data. A feature of this model is presence of only two adjustable parameters and high accuracy of experimental data approximation by this model for a wide temperature range. The basis of the scaling description is an original temperature scale. Within this scaling description we obtain the transformed Angell plot, in which the a…
All liquids are topologically disordered materials; however, the degree of disorder can vary as a result of internal fluctuations in structure and topology. These fluctuations depend on both the composition and temperature of the system. Most prior work has considered the mean values of liquid or glass properties, such as the average number of topological degrees of freedom per atom; however, the localized fluctuations in properties also play a key role in governing the macroscopic characteristics of any glass-forming system. This paper proposes a generalized approach for modeling topological …
