Fragile Glass-forming Liquids Research Articles

Light scattering and dielectric susceptibility spectra of glassforming liquids

We propose a simple explanation for the marked difference in the spectral shapes of the dielectric and the depolarized light scattering susceptibilities of the fragile molecular glassforming liquids: it is the direct consequence of the probing of the same molecular orientational dynamics through techniques having different tensorial properties. We deduce a relationship between the corresponding nonergodicity parameters which is well verified in molecular dynamics simulations available so far. We also show that the corresponding orientational susceptibilities of the NO3−-ions in the glassformer CKN, obtained in a recent simulation, exhibit the same spectral difference as in the molecular liquids.

Comment on ``Fast dynamics of glass-forming glycerol''

The coupling model predicts the existence of a prominent fast \ensuremath{\alpha} process at times 2 ps in fragile glass-forming liquids such as polymers. A further prediction is that this process will be only weakly apparent in intermediate and strong liquids such as glycerol. In light of these predictions, we have reexamined the neutron scattering data of glycerol published by Wuttke, Petry, Coddens, and Fujara (WPCF) [Phys. Rev. E 52, 4026 (1995)], leading to the conclusion that the fast \ensuremath{\alpha} process is indeed quite weak therein. Thus, the coupling model predictions are fully consistent with the glycerol data, notwithstanding WPCF's statement to the contrary.

Dynamics of glass-forming liquids at high temperatures: a general scenario

An analysis of light scattering spectra of different glass-forming liquids is presented. It is shown that at high temperatures one finds a scenario for the temperature variations of dynamic spectra at high frequencies which is common for strong and fragile liquids. This scenario breaks down at some crossover temperature . One conclusion is that the difference between strong and fragile glass-forming liquids is the temperature range between and : it is small for fragile and large for strong glass-forming systems.

Reply to "Comment on 'Translation-rotation paradox for diffusion in fragile glass-forming liquids' "

We discuss several aspects of the fluidized domain model advanced earlier to explain rotational and translational diffusion rates for fragile glass formers and, in particular, we point out weaknesses in the preceding Comment [Sillescu, Phys. Rev. E 53, 2992 (1996)]. Recent experimental results are cited to refine estimates of domain characteristics for orthoterphenyl at ${\mathit{T}}_{\mathit{g}}$ and to suggest that domain heterogeneity in space and time underlies stretched exponential relaxation behavior for rotational motion. \textcopyright{} 1996 The American Physical Society.

Possible Causes of the Change of Dynamics in Glass-Forming Materials Subjected to Reduced Dimension

ABSTRACTThere is currently many ongoing investigations of the change in the glass transition temperature when a material is reduced in dimension from the normal bulk state. The reduction in dimension can be accomplished by casting the material as thin films with or without a substrate or putting it in nanometer size pores. In this work, we explore possible causes of the change in dynamics of the bulk material when the glass-former is subjected to such modifications. The existence of a growing cooperative length scale L(T) with decreasing temperature in bulk fragile glass-forming liquids reaching the size of approximately 1.5–2.0 nm at the glass transition temperature is the basis of our consideration. When the reduced dimension is comparable to L(Tg), cooperative dynamics within a lengthscale equal to L(Tg) can no longer be maintained in all three dimensions throughout the sample. The imposed reduction of the cooperative length scale speeds up the dynamics and causes a reduction of the glass transition temperature. For polymeric glass-formers particularly at higher molecular weights, reduction of one dimension in thin films engenders orientation of the polymer chains when their radius of gyration becomes comparable to the film thickness. The latter is known to cause also a reduction of the glass transition temeperature.

Model of the glass transition

This paper examines the three well-known empiricisms that accompany the glass transition of fragile glass-forming liquids: the unusual thermodynamic behavior of supercooled liquids that is usually called the Kauzmann paradox, the temperature dependence of the relaxation time as embodied in the Vogel–Fulcher law, and the time-domain relaxation law usually known as the Kohlrausch–Williams–Watts relaxation function. Mutually consistent descriptions of all three phenomena are presented. The Kauzmann paradox can be explained by recognizing that the ground states of both the liquid and the crystal, and also the dominant excitations of these two ground states, are similar in many ways. This implies that the two phases have similar thermodynamics at low T. The Vogel–Fulcher law results from the assumption that localized regions of the liquid must be excited above a certain threshold enthalpy before they can relax; as the temperature falls, this threshold enthalpy becomes less accessible and the apparent activation enthalpy to affect relaxation increases. The resulting model is also consistent with the Montroll–Schlesinger–Bendler description of Kohlrausch–Williams–Watts relaxation, which explains the third empiricism.

The glass transition possible research directions

In these proceedings, I will outline what I think is the present state of our understanding of the glass transition in fragile glass-forming liquids, and the most important and promising directions for research. These considerations are based on numerous conversations with colleagues as well as the panel discussions held at the Kent Island Workshop.

High-temperature dynamics in glass-forming liquids.

The light scattering spectra of several recently investigated glass-forming systems are reanalyzed. The analysis shows that for both fragile and nonfragile glass-forming liquids, there is some general scenario for the dynamics at high temperatures that seems to follow the qualitative scenario suggested by the mode-coupling theory (MCT) of the liquid-glass transition. However, significant quantitative deviations from MCT predictions occur. They are small for fragile and large for nonfragile systems. Moreover, from the analysis a dynamic crossover temperature clearly appears as a universal feature and in all cases it is close to the critical temperature reported within standard MCT analyses.

Long-Lived Structures in Fragile Glass-Forming Liquids

We present molecular dynamics results for the existence of long-lived clusters near the glass transition in a two component, two-dimensional Lennard-Jones supercooled liquid. Several properties of this system are similar to a mean-field glass-forming liquid near the spinodal. This similarity suggests that the glass ``transition'' in the supercooled liquid is associated with an incipient thermodynamic instability. Our results also suggest that single particle properties are not relevant for characterizing the instability, but are relevant to the kinetic transition that occurs at a lower temperature than the glass transition.

Stokes-Einstein violation in glass-forming liquids.

The Stokes-Einstein relation D=${\mathit{k}}_{\mathit{B}}$T/(C\ensuremath{\eta}a) (\ensuremath{\eta} is the shear viscosity; D is the diffusion constant; C=6\ensuremath{\pi} for no-slip and 4\ensuremath{\pi} for slip boundary conditions; a is the molecular diameter) holds over a wide temperature range in many liquids. However, in a variety of fragile glass-forming liquids, a, as defined by the above expression, becomes smaller with decreasing temperature as the glass transition is approached. In an attempt to explain this experimental result, we propose that special thermal fluctuations cause domains in the liquid to become temporarily more fluidized, so that a diffusing particle can move through fluidized regions, but is inhibited from moving in the unfluidized region. We introduce a mean-field picture of this fluctuating fluid, and solve two versions for their hydrodynamic flow fields. The resulting reduced drag force can account for the violation of the Stokes-Einstein relation seen in fragile glass-forming liquids, with plausible values for the size of the fluidized region and the mean-field reduction in viscosity.

Temperature dependence of the stretched exponent in structural relaxation of fragile glass-forming molecular liquids

The Adam-Gibbs theory of structural relaxation in glass-forming liquids based on independent transitions of cooperative rearranging regions has been modified by Ngai, Rendell and Plazek to include possible interactions between them, using the coupling model. From this modified theory it has been shown that the structural relaxation function has the stretched exponential form. In this work, the possible temperature dependence of the stretch exponent, β, is inferred and its trend deduced from the modified Adam-Gibbs theory.

Strength and fragility in glass-forming liquids

Etude montrant que l'influence de la temperature sur la viscosite dans la zone sous refroidie pour une grande variete de materiaux vitrifiables peut etre explicite par une equation loi puissance. La valeur de l'entropie 5 est utilisee pour classifier les materiaux vitrifiables en materiaux fragiles ou non

Relaxation and flow mechanisms in ‘‘fragile’’ glass-forming liquids

Fragile glass-forming liquids are characterized by strongly non-Arrhenius temperature dependence of shear viscosity, and by unusually high heat capacity in the supercooled liquid above the glass transition. Using the inherent structure formalism, a theoretical model is developed to explain these characteristics. The vitrifying liquid is viewed as a dynamic patchwork of relatively strongly bonded (but amorphous) molecular domains that are separated by irregular walls of weakened bonds. Mean domain diameter ξ rises as temperature declines to minimize wall free energy, but the process is self-limiting due to intradomain frustration energy. Shear flow occurs by a ‘‘tear and repair’’ mechanism involving elemental wall areas of mean size ξ2. The relation of the present model to the tiling models for glass formation is explored, and helps to establish the presence of anomalous heat capacity in the supercooled liquid. The analysis suggests in strongly supercooled fragile liquids that the Stokes–Einstein formula may underestimate self-diffusion constants, and that the Adam–Gibbs relation for mean relaxation time in terms of calorimetric entropy may display systematic errors.