Thermodynamic Study of Simple Molecular Glasses: Universal Features in Their Heat Capacity and the Size of the Cooperatively Rearranging Regions

Brillouin light scattering spectra from transverse and longitudinal acoustic waves in liquid and supercooled 3-methylpentane have been collected from room temperature down to 80 K, just above the glass transition. Spectra at different wave vectors have been obtained using 532 nm and 266 nm excitation. We found evidence of a shear relaxation with a characteristic time of 100 s at the glass transition which only partly accounts for the relaxation observed in the propagation and attenuation of the longitudinal modes. The inclusion of a relaxing bulk viscosity contribution with a relaxation time of the order of 10(2) ns at the glass transition is found to adequately reproduce the experimental data including transient grating data at a much lower frequency. A consistent picture of relaxed shear and bulk moduli as a function of temperature is derived. These two quantities are found to be related by a linear relation suggesting that a Cauchy-like relation holds also above the glass transition.

Section: Discussionmentioning

confidence: 99%

Ultraviolet and visible Brillouin scattering study of viscous relaxation in 3-methylpentane down to the glass transition

Benassi

Nardone

Giugni

2012

“…Despite these caveats, the general consensus is that the Adam-Gibbs relation is generally valid in the regime accessed by the simulations. In experiments, which typically analyse temperatures close to T g , the Adam-Gibbs relation seems again to be well obeyed for a range of materials [10,11,[26][27][28][29][30][31][32]]. Yet, experiments indicate as well that the Adam-Gibbs relation does not hold anymore above a temperature scale close to T mct [11,28], * ludovic.berthier@umontpellier.fr in stark contrast with the numerical results.…”

Section: Introductionmentioning

confidence: 93%

Does the Adam-Gibbs relation hold in simulated supercooled liquids?

Ozawa

Scalliet

Ninarello³

et al. 2019

We perform stringent tests of thermodynamic theories of the glass transition over the experimentally relevant temperature regime for several simulated glass-formers. The swap Monte Carlo algorithm is used to estimate the configurational entropy and static point-to-set lengthscale, and careful extrapolations are used for the relaxation times. We first quantify the relation between configurational entropy and the point-to-set lengthscale in two and three dimensions. We then show that the Adam-Gibbs relation is generally violated in simulated models for the experimentally relevant time window. Collecting experimental data for several supercooled molecular liquids, we show that the same trends are observed experimentally. Deviations from the Adam-Gibbs relation remain compatible with random first order transition theory, and may account for the reported discrepancies between Kauzmann and Vogel-Fulcher-Tammann temperatures. Alternatively, they may also indicate that even near Tg thermodynamics is not the only driving force for slow dynamics.

“…We compile state-of-the-art experimental 32,34,35 and numerical 16,33 data of S conf , and their extrapolation to low temperatures in Fig. 2.…”

Section: B Why the Configurational Entropy?mentioning

confidence: 99%

“…In calorimetric experiments, the configurational entropy becomes constant below T g upon entering the non- Experimental and numerical determinations of the equilibrium configurational entropy in various models 16,33 and materials. 32,34,35 Data points extracted from vapor deposition experiments 35 are indicated by the ellipse. Both axis are rescaled using the mode-coupling crossover as a reference temperature at which the relaxation time is about 10 −7 s. For hard spheres, the inverse of the reduced pressure, 1/p, replaces temperature.…”

Section: B Why the Configurational Entropy?mentioning

confidence: 99%

“…equilibrium glass regime, defining a residual entropy. 29,34 The glass residual entropy is a non-equilibrium effect that has been extensively discussed. [37][38][39][40][41] Here, we focus on equilibrium supercooled liquids and do not discuss further the glass residual entropy and remove nonequilibrium measurements in Fig.…”

Section: B Why the Configurational Entropy?mentioning

confidence: 99%

See 1 more Smart Citation

Configurational entropy of glass-forming liquids

Berthier

Ozawa

Scalliet

2019

102

The configurational entropy is one of the most important thermodynamic quantities characterizing supercooled liquids approaching the glass transition. Despite decades of experimental, theoretical, and computational investigation, a widely accepted definition of the configurational entropy is missing, its quantitative characterization remains fraud with difficulties, misconceptions and paradoxes, and its physical relevance is vividly debated. Motivated by recent computational progress, we offer a pedagogical perspective on the configurational entropy in glass-forming liquids. We first explain why the configurational entropy has become a key quantity to describe glassy materials, from early empirical observations to modern theoretical treatments. We explain why practical measurements necessarily require approximations that make its physical interpretation delicate. We then demonstrate that computer simulations have become an invaluable tool to obtain precise, non-ambiguous, and experimentally-relevant measurements of the configurational entropy. We describe a panel of available computational tools, offering for each method a critical discussion. This perspective should be useful to both experimentalists and theoreticians interested in glassy materials and complex systems. I. CONFIGURATIONAL ENTROPY AND GLASS FORMATION A. The glass transitionWhen a liquid is cooled, it can either form a crystal or avoid crystallization and become a supercooled liquid. In the latter case, the liquid remains structurally disordered, but its relaxation time increases so fast that there exists a temperature, called the glass temperature T g , below which structural relaxation takes such a long time that it becomes impossible to observe. The liquid is then trapped virtually forever in one of many possible structurally disordered states: this is the basic phenomenology of the glass transition. 1-4 Clearly, T g depends on the measurement timescale and shifts to lower temperatures for longer observation times. The experimental glass transition is not a genuine phase transition, as it is not defined independently of the observer.The rich phenomenology characterizing the approach to the glass transition has given rise to a thick literature. It is not our goal to review it, and we refer instead to previous articles. 1-9 There are convincing indications that the dynamic slowing down of supercooled liquids is accompanied by an increasingly collective relaxation dynamics. This is seen directly by the measurement of growing lengthscales for these dynamic heterogeneities, 10-12 or more indirectly by the growth of the apparent activation energy for structural relaxation, as seen in its non-Arrhenius temperature dependence. These observations suggest an interpretation of the experimental glass transition in terms of a generic, collective mechanism possibly controlled by a sharp phase transition. 13 'Solving the glass problem' thus amounts to identifying and obtaining direct experimental signatures about the fundamental nature and the mathematical des...