Role of entropy in the thermodynamic evolution of the time scale of molecular dynamics near the glass transition (original) (raw)
Related papers
Entropy Scaling of Molecular Dynamics in a Prototypical Anisotropic Model near the Glass Transition
Mysteries of dynamics and thermodynamics of molecular systems in the vicinity of the boundary between thermodynamically nonequilibrium glassy and metastable supercooled liquid states are still incompletely discovered and their theoretical and simulation models are imperfect despite many previous efforts. Among them, the role of total system entropy, configurational entropy, and excess entropy in the temperature-pressure or temperature-density dependence of global molecular dynamics timescale relevant to the glass transition is an essential topic intensively studied for over half of a century. By exploiting a well-known simple ellipsoidal model recently successfully applied to simulate the supercooled liquid state and the glass transition, we gain a new insight into the different views on the relationship between entropy and relaxation dynamics of glass formers, showing the molecular grounds for the entropy scaling of global molecular dynamics timescale.
Structural atomistic mechanism for the glass transition entropic scenario
arXiv: Soft Condensed Matter, 2019
A popular Adam--Gibbs scenario has suggested that the excess entropy of glass and liquid over crystal dominates the dynamical arrest at the glass transition with exclusive contribution from configurational entropy over vibrational entropy. However, an intuitive structural rationale for the emergence of frozen dynamics in relation to entropy is still lacking. Here we study these issues by atomistically simulating the vibrational, configurational, as well as total entropy of a model glass former over their crystalline counterparts for the entire temperature range spanning from glass to liquid. Besides confirming the Adam--Gibbs entropy scenario, the concept of Shannon information entropy is introduced to characterize the diversity of atomic-level structures, which undergoes a striking variation across the glass transition, and explains the change found in the excess configurational entropy. Hence, the hidden structural mechanism underlying the entropic kink at the transition is reveal...
A real-space description of the glass transition based on heterogeneities and entropy barriers
Eprint Arxiv Cond Mat 0102104, 2001
An alternative scenario for the glass transition based on the cooperative nature of nucleation processes and the role of entropic effects is presented. The new ingredient is to relate the dissipation during the relaxation process to the release of strain energy driven by the nucleation of progressively larger cooperative spatial regions. Using an equiprobability hypothesis for the transition between different metastable configurations, we obtain a relation between the free energy dissipation rate and the size of the largest cooperative regions. This leads to a new phenomenological relation between the largest relaxation time in the supercooled liquid phase and the effective temperature. This differs from the classical Adam-Gibbs relation in that predicts no divergence of the primary relaxation time at the Kauzmann temperature but a crossover from fragile to strong behavior.
The static lengthscale characterizing the glass transition at lower temperatures
EPL (Europhysics Letters), 2015
The existence of a static lengthscale that grows in accordance with the dramatic slowing down observed at the glass transition is a subject of intense interest. Due to limitations on the relaxation times reachable by standard molecular dynamics techniques (i.e. a range of about 4-5 orders of magnitude) it was until now impossible to demonstrate a significant enough increase in any proposed length scale. In this Letter we explore the typical scale at unprecedented lower temperatures. A swap Monte Carlo approach allows us to reach a lengthscale growth by more than 500%. We conclude by discussing the relationship between the observed lengthscale and various models of the relaxation time, proposing that the associated increase in relaxation time approaches experimental values.
Journal of Non-Crystalline Solids, 2009
The structural relaxation dynamics of two molecular glass forming systems have been analyzed by means of dielectric spectroscopy, under cooling and compression conditions. The relation of the dynamic slowing down with the reduction of the configurational entropy, S C , as predicted by Adam and Gibbs (AG), was also investigated. As S C is not directly accessible by experiments, it was estimated, following a common procedure in literature, from the excess entropy S exc of the supercooled liquid with respect to the crystal, determined from calorimetric and expansivity measurements over the same T-P range of dynamics investigation. The AG relation, predicting linear dependence between the logarithmic of structural relaxation time and the product of temperature with configurational entropy, was successfully tested. Actually, a bilinear relation between S exc and S C was found, with different proportionality factors in isothermal and isobaric conditions. Using such results, we derived an equation for predicting the pressure dependence of the glass transition temperature, in good accordance with the experimental values in literature.
Relaxation pathway confinement as a determinant of glassy dynamics
The nature of the glass transition, that is, the dramatic dynamic slowing down that a glass-forming system experiences when supercooled below its melting point, continues to be one of the major open questions in condensed-matter physics. To elucidate the physical underpinnings of such slowing down, we compute for an archetypical glass-forming system the excess of particle mobility distributions over the corresponding distribution of dynamic propensity, a quantity that measures the tendency of the particles to be mobile and reflects the local structural constraints. This enables us to demonstrate that glassiness stems from the fact that, on supercooling, the dynamical trajectory in search for a relaxation event must deal with an increasing confinement of relaxation pathways. This "entropic funnel" of relaxation pathways built upon a restricted set of mobile particles is also made evident from the decay and further collapse of the associated Shannon entropy.
Journal of Non-Crystalline Solids, 2005
We analyze the slowing down of structural relaxation dynamics of two small molecular glass formers and one polymer: o-terphenyl, triphenylchloromethane, and poly(methylmethacrylate). Considering the literature data of expansivity and heat capacity we calculate configurational entropy using a previously proposed relation between the configurational and the excess entropy, and we directly check the Adam and Gibbs theory for glass transition. In particular, we clearly show that using such expression for configurational entropy the predicted linear dependence between the logarithmic of structural relaxation time and the product of temperature with configurational entropy is well satisfied and it does not depend on pressure. Moreover, we also derive an equation for calculating the pressure dependence of the glass transition temperature, which in these systems is in good accordance with the experimental values.
Critical Dynamics of Dimers: Implications for the Glass Transition †
The Journal of Physical Chemistry B, 2005
The Adam-Gibbs view of the glass transition relates the relaxation time to the configurational entropy, which goes continuously to zero at the so-called Kauzmann temperature. We examine this scenario in the context of a dimer model with an entropy vanishing phase transition, and stochastic loop dynamics. We propose a coarse-grained master equation for the order parameter dynamics which is used to compute the time-dependent autocorrelation function and the associated relaxation time. Using a combination of exact results, scaling arguments and numerical diagonalizations of the master equation, we find non-exponential relaxation and a Vogel-Fulcher divergence of the relaxation time in the vicinity of the phase transition. Since in the dimer model the entropy stays finite all the way to the phase transition point, and then jumps discontinuously to zero, we demonstrate a clear departure from the Adam-Gibbs scenario. Dimer coverings are the "inherent structures" of the canonical frustrated system, the triangular Ising antiferromagnet. Therefore, our results provide a new scenario for the glass transition in supercooled liquids in terms of inherent structure dynamics. PACS numbers:
Physical Review Letters, 2014
We present a study of two model liquids with different interaction potentials, exhibiting similar structure but significantly different dynamics at low temperatures. By evaluating the configurational entropy, we show that the differences in the dynamics of these systems can be understood in terms of their thermodynamic differences. Analyzing their structure, we demonstrate that differences in pair correlation functions between the two systems, through their contribution to the entropy, dominate the differences in their dynamics, and indeed overestimate the differences. Including the contribution of higher order structural correlations to the entropy leads to smaller estimates for the relaxation times, as well as smaller differences between the two studied systems.