Qualitative change in structural dynamics of some glass-forming systems (original) (raw)
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Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers
Entropy
Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature Tg, whether there is an underlying thermodynamic transition at some finite temperature below Tg, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section disc...
SuperArrhenius character of supercooled glass-forming liquids
Journal of Non-Crystalline Solids, 1998
We picture the viscosity and a-relaxation time of a supercooled liquid as Arrhenius-like at temperatures well above the melting point, and as superArrhenius, i.e., describable in terms of a temperature-dependent activation free energy, E(T), at temperatures near the glass transition temperature, T g. We discuss this in terms of a superArrhenius factor D g i g À i I a g , where E I is the activation free energy at high T. We relate D(T g) to the property called``fragility'', and discuss its relation to non-exponential relaxation and to activated dynamics. We postulate that supercooled liquids are broken into supermolecular domains, frustrated phase-related entities which are to be distinguished from species-speci®c local clusters.
Relaxation dynamics of glasses along a wide stability and temperature range
Scientific Reports, 2016
While lots of measurements describe the relaxation dynamics of the liquid state, experimental data of the glass dynamics at high temperatures are much scarcer. We use ultrafast scanning calorimetry to expand the timescales of the glass to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-temperature regime above the conventional devitrification temperature heating at 10 K/min. The liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws originally derived for the liquid. This work provides a strong connection between the dynamics of the equilibrium supercooled liquid and non-equilibrium glassy states.
The Journal of Chemical Physics, 2005
The primary relaxation time scale ͑T͒ derived from the glass forming supercooled liquids ͑SCLs͒ is discussed within ergodic-cluster Gaussian statistics, theoretically justified near and above the glass-transformation temperature T g . An analysis is given for the temperature-derivative data by Stickel et al. on the steepness and the curvature of ͑T͒. Near the mode-coupling-theory ͑MCT͒ crossover T c , these derivatives separate by a kink and a jump, respectively, the moderately and strongly SCL states. After accounting for the kink and the jump, the steepness remains a piecewise conitnuous function, a material-independent equation for the three fundamental characteristic temperatures, T g , T c , and the Vogel-Fulcher-Tamman ͑VFT͒ T 0 , is found. Both states are described within the heterostructured model of solidlike clusters parametrized in a self-consistent manner by a minimum set of observable parameters: the fragility index, the MCT slowing-down exponent, and the chemical excess potential of Adam and Gibbs model ͑AGM͒. Below the Arrhenius temperature, the dynamically and thermodynamically stabilized clusters emerge with a size of around of seven to nine and two to three molecules above and close to T g and T c , respectively. On cooling, the main transformation of the moderately into the strongly supercooled state is due to rebuilding of the cluster structure, and is attributed to its rigidity, introduced through the cluster compressibility. It is shown that the validity of the dynamic AGM ͑dynamically equivalent to the standard VFT form͒ is limited by the strongly supercooled state ͑T g Ͻ T Ͻ T c ͒ where the superrigid cooperative rearranging regions are shown to be well-chosen parametrized solidlike clusters. Extension of the basic parameter set by the observable kinetic and diffusive exponents results in prediction of a subdiffusion relaxation regime in SCLs that is distinct from that established for amorphous polymers. Other overall T fitting forms were suggested by Kivelson et al. in Ref. 11 and by Schulz in Ref. 12 that, respectively, admit the "avoided critical points" 13 ͑T cr is above the melting point T m ͒, and rule out the nonzero critical temperatures ͑T cr =0͒, although their real observation windows were not tested by the T-derivative analysis.
Short time dynamics of glass-forming liquids
The Journal of Chemical Physics, 1995
Calculations have been presented for the intermediate scattering function, dynamic structure factor, and dynamic susceptibility of a complex correlated system undergoing relaxation with independent vibrations. The vibrational contribution was approximated by a Debye spectrum, smoothed at high frequency, while the coupling model was used to describe the relaxation. This model asserts for nonpolymeric glass-forming liquids a crossover at a microscopic time from intermolecularly uncorrelated relaxation at a constant rate to intermolecularly coupled relaxation with a time-dependent, slowed-down rate. Although the model has previously been employed to successfully predict and otherwise account for a number of macroscopic relaxation phenomena, critical phenomena are not included in, and cannot be addressed by, the coupling model. Notwithstanding an absence of any change in transport mechanism for the supercooled liquid at a critical temperature, the coupling model data, when analyzed in t...
Growing length and time scales in glass-forming liquids
Proceedings of the National Academy of Sciences, 2009
The glass transition, whereby liquids transform into amorphous solids at low temperatures, is a subject of intense research despite decades of investigation. Explaining the enormous increase in relaxation times of a liquid upon supercooling is essential for understanding the glass transition. Although many theories, such as the Adam-Gibbs theory, have sought to relate growing relaxation times to length scales associated with spatial correlations in liquid structure or motion of molecules, the role of length scales in glassy dynamics is not well established. Recent studies of spatially correlated rearrangements of molecules leading to structural relaxation, termed ''spatially heterogeneous dynamics,'' provide fresh impetus in this direction. A powerful approach to extract length scales in critical phenomena is finite-size scaling, wherein a system is studied for sizes traversing the length scales of interest. We perform finite-size scaling for a realistic glass-former, using computer simulations, to evaluate the length scale associated with spatially heterogeneous dynamics, which grows as temperature decreases. However, relaxation times that also grow with decreasing temperature do not exhibit standard finite-size scaling with this length. We show that relaxation times are instead determined, for all studied system sizes and temperatures, by configurational entropy, in accordance with the Adam-Gibbs relation, but in disagreement with theoretical expectations based on spin-glass models that configurational entropy is not relevant at temperatures substantially above the critical temperature of mode-coupling theory. Our results provide new insights into the dynamics of glass-forming liquids and pose serious challenges to existing theoretical descriptions.
A single relaxation time description for glasses and their liquid state
arXiv: Soft Condensed Matter, 2016
We use relaxation data taken out of equilibrium on glasses of different stability and equilibrium relaxation times from the supercooled liquid to propose a common description for both liquid and glass states. Using ultrafast scanning calorimetry, the accessible timescales of the glass are expanded to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-temperature regime above the conventional devitrification temperature heating at 10 K/min. Surprisingly, both the liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws derived for the liquid. This work provides a strong connection between the glass and liq...
Relaxation in glassforming liquids and amorphous solids
Journal of Applied Physics, 2000
The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg, and a fourth dealing with “short time processes.” The first part tackles the high temperature regime T>Tg, in which the system is ergodic and the evolution of the viscous liquid toward the condition at Tg is in focus. The second part deals with the regime T∼Tg, where the system is nonergodic except for very long annealing times, hence has time-dependent properties (aging and annealing). The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for “superionic” conductivit...