Liquids, Glasses, and the Glass Transition: A Free-Volume Approach

2007 ◽  
pp. 455-525 ◽  
Author(s):  
Gary S. Grest ◽  
Morrel H. Cohen
Keyword(s):  
Glass Transition ◽  
Free Volume

scholarly journals A theoretical interpretation of free volume at glass transition

2017 ◽  
Vol 35 (11) ◽  
pp. 1415-1427 ◽  
Author(s):  
Yi-kun Ren ◽  
Yun-tao Li ◽  
Liang-bin Li
Keyword(s):  
Glass Transition ◽  
Free Volume ◽  
Theoretical Interpretation

A molecular dynamics model of melting and glass transition in an idealized two-dimensional material I

1989 ◽  
Vol 329 (1608) ◽  
pp. 549-573 ◽  
Keyword(s):  
Molecular Dynamics ◽  
Thermal Expansion ◽  
Glass Transition ◽  
Free Volume ◽  
Dimensional Structure ◽  
Valuable Insight ◽  
Two Dimensional ◽  
Discrete Elements ◽  
Dynamics Model ◽  
Topological Features

In a preparatory study of structural relaxations and plastic flow in a two-dimensional idealized atomic glass, the process of melting and quenching through a glass transition has been studied by computer simulation using a molecular dynamics model. In this model, the transition from a solid to a melt was observed to take place when liquid-like structural elements composed of dipoles of five- and seven-sided Voronoi polygons percolate through the two-dimensional structure of distorted hexagons in the form of strings. Such dipoles constitute discrete elements of excess free volume within which liquid like behaviour is established in the sense of reduced cohesion or local elastic moduli. Upon quenching the melt, the percolation condition of liquid-like regions is retained for under-cooled melts between the melting point and a glass transition temperature below which the percolation condition is broken and the thermal expansion is sharply reduced. The simulation that has used empirical pair potentials characteristic of Cu and Zr has substantially underpredicted the melting and glass transition temperatures and overpredicted the thermal expansion of C u x Zr 1-x type glasses. These defects of the model can be partly attributed to the two-dimensional nature of the material, which stores larger concentrations of free volume than a corresponding three-dimensional material. In spite of these quantitative shortcomings, the model gives valuable insight into the topological features of the local atomic configurations at melting and upon vitrification.

Applicability of Free Volume Concept to Relaxation Phenomena in the Glass Transition Range

1968 ◽  
Vol 41 (3) ◽  
pp. 555-563
Author(s):  
A. J. Kovacs
Keyword(s):  
Glass Transition ◽  
Free Volume ◽  
Excess Entropy ◽  
Heat Content ◽  
Configurational Entropy ◽  
Wide Distribution ◽  
Dielectric Constants ◽  
Experimental Point ◽  
Transition Range ◽  
Glass Transition Range

Abstract Except for discrepancies mentioned with respect to pressure investigations, which need future clarification, we can conclude in a general way, as follows. As far as only average parameters of macroscopic specimens are considered (complex moduli, or dielectric constants, volume or heat content etc...), the free volume concept can relate variations of molecular mobility to changes of an average free volume in a semiquantitative way. This average free volume can no longer fully characterize the wide variety of molecular motions involved in the kinetics of redistribution of holes in the liquid during recovery experiments. These kinetic processes involve a wide distribution of retardation times, which may be associated with local distribution of holes, or with that of cooperating groups of molecules, or molecular segments. On the other hand, free volume is not necessarily the fundamental molecular parameter which controls rate of configurational changes, characterized by variation of entropy of the liquid. Even if this is the case, most of the above discussion may be applied to any other average excess parameter, as far as the Doolittle equation is formally adopted, in which f/b is expressed in terms of the new parameter, rather than that of free volume. However, since relaxational free volume, as determined from the WLF equation, and independently measured volume changes are often in close agreement, this means that variations of excess entropy, or those of configurational free energy, and changes in volume are closely related. This conclusion is in agreement with that of Eisenberg and Saito, who found that the Gibbs—Dimarzio theory, based on configurational entropy, is practically equivalent with the free-volume approach. Thus, the free volume concept remains still a valuable tool for unifying different kinds of rate processes from both a theoretical and an experimental point of view, especially in the glass transition range.

Sign in / Sign up

Export Citation Format

Share Document