The different physical aspects of glass transitions are reviewed and models aiming at their explanation are described. The following three main aspects are distinguished: the degree of stability of supercooled liquids with respect to crystallisation; the variation of physical properties of supercooled liquids in metastable equilibrium above the glass transition; the arrest of structural relaxation at the glass transition. The physical significance of computer simulations of glass transitions in simple liquids and the question of a hidden phase transition underlying an observed glass transition are examined critically. In relation to the stability problem, the geometrical constraints operative in typical disordered structures, such as random sphere packings and random covalent networks, are also discussed. A general survey of glass transition phenomena and concepts is presented in an introductory section.

https://iopscience.iop.org/article/10.1088/0034-4885/49/2/002/pdf

Models of the glass transition

Molecular dynamics simulation of bulk liquid and glass of long‐chain molecules has been performed. The system consists of linear chains of up to 50 spherical segments, each subject to forces due to bond stretching, bending, and torsion, and to nonbonded interaction, according to a truncated Lennard‐Jones potential, between segments in neighboring chains and between segments separated by more than three bonds along the chain. The parameters are chosen to mimic polymethylene, the segment representing a CH2 unit. Behaviors suggestive of liquid‐to‐glass transition were exhibited by (i) cessation of trans–gauche conformational transitions, (ii) changes in the temperature coefficients of the density and internal energy, and (iii) effective vanishing of the segmental self‐diffusion coefficient. The ‘‘freezing in’’ of these properties occurs at decreasing temperatures in the order given above, indicating the decreasing size of domains of cooperative motion required. The dependence of the transition temperature on...

Molecular dynamics simulation of polymer liquid and glass. I. Glass transition

Properties of the crystal and liquid phases have been measured for a system of particles interacting through a modified Lennard‐Jones potential. Through constant pressure molecular dynamics, we have evaluated the density and internal energy of these phases at a pressure that approximates that of the vapor phase. The free energy of the crystal is obtained with the Einstein crystal as a reference state, and the liquid free energy is measured relative to the ideal gas. The triple point temperature is obtained. Compressibilities and Gruneisen parameters are obtained at zero temperature and the triple point. Dynamic properties of the supercooled liquid state are also calculated. These results are applied in forthcoming publications which calculate surface excess quantities and dynamic properties of the fcc (111), (100), and (110) faces.

Molecular dynamics investigation of the crystal–fluid interface. I. Bulk properties

Recent Brillouin scattering studies of relaxation in the simple ion system Ca++–K+/NO−3 (two particles of argon structure and a small anion) provide the experimental basis for a direct comparison of glass transformation phenomenology in computer simulated LJ argon ‘‘glass,’’ with that in laboratory substances studied on the same time scale. Most of the attenuated glass transition characteristics observed for LJ argon are found in the ionic system, and the same relation of the dispersion midpoint to the so‐called ‘‘ideal’’ glass transition temperature is observed. Analysis of the real and imaginary parts of the complex longitudinal modulus shows that at high temperatures the relaxation function for Ca++–K+–NO−3, in strong contrast with that at normal low temperature behavior, closely approaches a simple exponential decay with Arrhenius form for the relaxation time. Furthermore, the high temperature Arrhenius plot extrapolates naturally to the reciprocal quasilattice vibration frequency determined by far in...

Short time structural relaxation processes in liquids: Comparison of experimental and computer simulation glass transitions on picosecond time scales

A full-scale simulation approach for atom probe tomography.

The molecular dynamics technique was used to study the thermodynamic and structural properties of non-crystalline solid structures. The structures were obtained by quenching a model liquid consisting of 336 atoms arranged in a computational cell with periodic boundaries and with the atomic intereaction represented by the Lennard-Jones pair potential. The characteristics of the resulting non-crystalline atomic configurations were found to depend weakly on the applied quenching rate.

Willy Damgaard Kristensen

Papers

Computer-simulated amorphous structures (I). Quenching of a Lennard-Jones model system