Deviations from thermally activated and from exponential response are typical features of the vitrification phenomenon and previously have been studied using viscoelastic, dielectric, calorimetric, optical, and other techniques. Linear response data from literature on about 70 covalent glass formers, ionic melts, supercooled liquids, amorphous polymers, and glassy crystals are surveyed. Except for orientational glasses and monohydric aliphatic alcohols a distinct but broad correlation of non‐Debye behavior with non‐Arrhenius relaxations is found. Within the broad trend several groups of materials, distinguished by their respective molecular complexity, can be identified and are shown to exhibit narrow correlations. At a given degree of deviation from Arrhenius behavior externally imposed stresses are relaxed with a departure from exponential behavior which is stronger the more the molecular or atomic subunits of the glassforming material are interconnected with each other.

Nonexponential relaxations in strong and fragile glass formers

Theoretical concepts and experimental evidence of heterogeneity in glass-forming liquids and polymers are reviewed. The main purpose is to provide an introduction to theoretical developments and recent experiments which have led to rapidly increasing knowledge. Realizing that there is no consensus in regard to the various scenarios of the glass transition starting from rather different assumptions we try to give a balanced overview although we also compare and interrelate some of the approaches. The experimental part describes recent nuclear magnetic resonance, dielectric, and optical experiments from which dynamically distinguishable subensembles can be selected thus proving the existence of a well defined dynamical heterogeneity.

Heterogeneity at the glass transition: a review

An intriguing problem in condensed matter physics is understanding the glass transition, in particular the dynamics in the equilibrium liquid close to vitrification Recent advances have been made by using hydrostatic pressure as an experimental variable These results are reviewed, with an emphasis in the insight provided into the mechanisms underlying the relaxation properties of glass-forming liquids and polymers

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R03/pdf

Supercooled dynamics of glass-forming liquids and polymers under hydrostatic pressure

We review the important general dynamic and thermodynamic properties of structural glass-forming substances and classify them into 12 different categories. Our understanding of glass-forming substances is incomplete until all these properties have been explained. The dynamic properties considered include those relating to the high frequency fast relaxation, the Johari–Goldstein β-relaxation, and the slow structural α-relaxation. Practically all the important relaxation and diffusion processes in the time region extending from 10−13 to 106 s have been discussed. We show from experimental data that the properties in different categories bear some relations to each other. Some issues that impede progress of our understanding of the behavior of glass-forming substances are accentuated by formulating them as questions. Answers to these questions by future experimental and theoretical investigations will improve our understanding. Specific suggestions for future research efforts that can provide some answers to these questions are also made. Within the same property in each category, by examining a large number of glass-formers, a pattern of variation is found which correlates with the departure of the structural relaxation of the glass-former from exponential decay or the quantity, (1−β), where β is the fractional exponent in the Kohlrausch expression, exp[−(t/τ)β]. These patterns, as well as their correlations with (1−β), suggest that the salient dynamic properties (or phenomenology) of glass-forming substances are all governed by the non-exponential nature of the structural relaxation, i.e., the quantity (1−β). Therefore, a theory or model capable of relating the structural relaxation time τ to (1−β) has the potential of explaining many of the salient dynamic properties of glass-forming substances.

Dynamic and thermodynamic properties of glass-forming substances

Molecular-dynamics simulations were carried out for the SPC, SPCE, TIP4P, and TIP5P models of water at 298 K. From these results we determine the following quantities: the absolute entropy using the two-particle approximation, the mean lifetime of the hydrogen bond, the mean number of hydrogen bonds per molecule, and the mean energy of the hydrogen bond. From the entropy calculations we find that nearly all contributions to the total entropy originates from the orientation effects. Moreover, we determine the contributions to the total entropy which originate from the first, second, and higher solvation shells. It is interesting that the limits between solvation shells are clearly visible. The first solvation shell (0.22<r<0.36nm) contributes approximately 43J∕molK to the total entropy; the second solvation shell (0.36<r<0.60nm) contributes approximately 12J∕molK, while contributions from the third and other solvation shells are very small, approximately 2J∕molK in summary. This indicates that water molecu...

Structural properties of water: Comparison of the SPC, SPCE, TIP4P, and TIP5P models of water

The laser light scattering intensity fluctuations of supercooled liquid o‐terphenyl at temperatures between 12 and −15 °C have been studied using the photon‐correlation technique. The time correlation functions obtained using the VV and VH scattering configurations at 90° scattering angle show a wide distribution of relaxation times. Angular‐dependent measurements at 45°, 90° and 135° scattering angles show that the time correlation functions for VH and for VV are independent of scattering angles. The VV and VH correlation functions cannot be fit to a bimodal distribution function; neither can they be fit to a cumulant expansion expression with less than four cumulants, but they can be satisfactorily fit to the William–Watts distribution function. The mean relaxation times derived for both isotropic and anisotropic scattering processes from the VV and VH correlation functions are nearly equal. The temperature dependence of the relaxation times cannot be described by an Arrhenius equation with a constant activation energy. The average orientational relaxation times change by more than ten orders of magnitude in the temperature range between 147 and −16 °C but the temperature dependence of the orientational relaxation times over this wide dynamic range can be excellently correlated with the modified Debye–Stokes–Einstein equation. The closeness of the mean orientational relaxation time to the mean structural relaxation time is interpreted as due to strong rotation–translation coupling in the viscoelastic state. It is found that within the dynamic range of the autocorrelator, the collective shear wave plays no role in the time dependence of the VV and VH correlation functions. Quentrec’s theory of viscoelasticity based on the coupling of the shear wave to local translational order and orientational order is found to disagree with the experimental result. Comparison with the dielectric relation time data shows that in o‐terphenyl the dielectric relaxation and the present light beating techniques probe different types of motions.

Homodyne light beating spectroscopy of o-terphenyl in the supercooled liquid state

We report results of the photon correlation polarized light scattering study on the dynamics of bulk polymethylacrylate (PMA) in the temperature range of 7–76 °C and in the pressure range 1–2100 bar. A single relaxation process with a distribution of relaxation times represents the experimental photon correlation functions. The temperature and pressure dependence of the mean relaxation time are analyzed in terms of the VFT and WLF equations. The parameters obtained are discussed in terms of the rotational isomerization model and the free volume model. The results obtained in light scattering agree well with dielectric and mechanical relaxation data. In the case of polyethylmethacrylate (PEMA) and polybutylmethacrylate (PBMA), two overlapping relaxation modes are found. The values of the activation parameters and the relaxation time at the glass transition temperature (Tg) thus obtained characterizing the dynamics of a polymer melt above Tg are compared for PMA, PEA, PEMA, and PBMA.

A high pressure photon correlation study of bulk poly(methylacrylate). Comparison with relaxation processes in poly(ethylacrylate) and related polymethacrylates

Molecular dynamics computer simulation was used to study depolarized light scattering in liquid OCS at temperatures of 170, 243, and 298 K. Applying the first‐order dipole‐induced dipole model for the interaction‐induced polarizabilities, the time correlation functions and spectral line shapes corresponding to the depolarized Rayleigh and nonvibrational contributions to the symmetric stretch (ν1) mode Raman spectra were calculated. The results were compared with experimental findings and simulations on the liquids CO2 and CS2. We find that the OCS simulations reflect the main spectral features in satisfactory agreement with experiment and that an intermediate position concerning all the studied particularities is assigned to liquid OCS between CO2 and CS2. For these three liquids the difference correlation functions between Rayleigh and Raman light scattering were computed and discussed in terms of their component many‐body contributions.

Molecular dynamics simulations of the depolarized light scattering spectra of liquid OCS in comparison with experiment and simulations of CO2 and CS2

The photon correlation spectra of glycerol were obtained between −30° and −75°C. The autocorrelation functions obtained with VV and VH geometry display both a decay deviating from a single relaxation time process. By using a model based upon the contribution of a diffusion and a pure relaxation, the experimental correlation function could be accurately described. With this model, we obtained mean relaxation times which were compared to those obtained from dielectric relaxation. The comparison of these quantities involving the l = 1 and l = 2 components of the Legendre polynomials has led us to the conclusion that at low temperatures the motion can be described by a jump diffusion whereas it tends to be more diffusive at higher temperatures.

Reorientation in associated liquids. I glycerol

The Brillouin lines in the polarized light scattering spectrum of liquid CCI4, SiCl4 and SnCl4 were analysed and relaxation times and relaxation strengths were calculated in the temperature range between the melting and the boiling points of these liquids. The results were interpreted on the basis of an isolated binary collision model. The connection between experimental relaxation times, theoretical relaxation probabilities and molecular collision numbers as obtained from model considerations are discussed. The relative significance of factors as the anisotropy of intermolecular forces and multiple collision is assessed.

Th. Dorfmüller

Papers

Homodyne light beating spectroscopy of o-terphenyl in the supercooled liquid state

A high pressure photon correlation study of bulk poly(methylacrylate). Comparison with relaxation processes in poly(ethylacrylate) and related polymethacrylates

Molecular dynamics simulations of the depolarized light scattering spectra of liquid OCS in comparison with experiment and simulations of CO2 and CS2

Reorientation in associated liquids. I glycerol

A light scattering study of vibrational relaxation in liquids