Table of contents

There has been considerable interest in the impact of mechanical stresses on the thermodynamic state of glassy materials over the past quarter century—beginning with Struik's hypothesized erasure of ageing by large stresses. In particular, there has been a recent application of the ideas of erasure or rejuvenation in the general literature on glasses and glass-like substances. For this reason we here examine the evidence for and against rejuvenation and show several sets of data that argue strongly against rejuvenation or erasure of ageing as viable explanations for the interactions between large mechanical stimuli and the structure of the glass. Two particular experimental conditions are of significance here. First, measurements that are made in the sub-yield region and close to the glass transition allow the direct measurement of the evolution of the glassy response both with and without applying any stress. For example, in torsional dilatometry experiments where the volume recovery is measured simultaneously and in the same apparatus as the mechanical response, it is found that the baseline volume recovery is unaffected by the applied stress. The second class of experiments has been post-yield experiments, where the rejuvenation hypothesis is more difficult to reject. However, observations that post-yield ageing leads to evolution into a different 'equilibrium' state as measured by the recovered yield stress suggest that yielding does not rejuvenate the glass, but it may lead to a sort of polyamorphism or new deformation induced phase. We also present results of mechanically induced 'implosion' in polymers far below the glass temperature that suggest that mechanical stimuli may actually accelerate ageing rather than rejuvenating the glass. Finally, the origins of the power-law dependence on ageing time (also termed elapsed or waiting time) of the mechanical relaxation time are discussed, and the use of the elapsed time as a natural variable to describe ageing is shown not to be appropriate for structural glasses.

We show that the phonon–saddle transition in the ensemble of generalized inherent structures (minima and saddles) happens at the same point as the dynamical phase transition in glasses, that has been studied in the framework of the mode coupling approximation. The boson peak observed in glasses at low temperature is a remnant of this transition.

The freezing-in phenomena that occurred in some orientationally disordered crystals and liquid crystals, either in the metastable or in the stable state, are described in comparison with those in liquids. These observations indicate clearly that the glass transitions observed exclusively in liquids are just one example of 'transitions' which must occur widely in condensed states of matter, irrespective of the structural regularity. The interplay between phase and glass transitions in crystals is discussed in relation to useful dopants that accelerate some molecular motions resulting in failure to maintain equilibrium at low temperatures. The structural relaxation occurring in vapour-quenched amorphous solids is studied using an adiabatic calorimeter operated as a low-frequency spectrometer. The anomalous behaviour of the configurational entropy S_c observed for some molecular systems is discussed. The solid-state vitrification of molecular crystals caused by mechanical milling and the formation of molecular alloys for some binary systems are described, with potential applications.

Since the glass temperature T_g is a corresponding temperature for amorphous materials, if measured properly, precise values are needed. When determinations are made by cooling from equilibrium, material characterizing values are obtained. Most reported values are obtained in heating experiments which yield specimen-specific values, which approximate fictive temperatures T_f. Examples of consequent discrepancies of relaxation times at T_g are given. Results obtained in cooling show that it is likely that the mobility at T_g of local molecular motions is the same for all of the amorphous materials examined. In addition, it is shown that relatively precise values of T_g can be obtained from the recoverable compliance.

We present a novel theoretical approach to understanding the complex dynamics of glass-forming liquids, granular packings and amorphous solids. This theory, which is an elaboration of the free volume and inherent structure approaches, allows one to retrieve the thermodynamical properties of these systems from studies of geometrical and topological properties of local, static configurations alone. When applied to hard-sphere systems, the present theory reproduces with a good quantitative agreement the equation of state for the crystalline and the disordered glassy phases. Moreover, we find that, as the density approaches a critical value close to the random close-packing density, the configurational entropy approaches zero and the large relaxation time diverges according to the Vogel–Fulcher behaviour, following also the Adam–Gibbs relation.

The mechanism of the light-induced orientation process of azobenzene-containing polymers caused by irradiation with linearly polarized red light is investigated. This process is surprising because there is almost no absorption at 633 nm. Depending on the photochemical pre-treatment and the exposure time, the azobenzene moieties can undergo two different orientation processes resulting in either a parallel or a perpendicular orientation with respect to the electric field vector of the incident light. The fast orientation of the photochromic groups with their long axis in the direction of the light polarization requires a photochemical pre-treatment in which non-polarized UV light generates Z-isomers. Due to this procedure the film becomes 'photochemically activated' for the subsequent polarized irradiation with red light. But on continued exposure a second, much slower reorientation process occurs which establishes an orientation of the azobenzene groups perpendicular to the electric field vector. The fast mechanism is probably caused by an angle-selective photo-isomerization of the Z-isomers to the E-isomers, while the subsequent slow reorientation process is caused by the well-known conventional photo-orientation taking place via the accumulation of a number of photoselection steps and the rotational diffusion minimizing the absorbance of the E-isomer. This process occurs in the steady state but at this wavelength with a very small concentration of Z-isomers. The competing mechanisms take place in the same polymer film under almost identical irradiation conditions, differing only in the actual concentration of the Z-isomers.

The coupling of the rotational dynamics of anisotropic molecules to longitudinal phonons gives rise to different effects in the interaction of light with supercooled liquids formed of such molecules. One is the opening of new light scattering channels in Brillouin scattering; this can alter the corresponding phonon lineshape. Another one is the possible appearance of a depolarized signal in transient grating experiments (TG), also called impulsive stimulated scattering. We give a brief theoretical description of these two effects for supercooled liquids formed of linear rigid molecules. We also present preliminary results on the analysis of such a TG experiment performed on m-toluidine.

We discuss dielectric and light scattering susceptibility spectra of simple glass formers at temperatures above as well as below the critical temperature of the mode coupling theory (MCT). Close to T_g the systems are characterized by the presence of a pronounced excess wing (type A glass formers). The data are analysed within a phenomenological approach, on the one hand, and within MCT, on the other. Among other work we present a complete interpolation of the dielectric data for glycerol (Lunkenheimer et al2000 Contemp. Phys.41 15). The crossover temperature T_x, defined by the emergence of the excess wing upon cooling, is extracted from the phenomenological analysis and found to agree well with the critical temperature T_c, extracted from the MCT analysis at high temperatures. Below T_x the evolution of the susceptibility is characterized by a universal appearance of the excess wing. No difference is observed for the non-fragile system with respect to fragile glass formers provided that the wing parameters are studied as a function of the correlation time τ_α. Finally, a generalized scaling for the susceptibility minimum is proposed which is a phenomenological extension of that of MCT but now also includes the data below T_c.

We study the influence of the system size on various static and dynamic properties of a supercooled binary Lennard-Jones liquid via computer simulations. In this way, we demonstrate that the treatment of systems as small as N = 65 particles yields results relevant for the understanding of bulk properties. In particular, we find that a system of N = 130 particles behaves basically as two non-interacting systems of half the size.

Dielectric relaxation times over a broad range of temperature and pressure for the glass former phenolphthalein-dimethylether (PDE) reveal a change of dynamics at a characteristic relaxation time τ_B. The value of τ_B was found to be largely insensitive to the particular combination of pressure and temperature of the measurement. Data for a second glass former, cresol-phthalein-dimethylether, having a molecular structure very close to that of PDE, were also analysed. In this case, τ_B is much smaller, so the change of dynamics could not be observed in the elevated pressure experiments. The PDE data were in good agreement with the Adam–Gibbs model near T_g (τ > τ_B), while deviating for τ < τ_B. Finally, a possible connection between the observed T_B and theoretical models is presented.

Within the mode-coupling theory for the evolution of structural relaxation, the A₄-glass-transition singularities are identified for systems of particles interacting with a hard-sphere repulsion complemented by different short-ranged potentials: Baxter's singular potential regularized by a large-wavevector cut-off, a model for the Asakura–Oosawa depletion attraction, a triangular potential, a Yukawa attraction, and a square-well potential. The regular potentials yield critical packing fractions, critical Debye–Waller factors, and critical amplitudes very close to each other. The elastic moduli and the particle localization lengths for corresponding states of the Yukawa system and the square-well system may differ by up to 20 and 10%, respectively.

We review a statistical picture of the glassy state derived from the analysis of the off-equilibrium fluctuation-dissipation relations. We define an ultralong-time limit where 'one-time quantities' are close to equilibrium while the response and correlation can still display ageing.

In this limit it is possible to relate the fluctuation response relation to static breaking of ergodicity. The resulting picture suggests that even far from that limit, the fluctuation-dissipation ratio relates to the rate of growth of the configurational entropy with the free-energy density.

We present a phenomenological time-dependent Ginzburg–Landau theory of nonlinear plastic deformations in solids. Because the problem is very complex, we first give models in one and two dimensions without vacancies and interstitials, where large strains produce densely distributed slips but the mass density deviations remain small except near the tips of slips. Next we set up a two-dimensional model including a vacancy field (or local free-volume fraction), where the sensitive dependence of the elastic shear modulus on the vacancy density is relevant. In our simulation, if strains are applied to nearly defectless solids but in the presence of such an elastic inhomogeneity, the vacancy density and the mass density can become considerably heterogeneous for large strains on spatial scales much longer than the atomic size. These strain-induced disordered states are metastable or long lived once they are created.

We present the results of large-scale computer simulations in order to discuss the structural and dynamic properties of sodium silicate melts with the compositions (Na₂O)2(SiO₂) (NS2) and (Na₂O)20(SiO₂) (NS20). We show that, compared to silica (SiO₂), these systems exhibit additional intermediate range order on intermediate length scales that stem from the tetrahedral network structure. By means of intermediate-scattering functions, we characterize the dynamics of sodium in the system under consideration. Whereas in NS2 the incoherent scattering functions for Na decay much faster to zero than the coherent ones for Na–Na, in NS20 this different behaviour of the incoherent and coherent functions is not very pronounced. On the other hand, the incoherent functions of the two systems share a very peculiar feature: their long-time decay can be described by a Kohlrausch law with a constant exponent β for q > q_th where q_th is significantly below the location of the main peak in the static structure factor for the Na–Na correlations.

This paper gives a brief review of recent results of rheo-dielectric studies for oligo-styrene (OS) and cis-polyisoprene (PI). OS has type-B dipoles perpendicular to the backbone, and its dielectric α relaxation (glassy mode relaxation) is accelerated under the shear flow even at rates much smaller than the equilibrium relaxation frequency. This acceleration, resulting in the decrease of the viscosity (thinning) observed at those rates, is related to flow-induced reduction of the cooperativity of the segmental motion. For PI chains, having type-A dipoles parallel to the backbone, the dielectric relaxation detects the global chain motion. For well entangled PI chains, this relaxation is only moderately accelerated and its intensity is only mildly reduced even under fast flow in the non-Newtonian thinning regime. This result is related to a flow-induced orientational cross-correlation of entanglement segments. Within the context of the tube model for entangled chains, this cross-correlation can be related to the dynamic tube dilation induced by the convective constraint release.

Linear and Fourier-transform rheology were used to study the influence of the oscillatory shear amplitude, γ₀, on the isothermal crystallization at 140^° C of three commercial isotactic polypropylenes. The development of the crystallization was monitored through the time dependence of the dynamic storage modulus, G'(t), and the normalized intensity of the third harmonic of the stress waveform, I₃ (t), a quantification of the degree of nonlinearity under oscillatory shear conditions. A change in the exponent, n, of the power law describing growth, according to G_norm '(t) ∼ tⁿ, was observed at t ∼ 4 h. For t < 4 h, n was between 3 and 4. This change in growth kinetics was also accompanied by a maximum in I₃ (t). Below γ₀ = 0.05, these results were independent of the applied strain amplitude. Above γ₀ = 0.05, failure of the polypropylene in the parallel plate geometry due to stress build-up was often observed in the late stages of crystallization. This was accompanied by a sharp decrease of G'(t), and a simultaneous sharp increase of I₃ (t). Additionally, the presence of even harmonics in the spectrum was observed after failure. Notably, a plateau of I₃ (t) at least 1 h before actual failure indicating a greater sensitivity to its onset than that of G'(t).

The rheological behaviour of soft glassy materials results essentially from the interplay between shearing forces and an intrinsic slow dynamics. This competition can be described by a microscopic theory, which can be viewed as a nonequilibrium schematic mode-coupling theory. This statistical mechanics approach to rheology results in a series of detailed theoretical predictions, some of which are yet to be experimentally verified. We present new, preliminary, results about the description of yield stress, flow heterogeneities and activated processes within this theoretical framework.

Newtonian viscosities of 19 multicomponent natural and synthetic silicate liquids, with variable contents of SiO₂ (41–79 wt%), Al₂O₃ (10–19 wt%), TiO₂ (0–3 wt%), FeO_tot (0–11 wt%); alkali oxides (5–17 wt%), alkaline-earth oxides (0–35 wt%), and minor oxides, obtained at ambient pressure using the high-temperature concentric cylinder, the low-temperature micropenetration, and the parallel plates techniques, have been analysed. For each silicate liquid, regression of the experimentally determined viscosities using the well known Vogel–Fulcher–Tammann (VFT) equation allowed the viscosity of all these silicates to be accurately described. The results of these fits, which provide the basis for the subsequent analysis here, permit qualitative and quantitative correlations to be made between the VFT adjustable parameters (A_VFT, B_VFT, and T₀).

The values of B_VFT and T₀, calibrated via the VFT equation, are highly correlated. Kinetic fragility appears to be correlated with the number of non-bridging oxygens per tetrahedrally coordinated cation (NBO/T). This is taken to infer that melt polymerization controls melt fragility in liquid silicates. Thus NBO/T might form an useful ingredient of a structure-based model of non-Arrhenian viscosity in multicomponent silicate melts.

There is a growing belief that the mode coupling theory is the proper microscopic theory for the dynamics of the undercooled liquid above and around a critical temperature T_c. In addition, there is some evidence that the system leaves the saddle points of the energy landscape to settle in the valleys at this critical temperature. Finally, there is a microscopic theory for the entropy well below T_c (i.e. close to the calorimetric glass transition T_g), of Mézard and Parisi, which allows one to calculate the Kauzmann temperature from the atomic pair potentials.

Description of the dynamics of the frozen glass phase is at present limited to phenomenological models. In the spirit of the energy landscape concept, one considers an ensemble of independent asymmetric double-well potentials with a wide distribution of barrier heights and asymmetries (the ADWP or Gilroy–Phillips model). The model gives an excellent description of the relaxation of glasses up to about T_g /4. Above this temperature, the interaction between different relaxation centres begins to play a role. In a mean-field treatment, the interaction reduces the number of relaxation centres needed to bring the shear modulus down to zero by a factor of three.

We present a first step toward a microscopic theory for the glass transition in systems with trivial static correlations. As an example we have chosen N infinitely thin hard rods with length L, fixed with their centres on a periodic lattice with lattice constant a. Starting from the N-rod Smoluchowski equation we derive a coupled set of equations for fluctuations of reduced k-rod densities. We approximate the influence of the surrounding rods on the dynamics of a pair of rods by introduction of an effective rotational diffusion tensor D(Ω₁, Ω₂) and in this way we obtain a self-consistent equation for D. This equation exhibits a feedback mechanism leading to a slowing down of the relaxation. It involves as an input the Laplace transform υ₀ (l/r) at z = 0, l = L/a, of a torque–torque correlator of an isolated pair of rods with distance R = ar. Our equation predicts the existence of a continuous ergodicity-breaking transition at a critical length l_c = L_c /a. To estimate the critical length we perform an approximate analytical calculation of υ₀ (l/r) based on a variational approach and obtain l_c^var ∼ = 5.68, 4.84 and 3.96 for an sc, bcc and fcc lattice. We also evaluate υ₀ (l/r) numerically exactly from a two-rod simulation. The latter calculation leads to l_c^num ∼ = 3.45, 2.78 and 2.20 for the corresponding lattices. Close to l_c the rotational diffusion constant decreases as D(l) ∼ (l_c − l)^γ with γ = 1 and a diverging timescale t ∼ |l_c − l|^−δ, δ = 2, appears. On this timescale the t- and l-dependence of the one-rod density is determined by a master function depending only on t/t. In contrast to present microscopic theories our approach predicts a glass transition despite the absence of any static correlations.

By analysing the specific heats and the mechanical characteristics of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous single-state blends in the glass transition region, it has been found that increasing PVDF content causes a small reduction of the fragility of these systems. The degree of fragility has been evaluated from ΔC_p /C_p,l, the normalized change of the heat capacity at T_g, and by modelling the mechanical α_a-relaxation. The observed behaviour has been ascribed to a more coherent structure of the blends due to strong specific interactions between PMMA and PVDF polymeric chains.

We present structural and dynamical results of molecular dynamics simulation of vitreous silica undergoing a hydrostatic compression and decompression cycle at room temperature. Structural results as a function of density compare fairly well with experiments as well as with the longitudinal and transverse sound velocity pressure dependence. A shift of the boson peak (BP) toward higher energies and its simultaneous weakening is observed as in experiments. A detailed study of the dispersion of the glass vibration is presented at several densities and for the densified state. Evidence of phonon-like character with two distinct pseudo-periods is shown for longitudinal and transverse dynamics. The relationship between the BP vibrations and the correlation length scale indicated by the first sharp diffraction peak is discussed.

In this work, we review, extend and discuss low-temperature specific-heat experiments, that we have conducted on different molecular (hydrogen-bonded) solids both in crystalline and glassy (either amorphous or orientationally disordered) phases. In particular, we have measured the low-temperature specific heat C_p for a set of simple, well known alcohols: glycerol, propanol and ethanol. For glycerol, we have prepared and measured C_p of both glass and crystal phases down to 0.5 K. The same has been done for propanol, in this case comparing the strikingly different values observed for the two chemical isomers, 1-propanol and 2-propanol. Moreover, ethanol exhibits a very interesting polymorphism presenting three different solid phases at low temperature: a fully ordered (monoclinic) crystal, an orientationally disordered (cubic) crystal or 'orientational glass' and the ordinary structural (amorphous) glass. By measuring and comparing the low-temperature specific heat of the three phases, in the boson peak range 2–10 K as well as in the tunnelling-state range below 1 K, we provide a quantitative confirmation that 'glassy behaviour', either concerning low-temperature properties or the glass-transition phenomenon itself, is not directly related to the lack of long-range crystalline order occurring in amorphous solids.

An off-lattice simulation of the effect of confinement geometry on segmental mobility and the glass transition temperature has been conducted using Monte Carlo techniques. The off-lattice random coil Monte Carlo model represents the polymer chains, N − 1 units long, as a string of N impenetrable spheres. The segmental mobility is characterized by monitoring the mean square displacement of a bead over two different time intervals, one and fifty Rouse relaxation times, as the temperature is lowered. The change in volume fraction of occupied space, a measure of density, is monitored to detect the glass transition temperature T_g. Simulations have been conducted on freely jointed polymer chains, which have been spatially confined by one dimension (between parallel plates) and by three dimensions (in a sphere).

For both environments the value of T_g decreases as the confining dimension decreases in length. The onset of the decrease occurs first when the confinement is 3D and the magnitude of the effect is largest for 3D confinement. For 3D spherical confinement, throughout the cooling cycle, the segmental beads have a significantly higher mobility in the outer spherical region compared to those near the centre of the confinement space. For the 1D confinement, between two plates, any differences in mobility for regions near the plates compared to the centre region disappear as the temperature is lowered.

We present here results which indicate that polymeric monolayers confined at the air–water interface may provide interesting model systems for the study of glass transition phenomenology in a strongly confined 2D geometry. We report on photoinduced relaxation of the pressure–area isotherms in Langmuir monolayers of a photosensitive polyacrylate, using null-ellipsometry to study the evolution of the film thickness in the neighbourhood of the bulk glass transition. Furthermore, viscoelastic behaviour is investigated by a combination of surface quasielastic light scattering and expansion–compression isothermal cycles.

Triplet state solvation dynamics experiments on geometrically confined glass-forming liquids are performed in order to study the effects of confinement on length scales of several nanometres. Variations of the surface chemistry of porous glasses and the spatially selective observation of the interfacial liquid layer indicate that the surface interactions determine the difference between the dynamics of the bulk and confined situations. For 3-methylpentane in pores of 7.5 nm diameter, the timescale of interfacial dynamics is slowed down by three orders of magnitude, while structural relaxation already remains bulk-like just a few nanometres away from the silica surface. The time-resolved optical linewidths indicate dynamical heterogeneity in confined liquids, but the signature differs from the bulk case.

We describe a combination of laboratory and simulation studies that give quantitative information on the energy landscape for glass-forming liquids. Both types of study focus on the idea of suddenly extracting the thermal energy, so that the system obtained for subsequent study has the structure, and hence potential energy, of a liquid at a much higher temperature than the normal glass temperature T_g. One type of study gives information on the energy that can be trapped in experimental glasses by hyperquenching, relative to the normal glass, and on the magnitude of barriers separating basins of attraction on the landscape. Stepwise annealing studies also give information on the matter of energy heterogeneity and the question of 'nanogranularity' in liquids near T_g. The other type of study gives information on the vibrational properties of a system confined to a given basin, and particularly on how that vibrational structure changes with the state of configurational excitation of the liquid. A feature in the low frequency ('boson peak') region of the density of vibrational states of the normal glass becomes much stronger in the hyperquenched glass. Qualitatively similar observations are made on heating fragile glass-formers into the supercooled and stable liquid states. The vibrational dynamics findings are supported and elucidated by constant pressure molecular dynamics/normal mode MD/NM simulations/analysis of the densities of states of different inherent structures of a model fragile liquid (orthoterphenyl (OTP) in the Lewis–Wahnstrom approximation). These show that, when the temperature is raised at constant pressure, the total density of states changes in a manner that can be well represented by a two-Gaussian 'excitation across the centroid', leaving a third and major Gaussian component unchanging. The low frequency Gaussian component, which grows with increasing temperature, has a constant peak frequency of 18 cm⁻¹ and is identified with the Boson peak. It is suggested that the latter can serve as a signature for configurational excitations of the ideal glass structure, i.e. the topologically diverse defects of the glassy solid state. The excess vibrational heat capacity associated with this generation of low frequency modes with structural excitation is shown to be responsible for about 60% of the jump in heat capacity at T_g, most of the remainder coming from configurational excitation.

The nonlinear dielectric response of a lead titanate-doped lead magnesium niobate relaxor ferroelectric was studied using pseudo-stochastic binary electrical field excitation. The polarization was recorded for various pulse spacings and electrical field amplitudes. The decay of the field-polarization cross-correlation function could be accelerated by increasing the amplitude of the pulse fields. The extension of these experiments in order to record multidimensional dielectric spectra is illustrated.

We propose that the concept of liquids characterized by a given locally preferred structure (LPS) could help in understanding the observed phenomenon of polyamorphism. 'True polyamorphism' would involve the competition between two (or more) distinct LPSs, one favoured at low pressure because of its low energy and one favoured at high pressure because of its small specific volume, as in tetrahedrally coordinated systems. 'Apparent polyamorphism' could be associated with the existence of a poorly crystallized defect-ordered phase with a large unit cell and small crystallites, which may be illustrated by the metastable glacial phase of the fragile glass-former triphenylphosphite; the apparent polyamorphism might result from structural frustration, i.e., a competition between the tendency to extend the LPS and a global constraint that prevents tiling of the whole space by the LPS.

The techniques which allow the numerical evaluation of the statistical properties of the potential energy landscape for models of simple liquids are reviewed and critically discussed. Expressions for the liquid free energy and its vibrational and configurational components are reported. Finally, a possible model for the statistical properties of the landscape, which appears to describe correctly fragile liquids in the region where equilibrium simulations are feasible, is discussed.

We study the phenomenon of size segregation, observed in models of vibrated granular mixtures such as powders or sand. This consists of the de-mixing of the different components of the system under shaking. Several mechanisms have been proposed to explain this phenomenon. However, the criteria for predicting segregation in a mixture, an issue of great practical importance, are largely unknown. In the present paper we study a binary hard-sphere mixture under gravity on a three-dimensional lattice using Monte Carlo simulations. The vertical and horizontal segregation observed during the tap dynamics is interpreted in the framework of a statistical mechanics approach to granular media in the manner of Edwards. A phase diagram for the vertical segregation is derived, and compared with the simulation data.

In the past the coupling model (CM) was focused on the dynamics at long times, when all relaxing units of an interacting system participate in the terminal, cooperative Kohlrausch relaxation. No attention was paid to the dynamics at short times when all the relaxing units are caged, nor to intermediate times when an increasing number of them are no longer caged. We now extend the CM to also address the dynamics in these earlier time regimes. The crux of the extended CM is the quantitatively determinable independent relaxation time, from which the characteristics of the dynamics in the short-time and intermediate-time regimes can be deduced. This description of the evolution of the dynamics by the extended CM is supported by broadband dielectric relaxation spectra of two archetypal systems, the glassy/molten ionic conductors and supercooled liquids. In supercooled liquids, the 'universal' Johari–Goldstein β-relaxation provides evidence for the physical reality of the independent relaxation of the extended CM.

The momentum transfer dependence of the self-motion of main chain hydrogens in the α-relaxation regime of a glass forming polymer, polyisoprene, has been thoroughly investigated by a combined effort involving fully atomistic molecular dynamic simulations and quasielastic neutron scattering measurements. In this way, we have established the existence of a crossover from a Gaussian regime of sublinear diffusion to a strongly non-Gaussian regime at short distances. We show that an anomalous jump diffusion model with a distribution of jump lengths gives rise to such a crossover. This model leads to a time-dependent non-Gaussian parameter exhibiting all features revealed so far from various simulations of different glass forming systems.

Dielectric spectroscopy and temperature-modulated differential scanning calorimetry are employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) melts of different molecular weights confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). Moreover, the results obtained for the polymers are compared with those for the corresponding monomer. The experimental results are discussed in the framework of the interplay of confinement and adsorption effects. For large pore sizes (> 5 nm) a speeding up of the molecular dynamics is observed (the confinement effect), whereas for small pore sizes (< 5 nm) a slowing down of the segmental fluctuations is found (the adsorption effect). In addition, a minimal length scale for the glass transition of 1.6 nm is estimated for PPG confined in nanoporous glasses. This supports the idea that the molecular motions responsible for the glassy dynamics must be describable by a characteristic length scale.

Polarized Raman scattering investigations are carried out to investigate the conformations of the macromolecules inside the pores. These experiments show that the confined polymer chains are locally stretched. This effect increases with decreasing pore size.

The miscibility of poly(methyl acrylate)–poly(methyl methacrylate) sequential interpenetrating polymer networks (IPNs) has been studied by probing the conformational mobility of the component polymer chains. These IPNs exhibit the phenomenon of forced compatibilization. In a conventional heating differential scanning calorimetry (DSC) thermogram, the highly cross-linked IPN shows a single glass transition which covers a temperature interval of around 100^° C; in contrast, loosely cross-linked IPNs show two glass transitions. The conformational mobility in these IPNs is studied by subjecting them to isothermal annealings at temperatures in the region of the glass transition and below it. The DSC scans measured after these treatments allow one to determine the temperature interval in which the sample is out of thermodynamic equilibrium but keeps enough conformational mobility to relax during the isothermal annealing in such a way that the enthalpy loss is measurable with the sensitivity of a conventional DSC. The results allow one to reach some conclusions about the compositional distribution of the IPN on the nanometre scale.

The fluctuation–dissipation relation is measured on the dielectric properties of a gel (Laponite) and of a polymer glass (polycarbonate). For the gel it is found that during the transition from a fluid-like to a solid-like state the fluctuation–dissipation theorem is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around 1 Hz it may persist for several hours. A very similar behaviour is observed in polycarbonate after a quench below the glass transition temperature. In both cases the origin of this violation is a highly intermittent dynamics characterized by large fluctuations. The relevance of these results for recent models of ageing is discussed.

We studied the relaxation processes of glycerol by a transient grating experiment with polarization selectivity and optical heterodyne detection. The density response of supercooled glycerol, for a wavevector q = 0.63 μm⁻¹, has been studied in that temperature range (T = 200– 340 K), where the rearrangement of the structure (α-relaxation) and thermal diffusion occur on the same timescale. A strong interaction between the two modes is observed, manifesting in a dip in the T dependence of the apparent thermal conductivity and in a flattening of the apparent α-relaxation time upon cooling. A parameter-free thermo-hydrodynamic model for the long-time response is developed. The model is capable of quantitatively reproducing the data and of explaining the observed phenomenology.

The ageing of various glass forming materials (polycarbonate, polystyrene, polyvinyl acetate and triphenylethene) has been studied by differential scanning calorimetry. The variation of the relaxation enthalpy ΔH with the ageing time measured in a large domain of time is compared to the variation of volume ΔV reported in the literature. Simple and complex thermal treatments (memory effect) can be distinguished.

In simple thermal treatments (annealing at a unique temperature) the following can be concluded.

(a) The glass at equilibrium is a liquid: the relaxation time τ_f to reach equilibrium, the saturation of enthalpy, is the time predicted by the Vogel–Fulcher–Tamman (VFT) law and the heat capacity is equal to that of the liquid.

(b) The kinetic laws giving the enthalpy, ΔH = k_H log t, and the volume, ΔV = k_V log t, are observed between the initial τ_i (H) and τ_i*(V) and final times τ_f (H) and τ_f* (V). For PS and PVAc we show that the times obtained by these two techniques are comparable, τ_i = τ_i* and τ_f = τ_f*. The initial relaxation times (or incubation times) follow the Arrhenius law and the final times (or equilibrium times) verify the VFT law. It is shown that for polymers the experimental slopes k_H and k_V can be deduced from this VFT (WLF) law, k_H = C₂ ΔC_p /C₁ and k_V = C₂ Δα/C₁, C₁ and C₂ being the coefficients of the WLF equation and ΔC_p and Δα being the jumps of the heat capacity and of the expansion coefficient at T_g.

In complex thermal treatments (annealing at two different temperatures) the enthalpy relaxation is compared to the volume relaxation (the memory effects) studied by Kovacs and Struik. In any case the kinetics presents two regimes, which defines an equivalent time t_eq; this is the annealing time necessary to erase the previous thermal history of the glass; the origin of this time is discussed. Above t_eq the kinetics is not different from that of simple treatments; below t_eq no variation of ΔH is observed.

The enthalpy recovery mechanism of a low molecular weight synthesis of polymethylmethacrylate is investigated by means of differential scanning calorimetry (DSC) experiments. The experimental results can be described satisfactorily in terms of the Tool–Narayanaswamy–Moynihan theory. This work is mainly focused on developing a strategy for evaluation of the best set of parameters for the model. The approach starts with a simultaneous fitting procedure of several experimental DSC traces. Sets of parameters are obtained which exhibit agreement with experiments. The enthalpy lost on ageing of the sample in the glassy state as a function of the annealing time is then compared with the predictions provided by using the different sets of parameters. We show that this procedure is able to single out the best set of parameters and to provide a good estimation of the Adam–Gibbs temperature.

The potential energy landscape in the Kob–Andersen Lennard-Jones binary mixture model has been studied carefully from the liquid down to the supercooled regime, from T = 2 down to 0.46. One thousand independent configurations along the time evolution locus have been examined at each temperature investigated. From the starting configuration, we searched for the nearest saddle (or quasi-saddle) and minimum of the potential energy. The vibrational densities of states for the starting and the two derived configurations have been evaluated. Besides the number of negative eigenvalues of the saddles other quantities show some signature of the approach of the dynamical arrest temperature.

We study the dynamics in a simple hierarchical energy landscape. We compare a straightforward analytical approximation with the results of Monte Carlo simulations. The model is devised to mimic some aspects of the dynamics in supercooled liquids. We show that the concept of metabasins, as recently discussed in the framework of the potential energy landscape of glasses, emerges quite naturally.

For soft spheres the number of minima on the potential energy landscape is independent of the volume, and the shape of each basin in the potential energy landscape varies with volume in a predictable way. Two simple assumptions, (1) that the basins are harmonic and (2) that the distribution of basin depths is Gaussian, yield a model for the cold dense states of matter: crystals, glasses, the supercooled fluid and the glass transition. The model agrees with simulation data at temperatures below freezing but fails above the freezing temperature, where the harmonic approximation breaks down.

The dynamical behaviours of glass-forming liquids have been analysed extensively via computer simulations of model liquids, among which the Kob–Andersen binary Lennard-Jones mixture has been a widely studied system. Typically, studies of this model have been restricted to temperatures above the mode coupling temperature. Preliminary results concerning the dynamics of the Kob–Andersen binary mixture are presented at temperatures that extend below the mode coupling temperature, along with properties of the local energy minima sampled. These results show that a crossover in the dynamics occurs alongside changes in the properties of the inherent structures sampled. Furthermore, a crossover is observed from non-Arrhenius behaviour of the diffusivity above the mode coupling temperature to Arrhenius behaviour at lower temperatures.

The high-frequency dynamics of the molecular glass-former propylene carbonate has been investigated by the use of inelastic x-ray scattering (IXS) spectroscopy. The inelastic contribution to the spectra was analysed with a damped harmonic oscillator model. Up to a momentum transfer of Q_c ≃ 5 nm⁻¹ and an energy of Ω_c ≃ 8 meV, acoustic-like excitations with a Q²-dependence of the linewidth and a linear dispersion, consistent with the low-frequency sound velocity as measured by means of Brillouin light scattering, were found. At Q_c, a clear change in the excitation behaviour is found, which is characterized by a bending of the dispersion and a deviation from the Q²-behaviour of the linewidth. A comparison with reported IXS data for other glass-formers reveals a common tendency to bend upon approaching a pseudo-Brillouin zone boundary defined at Q_FSDP /2. The comparison also shows that the energy of the acoustic excitations, relative to that of the boson peak (Ω/E_BP), extends to larger values for glasses without a strong directional bonding topology.

The improvement in performance of the inelastic x-ray scattering technique has led to accurate determination of the dynamic structure factors of glassy and liquid glycerol, in the region of exchanged momentum Q = 2– 23 nm⁻¹ and in the temperature range 80–570 K. Thanks to the improved statistical accuracy, it has been possible to identify, in the spectra at the lower temperatures, besides the propagating longitudinal mode, a second non-Q-dispersing peak at ℏΩ_T ≈ 8.5 meV. We interpret this peak as a signature of the transverse dynamics that, in topologically disordered systems, acquires a longitudinal symmetry component. This assignment is substantiated by the observation that this peak still survives, across the glass transition, in the liquid state, and vanishes when the structural relaxation time τ_α approaches Ω_T⁻¹: a behaviour consistent with the condition τ_α Ω_T ≫ 1 required for the existence of a non-relaxational transverse-like dynamics in the liquid state.

Spectroscopic results on low-frequency excitations of densified silica are presented and related to characteristic thermal properties of glasses. The end of the longitudinal acoustic branch is marked by a rapid increase of the Brillouin linewidth with the scattering vector. This rapid growth saturates at a crossover frequency Ω_co which nearly coincides with the centre of the boson peak. The latter is clearly due to additional optic-like excitations related to nearly rigid SiO₄ librations as indicated by hyper-Raman scattering. Whether the onset of strong scattering is best described as resulting from hybridization of acoustic modes with these librations, from their elastic scattering (Rayleigh scattering) on the local excitations, or from soft potentials remains to be established.

This special issue contains the Proceedings of the Third Workshop on Non-equilibrium Phenomena in Supercooled Fluids, Glasses and Amorphous Materials which was held in Pisa, Italy from 22 to 27 September 2002. This was the third of a series of workshops on this theme which started in 1995 as a joint initiative of the University of Pisa and of the Scuola Normale Superiore di Pisa. The history of the subject goes back to at least the times of J C Maxwell, whose basic formula τ = η/G_∞ has been taken as the symbol of these workshops.

The dynamical processes occurring in supercooled fluids and other disordered materials in non-equilibrium states involve different length and time scales. Fast and ultra-slow molecular rearrangements occur in the presence of microscopic, mesoscopic and macroscopic organization of the system. These phenomena are of particular relevance in several branches of science, technology and engineering, and their characterization appeals to a variety of experimental techniques and to simulational and theoretical studies. The underlying conceptual unity of the field provides a common ground for the scientific community working in its various areas.

As for the earlier workshops on the same theme, the main objective of the third workshop was to bring together liquid state physicists, chemists and engineers to comparatively discuss experimental facts and theoretical predictions. In this edition the main themes included topics such as the phenomena of ageing, relaxation processes in materials ranging from systems of low molecular weight and oligomers to polymers, the dependence of relaxation processes on material thickness, the behaviour of multicomponent systems, and several others. Special attention was devoted to features and potentialities for applications of disordered systems. Round table discussions covered two presently hot topics: `Recent developments in potential energy landscape description of slow dynamics' and `Vibrational excitations in glasses at terahertz frequencies'.

The core of the workshop was a set of general lectures followed by more specific presentations on current issues in the main areas of the field. These were covered in sessions devoted sequentially to viscous flow and microscopic relaxation, theoretical aspects, numerical simulation, rheology, glassy materials, confined materials, heterogeneities and energy landscape, polymers, and ageing. Three poster sessions further dealt with the above subjects. The structure of the workshop has been preserved in this publication, in which the set of papers from the plenary lectures is followed by topical contributions in the order in which they were presented. Five contributions to the proceedings come from a selection of the posters. The proceedings end with the round table contributions.

We wish here to thank all participants in the workshop, who made it enjoyable and scientifically profitable, and all referees who worked in preparing this special issue of the Journal of Physics: Condensed Matter. We also wish to acknowledge the generous support that we have received from our institutions and the sponsorship and support given by the Istituto Nazionale di Fisica della Materia and by its section Fisica dei Liquidi e Sistemi Disordinati.

The Cassa di Risparmio di Pisa hosted the workshop in the splendid location provided by the Convento delle Benedettine. On the behalf of the international scientific community we express our gratitude to all these institutions as well as to those individuals-we mention here in particular Dr Massimo Faetti, Dr Diego Palazzuoli, Dr Fabio Zulli, Ms Beatrice Pistoresi and Ms Patrizia Pucci-who have contributed to organizing and running the workshop.