Fragile Glass Research Articles

Linking rigidity transitions with enthalpic changes at the glass transition and fragility:insight from a simple oscillator model

A low temperature Monte Carlo dynamics of a Keating-like oscillator model is used tostudy the relationship between the nature of network glasses from the viewpoint of rigidity,the thermal reversibility during the glass transition and the strong–fragile behaviour ofglass-forming liquids. The model shows that a Phillips optimal glass formation withminimal enthalpic changes is obtained under a cooling/annealing cycle when the system isoptimally constrained by the harmonic interactions, i.e. when it is isostatically rigid. Forthese peculiar systems with a nearly reversible glass transition, the computed activationenergy for relaxation time shows also a minimum, which demonstrates that isostaticallyrigid glasses are strong (Arrhenius-like) glass-forming liquids. Experiments onchalcogenide and oxide glass-forming liquids are discussed under this new perspectiveand confirm the theoretical prediction for chalcogenide network glasses whereaslimitations of the approach appear for weakly interacting (non-covalent, ionic) systems.

A Comparison of the Dynamical Relaxations in a Model for Glass Transition in Polymer Nanocomposites and Polymer Thin Films

We present the results of a facilitated kinetic Ising model based investigation of the confinement and filler-induced changes in the glass transition, fragility, and aging dynamics of a model system. This work is motivated by the issue of relationships, if any, in the glass transition phenomena of polymer nanocomposites and polymer thin films. Our results suggest that addition of plasticizing fillers lower the Tg and increase the fragility of the system, while antiplasticizing fillers have an opposite effect. Confinement arising in freely suspended films of both filled and unfilled systems were seen to reduce the overall Tg of the system. Confinement and filler-induced shifts in the Tg were not found to be quantitatively equivalent when compared at the appropriate film thicknesses and interparticle spacings. However, the fragilities exhibited a trend that suggests a closer quantitative equivalence between the PNCs and thin films. In all the cases we examined, the aging dynamics and nonequilibrium relaxati...

Free volume from positron lifetime and pressure–volume–temperature experiments inrelation to structural relaxation of van der Waals molecular glass-forming liquids

Positron annihilation lifetime spectroscopy (PALS) is employed to characterizethe temperature dependence of the free volume in two van der Waals liquids:1, 1′-bis(p-methoxyphenyl)cyclohexane(BMPC) and 1, 1′-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC). From the PALSspectra analysed with the routine LifeTime9.0, the size (volume)distribution of local free volumes (subnanometer size holes), its mean,⟨vh⟩, and meandispersion, σh, were calculated. A comparison with the macroscopic volume from pressure–volume–temperature (PV T) experiments delivered the hole density and the specific hole free volumeand a complete characterization of the free volume microstructurein that sense. These data are used in correlation with structural (α)relaxation data from broad-band dielectric spectroscopy (BDS) in terms of the Cohen–Grest andCohen–Turnbull free volume models. An extension of the latter model allows us to quantifydeviations between experiments and theory and an attempt to systematize these in terms ofTg or of the fragility. The experimental data for several fragile and less fragile glass formers areinvolved in the final discussion. It was concluded that, for large differences in the fragilityof different glass formers, the positron lifetime mirrors clearly the different character ofthese materials. For small differences in the fragility, additional properties like the characterof bonds and chemical structure of the material may affect size, distribution and thermalbehaviour of the free volume.

Glass transition and fragility in the simple molecular glassformer CS2 from CS2–S2Cl2 solution studies

With an interest in finding the fragility for a simple, single component, molecular glassformer, we have determined the dielectric relaxation and glass transition behavior for a series of glasses in the CS(2)-S(2)Cl(2) and CS(2)-toluene systems. Crystallization of CS(2) can be completely avoided down to the composition 20 mol% second component, and the fragility proves almost independent of CS(2) content in each system. Since the glass temperature T(g) obtained from both thermal studies and from dielectric relaxation (using T(g,diel)=T(tau=100 s)) is quite linear over the whole composition range in each system, and since relaxation time data for pure CS(2) fall on the same master plot when scaled by the linearly extrapolated T(g) value, we deduce that pure CS(2) has the same high fragility as the binary solutions. The value is m=86, as for ortho-terphenyl (OTP). Based on observations of independent studies for the vibrational density of states (VDoS) (of inherent structures for OTP and instantaneous, at-temperature structures for CS(2)), we attribute the high fragility to an excess vibrational heat capacity (defined by C(p) (vib, excess)=dS(vib, excess)/d ln T) originating in the behavior of the low frequency modes of the VDoS (the boson peak modes). Both low frequency DoS and anharmonicity increase with increasing temperature, augmenting the configurational entropy drive to the top of the system energy landscape. The surprising implication is that fragility is determined in the vibrational, not configurational, manifold of microstates.

Pressure dependence of the fragility in metallic glass forming liquids: Predictions from a theoretical model and materials properties correlations

By studying the pressure dependence of the fragility, it is expected that new horizons will open to understand the relaxation behaviour of supercooled liquids. However, as far as the authors are informed, no study has been performed on pressure dependence of the fragility in bulk metallic glass forming systems. In the present study, such dependence is investigated based on materials properties correlations. It is suggested that the effect of pressure on the fragility of bulk metallic glasses is very small. Our analysis indicates also that the fragility of metallic glasses will increase with the application of pressure. This behaviour contrasts with the behaviour observed in molecular systems, where the fragility remains almost constant or decreases slightly with the application of pressure. The result found is discussed in terms of the bond strength-coordination number fluctuation model of the viscosity.

Inhomogeneous Amorphous Structure of Bulk Metallic Glasses Examined from Structural Relaxation Kinetics

The kinetics of structural relaxation in fragile glass former, Pd46Cu35.5P18.5 BMG, and strong glass former, Zr50Cu40Al10 BMG, was investigated by volume relaxation. The former exhibited a relaxation phenomenon that is well understood by the local topological instability model, while the latter showed monotonous relaxation behavior over a wide range down to Tg-60 K. The discrepancy may be closely related to the difference in the fragility of both glasses.

Brillouin Light Scattering Study of Hypersonic Velocity Dispersion in Aqueous Sucrose Solutions

Sucrose (C12H22O11) is a nonreducing disaccharide comprising of one glucose ring (C6H12O6) and one fructose ring (C6H12O6) bridged through a C–O–C linkage. The physical properties of aqueous sucrose solutions have been intensively investigated over a long period of time. The temperature vs concentration (d in wt%) phase diagram of the sucrose-water system reveals a stable unsaturated liquid region with concentration below 60%, a metastable supercooled region between 60 and 95%, and a glass region above 95% at room temperature. Temperature studies of viscosity indicate that the sucrose-water system can be classified into a fragile glass former. Sugars as glass-forming materials have attracted some attention during the last two decades. The technological and biological importance of the glass transition in sugars, as a food, is now widely recognized, and intensive research activities are being directed towards understanding the glass transition in various sugar-water systems utilizing various experimental techniques. The acoustic properties of glass-forming liquids have been one of the important subjects of Brillouin light scattering (BLS) since the late 1960s. BLS studies on glucose solutions and trehalose solutions have been performed to determine the glass transition temperature TG. However, the acoustic dispersion of sugar solutions has not yet been fully investigated. We examined the acoustic dispersion of sucrose solutions by means of BLS as a function of d up to 79% at 23 C. Unsaturated solutions below d 60% can be prepared at 23 C by dissolving granulated sugar into pure water. For metastable 70 and 79% solutions, granulated sugar was solved into water at 50 and 80 C, and then cooled to 23 C. The sucrose concentration was determined using a conventional refractometer. BLS spectra were probed using the 488 nm line (1⁄4 0) from an Arþ laser in a single cavity mode with an output power of 15mW. Backscattered BLS spectra were obtained using a (3þ 3)-pass tandem Fabry– Perot interferometer. The depolarized BLS component is more than two orders of magnitude weaker than the polarized component. Figure 1 shows the development of the BLS spectrum normalized by spectrum accumulation time as a function of d of up to 70%. Strong modification of the BLS spectrum through viscoelastic coupling between the sound wave and a relaxation mode can be clearly seen above d 1⁄4 40%. The observed BLS spectrum Oðq; !Þ can be written as

Correlation of dynamic and quasistatic relaxations: The Cox–Merz rule for metallic glass

The correlation of quasistatic and dynamic relaxations was discussed in a typical strong Zr55Al10Ni5Cu30 metallic glass from room temperature to Tg. The quasistatic relaxation behavior, investigated by high temperature compressive testing at a constant strain rate, was compared with dynamic tensile relaxation behavior. A correlation equation of the dynamic frequency and quasistatic strain rate was successfully deduced, and then its validity was experimentally confirmed in a fragile metallic glass. Using this correlation, the Cox–Merz rule, derived for correlating the steady-state and dynamic viscosities of the polymers, is found to be applicable to metallic glasses.

Relaxation map of a 100% green thermoplastic film. Glass transition and fragility

A 100% green thermoplastic obtained by extrusion of a mixture of wheat flour and plasticizers has been realized. The existence of two vitreous phases in this 100% green thermoplastic film has been pointed out by means of calorimetric measurements ( T g =−56 °C and T g =10 °C) and confirmed by electron microscopy. The molecular dynamics have been investigated by means of DMA measurements for temperature domains in the vicinity of the glass transition of each phase. We show that each phase exhibits a molecular dynamic characteristic of a fragile glass liquid former. The size of the cooperative domain engaged in the relaxation processes have also been estimated and we show that this new green thermoplastic exhibits relaxation mechanisms as expected for a conventional thermoplastic, as for instance a PMMA.

Reversible giant photocontraction in chalcogenide glass

It is shown that chalcogenide glasses with suitably underconstrained network can undergo reversible giant photocontractions up to a micron depth. These effects result from the combination of two attributes particular to these glasses, (i) the high photosensitivity characteristic of low coordination floppy networks and (ii) the wide window of structural configuration characteristic of fragile glass former. Interestingly these effects are reversible and subsequent irradiation with high intensity results in giant photoexpansion in the same glass. The combination of subsequent photocontraction and photoexpansion on the same glass surface has good potential for the design of complex optical elements.

Universality in multicomponent glass-forming liquids near the glass transition

The slow dynamics of a single particle in multicomponent glass-forming systems including fragile and strong glasses is studied from a unified point of view. The simulation results on two different systems, bulk glass-forming Cu60Ti20Zr20 melt and network-forming SiO2 , melt are analyzed by the mean-field theory (MFT) recently proposed and are compared with other systems near the glass transition. It is shown that the simulation results for the mean-square displacement are all collapsed into a master curve given by MFT if a long-time self-diffusion coefficient has the same value in each system. It is also shown that each long-time self-diffusion coefficient is described well by a singular function predicted recently from first principles. Thus, we conclude that there exists a simple universal mechanism near the glass transition even among any diversely different glass-forming systems.

Professor Michael J. Pikal: Scientist, family man, educator, colleague and friend

Professor Michael J. Pikal: Scientist, family man, educator, colleague and friend

Composition dependence of glass transition temperature and fragility. II. A topological model of alkali borate liquids

Glass transition temperature and fragility are two important properties derived from the temperature dependence of the shear viscosity of glass-forming melts. While direct calculation of these properties from atomistic simulations is currently infeasible, we have developed a new topological modeling approach that enables accurate prediction of the scaling of both glass transition temperature and fragility with composition. A key feature of our approach is the incorporation of temperature-dependent constraints that become rigid as a liquid is cooled. Using this approach, we derive analytical expressions for the composition (x) dependence of glass transition temperature, T(g)(x), and fragility, m(x), in binary alkali borate systems. Results for sodium borate and lithium borate systems are in agreement with published values of T(g)(x) and m(x). Our modeling approach reveals a natural explanation for the presence of the constant T(g) regime observed in alkali borate systems.

Self-diffusion in multi-component glass-forming systems

Self-diffusion in multi-component glass-forming systems including fragile and strong glasses is studied from a unified viewpoint. A simple analytic form of long-time self-diffusion coefficient D S L is proposed. The equations for the mean-square displacements recently derived for two types of systems, (S) suspensions of colloids and (M) molecular systems, from a first principle by employing the Tokuyama–Mori projection-operator method are used to define D S L in each type formally, both of which are uniquely determined by the correlation function of the fluctuating forces. Analyses of the correlation functions in two types in terms of many-body interactions thus lead in type (M) to D S L ( λ ) ≃ κ − 1 ( λ c / λ ) ( 1 − λ / λ c ) 2 , where λ is a control parameter, such as an inverse temperature and a volume fraction. Here κ is simply written in terms of the potential parameters and λ c an adjustable parameter to be determined. The predictions for λ dependence of D S L in multi-component glass-forming systems are in excellent agreement with available experimental data and simulation results in equilibrium states.

Composition dependence of glass transition temperature and fragility. I. A topological model incorporating temperature-dependent constraints

We present a topological model for the composition dependence of glass transition temperature and fragility. Whereas previous topological models are derived for zero temperature conditions, our approach incorporates the concept of temperature-dependent constraints that freeze in as the system is cooled from high temperature. Combining this notion of temperature-dependent constraints with the Adam-Gibbs model of viscosity, we derive an analytical expression for the scaling of glass transition temperature and fragility in the binary Ge(x)Se(1-x) system. In the range of 0<or=x<or=1/3, we reproduce the modified Gibbs-DiMarzio equation of Sreeram et al. [J. Non-Cryst. Solids 127, 287 (1991)] but without any empirical fitting parameters. The modified Gibbs-DiMarzio equation breaks down for 1/3<x<or=2/5, where the glass transition temperature decreases with increasing germanium content.

Corresponding States of Structural Glass Formers

The variation with respect to temperature T of transport properties of 58 fragile structural glass-forming liquids (67 data sets in total) are analyzed and shown to exhibit a remarkable degree of universality. In particular, super-Arrhenius behaviors of all supercooled liquids appear to collapse to one parabola for which there is no singular behavior at any finite temperature. This behavior is bounded by an onset temperature T(o) above which liquid transport has a much weaker temperature dependence. A similar collapse is also demonstrated over the smaller available range for existing numerical simulation data.

Compact-slab radio-frequency discharge CO laser

Carbon monoxide (CO) lasers1 are very attractive sources of mid-IR radiation for laser spectroscopy and multicomponentgas analysis. The spectral range (2.5–4.2μm) of first-overtone (FO) CO lasers (characterized by vibrational transitions from V + 2 → V) covers those of the well-established hydrogen fluoride and deuterium fluoride lasers, but with significantly smaller vibrational-rotational line spacings. Their full output spectra may contain more than 400 emission lines,2 many of which coincide with absorption lines exhibited by a wide range of organic and inorganic materials such as, H2O, CO2, CH4, NO2, NO, SO2, acetone, benzene, andmethanol. FO CO lasers operating at these wavelengths can thus be used to assess the impact of resonances on various media in fields as diverse as, nonlinear spectroscopy, atmospheric remote sensing, and laser chemistry. FO CO lasers are among the best devices for laser-based spectroscopic analysis of multicomponent-gas mixtures.3 In addition, the longerwavelength section of their output spectrum coincides with the atmospheric ‘transparency window.’ Successful applications require the development of compact, sealed-off FO CO lasers. Present-generation FO CO lasers use either fragile glass components, e.g., for continuous-wave (cw) gas-flow high-voltage DC discharge, or very complicated, largescale designs (e.g., for high-voltage electron-beam-sustained discharge or supersonic cooling), and are not suitable for integration in compact multiwavelength-laser gas analyzers.3 One of the most promising approaches to producing compact gas lasers uses radio-frequency (RF) discharge in a slab geometry to pump an activemedium.We developed a cryogenically cooled slab RFdischarge CO laser4, 5 (see Figure 1). Significant operational advantages of our FOCO laser include its slab RF-discharge pumping operation, robust compact stainless-steel design, sealedoff performance, cryogenic cooling of the electrode system Figure 1. Slab radio-frequency discharge first-overtone carbon monoxide (CO)-laser setup. (a) General view. (b) Open view.

Art Forms in Nature EXAMINATION AND CONSERVATION OF A BLASCHKA GLASS MODEL OF THE PROTOZOAN AULOSPHAERA ELEGANTISSIMA

The paper describes the examination and conservation treatment of a glass model of a micro-organism made by the German glass artisans Leopold and Rudolf Blaschka in the late nineteenth century. The beautiful object is part of a collection held at the Natural History Museum in London and has been chosen for priority conservation due to its relatively poor condition. The model is assembled from several hundred parts with the help of organic materials. Major conservation issues identified were breakages and detachments of extremely fragile glass spines, the partially unstable glass composition and consequent deterioration, the presence of a water-soluble organic coating and inappropriate previous storage conditions. Characterization of the original materials and their deterioration processes was carried out using a range of analytical facilities, allowing the development of suitable treatments. These included a two-stage cleaning process, reattachment of broken spines with Paraloid™ B72 and reversible remounting of fragments using nylon micro-tubing.

Dynamic glass transition and fragility of lithium borate binary glass

The compositional variations of the fragility of lithium borate binary glasses have been investigated over a wide composition range up to 64 mol% of Li 2O. The frequency dependences of the dynamic heat capacity C p( ω) above the dynamic glass transition temperature T g have been observed by Temperature-Modulated DSC. The fragility or steepness index m has been determined. The composition dependence has been discussed on the basis of the fragility model proposed by Vilgis, and it has been revealed that the trends of the fragility and the fluctuation of the coordination number of boron atom have the correlation over a wide composition range. The result indicates the origin of the fragility relates to the degree of the fluctuation of the coordination number. The formation of the 4-coordinated boron atom plays the dominant role in the fragility of the borate glassy system.

Connections of activated hopping processes with the breakdown of the Stokes-Einstein relation and with aspects of dynamical heterogeneities

We develop an extended version of the mode-coupling theory (MCT) for glass transition, which incorporates activated hopping processes via the dynamical theory originally formulated to describe diffusion-jump processes in crystals. The dynamical-theory approach adapted here to glass-forming liquids treats hopping as arising from vibrational fluctuations in the quasiarrested state where particles are trapped inside their cages, and the hopping rate is formulated in terms of the Debye-Waller factors characterizing the structure of the quasiarrested state. The resulting expression for the hopping rate takes an activated form, and the barrier height for the hopping is "self-generated" in the sense that it is present only in those states where the dynamics exhibits a well defined plateau. It is discussed how such a hopping rate can be incorporated into MCT so that the sharp nonergodic transition predicted by the idealized version of the theory is replaced by a rapid but smooth crossover. We then show that the developed theory accounts for the breakdown of the Stokes-Einstein relation observed in a variety of fragile glass formers. It is also demonstrated that characteristic features of dynamical heterogeneities revealed by recent computer simulations are reproduced by the theory. More specifically, a substantial increase of the non-Gaussian parameter, double-peak structure in the probability distribution of particle displacements, and the presence of a growing dynamic length scale are predicted by the extended MCT developed here, which the idealized version of the theory failed to reproduce. These results of the theory are demonstrated for a model of the Lennard-Jones system, and are compared with related computer-simulation results and experimental data.