Fragile Glass Formers Research Articles

Ultrafastα-Like Relaxation of a Fragile Glass-Forming Liquid Measured Using Two-Dimensional Infrared Spectroscopy

Ultrafast two-dimensional infrared (2D-IR) spectroscopy is used to study the picosecond dynamics of a vibrational probe molecule dissolved in a fragile glass former. The spectral dynamics are observed as the system is cooled to within a few degrees of the glass transition temperature (T(g)). We observe nonexponential relaxation of the frequency-frequency correlation function, similar to what has been reported for other dynamical correlation functions. In addition, we see evidence for α-like relaxation, typically associated with long-time, cooperative molecular motion, on the ultrafast time scale. The data suggests that the spectral dynamics are sensitive to cooperative motion occurring on time scales that are necessarily longer than the observation time.

Coherent dynamics of meta-toluidine investigated by quasielastic neutron scattering

The coherent dynamics of a typical fragile glass former, meta-toluidine, was investigated at the molecular level using quasielastic neutron scattering, with time-of-flight and neutron spin echo spectrometers. It is well known that the static structure factor of meta-toluidine shows a prepeak originating from clustering of the molecules through hydrogen bonding between the amine groups. The dynamics of meta-toluidine was measured for several values of the wavevector transfer Q, which is equivalent to an inverse length scale, in a range encompassing the prepeak and the structure factor peak. Data were collected in the temperature range corresponding to the liquid and supercooled states, down to the glass transition. At least two dynamical processes were identified. This paper focuses on the slowest relaxation process in the system, the α-relaxation, which was found to scale with the macroscopic shear viscosity at all the investigated Q values. No evidence of "de Gennes" narrowing associated with the prepeak was observed, in contrast with what happens at the Q value corresponding to the interparticle distance. Moreover, using partially deuterated samples, the dynamics of the clusters was found to be correlated to the single-particle dynamics of the meta-toluidine molecules.

Locally preferred structures and many-body static correlations in viscous liquids

The influence of static correlations beyond the pair level on the dynamics of selected model glass formers is investigated. The pair structure, angular distribution functions, and statistics of Voronoi polyhedra of two well-known Lennard-Jones mixtures as well as of the corresponding Weeks-Chandler-Andersen variants, in which the attractive part of the potential is truncated, are compared. By means of the Voronoi construction, the atomic arrangements corresponding to the locally preferred structures of the models are identified. It is found that the growth of domains formed by interconnected locally preferred structures signals the onset of the slow-dynamics regime and allows the rationalization of the different dynamic behaviors of the models. At low temperature, the spatial extension of the structurally correlated domains, evaluated at fixed relaxation time, increases with the fragility of the models and is systematically reduced by truncating the attractions. In view of these results, proper inclusion of many-body static correlations in theories of the glass transition appears crucial for the description of the dynamics of fragile glass formers.

Relaxation dynamics and ionic conductivity in a fragile plastic crystal

We report a thorough characterization of the dielectric relaxation behavior and the ionic conductivity in the plastic-crystalline mixture of 60% succinonitrile and 40% glutaronitrile. The plastic phase can be easily supercooled and the relaxational behavior is investigated by broadband dielectric spectroscopy in the liquid, plastic crystalline, and glassy crystal phases. The α-relaxation found in the spectra is characterized in detail. A well-pronounced secondary and faint indications for a third relaxation process were found. The latter most likely is of Johari-Goldstein type. From the temperature dependence of the α-relaxation time, a fragility parameter of 62 was determined. Thus, together with Freon112, this material stands out among all other plastic crystals by being a relatively fragile glass former. This finding provides strong support for an energy-landscape related explanation of the fragility of glass formers. In addition, unusually strong conductivity contributions were detected in the spectra exhibiting the typical features of ionic charge transport making this material a good basis for solid-state electrolytes.

The Role of Chain Length in Nonergodicity Factor and Fragility of Polymers

The mechanism that leads to different fragility values upon approaching the glass transition remains a topic of active discussion. Many researchers are trying to find an answer in the properties of the frozen glassy state. Following this approach, we focus here on a previously proposed relationship between the fragility of glass-formers and their nonergodicity factor, determined by inelastic X-ray scattering (IXS) in the glass. We extend this molecular liquid study to two model polymers; polystyrene (PS) and poly- isobutylene (PIB);for which we change the molecular weight. Polymers offer the opportunity to change the fragilitywithoutalteringthechemicalstructure,justbychangingthechainlength.Thus,wespecificallychose PS and PIB because they exhibit opposite dependences of fragility with molecular weight. Our analysis for thesetwopolymersrevealsnouniquecorrelationbetweenthefragilityandthenonergodicityparameter.Even after a recently suggested correction for a possible contribution of the β relaxation, the correlation is not restored. We discuss possible causes for the failure of the fragility-nonergodicity factor correlation, emphasizing the features that are specific to polymers. We speculate that polymers might have specific contributions to fragility related to the chain connectivity that are absent in nonpolymeric systems.

Molecular Dynamics and Physical Stability of Amorphous Anti-Inflammatory Drug: Celecoxib

By using dielectric spectroscopy we analyzed the relation between molecular mobility and tendency of the amorphous celecoxib to recrystallize. We found that celecoxib is kinetically a fragile glassformer, contrary to the conclusion reached by others from thermodynamic fragility. The possible correlation of the large tendency of celecoxib to crystallize with various molecular motions have been investigated. Our study shows that the structural relaxation seems to be responsible for devitrification of celecoxib if stored at room temperature ∼293 K. Notwithstanding, the crystallization can be considered to ultimately be affected by the β-process (JG-relaxation) because it is the precursor of the structural α-relaxation.

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.

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

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.

Dynamics and energy landscape in a tetrahedral network glass-former: direct comparisonwith models of fragile liquids

We report molecular dynamics simulations for a new model of tetrahedral networkglass-former, based on short-range spherical potentials. Despite the simplicity of theforcefield employed, our model reproduces some essential physical properties of silica, anarchetypal network-forming material. Structural and dynamical properties, includingdynamic heterogeneities and the nature of local rearrangements, are investigated in detailand a direct comparison with models of close-packed, fragile glass-formers is performed.The outcome of this comparison is rationalized in terms of the properties of the potentialenergy surface, focusing on the unstable modes of the stationary points. Our resultsindicate that the weak degree of dynamic heterogeneity observed in network glass-formersmay be attributed to an excess of localized unstable modes, associated with elementarydynamical events such as bond breaking and reformation. In contrast, the morefragile Lennard-Jones mixtures are characterized by a larger fraction of extendedunstable modes, which lead to a more cooperative and heterogeneous dynamics.

Brillouin light scattering study of glassy sorbitol

Brillouin light scattering has been used to investigate the density fluctuations of the fragile glass-former sorbitol in the GHz frequency range and at temperatures between 20 and 294 K. Longitudinal acoustic sound velocity and attenuation data are here reported and discussed. These data are used to test the existence of a correlation between elastic properties of glasses and viscous properties of the corresponding deeply undercooled melts above the glass transition temperature, T g . In particular, and as for other fragile systems, the ratio of the vibrational to the relaxational compressibility at T g is found to be basically independent of fragility.

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.

Molecular Motions in Amorphous Ibuprofen As Studied by Broadband Dielectric Spectroscopy

The molecular mobility of amorphous ibuprofen has been investigated by broadband dielectric relaxation spectroscopy (DRS) covering a temperature range of more than 200 K. Four different relaxation processes, labeled as alpha, beta, gamma, and D, were detected and characterized, and a complete relaxation map was given for the first time. The gamma-process has activation energy E a = 31 kJ.mol (-1), typical for local mobility. The weak beta-relaxation, observed in the glassy state as well as in the supercooled state was identified as the genuine Johari-Goldstein process. The temperature dependence of the relaxation time of the alpha-process (dynamic glass transition) does not obey a single VFTH law. Instead two VFTH regimes are observed separated by a crossover temperature, T B = 265 K. From the low temperature VFTH regime, a T g (diel) (tau =100 s) = 226 K was estimated, and a fragility or steepness index m = 93, was calculated showing that ibuprofen is a fragile glass former. The D-process has a Debye-like relaxation function but the temperature dependence of relaxation time also follows the VFTH behavior, with a Vogel temperature and a pre-exponential factor which seem to indicate that its dynamics is governed by the alpha-process. It has similar features as the Debye-type process observed in a variety of associating liquids, related to hydrogen bonding dynamics. The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ibuprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.

Molecular Dynamics Simulation of the Energetic Room-Temperature Ionic Liquid, 1-Hydroxyethyl-4-amino-1,2,4-triazolium Nitrate (HEATN)

Molecular dynamics (MD) simulations have been performed to investigate the structure and dynamics of an energetic ionic liquid, 1-hydroxyethyl-4-amino-1,2,4-triazolium nitrate (HEATN). The generalized amber force field (GAFF) was used, and an electronically polarizable model was further developed in the spirit of our previous work (Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877). In the process of simulated annealing from a liquid state at 475 K down to a glassy state at 175 K, the MD simulations identify a glass-transition temperature region at around 250-275 K, in agreement with experiment. The self-intermediate scattering functions show vanishing boson peaks in the supercooled region, indicating that HEATN may be a fragile glass former. The coupling/decoupling of translational and reorientational ion motion is also discussed, and various other physical properties of the liquid state are intensively studied at 400 K. A complex hydrogen bond network was revealed with the calculation of partial radial distribution functions. When compared to the similarly sized 1-ethyl-4-methyl-1,4-imidazolium nitrate ionic liquid, EMIM+/NO3-, a hydrogen bond network directly resulting in the poorer packing efficiency of ions is observed, which is responsible for the lower melting/glass-transition point. The structural properties of the liquid/vacuum interface shows that there is vanishing layering at the interface, in accordance with the poor ion packing. The effects of electronic polarization on the self-diffusion, viscosity, and surface tension of HEATN are found to be significant, in agreement with an earlier study on EMIM+/NO3- (Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877).

Anharmonicity in a fragile glass-former probed by inelastic neutron scattering

Within the overall understanding of the glass transition, the relationship between microscopic dynamics and fragility is still to be clarified. Decalin is an organic glass former, for which a cis/trans mixture exhibits the highest known degree of fragility in a molecular system. It is therefore an ideal system for the investigation of microscopic dynamics in fragile systems. In the present study, the microscopic dynamics of pure cis-decalin has been measured by inelastic and quasi-elastic incoherent neutron scattering, giving the single particle self-correlation function. The fast relaxation dynamics and low-frequency vibrational modes are reported here. Both in the glass and in the crystal the vibrations show strong anharmonic behavior. In the glass phase, the short time microscopic dynamics evolve rapidly with temperature, however do not exhibit any significant change around the glass transition temperature T(g). The elastic intensity provides a measure of the mean square displacements which are comparable to those measured in other fragile glass formers, in particular, the archetypical fragile glass former orthoterphenyl. It appears that the microscopic relaxation gets unfrozen, relative to T(g), at much lower temperature than in other fragile systems.

Translational diffusion in sucrose benzoate near the glass transition: Probe size dependence in the breakdown of the Stokes-Einstein equation

The translational diffusion coefficient D(trans) for rubrene, 9,10-bis(phenylethynyl)anthracene (BPEA), and tetracene in the fragile molecular glass-former sucrose benzoate (SB) (Tg=337 K) was studied as a function of temperature from Tg+3 K to Tg+71 K by use of the holographic fluorescence recovery after photobleaching technique. The values of D(trans) vary by five to six orders of magnitude in this temperature range. Contrary to the predictions of the Stokes-Einstein equation, the temperature dependence of probe diffusion in SB over the temperature range of the measurements is weaker than that of T/eta, where eta is the shear viscosity. In going from the crossover temperature Tx approximately 1.2Tg to Tg, D(trans)eta/T increases by factors of 2.4+/-0.2 decades for rubrene, 3.4+/-0.2 decades for BPEA, and 3.8+/-0.4 decades for tetracene. The decoupling between probe diffusion in SB and viscosity is characterized by the scaling law D(trans) approximately T/eta(xi), with xi=0.621 for tetracene, 0.654 for BPEA, and 0.722 for rubrene. Data for probe diffusion in SB are combined with data from the literature for probe diffusion in ortho-terphenyl and alphaalphabeta-tris(naphthyl)benzene in a plot of enhancement versus the relative probe size parameter rho(m)=(m(p)m(h))(1/3), where m(p) and m(h) are, respectively, the molecular weights of the probe and host solvent. The plot clearly shows a sharp increase in enhancement of translational diffusion at rho(m) approximately 1. By applying temperature shifts, D(trans) for probe diffusion in SB and the dielectric relaxation time tau(D) can be superimposed on a single master curve based on the Williams-Landel-Ferry equation. This suggests that the dynamics of probe diffusion in SB is described by the scaling relationship D(trans) approximately 1/tau(D)(T+DeltaT), where tau(D)(T+DeltaT) is the temperature-shifted dielectric relaxation time. The results from this study are discussed within the context of dynamic heterogeneity in glass-forming liquids.

Enhanced diffusivity in supercooled liquids

The enhancement of self and probe diffusivity over the Stokes–Einstein values by orders of magnitude is observed in the viscous regime of polymers, molecular glass-formers and supercooled metallic systems. We investigate an alternative to identifying enhanced diffusivity with the fast components of the primary structural relaxation process: high translational mobility is being associated with cooperative dynamics in string-like topologies, which is responsible for fast diffusive particle motion and the slow secondary β-relaxation observed below the dynamic crossover temperature in fragile glass-formers. This approach provides a unifying view of diffusivity across the different classes of glass-forming materials.

Generalized excluded volume and the diffusivity and viscosity of supercooled liquids and glasses over the entire fragility spectrum

Transport properties of glass-formers near glass transition reflect the varying degrees of the sensitivity of the solid-like dynamics and structures with respect to temperature, depending on their fragility. Notably, however, most glasses resume Arrehenius transport behavior upon onset of vitrification. To address this phenomenon a theory of the self-diffusion coefficient and viscosity is developed on the basis of a model constructed for the generalized excluded volume of glass-formers described by the generic van der Waals equation of state. The molecular clustering behavior of a glass-former is exploited in terms of an order parameter that measures the concentration of glassy, clustered molecules, which is then related to the excluded volume. The formulas arrived therefrom are shown to excellently account for the self-diffusion coefficient and viscosity of various glass-formers over the entire fragility spectrum studied experimentally: e.g., GeO 2 , silica, ethanol, glycerol, diopside, propylene carbonate, o -terphenyl, tris-napthylbenzene, toluene, and so on. The excluded volume effect thus investigated is shown to essentially characterize the fragility of the glass-formers. The resulting theory not only predicts for fragile glass-formers to resume Arrehenius transport behavior upon the onset of the glass transition, but also explains a crossover between strong and fragile glass-formers in their diffusivity and viscosity profiles as vitrification sets in.

Correlation effects and super-Arrhenius diffusion in binary Lennard-Jones mixtures

An underlying Arrhenius temperature dependence of the diffusion constant can be extracted from the fragile, super-Arrhenius diffusion of a binary Lennard-Jones mixture. The Arrhenius form is found by considering time intervals and temperatures where diffusive motion is present but the system is nonergodic, with averages taken over an ergodic trajectory. The Arrhenius diffusion can be related to the true super-Arrhenius behavior by a factor that depends on the average angle between steps in successive time intervals. This correction factor accounts for the fact that on average, successive displacements are negatively correlated. This negative correlation can be linked directly with the higher apparent activation energy for diffusion in fragile glass formers at lower temperature. Arrhenius dependence of the diffusion constant for nonergodic systems is found over a range of time scales. As time intervals are increased, the time windows become long enough to register negative correlation and the correction decreases. The Arrhenius form changes with time as ergodicity is approached and this change can be characterized by the variation in gradient and intercept with time. We introduce a formalism to describe super-Arrhenius behavior based on Arrhenius and non-Arrhenius contributions to the diffusion constant.

Non-ergodicity in a locally ordered fragile glass former

We measure the dynamic structure factor of m-toluidine through inelastic X-ray scattering in the mesoscopic Q range between 1 and 10nm−1, where the static structure factor exhibits a prepeak resulting from a molecular spatial organization in nanometer size clusters, due to the formation of hydrogen bonds. We present experimental evidence of the square-root cusp in the temperature behavior of the non-ergodicity factor of m-toluidine which supports the view that the mode-coupling theory succeeds in describing the dynamics of supercooled molecular liquids, even for liquids with a local order, notwithstanding the cage formation is controlled by a mechanism different from the packing constraints typical of simple liquids. Moreover, we report on the behavior of the non-ergodicity factor in the low-temperature and wavevector regimes, finding a confirmation of the phenomenologic law proposed by Scopigno et al. [T. Scopigno, G. Ruocco, F. Sette, G. Monaco, Science 302 (2003) 849] which correlates the vibrational dynamics of the glass to the fragility of the supercooled liquid.