Fragile Glass Formers Research Articles

Structural Relaxation in Supercooled Water by Time‐Resolved Spectroscopy.

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Structural relaxation in supercooled water by time-resolved spectroscopy.

Water has many kinetic and thermodynamic properties that exhibit an anomalous dependence on temperature, in particular in the supercooled phase. These anomalies have long been interpreted in terms of underlying structural causes, and their experimental characterization points to the existence of a singularity at a temperature of about 225 K. Further insights into the nature and origin of this singularity might be gained by completely characterizing the structural relaxation in supercooled water. But until now, such a characterization has only been realized in simulations that agree with the predictions of simple mode-coupling theory; unambiguous experimental support for this surprising conclusion is, however, not yet available. Here we report time-resolved optical Kerr effect measurements that unambiguously demonstrate that the structural relaxation of liquid and weakly supercooled water follows the behaviour predicted by simple mode-coupling theory. Our findings thus support the interpretation of the singularity as a purely dynamical transition. That is, the anomalous behaviour of weakly supercooled water can be explained using a fully dynamic model and without needing to invoke a thermodynamic origin. In this regard, water behaves like many other, normal molecular liquids that are fragile glass-formers.

Photoinduced structural relaxation in chalcogenide glasses

Measurements of residual entropy were performed on irradiated chalcogenide glasses using a modulated differential scanning calorimeter. We show that glass from the Ge x Se 1− x system can undergo extensive structural relaxation under sub-bandgap irradiation. This ability to relax is correlated with the strong/fragile character of the glass-forming system. Fragile glassformers experience larger relaxation as expected from the Kauzmann entropy plot. The temperature dependence of this photorelaxation process is also consistent with entropy relaxation. These measurements demonstrate that the glass configurational entropy decreases under irradiation. This can be regarded as a light activated aging process.

Structural relaxation of polymers at the glass transition: conformational memory in poly(n-alkylmethacrylates).

Conformational memory is introduced as a new aspect of structural relaxation of polymers. In poly(n-alkylmethacrylates) extended backbone chain conformations are identified by advanced NMR techniques as the molecular units involved in structural relaxation. They retain conformational memory over many steps of restricted axial chain motion. Randomization of conformation and isotropization of backbone orientation occur on the same time scale, yet much slower than the slowest relaxation process identified so far. Behavior typical of fragile glass formers is found for this new process of chain relaxation.

Reexamination of the evolution of the dynamic susceptibility of the glass former glycerol

The dielectric data of glycerol compiled by Lunkenheimer et al. [Contemp. Phys. 41, 15 (2000)] are reanalyzed within a phenomenological approach on the one hand, and within mode coupling theory (MCT), on the other. We present a complete interpolation of the dielectric data covering 17 decades in frequencies. The crossover temperature extracted from the phenomenological analysis of the slow response at low temperatures and defined by the emergence of the excess wing upon cooling agrees well with the critical temperature extracted from a MCT analysis of the dynamics at high temperatures including data that were not used in the first MCT analysis of glycerol by Lunkenheimer et al. [Phys. Rev. Lett. 77, 318 (1996)]. The crossover temperature is found to be T(c)=288+/-3 K, which is significantly higher than previously reported. Extracting the nonergodicity parameter f, the characteristic anomaly is only found when 1-f is inspected, since f is very close to 1. No difference for the evolution of the dynamic susceptibility is observed for the nonfragile system glycerol with respect to fragile glass formers provided that the evolution of the dynamics is studied as a function of the correlation time tau(alpha).

Evolution of the dynamic susceptibility of simple glass formers in the stronglysupercooled regime

We discuss dielectric and light scattering susceptibility spectra ofsimple glass formers at temperatures above as well as below thecritical temperature of the mode coupling theory (MCT). Close toTgthe systems are characterized by the presence of a pronounced excess wing(type A glass formers). The data are analysed within a phenomenologicalapproach, on the one hand, and within MCT, on the other. Among other workwe present a complete interpolation of the dielectric data for glycerol(Lunkenheimer et al2000 Contemp. Phys. 41 15). The crossover temperatureTx,defined by the emergence of the excess wing upon cooling, is extracted from thephenomenological analysis and found to agree well with the critical temperatureTc,extracted from the MCT analysis at high temperatures. BelowTxthe evolution of the susceptibility is characterized by a universal appearanceof the excess wing. No difference is observed for the non-fragile systemwith respect to fragile glass formers provided that the wing parametersare studied as a function of the correlation time τα.Finally, a generalized scaling for the susceptibility minimum is proposed which is aphenomenological extension of that of MCT but now also includes the data belowTc.

Locally preferred structure and frustration in glass-forming liquids: a clue topolyamorphism?

We propose that the concept of liquids characterized by a given locally preferredstructure (LPS) could help in understanding the observed phenomenon ofpolyamorphism. ‘True polyamorphism’ would involve the competition betweentwo (or more) distinct LPSs, one favoured at low pressure because of its lowenergy and one favoured at high pressure because of its small specific volume, asin tetrahedrally coordinated systems. ‘Apparent polyamorphism’ could beassociated with the existence of a poorly crystallized defect-ordered phase with alarge unit cell and small crystallites, which may be illustrated by the metastableglacial phase of the fragile glass-former triphenylphosphite; the apparentpolyamorphism might result from structural frustration, i.e., a competitionbetween the tendency to extend the LPS and a global constraint that preventstiling of the whole space by the LPS.

Potential energy, relaxation, vibrational dynamics and the boson peak, ofhyperquenched glasses

We describe a combination of laboratory and simulation studies that givequantitative information on the energy landscape for glass-forming liquids. Bothtypes of study focus on the idea of suddenly extracting the thermal energy, so thatthe system obtained for subsequent study has the structure, and hence potentialenergy, of a liquid at a much higher temperature than the normal glass temperatureTg.One type of study gives information on the energy that can be trapped inexperimental glasses by hyperquenching, relative to the normal glass,and on the magnitude of barriers separating basins of attraction on thelandscape. Stepwise annealing studies also give information on the matter ofenergy heterogeneity and the question of ‘nanogranularity’ in liquids nearTg.The other type of study gives information on the vibrational properties of asystem confined to a given basin, and particularly on how that vibrationalstructure changes with the state of configurational excitation of the liquid. Afeature in the low frequency (‘boson peak’) region of the density of vibrationalstates of the normal glass becomes much stronger in the hyperquenched glass.Qualitatively similar observations are made on heating fragile glass-formers intothe supercooled and stable liquid states. The vibrational dynamics findings aresupported and elucidated by constant pressure molecular dynamics/normalmode MD/NM simulations/analysis of the densities of states of differentinherent structures of a model fragile liquid (orthoterphenyl (OTP) in theLewis–Wahnstrom approximation). These show that, when the temperature israised at constant pressure, the total density of states changes in a mannerthat can be well represented by a two-Gaussian ‘excitation across thecentroid’, leaving a third and major Gaussian component unchanging.The low frequency Gaussian component, which grows with increasingtemperature, has a constant peak frequency of 18 cm−1and is identified with the Boson peak. It is suggested that the latter can serve as asignature for configurational excitations of the ideal glass structure, i.e. thetopologically diverse defects of the glassy solid state. The excess vibrational heatcapacity associated with this generation of low frequency modes with structuralexcitation is shown to be responsible for about 60% of the jump in heat capacityat Tg,most of the remainder coming from configurational excitation.

The Avramov model of structural relaxation

The Avramov model, an entropy-based description of the effects of temperature and pressure on structural relaxation times, assumes separability of these two dependences. This implies that the fragility of glass-formers is independent of pressure. Herein we show that experimental results for polymethyltolylsiloxane are at odds with this assumption. By introducing a linear increase of the coordination number of the liquid state with pressure, the model can be modified, enabling good agreement with experiment to be achieved.

‘Unsticking’ a colloidal glass, and sticking it again

We study glass formation in hard spheres with short-range attraction. Thesystem consists of nearly-hard-sphere polymethylmethacrylate particles andnon-adsorbing random-coil polystyrene which induced a depletion attractionbetween the particles. The experiments reveal a re-entrant glass transitionand two qualitatively distinct glassy states. Dynamic light scattering,covering eleven orders of magnitude in time, gives insight into the kinds ofparticle motion responsible for these observations. The possible relevance ofour results to generic issues, such as the distinction between fragile andstrong glass formers, the nature of the underlying ‘free energy landscape’,and the relative importance of temperature and pressure, is discussed.

Quantitative tests of mode-coupling theory for fragile and strong glass formers

We calculate for a binary mixture of Lennard-Jones particles the time dependence of the solution of the mode-coupling equations in which the full wave vector dependence is taken into account. In addition we also take into account the short time dynamics, which we model with a simple memory kernel. We find that the so obtained solution agrees very well with the time and wave vector dependence of the coherent and incoherent intermediate scattering functions as determined from molecular dynamics computer simulations. Furthermore we calculate the wave vector dependence of the Debye–Waller factor for a realistic model of silica and compare these results with the ones obtained from a simulation of this model. We find that if the contribution of the three point correlation function to the vertices of the memory kernel are taken into account, the agreement between theory and simulation is very good. Hence we conclude that mode coupling theory is able to give a correct quantitative description of the caging phenomena in fragile as well as strong glass-forming liquids.

Dielectric relaxation of the alternating terpolymers of ethylene, propylene, and carbon monoxide

The alternating terpolymers of ethylene, propylene, and carbon monoxide were characterized by wide-angle x-ray scattering (WAXS), differential scanning calorimeter (DSC), thermally stimulated depolarization currents, and dielectric relaxation spectroscopy (DRS). At room temperature, the terpolymers were x-ray amorphous; however, the existence of weak endothermal effects over fairly broad temperature intervals above the glass transition suggested that they should be classified as “microcrystalline” (in a sense that the ultimate level of crystallinity lies beyond the resolution limits of ordinary WAXS instruments). The DRS data in the temperature intervals of α- and sub-glass relaxations were fitted quantitatively to the Havriliak–Negami equation. The characteristic parameters for the α-relaxation were consistent with classification of terpolymers as relatively fragile glass formers; however, their high fragility was attributed to steric constraints to the motion of chain dipoles by the residual network of microcrystals. Strong asymmetry of the dielectric sub-glass relaxation was regarded as additional experimental evidence for the interference of the spatial network of microcrystallites with the motion (in this case, noncooperative) of chain segments.

Layering and relaxation in molecular films of a photosensitive polyacrylate from X-ray reflectivity and in elastic neutron scattering experiments

Abstract In this paper we study the effect of ultraviolet (UV) illumination on the microscopic structure of molecular layers of a photosensitive fragile polymeric glass former. In particular we show that the increased time relaxation and the reduced mobility of thin molecular multilayers recently reported using high-precision pump-probe ellipsometric experiments correlate with a geometrically induced layering effect which was confirmed by synchrotron X-ray reflectivity experiments. Furthermore the results agree also with recent inelastic-quasielastic neutron scattering measurements of the coupling between the photosensitive side chains and the polymeric main chain. The observed layering is destroyed upon UV illumination. The significance of such results on the glass transition behaviour of this material is discussed.

Layering and relaxation in molecular films of a photosensitive polyacrylate from X-ray reflectivity and in elastic neutron scattering experiments

Abstract In this paper we study the effect of ultraviolet (UV) illumination on the microscopic structure of molecular layers of a photosensitive fragile polymeric glass former. In particular we show that the increased time relaxation and the reduced mobility of thin molecular multilayers recently reported using high-precision pump-probe ellipsometric experiments correlate with a geometrically induced layering effect which was confirmed by synchrotron X-ray reflectivity experiments. Furthermore the results agree also with recent inelastic-quasielastic neutron scattering measurements of the coupling between the photosensitive side chains and the polymeric main chain. The observed layering is destroyed upon UV illumination. The significance of such results on the glass transition behaviour of this material is discussed.

From strong to fragile glass formers: secondary relaxation in polyalcohols.

We have studied details of the molecular origin of slow secondary relaxation near T(g) in a series of neat polyalcohols by means of dielectric spectroscopy and (2)H NMR. From glycerol to threitol, xylitol, and sorbitol the appearance of the secondary relaxation changes gradually from a wing-type scenario to a pronounced beta peak. It is found that in sorbitol the dynamics of the whole molecule contributes equally to the beta process, while in glycerol the hydrogen bond forming OH groups remain rather rigid compared to the hydrogens bonded to the carbon skeleton.

Rotational probe relaxation and scaling in fragile glass formers

The rotational dynamics of a stiff paramagnetic tracer dissolved in supercooled SALOL is investigated via electron spin resonance spectroscopy. The study shows that the molecular rotation follows different dynamical regimes as the temperature is lowered. In particular, on cooling through the critical temperature T C of the SALOL, the coupling between rotational relaxation and viscosity weakens and enhanced rotational diffusion is observed. In this temperature interval, the relationship between rotational correlation times and viscosity is fairly well described by a power law τ∝ η ξ (Fractional Debye–Stokes–Einstein law). Activated reorientation is observed in the temperature region around the glass transition of the SALOL. The rotational dynamics of the tracer dissolved in SALOL are compared with its rotation in ortotherphenyl (OTP) investigated in previous studies, and a scaling procedure is proposed.

Molecular Glasses with High Fictive Temperatures for Energy Landscape Evaluations

With an interest in obtaining data on laboratory glass samples to compare with simulated glasses produced by molecular dynamics computer simulations, we have explored, using calorimetric techniques, the fictive temperatures that can be obtained using different laboratory quenching methods. We describe some useful analytical methods for characterizing quenched samples and, in the process, demonstrate a modified graphical treatment of DSC data that directly yields the m fragility index (“steepness” index) and permits the assembly of enthalpy relaxation data on different liquids in a fragility plot. Using these methods, we provide evidence for the trapping of high-Tg molecular glasses at fictive temperatures up to 1.16Tg and show that fictive temperatures up to and even beyond the crossover temperature for fragile glass formers should be possible using refined electrospray and fiber-spinning techniques. We discuss the relation of the low-T/Tg enthalpy relaxation, found in all hyperquenched glasses, to topographic features of the energy landscape for glass-forming liquids.

Fragility of unsaturated polyester resins cured with styrene: influence of the styrene concentration

Unsaturated polyester resins cured with varying styrene content (20–50% (w/w)) has been studied by calorimetry (DSC). We have evaluated the values of the heat capacity at the glass transition ΔC p =[ C p,l − C p,g ] T= T g , the values of apparent activation energy Δh *, and the values of the fragility index m for each sample. It is found a constant value of ΔC p=0.29 J g −1 K −1 for all the samples while Δh * and m increase with the styrene content. These results discussed in regard with the strong fragile glass-formers concept indicate a character becoming more fragile when the amount of styrene increases. Introducing the “bead concept” proposed by Wunderlich, we show that the average elementary energy barrier that the molecular species (or relaxing units) must overstep during their relaxation process is unchanged whatever the content of styrene. The origin of the variations with the styrene content of the fragility behavior is understood by the increase of the bead number.

Glass transition and density fluctuations in the fragile glass former orthoterphenyl.

High-resolution Brillouin light scattering is used to measure the dynamic structure factor of the fragile glass former orthoterphenyl (OTP) in a wide temperature range around the glass transition region and up to the boiling point. The whole set of spectra is described in terms of a phenomenological generalized hydrodynamic model. In the supercooled phase, we show the contemporary existence of the structural process, whose main features come out to be consistent with the results obtained with other spectroscopies, and of a secondary, activated process, which occurs on the 10(-11) s time scale and has a low activation energy (E(f)(a)=0.28 kcal/mol). This latter process, which is also present in the glassy phase and seems to be insensitive to the glass transition, is attributed to the coupling between the density modes and intramolecular degrees of freedom. In the normal liquid phase, the two processes merge together, and the resulting characteristic time is no longer consistent with those derived with other spectroscopies. The analysis points to the conclusion that, for what concerns the long-wavelength density fluctuations in fragile glass formers such as OTP, the universal dynamical features related to the glass transition come out clearly only in the supercooled phase and at frequencies lower than approximately 10(6) Hz.

Does fragility depend on pressure? A dynamic light scattering study of a fragile glass-former

Relaxation times of the α-process in the fragile glass-forming liquid diglycidyl ether of bisphenol-A (EPON 828) were measured in a broad pressure (1–1500 bar) and temperature (264–293 K) ranges by means of the depolarized dynamic light scattering—photon correlation spectroscopy. Based on this experimental data the fragility of the supercooled liquid was calculated in two ways: as a steepness index m of the “Angell plot” and as the DT-parameter from the Vogel–Fulcher–Tammann Law, and was studied as a function of pressure. It was found, that while the steepness index depends on pressure, the DT parameter is pressure independent. The pressure dependence of the glass transition temperature Tg in EPON 828 was found to be nonlinear. Additionally, we established a relationship between the steepness index mT, the activation volume ΔV#, and the coefficient ∂Tg/∂Pg. In this pressure dependent study we found that also for EPON 828 the nonexponentiality of the correlation function of the α-process correlates well with the non-Arrhenius behavior (steepness index) of the relaxation times. An equation of state describing the temperature and pressure dependence of the structural relaxation time was proposed and verified using experimental data.