Nucleation and Growth of the Supercooled Liquid Phase Control Glass Transition in Bulk Ultrastable Glasses.

We report the anomalous bulk transformation of vapor deposited stable glasses into the liquid state. The transformation proceeds through two competing parallel processes: partial rejuvenation of the stable glass and nucleation and growth of liquid patches within the glass. The kinetics of the transformation extracted from heat capacity curves after isothermal runs is dominated by the heterogeneous nucleation and growth process that initiates at preexisting seeds and propagates radially at a velocity proportional to the alpha relaxation time. Remarkably, the distance between the activation seeds is independent of temperature within experimental uncertainty and amounts to several micrometers, a value in close agreement with the crossover length for TPD glasses. We speculate the initiation sites for the transformation of the glass into the supercooled liquid are localized regions of lower stability (or density).

Multiple glass transitions in vapor-deposited orientational glasses of the most fragile plastic crystal Freon 113.

We investigate by fast-scanning nanocalorimetry the formation of Freon 113 films from the vapor phase at deposition temperatures ranging from 50 to 120 K, that is, spanning above and below the transition temperature of the glassy crystal to the plastic crystal (Tgc = 72 K). Analysis of the heat capacity curves indicates that vapor deposition at T < Tgc of the highly fragile Freon 113 yields structural and orientational glasses in the as-deposited state depending on the temperature range of deposition. Interestingly, growing above Tgc produces plastic crystals with a conformational ratio C1/Cs that changes with Tdep above and below 110-120 K, the temperature at which previous works have identified the arrest of the transformations between the C1 and Cs conformers.

Relaxation dynamics of glasses along a wide stability and temperature range.

While lots of measurements describe the relaxation dynamics of the liquid state, experimental data of the glass dynamics at high temperatures are much scarcer. We use ultrafast scanning calorimetry to expand the timescales of the glass to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-temperature regime above the conventional devitrification temperature heating at 10 K/min. The liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws originally derived for the liquid. This work provides a strong connection between the dynamics of the equilibrium supercooled liquid and non-equilibrium glassy states.

Ultrastable glasses portray similar behaviour to ordinary glasses at high pressure.

Pressure experiments provide a unique opportunity to unravel new insights into glass-forming liquids by exploring its effect on the dynamics of viscous liquids and on the evolution of the glass transition temperature. Here we compare the pressure dependence of the onset of devitrification, Ton, between two molecular glasses prepared from the same material but with extremely different ambient-pressure kinetic and thermodynamic stabilities. Our data clearly reveal that, while both glasses exhibit different dTon/dP values at low pressures, they evolve towards closer calorimetric devitrification temperature and pressure dependence as pressure increases. We tentatively interpret these results from the different densities of the starting materials at room temperature and pressure. Our data shows that at the probed pressures, the relaxation time of the glass into the supercooled liquid is determined by temperature and pressure similarly to the behaviour of liquids, but using stability-dependent parameters.

Atomic-scale relaxation dynamics and aging in a metallic glass probed by x-ray photon correlation spectroscopy.

We use x-ray photon correlation spectroscopy to investigate the structural relaxation process in a metallic glass on the atomic length scale. We report evidence for a dynamical crossover between the supercooled liquid phase and the metastable glassy state, suggesting different origins of the relaxation process across the transition. Furthermore, using different cooling rates, we observe a complex hierarchy of dynamic processes characterized by distinct aging regimes. Strong analogies with the aging dynamics of soft glassy materials, such as gels and concentrated colloidal suspensions, point at stress relaxation as a universal mechanism driving the relaxation dynamics of out-of-equilibrium systems.

Glass transition in ultrathin films of amorphous solid water.

Nanocalorimetry at ultrafast heating rates is used to investigate the glass transition of nanometer thick films of metastable amorphous solid water grown by vapor deposition in an ultrahigh vacuum environment. Apparent heat capacity curves exhibit characteristic features depending on the deposition temperature. While films grown at T ≥ 155 K are completely crystallized, those deposited at 90 K show a relaxation exotherm prior to crystallization. Films grown between 135 and 140 K and subsequently cooled down to 90 K reveal a clear endothermic feature before crystallization, which is compatible with a glass-to-liquid transition. The onset temperature is located at 174 K at a heating rate of 2.4 × 10(4) K/s and is independent of film thickness in the range of 16-150 nm. Comparison of our data with other calorimetric measurements at various heating rates suggests that water is a strong glass former in the deeply supercooled state.

Anomalous Transformation of Vapor-Deposited Highly Stable Glasses of Toluene into Mixed Glassy States by Annealing Above Tg.

Vapor-deposited glasses have recently emerged as a remarkable new class of materials that can form much denser and stable glasses than those obtained by cooling the liquid. These new amorphous materials reach lower regions of the energy landscape and may impact important technologies that use vapor-deposition. Here, we report on the formation of a glass with two distinct glassy states obtained through the partial annealing of highly stable vapor-deposited glassy films of toluene. The resulting glass exhibits two clear heat capacity overshoots with different onset and fictive temperatures. The transformation times of the ultrastable glass are around 10(5) times slower than the structural relaxation time (τα) of supercooled liquid toluene. We show that the nature of the transformed glass depends on the annealing temperature above Tg. This finding suggests the formation of distinct supercooled liquids at temperatures slightly above Tg during the transformation of the highly stable glass. Our results are compatible with the existence of polyamorphism in toluene.

Accelerated aging in ultrathin films of a molecular glass former.

We report the thermodynamic measurement of the enthalpy released during the aging of supported films of a molecular glass former, toluene, at temperatures well below the glass transition temperature. By using microfabricated devices with very short equilibration times (below 1 s), we evidence a remarkable variation of the relaxation rate on decreasing film thickness from 100 nm down to a 7 nm thick film. Our results demonstrate that surface atoms are more efficient than bulk atoms in attaining low energy configurations within the potential energy landscape.

Destabilization of the Mg-H system through elastic constraints.

We tune the thermodynamics of hydrogen absorption in Mg by means of elastic clamping. The loading isotherms measured by hydrogenography show that Mg films covered with Mg-alloy-forming elements, such as Pd and Ni, have hydrogen plateau pressures more than 2 orders of magnitude higher than bulk Mg at the same temperature. An elastic model allows us to interpret the Mg thickness dependence of the hydrogen plateau pressure. Our results suggest an alternative route for the development of new hydrogen storage materials with optimized thermodynamic properties.