Strong Glass Formers Research Articles

Correlation between fragility and surface glass transition temperature of polymers

The fragility of glass describes how rapidly its molecules slow down as it is cooled near its glass transition temperature. In nanoscale films, polymer glasses with higher fragility experience larger reductions in their Tg compared to those with lower fragility. We investigated whether this is due to the free surface of the polymers, which can cause the surface Tg (Tgsurf) to decrease relative to the bulk Tg. By measuring Tgsurf of various polymers, we found that the shift in Tgsurf relative to the bulk Tg increased with fragility. This suggests that more fragile polymers are more susceptible to the free surface effect. We explain this using the concept of energy landscape, as it is used to explain the different slowdown rates between strong and fragile glass formers at Tg.

Effect of Concentration, Cooling, and Warming Rates on Glass Transition Temperatures for NaClO4, Ca(ClO4)2, and Mg(ClO4)2 Brines with Relevance to Mars and Other Cold Bodies.

The hygroscopic and supercooling properties of perchlorates make them potentially important for sustaining liquid water on Mars. To understand the possibility for supercooled liquids and glasses on Mars and other cold bodies, we have characterized the supercooling and vitrification features using differential scanning calorimetry for Na, Ca, and Mg perchlorate brines in a temperature range relevant to Mars. Results show that the glass transition temperature (Tg) depends on the salt composition, concentration, and cooling or warming rate. The difference in Tg may be significant even in a single composition, producing glass transitions with over 40 K difference. A new model was developed to describe these Tg dependencies, with the warmest Tg values found for high concentrations and fast cooling rates. These results emphasize the importance of considering Tg as a range rather than a discrete temperature. For all perchlorates measured, the degree of supercooling was extensive at high concentrations, exceeding 100 K from the liquidus. With a highly reduced glass temperature (Tg/liquidus temperature) and low critical rate of temperature change to avoid crystallization, concentrated perchlorate brines are strong glass formers when compared to other glass-forming materials. The consideration of cooling rates in the context of cellular cryopreservation suggests that cooling and warming rates may be an important astrobiological factors in a diverse set of planetary environments. These findings provide additional constraints on the possibility of liquid water on Mars in terms of concentration, different latitudes, seasons, and times of day.

Many-Body Correlations Are Non-negligible in Both Fragile and Strong Glassformers.

It is widely believed that the emergence of slow glassy dynamics is encoded in a material's microstructure. First-principles theory [mode-coupling theory (MCT)] is able to predict the dramatic slowdown of the dynamics from only static two-point correlations as input, yet it cannot capture all of the observed dynamical behavior. Here we go beyond two-point spatial correlation functions by extending MCT systematically to include higher-order static and dynamic correlations. We demonstrate that only adding the static triplet direct correlations already qualitatively changes the predicted glass-transition diagram of binary hard spheres and silica. Moreover, we find a nontrivial competition between static triplet correlations that work to stabilize the glass state and dynamic higher-order correlations that destabilize it for both materials. We conclude that the conventionally neglected static triplet direct correlations as well as higher-order dynamic correlations are, in fact, non-negligible in both fragile and strong glassformers.

Cyclic Olefin Copolymers (COC)—Excellent Glass Formers with Low Dynamic Fragility

AbstractApplications of cyclic olefin copolymers (COC) are manifold, and developing a complete understanding of the characteristics of the glass transition behavior of these materials is essential to their processing and use in a variety of those applications. Fast scanning calorimetry is employed to investigate the glass transition of three commercial TOPAS COCs of different norbornene content between 37 and 54 mol% at cooling rates varying from 0.01 to 100 000 K s−1. The glass transition temperatures as a function of cooling rate are determined as limiting fictive temperatures from reheating scans at 100 000 K s−1. The data, covering seven orders of magnitude in cooling rate, allows the determination of the dynamic fragilities. The dynamic fragilities of TOPAS 6013, 6015, and 8007 are in the range of 50–60 and increase with increasing norbornene content. Compared to literature data of other polymers, the dynamic fragilities are rather low, and the COCs are classified as relatively strong glass formers. In other words, relating this information to processability, they have a relatively large “working range” and are classified as “long” glasses. Such classification, borrowed from glass making (blowing) technologies, may also help to describe the processability of polymeric glass formers in general.

Catalyst Control of Nanoscale Characteristic Length of the Glass Transition in Organic Strong Glass-Formers.

We report the nanoscale characteristic length (ξα) in organic strong glass-formers at glass transition. Catalyst-dependent ξα of the epoxy-based covalent adaptable networks (CANs) is observed, that is, the more efficient the catalyst for the cross-linking reactions, the larger the ξα is, and upon thermal heating, the more broad the glass transition will be. The observed structural properties at glass transition can be correlated with the topology freezing transition where the fluctuations of network topology aroused from the bond exchange reactions are frozen. This study proposes the catalyst-dependent structural properties in CANs and may fill the structural gap between the glass transition and topology freezing transition of organic strong glass-formers.

Vitrification of octonary perylene mixtures with ultralow fragility.

Strong glass formers with a low fragility are highly sought-after because of the technological importance of vitrification. In the case of organic molecules and polymers, the lowest fragility values have been reported for single-component materials. Here, we establish that mixing of organic molecules can result in a marked reduction in fragility. Individual bay-substituted perylene derivatives display a high fragility of more than 70. Instead, slowly cooled perylene mixtures with more than three components undergo a liquid-liquid transition and turn into a strong glass former. Octonary perylene mixtures display a fragility of 13 ± 2, which not only is a record low value for organic molecules but also lies below values reported for the strongest known inorganic glass formers. Our work opens an avenue for the design of ultrastrong organic glass formers, which can be anticipated to find use in pharmaceutical science and organic electronics.

Anomalous fast atomic dynamics in bulk metallic glasses

It is commonly believed that the strong glass formers tend to be dense packing and have rather slow atomic dynamics. Here, we report that the anomalous fast atomic dynamics exist in an Au55Cu20Ag5Si20 metallic glass (MG) with good glass forming ability (GFA) by combining X-ray photon correlation spectroscopy measurements with ab initio molecular dynamics simulations. The fast dynamics are mainly originated from the increased mobility of Cu atoms by Ag addition. Ag atoms are inclined to locate at Cu-centered <0, 2, 8, 1> motifs, which slow or accelerate the dynamics of shell Cu atoms, depending on whether Ag atoms appear at the first shells or not, leading to the formation of structural heterogeneity at the atomic level. The results enrich the understanding of atomic dynamics in Cu–Ag-containing MGs and will trigger more studies on the dynamics-GFA relationship in MGs.

Transition zone theory of the glass transition

An energy-based theory of the glass transition is described and validated by evaluation of crystallization and viscosity data of sixteen diverse systems, ranging from the fragile o-terphenyl to the strong SiO2 glass formers, including glass formation in non-crystallizing polymers. Transition zone theory demonstrates an inverse temperature dependence of the entropy of activation for crystallization and viscous relaxation which results in a temperature at which their free energies of activation are equivalent. Below this temperature there is a greater probability of crystallization than of relaxation. Under such conditions, crystal-like organization can propagate only over the internal length scales for which the system has relaxed, i.e. a few nanometers. However, that structural organization makes the barrier to bulk liquid relaxation insurmountable, resulting in glass formation. The temperature of the crystallization-relaxation free energy of activation equivalence point, defines the glass transition temperature. The temperature dependent slope of the free energy of activation for relaxation at the equivalence point reflects the fragility of the system. The time-temperature dependence of viscous relaxation shifts the crystallization-relaxation free energy of activation equivalence point, consistent with the sample history dependence of Tg. Crystal-like formation over nanometer-length scales, which can still occur below Tg, is shown to account for glass aging.

Molecular dynamics in amorphous pharmaceutically important protic ionic liquid–benzalkonium chloride

In this paper, we report the dielectric measurements at ambient pressure to portray the molecular dynamics of a glass forming pharmaceutical as well as a protic ionic conductor, benzalkonium chloride [BaCl] by means of broadband dielectric spectroscopy. The measurement in macroscopic nature of a stochastic transport theory and of the electric modulus formalism stanchly reproduces the shape of the dispersion of the microscopic ionic movement in terms of translational and ionic diffusive motion of molecular ions. The situation is analogous to most ionic conductors, BaCl showed only single resolved peak in the electric modulus formalism and doesn't show any crystallisation tendency during cooling as well as heating. The temperature dependence of the conductivity relaxation has been fitted by single Vogel–Fulcher–Tamman (VFT) equation. It is found that the value of Tg observed from DSC is significantly higher than one obtained from the α-conductivity relaxation corresponding to 100s indicating that the structural τα(Τ) is longer than conductivity τσα(Τ) for about 10K. Finally, the results were compared with data for other protic ionic conductors like Procaine HCl, Procainamide HCl, Verapamile HCl, Lidocaine HCl, Tramadole HCl and Chlorpromazine HCl and found that BaCl is a good strong glass former.

Universal glass-forming behavior of in vitro and living cytoplasm

Physiological processes in cells are performed efficiently without getting jammed although cytoplasm is highly crowded with various macromolecules. Elucidating the physical machinery is challenging because the interior of a cell is so complex and driven far from equilibrium by metabolic activities. Here, we studied the mechanics of in vitro and living cytoplasm using the particle-tracking and manipulation technique. The molecular crowding effect on cytoplasmic mechanics was selectively studied by preparing simple in vitro models of cytoplasm from which both the metabolism and cytoskeletons were removed. We obtained direct evidence of the cytoplasmic glass transition; a dramatic increase in viscosity upon crowding quantitatively conformed to the super-Arrhenius formula, which is typical for fragile colloidal suspensions close to jamming. Furthermore, the glass-forming behaviors were found to be universally conserved in all the cytoplasm samples that originated from different species and developmental stages; they showed the same tendency for diverging at the macromolecule concentrations relevant for living cells. Notably, such fragile behavior disappeared in metabolically active living cells whose viscosity showed a genuine Arrhenius increase as in typical strong glass formers. Being actively driven by metabolism, the living cytoplasm forms glass that is fundamentally different from that of its non-living counterpart.

Pathways for diffusion in the potential energy landscape of the network glass former SiO2.

We study the dynamical behaviour of a computer model for viscous silica, the archetypal strong glass former, and compare its diffusion mechanism with earlier studies of a fragile binary Lennard-Jones liquid. Three different methods of analysis are employed. First, the temperature and time scale dependence of the diffusion constant is analysed. Negative correlation of particle displacements influences transport properties in silica as well as in fragile liquids. We suggest that the difference between Arrhenius and super-Arrhenius diffusive behaviour results from competition between the correlation time scale and the caging time scale. Second, we analyse the dynamics using a geometrical definition of cage-breaking transitions that was proposed previously for fragile glass formers. We find that this definition accurately captures the bond rearrangement mechanisms that control transport in open network liquids, and reproduces the diffusion constants accurately at low temperatures. As the same method is applicable to both strong and fragile glass formers, we can compare correlation time scales in these two types of systems. We compare the time spent in chains of correlated cage breaks with the characteristic caging time and find that correlations in the fragile binary Lennard-Jones system persist for an order of magnitude longer than those in the strong silica system. We investigate the origin of the correlation behaviour by sampling the potential energy landscape for silica and comparing it with the binary Lennard-Jones model. We find no qualitative difference between the landscapes, but several metrics suggest that the landscape of the fragile liquid is rougher and more frustrated. Metabasins in silica are smaller than those in binary Lennard-Jones and contain fewer high-barrier processes. This difference probably leads to the observed separation of correlation and caging time scales.

Molecular dynamics of 4-propyl-4′-thiocyanatobiphenyl (3BT) in the strong glass-forming smectic E phase

Broadband dielectric spectroscopy was employed to study glass transition dynamics in highly ordered smectic E phase of 4-propyl-4′-thiocyanato-1,1′-biphenyl (3BT). Three relaxation processes exhibiting Arrhenius-like thermal activation were detected: (1) δ-relaxation assigned to molecular reorientation around the short axis, (2) α-relaxation attributed to rigid molecular cores tumbling around the long axis and (3) secondary β-relaxation. Our investigations revealed that the smectic E phase (SmE) is a strong glass-former with the smallest possible fragility index mf~16. Finally, an analysis of loss spectra (ε″(f)) in terms of their shape parameters (1−m) and n demonstrated long and short range coupling between molecular motions in the glassy state.

Real-time observation of the isothermal crystallization kinetics in a deeply supercooled liquid

Below the melting temperature Tm, crystals are the stable phase of typical elemental or molecular systems. However, cooling down a liquid below Tm, crystallization is anything but inevitable. The liquid can be supercooled, eventually forming a glass below the glass transition temperature Tg. Despite their long lifetimes and the presence of strong barriers that produces an apparent stability, supercooled liquids and glasses remain intrinsically a metastable state and thermodynamically unstable towards the crystal. Here we investigated the isothermal crystallization kinetics of the prototypical strong glassformer GeO2 in the deep supercooled liquid at 1100 K, about half-way between Tm and Tg. The crystallization process has been observed through time-resolved neutron diffraction for about three days. Data show a continuous reorganization of the amorphous structure towards the alpha-quartz phase with the final material composed by crystalline domains plunged into a low-density, residual amorphous matrix. A quantitative analysis of the diffraction patterns allows determining the time evolution of the relative fractions of crystal and amorphous, that was interpreted through an empirical model for the crystallization kinetics. This approach provides a very good description of the experimental data and identifies a predator-prey-like mechanism between crystal and amorphous, where the density variation acts as a blocking barrier.

The amorphous state: first-principles derivation of the Gordon-Taylor equation for direct prediction of the glass transition temperature of mixtures; estimation of the crossover temperature of fragile glass formers; physical basis of the "Rule of 2/3".

Predicting the glass transition temperature (Tg) of mixtures has applications that span across industries and scientific disciplines. By plotting experimentally determined Tg values as a function of the glass composition, one can usually apply the Gordon-Taylor (G-T) equation to determine the slope, k, which subsequently can be used in Tg predictions. Traditionally viewed as a phenomenological/empirical model, this work proposes a physical basis for the G-T equation. The proposed equations allow for the calculation of k directly and, hence, they determine/predict the Tg values of mixtures algebraically. Two derivations for k are provided, one for strong glass-formers and the other for fragile mixtures, with the modeled trehalose-water and naproxen-indomethacin systems serving as examples of each. Separately, a new equation is described for the first time that allows for the direct determination of the crossover temperature, Tx, for fragile glass-formers. Lastly, the so-called "Rule of 2/3", which is commonly used to estimate the Tg of a pure amorphous phase based solely on the fusion/melting temperature, Tf, of the corresponding crystalline phase, is shown to be underpinned by the heat capacity ratio of the two phases referenced to a common temperature, as evidenced by the calculations put forth for indomethacin and felodipine.

Significant difference in the dynamics between strong and fragile glass formers.

Glass-forming liquids are often classified into strong glass formers with nearly Arrhenius behavior and fragile ones with super-Arrhenius behavior. We reveal a significant difference in the dynamics between these two types of glass formers through molecular dynamics simulations: In strong glass formers, the relaxation dynamics of density fluctuations is nondiffusive, whereas in fragile glass formers it exhibits diffusive behavior. We demonstrate that this distinction is a direct consequence of the fundamental difference in the underlying elementary relaxation process between these two dynamical classes of glass formers. For fragile glass formers, a density-exchange process proceeds the density relaxation, which takes place locally at the particle level in normal states but is increasingly cooperative and nonlocal as the temperature is lowered in supercooled states. On the other hand, in strong glass formers, such an exchange process is not necessary for density relaxation due to the presence of other local relaxation channels. Our finding provides a novel insight into Angell's classification scheme from a hydrodynamic perspective.

Cooling rate dependence of structural order in Al90Sm10 metallic glass

The atomic structure of Al90Sm10 metallic glass is studied using molecular dynamics simulations. By performing a long sub-Tg annealing, we developed a glass model closer to the experiments than the models prepared by continuous cooling. Using the cluster alignment method, we found that “3661” cluster is the dominating short-range order in the glass samples. The connection and arrangement of “3661” clusters, which define the medium-range order in the system, are enhanced significantly in the sub-Tg annealed sample as compared with the fast cooled glass samples. Unlike some strong binary glass formers such as Cu64.5Zr35.5, the clusters representing the short-range order do not form an interconnected interpenetrating network in Al90Sm10, which has only marginal glass formability.

Nonisothermal crystallization kinetics, fragility and thermodynamics of Ti20Zr20Cu20Ni20Be20 high entropy bulk metallic glass

Abstract

The Modified VFT law of glass former materials under pressure: Part II: Relation with the equation of state.

The dynamical properties of glass formers (GFs) as a function of P, V, and T are reanalyzed in relation with the equations of state (EOS) proposed recently (Eur. Phys. J. E 37, 113 (2014)). The relaxation times τ of the cooperative non-Arrhenius α process and the individual Arrhenius β process are coupled via the Kohlrausch exponent n S(T, P). In the model n S is the sigmoidal logistic function depending on T (and P, and the α relaxation time τ α of GFs above T g verifies the pressure-modified VFT law: log τ α ∼ E β /nsRT, which can be put into a form with separated variables: log τ α ∼ f(T)g(P). From the variation of n S and τ α with T and P the Vogel temperature T 0 (τ α → ∝, n S = 0) and the crossover temperature (also called the merging or splitting temperature) T B (τ α ∼ τ β, n S ∼ 1) are determined. The proposed sm-VFT equation fits with excellent accuracy the experimental data of fragile and strong GFs under pressure. The properties generally observed in organic mineral and metallic GFs are explained: a) The Vogel temperature is independent of P (as suggested by the EOS properties), the crossover is pressure-dependent. b) In crystallizable GFs the T B (P) and Clapeyron curves T m(P) coincide. c) The α and β processes have the same ratio of the activation energies and volume, E*/V* (T- and P-independent), the compensation law is observed, this ratio depends on the anharmonicity Slater-Grüneisen parameter and on the critical pressure P* deduced from the EOS. d) The properties of the Fan Structure of the Tangents (FST) to the isotherms and isobars curves log τ versus P and T and to the isochrones curves P(T). e) The scaling law log τ = f(V (Λ) ) and the relation between Γ and γ. We conclude that these properties should be studied in detail in GFs submitted to negative pressures.

34. Designing next generation protectants

In order to advance towards the vitrification of complex organs, further advances in cryoprotectant compositions will be necessary. Systematic solution optimization has enabled the identification of superior combinations of cryoprotectants for various cell and tissue types, yet the range of chemicals used as protectants is still quite narrow and it is oftentimes not well understood why some compositions work better than others. The White House recently launched a “Materials Genome Initiative” to catalyze an integrated design process that uses feedback from computational studies to improve the efficiency of advanced materials development. This approach is very suitable to the design of Next Generation Protectants. For vitrification applications the main materials optimization parameters are cryoprotectant toxicity and glassy state properties. While advances have been made in understanding and predicting CPA toxicity, it is much harder to predict the glassy state characteristics of complex mixtures. Additive mixing rules provide little insight, as superior compositions often exhibit strong deviations from ideal solution behavior. In recent work we have combined advanced thermal analysis techniques, specifically Dynamic Mechanical Analysis (DMA) and Differential Scanning Calorimetry (DSC), together with molecular modelling to explore the underling molecular mechanisms that give rise to Tg enhancements in protectant compositions in an effort to better understand the design rules that lead to improved compositions. Using molecular dynamics (MD) simulation, we have recently shown that divalent cations can strengthen the interacting network in electrolyte/glycerol mixtures via strong cation-dipole attractions, leading to an increase in T g 1 . In contrast, monovalent cations were observed to have little effect on Tg as the cation-dipole attraction was only slightly stronger than the original hydrogen-bonding network amongst glycerol molecules. The precursor of salt crystallization was also observed in these monovalent ion compositions, potentially contributing to weak Tg-enhancing ability. Using both thermal analysis and MD simulation we were also able to better understand the effects of phosphate anions on the glassy state properties of trehalose2. We observed that the addition of NaH2PO4 decreased both the glass transition temperature and the α -relaxation temperature of the dehydrated trehalose/NaH2PO4 mixture compared to trehalose alone while both Tg and Tα were increased by adding Na2HPO4 to pure trehalose. The hydrogen-bonding interactions between trehalose and HPO 4 2 - were found to be stronger than both the trehalose-trehalose hydrogen bonds and those formed between trehalose and H2 PO 4 - . The HPO 4 2 - ions also aggregated into smaller clusters than H2 PO 4 - ions, contributing to a more homogeneous glass network. Using DMA techniques we also demonstrated that when pure trehalose, a fragile glass-former, was diluted with a small amount of the strong glass-former, glycerol, the overall mixture was strengthened significantly. The fragility index (m) was observed to be minimized when glycerol was added at a 5–10 wt% concentration, the same formulation that has been shown by others to be maximally effective for preserving proteins in the glassy state. The combination of MD simulation and thermal analysis should prove to be very valuable for the development of Next Generation Protectants.

Glassy Dynamics in Geometrically Frustrated Coulomb Liquids without Disorder.

We show that introducing long-range Coulomb interactions immediately lifts the massive ground state degeneracy induced by geometric frustration for electrons on quarter-filled triangular lattices in the classical limit. Important consequences include the stabilization of a stripe-ordered crystalline (global) ground state, but also the emergence of very many low-lying metastable states with amorphous "stripe-glass" spatial structures. Melting of the stripe order thus leads to a frustrated Coulomb liquid at intermediate temperatures, showing remarkably slow (viscous) dynamics, with very long relaxation times growing in Arrhenius fashion upon cooling, as typical of strong glass formers. On shorter time scales, the system falls out of equilibrium and displays the aging phenomena characteristic of supercooled liquids above the glass transition. Our results show remarkable similarity with the recent observations of charge-glass behavior in ultraclean triangular organic materials of the θ-(BEDT-TTF)(2) family.