Kinetic Fragility Research Articles

Unveiling the boson peaks in amorphous phase-change materials

The Boson peak is a universal phenomenon in amorphous solids. It can be observed as an anomalous contribution to the low-temperature heat capacity over the Debye model. Amorphous phase-change materials (PCMs) such as Ge-Sb-Te are a family of poor glass formers with fast crystallization kinetics, being of interest for phase-change memory applications. So far, whether Boson peaks exist in PCMs is unknown and, if they do, their relevance to PCM properties is unclear. Here, we investigate the thermodynamic properties of the pseudo-binary compositions on the tie-line between Ge15Te85and Ge15Sb85from a few Kelvins to the liquidus temperatures. Our results demonstrate the evidence of the pronounced Boson peaks in heat capacity below 10 K in the amorphous phase of all compositions. By fitting the data using the Debye model combined with a modification of the Einstein model, we can extract the characteristic parameters of the Boson peaks and attribute their origin to the excess vibrational modes of dynamic defects in the amorphous solids. We find that these parameters correlate almost linearly with the Sb-content of the alloys, despite the nonmonotonic behaviors in glass forming abilities and thermal stabilities. In a broader context, we show that the correlations of the characteristic parameters of the Boson peaks with Tg and kinetic fragility, vary according to the type of bonding. Specifically, metallic glasses and conventional covalent glasses exhibit distinct patterns of dependence, whereas PCMs manifest characteristics that lie in between. A deeper understanding of the Boson peaks in PCMs holds the promise to enable predictions of material properties at higher temperatures based on features observed in low-temperature heat capacity.

Structure and rheological behavior of mixed-alkali molybdate and tungstate glasses and liquids

The structure of mixed-alkali (Li,Na,K) molybdate and tungstate glasses with 55-62.5 mol% MoO3 and 53-65 mol% WO3 is studied using Raman spectroscopy, while the rheological behavior of the corresponding supercooled liquids is measured using parallel plate rheometry. The M-O (M = Mo,W) polyhedral units in these glasses are found to be similar to those characteristic of the crystalline alkali ortho- and pyro- molybdate and tungstate phases. The structure of glasses with ≤55 % MO3 is dominated by isolated [MO4]2− tetrahedra. Increasing MO3 content results in the formation of chain-like moieties of corner- and edge- sharing MO6 octahedra and MO4 tetrahedra. This compositional evolution of the glass structure is found to be consistent with the corresponding variation in Tg and viscosity of the parent supercooled liquids. Moreover, the low degree of structural connectivity of these liquids is manifested in rather high kinetic fragility index that is somewhat unprecedented in archetypal oxide systems.

Temperature dependence of polymer melt viscosity obtained based on DSC data and the consistency between thermodynamic and kinetic fragility

AbstractThe viscosity of the polymer melt has an impact on both the processing and properties of the product. However, conventional viscosity measurement methods have limitations such as strict testing conditions, large sample sizes, and long response times. This paper proposes a simplified method that utilizes differential scanning calorimetry to determine the viscosity–temperature relationship of the polymer melt. The research subjects consist of four noncrystalline polymers and five crystalline polymers. A new thermodynamic fragility formula of crystalline polymer is obtained. Additionally, high‐temperature limiting viscosity shows a good linear relationship with fragility. Compared with the traditional viscosity measurement, this method is a simpler and more effective way to model the viscosity of polymer melt and has broad prospects in application.

Stability of Rapidly Crystallizing Sulfonamides Glasses by Fast Scanning Calorimetry: Crystallization Kinetics and Glass-Forming Ability

Production and evaluation of the kinetic stability of the amorphous forms of active pharmaceutical ingredients are among the current challenges of modern pharmaceutical science. In the present work, amorphous forms of several sulfonamides were produced for the first time using Fast Scanning calorimetry. The parameters, characterizing the glass-forming ability of the compounds, i.e. the critical cooling rate of the melt and the kinetic fragility, were determined. The cold crystallization kinetics was studied using both isothermal and non-isothermal approaches. The results of the present study will contribute to the development of approaches for producing amorphous forms of rapidly crystallizing active pharmaceutical ingredients.

Control of thermodynamic liquid-liquid phase transition in a fragility-tunable glassy model.

We propose a distinguishable-particle glassy model suitable for the molecular dynamics simulation of structural glasses. This model can sensitively tune the kinetic fragility of supercooled liquids in a wide range by simply changing the distribution of particle interactions. In the model liquid, we observe the occurrence of thermodynamic liquid-liquid phase transitions above glass transition. The phase transition is facilitated by lowering fragility. Prior to the liquid-liquid phase transition, our simulations verify the existence of a constant-volume heat capacity maximum varying with fragility. We reveal the characteristics of the equilibrium potential energy landscape in liquids with different fragility. Within the Gaussian excitation model, the liquid-liquid transition as well as the response to fragility is reasonably interpreted in configuration space.

Coformer-Dependent Physical Stability in a Series of Naringenin-Based Coamorphous Materials with Caffeine, Theophylline, and Theobromine.

To investigate the production and physical stability of coamorphous materials (CAM) of naringenin (NAR) and coformers-caffeine, theophylline or theobromine (CAF/THY/THE, respectively). We independently assessed the impact of moisture and temperature on the physical stability of CAMs, and transformation products after destabilization were examined. Neat grinding, liquid assisted grindingand water slurry were selected to prepare multi-component materials with NAR and CAF, THY or THE. The physical stability of CAMs was investigated at 65°C/<10%RH, 21°C/85% RH and 21°C/<10% RH. Differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD) were employed to monitor for recrystallization during the stability studies. Glass forming ability of amorphous NAR was assessed to understand CAM formation and physical stability. NAR:THY and NAR:THE CAMs showed physical stability for approximately nine months, under 21°C/<10% RH while NAR:CAF CAMs destabilized in 2.5weeks. All CAMs recrystallized within a week at 65°C/<10%RH, and the physical stability at 21°C/85% RH was in the order of - NAR:THY > NAR:THE > NAR:CAF. NAR:THY produced 1:1 cocrystal under all storage conditions, while NAR:CAF destabilized to a 1:1 cocrystal at high RH but a physical mixture at high temperature. NAR:THE was found to recrystallize as a physical mixture in all conditions. NAR was found to be strong glass, with moderate kinetic fragility and good glass forming ability. Five naringenin-based multi-component solids were generated in this study: 3 new CAMs, 1 new cocrystal, and 1 previously reported cocrystal. Destabilization of CAMs was found to be exposure specific and coformer dependent.

Predicting self-diffusion coefficients in semi-crystalline and amorphous solid dispersions using free volume theory

The self-diffusion coefficient of active ingredients (AI) in polymeric solid dispersions is one of the essential parameters for the rational formulation design in life sciences. Measuring this parameter for products in their application temperature range can, however, be difficult to realise and time-consuming (due to the slow kinetics of diffusion). The aim of this study is to present a simple and time-saving platform for predicting the AI self-diffusivity in amorphous and semi-crystalline polymers on the basis of a modified version of Vrentas’ and Duda’s free volume theory (FVT) [A. Mansuri, M. Völkel, T. Feuerbach, J. Winck, A.W.P. Vermeer, W. Hoheisel, M. Thommes, Modified free volume theory for self-diffusion of small molecules in amorphous polymers, Macromolecules. (2023)]. The predictive model discussed in this work requires pure-component properties as its input and covers the approximate temperature range of T < 1.2 Tg, the whole compositional range of the binary mixtures (as long as a molecular mixture is present), and the whole crystallinity range of the polymer. In this context, the self-diffusion coefficients of the AIs imidacloprid, indomethacin, and deltamethrin were predicted in polyvinylpyrrolidone, polyvinylpyrrolidone/vinyl acetate, polystyrene, polyethylene, and polypropylene. The results highlight the profound importance of the kinetic fragility of the solid dispersion on the molecular migration; a property which in some cases might entail higher self-diffusion coefficients despite an increase in the molecular weight of the polymer. We interpret this observation within the context of the theory of heterogeneous dynamics in glass-formers [M.D. Ediger, Spatially heterogeneous dynamics in supercooled liquids, Annu. Rev. Phys. Chem. 51 (2000) 99–128] by attributing it to the stronger presence of “fluid-like” mobile regions in fragile polymers offering facilitated routes for the AI diffusion within the dispersion. The modified FVT further allows for identifying the influence of some structural and thermophysical material properties on the translational mobility of AIs in binary dispersions with polymers. In addition, estimates of self-diffusivity in semi-crystalline polymers are provided by further accounting for the tortuosity of the diffusion paths and the chain immobilisation at the interface of the amorphous and crystalline phases.

Poly(cyclohexylsilsesquioxane)-Based Hydrophilic Thermoset-Resistant Deep-Ultraviolet-Transparent Glasses with Low Melting Temperatures.

Silsesquioxane (SQ)-based glasses with low melting temperatures were prepared by the cosolvent-free (solventless) hydrolytic polycondensation of organotrimethoxysilanes with cyclopentyl (c-Pe) and cyclohexyl (c-Hx) groups. Copolymers consisting of phenylsilsesquioxane (Ph-SQ) units and c-Pe-SQ units [poly(Ph-co-c-Pe-SQ)] or c-Hx-SQ units [poly(Ph-co-c-Hx-SQ)] were melted at 140 °C and formed clear glasses. The glasses prepared by this method contained many residual SiOH groups and exhibited high adhesive strength to microscope glass plates, metals, and several polymers. The glass-transition temperature of poly(c-Hx-SQ) was lower than that of poly(Ph-SQ) by only a small margin, whereas that of poly(c-Pe-SQ) was much lower. The poly(c-Hx-SQ)-based glasses were stiff at room temperature and transparent in the deep-ultraviolet spectral region (≲300 nm). They formed fragile melts with kinetic fragility parameters as high as ∼0.8. The melts of poly(c-Hx-SQ) and poly(c-Hx-co-Et-SQ) exhibited better resistance to thermal curing than that of poly(Ph-SQ) and maintained thermoplasticity even after heat treatment at 200 °C for 6 h.

Viscosity of the supercooled liquid and crystallization kinetic analysis in the Fe74B20Nb2Hf2Si2 amorphous alloy

The non-isothermal thermal behavior of Fe74B20Nb2Hf2Si2 bulk metallic glass (BMG) was studied utilizing various thermal analytical methods. The obtained results showed that the viscosity behavior of this alloy in the supercooled region was strong due to the high kinetic fragility parameter (D* = 49.5) and low fragility parameter (m = 20.2). Also, the apparent activation energy (Ep) values for crystallization process were calculated in the range of 188.7–394.6 kJ/mol using the Gao-Wang method. Additionally, the Avrami's exponent (n) values increased from 2 to 4 with the progress of the crystallization process. The two-parameter Šesták-Berggren (SB) model was used to demonstrate the autocatalytic behavior of crystallization. Furthermore, the kinetic analysis revealed that all crystallization stages were controlled by the interface. The phase analysis showed that the Fe23B6, α-Fe, Fe2(Hf, Nb), and Fe2B phases were formed during the crystallization process. As obtained by the microstructural characterization, it was founded that the low dimensional growth (m = 2) is due to the complex structure of the main phase of Fe23B6 in the first crystallization stage. Moreover, the high dimensional growth (m = 3) during the second and third crystallization stages attributed to the Fe2B phase as the heterogeneous nucleation, as well as a significant increase in the volume fraction of the α-Fe phase, and the preferential orientation of the various crystalline phases. Hence, the values of Avrami's exponent (n) in the second and third crystallization stages were calculated equal to 3 and 4, respectively.

Kinetic stability and glass-forming ability of phenacetin by fast scanning calorimetry

In the present work, an amorphous active pharmaceutical ingredient, phenacetin, was obtained by fast scanning calorimetry. The critical cooling rate and kinetic fragility of its supercooled melt were determined. The process of cold crystallization of phenacetin was studied by methods of isothermal and non-isothermal kinetics. It was found that the best correspondence between the two kinetic approaches is observed in the case of using the Nakamura crystallization model. The results obtained can find their application in the development of approaches to obtaining amorphous forms of drugs prone to crystallization.

Crystallization kinetics of GdYScAlCo high-entropy bulk metallic glass

The thermal stability and non-isothermal crystallization of a new bulk-amorphous high-entropy (HE-BMG) equiatomic GdYScAlCo alloy were studied by differential scanning calorimetry (DSC). The alloy shows a four-stage crystallization process. The kinetic parameters (activation energy (Eα)), the pre-exponential factor (logA) and glass-forming ability indicators (kinetic fragility index, characteristic temperatures) for the GdYScAlCo alloy were obtained. The Eα values obtained by isoconversional methods indicate a nonlinear Arrhenian behaviour and a complex process. The Avrami equation modification proposed by Jeziorny and the multivariate nonlinear regression method were applied on the nonisothermal crystallization. In the case of primary crystallization of the amorphous GdYScAlCo alloy under nonisothermal conditions, the kinetics of the nucleation process is best described by an autocatalytic reaction.

Viewing high entropy alloys through glasses: Linkages between solid solution and glass phases in multicomponent alloys

High entropy alloys (HEAs) represent highly promising multicomponent crystals that form concentrated solid solutions (CSSs) and may violate traditional thermodynamic rules of mixing, ultimately leading to excellent physical properties. For a deeper understanding, we investigate seven CSSs, including Co-Cr-Ni-Fe-Mn elements, at experimentally relevant compositions and conditions, through molecular simulations, and we use 1-1 comparisons to corresponding glass state characteristics, attained through rapid cooling protocols. We determine the behavior of various structural features, including the configurational entropy for a set of CSSs in their crystalline and vitreous states numerically. We employ swap Monte Carlo (MC) simulations, in combination with the reversible scaling method, to efficiently compute the configurational entropy (${S}_{\text{conf}}$), and show that the entropic rule of mixing is not always adequate for predicting alloy formation. We study the stability and formability of crystalline solid solutions, as well as glasses, while following the thermodynamics of ${S}_{\text{conf}}$. An apparent entropic similarity between CSSs and corresponding glasses leads us to use a Kauzmann-like ansatz, relating the CSSs at ${S}_{\text{conf}}\ensuremath{\rightarrow}0$ with the emergence of a CSS order-disorder transition, at temperature ${T}_{OD}$. In the context of glasses, a comparison between kinetic and thermodynamic fragilities allows the association of sluggish diffusion onset to a drop in ${S}_{\text{conf}}$ at ${T}_{K}$. Analogously, we classify CSSs as ``strong'' or ``fragile'' in the sense of their ability to migrate across CSS crystal configurations at high temperatures, distinguishing its formability. We argue that the magnitude of ${T}_{OD}$ may be an excellent predictor of CSS single-phase stability, which appears to scale with well-known HEA predictors, in particular we notice that VEC and ${T}_{OD}$ have in relation to the others a significantly large Pearson correlation coefficient, much larger than most other observables (except $\mathrm{\ensuremath{\Delta}}{H}_{\text{mix}}$).

Structure ordering and glass transition in size-asymmetric ternary mixtures of hard spheres: Variation from fragile to strong glasses.

We investigate the structure and activated dynamics of a binary mixture of colloidal particles dispersed in a solvent of much smaller-sized particles. The solvent degrees of freedom are traced out from the grand partition function of the colloid-solvent mixture which reduces the system from ternary to effective binary mixture of colloidal particles. In the effective binary mixture colloidal particles interact via effective potential that consists of bare potential plus the solvent-induced interaction. Expressions for the effective potentials and pair correlation functions are derived. We used the result of pair correlation functions to determine the number of particles in a cooperatively reorganizing cluster (CRC) in which localized particles form "long-lived" nonchemical bonds with the central particle. For an event of relaxation to take place these bonds have to reorganize irreversibly, the energy involved in the processes is the effective activation energy of relaxation. Results are reported for hard sphere colloidal particles dispersed in a solvent of hard sphere particles. Our results show that the concentration of solvent can be used as a control parameter to fine-tune the microscopic structural ordering and the size of CRC that governs the glassy dynamics. We show that a small variation in the concentration of solvent creates a bigger change in the kinetic fragility which highlights a wide variation in behavior, ranging from fragile to strong glasses. We conclude that the CRC which is determined from the static pair correlation function and the fluctuations embedded in the system is probably the sole player in the physics of glass transition.

Disentangling structural and kinetic components of the α-relaxation in supercooled metallic liquids

The particle motion associated to the α-relaxation in supercooled liquids is still challenging scientists due to its difficulty to be probed experimentally. By combining synchrotron techniques, we report the existence of microscopic structure-dynamics relationships in Pt42.5Cu27Ni9.5P21 and Pd42.5Cu27Ni9.5P21 liquids which allows us to disentangle structural and kinetic contributions to the α-process. While the two alloys show similar kinetic fragilities, their structural fragilities differ and correlate with the temperature dependence of the stretching parameter describing the decay of the density fluctuations. This implies that the evolution of dynamical heterogeneities in supercooled alloys is determined by the rigidity of the melt structure. We find also that the atomic motion not only reflects the topological order but also the chemical short-range order, which can lead to a surprising slowdown of the α-process at the mesoscopic length scale. These results will contribute to the comprehension of the glass transition, which is still missing.

Room Temperature Dye Glasses: A Guideline Toward the Fabrication of Amorphous Dye Films with Monomeric Absorption and Emission

The morphology of films containing photoactive materials is crucial for the performance of solid-state dye applications. Organic dyes tend to crystallize due to their usually planar molecular structure and the resulting intermolecular interactions. This leads to inhomogeneous films with crystalline, aggregated, and amorphous regions, decreasing device efficiency and complicating spectral analysis. Improving the glass-forming ability of organic dyes therefore presents a major challenge for solid-state dye applications. Here, we present a guideline to create organic dye glasses using BODIPY as a model dye. The method is based on the strategic design of BODIPY derivatives, equipped with short alkyl chains, in combination with blending of two or more derivatives. Mixing increases the entropy of the liquid state and lowers the thermodynamic driving force for crystallization as well as the kinetic fragility of the system. This enables the fabrication of homogeneous thin films without any additives. In these films, the dye molecules are trapped in a glassy state, featuring monomeric absorption and emission. This strategy leads to a BODIPY material with an amorphous character in thin films, dropcast films, and bulk. Further, the strategy is based on thermodynamics and is therefore expected to be general, enabling the transformation of any dye molecule into a glass former.

Segmental Relaxation Dynamics in Amorphous Polylactide Exposed to UV Light

AbstractThe degradation of polylactide (PLA) under UV exposure is investigated in terms of cooperativity and kinetic fragility at the glass transition. In the first part, possibly coexisting degradation mechanisms are evoked from the interpretation of the infrared spectroscopy analyses. Furthermore, the reduction of PLA chain length, owing to photolytic scissions predominant over local crosslinks, is assessed from chromatography, and confirmed by the shift of the glass transition temperature toward lower temperature. Modulated temperature thermogravimetric analysis (MT‐TGA) also shows that the activation energy needed to initiate thermal degradation falls after UV exposure. In the second part, the impact of UV‐induced degradation on the cooperative rearranging region (CRR) size and the kinetic fragility, respectively, calculated thanks to calorimetric and dielectric measurements, is discussed. Despite the assumed concomitance of several degradation mechanisms, it is observed that the glass transition, the kinetic fragility, and the CRR size decrease together with the exposure time. Moreover, it is found that the data align well on another trend depicting the change in the relaxation properties caused by plasticization of PLA. Thus, the variations of segmental relaxation properties caused by UV may be related to the increase of free volume linked to the damaging of the PLA structure.

Glass transition kinetics and fragility of ZrCuAlNi(Nb) metallic glasses

The kinetics of glass transition and the fragility of Zr55Cu30Al10Ni5 and (Zr55Cu30Al10Ni5)98Nb2 metallic glasses were investigated by the calorimetric measurements. The Nb minor addition narrows the width of glass transition, accelerates the relaxation process during glass transition, and decreases the energy barrier of glass transition. In addition, the fragility was evaluated in terms of the kinetic fragility index. While both alloys can be classified as intermediate glass formers, the Nb minor addition remarkably decreases the fragility and a stronger glass is achieved by the microalloying process. The lower fragility in the Nb-containing alloy is accompanied by higher degree of short-range ordering inferred from structural characterizations, formation of nanoscale chemical heterogeneities evidenced from the energy-dispersive X-ray spectroscopy (EDS) results, and deteriorated plasticity evaluated from the bending experiments. Our results highlight positive correlations between the fragility and the plasticity of the studied glasses.

Kinetic fragility directly correlates with the many-body static amorphous order in glass-forming liquids

The term ``fragility'' describes the rate at which viscosity grows when a supercooled liquid approaches its putative glass transition temperature. The field of glassy materials is actively searching for a structural origin that governs this dynamical slowing down in the supercooled liquid, which occurs without any discernible change in structure. Our work shows clear evidence that growing many-body static amorphous order is intimately correlated with the kinetic fragility of glass-forming liquids. It confirms that the system's dynamical response to temperature is concealed in its microstructures. This finding may pave the way for a deeper understanding of the different temperature dependence of the relaxation time or viscosity in a wide variety of glass-forming liquids.

Non-isothermal crystallisation behaviour of a Cr–Fe–Ni–Co–Mo–Si amorphous coating

The kinetics and mechanism of crystallisation of a Cr–Fe–Ni–Co–Mo–Si amorphous coating were studied using differential scanning calorimetry, owing to its relatively complex crystallisation behaviour. The premise of the production and application of amorphous materials is to understand the crystallization process of amorphous materials. It is closely related to the thermal stability of materials. Thermal stability is an important parameter to determine the effective working limit of materials. This study provides significant guidance for obtaining amorphous materials with excellent performance by controlling the crystallisation process and predicting the crystallisation failure of the coating in service. In the non-isothermal crystallisation process, results showed that the apparent activation energy (Ex) corresponding to the temperature of the onset of crystallisation was greater than those corresponding to the glass transition (Eg) and peak crystallisation (Ep) temperatures; Ex, Eg, and Ep were calculated using the Kissinger and Ozawa methods. This finding revealed that for the amorphous coating, the nucleation process was more difficult than overcoming the energy barrier for the rearrangement of atoms and the grain growth process of crystallisation. The relational plots between the local activation energy and crystallisation volume fraction under non-isothermal conditions illustrated that the crystallisation process of the amorphous coating was initially difficult but became easy in the succeeding stages. The kinetic fragility indices calculated by the Angell’s method and DA equation were 16.75 and 16.42, respectively; the results suggested that the Cr–Fe–Ni–Co–Mo–Si amorphous coating was a ‘strong material’. Furthermore, the two kinetic fragility indices indicate the good glass forming ability and thermal stability of the Cr–Fe–Ni–Co–Mo–Si amorphous coating.

Inter-enantiomer conversion dynamics and Johari\u2013Goldstein relaxation of benzophenones

We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted ortho-bromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules. Despite the significantly higher mass of the brominated derivative, its dynamic and calorimetric glass transition temperatures are only ten degrees higher than those of benzophenone. The kinetic fragility index of the halogenated molecule is lower than that of the parent compound, and is found to decrease with increasing pressure. By a detailed analysis of the dielectric loss spectra, we provide evidence for the existence of a Johari–Goldstein (JG) relaxation in both compounds, thus settling the controversy concerning the possible lack of a JG process in benzophenone and confirming the universality of this dielectric loss feature in molecular glass-formers. Both compounds also display an intramolecular relaxation, whose characteristic timescale appears to be correlated with that of the cooperative structural relaxation associated with the glass transition. The limited molecular flexibility of ortho-bromobenzophenone allows identifying the intramolecular relaxation as the inter-enantiomeric conversion between two isoenergetic conformers of opposite chirality, which only differ in the sign of the angle between the brominated aryl ring and the coplanar phenyl-ketone subunit. The observation by dielectric spectroscopy of a similar relaxation also in liquid benzophenone indicates that the inter-enantiomer conversion between the two isoenergetic helicoidal ground-state conformers of opposite chirality occurs via a transition state characterized by a coplanar phenyl-ketone moiety.