Glassy Clusters Research Articles

Sequence-Enhanced Self-Healing in "Lock-and-Key" Copolymers.

Van der Waals-driven self-healing in copolymers with "lock-and-key" architecture has emerged as a concept to endow engineering-type polymers with the capacity to recover from structural damage. Complicating the realization of "lock-and-key"-enabled self-healing is the tendency of copolymers to form nonuniform sequence distributions during polymerization reactions. This limits favorable site interactions and renders the evaluation of van der Waals-driven healing difficult. Here, methods for the synthesis of lock-and-key copolymers with prescribed sequence were used to overcome this limitation and enable the deliberate synthesis of "lock-and-key" architectures most conducive to self-healing. The effect of molecular sequence on the material's recovery behavior was evaluated for the particular case of three poly(n-butyl acrylate/methyl methacrylate) [P(BA/MMA)] copolymers with similar molecular weights, dispersity, and overall composition but with different sequences: alternating (alt), statistical (stat), and gradient (grad). They were synthesized using atom transfer radical polymerization (ATRP). Copolymers with alt and stat sequence displayed a 10-fold increase of recovery rate compared to the grad copolymer variant despite a similar overall glass transition temperature. Investigation with small-angle neutron scattering (SANS) revealed that rapid property recovery is contingent on a uniform microstructure of copolymers in the solid state, thus avoiding the pinning of chains in glassy MMA-rich cluster regions. The results delineate strategies for the deliberate design and synthesis of engineering polymers that combine structural and thermal stability with the ability to recover from structural damage.

Phonon transport properties of particulate physical gels.

Particulate physical gels are sparse, low-density amorphous materials in which clusters of glasses are connected to form a heterogeneous network structure. This structure is characterized by two length scales, ξs and ξG: ξs measures the length of heterogeneities in the network structure and ξG is the size of glassy clusters. Accordingly, the vibrational states (eigenmodes) of such a material also exhibit a multiscale nature with two characteristic frequencies, ω* and ωG, which are associated with ξs and ξG, respectively: (i) phonon-like vibrations in the homogeneous medium at ω<ω*, (ii) phonon-like vibrations in the heterogeneous medium at ω*<ω<ωG, and (iii) disordered vibrations in the glassy clusters at ω > ωG. Here, we demonstrate that the multiscale characteristics seen in the static structures and vibrational states also extend to the phonon transport properties. Phonon transport exhibits two distinct crossovers at frequencies ω* and ωG (or at wavenumbers of ∼ξs -1 and ∼ξG -1). In particular, both transverse and longitudinal phonons cross over between Rayleigh scattering at ω<ω* and diffusive damping at ω>ω*. Remarkably, the Ioffe-Regel limit is located at the very low frequency of ω*. Thus, phonon transport is localized above ω*, even where phonon-like vibrational states persist. This markedly strong scattering behavior is caused by the sparse, porous structure of the gel.

Itinerant ferromagnetism in van der Waals Fe5−xGeTe2 crystals above room temperature

Two-dimensional (2D) van der Waals (vdW) magnets have recently attracted increasing attention, as they provide a novel system for exploring 2D magnetism. However, intrinsic ferromagnetism in 2D systems has almost exclusively been observed at low temperatures, limiting their technological relevance. ${\mathrm{Fe}}_{\mathrm{N}}{\mathrm{Ge}\mathrm{Te}}_{2}$ ($N=3$, 4, and 5) systems are currently becoming the most attractive 2D vdW materials due to their relatively high Curie temperatures and large saturation magnetization. However, the nature of their complex yet intriguing magnetic behaviors is still unclear, in part due to the multiple inequivalent iron sites and iron vacancies. Here, we show evolution of magnetic ordering transitions in ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ with high Curie temperature and a strong saturation magnetization using photoemission electron microscopy and transport measurements. At 275 K, the ferromagnet transitions to a ferrimagnet, and below 110 K transitions to a state with glassy clusters. These are evidenced from temperature-dependent magnetic stripe domain evolution and anisotropic magnetoresistance measurements. Our findings show a clear magnetic ground state of ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ at room temperature which signals that ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ system is a very promising candidate for spintronic devices and provides a material design pathway to further increase the Curie temperature and saturation moments in vdW ferromagnets.

Crystal, Fivefold and Glass Formation in Clusters of Polymers Interacting with the Square Well Potential.

We present results, from Monte Carlo (MC) simulations, on polymer systems of freely jointed chains with spherical monomers interacting through the square well potential. Starting from athermal packings of chains of tangent hard spheres, we activate the square well potential under constant volume and temperature corresponding effectively to instantaneous quenching. We investigate how the intensity and range of pair-wise interactions affected the final morphologies by fixing polymer characteristics such as average chain length and tolerance in bond gaps. Due to attraction chains are brought closer together and they form clusters with distinct morphologies. A wide variety of structures is obtained as the model parameters are systematically varied: weak interactions lead to purely amorphous clusters followed by well-ordered ones. The latter include the whole spectrum of crystal morphologies: from virtually perfect hexagonal close packed (HCP) and face centered cubic (FCC) crystals, to random hexagonal close packed layers of single stacking direction of alternating HCP and FCC layers, to structures of mixed HCP/FCC character with multiple stacking directions and defects in the form of twins. Once critical values of interaction are met, fivefold-rich glassy clusters are formed. We discuss the similarities and differences between energy-driven crystal nucleation in thermal polymer systems as opposed to entropy-driven phase transition in athermal polymer packings. We further calculate the local density of each site, its dependence on the distance from the center of the cluster and its correlation with the crystallographic characteristics of the local environment. The short- and long-range conformations of chains are analyzed as a function of the established cluster morphologies.

Colloidal gel elasticity arises from the packing of locally glassy clusters

Colloidal gels formed by arrested phase separation are found widely in agriculture, biotechnology, and advanced manufacturing; yet, the emergence of elasticity and the nature of the arrested state in these abundant materials remains unresolved. Here, the quantitative agreement between integrated experimental, computational, and graph theoretic approaches are used to understand the arrested state and the origins of the gel elastic response. The micro-structural source of elasticity is identified by the l-balanced graph partition of the gels into minimally interconnected clusters that act as rigid, load bearing units. The number density of cluster-cluster connections grows with increasing attraction, and explains the emergence of elasticity in the network through the classic Cauchy-Born theory. Clusters are amorphous and iso-static. The internal cluster concentration maps onto the known attractive glass line of sticky colloids at low attraction strengths and extends it to higher strengths and lower particle volume fractions.

Growing length scale accompanying vitrification: A perspective based on nonsingular density fluctuations.

A model for describing growing length scale accompanying the vitrification is introduced, in which we assume that in a subsystem whose density is above a certain threshold value, ρ_{c}, due to steric constraints, particle rearrangements are highly suppressed for a sufficiently long time period (∼structural relaxation time). We regard such a subsystem as a glassy cluster. With this assumption and without introducing any complicated thermodynamic arguments, we predict that with compression (increasing average density ρ) at a fixed temperature T in supercooled states, the characteristic length of the clusters, ξ, diverges as ξ∼(ρ_{c}-ρ)^{-2/d}, where d is the spatial dimensionality. This ξ measures the average persistence length of the steric constraints in blocking the rearrangement motions and is determined by the subsystem density. Additionally, with decreasing T at a fixed ρ, the length scale diverges in the same manner as ξ∼(T-T_{c})^{-2/d}, for which ρ is identical to ρ_{c} at T=T_{c}. The exponent describing the diverging length scale is the same as the one predicted by some theoretical models and indeed has been observed in some simulations and experiments. However, the basic mechanism for this divergence is different; that is, we do not invoke thermodynamic anomalies associated with the thermodynamic phase transition as the origin of the growing length scale. We further present arguements for the cooperative properties of the structural relaxation based on the clusters.

Thermal stability of the Ti-Zr-Cu-Pd nano-glassy thin films

Nano-glassy thin films consist of nanometer-sized glassy regions (clusters) with a structure corresponding to melt-quenched glasses and amorphous interfacial regions characterized by a reduced density, or locally enhanced free volume, and in many cases a modified chemical composition. Ti-Zr-Cu-Pd nano-glass thin films were synthesized by using direct current (DC) magnetron sputtering on Si substrates. The influence of the sputtering conditions (sputtering power and Ar gas pressure) on the nano- and microstructure of deposited films and on their thermal behavior was analyzed. The thin films were noted to exhibit a homogeneous amorphous structure when sputtered at low Ar pressure (0.2 Pa) and a nano-glass structure with various sizes of the glassy cluster at higher Ar pressures (0.5–0.8 Pa). By raising the Ar pressure, the average size of the glassy clusters was noted to decrease corresponding to an increase of the volume fraction of the interfacial regions. The glass transition and crystallization temperatures were determined by using differential scanning calorimetry (DSC) at heating rates up to 40 Kmin−1. The thermal stability of the thin films was studied by annealing at temperatures above the glass transition and crystallization temperatures. The microstructure of the thin films was studied by means of X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy. The results obtained indicate that the nano-glassy thin films exhibit higher thermal stability than homogeneous glassy thin films. The fact that an increase of the volume fraction of the interfacial component in nano-glassy thin films results in an ultra-stable thermal behavior is indicating that the presence of nano-glassy interfaces impedes crystallization.

Investigation of the deposition conditions on the microstructure of TiZrCuPd nano-glass thin films

Nano-grained metallic glass Ti-Zr-Cu-Pd thin films were deposited at room temperature on Si substrate by using direct current (DC) magnetron sputtering with an alloy target. The influence of the sputtering parameters like the DC power and sputtering pressure on the chemical composition and the microstructure of the deposited films was analyzed. Experiments were carried out by changing the sputtering pressure (from 0.2 to 0.8Pa) and the sputtering power used was 100 or 200W. The compositional variation with sputtering pressure and applied power was found to be due to the dissipation of the kinetic energy of the sputtered atoms by the interatomic collision. Microstructural studies of the as-deposited thin films by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) exhibited that the films have an amorphous structure. Nanometer-sized glassy clusters were observed in the thin films deposited at higher Ar pressure. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the surface morphology and topography of the Ti-Zr-Cu-Pd thin films. The as-deposited thin films exhibited granular morphology and its surface roughness was influenced by the Ar pressure. At low sputtering pressures, the surface morphology is characterized by low surface roughness. Moreover, the chemical composition and the size of glassy regions in the nano-glass thin films can be controlled by changing the sputtering parameters.

Two-phase amorphous-amorphous solid drug dispersion with enhanced stability, solubility and bioavailability resulting from ultrasonic dispersion of an immiscible system.

Amorphization of active pharmaceutical ingredients (APIs) and the preparation of solid dispersions are strategies that can be synergized to improve the solubility of oral drugs. Immiscibility between an API and a carrier in the molten state that could be perceived as a problem in the preparation of solid dispersions, may actually introduce an advantage. In the present work, a two-phase amorphous-amorphous solid dispersion (AASD) was prepared by ultrasonicating a molten immiscible mixture of indomethacin (IND) and glucose (GLU) prior quenching. By introducing this novel ultrasound assisted method, the immiscible API particles were uniformly dispersed as microscopic glassy clusters of the drug in the solid amorphous GLU matrix; particle sizes of IND in the AASD range from 600nm to 1.4μm. As a result of the amorphization and particle size reduction of IND, its aqueous solubility increased to reach almost 40ppm (8 times more soluble compared to indomethacin in its crystalline state). In addition, the oral bioavailability and its resistance against crystallization were also enhanced; AASD samples have remained amorphous for more than two years of storage.

ChemInform Abstract: Nanostructured Solids — From Nano‐Glasses to Quantum Transistors

AbstractReview: 212 refs.

Structural signatures of dynamic heterogeneities in monolayers of colloidal ellipsoids.

When a liquid is supercooled towards the glass transition, its dynamics drastically slows down, whereas its static structure remains relatively unchanged. Finding a structural signature of the dynamic slowing down is a major challenge, yet it is often too subtle to be uncovered. Here we discover the structural signatures for both translational and rotational dynamics in monolayers of colloidal ellipsoids by video microscopy experiments and computer simulations. The correlation lengths of the dynamic slowest-moving clusters, the static glassy clusters, the static local structural entropy and the dynamic heterogeneity follow the same power-law divergence, suggesting that the kinetic slowing down is caused by a decrease in the structural entropy and an increase in the size of the glassy cluster. Ellipsoids with different aspect ratios exhibit single- or double-step glass transitions with distinct dynamic heterogeneities. These findings demonstrate that the particle shape anisotropy has important effects on the structure and dynamics of the glass.

Relation between the Dynamics of Glassy Clusters and Characteristic Features of their Energy Landscape

Relation between the Dynamics of Glassy Clusters and Characteristic Features of their Energy Landscape

Relationship between neighbor number and vibrational spectra in disordered colloidal clusters with attractive interactions

We study connections between vibrational spectra and average nearest neighbor number in disordered clusters of colloidal particles with attractive interactions. Measurements of displacement covariances between particles in each cluster permit calculation of the stiffness matrix, which contains effective spring constants linking pairs of particles. From the cluster stiffness matrix, we derive vibrational properties of corresponding "shadow" glassy clusters, with the same geometric configuration and interactions as the "source" cluster but without damping. Here, we investigate the stiffness matrix to elucidate the origin of the correlations between the median frequency of cluster vibrational modes and average number of nearest neighbors in the cluster. We find that the mean confining stiffness of particles in a cluster, i.e., the ensemble-averaged sum of nearest neighbor spring constants, correlates strongly with average nearest neighbor number, and even more strongly with median frequency. Further, we find that the average oscillation frequency of an individual particle is set by the total stiffness of its nearest neighbor bonds; this average frequency increases as the square root of the nearest neighbor bond stiffness, in a manner similar to the simple harmonic oscillator.

Phonon Spectra, Nearest Neighbors, and Mechanical Stability of Disordered Colloidal Clusters with Attractive Interactions

We investigate the influence of morphology and size on the vibrational properties of disordered clusters of colloidal particles with attractive interactions. Spectral features of the vibrational modes are found to depend strongly on the average number of nearest neighbors, NN, but only weakly on the number of particles in each glassy cluster. In particular, the median phonon frequency, ω(med), is constant for NN<2 and then grows linearly with NN for NN>2. This behavior parallels concurrent observations about local isostatic structures, which are absent in clusters with NN<2 and then grow linearly in number for NN>2. Thus, cluster vibrational properties appear to be strongly connected to cluster mechanical stability, and the scaling of ω(med) with NN is reminiscent of the jamming transition. Simulations of random networks of springs corroborate observations.

Dynamics of Electron Localization in Warm versus Cold Water Clusters

The process of electron localization on a cluster of 32water molecules at 20, 50, and 300K is unraveled using abinitio molecular dynamics simulations. In warm, liquid clusters, the excess electron relaxes from an initial diffuse and weakly bound structure to an equilibrated, strongly bound species within 1.5ps. In contrast, in cold, glassy clusters the relaxation processes is not completed and the electron becomes trapped in a metastable surface state with an intermediate binding energy. These results question the validity of extrapolations of the properties of solvated electrons from cold clusters of increasing size to the liquid bulk.

Colloidal glass transition in unentangled polymer nanocomposite melts

The linear and non-linear rheology of a high volume fraction particle filled unentangledpolymer melt is measured. The particles in the polymer melt behave like hard spheres asthe particle volume fraction is raised. At high volume fractions, the suspension develops aplateau elastic modulus. Over the frequency range of the elastic modulus plateau, theviscous modulus develops a minimum and a maximum. The frequencies of the two localextrema initially have critical power law scaling, suggesting the approach of a singular glasstransition. At higher volume fractions in excess of the glass transition, the viscous moduluscontinues to show a well defined minimum and a well defined maximum. The non-linearmoduli show a single perturbative yield point beyond which the suspension softens. Theyielding behavior of the nanocomposite is shown to be sensitive to the strainfrequency and the proximity of the strain frequency to the maximum frequencyfor the linear viscous modulus from linear rheology which characterizes thermalrelaxation of glassy particle clusters in the zero strain limit. The linear and non-linearmeasurements are compared against a recently developed mechanical theory for colloidalglasses.

Evaluation of Inutec SP1 as a new carrier in the formulation of solid dispersions for poorly soluble drugs

Solid dispersions made up of itraconazole and Inutec SP1, a new polymeric surfactant, were prepared by spray drying and hot-stage extrusion. Differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) were used to evaluate the miscibility of the components of the dispersions, and dissolution experiments were performed in simulated gastric fluid without pepsin (SGF sp) to evaluate the pharmaceutical performance of itraconazole from the solid dispersions. DSC analysis showed that the solid dispersions are phase separated systems made up of glassy and crystalline itraconazole and amorphous Inutec SP1. The amount of crystalline drug substance was higher in the dispersions prepared by hot-stage extrusion and was clearly a function of the drug concentration. Since no crystallinity could be detected by XRD points to the fact that the crystallites formed are very small in size. Despite the presence of glassy and crystalline clusters, the dissolution properties of the solid dispersions were significantly improved in comparison to pure itraconazole (glassy or crystalline) or physical mixtures with Inutec SP1. This study proves the potential of the new polymeric surfactant as a carrier in the formulation of solid dispersions for poorly soluble drugs.

Increased Physical Stability and Improved Dissolution Properties of Itraconazole, a Class II Drug, by Solid Dispersions that Combine Fast‐ and Slow‐Dissolving Polymers

Solid dispersions were prepared of itraconazole‐Eudragit E100, itraconazole‐PVPVA64, and itraconazole‐Eudragit E100/PVPVA64 using a corotating twin‐screw hot‐stage extruder. Modulated temperature differential scanning calorimetry (MTDSC) was used to evaluate the miscibility of the extrudates, and dissolution experiments were performed in simulated gastric fluid without pepsin (SGFsp). Itraconazole and Eudragit E100 are miscible up to 13% w/w drug loading. From that concentration on, phase separation is observed. Pharmaceutical performance of this dispersion was satisfactory because 80% of the drug dissolved after 30min. Extrudates of itraconazole and PVPVA64 were completely miscible but the pharmaceutical performance was low, with 45% of drug dissolved after 3h. Combination of both polymers in different ratios, with a fixed drug loading of 40% w/w, was evaluated. MTDSC results clearly indicated a two‐phase system consisting of itraconazole‐Eudragit E100 and itraconazole‐PVPVA64 phases. In these extrudates, no free crystalline or glassy clusters of itraconazole were observed; all itraconazole was mixed with one of both polymers. The pharmaceutical performance was tested in SGFsp for different polymer ratios, and Eudragit E100/PVPVA64 ratios of 50/50 and 60/40 showed significant increases in dissolution rate and level. Polymer ratios of 70/30 and 80/20, on the other hand, had a release of 85% after 30min. Precipitation of the drug was never observed. The combination of the two polymers provides a solid dispersion with good dissolution properties and improved physical stability compared with the binary solid dispersions of itraconazole. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:124–131, 2004

Ergodicity failure near structural glass transformations

Relaxation timescale for glass-forming materials is analyzed within a self-consistent description introduced within percolation theory treatment of the Adam–Gibbs model. The ergodic–non-ergodic phase diagram is proposed for the case of molecular supercooled liquids in terms of the ergodic-phase instability temperature T E vs fragility. T E, being below and close to the glass-transformation temperature T g, is established through violation of the ergodic hypothesis, i.e. by a crossover from the Gaussian (‘ergodic’) to a non-Gaussian dynamics of evolution of clusters. The finite-size fractal cluster distribution is deduced from the known Stauffer scaling form. Crossover of the compact-structure (‘ergodic’) clusters to the hole-like glassy clusters is attributed to their critical-size thermal fluctuations.

Dynamics of glassy clusters appearing by nonlinear dielectric effect studies

Preliminary results of time-resolved nonlinear dielectric effect (NDE) studies in the liquid, glassforming epoxy resin EPON 5 are presented. For all tested temperatures the NDE response of the sample parametrizes the stretched exponential function with exponent $\ensuremath{\beta}=0.38\ifmmode\pm\else\textpm\fi{}0.05.$ The temperature evolution of relaxation times can be parametrized by means of the Vogel-Fulcher-Tammann function ${\ensuremath{\tau}}^{\mathrm{NDE}}={\ensuremath{\tau}}_{0}^{\mathrm{NDE}}\mathrm{exp}{(D}_{\mathrm{NDE}}{T}_{0}^{\mathrm{NDE}}/T\ensuremath{-}{T}_{0}^{\mathrm{NDE}}),$ with ${\ensuremath{\tau}}_{0}^{\mathrm{NDE}}\ensuremath{\approx}0.3\mathrm{s},$ ${D}_{E}\ensuremath{\approx}1.2,$ and ${T}_{0}^{\mathrm{NDE}}\ensuremath{\approx}261\mathrm{K}.$ The latter value is, within the limit of the experimental error, equal to the glass temperature ${T}_{g}.$ The relaxation of ``nonlinear'' changes of the dielectric permittivity induced by a strong, steady electric field may be associated with the dynamic equilibrium of glassy clusters and the surrounding fluidlike region.