Structural Relaxation Processes Research Articles

Concentrated suspensions of Brownian beads in water: dynamic heterogeneities through a simple experimental technique

Concentrated suspensions of Brownian hard-spheres in water are an epitome for understanding the glassy dynamics of both soft materials and supercooled molecular liquids. From an experimental point of view, such systems are especially suited to perform particle tracking easily, and, therefore, are a benchmark for novel optical techniques, applicable when primary particles cannot be resolved. Differential variance analysis (DVA) is one such novel technique that simplifies significantly the characterization of structural relaxation processes of soft glassy materials, since it is directly applicable to digital image sequences of the sample. DVA succeeds in monitoring not only the average dynamics, but also its spatio-temporal fluctuations, known as dynamic heterogeneities. In this work, we study the dynamics of dense suspensions of Brownian beads in water, imaged through digital video-microscopy, by using both DVA and single-particle tracking. We focus on two commonly used signatures of dynamic heterogeneities: the dynamic susceptibility, χ4, and the non-Gaussian parameter, α2. By direct comparison of these two quantities, we are able to highlight similarities and differences. We do confirm that χ4 and α2 provide qualitatively similar information, but we find quantitative discrepancies in the scalings of characteristic time and length scale on approaching the glass transition.

Structure-property relations characterizing the devitrification of Ni-Zr glassy alloy thin films

The investigation of devitrification in thermally annealed nanodimensional glassy alloy thin films provides a comprehensive understanding of their thermal stability, which can be used to explore potential applications. The amorphous to crystalline polymorphous transformation of cosputtered Ni-Zr alloy (Ni78Zr22 at. %) films, with a thickness lower than the reported critical limit of devitrification, was studied through detailed structural characterization and molecular dynamics (MD) simulations. Devitrification to a nanocrystalline state (Ni7Zr2 structure) was observed at 800 °C, with an increase in density (∼3.6%) much higher than that achieved in bulk alloys. Variation in the magnetic property of the films and the overall physical structure including morphology and composition were examined before and after annealing. MD simulations were employed to effectively elucidate not only the high densification but also the increased magnetic moment after annealing, which was correlated with the simulated change in the coordination number around Ni atoms. The structural relaxation process accompanying devitrification was described as a disorder-to-order transformation while highlighting the crucial role played by chemical short range order prevalent in glassy materials.

Viscoelastic study of the creep behavior of GeS2-SbS3/2-SnS and GaS3/2-SbS3/2-SnS glasses around their deformation temperatures

A viscoelastic study of the 25GeS2-55SbS3/2-20SnS and 12.5GaS3/2-75SbS3/2–12.5SnS sulfide glasses was carried out around their deformation temperatures using a uniaxial compression creep test. Their viscosities showed similar temperature dependences, but slightly lower sensitivity was observed for the latter glass than for the former glass, which may be explained by the theory of free volume. The creep function, derived from the displacement of the height of the sample subjected to a creep test, was converted into relaxation moduli by performing a Laplace transformation and its inversion. The shear relaxation moduli G(t) revealed two distinct structural relaxation processes, specifically with short and long relaxation times. Both the major fast-relaxation process and minor slow-relaxation process obeyed the time–temperature superposition principle. The fast-relaxation activation energies for both glasses were estimated to be about 390 kJ/mol. This value was close to the dissociation energy of the Sb—S bond, which formed the major cross-links of the glass network. In contrast, slow-relaxation activation energies of approximately 590 and 550 kJ/mol were determined for the GeS2-containing and GaS3/2-containing glasses, respectively. The slow-relaxation process, having a higher activation energy for the GeS2-containing glass than for the GaS3/2-containing glass, was concluded to have involved dissociation and recombination, even at high temperatures, of the stronger crosslinks made of the Ge—S and Ga—S bonds in the glass network.

Structural relaxation in the wave-vector dependence of the longitudinal rigidity modulus.

Brillouin microscopy recently attracted much attention for being a promising tool for all-optical label-free determination of mechanical properties of biological samples. Before its widespread utilization for biomedical applications, numbers of nuances related with this technique need to be recognized. In this article, we discuss the process of structural relaxation, the phenomena not commonly addressed by the emerging bio-Brillouin community, and its effect on longitudinal rigidity modulus. Using a model aqueous polymer mixture, we show how scattering measurements performed on the same specimen using different experimental geometries can lead to different results.

Heat-treatment-assisted approach towards scalable synthesis of mesoporous carbons for high-performance lithium-sulfur battery

A heat-treatment-assisted approach towards scalable synthesis of mesoporous carbons by direct solid-solid mixing of microcrystalline cellulose (MCC) and nano-sized SiO2, subsequent pyrolysis and SiO2 removal process, was reported in this study. The obtained mesoporous carbon presented a well-defined mesopore structure with high surface area and large mesopore size. During synthesis, MCC underwent structural relaxation process before degradation, which could provide a capillarity condensation force inducing the imprint of mesopore on final carbons. Due to the proper structure, the as-prepared mesoporous carbon showed prominent performance as sulfur host for lithium-sulfur (Li-S) battery, exhibiting a high reversible capacity of 860 mA h g−1 at 0.5 C after 200 cycles, and exceptional rate capability with 600 mA h g−1 at 5.0 C.

Linear and nonlinear shear studies reveal supramolecular responses in supercooled monohydroxy alcohols with faint dielectric signatures.

Monohydroxy alcohols (MAs) with methyl and hydroxyl side groups attached to the same carbon atom in the alkyl backbone can display very weak structural and supramolecular dielectric relaxation processes when probed in the regime of small electrical fields. This can render their separation and assignment difficult in the pure liquids. When mixing with bromoalkanes, a faint Debye-like process can be resolved dielectrically for 4-methyl-4-heptanol. To achieve a separation of structural and supramolecular processes in pure 4-methyl-4-heptanol and 3-methyl-3-heptanol, mechanical experiments are carried out in the linear-response regime as well as using medium-angle oscillatory shear amplitudes. It is demonstrated that first-order and third-order nonlinear mechanical effects allow for a clear identification of supramolecular viscoelastic modes even for alcohols in which they leave only a weak signature in the linear-response shear modulus. Additionally, the nonlinear rheological behavior of 2-ethyl-1-hexanol is studied, revealing that its linearly detected terminal mode does not coincide with that revealed beyond the linear-response regime. This finding contrasts with those for the other MAs studied in this work.

Stable High-Energy Excited States Observed in a Conjugated Molecule with Hindered Internal Conversion Processes

When the excited states of a molecule above its lowest excited state (Sn or Tn, n > 1) have a sufficient lifetime, photophysical and photochemical processes may occur directly in these high-energy levels, bypassing the Kasha’s rule and significantly influencing the system properties. Investigation of the relationship between molecular structure and intramolecular electronic relaxation processes targets molecular design to achieve a long-lifetime higher excited states. Here, we report stable high-energy excited states with lifetimes approaching several nanoseconds in our newly designed and synthesized compound PPI–AnCN. Through experimental and theoretical investigation, we reveal that both large energy gap and poor electronic coupling between the high excited state and the S1 state are responsible for this stability. More importantly, we find that the emission from the higher exited states can be promoted when the lowest excited state is quenched. This study provides new insights into understanding long-l...

Peculiarities of Phase Formation Processes in Activated Ti + Al Powder Mixture during Transition from Combustion Synthesis to High-temperature Annealing

ABSTRACTIn the present work, the experimental study of the features of phase formation processes in preliminary mechanically activated powder mixture Ti+Al was conducted. The synthesis was realized in the thermal explosion mode with the use of microwave heating. It was established that in the case of a continuous transition from the fast heating mode (thermal explosion) to the high-temperature annealing regime the structural relaxation processes occurs. These processes lead to equalization of the phase composition in the volume of the mixture and formation of the almost single phase compound TiAl. At later stages of annealing, the transition to thermodynamic equilibrium state takes place. This process is accompanied by formation of compounds which are in equilibrium with each other at a given temperature.

Method for calculating stress evolution in glass-metal composite taking into account structural and mechanical relaxation processes

Technological modes of annealing layered glass-metal composite material are investigated. A cylindrical glass-metal composite vessel with an internal glass cylinder is used as a model sample in the experimental studies of highly compressed fragile rocks mass. A mathematical model of the evolution of technological and residual stresses during its annealing is considered. The difficulty in modeling is due to the glass transition process and the elastic-plastic behavior of the metal. Structural and mechanical relaxation processes in glass are calculated using the Tula-Naariswami-Moynihan-Mazurin method, which is based on the concept of structural temperature, and the Boltzman-Volterra superposition principle. The dependence of viscosity and coefficient of linear temperature expansion on the structural temperature is taken into account. Elastic state in the stress space is limited by the surface of the Mises limit state for metal shell. In this paper, a finite-difference scheme for calculating structural changes and process stresses over the entire temperature treatment interval is proposed. To assess the accuracy of the developed calculation algorithm, we used an analytical solution of the problem with the Maxwell core in the absence of temperature-time analogy in glass and elastic deformation of the metal layer. Calculations of technological stresses in glass metal composite based on borosilicate glass and low carbon steel for various temperature regimes of annealing were carried out. The proposed mathematical model and calculation method can be used for evaluating residual stresses in the technology of applying glass coatings on metal pipes.

Structural Relaxation Processes and Collective Dynamics of Water in Biomolecular Environments

In this simulation study, we investigate the influence of biomolecular confinement on dynamical processes in water. We compare water confined in a membrane protein nanopore at room temperature to pure liquid water at low temperatures with respect to structural relaxations, intermolecular vibrations, and the propagation of collective modes. We observe distinct potential energy landscapes experienced by water molecules in the two environments, which nevertheless result in comparable hydrogen bond lifetimes and sound propagation velocities. Hence, we show that a viscoelastic argument that links slow rearrangements of the water-hydrogen bond network to ice-like collective properties applies to both, the pure liquid and biologically confined water, irrespective of differences in the microscopic structure.

Influence of cyclic loading on the structure and double-stage structure relaxation behavior of a Zr-Cu-Fe-Al metallic glass

The thermal and mechanical relaxation spectra of a Zr-Cu-Fe-Al bulk metallic glass were obtained before and after room-temperature cyclic loading up to 0.4% of elastic strain. The loading was performed using a dynamic mechanical analyzer at an oscillation frequency of 3 Hz to induce the structural changes within the glassy phase at room temperature. A double-stage structure relaxation behavior was monitored using the same dynamic mechanical analyzer at a lower strain on heating. These structure changes within the glassy phase were studied with X-ray diffractometry and high-resolution transmission electron microscopy. The cycling loading at room temperature lead to drastic changes in the atomic structure of the bulk metallic glassy samples. In particular, it caused partial nanocrystallization after 10,000 elastic cycles, which in turn influenced the relaxation behavior. The structural relaxation process of the Zr-Cu-Fe-Al bulk metallic glass monitored by thermal measurements is different from that observed by the dynamic mechanical analysis.

Studies on the molecular dynamics of acetylated oligosaccharides of different topologies (linear versus cyclic)

In this paper, the molecular dynamics and thermal properties of representative acetylated linear and cyclic oligosaccharides: acTRE, acRAF, acSTA, ac-α-CD, ac-β-CD, ac-γ-CD, have been investigated by using broadband dielectric spectroscopy and differential scanning calorimetry. We found that there are marked differences in the dynamics of the structural and secondary relaxation processes in both groups of materials. Just to mention a variation in the distribution of the structural relaxation times as well as different evolutions of the glass transition temperature (Tg) and fragility (m) versus molecular weight (Mw), which seem to be affected by the shape of the molecule, strain in the carbohydrate ring and mobility of side acetyl moieties.

Structure and dynamics of polypyrrole/chitosan nanocomposites

AbstractPolypyrrole/chitosan (PPy/Cs) nanocomposites with various amounts of polypyrrole nanoparticles (PPy‐NPs) have been synthesized and afterwards exposed to UV irradiation for 240 min. Structure, dynamics and antibacterial activity of pure compounds and the corresponding nanocomposites were investigated by means of wide‐angle X‐ray scattering, Fourier transform infrared spectroscopy, transmission electron microscopy, thermogravimetric analysis, broadband dielectric spectroscopy and antibacterial activity assay. The occurrence of a pronounced interaction between PPy‐NPs and Cs has been verified. Broadband dielectric spectroscopy measurements revealed two relaxation processes at low and high frequencies. The low‐frequencyα‐relaxation associated with the glass–rubber transition is attributed to the presence of hydrogen‐bonded water (plasticizing effect) appearing at low temperature (<373 K). The high‐frequencyσ‐relaxation is related to water molecules which become desorbed upon heating atT > 373 K and demonstrates Arrhenius temperature dependence with activation energy in the range 70–90 kJ mol−1in good agreement with previous reports. On the contrary, PPy‐NPs do not show any structural relaxation process and the charge transport mechanism follows the correlated barrier hopping model. All PPy/Cs nanocomposites exhibit an improved antibacterial activity against bothStaphylococcus aureusandEscherichia colibacteria as compared with pure Cs and PPy‐NPs. © 2018 Society of Chemical Industry

Novel features of pressure relaxation in nonequilibrium helium crystals

The method of precision barometry has been used to study the structural relaxation processes in nonequilibrium helium crystals obtained by liquid crystallization during rapid cooling in the temperature range of 1.3–1.7 K. The pressure at the ends of a cylindrical measuring cell with solid helium and their variation with time during stepwise heating of the sample were measured. The experimental results for the relaxation kinetics, when the pressure values recorded by both sensors increased with time, were analyzed. Analysis of the experimental results provides estimates of the average atomic concentration of vacancies in the crystal, the vacancy diffusion coefficient values, and the activation energy of vacancy motion. The present work continues the study of structural pressure relaxation initiated in a previous work (LTP 44, 402 (2018)), where a model describing the relaxation of internal stresses in nonequilibrium helium crystals associated with the diffusion of vacancies was proposed for interpretation of the experimental data.

Transient Nonlinear Response of Dynamically Decoupled Ionic Conductors.

The present study demonstrates that large electric fields progressively enhance the conductivity of ionic systems up to timescales corresponding to those on which their structural rearrangements take place. Yet, in many ionic materials, some regarded as candidates for electrical energy storage applications, the structural relaxation process can be tremendously slower than (or highly decoupled from) the charge fluctuations. Consequently, nonlinear dielectric spectroscopy may be employed to access rheological information in dynamically decoupled ionic conductors, whereas the combination of large electric power density and good mechanical stability, both technologically highly desired, imposes specific experimental constraints to reliably determine the steady-state conductivity of such materials.

Coexistence of two structural relaxation processes in monohydroxy alcohol-alkyl halogen mixtures: Dielectric and rheological studies.

Evidence for the existence of two glass transitions is found in binary mixtures of monohydroxy alcohols with an aprotic alkyl halide by means of dielectric spectroscopy and, markedly, also shear rheology. In the mechanical data, an enormous separation of two components becomes obvious for suitable compositions. The observation of bimodal motional heterogeneity is possible despite the fact that the glass transition temperatures of these substances differ by only 40 K. Obviously, the hydrogen-bond driven formation of supramolecular structures in one of the mixture components facilitates the emergence of dynamic contrast which for other binary liquids was so far only observed in the presence of much larger glass transition temperature differences.

Magnetic Properties of Amorphous Nanocrystalline Alloys Based on Iron with Different Contents of Nobium in Structural Relaxation Processes

Magnetic properties of amorphous nanocrystalline alloy Fe–(Cu, Nb)–(Si, B) during stepwise annealing are investigated by means of X-ray diffraction analysis, vibration magnetometry, and Mossbauer spectroscopy. It is shown that an increase of the Nb content reduces the amount of the nanocrystalline component in the alloy, stabilizing the amorphous state and magnetic softness during structural relaxation.

Studying molecular dynamics of the slow, structural, and secondary relaxation processes in series of substituted ibuprofens.

In this paper, the molecular dynamics of a series of ester derivatives of ibuprofen (IBU), in which the hydrogen atom from the hydroxyl group was substituted by the methyl, isopropyl, hexyl, and benzyl moieties, has been investigated using Broadband dielectric (BD), Nuclear magnetic resonance (NMR), and Raman spectroscopies. We found that except for benzyl IBU (Ben-IBU), an additional process (slow mode, SM) appears in dielectric spectra in all examined compounds. It is worth noting that this relaxation process was observed for the first time in non-modified IBU (a Debye relaxation). According to suggestions by Affouard and Correia [J. Phys. Chem. B. 114, 11397 (2010)] as well as further studies by Adrjanowicz et al. [J. Chem. Phys. 139, 111103 (2013)] on Met-IBU, it was attributed to synperiplanar-antiperiplanar conformational changes within the molecule. Herein, we have shown that with an increasing molecular weight of the substituent, the relaxation times of the SM become longer and its activation energy significantly increases. Moreover, this new relaxation mode was found to be broader than a simple Debye relaxation in Iso-IBU and Hex-IBU. Additional complementary NMR studies indicated that either there is a significant slowdown of the rotation around the O=C-O-R moiety or this kind of movement is completely suppressed in the case of Ben-IBU. Therefore, the SM is not observed in the dielectric loss spectra of this compound. Finally, we carried out isothermal experiments on the samples which have a different thermal history. Interestingly, it turned out that the relaxation times of the structural processes are slightly shorter with respect to those obtained from temperature dependent measurements. This effect was the most prominent in the case of Hex-IBU, while for Ben-IBU, it was not observed at all. Additional time-dependent measurements revealed the ongoing equilibration manifested by the continuous shift of the structural process, until it finally reached its equilibrium position. Further Raman investigations showed that this effect may be related to the rotational/conformational equilibration of the long hexyl chains. Our results are the first ones demonstrating that the structural process is sensitive to the conformational equilibration occurring in the specific highly viscous systems.

Tracer Transport Probes Relaxation and Structure of Attractive and Repulsive Glassy Liquids.

Dynamic coupling of small penetrants to slow, cooperative relaxations within crowded cells, supercooled liquids, and polymer matrices has broad consequences for applications ranging from drug delivery to nanocomposite processing. Interactions between the constituents of these and other disordered media alter the cooperative relaxations, but their effect on penetrant dynamics remains incompletely understood. We use molecular dynamics simulations to show that the motions of hard-sphere tracer particles probe differences in local structure and cooperative relaxation processes in attractive and repulsive glassy liquid matrices with equal bulk packing fractions and long-time diffusivities. Coupling of the tracer dynamics to collective relaxations in each matrix affects the shape of tracer trajectories, which are fractal within the repulsive matrix and more compact in the attractive. These results reveal that the structure of relaxations controls penetrant transport and dispersion in cooperatively relaxing systems and provide insight into dynamical heterogeneity within glassy liquids.

Two types of structural relaxations around the deformation temperature of Ge20Sb15Se65 glass

Chalcogenide glasses have attracted a great deal of attention for the optical elements of infrared devices. The viscosity and elasticity of glasses are important characteristics for the use of precision molding techniques. The uniaxial compression creep behavior of Ge20Sb15Se65 glass was studied around its deformation temperature. The derived creep function was converted into relaxation moduli by Laplace transformation and its inversion. Analysis using a two-term Maxwell model revealed the shear relaxation modulus G(t) as two distinct structural relaxation processes with short and long relaxation times on the order of 101 s and 103 s, respectively, at 313 °C. The fast relaxation with high elasticity obeyed the time–temperature superposition principle and its activation energy ΔH1 was estimated to be 350 ± 10 kJ/mol using Narayanaswamy's relation. This energy is mainly comparable to the bond dissociation energies of SeSe (~320 kJ/mol). The process was therefore suggested to be due to motion around these thermally broken inter-atomic bonds, as in many glassy systems. In contrast, the low-elasticity slow relaxation process had activation energy of approximately 480 ± 60 kJ/mol and was considered to represent breaking and rearrangement of heteronuclear GeSe and SbSe bonds in the glass network during pressure-induced entanglement.