Structural Relaxation Parameters Research Articles

Numerical simulation of the structural relaxation characteristics of K-LCV161 glass

Only a few glass structure relaxation parameters are applicable for finite element simulation. The structural relaxation characteristics of K-LCV161 glass were described using the Tool–Narayanaswamy–Moynihan (TNM) model in multiple differential scanning calorimetry(DSC) experiments. First, the proposed method of narrowing the fitting parameter range was based on the influence of TNM model parameters on DSC curve. The position of the DSC curve peak is affected by changing Δh*/R and A, the height of the DSC curve peak is affected by changing x and β. The least squares fitting method to solve the TNM model parameters was then systematically presented systematically presented, and the following results were obtained: ∆h*/R = 52,394.91 K, A = 6.15e−31 s, x = 0.77, β=0.70. Finally, the volume change of K-LCV161 glass during cooling was predicted by using the thermal expansion and structural relaxation models, and the difference between these two models was approximately 23%.

Renewable polyol obtained by microwave-assisted alcoholysis of epoxidized soybean oil: Preparation, thermal properties and relaxation process

The soybean oil polyol (SOP) use as feedstock in the polyurethane industry has been recently emphasized due to its excellent resistance to hydrolysis, which is also applicable in coatings and thermal insulation. In this article, the SOP was obtained by a very fast microwave-assisted alcoholysis of epoxidized soybean oil (ESO). The preparation method, thermal properties, and relaxation process were evaluated. High yields as opening and consumption epoxy group and selectivity of 99.8 mol%, 98.5 mol%, and 71.2 mol% were obtained. Through titrations, nuclear magnetic resonance and gel permeation chromatography were identified parameters as 0.32 mg KOH·g−1 acid number, 190 mg KOH·g−1 hydroxyl number, 150 mg KOH·g−1 saponification index, 0.17 wt% water content, 1463 g·mol−1 molecular weight, 4.98 average functionality, 2.4 × 10−5 mPa·s−1 viscosity at 333 K and 1.00 g·cm−3 density. The dielectric relaxation spectroscopy allowed identifying the α-relaxation process with a 193.5 K glass transition (Tg), 63.2 fragility index and 234.1 kJ mol−1 activation energy associated with Tg from the dynamic fragility index. The ionic conductivity temperature dependence on SOP obeys Arrhenius behavior. In summary, the SOP structure and thermal relaxation parameters determination are fundamental for the understanding of the structure-properties relationship of renewable polyurethanes.

Utilization of constant heating rate DSC cycles for enthalpy relaxation studies and their influenceability by erroneous experimental operations

The utilization of the constant heating rate cyclic temperature history was examined with respect to enthalpy relaxation studies. In this regard, three main methodologies for the determination of structural relaxation parameters were described in detail — curve-fitting, simulation-comparative methods and determination of activation energy according to Moynihan. Advantages and disadvantages of the constant heating rate cycles were introduced for each of the methodologies. In addition, Moynihan's method was extensively tested for all types of relaxation behavior and its accuracy has been determined. Main emphasis was then put on the utilization of the unique shape of the constant heating rate cycles for the identification of certain data-distortive effects caused by erroneous experimental operations. A discussion of all possible data-distortive effects was conducted; the manifestation of these distortions with respect to the relevant evaluation methodologies was demonstrated on theoretically simulated data. For each data-distortive effect, a universal and easy-to-apply practice was suggested to recognize and account for the given data-distortion.

Atomistic simulations of mixing properties and the local structure of the (Ca, Sr)10[PO4]6F2 solid solution

The thermodynamics of mixing is computed for eight different distributions of Sr and Ca over the M1 and M2 cation sites in the fluorapatite solid solution. Calculations are carried out for a 4×4×4 supercell via the semi-empirical approach using the GULP package. In the solid solution with the most energetically favorable distribution, Sr tends to occupy the M2 site rather than the M1 site and the x (Sr1): x (Sr2) ratio varies as a function of the Sr content. These structures are used for further investigation of the effect of mixing on the lattice parameters, local structure and elasticity of the Ca-Sr fluorapatite solid solution. Changes in the Sr2-F bond lengths as the Sr content increases turn out not to comply with Vegard’s rule. However, this deviation occurs in order to keep the linear dependence of the polyhedral volumes with the solid-solution composition. The obtained data allow us to estimate the structural relaxation parameters for both independent cation positions. Analysis of the bond sums also backs up the preference of Sr cations for the M2 site.

Atomistic computer modeling of the local structure and mixing properties of Sr-bearing fluorapatite

The mixing properties and the local structure of the Ca-Sr fluorapatite isomorphic system were studied using the original atomistic computer modeling approach for solid solutions. Calculations were carried out for a 4 × 4 × 4 supercell (2688 atoms) in conjunction with the high-performance computer SKIF MSU Chebyshev. In order to find the most probable (energetically preferable) distribution of Sr over the M1 and M2 cation sites in the fluorapatite solid solution, eight different distributions were tested. Structures with the most preferable x(Sr1): x(Sr2) ratios were used for further investigation of the effect of mixing on the lattice parameters and elasticity of the Ca-Sr fluorapatite solid solution and for the detailed analysis of the local structure. The local structure analysis included investigation of the individual and average M1-O, M2-O, M2-F bond lengths distortions, changes in the M1 and M2 polyhedra volumes as function of Sr content as well as the estimation of the structural relaxation parameters for all compositions of the solid solutions considered in this work.

Step-wise kinetics of natural physical ageing in arsenic selenide glasses

The long-term kinetics of physical ageing at ambient temperature is studied in Se-rich As–Se glasses using the conventional differential scanning calorimetry technique. It is analysed through the changes in the structural relaxation parameters occurring during the glass-to-supercooled liquid transition in the heating mode. Along with the time dependences of the glass transition temperature (Tg) and partial area (A) under the endothermic relaxation peak, the enthalpy losses (ΔH) and calculated fictive temperature (TF) are analysed as key parameters, characterizing the kinetics of physical ageing. The latter is shown to have step-wise character, revealing some kinds of subsequent plateaus and steep regions. A phenomenological description of physical ageing in the investigated glasses is proposed on the basis of an alignment–shrinkage mechanism and first-order kinetic equations.

Initial stage of physical ageing in network glasses

An atomistic view on Johari–Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (Tg ) in network glasses is developed for the archetypal chalcogenide glass, As20Se80, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below Tg . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network.

Correlation between the nonlinear optical constants and the structural relaxation parameters in chalcogenide glasses

It is well known that chalcogenide glasses exhibit high values of nonlinear optical constants in addition to the widely studied photostructural changes. However, the relations between these properties are not sufficiently understood. On the other hand, according to the bond strength-coordination number fluctuation (BSCNF) model proposed by one of the authors, the viscous behavior is described in terms of the average bond strength, coordination number, and their fluctuations of the structural units that form the system. In the present study, the interrelation between the nonlinear optical constants and the structural relaxation parameters defined from the BSCNF model has been investigated for the case of chalcogenide systems. It is found that the third order nonlinear optical susceptibility increases with the fluctuations of the structural relaxation parameters. The correlation found is discussed based on the BSCNF model and the average electronegativity.

PAL signature of physical ageing in chalcogenide glasses

AbstractKinetics of physical ageing (PhA) far below the glass transition temperature (Tg) is studied by positron annihilation lifetime and differential scanning calorimetry techniques for vitreous As20Se80 as typical representative of network glasses. The increased density fluctuations are shown to be responsible for the initial stage of PhA in this glass at below‐Tg temperatures. These fluctuations are correlated with changes in thermodynamic parameters of structural relaxation through the glass‐to‐supercooled liquid transition interval. General shrinkage, occurred during the next stage of PhA, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through densification process of glass network.

Method to Estimate Thermal Shrinkage Behavior of Glasses

We present a method to estimate the effect of heat treatment on the shrinkage behavior of glasses. As a pre‐requisite, sensitivity of the glass density as a function of glass fictive temperature is measured using the sink–float method and the slope of the relationship is used to determine the linear thermal strain proportionality factor. Evolution of the fictive temperature for different temperature–time history is measured using the infrared spectroscopy method and the results are used to estimate the structural relaxation parameters in the temperature range of interest. The overall shrinkage behavior is predicted using the linear thermal strain factor and estimated change in fictive temperature due to the thermal treatment. The predicted shrinkage behavior is observed to be in good agreement with the independent dimensional change measurements performed on large glass sheets that have undergone similar thermal treatments.

The Spin Probe Dynamics and the Free Volume in a Series of Amorphous Polymer Glass‐Formers

AbstractWe report the results of a combined study of the local structure and the reorientation dynamics in a series of five amorphous polymers of different fragility: cis‐trans‐1,4‐poly(butadiene) (c‐t‐1,4‐PBD), cis‐1,4‐poly(isoprene) (cis‐1,4‐PIP), poly(isobutylene) (PIB), poly(vinyl methylether)(PVME) and poly (propylene glycol) (PPG) by using two different probe methods. The reorientation dynamics of the molecular spin probe 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) from electron spin resonance (ESR) is related to the annihilation behaviour of the atomic ortho‐positronium (o‐Ps) one as obtained by positron annihilation lifetime spectroscopy (PALS). It was found that a slow to fast transition in the spin probe rotation mobility at the operationally defined spectral temperature parameter, T50G, is connected with the mean o‐Ps lifetime, τ3 (T50G) = (2.04 ± 0.26) ns. Consequently, using the free‐volume concept of the o‐Ps annihilation in terms of a quantum‐mechanical model of o‐Ps lifetime this transition can be connected with the occurrence of the mean free volume hole, Vh (T50G) = (102 ± 17) Å3, nearly independent of the chemical composition and the basic structural relaxation parameters of the amorphous polymers investigated. Finally, the free volume hole distribution aspect of the slow to fast transition indicates the presence of a sufficient free volume fluctuation at T50G for both typical fragile PVME and strong PIB polymer and emphasizes the essential role of free volume in the spin probe dynamics.

Conformational relaxation and water penetration coupled to ionization of internal groups in proteins.

Molecular dynamics simulations were used to examine the effects of ionization of internal groups on the structures of eighteen variants of staphylococcal nuclease (SNase) with internal Lys, Asp, or Glu. In most cases the RMSD values of internal ionizable side chains were larger when the ionizable moieties were charged than when they were neutral. Calculations of solvent-accessible surface area showed that the internal ionizable side chains were buried in the protein interior when they were neutral and moved toward crevices and toward the protein-water interface when they were charged. The only exceptions are Lys-36, Lys-62, and Lys-103, which remained buried even after charging. With the exception of Lys-38, the number of internal water molecules surrounding the ionizable group increased upon charging: the average number of water oxygen atoms within the first hydration shell increased by 1.7 for Lys residues, by 5.2 for Asp residues, and by 3.2 for Glu residues. The polarity of the microenvironment of the ionizable group also increased when the groups were charged: the average number of polar atoms of any kind within the first hydration shell increased by 2.7 for Lys residues, by 4.8 for Asp residues, and by 4.0 for Glu residues. An unexpected correlation was observed between the absolute value of the shifts in pK(a) values measured experimentally, and several parameters of structural relaxation: the net difference in the polarity of the microenvironment of the charged and neutral forms of the ionizable groups, the net difference in hydration of the charged and neutral forms of the ionizable groups, and the difference in RMSD values of the charged and neutral forms of the ionizable groups. The effects of ionization of internal groups on the conformation of the backbone were noticeable but mostly small and localized to the area immediately next to the internal ionizable moiety. Some variants did exhibit local unfolding.

Kinetics of isothermal structural relaxation in metallic glasses measured by real-time diffraction using synchrotron radiation

Quantitative measurements of the excess free volume in Pd40Cu30Ni10P20 and Cu55Zr30Ti10Pd5 metallic glasses during in situ isothermal annealing at various temperatures are used to identify physical parameters of structural relaxation in metallic glasses and to discuss proposed models describing this process. The free-volume model is found to provide a simple and predictive description for structural relaxation phenomena. We show that structural relaxation kinetics follows a second-order law in a satisfactory manner. The activation energy for relaxation is found to depend on annealing temperature and the extent of structural relaxation.

First-principles study of the (001) surface of cubic PbHfO3 and BaHfO3

Using first-principles techniques, we investigate the (001) surfaces of cubic PbHfO3 (PHO) and BaHfO3 (BHO) terminated with both AO (A = Pb and Ba) and HfO2. Surface structure, partial density of states, band structure, and surface energy are obtained. The BaO surface is found to be similar to its counterpart in BHO. For the HfO2-terminated surface of cubic PHO, the largest relaxation appears on the second-layer atoms but not on the first-layer ones. The analysis of the structure relaxation parameters reveals that the rumpling of the (001) surface for PHO is stronger than that for BHO. The surface thermodynamic stability is explored, and it is found that both the PbO- and the BaO-terminated surfaces are more stable than the HfO2-terminated surfaces for PHO and BHO, respectively. The surface energy calculations show that the (001) surface of PHO is more easily constructed than that of BHO.

1H and 17O NMR relaxometric study in aqueous solution of Gd(III) complexes of EGTA-like derivatives bearing methylenephosphonic groups

The Gd(III) complexes of three new octadentate chelators, prepared by substitution of four, two, and one carboxylate groups of EGTA with phosphonate groups, have been investigated by 1H and 17O NMR relaxometric techniques in aqueous solutions. The analysis of the solvent proton relaxivity data as a function of pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion (NMRD) profiles) in combination with the 17O transverse relaxation rate data at variable temperature allowed assessing the hydration state of the complexes, the occurrence of pH-dependent oligomerization processes for the tetraphosphonate derivative, the presence of a well-defined second sphere of hydration that markedly contributes to the relaxivity, and the values of the structural and dynamic relaxation parameters. In addition, in the case of the monophosphonate derivative the presence of a coordinated water molecule has allowed evaluation of the kinetic parameters of the exchange process, highly relevant for the possible use of this Gd(III) complex as an MRI probe. The rate of exchange of the water molecule, (298)k(ex) = 4.2 x 10(8)s(-1), is one of the highest measured so far for a nonacoordinate Gd(III) chelate and optimal for developing contrast-enhancing probes of high efficacy at high magnetic fields.

Glass Transition and Structure Relaxation of 1, 2-Propanediol Aqueous Solutions

In order to examine the effects of water contents on glass transition and structure relaxation parameters of 1, 2-propanediol (PD) aqueous glasses, apparent specific heat capacity during glass transition processes of five PD aqueous solutions with high concentrations (60%, 70%, 80%, 90%, and 100%, w) were determined using the differential scanning calorimetry (DSC). Five cooling rates (1, 2, 5, 10, 20 K·min-1) and one heating rate (10 K·min-1) were used to obtain the glass transition and structure relaxation parameters. Water seemed to be no obvious plasticization effect on 1, 2-propanediol (compared to some hydroxyl group linear polyalcohol) since the glass transition temperatures of the aqueous solutions were only slightly decreased with the increasing of water content. The structure relaxation activation energies were found to decrease as the water content increased. A comparison of thermodynamic fragility(ΔCp) and dynamic fragility (Angell′s fragility) was present and no similar trend was found with the changing of the component, since the former increased first and decreased last. The calculating results of dynamic fragility indicated that 1, 2-propanediol aqueous solutions behaved as though they were stronger than the pure solute. The sizes of cooperative rearrangement region (CRR) discussed in the term of glass transition characteristic length were determined using Donth's thermodynamic temperature fluctuant method. It was found that the sizes of CRR decreased as the water content increasing in 1,2-propanediol aqueous solutions.

First-principles study of the (001) surface of cubic BaZrO3 and BaTiO3

First-principles density functional calculations are employed to study the (001) surface of BaZrO3 with both BaO and ZrO2 termination. Surface structure and band structure have been obtained. For the ZrO2-terminated surface, the large displacement of the O atoms in the first layer results in the small surface rumpling. In addition, the largest relaxation appears on the second layer atoms but not on the very first layer ones. The analysis of the structure relaxation parameters reveals that the rumpling of the (001) surface for BaZrO3 with the BaO termination is stronger than that of BaTiO3. The origin of the different surface relaxation behavior between the two materials is discussed. The surface state appears in the band structure of the ZrO2-terminated surface of BaZrO3.

First-Principles Study of the (001) Surface of Cubic SrHfO3 and SrTiO3

We have performed first-principles calculations on the (001) surface of cubic SrHfO(3) and SrTiO(3) with SrO and BO(2) (B = Ti or Hf) terminations. Surface structure, partial density of states, band structure, and surface energy have been obtained. For the BO(2)-terminated surface, the largest relaxation appears on the second-layer atoms but not on the first-layer ones. The analysis of the structure relaxation parameters reveals that the rumpling of the (001) surface for SrHfO(3) with SrO termination is stronger than that for SrTiO(3). For the HfO(2)-terminated surface of SrHfO(3), the surface state appears near the M point of its band structure.

First-principles study on the (0 0 1) surface of cubic PbZrO 3 and PbTiO 3

We present first-principles calculations on the (0 0 1) surface of cubic PbZrO 3 and PbTiO 3 with the PbO and B (Ti or Zr) O 2 terminations. For the BO 2 terminated surface, the largest relaxations are on the second layer atoms and not on the first layer ones. This behavior is very different to SrTiO 3 and BaTiO 3. The analysis of the structure relaxation parameters reveals that the rumpling of the (0 0 1) surface for PbZrO 3 with the PbO termination is stronger than that of PbTiO 3. For the PbO-terminated surface of PbZrO 3, the surface state appears at the X point of its band structure. The result of the charge density shows the Pb–O covalent interaction in the surface is stronger than that of the bulk system for both the types of the surface of both the materials.

Structural Interpretation of the Results of Investigations into the Influence of Residual Water on the Properties of One-Alkali Borate Glasses

The results obtained in earlier investigations into the influence of water on the viscosity, thermal expansion coefficients, structural relaxation parameters, and fragility parameters of one-alkali borate glasses are analyzed, and schemes of the interaction of water with the glass network are proposed. The variations observed in the properties of glasses are explained in terms of possible changes in the ratio between different borate groups in the structure of the glass with a change in its composition.