Home About us Contact
|
Home About us Contact | ||
|
|
||
Structural Relaxation (structural + relaxation)
Selected AbstractsA Nonequilibrium Statistical Mechanical Model of Structural Relaxation in GlassJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2006John C. Mauro We derive a new model of structural relaxation in glass based on nonequilibrium statistical mechanics and the Stillinger model of inherent structures. Our model follows the evolution of a system from its equilibrium liquid state through an arbitrary cooling path and allows for the computation of macroscopic properties as a function of time. Using this new model, we have numerically demonstrated for the first time the connection between the topography of a potential energy landscape in 1-D and its corresponding fragility. [source] Structural relaxation and evolution of yield stress in epoxy glass aged under shear strain,POLYMER ENGINEERING & SCIENCE, Issue 1 2005Hiroshi Kawakami Hollow cylindrical specimens of annealed epoxy glass were twisted and then aged for various periods of time under shear strain. At the end of the aging process, we twisted the specimens again to determine the stress,strain relations. For specimens aged under a shear strain of 0.005 or 0.01, the stress relaxation behavior was almost independent of the amount of strain imposed, and the value of stress at the upper yield point, regardless of aging time, was almost the same as that of the annealed specimen. On the other hand, for specimens aged under a strain of 0.02 or 0.04, the stress relaxation behavior depended on the value of the strain applied, and the value of stress at the upper yield point first decreased and subsequently increased with increasing aging time. These results led us to the following conclusions: If epoxy glass is strained largely, the originally stable structure becomes unstable. Also, when epoxy glass is aged under strain, the stability of the structure continues to decrease for a short period of time after deformation ceases, and then increases with increasing aging time. POLYM. ENG. SCI. 45:20,24, 2005. © 2004 Society of Plastics Engineers. [source] Diffusion in Nanocrystalline Metals and Alloys,A Status Report,ADVANCED ENGINEERING MATERIALS, Issue 5 2003R. Würschum Abstract Diffusion is a key property determining the suitability of nanocrystalline materials for use in numerous applications, and it is crucial to the assessment of the extent to which the interfaces in nanocrystalline samples differ from conventional grain boundaries. The present article offers an overview of diffusion in nanocrystalline metals and alloys. Emphasis is placed on the interfacial characteristics that affect diffusion in nanocrystalline materials, such as structural relaxation, grain growth, porosity, and the specific type of interface. In addition, the influence of intergranular amorphous phases and intergranular melting on diffusion is addressed, and the atomistic simulation of GB structures and diffusion is briefly summarized. On the basis of the available diffusion data, the diffusion-mediated processes of deformation and induced magnetic anisotropy are discussed. [source] Phase and Rheological Behavior of High-Concentration Colloidal Hard-Sphere and Protein DispersionsJOURNAL OF FOOD SCIENCE, Issue 7 2007S.M. Loveday ABSTRACT:, Colloidal hard-sphere (HS) particles of narrow-size distribution exhibit crystalline and glassy states beginning at the particle volume fractions ,= 0.494 and ,G= 0.58, respectively. Dynamic rheological data on the dispersions were strongly modified to solid-like behavior as , approached ,G. In addition, cooperative motion in structural relaxation has been observed microscopically in the colloidal dispersions near the glassy state. Very high viscosities and glassy states were also found in high-concentration dispersions of sodium caseinate and the globular proteins: bovine serum albumin and ,-lactoglobulin. Viscosity models developed for HS dispersions predicted accurately the trends but not the absolute values of protein dispersions. Dispersions of food colloidal particles may be employed in studies, in which volume fraction is the thermodynamic variable, for understanding the relaxation and transport processes related to 1st-order and colloidal glass transitions. [source] Estimation of the fragility index of indomethacin by DSC using the heating and cooling rate dependency of the glass transitionJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 6 2004Joaquim J. Moura Ramos Abstract In this study we have investigated the features of the glass transition relaxation of indomethacin using Differential Scanning Calorimetry (DSC). The purpose of this work is to provide an estimation of the activation energy at the glass transition temperature, as well as of the fragility index, of amorphous indomethacin from DSC data. To do so, the glass transition temperature region of amorphous indomethacin was characterized in both cooling and heating regimes. The activation energy for structural relaxation (directly related to glass fragility) was estimated from the heating and cooling rate dependence of the location of the DSC profile of the glass transition. The obtained results were similar in the heating and in the cooling modes. The results on the fragility index of indomethacin obtained in the present study, m,=,60 in the cooling mode and m,=,56 in the heating mode, are compared with other values previously published in the literature. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:1503,1507, 2004 [source] Intramolecular structural relaxation in excited hetarylazole cationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2005Valery V. Volchkov Abstract Neutral quinolyl-and pyridyl-substituted hetarylazoles and their protonated (ethylated) cations with anomalously high Stokes shifts were investigated. Fluorescence measurements at different temperatures and pressures confirmed the viscosity-dependent activation relaxation of the latter. Fluorescence kinetic analyses at various temperatures showed that in the range 293,77,K, the k,f values of cations increase 2,8-fold whereas those of neutral molecules change only 0.3,1.5-fold. The low-temperature time-resolved emission spectra of 1a, 1H+ and 5a were found to be consistent with the model: A,A*,B*, where A* is the local excited planar and B* is the relaxed twisted state of the molecule. Copyright © 2004 John Wiley & Sons, Ltd. [source] Refractive Index Drop Observed After Precision Molding of Optical Elements: A Quantitative Understanding Based on the Tool,Narayanaswamy,Moynihan ModelJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008Ulrich Fotheringham The room-temperature refractive index is measured for three different prior cooling rates (approximately 10, 50, and 250 K/h) for two glasses especially developed for precision molding. The empirical logarithmic relationship between the cooling rate and the refractive index is also reproduced for the comparatively high cooling rate of ca. 250 K/h. The same relationship is found in a simulation of these cooling rates by the semiempirical Tool,Narayanaswamy,Moynihan model for structural relaxation, with the necessary parameters obtained from differential scanning calorimetry and temperature jump experiments. The measured and the simulated refractive indices coincide within the limits of experimental error. The results demonstrate that the index drop, which is observed when these glasses are first cooled at a regular optical cooling rate (e.g., 2 K/h), and then precision molded (typical cooling rate 1000 K/h), can be understood considering the concepts of structural relaxation. [source] A Nonequilibrium Statistical Mechanical Model of Structural Relaxation in GlassJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2006John C. Mauro We derive a new model of structural relaxation in glass based on nonequilibrium statistical mechanics and the Stillinger model of inherent structures. Our model follows the evolution of a system from its equilibrium liquid state through an arbitrary cooling path and allows for the computation of macroscopic properties as a function of time. Using this new model, we have numerically demonstrated for the first time the connection between the topography of a potential energy landscape in 1-D and its corresponding fragility. [source] Control of Glass-Forming Process During Fiber-Drawing to Reduce the Rayleigh Scattering LossJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2006K. Saito Limit of the Rayleigh scattering loss in Ge-doped silica core fiber was estimated from the results of structural relaxation and the Rayleigh scattering measurements, and the most suitable fiber-drawing condition to reduce the Rayleigh scattering loss was determined. An annealing furnace, which could be attached to a fiber-drawing tower, was developed for realizing the optimum fiber-drawing condition. Drawing tests with this furnace confirmed our estimation of the loss. [source] First-Principles Calculations of Anion Vacancies in Oxides and NitridesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002Isao Tanaka The formation energy, structural relaxation, and defect-induced states of neutral anion vacancies of five oxides (i.e., MgO, Al2O3, ZnO, In2O3, and SnO2) and four nitrides (i.e., AlN, Si3N4, Ge3N4, and InN) are systematically discussed, based on first-principles plane-wave pseudopotential calculations. Two types of polymorphs for each compound are compared. The number of atoms included in the supercells ranged from 54 to 96. When a localized vacancy-induced state appears within the band gap, as in a typical ionic crystal, the formation energy can be well scaled by the band gap of the perfect crystal. On the other hand, when an empty and localized vacancy-induced state is located above the highest occupied band or no localized state is formed, the formation energy has a tendency to be smaller. In compounds such as ZnO and SnO2, the formation energy is dependent largely on the crystal structure. This result can be explained by the transition of the vacancy-induced state from occupied to unoccupied, which is caused by the change in atomic arrangement, as represented by the cation coordination number. [source] Decoupling between Enthalpy Relaxation and Viscous Flow and Its Structural Origin in Fragile Oxide Glass-Forming LiquidsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002Takayuki Komatsu The structural relaxation kinetics at the glass transition in tellurium oxide (TeO2)-based glasses has been examined from viscosity and heat-capacity measurements to clarify the features of the structural relaxation in fragile oxide glass-forming liquids. A large decoupling between enthalpy relaxation and viscous flow, i.e., a large discrepancy between the activation energies for the enthalpy relaxation (recovery), ,H, and viscous flow, E,, has been demonstrated in TeO2 -based glasses. The values in xK2O·xMgO·(100 , 2x)TeO2 glasses, for example, are ,919,1051 kJ/mol for ,H and , 577,701 kJ/mol for E,, given the ratio of ,H/E,, 1.44,1.59. Some viscosity and heat-capacity data (all data have been reported previously) obtained from similar experiments in Sb2O3,B2O3 glasses belonging to the category of strong glass-forming liquids have been reanalyzed in this paper for comparison; a strong coupling was found to exist between ,H and E,, i.e., ,H/E,, 0.98,1.18. An origin of decoupling between ,H and E, in fragile glass-forming systems such as TeO2 -based glasses has been discussed by considering the glass structure model for fragile glasses; strongly bonded correlated (highly constrained) regions are surrounded or connected by weakly bonded noncorrelated (unconstrained) parts. [source] Effect of the Growth Treatment on Two-Stage Nucleation ExperimentsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2001Mark J. Davis Numerical simulations are presented that document the strong effect of a previously underappreciated portion of two-stage thermal treatments used in the study of nucleation processes: the "heat-up" process whereby samples are heated from "nucleation" conditions to "growth" conditions. The simulations indicate that two limiting regimes exist, dependent on (a) the cluster size distribution of as-quenched glasses, (b) the temperatures used for nucleation and growth, and (c) the rates of heating and cooling: (1) all clusters larger than the critical size at growth conditions (n*gr) will grow to macroscopic size (the "standard" case); and (2) all clusters larger than the critical size at nucleation conditions (n*nuc) will grow to macroscopic size. In addition, cases in which the "effective critical size" (n*eff) is intermediate between n*gr and n*nuc can also occur. Cases in which n*eff < n*gr is manifested during nucleation experiments as an abrupt boost in crystal number density during the heat-up from nucleation to growth conditions, as all clusters larger than n*eff are rapidly "flushed" past n*gr. For the system studied herein, this can lead to a 106 -fold increase in final number density within seconds to a few minutes. Finally, the importance of structural relaxation for this process is demonstrated by examining a case in which the nucleation temperature is below the nominal glass transition temperature. [source] Study of the Segmental Dynamics in Semi-Crystalline Poly(lactic acid) using Mechanical SpectroscopiesMACROMOLECULAR BIOSCIENCE, Issue 4 2005João F. Mano Abstract Summary: The glass transition of poly(L -lactic acid) (PLLA) occurs not far above typical service temperatures (room or body temperatures) which has consequences on the material properties during its use, such as damping or the occurrence of structural relaxation. This work aims at characterising the glass transition dynamics of a semi-crystalline PLLA using both dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). The main viscoelastic parameters have been characterised at 1 Hz using DMA and the master curve obtained after isothermal experiments at different temperatures provided a full picture of the solid-state rheological behaviour throughout a wide frequency range. The activation energies calculated from the shift factors agree with the TSR ones, exhibiting a maximum near the Tg. Above the Tg, the results can be described with the WLF model. In the glassy state, the activation energy decreases with decreasing temperatures being always higher than the prediction of the Adam and Gibbs theory, at least down to temperatures 30,°C below the Tg. This suggests the existence of non-arrested degrees of freedom in the glassy state, being consistent with the existence of a significant degree of cooperativity in the TSR results. Temperature dependence of the apparent activation energy across the glass transition. [source] Crystallization study of amorphous Pd43Ni10Cu27P20 alloy by internal friction measurementPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2007K. W. Yang Abstract We have successfully measured the internal friction and electrical resistivity of Pd43Ni10Cu27P20 bulk metallic glass during a constant heating process. The results of electrical resistivity suggest that the position of the internal friction peak is the onset temperature of crystallization. The dependence of internal friction on frequencies show both linear and nonlinear relations. The internal friction at 623 K in the supercooled liquid region decreases monotonously with the increase of annealing time, indicating structural relaxation and subsequent crystallization occurs during isothermal annealing near the crystallization temperature. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Enthalpy relaxation in bisphenol-A polycarbonate/poly(methyl methacrylate) blendsPOLYMER ENGINEERING & SCIENCE, Issue 3 2007Maurizio Penco Bisphenol-A polycarbonate (PC)/poly(methyl methacrylate) (PMMA) blends (PC/PMMA) were prepared by melt mixing with and without a trans-esterification catalyst (tetrabutylammoniun tetraphenylborate), which is able to promote inter-exchange reactions between PC and PMMA. Evidences of the ester,ester exchange reaction taking place were pointed out via Fourier Transform Infra-Red and size exclusion chromatography analyses. A series of enthalpy relaxation measurements were carried out for the pure polymers and for blends thereof. The data were analyzed on the basis of the Tool,Narayanaswamy,Moynihan/Kohlraush,Williams,Watts model. This model characterizes the structural relaxation in the glass transition region by means of four parameters: the apparent activation energy (,h*), the nonlinearity (x), the nonexponentiality (,), and the pre-exponential factor (,0). The apparent activation energy can be determined from the cooling rate dependence of the fictive temperature (Tf) measured using DSC. ,h* values of homogeneous blend and of the starting polymers were determined in this way, while an estimation of x, , and ,0 was proposed. Comparisons were made between a single glass transition temperature PC/PMMA blend and homopolymers data. The experimental values of ,h* suggest that the degree of cooperativity decreases on going from the starting polymers to the blend. POLYM. ENG. SCI., 47:218,224, 2007. © 2007 Society of Plastics Engineers. [source] Time-resolved resonance Raman study on ultrafast structural relaxation and vibrational cooling of photodissociated carbonmonoxy myoglobinBIOPOLYMERS, Issue 4-5 2002Teizo Kitagawa Abstract A localized small structural change is converted to a higher order conformational change of protein and extends to a mesoscopic scale to induce a physiological function. To understand such features of protein, ultrafast dynamics of myoglobin (Mb) following photolysis of carbon monoxide were investigated. Recent results are summarized here with a stress on structural and vibrational energy relaxation. The core expansion of heme takes place within 2 ps but the out of plane displacement of the heme iron and the accompanying protein conformational change occur in 10 and 100 s of the picosecond regimes, respectively. Unexpectedly, it was found from UV resonance Raman spectra that Trp7 in the N-terminal region and Tyr151 in the C-terminal region undergo appreciable structural changes upon ligand binding,dissociation while Tyr104, Tyr146, and Trp14 do not. Because of the communication between the movements of these surface residues and the heme iron, the rate of spectral change of the iron-histidine (Fe- His) stretching band after CO photodissociation is influenced by the viscosity of solvent. Temporal changes of the anti-Stokes Raman intensity demonstrated immediate generation of vibrationally excited heme upon photodissociation and its decay with a time constant of 1,2 ps. © 2002 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 67: 207,213, 2002 [source] A Structure-Consistent Mechanism for Dioxygen Formation in Photosystem IICHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2008Abstract In recent DFT studies a new mechanism for OO bond formation at the oxygen evolving center (OEC) in photosystem II has been suggested. With the structure of the S4 state required for that mechanism, the structures of the lower S states are investigated herein by adding protons and electrons. A model was used including the full amino acids for the ones ligating the OEC, and in which the backbone positions were held fixed from the X-ray structure. The only charged second-shell ligand Arg357 was also included. An optimized structure for the S1 state was reached with a large similarity to one of those suggested by EXAFS. A full catalytic cycle was derived which can rationalize the structural relaxation in the S2 to S3 transition, and the fact that only an electron leaves in the transition before. Water is suggested to bind to the OEC in the S2 to S3, and S4 to S0 transitions. A new possibility for water exchange is suggested from the final energy diagram. The optimal OO bond formation occurs between an oxygen radical and an oxo ligand. The alternative mechanism, where the oxygen radical reacts with an external water, has a barrier about 20,kcal,mol,1 higher. [source] | |||||||||||||