Showing papers on "Glass transition published in 1997"

Controlling Polymer-Surface Interactions with Random Copolymer Brushes

Abstract: A simple technique for precisely controlling the interfacial energies and wetting behavior of polymers in contact with solid surfaces is described. End-functionalized statistical random copolymers of styrene and methylmethacrylate were synthesized, with the styrene fraction f varying from 0 to 1, and were end-grafted onto silicon substrates to create random copolymer brushes about 5 nanometers thick. For f < 0.7, polystyrene (PS) films (20 nanometers thick) rapidly dewet from the brushes when heated well above the glass transition temperature. The contact angle of the resulting polymer droplets increased monotonically with decreasing f . Similar behavior was observed for poly(methylmethacrylate) (PMMA) films but with an opposite dependence on f . The interfacial energies of the random copolymer brushes with PS and PMMA were equal when f was about 0.6. Thus, precise control of the relative surface affinities of PS and PMMA was possible, demonstrating a way to manipulate polymer-surface interactions.

Problems associated with spray drying of sugar-rich foods

Abstract: Stickiness is a major reason that limits the spray drying of various sugar-rich food products.Higher hygrmopicity of amorphous powder, increase in solubility of sugars with temperature, and lower melting point and glass transition temperature, contribute to the aickiness problem. So far, the glass transition temperature has been widely accepted as a ben indicator for stickiness. There are various manawm that have been applied to spray dry such product. Some of them are the addition of drying aids, modilication of drier design and use of mild drying temperature conditions. This review paper highlights the major research works that deal with the stickiness property of sugar-rich foods.

Direct Evidence for Stiffness Threshold in Chalcogenide Glasses

Abstract: Raman scattering in ${\mathrm{Ge}}_{x}{X}_{1\ensuremath{-}x}$ glasses, $X\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{S}$ or Se, reveals that the frequency of ${A}_{1}$ modes of corner-sharing $\mathrm{Ge}({X}_{1/2}{)}_{4}$ tetrahedra displays a discontinuous jump between $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.225$ and $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.230$, which coincides with a minimum in the nonreversing heat flow at the glass transition ${T}_{g}$ established from modulated differential scanning calorimetry. These results constitute direct evidence for a stiffness threshold at a mean coordination $〈r{〉}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.46(1)$, which is well described by mean-field constraint counting procedures.

Heterogeneity at the Glass Transition: Translational and Rotational Self-Diffusion

Abstract: Self-diffusion coefficients, D, have been measured in the glass forming liquids salol, glycerol, phenolphthaleine dimethyl ether (PDE), cresolphthaleine dimethyl ether (CDE), and ααβ-trinaphthylbenzene (TNB) in the supercooled regime. The NMR static magnetic field gradient technique was applied where D >10-14 m2 s-1 can be attained. The results are similar to previous diffusion experiments where an enhancement of translational diffusion was found in comparison with rotational diffusion and shear viscosity. Various models of spatial heterogeneity are related to a phenomenological environmental fluctuation model in view of recent diffusion and relaxation data close to the glass transition.

Length Scale of Cooperativity in the Dynamic Glass Transition

Abstract: The molecular dynamics in the glass transition of the “quasi” ‐ van der Waals glass salol confined to nanopores (2.5, 5.0, and 7.5 nm) with lubricated inner surfaces is found to be faster ( by up to 2 orders of magnitude) than in the bulk liquid. This effect of confinement is more pronounced for smaller pores. It reflects the cooperativity of molecular motions in the glass transition and enables its length scale to be determined quantitatively. [S0031-9007(97)04009-X]

Relaxations of confined chains in polymer nanocomposites: Glass transition properties of poly(ethylene oxide) intercalated in montmorillonite

Abstract: The relaxation behavior of poly(ethylene oxide) (PEO), intercalated in montmorillonite, a naturally occurring mica-type silicate, was studied by differential scanning calorimetry ( DSC and thermally stimulated dielectric depolarization ( or thermally stimulated current, TSC). The materials were synthesized by melt or solution-mediated intercalation. In both intercalates, the PEO chains were confined to ca. 0.8-nm galleries between the silicate layers. The solution intercalate contained a fraction of uninterca-lated PEO chains which exhibited a weak and depressed PEO melting endotherm in DSC. In contrast, the melt intercalate was starved such that almost all the PEO chains were effectively intercalated. For these melt intercalates, no thermal events were detected by DSC. TSC thermal sampling technique was used to examine the glass transition regions and to estimate the extent of cooperativity of chain motions. The motions of the intercalated PEO chains are inherently noncooperative relative to the cooperative T g motions in the amorphous portion of the bulk polymer. This is presumably due to the strong confining effect of the silicate layers on the relaxations of the intercalated polymer.

Preparation and thermal stability of bulk amorphous Pd40Cu30Ni10P20 alloy cylinder of 72 mm in diameter

Abstract: The glass-forming ability of a Pd 4 Cu 30 Ni 10 P 20 alloy was found to increase significantly by B 2 O 3 flux treatment, as is evidenced by the decrease in the critical cooling rate from 1.57 K/s in the non-fluxed state to 0.100 K/s in the fluxed state. The flux treatment also causes the extension of the supercooled liquid region by the increase in the onset temperature of crystallization (T x ). The effect of the flux treatment is presumably due to the increase in the thermal stability of the supercooled liquid by the suppression of heterogeneous nucleation. The critical cooling rates in the non-fluxed and fluxed states for a Pd 40 Ni 40 P 20 alloy are measured to be 128 and 0.167 K/s, respectively, both of which are larger than those for the Pd-Cu-Ni-P alloy. The use of the molten Pd-Cu-Ni-P alloy subjected to the flux treatment enabled the production of bulk amorphous alloys in cylindrical forms of 50 to 72 mm in diameter and 52 to 75 mm in length. The glass transition temperature (T g ) and T x values of the bulk amorphous alloys are the same as those for the melt-spun amorphous ribbon prepared from the fluxed molten alloy. The success of synthesizing an amorphous alloy of 72 mm in diameter is encouraging both for the future development of basic science of bulk amorphous alloys and for their engineering application.

A distributed reactivity model for sorption by soils and sediments. 8. Sorbent organic domains : Discovery of a humic acid glass transition and an argument for a polymer-based model

Abstract: Analysis of a humic acid by differential scanning calorimetry has revealed the existence of a glass transition point. Glass transition temperatures, Tg, of water-wet and desiccator-dry specimens we...

Thermal stability in oligomeric triphenylamine/tris(8-quinolinolato) aluminum electroluminescent devices

Abstract: Thermal stability of the electroluminescent (EL) devices using various hole-transporting materials based on triphenylamine, and a typical emitting material, tris(8-quinolinolato) aluminum has been systematically studied. The thermal stability of the EL devices is clearly seen to depend on the glass transition temperature (Tg) of the hole-transporting material. The highest thermal stability up to 155 °C is obtained in the device using the pentamer of triphenylamine. It has been found that the linear linkage of triphenylamine is useful to attain high Tg rather than the branch linkage.

Influence of glycerol and water content on the structure and properties of extruded starch plastic sheets during aging

Abstract: The properties of starch plastic sheets were investigated by stress—strain measurements in relation with starch crystallinity. Granular potato starch was plasticized with different amounts of glycerol and water by extrusion. The materials were amorphous directly after processing. During aging above the glass transition temperature at various humidities single helical (V and E-type) and double helical (B-type) crystallinity was formed. The rate of crystallization is a function of water and glycerol content. The amorphous rubbery materials were soft and weak with high elongations. During aging the materials became less flexible with higher elastic modulus and tensile stress. The changes are related to changes in water content and glass transition temperature and to changes in B-type crystallinity. The changes in stress—strain properties are explained by the formation of helical structures and crystals, which results in a reinforcement of the starch network by physical crosslinking. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1411–1422, 1997

Isothermal crystallization and chain mobility of poly(L-lactide)

Abstract: Isothermal melt crystallization of poly(L-lactide) (PLLA) has been studied in the temperature range of 90 to 135°C. A maximum in crystallization kinetic was observed around 105°C. A transition from regime II to regime III is present around 115°C. The crystal morphology is a function of the degree of undercooling. At crystallization temperatures (Tc) below 105°C, further crystallization occurs upon heating; this behavior is not detected for Tc above 110°C. The analysis of the heat capacity increment at glass transition temperature (Tg) and of dielectric properties of PLLA indicates the presence of a fraction of the amorphous phase which does not relax at the Tg, and the amount of this so-called rigid amorphous phase is a function of Tc. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 911–919, 1997

Universal criterion for metallic glass formation

Abstract: It is proposed that a simple criterion for topological instability applied to local atomic structure can explain a wide variety of phase transition phenomena involving metallic elements and alloys. These include melting, alloy formation, glass transition, solid state amorphization, and glass formation by rapid quenching. It can also provide a microscopic basis to explain the local structure of metallic glasses and their relaxation phenomena. Such simple theories are complementary to precise numerical computational theories in facilitating full understanding of the phenomena.

Stable glassy state powder formulations

Abstract: A powdered, dispersible composition having stable dispersibility over time is provided. The composition exhibits a characteristic glass transition temperature (Tg) and a recommended storage temperature (Ts), wherein the difference between Tg and Ts is at least about 10 °C (i.e. Tg-Ts is greater than 10 °C). The composition comprises a mixture of a pharmaceutically-acceptable glassy matrix and at least one pharmacologically active material within the glassy matrix. It may be further mixed with a powdered, pharmaceutically-acceptable carrier. It is particularly valuable in unit dosage form having a moisture barrier, in combination with appropriate labelling instructions. A process for producing a powdered dispersible composition is also provided, wherein the process comprises removing the solvent from a solution comprising a solvent, a glass former and a pharmacologically active material under conditions sufficient to form a glassy matrix having the pharmacologically active material within the matrix.

Yield criteria for amorphous glassy polymers

Abstract: Three amorphous polymers, polymethyl methacrylate, polystyrene and polycarbonate were tested in uniaxial tension, uniaxial compression, plane strain compression and simple shear, over a range of temperatures. In each test, the yield point was precisely determined via residual strain measurements after unloading. With the yield stresses determined for these four different stress states, two pressure dependent shear yield criteria, i.e, the modified Von Mises and the modified Tresca criteria, were checked and compared. It is shown that (i) in each case (material, temperature, initial ageing state), the yield locus is satisfactorily described by either one or the other of the two criteria, and (ii) each criterion can be associated with a specific deformation mode (either homogeneous or localized in shear bands). As for the temperature dependence of the yield stress sensitivity to the hydrostatic pressure, it appears to be related to the glass transition temperature (Tg) and more precisely to the α and β relaxations. Finally, the pressure dependence of the yield stress can be possibly explained as being due to two effects: (i) the influence of pressure on molecular motions leading to yielding and (ii) the influence of pressure on the microstructural state.

Low-temperature synthesized aluminosilicate glasses. Part III Influence of the composition of the silicate solution on production, structure and properties

Abstract: The low-temperature reaction between an aqueous sodium or potassium silicate solution and metakaolinite yields a solid aluminosilicate. The influence of the molar ratios H2O/R2O (between 6.6 and 21.0) and SiO2/R2O (between 0.0 and 2.3) of the silicate solution (R=Na or K) on the aluminosilicate's production, on the reaction stoichiometry and on the aluminosilicate's molecular structure is studied with differential scanning calorimetry, 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR), cross-polarization MAS NMR, Fourier transform infrared spectroscopy and X-ray diffractometry. The reaction stoichiometry is determined by a one to one ratio for R/Al. H2O/R2O has no influence on the molecular structure of the aluminosilicate. Aluminium in the aluminosilicate is four-fold coordinated for the whole range of silicate solutions investigated. Moreover, Si and Al are homogeneously distributed and the ratio Al/Si in the aluminosilicate is the same as in the reaction mixture if the stoichiometric one-to-one ratio for R/Al is used. If SiO2/R2O in the Na-silicate solution is equal to or higher than 0.8, the low-temperature reaction yields an amorphous aluminosilicate or “inorganic polymer glass”. For smaller values of SiO2/R2O the Na-aluminosilicates are partially crystalline. Thermomechanical analysis and dynamic mechanical analysis indicate that a variation in the composition of the amorphous aluminosilicates can shift the glass transition over a few hundreds of degrees, with a minimum value of 650°C.

Crystallization kinetics of suspensions of hard colloidal spheres

Abstract: The crystallization of suspensions of sterically stabilized polymer particles, with hard-sphere-like interactions, is studied by laser light scattering. Over the range of volume fractions examined, from just below melting to the glass transition, crystallization occurs by homogeneous nucleation. After the suspensions are shear melted, the intensity, position, and width of the main interlayer Bragg reflection are measured as functions of time. From these the amount of crystal, the average linear crystal dimension, the number of crystals, and the volume fraction of the crystal phase are obtained. No assumptions are made concerning the functional time dependence of nucleation or growth processes. Below the melting concentration the observed crystallization process is compatible with the classical picture of sequential nucleation and growth of isolated crystals. However, when the melting concentration is exceeded, nucleation events are correlated, nucleation is accelerated, and high nucleation rate densities suppress crystal growth. Above the melting concentration we infer, with the aid of the equations of state for the hard-sphere fluid and solid, that the first identifiable crystals are in mechanical equilibrium with the embedding fluid and, consequently, strongly compressed by it. Ensuing nucleation lags expansion of the crystal lattice.

Configurational Entropy Approach to the Kinetics of Glasses.

Abstract: A kinetic theory of glasses is developed using equilibrium theory as a foundation. After establishing basic criteria for glass formation and the capability of the equilibrium entropy theory to describe the equilibrium aspects of glass formation, a minimal model for the glass kinetics is proposed. Our kinetic model is based on a trapping description of particle motion in which escapes from deep wells provide the rate-determining steps for motion. The formula derived for the zero frequency viscosity η (0,T) is log η (0,T) = B - AF(T)kT where F is the free energy and T the temperature. Contrast this to the Vogel-Fulcher law log η (0,T) = B + A/(T - Tc). A notable feature of our description is that even though the location of the equilibrium second-order transition in temperature-pressure space is given by the break in the entropy or volume curves the viscosity and its derivative are continuous through the transition. The new expression for η (0,T) has no singularity at a critical temperature Tc as in the Vogel-Fulcher law and the behavior reduces to the Arrhenius form in the glass region. Our formula for η (0,T) is discussed in the context of the concepts of strong and fragile glasses, and the experimentally observed connection of specific heat to relaxation response in a homologous series of polydimethylsiloxane is explained. The frequency and temperature dependencies of the complex viscosity η (ω< T), the diffusion coefficient D(ω< T), and the dielectric response e (ω< T) are also obtained for our kinetic model and found to be consistent with stretched exponential behavior.

Effect of glass transition temperature on the stability of lyophilized formulations containing a chimeric therapeutic monoclonal antibody.

Abstract: Purpose The purpose of this study is to highlight the importance of knowing the glass transition temperature, Tg, of a lyophilized amorphous solid composed primarily of a sugar and a protein in the interpretation of accelerated stability data Methods Glass transition temperatures were measured using DSC and dielectric relaxation spectroscopy Aggregation of protein in the solid state was monitored using size-exclusion chromatography Results Sucrose formulation (Tg ~ 59°C) when stored at 60°C was found to undergo significant aggregation, while the trehalose formulation (Tg ~ 80°C) was stable at 60°C The instability observed with sucrose formulation at 60°C can be attributed to its Tg (~59°C) being close to the testing temperature Increase in the protein/sugar ratio was found to increase the Tgs of the formulations containing sucrose or trehalose, but to different degrees Conclusions Since the formulations exist in glassy state during their shelf-life, accelerated stability data generated in the glassy state (40°C) is perhaps a better predictor of the relative stability of formulations than the data generated at a higher temperature (60°C) where one formulation is in the glassy state while the other is near or above its Tg

Liquid water in the domain of cubic crystalline ice Ic.

Abstract: Vapor-deposited amorphous water ice when warmed above the glass transition temperature (120-140 K), is a viscous liquid which exhibits a viscosity vs temperature relationship different from that of liquid water at room temperature. New studies of thin water ice films now demonstrate that viscous liquid water persists in the temperature range 140-210 K. where it coexists with cubic crystalline ice. The liquid character of amorphous water above the glass transition is demonstrated by (1) changes in the morphology of water ice films on a nonwetting surface observed in transmission electron microscopy (TEM) at around 175 K during slow warming, (2) changes in the binding energy of water molecules measured in temperature programmed desorption (TPD) studies, and (3) changes in the shape of the 3.07 micrometers absorption band observed in grazing angle reflection-absorption infrared spectroscopy (RAIRS) during annealing at high temperature. whereby the decreased roughness of the water surface is thought to cause changes in the selection rules for the excitation of O-H stretch vibrations. Because it is present over such a wide range of temperatures, we propose that this form of liquid water is a common material in nature. where it is expected to exist in the subsurface layers of comets and on the surfaces of some planets and satellites.

Viscosity, fragility, and configurational entropy of melts along the join SiO2-NaAlSiO4

Abstract: Viscosities of fourteen melts close to the join SiO 2-NaAlO2 were measured in the range 1‐10 12 Pa·s (700‐1650 8C) using a combination of concentric cylinder and micropenetration techniques. These compositions cover five isopleths in silica content from 50 to 82 mol% and vary from mildly peralkaline to mildly peraluminous. Greatly improved constraints on the temperature dependence of viscosity in the system SiO2-NaAlO2 result because exactly the same compositions were used for both high- and low-temperature measurements, viscosities over an extended range of silica contents were measured at temperatures close the glass transition, and several compositions at constant silica content and variable alkali/Al ratio were measured, allowing interpolation of data to compositions exactly along the join SiO2-NaAlO2. At high temperature (1600 8C) viscosity and activation energy are shown to be approximately a linear function of silica content, but large nonlinearities occur at temperatures close to the glass transition range. Defining fragility as the gradient of the viscosity curve at the glass transition temperature ( Tg taken to be the 10 12 Pa·s isokom) on a reduced temperature scale (Tg/T), it is found that the fragility increases in a nonlinear fashion as NaAlO2 is substituted for SiO 2, with fragility increasing more rapidly at lower SiO2 contents. The viscosity data are combined with heat capacity data available in the literature to estimate configurational entropies of albite, jadeite, and nepheline glasses using the Adam-Gibbs theory. Fragility, when defined in terms of the AdamGibbs parameters, is shown to increase with configurational heat capacity (difference in heat capacity between the liquid and the glassy states) but to decrease with increasing configurational entropy at the glass transition. In the light of independent phase equilibria and spectroscopic and calorimetric evidence that suggests the Al-Si ordering increases as silica content decreases from SiO2 to nepheline, the modeling of configurational entropy in terms of Al-Si mixing suggests the following: (1) Melt configurational entropy has contributions from both cation mixing (chemical contribution), as well as variations in the topology of the O network (topological contribution), of which the latter dominates. (2) The chemical contribution is due to mixing of tetrahedral rather than O sites. (3) At the glass transition (10 12 Pa·s isokom) the topological contribution shows little, if any, variation.