Showing papers on "Glass transition published in 2006"

Fundamentals of Inorganic Glasses

Abstract: Introduction. Fundamentals of the Glassy State. Glass Formation Principles. Glass Microstructure: Phase Separation and Liquid Immiscibility. Glass Compositions and Structures. Composition-Structure-Property Relationship Principles. Density and Molar Volume. Elastic Properties and Microhardness of Glass. The Viscosity of Glass. Thermal Expansion of Glass. Heat Capacity of Glass. Thermal Conductivity and Heat Transfer in Glass. Glass Transition Range Behavior. Permeation, Diffusion and Ionic Conduction in Glass. Dielectric Properties. Electronic Conduction. Chemical Durability. Strength and Toughness. Optical Properties. Fundamentals of Inorganic Glassmaking. Appendix I: Elements of Linear Elasticity. Appendix II: Units and General Data Conversions. Subject Index.

Determining Beta-Sheet Crystallinity in Fibrous Proteins by Thermal Analysis and Infrared Spectroscopy

Abstract: We report a study of self-assembled beta-pleated sheets in B. mori silk fibroin films using thermal analysis and infrared spectroscopy. B. mori silk fibroin may stand as an exemplar of fibrous proteins containing crystalline beta-sheets. Materials were prepared from concentrated solutions (2−5 wt % fibroin in water) and then dried to achieve a less ordered state without beta-sheets. Crystallization of beta-pleated sheets was effected either by heating the films above the glass transition temperature (Tg) and holding isothermally or by exposure to methanol. The fractions of secondary structural components including random coils, alpha-helices, beta-pleated sheets, turns, and side chains were evaluated using Fourier self-deconvolution (FSD) of the infrared absorbance spectra. The silk fibroin films were studied thermally using temperature-modulated differential scanning calorimetry (TMDSC) to obtain the reversing heat capacity. The increment of the reversing heat capacity ΔCp0(Tg) at the glass transition fo...

Supported absorption of CO2 by tetrabutylphosphonium amino acid ionic liquids.

Abstract: A new type of "task specific ionic liquid", tetrabutylphosphonium amino acid [P(C 4 ) 4 ][AA], was synthesized by the reaction of tetrabutylphosphonium hydroxide [P(C 4 ) 4 ][OH] with amino acids, including glycine, L-alanine, L-β-alanine, L-serine, and L-lysine. The liquids produced were characterized by NMR, IR spectroscopies, and elemental analysis, and their thermal decomposition temperature, glass transition temperature, electrical conductivity, density, and viscosity were recorded in detail. The [P(C 4 ) 4 ][AA] supported on porous silica gel effected fast and reversible CO 2 absorption when compared with bubbling CO 2 into the bulk of the ionic liquid. No changes in absorption capacity and kinetics were found after four cycles of absorption/ desorption. The CO 2 absorption capacity at equilibrium was 50 mol % of the ionic liquids. In the presence of water (1 wt%), the ionic liquids could absorb equimolar amounts of CO 2 . The CO 2 absorption mechanisms of the ionic liquids with and without water were different.

Protic Ionic Liquids: Solvents with Tunable Phase Behavior and Physicochemical Properties

Abstract: The phase behavior, including glass, devitrification, solid crystal melting, and liquid boiling transitions, and physicochemical properties, including density, refractive index, viscosity, conductivity, and air−liquid surface tension, of a series of 25 protic ionic liquids and protic fused salts are presented along with structure−property comparisons. The protic fused salts were mostly liquid at room temperature, and many exhibited a glass transition occurring at low temperatures between −114 and −44 °C, and high fragility, with many having low viscosities, down to as low as 17 mPa·s at 25 °C, and ionic conductivities up to 43.8 S/cm at 25 °C. These protic solvents are easily prepared through the stoichiometric combination of a primary amine and Bronsted acid. They have poor ionic behavior when compared to the far more studied aprotic ionic liquids. However, some of the other physicochemical properties possessed by these solvents are highly promising and it is anticipated that these, or analogous protic s...

Polymer-Nanoparticle Interfacial Interactions in Polymer Nanocomposites: Confinement Effects on Glass Transition Temperature and Suppression of Physical Aging.

Abstract: The effects of confinement on glass transition temperature (Tg) and physical aging are measured in polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) nanocomposites containing 10- to 15-nm-diameter silica nanospheres or 47-nm-diameter alumina nanospheres. Nanocomposites are made by spin coating films from sonicated solutions of polymer, nanofiller, and dye. The Tgs and physical aging rates are measured by fluorescence of trace levels of dye in the films. At 0.1–10 vol % nanofiller, Tg values can be enhanced or depressed relative to neat, bulk Tg (Tg,bulk) or invariant with nanofiller content. For alumina nanocomposites, Tg increases relative to Tg,bulk by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. By analogy with thin polymer films, these results are explained by wetted P2VP–nanofiller interfaces with attractive interactions, nonwetted PMMA–nanofiller interfaces (free space at the interface), and wetted PS–nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or nonwetted interfaces is controlled by choice of solvent. For example, 0.1–0.6 vol % silica/PMMA nanocomposites exhibit Tg enhancements as large as 5 K or Tg reductions as large as 17 K relative to Tg,bulk when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 17 reduction of physical aging rate relative to that of neat, bulk P2VP is demonstrated in a 4 vol % alumina/P2VP nanocomposite. This suggests that a strategy for achieving nonequilibrium, glassy polymeric systems that are stable or nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2935–2943, 2006

Frustration on the way to crystallization in glass

Abstract: Some liquids do not crystallize below the melting point, but instead enter into a supercooled state and on cooling eventually become a glass at the glass-transition temperature. During this process, the liquid dynamics not only drastically slow down, but also become progressively more heterogeneous. The relationship between the kinetic slowing down and growing dynamic heterogeneity is a key problem of the liquid–glass transition. Here, we study this problem by using a liquid model, with a crystalline ground state, for which we can systematically control frustration against crystallization. We found that slow regions having a high degree of crystalline order emerge below the melting point, and their characteristic size and lifetime increase steeply on cooling. These crystalline regions lead to dynamic heterogeneity, suggesting a connection to the complex free-energy landscape and the resulting slow dynamics. These findings point towards an intrinsic link between the glass transition and crystallization.

Polymer-nanoparticle interfacial interactions in polymer nanocomposites: Confinement effects on glass transition temperature and suppression of physical aging

Abstract: The effects of confinement on glass transition temperature (Tg) and physical aging are measured in polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) nanocomposites containing 10- to 15-nm-diameter silica nanospheres or 47-nm-diameter alumina nanospheres. Nanocomposites are made by spin coating films from sonicated solutions of polymer, nanofiller, and dye. The Tgs and physical aging rates are measured by fluorescence of trace levels of dye in the films. At 0.1–10 vol % nanofiller, Tg values can be enhanced or depressed relative to neat, bulk Tg (Tg,bulk) or invariant with nanofiller content. For alumina nanocomposites, Tg increases relative to Tg,bulk by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. By analogy with thin polymer films, these results are explained by wetted P2VP–nanofiller interfaces with attractive interactions, nonwetted PMMA–nanofiller interfaces (free space at the interface), and wetted PS–nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or nonwetted interfaces is controlled by choice of solvent. For example, 0.1–0.6 vol % silica/PMMA nanocomposites exhibit Tg enhancements as large as 5 K or Tg reductions as large as 17 K relative to Tg,bulk when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 17 reduction of physical aging rate relative to that of neat, bulk P2VP is demonstrated in a 4 vol % alumina/P2VP nanocomposite. This suggests that a strategy for achieving nonequilibrium, glassy polymeric systems that are stable or nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2935–2943, 2006

Correlation between dynamic fragility and glass transition temperature for different classes of glass forming liquids

Abstract: Here we compile literature data for dynamic fragility m for six types of glass forming liquids: polymers, small molecule organics, hydrogen bonding organics, inorganics, ionic and metallic glass formers. Our analysis of the data shows that different categories of glass forming liquids exhibit different behaviors in terms of the correlation between m and Tg, a correlation not previously examined. For example, for hydrogen bonding organics, polymeric and metallic glass formers, there is an approximately linear increase in m with increasing Tg. While for inorganic glass formers, m appears almost independent of Tg, remaining nearly constant over a wide range in Tg. At the same time, another important parameter, the apparent activation energy Eg at Tg has been investigated. It was found that Eg increases with Tg to the 2nd power for hydrogen bonding organics, polymeric and metallic glass forming liquids, while Eg of inorganic glasses has a linear dependence on Tg.

The shapes of cooperatively rearranging regions in glass-forming liquids

Abstract: The cooperative rearrangement of groups of many molecules has long been thought to underlie the dramatic slowing of liquid dynamics on cooling towards the glassy state. For instance, there exists experimental evidence for cooperatively rearranging regions (CRRs) on the nanometre length scale near the glass transition. The random first-order transition (RFOT) theory of glasses predicts that, near the glass-transition temperature, these regions are compact, but computer simulations and experiments on colloids suggest CRRs are string-like. Here, we present a microscopic theory within the framework of RFOT, which unites the two situations. We show that the shapes of CRRs in glassy liquids should change from being compact at low temperatures to fractal or ‘stringy’ as the dynamical crossover temperature from activated to collisional transport is approached from below. This theory predicts a correlation of the ratio of the dynamical crossover temperature to the laboratory glass-transition temperature, and the heat-capacity discontinuity at the glass transition. The predicted correlation quantitatively agrees with experimental results for 21 materials.

Synthesis, characterization and a.c. conductivity of polypyrrole/Y 2 O 3 composites

Abstract: Conducting polymer composites of polypyrrole/yttrium oxide (PPy/Y2O3) were synthesized byin situ polymerization of pyrrole with Y2O3 using FeCl3 as an oxidant. The Y2O3 is varied in five different weight percentages of PPy in PPy/Y2O3 composites. The synthesized polymer composites are characterized by infrared and X-ray diffraction techniques. The surface morphology of the composite is studied by scanning electron microscopy. The glass transition temperature of the polymer and its composite is discussed by DSC. Electrical conductivity of the compressed pellets depends on the concentration of Y2O3 in PPy. The frequency dependent a.c. conductivity reveals that the Y2O3 concentration in PPy is responsible for the variation of conductivity of the composites. Frequency dependent dielectric constant at room temperature for different composites are due to interfacial space charge (Maxwell Wagner) polarization leading to the large value of dielectric constant. Frequency dependent dielectric loss, as well as variation of dielectric loss as a function of mass percentage of Y2O3 is also presented and discussed.

Chemical, Physical, and Mechanical Characterization of Isocyanate Cross-linked Amine-Modified Silica Aerogels

Abstract: We describe a new mechanically strong lightweight porous composite material obtained by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea. The conformal polymer coating preserves the mesoporous structure of the underlying silica framework and the thermal conductivity remains low at 0.041 plus or minus 0.001 W m(sup -1 K(sup -1). The potential of the new cross-linked silica aerogels for load-carrying applications was determined through characterization of their mechanical behavior under compression, three-point bending, and dynamic mechanical analysis (DMA). A primary glass transition temperature of 130 C was identified through DMA. At room temperature, results indicate a hyperfoam behavior where in compression cross-linked aerogels are linearly elastic under small strains (less than 4%) and then exhibit yield behavior (until 40% strain), followed by densification and inelastic hardening. At room temperature the compressive Young's modulus and the Poisson's ratio were determined to be 129 plus or minus 8 MPa and 0.18, respectively, while the strain at ultimate failure is 77% and the average specific compressive stress at ultimate failure is 3.89 x 10(exp 5) N m kg(sup -1). The specific flexural strength is 2.16 x 10(exp 4) N m kg(sup -1). Effects on the compressive behavior of strain rate and low temperature were also evaluated.

Polyethylene glycol functionalized dicationic ionic liquids with alkyl or polyfluoroalkyl substituents as high temperature lubricants

Abstract: A series of new polyethylene glycol functionalized dicationic ionic liquids with alkyl or polyfluoroalkyl substitutents (9–17 and 19–24) has been prepared. Important physical properties of these liquids, including glass transition (Tg) and decomposition temperatures (Td), solubility in common solvents, density (d) and viscosity (η), were measured. These ionic liquids show high thermal stability and good lubricity. In general, imidazolium based dicationic liquids have higher Td (>415 °C) than their triazolium analogues. The introduction of polyfluoroalkyl groups boosts antiwear properties but also leads to a decrease in Td. These ionic liquids also exhibit excellent tribological characteristics even at 300 °C, which suggests use as high temperature lubricants.

The structure of amorphous, crystalline and liquid GeO2

Abstract: Germanium dioxide (GeO2) is a chemical analogue of SiO2. Furthermore, it is also to some extent a structural analogue, as the low- and high-pressure short-range order (tetrahedral and octahedral) is the same. However, a number of differences exist. For example, the GeO2 phase diagram exhibits a smaller number of polymorphs, and all three GeO2 phases (crystalline, glass, liquid) have an increased sensitivity to pressure, undergoing pressure-induced changes at much lower pressures than their equivalent SiO2 analogues. In addition, differences exist in GeO2 glass in the medium-range order, resulting in the glass transition temperature of germania being much lower than for silica. This review highlights the structure of amorphous GeO2 by different experimental (e.g., Raman and NMR spectroscopy, neutron and x-ray diffraction) and theoretical methods (e.g., classical molecular dynamics, ab initio calculations). It also addresses the structures of liquid and crystalline GeO2, that have received much less attention. Furthermore, we compare and contrast the structures of GeO2 and SiO2, as well as along the GeO2–SiO2 join. It is probably a very timely review, as interest in this compound, that can be investigated in the liquid state at relatively low temperatures and pressures, continues to increase.

Self-diffusion of supercooled o-terphenyl near the glass transition temperature.

Abstract: Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics. Self-diffusion controls crystallization in o-terphenyl for most of the supercooled liquid regime, but at temperatures below Tg + 10 K, the reported crystallization rate increases suddenly while the self-diffusion coefficient does not. This work and previous work on trisnaphthylbenzene both find a self-diffusion-controlled crystal growth regime and an enhancement in self-diffusion near Tg, suggesting that these phenomena are general characteristics of fragile low molecular weight glass formers. We discuss the width of the relaxation time distributions of o-terphenyl and trisnaphthylbenzene as they relate to the observation of enhanced translational diffusion.

Glass transition in biomolecules and the liquid-liquid critical point of water.

Abstract: Using molecular dynamics simulations, we investigate the relation between the dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and thermodynamic properties of hydration water. We find that the dynamic transition of the macromolecules, sometimes called a "protein glass transition," occurs at the temperature of dynamic crossover in the diffusivity of hydration water and also coincides with the maxima of the isobaric specific heat C_{P} and the temperature derivative of the orientational order parameter. We relate these findings to the hypothesis of a liquid-liquid critical point in water. Our simulations are consistent with the possibility that the protein glass transition results from crossing the Widom line, which is defined as the locus of correlation length maxima emanating from the hypothesized second critical point of water.

Magnetic imaging of a supercooling glass transition in a weakly disordered ferromagnet.

Abstract: Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems where replica symmetry is broken. Although magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin-glass picture, our results show that the observed static magnetic configuration seen below the glass-transition temperature arises from the cooperative freezing of the first-order antiferromagnetic (charge ordered) to ferromagnetic transition. Our data also suggest that accommodation strain is important in the kinetics of the phase transition. This cooperative freezing idea has been applied to structural glasses including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free-energy landscape.

Unified equation for the strength of bulk metallic glasses

Abstract: In the present study, a conceptual approach to evaluate the strength of metallic glass systems is proposed from a free volume point of view. Based on the physical analogy between the plastic deformation and glass transition, the strength of amorphous structures was found to depend on both the localized shear mechanism and the atomic cohesive energy. Interestingly, we find that the strength at the ambient temperature (T0) can be determined by the glass transition temperature (Tg) and molar volume (V), and can be specifically predicted by a unified parameter of (Tg−T0)∕V. The predicted strength was unambiguously verified from experimental data reported for a number of metallic glass systems.

Controlling the thermomechanical properties of polymer nanocomposites by tailoring the polymer–particle interface

Abstract: We show that the thermomechanical properties of polymer nanocomposites are critically affected by polymer-particle wetting behavior. Silica nanoparticles grafted with dense polystyrene brushes of degree of polymerization 1050 are blended with polystyrene melts to form nanocomposites. It was found that low molecular weight (MW) polystyrene melts with lengths <880 wet these particles. Concurrently, the glass transition temperature (Tg) of the nanocomposite increases. At higher MW, the matrix does not wet the particles and the Tg decreases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2944–2950, 2006

Thickness-dependent reduction of the glass-transition temperature in thin polymer films with a free surface

Abstract: We present results of molecular dynamics simulations for free-standing and supported thin films of a nonentangled polymer melt using a coarse-grained (bead-spring) model. Our discussion is mainly concerned with the equilibrium properties of the films above the critical temperature (Tc) of mode-coupling theory, although we also determine the glass-transition temperature (Tg) by measurements of the film thickness h upon cooling. We explore the influence of confinement on the structure and dynamics of the polymer films. We find that the dynamics in the films is accelerated compared to the bulk, that this enhanced mobility originates from the surfaces, and that the effect is larger at the free than at the supported surface. Thus, the films have lower Tc values relative to the bulk. Tc depends on film thickness h; this dependence can be well parametrized by Tc(h) = T/(1 + h0/h), a function proposed by experiments on supported polystyrene films. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2951–2967, 2006