Showing papers on "Glass transition published in 2001"

Supercooled liquids and the glass transition

Abstract: Glasses are disordered materials that lack the periodicity of crystals but behave mechanically like solids. The most common way of making a glass is by cooling a viscous liquid fast enough to avoid crystallization. Although this route to the vitreous state-supercooling-has been known for millennia, the molecular processes by which liquids acquire amorphous rigidity upon cooling are not fully understood. Here we discuss current theoretical knowledge of the manner in which intermolecular forces give rise to complex behaviour in supercooled liquids and glasses. An intriguing aspect of this behaviour is the apparent connection between dynamics and thermodynamics. The multidimensional potential energy surface as a function of particle coordinates (the energy landscape) offers a convenient viewpoint for the analysis and interpretation of supercooling and glass-formation phenomena. That much of this analysis is at present largely qualitative reflects the fact that precise computations of how viscous liquids sample their landscape have become possible only recently.

Ionic conductivity in crystalline polymer electrolytes

Abstract: Polymer electrolytes are the subject of intensive study, in part because of their potential use as the electrolyte in all-solid-state rechargeable lithium batteries. These materials are formed by dissolving a salt (for example LiI) in a solid host polymer such as poly(ethylene oxide) (refs 2, 3, 4, 5, 6), and may be prepared as both crystalline and amorphous phases. Conductivity in polymer electrolytes has long been viewed as confined to the amorphous phase above the glass transition temperature, Tg, where polymer chain motion creates a dynamic, disordered environment that plays a critical role in facilitating ion transport. Here we show that, in contrast to this prevailing view, ionic conductivity in the static, ordered environment of the crystalline phase can be greater than that in the equivalent amorphous material above Tg. Moreover, we demonstrate that ion transport in crystalline polymer electrolytes can be dominated by the cations, whereas both ions are generally mobile in the amorphous phase. Restriction of mobility to the lithium cation is advantageous for battery applications. The realization that order can promote ion transport in polymers is interesting in the context of electronically conducting polymers, where crystallinity favours electron transport.

Dependence of the Glass Transition Temperature of Polymer Films on Interfacial Energy and Thickness

Abstract: The glass transition temperatures (Tg's) of ultrathin films (thickness 80−18 nm) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were measured on surfaces with interfacial energies (γSL) ranging from 0.50 to 6.48 mJ/m2. The surfaces consisted of self-assembled films of octadecyltrichlorosilane (OTS) that were exposed to X-rays in the presence of air. Exposure to X-ray radiation systematically modified the OTS by incorporating oxygen-containing groups on the surface. The interfacial energy for PS and PMMA on the OTS surface was quantified as a function of X-ray dose using the Fowkes−van Oss−Chaudhury−Good model of surface tension. The Tg values of the films were characterized by three complementary techniques: local thermal analysis, ellipsometry, and X-ray reflectivity. Within the resolution of the techniques, the results were in agreement. At low values of γSL, the Tg values of PS and PMMA films were below the respective bulk values of the polymers. At high values of γSL, the Tg values of PS an...

Character of the glass transition in thin supported polymer films.

Abstract: We have used ellipsometry to study the thermal expansivity of thin polystyrene films on silicon substrates with thicknesses of 10--200 nm. We find well-defined glass transitions, and detailed analysis of the expansivities shows that for thinner films the transition width is broadened, while the strength of the transition, defined by the difference between the expansivities in the liquid and glassy state, is reduced; the expansivity in the glassy state is higher than in the bulk. These phenomena are consistent with the idea that a layer of roughly constant thickness, of order 10 nm, near the surface of the film has liquidlike thermal properties at all experimental temperatures.

Fragile-to-strong transition and polyamorphism in the energy landscape of liquid silica

Abstract: Liquid silica is the archetypal glass former, and compounds based on silica are ubiquitous as natural and man-made amorphous materials. Liquid silica is also the extreme case of a 'strong' liquid, in that the variation of viscosity with temperature closely follows the Arrhenius law as the liquid is cooled toward its glass transition temperature. In contrast, most liquids are to some degree 'fragile', showing significantly faster increases in their viscosity as the glass transition temperature is approached. Recent studies have demonstrated the controlling influence of the potential energy hypersurface (or 'energy landscape') of the liquid on the transport properties near the glass transition. But the origin of strong liquid behaviour in terms of the energy landscape has not yet been resolved. Here we study the static and dynamic properties of liquid silica over a wide range of temperature and density using computer simulations. The results reveal a change in the energy landscape with decreasing temperature, which underlies a transition from a fragile liquid at high temperature to a strong liquid at low temperature. We also show that a specific heat anomaly is associated with this fragile-to-strong transition, and suggest that this anomaly is related to the polyamorphic behaviour of amorphous solid silica.

The glass transition of water, based on hyperquenching experiments.

Abstract: The glass transition temperature (Tg) in water is still uncertain, with conflicting values reported in the literature. As with other hyperquenched glasses, water exhibits a large relaxation exotherm on reheating at the normal rate of 10 kelvin (K) per minute. This release of heat indicates the transformation of a high enthalpy state to a lower one found in slow-cooled glasses. When the exotherm temperature is scaled by Tg, the good glass-formers show a common pattern. However, for hyperquenched water, when this analysis is performed using the commonly accepted Tg = 136 K, its behavior appears completely different, but this should not be the case because enthalpy relaxation is fundamental to the calorimetric glass transition. With Tg = 165 +/- 5 K, normal behavior is restored in comparison with other hyperquenched glasses and with the binary solution behavior of network-former systems (H2O, ZnCl2, or BeF2 plus a second component). This revised value has relevance to the understanding of water- biomolecule interactions.

Glasslike Kinetic Arrest at the Colloidal-Gelation Transition

Abstract: We show that gelation of weakly attractive colloids is remarkably similar to the colloidal glass transition. Like the glass transition, dynamic light scattering functions near gelation scale with scattering vector, and exhibits a two-step decay with a power-law divergence of the final decay time. Like the glass transition, static light scattering does not change upon gelation. These results suggest that, like the glass transition, gelation results from kinetic arrest due to crowding of clusters, and that both gelation and the glass transition are manifestations of a more general jamming transition.

Heterogeneous dynamics at the glass transition in van der Waals liquids, in the bulk and in thin films

Abstract: It has been shown over the last few years that the dynamics close to the glass transition is strongly heterogeneous, both by measuring the diffusion coefficient of tagged particles or by NMR studies. Recent experiments have also demonstrated that the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. We propose here first a thermodynamical model for van der Waals liquids, which accounts for experimental results regarding the bulk modulus of polymer melts and the evolution of the density with temperature. This model allows us to describe the density fluctuations in such van der Waals liquids. Then, by considering the thermally induced density fluctuations in the bulk, we propose that the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous dynamics close to T g. We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and we calculate the corresponding glass transition temperature reduction, in quantitative agreement with experimental results of Jones and co-workers. In the case of strongly adsorbed films, we show that the strong adsorption amounts to enhance the slow domains percolation. This effect leads to 1) a broadening of the glass transition and 2) an increase of T g in quantitative agreement with experimental results. For both strongly and weakly adsorbed films, the shift in T g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation.

Blue-Light-Emitting Fluorene-Based Polymers with Tunable Electronic Properties

Abstract: A series of soluble alternating polyfluorene copolymers with different main chain structures and those of the same main chain structure polyfluorene-co-alt-phenylene with different functional groups attached at the 2- and/or 5-positions of the phenylene ring were synthesized by a palladium-catalyzed Suzuki coupling reaction. All 10 polymers had the band gaps ranging from 2.81 to 3.35 eV, corresponding to blue-light emission. Through controllable modification for both the main chain structures and the side chains, not only the optical and electronic properties of the blue emissive polymers had been tuned, but also the structure−property relationships, especially the HOMO and LUMO energy level engineering, had been studied. Relatively high PL efficiency in both solution and film states, good thermal stability, and relatively high glass transition temperatures were demonstrated on these polymers. In general, polymers with the main chain structure of polyfluorene-co-alt-phenylene were found to have higher Φfl...

Effect of Interfacial Interactions on the Glass Transition of Polymer Thin Films

Abstract: Previous studies had demonstrated that the Tg of polymer thin films is strongly dependent on the interactions, γs, between the polymer and the underlying substrate. We present a study of the glass transition temperature, Tg, in thin films of polystyrene, PS, as a function of film thickness and as a function of γs by measuring the change in the thermal expansion using X-ray reflectivity. The Tg for PS on native silicon oxide was found to decrease with decreasing film thickness, consistent with results by others. Using random copolymers of styrene and methyl methacrylate anchored to the substrate, γs could be varied by changing the styrene fraction, f. With a constant PS film thickness of 330 A, the Tg was depressed by ∼20 °C as f was decreased from 1 to 0.75. An analysis analogous to the Gibbs−Thompson model indicated that the surface energy was not a suitable parameter to use to describe the effects of interfacial interactions on the Tg of polymer thin films. An associated local fractional change in the s...

Mobile silver ions and glass formation in solid electrolytes

Abstract: Solid electrolytes are a class of materials in which the cationic or anionic constituents are not confined to specific lattice sites, but are essentially free to move throughout the structure. The solid electrolytes AgI and Ag2Se (refs 1, 2, 3, 4, 5, 6, 7) are of interest for their use as additives in network glasses8,9,10,11,12, such as chalcogenides and oxides, because the resulting composite glasses can show high electrical conductivities with potential applications for batteries, sensors and displays. Here we show that these composite glasses can exhibit two distinct types of molecular structures—an intrinsic phase-separation that results in a bimodal distribution of glass transition temperatures, and a microscopically homogeneous network displaying a single glass transition temperature. For the first case, the two transition temperatures correspond to the solid-electrolyte glass phase and the main glass phase (the ‘base glass’), enabling us to show that the glass transition temperatures for the AgI and Ag2Se phases are respectively 75 and 230 °C. Furthermore, we show that the magnitude of the bimodal glass transition temperatures can be quantitatively understood in terms of network connectivity, provided that the Ag+ cations undergo fast-ion motion in the glasses. These results allow us to unambiguously distinguish base glasses in which these additives are homogeneously alloyed from those in which an intrinsic phase separation occurs, and to provide clues to understanding ion-transport behaviour in these superionic conductors.

Microscopic theory of heterogeneity and nonexponential relaxations in supercooled liquids.

Abstract: Recent experiments show that supercooled liquids around the glass transition temperature are "dynamically heterogeneous" [H. Sillescu, J. Non-Cryst. Solids 243, 81 (1999)]. Such heterogeneity is expected from the random first order transition theory of the glass transition. Using a microscopic approach based on this theory, we derive a relation between the departure from Debye relaxation as characterized by the beta value of a stretched exponential response function, phi(t) = e(-(t/tau(KWW))beta), and the fragility of the liquid. The beta value is also predicted to depend on temperature and to vanish as the ideal glass transition is approached at the Kauzmann temperature.

Effects of a nanoscopic filler on the structure and dynamics of a simulated polymer melt and the relationship to ultrathin films.

Abstract: ~Received 19 July 2000; revised manuscript received 9 January 2001; published 27 July 2001! We perform molecular dynamics simulations of an idealized polymer melt surrounding a nanoscopic filler particle. We show that the glass transition temperature Tg of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and filler. A gradual change of the polymer dynamics approaching the filler surface causes the change in the glass transition. We also find that polymers close to the surface tend to be elongated and flattened. Our findings show a strong similarity to those obtained for ultrathin polymer films.

Nature and properties of the Johari–Goldstein β-relaxation in the equilibrium liquid state of a class of glass-formers

Abstract: Previous dielectric relaxation measurements of glycerol and propylene carbonate and new results on propylene glycol performed below the conventional glass transition temperatures T_(g) after long periods of aging all show that the excess wing (a second power law at higher frequencies) in the isothermal dielectric loss spectrum, develops into a shoulder. These results suggest that the excess wing, a characteristic feature of a variety of glass-formers, is the high frequency flank of a Johari–Goldstein β-relaxation loss peak submerged under the α-relaxation loss peak. With this interpretation of the excess wing assured, the dielectric spectra of all three glass-formers measured at temperatures above T_(g) are analyzed as a sum of a α-relaxation modeled by the Fourier transform of a Kohlrausch–Williams–Watts function and a β-relaxation modeled by a Cole–Cole function. Good fits to the experimental data have been achieved. In addition to the newly resolved β-relaxation on propylene glycol, the important results of this work are the properties of the β-relaxation in this class of glass-formers in the equilibrium liquid state obtained over broad frequency and temperature ranges.

Neutron scattering studies of the model glass former ortho-terphenyl

Abstract: The van der Waals liquid ortho-terphenyl has long been used as a model system in the study of the glass transition. Motivated by mode-coupling theory, extensive experiments have been undertaken to monitor the onset of structural relaxation on microscopic time- and lengthscales. Using in particular quasielastic neutron scattering, the decay of density and tagged-particle correlations has been measured as a function of temperature, pressure and wavenumber. A consistent picture is developed, in which the mode-coupling singularity appears as a change of transport mechanism in the moderately viscous liquid, at temperatures far above the conventional (caloric) glass transition.