Showing papers on "Glass transition published in 1988"

pH-Dependent Equilibrium Swelling Properties of Hydrophobic Polyelectrolyte Copolymer Gels

Abstract: Etude a 25°C de polymeres reticules a base de methacrylates de n-alkyle et de (dimethyl-amino) ethylmethacrylate Gonflement dans l'eau Transition de l'etat hydrophobe a l'etat hydrophile Influence de la composition du comonomere

Icosahedral ordering in the Lennard-Jones liquid and glass.

Abstract: We have simulated the cooling of Lennard-Jones fluids from liquid temperatures to below the glass transition by using molecular-dynamics calculations and periodic boundary conditions. At higher temperatures the "inherent" structure is independent of temperature, but slightly above the glass transition temperature ${T}_{g}$ the structure changes and local fivefold symmetry becomes more prominent. At ${T}_{g}$ this process is slow and a hysteresis is observed on heating. The low-temperature two-component glass has a cluster of interpenetrating and face-sharing icosahedra percolating throughout the sample.

Structural instability and relaxation in liquid and glassy phases near the fragile liquid limit

Abstract: We consider two sorts of structural instability of glassy substances, each of which may be released by a relaxation process with its own characteristic relaxation time, and specific kinetic features. The first of these is the instability against relaxation out of the amorphous state into the crystalline state, while the second is the instability against relaxation within the amorphous state itself. The latter may often involve relaxation out of a homogeneous amorphous phase into a two-phase amorphous structure, but we will not specifically consider this liquid-liquid phase separation process here. In most glasses, the former (which is no more than the characteristic nucleation time) is much longer than the latter time. However, there are important classes of glasses, for instance the metallic glasses, in which the former is in fact the shorter time, a fact which is responsible for the inability to observe the glass transition phenomenon in such substances. In this paper we will be considering the relation between these two times and the specific kinetics of each. The nucleation time has been the subject of theoretical developments over a number of decades, and details will be omitted in order to concentrate on experimental studies of this phenomenon. We will described briefly the recently developed DSC techniques for determining the classical time-temperature-transformation curves for a variety of supercooled liquids, and the relation of these to the nucleation curves. The relaxation process within the amorphous state, which can be observed for cases where the nucleation time is relatively long, has a number of features which currently lack a complete explanation. In most cases the relaxation process is non-Arrhenius in its temperature dependence, nonexponential in its time dependence, and nonlinear in its structural state dependence. Some examples taken from glasses at the “fragile” edge of the deduced viscosity-temperature pattern for glassforming liquids are dealt with in detail, and the distinction between shear stress relaxation and thermodynamic stress relaxation is made. The possibility that near T g the latter relaxation time remains Vogel-Fulcher in form with T 0 ≡ T K (the Kauzmann temperature), in contrast with the common observations for (the decoupled) shear relaxation, is raised. Strong support for this notion is found in the current “specific heat spectroscopy” results of Nagel and co-workers. Microscopic relaxation processes, as observed using spectroscopic probes and neutron scattering techniques, are reviewed, and the difference in non-exponentiality from macroscopic relaxation are examined in the light of current theories. Finally, secondary relaxations in ionic and molecular glasses, and their relation to the fastest of all glassy state relaxation processes, the tunnelling modes (TLS), are briefly considered.

Entropy and enthalpy catastrophe as a stability limit for crystalline material

Abstract: The transition from a crystalline material with long-range order to a glass-like disordered structure has been observed in several metal alloys and minerals using experimental techniques such as solid-state reaction, mechanical alloying, pressure application, ion-beam mixing and hydriding1–3. In all these examples the vitrification occurs below the glass transition temperature, and because the glass can be thought of as a highly undercooled liquid, one may draw an analogy between the vitrification process and melting. The melting process is often viewed as a catastrophic instability of the crystal lattice4–6, although none of these theories predicts the melting temperature correctly. The transition from crystal to glass could also be triggered by some type of instability7; indeed, Kauzmann8 has argued that an undercooled liquid whose entropy falls below that of the crystalline phase must undergo massive freezing to a glass. Applying an 'inverse' Kauzmann argument to the problem of melting, an entropy catastrophe is predicted when the entropy of a superheated crystal exceeds that of the liquid phase. We propose that this temperature represents an ultimate stability limit for superheated or supersaturated crystals.

Thermal analysis of poly(butylene terephthalate) for heat capacity, rigid-amorphous content, and transition behavior

Abstract: Thermal analysis was performed on poly(butylene terephthalate), PBT, between 210 and 560 K. By combination of experimental heat capacities with computations with an approximate frequency spectrum of 65 group and 19 skeletal vibrations, preliminary recommended ATHAS (1988) heat capacities are proposed for the solid state from 0 to 600 K. The Tarasov parameters used for the computation of the skeletal vibrations were θ1 = 542 K and θ3 = 80 K for crystalline PBT and θ3 = 40 K for amorphous PBT. The glass transition temperature of amorphous PBT was found on efficiently quenched samples to be 248 K, much lower than for semicrystalline PBT where a 310–325 K glass transition temperature is typical. The increase in heat capacity calculated for 100% amorphous samples is 107 J/(K · mol) at 248 K and 77 J/(K · mol) at 320 K. The equilibrium melting temperature is estimated to be 518 K. The unique existence of rigid-amorphous fractions of the semicrystalline polymers is discussed with quantitative data for samples crystallized from the glass and from the melt between 275 and 490 K. The rigidamorphous fraction varies between above 0,9 for cold-crystallized samples to 0,3 for samples crystallized at 490 K. The crystallinity varied from below 0,1 to 0,5. The crystallinity could be separated into four parts, melting at high, medium, and low temperatures, and a part crystallized on cooling after isothermal crystallization. The sequence of crystallization of differently melting crystals was established.

Glass transition and melting behavior of poly(ethylene 2,6-naphthalenedicarboxylate)

Abstract: Capacite calorifique du polymere a l'etat solide (230-350 K) et a l'etat liquide (390-600°K)

Neutron-spin-echo investigation on the dynamics of polybutadiene near the glass transition.

Abstract: We present neutron-spin-echo data on the structural relaxation of glass-forming polybutadiene near the glass transition. Microscopic dynamics on the scale of an interchain distance and the monomeric friction coefficient derived from macroscopic viscosity relaxation obey the time-temperature superposition principle following a common scale. The dynamic structure factor exhibits strong similarities with recent mode-coupling predictions.

Thermomechanical properties of small-carbohydrate–water glasses and ‘rubbers’. Kinetically metastable systems at sub-zero temperatures

Abstract: Aqueous solutions of 24 small carbohydrates have been analysed by low-temperature differential scanning calorimetry (d.s.c.). The method, based on analogue derivative thermograms, measured two thermomechanical properties characteristic of each non-crystallizing solute: T′g, the sub-zero glass transition temperature of a maximally freeze-concentrated solution and W′g, the amount of unfrozen water (UFW) kinetically immobilized in the glass at T′g. For 84 low-molecular-weight polyhydroxy compounds (PHCs) analysed to date, T′g ranged from –85 °C for ethylene glycol to –13.5 °C for maltoheptaose, and increased monotonically with increasing Mw. T′g plotted vs.M–1w showed a linear correlation characteristic of an homologous family of glass-forming linear oligomers. W′g ranged from 1.9 g UFW g–1 for ethylene glycol to 0.2–0.3 g UFW g–1 for several sugars and polyols, including maltoheptaose, and decreased with increasing T′g, showing fair linear correlations for several series of homologous solutes. We describe here the use of T′g and W′g, as invariant physico-chemical properties of glass-forming solutions at sub-zero temperatures, to interpret thermomechanical behaviour of small-carbohydrate–water systems in non-equilibrium glassy and ‘rubbery’ states, define structure–activity relationships and explain and often predict functional behaviour of such PHCs in various applications.

Observation of Scaling Behaviour of Dynamic Correlations near Liquid-Glass Transition

Abstract: A time-of-flight inelastic neutron scattering study of Ca0.4K0.6(NO3)1.4 of the temperature dependence of the dynamic structure factor revealed a clear frequency-independent signature of the glass transition at the conventional transition temperature Tg. Above Tg the data combined with our previous neutron spin echo results display two relaxation processes at well-separated frequencies. The faster process was now found to be consistent with a scaling behaviour corresponding to a critical-type slowing-down when approaching a certain temperature T0, as predicted by recent mode coupling theories. The variation of various properties consistently points to a critical temperature T0 of about 30 K above Tg = 333 K.

Solubility and miscibility of poly(ethyl oxazoline)

Abstract: The solubility of poly(ethyl oxazoline) in aqueous solutions was studied. The cloud point temperatures decreased in the presence of sodium chloride but increased by the addition of tetrabutylammonium bromide or dioxane. Solution-cast films of blends of the polymer and poly(acrylic acid) were miscible, but mutual precipitation occurred in water, methanol, and dioxane. The compositions of the complexes correspond in most cases to simple molar ratios of the interacting groups. The glass transition temperatures of the complexes are higher than the values for blends of the same compositions, and the high values are attributed to hydrogen bonds acting as physical crosslinks. Complex formation also occurs when the polymer is mixed with a styrene-acrylic acid copolymer and with low weight polymers containing phenol groups.

Heat capacity and glass transition behavior of amorphous ice

Abstract: The heat capacity of low-density amorphous ice, made by warming and annealing high-density amorphous ice, has been measured in the range 90-136 K. A glass transition was observed at 124 K for samples that were annealed in the range 124-130 K. The glass transition observed is presumably the same as that reported in the literature for amorphous ice prepared by vapor deposition or rapid quenching of water.

Analysis of the glass transition temperature of miscible polymer blends

Abstract: Etude theorique de la relation temperature de transition vitreuse-composition des melanges de polycaprolactone/polymere chlore

Polymer-Polymer Miscibility and Enthalpy Relaxations

Abstract: Annealing of polymers below the glass transition temperature results in a decrease in enthalpy that is recovered during heating. The enthalpy recovery is visible as an endothermic peak in a differential scanning calorimetry (DSC) scan. The position of this peak depends on the thermal treatment given and on the structure of the material itself. Because different polymers behave differently, the phenomenon can be utilized to investigate polymel-polymer miscibility of polymers with similar Tg values. Therefore, by annealing the blends at the temperature of interest and subsequent sub-T, annealing, one can monitor phase behavior resulting from the initial treatment by inspection of the enthalpy recovery. Two systems were investigated to illustrate this: an immiscible blend of poly(viny1 chloride) and poly(isopropy1 methacrylate) and a miscible blend of poly(viny1 chloride) and poly(methy1 methacrylate).

High-Tc Superconducting Glass Ceramics Based on the Bi-Ca-Sr-Cu-O System

Abstract: Various superconducting glass ceramics based on the Bi-Ca-Sr-Cu-O system with Tc(onset)=80~95 K and Tc(zero)=63~78 K were prepared by a melt-quenching method. The melt-quenched samples of Bi1.5CaSrCu2Ox and Bi1.5Ca1.5SrCu2Ox were found to be a glass with a glass transition of about 425°C; these samples became high-Tc superconductors by annealing at 820°C.

Relaxation and flow mechanisms in ``fragile'' glass-forming liquids

Abstract: Fragile glass‐forming liquids are characterized by strongly non‐Arrhenius temperature dependence of shear viscosity, and by unusually high heat capacity in the supercooled liquid above the glass transition Using the inherent structure formalism, a theoretical model is developed to explain these characteristics The vitrifying liquid is viewed as a dynamic patchwork of relatively strongly bonded (but amorphous) molecular domains that are separated by irregular walls of weakened bonds Mean domain diameter ξ rises as temperature declines to minimize wall free energy, but the process is self‐limiting due to intradomain frustration energy Shear flow occurs by a ‘‘tear and repair’’ mechanism involving elemental wall areas of mean size ξ2 The relation of the present model to the tiling models for glass formation is explored, and helps to establish the presence of anomalous heat capacity in the supercooled liquid The analysis suggests in strongly supercooled fragile liquids that the Stokes–Einstein formula may underestimate self‐diffusion constants, and that the Adam–Gibbs relation for mean relaxation time in terms of calorimetric entropy may display systematic errors

Ferroelectric liquid crystal device

Abstract: There is provided a ferroelectric liquid crystal device wherein the yellowing due to a cell thickness increase or occurrence of voids is suppressed. The ferroelectric liquid crystal device includes a pair of substrates each having thereon a group of electrodes for liquid crystal drive, and a layer of ferroelectric liquid crystal disposed between the substrates, wherein thermosetting adhesive particles and thermoplastic polymer particles having a diameter which is 1.5-5 times the liquid crystal layer thickness are dispersed and pressed between the substrates. The polymer particles preferably have a glass transition temperature of at most -20° C.

2H‐NMR study of the glass transition in supercooled ortho‐terphenyl

Abstract: The glass forming molecular liquid ortho‐terphenyl has been investigated by 2H‐NMR techniques providing spin‐relaxation times (T1, T2), and spin‐alignment data which yield information on the time scale and geometry of ultra‐slow molecular reorientation. The main results are as follows: The primary glass transition (α process) is characterized by rotational molecular jumps with a jump size distribution weighted in favor of large jump angles, and by a distribution of correlation times. In addition intramolecular flip–flop jumps of the lateral phenyl rings are found which do not take part in the α process. Apart from this (secondary) intramolecular dynamics no residual small angle reorientation persists below Tg on the time scale (10−4 to 102 s) of the spin‐alignment experiment.

Local fluctuations and ordering in liquid and amorphous metals.

Abstract: A molecular-dynamics study of the structure and dynamics of monatomic liquids and glasses is presented. The local atomic structure and its development during the quenching process are analyzed in terms of fluctuations of atomic-level stresses and their correlations. This approach extends the basis for the description of the local structure from the usually employed scalar quantity, the local density fluctuation, to a tensorial quantity, the local stress fluctuation. It is shown here that the local stress fluctuations and their spatial and temporal correlations provide a detailed picture of the dynamics of the liquid and of the transition from an ideal fluid to a viscous liquid, and then to a glass. In particular, it is demonstrated that the shear stresses which are spatially uncorrelated at high temperatures become correlated below a temperature, Ts, which is about twice the glass transition temperature. At the same time the dynamic behavior of the liquid, characterized by the diffusivity, viscosity, and phonon states, changes sharply at this temperature. Implications of this apparent structural transition and its origin are then discussed. Disciplines Atomic, Molecular and Optical Physics | Condensed Matter Physics | Engineering | Fluid Dynamics | Materials Science and Engineering | Physics | Semiconductor and Optical Materials This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/239 PHYSICAL REVIE%' 8 VOLUME 37, NUMBER 5 15 FEBRUARY 1988-I Local fluctuations and ordering in liquid and amorphous metals

Scaling properties in supercooled liquids near the glass transition

Abstract: A simple model for the liquid glass transition is solved numerically. Results for ideal and non-ideal transitions are compared. The data show two or three different scaling regions for the density correlation function or the corresponding susceptibility spectra in agreement with previous analytical results.

The gel to glass transition: Chemical and microstructural evolution

Abstract: The chemical and microstructural changes occurring in the conversion of a died gel to fully dense glass are reviewed. The main emphasis is on gels prepared from alkoxide precursors, including silica and more complex silicate compositions. The gel to glass conversion in these systems is contrasted with that in colloidal systems. The processes occurring in the conversion are crucially dependent on the composition of the starting solution and the chemistry of the sol to gel transformation. Shrinkage is governed by four processes operating at different stages during the gel to glass transition: capillary contraction, condensation-polymerization, structural relaxation and viscous sintering. A variety of techniques have recently been applied to study the changes in the porous gel as a result of heat treatment, including dilatometry, gas adsorption, DTA, TGA, TEM, infra-red spectroscopy (to monitor OH content, in particular), Raman spectroscopy, resonance techniques and SAXS. The conversion of dried gels into monolithic glass samples using the slow drying and firing method is discussed, including removal of hydroxyl content and prevention of bloating. Other processing routes are also briefly reviewed including hypercritical drying and sintering, the use of drying control additives prior to sintering, and colloidal techniques.