Showing papers on "Glass transition published in 1979"

Topology of covalent non-crystalline solids I: Short-range order in chalcogenide alloys

Abstract: The pronounced glass-forming tendencies of alloys of S and Se with Ge and/or As are discussed topologically. An atomic model is introduced which for predominantly covalent forces constitutes the first microscopic realization of Kauzmann's description of the glass transition as an entropy (not enthalpy or volume) crisis. The model contains no adjustable parameters and predicts the glass-forming tendency as a function of composition in excellent agreement with experiment. Several related properties, including phase diagrams, radial distribution functions and crystal structures are discussed in the context of chemical bonding and short-range order in the non-crystalline covalent networks of these materials.

Gas Sorption and Transport in Glassy Polymers

Abstract: Gas sorption and transport in amorphous polymers above their glass transition temperature are adequately described phenomenologically by Henry's and Fick's laws. However, below the glass transition temperature an additional sorption mechanism develops which follows the Langmuir isotherm, and, thus, there are two populations of gas molecules which may be regarded to be in equilibrium with each other. The Langmuir mechanism evidently arises from the non-equilibrium nature of the glassy state. Molecules sorbed by this mechanism have less diffusional mobility than molecules sorbed by the Henry's law mode. Because of these differences, the analyses of transient permeation experiments are more complex for glassy polymers compared to those used widely for rubbery polymers. A more comprehensive phenomeno-logical model for transport of gases in glassy polymers is developed here which has been successfully used to interpret the observed dependence of the permeability coefficient and diffusion time lag on upstream gas pressure. The state of knowledge about gas sorption and transport in glassy polymers is reviewed and contrasted with that of rubbery polymers. It is seen that such observations provide a unique method to probe the physical structure of the glassy state and that knowledge in this area is increasing rapidly.

Preparation and Properties of Yttrium-Silicon-Aluminum Oxynitride Glasses

Abstract: Glasses containing up to 7 at. % nitrogen were prepared in the system Y-Si-Al-O-N. The glass transition temperature, hardness, and relative fracture toughness increase and the thermal expansion coefficient decreases with increasing nitrogen content. Weight losses after 350 h in 95/sup 0/C distilled water are not simply related to nitrogen content, but for some compositions they are half that reported for fused silica. The electrical conductivity at 548/sup 0/C and 10 kHz is 7.8 x 10/sup -10/ (..cap omega...cm)/sup -1/ for one composition. Fourier transform ir spectroscopic analysis indicates the presence of Si-N bonds in the glasses. The data suggest that nitrogen substitutes for oxygen in the glass network to produce a more highly cross-linked structure.

The origin of residual internal stress in solvent‐cast thermoplastic coatings

Abstract: Experiments on polystyrene and poly(isobutyl methacrylate) coatings cast from toluene have shown that residual internal stress is independent of dried coating thickness and initial solution concentration. A theory is presented that predicts the magnitude of the stress and shows that it has no dependence on thickness or initial solution concentration. Internal strain is calculated from the volume of solvent lost after the coating has solidified. This solidification point is identified with the solvent concentration that is sufficient to depress the glass transition of the polymer to the prevailing experimental temperature. A one-to-one correlation is confirmed between the volume of solvent lost from solution during drying and the volume change of the polymer film.

Towards an understanding of the heat distortion temperature of thermoplastics

Abstract: A systematic understanding of the heat distortion temperature (HDT) of amorphous and semi-crystalline polymers is possible through a direct correlation with the modulustemperature behavior. For amorphous polymers, the precipitous drop in modulus at the glass transition temperature makes the HDT a well-defined, reproducible and predictable property. Furthermore, the addition of reinforcing fillers has a negligible effect on the HDT of the amorphous polymer. For semi-crystalline polymers, however, the exact opposite may hold true. The modulus exhibits a “plateau” region between the glass transition and the melting transition. Hence the HDT often is difficult to predict, is sensitive to thermal history and may be greatly increased through the addition of fillers. More importantly, the HDT may not be an accurate measure of the upper use temperature for semi-crystalline polymers in load bearing situations since considerable stiffness may still be retained even upon exceeding the HDT.

Molecular interactions and macroscopic properties of polyacrylonitrile and model substances

Abstract: The relationships between intra- and intermolecular forces in polyacrylonitrile on the one hand, and the macroscopic behavior of the polymer and fibers thereof on the other, are reviewed Characteristic properties such as the very high polymerization rate constant in water, the dissolution of the polymer in concentrated inorganic salt solutions, the high melting point, the strong depression of melting point and glass transition temperature by water, the plasticization by polar additives, etc, are traced back to their molecular origins, in particular to the strong intra- and intermolecular noncovalent bonding caused by the highly polar nitrile group The effects of dipole-dipole interaction, hydrogen bonding and electron-donor-acceptor complex formation are discussed separately

Polyester–Polycarbonate blends. IV. Poly(ε-caprolactone)

Abstract: Polycarbonate blends with poly(e-caprolactone) were prepared by both melt-blending and solution-blending techniques, and the properties of these blends were studied by thermal analytical and dynamic mechanical testing methods. Each blend composition was found to have a single glass transition temperature, and the temperature location of this transition was found to be a function only of blend composition and to be independent of the blending technique employed. This behavior led to the conclusions that these two polymers form blends containing a single amorphous phase comprised of the two materials and that this miscible phase results primarily from physical rather than chemical interactions between the two polymers. A reversible liquid-liquid-type phase separation was found to occur when the blend system was heated to high melt temperatures. The temperature required for phase separation, the lower critical solution temperature, was found to vary with blend composition and component molecular weight in the manner expected from thermodynamic considerations. The level of crystallinity of poly(e-caprolactone) was affected by the presence of the polycarbonate. The polycarbonate also crystallized to an appreciable extent in many of the blends.

Dynamic mechanical properties of elastin

Abstract: The dynamic mechanical properties of water-swollen elastin under physiological conditions have been investigated. When elastin is tested as a colsed, fixed-volume system, mechanical data could be temperature shifted to produce master curves. Master curves for elastin hydrated at 36°C (water content, 0.46 g water/g protein) and 55°C (water content, 0.41 g/g) were constructed, and in both cases elastin goes through a glass transition, with the glass transition temperatures of -46 and -21°C, respectively. Temperature shift data used to construct the master curves follow the WLF equation, and the glass transition appears to be characteristic of an amorphous, random-polymer network. For elastin tested as an open, variable-volume system free to change its swollen volume as temperature is changed, dynamic mechanical properties appear to be virtually independent of temperature. No glass transition is observed because elastin swelling increases with decreased temperature, and the increase in water content shifts elastin away from its glass transition. It is suggested that the hydrophobic character of elastin, which gives rise to the unusual swelling properties of elastin, evolved to provide a temperature-independent elastomer for the cold-blooded, lower vertebrates.

Glass transition and specific heats in the systems PS, PSe, AsS and AsSe

Abstract: The glass transition temperatures were measured in the systems AsS, As0.5P0.5S, PSe, AsSe and PAsSe. Heat capacities of the glasses in the selenium systems were obtained by differential scanning calorimetry. As shown by the residual entropies departures from ideality are high in the chalcogen glasses. The results are discussed in terms of the structure of glasses in these systems. The thermodynamic data of glasses and liquids in these systems indicate a balance of intra- and intermolecular saturation of bonds. The amount of polymerization increases with increasing average molecular weight in the glass and with increasing temperature in some of the investigated liquids.

Density, Electric Conductivity, and Viscosity of Several N‐Alkylpyridinium Halides and Their Mixtures with Aluminum Chloride

Abstract: The specific conductivities of molten N‐methyl‐, N‐ethyl‐, N‐(n‐propyl)‐, and N‐(n‐butyl)pyridinium chlorides and N‐ethylpyridinium bromide were measured from near the melting points to the thermal decomposition temperatures. Conductivities also were measured from 25° to 125°C for mixtures of each of these compounds with aluminum chloride. The composition of the mixtures was 1:2 . In addition, the equimolar melt 1:1 was investigated. Densities and viscosities were measured and equivalent conductivities calculated for each of the 1:2 melts. The viscosities of the mixtures displayed Arrhenius behavior, whereas the conductivities behaved as though a glass transition were involved.

Effect of free volume fluctuations on polymer relaxation in the glassy state. II. Calculated results

Abstract: The relaxation following a change in temperature of amorphous polymers near the glass transition has been calculated. The calculation uses a chain model consisting of cis and trans backbone rotational states. The relaxation is assumed to proceed by localized conformational changes whose rates are controlled by the fractional free volume in small enough regions of the polymer that thermal fluctuations need to be considered. The relaxation is treated as a stochastic process, and an approximate solution is obtained for a finite set of relaxation environments. Using what is believed to be the most plausible set of parameters for polystyrene, relaxation curves are computed for the internal energy that are very similar to the curves obtained by Kovacs and others for the volumetric relaxation of poly(vinyl acetate) and polystyrene.

Molecular dynamics studies of glass formation in the Lennard-Jones model of argon

Abstract: Using the method of molecular dynamics it is shown that glasses can be formed from a 216 particle sample of the Lennard-Jones (LJ) liquid either by isobaric cooling or by isothermal compression. The properties of the glasses are relatively independent of the mechanism of formation. At zero pressure a glass transition, characterised by the disappearance of diffusion, occurs at ρ*=Nσ3/V= 0.98 and kT/Iµ= 0.29 (ρ* is a reduced density and V is the volume of the N-particle system; σ and Iµ are LJ parameters). The non-Arrhenius temperature dependence of diffusion coefficients in the metastable liquid and the changes of second order thermodynamic properties characteristic of the transition are discussed in relation to laboratory glass-forming systems. For isothermal compression near the normal melting temperature a glass transition occurs at ρ*= 1.06 and ρσ3/NIµ= 8.8, where p is the pressure, although in this case it is less well-defined since crystal nucleation more readily occurs at higher temperature. Comparisons using perturbation theory show that the LJ system is ≈ 12% more dense at the glass transition than the equivalent hard sphere fluid. Analysis of radial distribution functions suggests that upon cooling and/or compression the soft LJ system can achieve a more compact second nearest neighbour packing geometry than hard spheres and may provide a better basis for modelling glass formation in, for instance, metal–metalloid systems.

The glass transition temperature of an epoxy resin and the effect of absorbed water

Abstract: The glass transition temperature, Tg, of a diamino-dipenyl methane (DDM, II) cured diglycidyl ether of bispenol-A epoxy resin (I, n=0) has been determined by proton NMR line width studies to be 410°K. In the presence of 2.5±0.1 w/o absorbed water Tg is reduced to 378°K. Such a large reduction of Tg by such a small amount of water requires specific interactions. It is postulated that segmental motion is increased as a result of the disruption of hydroxyl group hydrogen bonds by the absorbed water molecules.

Measurement of the elastic properties of polymer melts

Abstract: A method and apparatus for measuring the elastic and other properties of polymers in the melt state is presented. The recoverable strain magnitude and the rate of strain recovery have been measured as a function of: applied shear rate, applied shear magnitude, temperature and molecular weight. The elastic properties indicate that there is an abrupt change or “transition” in the response of polystyrene melts at temperatures well above the glass transition. This abrupt change is found to be molecular weight dependent. The results are interpreted qualitatively in terms of molecular structure and practical processing operations. The possible relationship of this “transition” to Tu, is briefly discussed.

ESR study of NiFe2O4 precipitation process from silicate glasses

Abstract: The precipitation process of nickel ferrite (NiFe2O4) from silicate glasses has been studied by electron spin resonance (ESR). The ESR linewidth ΔH1/2 and effective g value were measured as functions of heat‐treatment temperature and time. Both the linewidth and effective g value change drastically near the glass transition temperature and precipitation temperature. The results are interpreted on the basis of the development of ion ordering, i.e., magnetic‐order development, that occurs during the heat treatments.

Alkali Diffusion and Electrical Conductivity in Sodium Borate Glasses

Abstract: The diffusion coefficient of sodium, 22 Na, and silver, 110 Ag, and electrical conductivity for sodium borate glasses (4–24 mol% Na 2 O) has been measured from 100°C to slightly below the glass transition temperature, T g . Unlike silicate glasses where the self-diffusion coefficient is much larger than the impurity diffusion coefficient, D Na and D Ag had close to the same magnitude in the sodium borate glasses. In some glasses, D Ag was slightly larger than D Na . Na 2 O-Ag 2 O-B 2 O 3 glasses show a relatively small mixed alkali effect, despite the significant mass difference between Ag(≈108) and Na(≈23). It is concluded that the mixed alkali effect is more dependent upon the difference in ionic radii rather than differences in mass.

Conductive polymer processable as a thermoplastic

Abstract: A flexible conductive composition of carbon black or other fine conductive particles dispersed in a high molecular weight polymer matrix composed of two phases--a soft elastomeric, low glass transition temperature phase and a hard, crystalline and high glass transition temperature phase.

Relations between melting point, glass transition temperature, and thermal expansion for inorganic crystals and glasses

Abstract: Thermal expansion coefficients correlate with a number of physical properties of crystals. Cubic ionic insulators that have two completely filled sets of lattice sites have a maximum value for the product of the coefficient and melting point. Expansion is diminished by the presence of various internal degrees of freedom, singly or in combination. These factors depend upon the array, band gap, bond length, and character and coupling between orbital sets. For screwlike chain structures and glasses αT 2m is a constant.

Kinematical transformations of amorphous selenium by DTA measurement

Abstract: The effect of sample preparation procedures on the glass transition and crystallization of amorphous Se has been studied by differential thermal analysis. The glass transition mechanism is essentially identical in the amorphous Se samples prepared by meltquenching and vapour deposition. In contrast, melt-quenched amorphous Se exhibits a double-step crystallization process, as revealed by an asymmetric DTA peak with a shoulder, while there appears a narrow single peak in the crystallization exothermic curve of vapour-deposited Se. These characteristics in the kinematical transformation of amorphous Se samples are discussed using a disordered chain model in which a statistical occupation of Se atoms at the cis- and trans-site in a molecule is proposed.