Showing papers on "Glass transition published in 1993"

A highly processable metallic glass: Zr41.2Ti13.8Cu12.5Ni10.0Be22.5

Abstract: We report on the properties of one example of a new family of metallic alloys which exhibit excellent glass forming ability. The critical cooling rate to retain the glassy phase is of the order of 10 K/s or less. Large samples in the form of rods ranging up to 14 mm in diameter have been prepared by casting in silica containers. The undercooled liquid alloy has been studied over a wide range of temperatures between the glass transition temperature and the thermodynamic melting point of the equilibrium crystalline alloy using scanning calorimetry. Crystallization of the material has been studied. Some characteristic properties of the new material are presented. The origins of exceptional glass forming ability of these new alloys are discussed.

Glass-forming ability of alloys

Abstract: New amorphous alloys exhibiting a wide supercooled liquid region before crystallization were found to form by melt spinning in wide composition ranges of LaAlM, MgYM and ZrAlM (M = Ni or Cu) systems consisting of the constituent elements with significantly different atomic sizes. The temperature span between glass transition temperature, Tg, and crystallization temperature, Tx, ΔTx ( = Tx − Tg) is > 50 K in the compositional ranges around La2AlM, Mg6Ln3M and Zr3AlM and the largest ΔTx reaches 126 K. The critical cooling rate for the glass formation, Rc, is as low as 87–115 K/s and Tg/Tm is > 0.6 in the composition range where ΔTx > 50 K. There is a clear tendency for Rc to decrease with an increase of ΔTx and Tg/Tm. The crystallization of the alloys with large ΔTx occurs through the simultaneous precipitation of several compounds. Based on these results, it is presumed that the large glass-forming ability for these alloys is due to a combined effect of the difficulty of long-range atomic redistribution required for the precipitation of the compounds, the rapid increase of viscosity with decreasing temperature and the large liquidus-solidus interfacial energy which originates from the optimally bonding and packing states resulting from large negative heat of mixing and large atomic size ratios.

Preparation of 16 mm Diameter Rod of Amorphous Zr65Al7.5Ni10Cu17.5 Alloy

Abstract: Bulky amorphous alloys in a cylindrical shape with diameters up to 16 mm were found to form by water quenching a Zr 65 Al 75 Ni 10 Cu 175 melt in a quartz tube The amorphous phase of this alloy has the widest temperature interval of supercooled liquid region before crystallization The glass transition temperature, crystallization temperature and Vickers hardness for the bulky amorphous alloy with a diameter of 16 mm are 625 K, 750 K and 465, respectively, being nearly the same as those for the corresponding melt-spun ribbon with a thickness of 30 μm The extremely large glass-forming ability is presumably due to a combination of significantly different atomic size ratios among the constituent elements and the necessity of redistribution of Al for the progress of crystallization

Polymeric materials formed by precipitation with a compressed fluid antisolvent

Abstract: Polymer microspheres and fibers are formed with a versatile new process, precipitation with a compressed fluid antisolvent. By spraying a 1 wt. % polystyrene in toluene solution into CO2 through a 100-μm nozzle, microspheres are formed with diameters from 0.1 to 20 μm as the CO2 density decreases from 0.86 to 0.13 g/cm3. The uniform submicron spheres produced at high CO2 density are due in part to the rapid atomization produced by the large intertial and low interfacial forces. Fibers, with and without microporosity, are obtained at higher polymer concentrations where viscous forces stabilize the jet. The effect of CO2 density and temperature on the size, morphology and porosity of the resulting polymeric materials is explained in terms of the phase behavior, spray characteristics, and the depression in the glass transition temperature.

Molecular design for nonpolymeric organic dye glasses with thermal stability : relations between thermodynamic parameters and amorphous properties

Abstract: The molecular structures of low-molecular-weight organic compounds and their amorphous properties have been investigated to obtain a design rule for uniform amorphous films with high thermal stability. The glass transition temperature (Te/K), maximum crystal-growth velocity (MCV/m s -1 ), and maximum crystal-growth temperature (T c,max /K) are key parameters for characterizing the amorphous properties of organic materials. Some quantitative relations between these parameters and thermodynamic parameters were examined from both theoretical and experimental viewpoints

Nature of Alumina in Phosphate Glass: II, Structure of Sodium Alurninophosphate Glass

Abstract: We have used 27 Al and 31 P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to describe quantitatively the relationships between the composition, structure, and properties of glasses in the Na 2 O.Al 2 O 3 .P 2 O 5 (NAP) system. In general, the glass properties (evaluated in part I) are most sensitive to changes in Al coordination. 27 Al MAS NMR spectra reveal that octahedrally coordinated Al is most abundant in glasses with O/P ratios less than 3.5, the pyrophosphate structural limit. Tetrahedrally coordinated Al is most abundant in glasses with O/P greater than 3.5. Decreasing Al(OP) 6 /Al(OP) 4 ratio generally correlates with decreasing glass transition temperature and refractive index

Physical Properties of Dendritic Macromolecules: A Study of Glass Transition Temperature

Abstract: The variation of glass transition temperature with molecular weight and chain-end composition for dendritic polyethers and polyesters was studied. A new and modified version of the chain-end free volume theory was derived to account for the large number of chain ends in these unusual structures. For both homopolymers and a variety of novel block copolymers, the experimental variation in T with molecular weight was found to correlate well with theoretical predictions, thus demonstrating the wide applicability of the chain-end free volume theory. The glass transition was found to be greatly affected by the nature of the chain ends and internal monomer units.

Glass Transition and Infrared Spectra of Low‐Alkali, Anhydrous Lithium Phosphate Glasses

Abstract: Anhydrous glasses in the series xLi2O + (1 - x)P2O5 have been prepared and characterized in the range 0 ≤x≤ 0.5. FT-IR spectroscopy and glass transition temperature measurements have been used to explore the structure and a key physical property of the low-alkali phosphate glasses. The structure of v-P2O5 is proposed to consist of a 3-D network of trigonally connected tetrahedra decorated with a P=O unit. Contrary to what has long been proposed for these glasses, the addition of alkali degrades the 3-D network through the generation of nonbridging oxygens rather than strengthen the network through the proposed alkali ion bridging. The Tg of v-P2O5 is ∼653 K and decreases some 130 K with the addition of 10 mol% Li2O. Tg then reaches a minimum value at 20 mol% Li2O and increases with further Li2O additions. The increase in Tg, even though the fraction of nonbridging oxygens is still increasing, is interpreted in terms of an increasing entanglement of long-chain PO2 groups in the glass. Such a structural transition from a 3-D network of interconnected PO4 groups for P2O5 to a 1-D chain structure for LiPO3 is one of the first examples of the importance of intermediate-range order in governing the properties of glass.

Correlation between the Width of the Glass Transition Region and the Temperature Dependence of the Viscosity of High‐Tg Glasses

Abstract: A correlation exists between the width of the glass transition region measured using DTA or DSC during heating and the activation enthalpy ΔH η * for the shear viscosity of high-Tg inorganic glasses. This is demonstrated by the constancy of a dimensionless parameter C = (ΔH η * /R)(1/Tg − 1/Tg'), where Tg and Tg' are temperatures marking the extrapolated onset and finish of the transition region on the DTA or DSC trace. The logarithm of the shear viscosity at Tg, log η (Tg), is also approximately constant for high-Tg glasses. Using the universal values of C and log η (Tg), fairly reliable estimates of shear viscosities from the transition range through the working range can be made from the Tg and Tg' values obtained using DTA or DSC

Nature of alumina in phosphate glass. I: Properties of sodium aluminophosphate glass

Abstract: Glasses along four compositional tie lines, I: xAl(PO3)3’(1 – x)NaPO3, II: xAlPO4′(1 – x)NaPO3, III: xAl2O3′(1 – x)NaPO3, and IV: xNaAlO2,(1 – x)NaPO3, have been prepared and a number of their properties evaluated. In general, Al2O3 additions increase the glass transition temperature, decrease thermal expansion, and improve aqueous durability. However, the composition/property relationships are strongly influenced by relative cation ratios and thus the overall O/P ratio. The nature of these glasses undergoes significant transformations at both the pyrophosphate (O/P = 3.5) and the orthophosphate (O/P = 4.0) compositions, producing significant changes in the property behavior. Spectroscopic analyses of glass structure, described in part II of this report, reveal that changes in Al coordination coincide with the breaks in the property behavior.

Cure kinetics of a thermosetting liquid dicyanate ester monomer/high-Tg polycyanurate material

Abstract: The cure of a liquid dicyanate ester monomer, which reacts to form a high-Tg (≈200°C) polycyanurate network, has been investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and a dynamic mechanical technique, torsional braid analysis (TBA). The monomer is cured with and without catalyst. The same one-to-one relationship between fractional conversion and the dimensionless glass transition temperature is found from DSC data for both the uncatalyzed and catalyzed systems, independent of cure temperature, signifying that the same polymeric structure is produced. Tg is the parameter used to monitor the curing reactions since it is uniquely related to conversion, is sensitive, is accurately determined, and is also directly related to the solidification process. The rate of uncatalyzed reaction is found to be much slower than that of the catalyzed reaction. The apparent overall activation energy for the uncatalyzed reaction is found to be greater than that of the catalyzed reaction (22 and 13 kcal/mol, respectively) from time–temperature superposition of experimental isothermal Tg vs. In time data to form kinetically-controlled master curves for the two systems. Although the time–temperature superposition analysis does not necessitate knowledge of the rate expression, it has limitations, because if the curing process consists of parallel reactions with different activation energies, as is considered to be the case from analysis of the FTIR data, there should not be a kinetically-controlled master curve. Consequently, a kinetic model, which can be satisfactorily extrapolated, is developed from FTIR isothermal cure studies of the uncatalyzed reaction. The FTIR data for the uncatalyzed system at high cure temperatures, where the material is in the liquid or rubbery states throughout cure, 190 to 220°C, are fitted by a model of two parallel reactions, which are second-order and second-order autocatalytic (with activation energies of 11 and 29 kcal/mol), respectively. Using the model parameters determined from the FTIR studies and the relationship between Tg and conversion from DSC studies, Tg, vs. time curves are calculated for the uncatalyzed system and found to agree with DSC experimental results for isothermal cure temperatures from 120 to 200°C to even beyond vitrification. The DSC data for the catalyzed system are also described by the same kinetic model after incorporating changes in the pre-exponential frequency factors (due to the higher concentration of catalyst) and after incorporating diffusion-control, which occurs prior to vitrification in the catalyzed system (but well after vitrification in the uncatalyzed system). Time–temperature-transformation (TTT) isothermal cure diagrams for both systems are calculated from the kinetic model and compared to experimental TBA data. Experimental gelation is found to occur at a conversion of approximately 64% in the catalyzed system by comparison of experimental macroscopic gelation at the various curing temperatures and iso-Tg (iso-conversion) curves calculated from the kinetic model. © 1993 John Wiley & Sons, Inc.

Bulky La–Al–TM (TM=Transition Metal) Amorphous Alloys with High Tensile Strength Produced by a High-Pressure Die Casting Method

Abstract: The application of a high-pressure die casting technique to La-Al-TM (TM=Co, Ni, Cu) alloys was found to produce bulk amorphous alloys in cylinder and sheet forms. The maximum sample diameter (d,) for formation of an amorphous phase is 3 mm for La 55 Al 25 (Ni or Cu) 20 , 7 mm for La 55 Al 25 Ni 10 Cu 10 and above 9 mm for La 55 Al 25 Cu 10 Ni 5 Co 5 . There is a clear tendency for d, to increase with an increase in the temperature interval of a supercooled liquid region. The glass transition temperature (T g ), the crystallization temperature (T x ) and the heat of crystallization (ΔH x ) remain unchanged in the thickness range of 1 to 7 mm for the La 55 Al 25 Cu 10 Ni 10 alloy

Non-exponential structural relaxation, anomalous light scattering and nanoscale inhomogeneities in glass-forming liquids

Abstract: Light scattering from glass-forming liquids exhibits an anomalous time dependence in the glass transition region, e.g. maxima in the scattering intensity versus temperature curves during heating. It is shown that this behavior is consistent with the presence of nanoscale inhomogeneities (density fluctuations) which relax at different rates. It is suggested that this could be the source of non-exponential structural relaxation kinetics. An expression relating the size of these regions to structural relaxation kinetic parameters has been developed and predicts sizes in excellent agreement with those determined by other methods.

Ellipsometric study of the glass transition and thermal expansion coefficients of thin polymer films

Abstract: The glass transition temperature (T g ) of thin polystyrene films (ca. 3000 A) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films. T g was determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift in T g , possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films

Water II is a "strong" liquid

Abstract: Rationalization of anomalous devitrification relationships between hyperquenched vitreous water (and related ASW forms) and vitrified aqueous solutions requires the assumption that the viscosity-temperature relationship of water near but above its glass transition temperature is that of a «strong» liquid, like SiO 2 . This would support Speedy's claim that water II (ASW above T g ) is a separate and distinct state of liquid water. The conclusion is supported by analysis of low-temperature data on water-swollen polymers (hydrogels)

Effects of crosslinking on physical properties of phenol-formaldehyde novolac cured epoxy resins

Abstract: To clarify the relationship between crosslinking density and physical properties of phenol–formaldehyde novolac cured epxy resin and factors governing their physical properties, we studied various properties of cured resins having different crosslinking densities. The resins were prepared with various curing accelerators and raw epoxy resins having different molecular weights. We found that as the crosslinking density of a cured resin increases, glass transition temperature (Tg) rises and the relaxation time becomes longer. Furthermore, in the rubbery region, the coefficient of linear thermal expansion drops and the elastic modulus become larger, while, in the glassy region, the coefficient of linear thermal expansion, specific volume, water absorption, diffusion coefficient, and permeability all increase but the elastic modulus becomes smaller. The WLF analysis on the relaxation behaviors of typical cured resin showed that cured resin with a higher crosslinking density decreases in the fractional free volume. This behavior is completely opposite from the relationship predicted from the temperature dependency of specific volume. While the coefficient of thermal expansion of free volume decreases as the crosslinking density increases for the cured resin, it coincides well with the tendency predicted from the difference in coefficient of cubic thermal expansion in the rubbery and glassy regions of each cured resin. That the free volume obtained from WLF analysis shows a relationship opposite to the predicted free volume as based on the temperature dependency of specific volume is explained as follows: Namely, the free volume obtained from the WLF analysis is a hole free volume Vh which contributes to fluidity and Vh decreases with the crosslinking density. On the other hand, the free volume predicted from the specific volume is a sum of the interstitial free volume Vi and Vh. Vi increases with the crosslinking density and this Vi increase exceeds the decrease of Vh. Therefore, the free volume predicted from the specific volume increases with the crosslinking density. Consequently, the influence of free volume on the relationship between the crosslinking density and physical properties of cured resin can be interpreted as follows. As the crosslinking density increases on cured resins, Tg rises, the relaxation time is lengthened, and the coefficient of linear thermal expansion becomes smaller in the rubbery region because, as the crosslinking density increases, Vh decreases. Since crosslinking density increases on cured resins, the coefficient of linear thermal expansion, water absorption, diffusion coefficient, and permeability become larger, and the elastic modulus becomes smaller in the glassy region because, as the crosslinking density increases, Vi increases and, accordingly, molecular chain packing becomes looser; i.e., the specific volume increases. © 1993 John Wiley & Sons, Inc.

Change of the vibrational dynamics near the glass transition in polyisobutylene: Inelastic neutron scattering on a nonfragile polymer.

Abstract: Incoherent-inelastic-neutron-scattering studies were carried out on polyisobutylene, presumably the least-fragile polymeric glass former known so far. A fast dynamical process on a frequency scale of roughly 500 GHz is measured for temperatures higher than the glass transition essentially without being influenced by the \ensuremath{\alpha} relaxation. The present data show that this fast dynamical process can be inelastic and thus has no relaxational, i.e., quasielastic character. This strongly underlines the importance of changes in the vibrational behavior of glasses near the glass-transition temperature. We propose a librational model relating steric hindrance with the observed inelastic scattering.

Ethylene/alpha-olefin/7-methyl-1,6-octadiene copolymer rubber and composition of the same

Abstract: The ethylene/.alpha.-olefin/7-methyl-1,6-octadiene copolymer rubber of the invention is a random copolymer rubber prepared using a Group IVB transition metal type catalyst containing a specific zirconium catalyst component, and in this copolymer rubber, a molar ratio of ethylene to .alpha.-olefin, a diene content, an intrinsic viscosity [.eta.], an intensity ratio D of T.alpha..beta. to T.alpha..alpha. in a 13C-NMR spectrum of said copolymer rubber, a B value which is a measure of distribution of each monomers and a glass transition temperature are all within specific ranges. The rubber composition of the invention comprises the above-mentioned random copolymer rubber and a conjugated diene rubber. The random copolymer rubber has high stereoregularity, narrow composition distribution and excellent low-temperature flexibility, and shows extremely high covulcanizability with the conjugated diene rubber. In addition, this random copolymer rubber is excellent in weathering resistance, ozone resistance and thermal aging resistance.

Molecular mobility in mixtures of absorbed water and solid poly(vinylpyrrolidone).

Abstract: Poly(vinylpyrrolidone) (PVP) was used as model system to examine molecular mobility in mixtures of absorbed water with solid amorphous polymers. Water vapor absorption isotherms were determined, along with diffusion and proton NMR relaxation measurements of absorbed water. Concurrently, measurements of glass transition temperatures (Tg) and carbon-13 NMR relaxation times for PVP were determined as a function of water content. Two water contents were used as reference points: Wm, obtained from the fit of water absorption isotherms to the BET equation, corresponding to the first shoulder in the sigmoid isotherm; and Wg, the amount of water necessary to depress Tg to the isotherm temperature. Translational diffusion coefficients of water, along with proton T1 relaxation time constants, show that both the translational and the rotational mobility of the water is hindered by the presence of the solid polymer and that the absorbed water is most likely represented by two or more populations of water with different modes or time scales of motion. The presence of "tightly bound” or immobilized water at levels corresponding to Wm, however, is unlikely, since water molecules maintain a high degree of mobility, even at the lowest levels of water. Above Wg, water shows an increase in mobility with increasing water content, but it is always less mobile than bulk water. With increasing water content, carbon-13 T1 relaxation time constants for PVP, measured under the same conditions as above, indicate a major increase in the molecular mobility of carbon atoms associated with the pyrrolidone side chains.

Change in crystallization mechanism at the glass transition of colloidal spheres

Abstract: THE crystallization of 'hard' spheres, which interact only through repulsive forces on contact, is a purely entropic effect and provides a model system for testing theories of freezing transitions in liquids generally1. Micrometre-sized colloidal particles can be made to approximate hard spheres by grafting of polymers to their surfaces1. Their slow dynamics make them attractive systems for the study of crystallization kinetics2–4. As well as undergoing crystallization1,2, such particles also exhibit a glass transition at a well-defined volume fraction φg, which we have shown previously to be related to the cessation of large-scale diffusion on experimental timescales5,6.. Here we show that the glass transition coincides with a change in the mechanism of crystallization. At volume fractions less than φg isometric crystals are nucleated homogeneously throughout the sample, whereas above φg crystals forming slowly in the metastable glass phase are highly asymmetric. Regular rocking of the suspension above the glass transition induces more rapid formation of plate-like crystals, indicating that their nucleation is shear-induced.

Comparison of microwave and thermal cure of epoxy resins

Abstract: Stoichiometric mixtures of DGEBA (diglycidyl ether of bisphenol A)/DDS (diaminodiphenyl sulfone) and DGEBA/mPDA (meta phenylene diamine) have been isothermally cured by electromagnetic radiation and conventional heating using thin film sample configurations. Fourier transform infrared spectroscopy (FTIR) was used to measure the extent of cure. Thermal mechanical analysis (TMA) was used to determine the glass transition temperatures directly from the cured thin film samples. Well-defined glass transitions were observed in the TMA thermograph for both thermal and microwave cured samples. Significant increases in the reaction rates have been observed in the microwave cured DGEBA/DDS samples. Only slight increases in the reaction rates have been observed in the microwave cured DGEBA/mPDA samples. Higher glass transition temperatures were obtained in microwave cured samples compared to those of thermally cured ones after gelation. The magnitude of increases of glass transition temperature is much larger for the DGEBA/DDS system than DGEBA/mPDA system. The microwave radiation effect was much more significant in DGEBA/DDS system than in DGEBA/mPDA system. DiBenedetto's model was used to fit the experimental Tg data of both thermal and microwave cured epoxy resins.

DSC and Raman studies of lead borate and lead silicate glasses

Abstract: Several compositions in the xPbO·(1−x)B2O3 (0.20 ≤ x ≤ 0.65) and xPbO·(1−x)SiO2 (0.39 ≤ x ≤ 0.66) vitreous systems were examined by differential scanning calorimetry and by Raman scattering for the former system. Borate glasses with low PbO concentrations are built up in two and three dimensions with three- and four-coordinated boron atoms; Pb2+ acts as a charge balance in the covalent network. Glasses with high PbO content contain ring- and chain-metaborates and PbO4 entities. Differences in Raman scattering of alkali borate glasses are observed; in particular the boroxol rings still exist for x = 0.27. The heat capacities of the glasses at 200°C are an additive function of composition within the accuracy of the differential scanning calorimetry measurements. The heat capacity differences between the vitreous and supercooled liquid states are higher for lead borates than for lead silicates, indicating more profound changes in the lead borates during the glass transition. In the former glasses, the fictive temperature attains a maximum around 0.27PbO corresponding to the highest degree of polymerization. In lead silicates, the fictive temperature decreases continually upon adding PbO, and parallels the depolymerization of the silicate network.

Lithium-7 NMR and ionic conductivity studies of gel electrolytes based on poly(acrylonitrile)

Abstract: Composite gel electrolytes prepared from mixtures of poly(acrylonitrile) (PAN), ethylene carbonate (EC), propylene carbonate (PC), and LiClO[sub 4] or LiAsF[sub 6] have been investigated by complex impedance, differential scanning calorimetry (DSC), and [sup 7]Li nuclear magnetic resonance (NMR) spectroscopy The ionic conductivity of a gel containing LiAsF[sub 6] reaches 10[sup [minus]2] S/cm at 60[degrees]C Although the conductivity of the gels approach that found in EC/PC liquid electrolytes, NMR line width, and spin-lattice relaxation time (T[sub 1]) measurements indicate that even short-range ionic mobility is impeded by the presence of the PAN As in the case of amorphous polyether-salt polymer electrolytes, the onset of [sup 7]Li motional line narrowing in the gels is strongly correlated with the DSC-determined glass transition temperature 6 figs, 1 tab

Thermodynamic and rheological properties of rhyolite and andesite melts

Abstract: The heat capacities of a rhyolite and an andesite glass and liquid have been investigated from relative-enthalpy measurements made between 400 and 1800 K. For the glass phases, the experimental data agree with empirical models of calculation of the heat capacity. For the liquid phases, the agreement is less good owing to strong interactions between alkali metals and aluminum, which are not currently accounted for by empirical heat capacity models. The viscosity of both liquids has been measured from the glass transition to 1800 K. The temperature dependence of the viscosity is quantitatively related to the configurational heat capacity (determined calorimetrically) through the configurational entropy theory of relaxation processes. For both rhyolite and andesite melts, the heat capacity and viscosity do not differ markedly from those obtained by additive modeling from components with mineral compositions.