Showing papers on "Glass transition published in 1976"

Glass transition and secondary relaxations in molecular liquids and crystals

Abstract: It has been known for many years that substances in the glassy state retain some degree of molecular mobility that is detectable by a dielectric or a mechanical relaxation, or by an N M R experiment. These relaxations have been termed secondary or p relaxations and they occur on a time scale many times shorter than that of the main relaxation responsible for the glass transition itself.' The presence of such relaxations has generally been associated with the motion of a side group attached to a polymer chain, o r with a certain type of movement of the chain itself, and has been explained in terms of hindered internal molecular modes of motion that remain active even when the molecule as a whole is frozen in place in a glassy matrix.' Several years ago we studied a number of glasses made from molecular liquids2 lacking either a side chain or any other internal degree of freedom capable of giving rise to a relaxation traditionally invoked for polymers and network glasses. The study revealed a remarkable similarity in the dielectric relaxations of these glasses to those of polymers, and a surprisingly uniform pattern of behavior. The main conclusion drawn from the study was that the molecular mobility seen as p relaxations is intrinsic to the nature of the glassy state. It became apparent then that a theory for the structure of grossly disordered solids must necessarily consider such relaxations as arising from a motion over the potential energy barriers similar in origin to those involved in the glass transition itself. There is, however, another approach to the study of grossly disordered solids, such as glasses. One may start from a perfect crystal, relatively well understood both theoretically and experimentally because the translational symmetry allows enormous simplification of the algebra, and approach the glassy state gradually. An orientationally disordered or plastic crystal is intermediate between a perfect crystal and a glass in that, although there is a long-range order of molecular positions, there is no long-range order in the molecular orientations. This paper presents a brief review of the progress made in the study of relaxations of molecular liquids near and below the glass transition temperature in the context of present theories of glass structure, as well as some new results on the relaxation of rigid molecular liquids and plastic crystalline solids, and the implication these results have for the concepts of the glass transition phenomenon.

Thermodynamics of the glass transition: empirical aspects*

Abstract: The glass transition, a t which a viscous liquid settles into the glassy or structurally arrested state, in some respects appears to be a very obvious process, while in other respects it is both subtle and provocative. The process is obvious because the major phenomena observed, changes in heat capacity, expansion coeffiicient, and so forth, are no more than the consequences of the system under observation falling out of 'complete thermodynamic equilibrium a t some point in the experiment. I t is subtle and provocative because (i) the kinetics of the process whereby equilibrium is lost/recovered are in general not simple exponential (first order) processes even when the system i s very close to equilibrium, (ii) the magnitude of the changes, rather than the fact that they occur, are not predictable without a very detailed partition function for the high density liquid state, which is not available even for very simple liquids, and (iii) the question of what phenomena would be encountered under longer and longer experiment time scales remains to be properly resolved. The purpose of this paper is to provide some understanding of the magnitude of this problem by drawing attention to the great range and variety of thermodynamic changes by which the glass transition, i.e., the process of falling out of equilibrium, can be manifested. From the mass of data available, few consistent and general relationships can be salvaged. Those that can, however, are useful and may point the way to improved phenomenological description and ultimately to coherent theory for the phenomenon. We will consider, in turn, thermodynamic manifestations of Tg, their magnitude, variety, and interrelation, and finally their relation to dissipative (relaxation) processes in the liquid state. In these considerations it will be taken as read that the changes in properties observed at the glass transition actually occur over a range of temperatures that is larger the higher the temperature of the phenomenon. Quantities such as AC, characterizing the thermodynamic aspects of the transition will always be based on values measured outside (above and below) the transformation range.

Viscous liquids and the glass transition. V. Sources of the excess specific heat of the liquid

Abstract: The possibility that the excess ’’configurational’’ specific heat ΔCp of liquids above the glass transition temperature Tg might contain contributions either from changes in vibrational frequencies with configurational state, from changes in the anharmonicity of vibrations, or from the influence of changes in configurational state on the thermodynamic contributions of molecular rearrangement processes that also give rise to secondary relaxations below Tg is investigated with the use of calorimetric data on six quenched and annealed glasses. It is concluded that at least half of ΔCp arises from these sources, the greater part of this either from anharmonicity or from the molecular rearrangement processes.

Structure–property relationships in copolymers to composites: Molecular interpretation of the glass transition phenomenon

Abstract: Structure is taken as the main theme in outlining the mechanical properties of polymer composites. Examples of two-component polymer systems are selected from the literature showing their morphology, as evidenced from electron micrographs, and their corresponding mechanical properties, as evidenced by dynamic mechanical spectra. A compatibility number, Nc, is defined in a continuous scheme from a compatible system (one glass transition, Nc ∞) to an incompatible system (two glass transitions, Nc 0). The point at which semicompatibility occurs, Nc ≅ 1, is taken as the approximate universal segmental length associated with a glass transition. This length of 150 A allows for 100 to 5000 CC bonds for an associated glass transition. The ramifications of this molecular interpretation of a glass transition are discussed, resulting in a denouncement of the time–temperature correspondence principle and a new interpretation of short-segmental block copolymers.

Studies of amorphous GeSeTe alloys (I): Preparation and calorimetric observations

Abstract: We have used differential scanning calorimetry (DSC) to examine the thermally induced transformations of bulk and thin-film amorphous alloys within a large portion of the GeSeTe system. Most chalcogen-rich compositions showed a discontinuous increase of heat capacity when heated through the glass transition temperature T G . The Ge-rich compositions, which could only be prepared as sputtered amorphous films, were invariably characterized by an irreversible exothermic crystallization process on heating, beginning at the crystallization temperature T X . Values of T g and T X have been tabulated for all alloys investigated and the compositional dependence of T g has been examined in the light of recent models for viscous flow in glass-forming chalcogenide systems. In addition, a region of liquid immiscibility has been observed in the vicinity of Ge 20 Se 40 Te 40 in which a GeSe 2 -rich liquid phase segregates from a tellurium-rich liquid phase. The existence and limits of this immiscibility region have been rationalized on the basis of ionic perturbations to the covalent bonding. The segregation of a GeSe 2 -rich liquid increases the concentration of GeSe bonds which are the strongest and most ionic of the six angle-bond types which can occur in this system.

Stability of amorphous metallic alloys

Abstract: In a previous paper it was concluded that the migration of phosphorus was causing the embrittlement of amorphous Fe40Ni40P14B6 at temperatures as low as 100 °C. In this paper we compare the stability of the above alloy to amorphous Fe40Ni40B20, Fe50Ni30P14B6, and Fe50Ni30B20. The stability is evaluated from brittleness measurements after annealing at temperatures between 100 and 400 °C. The results confirm the prediction that the removal of phosphorus suppresses the embrittlement at low temperatures; temperatures above 225 °C are required to cause embrittlement in the phosphorus‐free alloys. At higher temperatures the fracture strain approaches that of the phosphorus‐containing alloy. It is concluded that the replacement of P by B inhibits the low‐activation processes involved in the embrittlement process. The changes are not related to the changes in the glass transition temperatures.

Binary Titania-Silica Glasses Containing 10 to 20 Wt% TiO2

Abstract: A series of glasses in the TiO2-SiO2 system was prepared by the flame hydrolysis boule process. Clear glasses containing as much as 16.5 wt% TiO2 were obtained. More TiO2 caused opacity due to phase separation and anatase/rutile crystallization during glass boule formation. Glasses in the 12 to ∼17 wt% TiO2 range were metastable and showed structural rearrangements on heat treatment at temperatures as low as 750CEC (∼200° below the annealing point). These changes were accompanied by large changes in thermal expansion. Thermal treatment can be designed to produce almost any desired expansion between α-200+700=−5 × 10-7/°C and +10 × 10-7/°C. Zero expansion between 0 and 550°C was obtained. Evidence that these changes are due to phase separation and anatase formation is presented. Viscosity data in the glass transition range, refractive index, and density are also presented.

Sequence Distribution-Glass Transition Effects

Abstract: While the deliberate copolymerization of various monomers to obtain properties that could not be obtained via homopolymerization has been going on for some 60 years, the effect of copolymerization on some properties has only recently been understood. The effect of copolymerization on the polymer glass transition (Tg) is just such a case.

Glass transition in the hard-sphere model

Abstract: Equations of state and transport data for the hard-sphere fluid suggest a quasi-thermodynamic transition from supercooled fluid to amorphous solid at a density slightly greater than the equilibrium freezing density This has been substantiated with a lengthy MD computation in which the density was increased very gradually from a low density fluid state to almost close-packing The amorphous close-packed density of 500 spheres with periodic boundary is found to be ρaσ3= 1217 ± 0004 A glass transition occurs in the density region ρgσ3= 100 ± 001; a (diffuse) thermodynamic transition of the third-order is indicated

Measurement of viscosity of the grain-boundary phase in hot-pressed silicon nitride

Abstract: An internal friction technique has been used to measure the viscosity of the grain-boundary amorphous phase in commercial hot-pressed silicon nitride. The viscosity in the region of the glass transition (850 to 900° C) was approximately 5×1015 P per unit thickness (cm) of the grain boundary, with an apparant activation energy of 163 kcal mol−1.

Heat Capacity and Structural Relaxation of Mixed‐Alkali Glasses

Abstract: Heat capacities of a series of mixed-alkali glasses of composition (in mol%) 24.4(Na2O + K2O)-75.6SiO2 were measured in the transition region by differential scanning calorimetry. The glass heat capacities at 700 K and the equilibrium liquid heat capacities are the same for all glasses on a per-g atom basis and equal, respectively, to 5.6 ± 0.1 and 6.8 ± 0.1 cal/g atom K. The glass transition temperatures exhibited negative deviations from additivity, but the heat capacity curves in the transition region of all the glasses for identical heating rates and thermal histories could be superimposed on the same reduced plot. This behavior indicates that the shapes of the structural relaxation functions are the same for all the glasses. These results support Shelby's conclusion that there is no unique “mixed-alkali effect” on thermodynamic or structural relaxation properties and that the term should be reserved for properties relating to ionic transport.

The effect of excess thermodynamic properties versus structure formation on the physical properties of glassy polymers

Abstract: The dynamic mechanical properties of glassy polymers, such as atactic polystyrene and poly(ethylene terephthalate), have been followed as a function of the excess enthalpy during isothermal annealing at selected temperatures below their respective glass transition temperatures. For glassy polymers prepared from crystallizable as well as noncrystallizable polymers, the changes in modulus parallel the extent of enthalpy relaxation that occurs during the annealing period as a result of the nonequilibrium nature of the glassy state, i.e., the modulus increases with time and approaches a limiting value that increases with decreasing temperature. These results suggest that the observed changes in the dynamic mechanical moduli are associated with changes in the excess thermodynamic properties of the polymeric glasses, and not with structure formation other than the normal liquidlike packing of the chain segments. Thus the physical properties for corresponding equilibrium glassy states can be determined ...

Structure and properties of abs polymers. X. Influence of particle size and graft structure on loss modulus temperature dependence and deformation behavior

Abstract: ABS systems which differ distinctly in particle size and degree of grafting were prepared and investigated by dynamic mechanical measurements as function of temperature in the glass transition region of the rubber phase. Variation of rubber content within different sample series results in effects which were mainly referred to thermal stresses as consequence of phase interactions. Basic aspects of the deformation behavior of some of these ABS systems are studied by mechanical and morphological methods, and the operation modes of the component of a bimodal/bigraft-system concerning toughness are discussed.

Some physical properties of GeAsSe infrared optical glasses

Abstract: This paper presents measured data on glass transition temperature, thermal expansion, density, hardness, refractive index and electron microscopy of GeAsSe glasses and bring together some literature data on viscosity and mechanical properties Interpretation of this information to identify suitable compositions for bulk synthesis is discussed

The hard sphere ’’glass transition’’

Abstract: We investigate the existence of a glass transition for the systems of hard spheres studied in computer simulations of molecular dynamics. An empirical best‐fit equation of state is established for the metastable (supercooled or supercompressed) hard sphere liquid. Configurational thermodynamic properties are calculated from this, and their significance is discussed. Evidence is presented to support the proposition that a system of hard spheres exhibits a phenomenon similar to a glass transition in which ’’glass formation’’ is purely a kinetic phenomenon. For hard spheres there is no underlying second order transition temperature T2. The effective ’’cooling rate’’ associated with these computer simulations of molecular dynamics is investigated and found to be enormous (∼1011 deg/sec). The implications of these results with respect to the configurational entropy theory of the glass transition are discussed.

Crystallization of Ca(PO3)2 Glass Below the Glass Transition Temperature

Abstract: Block specimens of Ca(PO3)2 glass crystallize easily at temperatures considerably lower than the glass transition temperature, Tg; thin blown films of the glass do not exhibit this behavior. The crystals, which are β-Ca(PO3)2 with a trace of the γ form, are identical to those formed above Tg; they exhibit “hemispherulitic” growth from the glass surface. The activation energy for crystal growth below Tg is 30 kcal/mol, which indicates that the growth mechanism differs from that in blown film samples above Tg (activation energy = 110 kcal/mol). A model for crystallization below Tgis proposed; the phenomenon is explained in terms of induced tension in the molecular chains at the glass- crystal interface.

Cold-rolling and annealing of amorphous ribbons

Abstract: Cold-rolling of Fe 40 Ni 40 P 14 B 6 amorphous alloy ribbon results in a large increase in coercive force and a large decrease in magnetization in low fields. It is suggested that these changes are the result of both the sliplike structures developed by the rolling and the strain-magnetostriction induced anisotropy. On annealing the rolled ribbon to temperatures below the crystallization temperature, the magnetic properties recover to the values obtained on annealing as-cast ribbon, but higher temperatures are required. The rolled ribbon showed no evidence for structural changes in high resolution electron micrographs; x-ray and electron diffraction patterns showed no change in the width or position of the diffuse rings; and the glass and crystallization temperatures remained unchanged. However, small angle x-ray scattering showed a decrease in intensity at small angles, and stress relief measurements showed a small increase in the stress-relief rate. Both as-cast and rolled ribbons when annealed even to within a few degrees of the start of crystallization showed no change in the width or position of the diffraction peaks, and no change in the heat of crystallization, glass transition, or crystallization temperature. However, on annealing, the small angle x-ray scattering increased, and stress-relief rate decreased. The change in small angle scattering is qualitatively interpreted as being due to the removal of scattering nuclei on rolling and their development during annealing. The stress relief results are discussed in terms of changes in short-range atomic order.

Effect of hydrogen bond formation on dynamic mechanical properties of amorphous cellulose

Abstract: The temperature dependence of the dynamic modulus (E′) and the mechanical loss tangent (tanδ) of amorphous cellulose prepared from cellulose triacetate by saponification was measured and compared with that of cellophane, recrystallized cellulose obtained by immersing amorphous cellulose in water, and cellulose triacetate. The E′ of amorphous cellulose decreased initially with increasing temperature and then began to increase at about 70°C with a maximum at 80°C, decreasing again at about 100°C. Another decrease in E′ was observed at 220°C accompanied by a discontinuity at 155°C. In the tan δ-versus-temperature curve, a medium peak at 60°C a shoulder peak at 146°C, and a broad peak at 200°C were observed. It was found that the transition at about 60°C was related to hydrogen bond formation by free OH groups. The transition at about 150°C was attributed to a recrystallization process by heating, and the relaxation at 200°C, to the glass transition of the polymer. The decrement in E′ observed at about 100°C was attributed to the cooperative motion of an individual pyranose ring in amorphous cellulose, juding from the E′ and tan δ assignment of other cellulose materials. The change in E′ was also measured isothermally as a function of time in the temperature range between 40°C and 80°C, where a maximum in tan δ and an increment in E′ were observed as the temperature dependence of the dynamic viscoelasticity. The change in E′ with elapsed time was analyzed kinetically, and an activation energy of 2.6 kcal/mole was calculated. This value is the expected activation energy of hydrogen bond formation.

Chain conformation and structure in amorphous polymers as revealed by X-ray, neutron, light and electron diffraction

Abstract: The physical structure of the melt, the supercooled melt, and the glassy state of amorphous polymers can be characterized by the chain conformation, the local order, and the morphology. The conformation was studied by small-angle neutron scattering. The local order was characterized by pair distribution functions which were derived from electron scattering curves. In addition, model calculations were used to gain information about short-range order. The morphology was studied by means of light scattering (density and anisotropy fluctuations), small-angle X-ray scattering (density fluctuations), and magnetic birefringence measurements (orientational order). Experiments were performed on polyethylene melts and polystyrene, polymethyl methacrylate, and polycarbonate above and below the glass transition temperature. The results show that the amorphous phase is a homogeneous phase and that there are no anisotropic structures of the kind proposed by various bundle models of the amorphous phase. The cha...