Showing papers on "Glass transition published in 1977"

Physical properties and supermolecular structure of perfluorinated ion‐containing (nafion) polymers

Abstract: The supermolecular structure and viscoelastic and diffusion properties of a perfluorinated polymer containing sulfonic acid (Nafion) were investigated. The breakdown of time–temperature super-position for the dry salt and and acid in the presence of 0.5 H2O/SO3H as well as the results of small-angle x-ray scattering suggest that the ions in this material are clustered. Above 180°C, the reestablishment of the time–temperature superposition in the salt suggests that ions in the clusters become mobile. Dynamic mechanical studies were performed over a temperature range from −190°C to above the glass transition temperatures Tg of the materials. The Tg of the salts is found at ca. 220°C, while in the acid it occurs at 110°C. A β peak in the acid is found at ca. 20°C, while in the salts it occurs between 140°C and 160°C. The β peak shifts to a lower temperature with the addition of water in both the acid and the salts, while the α and γ peaks are unaffected. The latter is located at ca. −110°C at 1 Hz. Dielectric behavior has also been studied as a function of water content for the acid sample and the potassium salt at frequencies of 100 Hz to 10 kHz. Two relaxations with different activation energies were observed. The position of both peaks shifts to a lower temperature as the water content increases. Finally, the diffusion of water in Nafion in the acid form has been determined. The diffusion coefficient can be represented by the equation

Modelling the structures of amorphous metals and alloys

Abstract: The application of random packing models to amorphous metals and metal–metalloid alloys is discussed, with particular reference to the effects of soft potentials, boundary conditions, and density. The local atomic organisations found leads to structural speculation concerning the glass transition, and the existence of ‘amorphous polymorphs’.

Non-equilibrium freezing behaviour of aqueous systems.

Abstract: The tendencies to non-equilibrium freezing behaviour commonly noted in representative aqueous systems derive from bulk and surface properties according to the circumstances. Supercooling and supersaturation are limited by heterogeneous nucleation in the presence of solid impurities. Homogeneous nucleation has been observed in aqueous systems freed from interfering solids. Once initiated, crystal growth is often slowed and, very frequently, terminated with increasing viscosity. Nor does ice first formed always succeed in assuming its most stable crystalline form. Many of the more significant measurements on a given system can be combined and displayed in the form of a 'supplemented phase diagram', the latter permitting the simultaneous representation of thermodynamic and non-equilibrium properties. The diagram incorporates equilibrium melting points, heterogeneous nucleation temperatures, homogeneous nucleation temperatures, glass transition and devitrification temperatures, recrystallization temperatures, and, where appropriate, solute solubilities and eutectic temperatures. Taken together, the findings on model systems aid the identification of the kinetic and thermodynamic factors responsible for the freezing - thawing survival of living cells.

The composition dependence of glass transition properties

Abstract: The configurational entropy theory of glass formation is used to derive the composition dependence of the glass transition temperature (Tg) and of supercooled liquid transport properties for binary mixtures which obey the laws of regular solutions. The relations between expressions derived subject to specified approximations and known empirical equations are shown, and the parameters of the latter are thereby identified with defined measurable quantities. The predicted compostion dependence of Tg is compared with experimental data from binary mixtures with satisfactory agreement.

Plastic deformation in polyimides, with new implications on the theory of plastic deformation of glassy polymers

Abstract: The plastic deformation behaviour of a series of glassy aromatic polyimides, derivatives of resorcinol, hydroquinone, oxydiphenyl, and pyromellitic acid, have been investigated between room temperature and a temperature 20 K below their respective glass transition temperature The results are analysed by means of a theory due to Argon (1973) for plastic flow in glassy polymers, and show that, as the spacing between natural hinges on polymer molecules increases with increasing chemical complexity, the thermally activated complex grows in size, and plastic deformation becomes less local

Moisture Effects in Epoxy Matrix Composites

Abstract: : The effect of absorbed moisture on the mechanical properties of a neat epoxy resin and derived graphite/epoxy composites is investigated. Glass transition temperature and modulus as a function of moisture content and temperature is determined for both the neat resin and composite. In addition, elastic modulus for the neat resin, and strength and elastic moduli for unidirectional composites and quasi-isotropic laminates are determined as a function of absorbed moisture and temperature. A method for predicting moisture content and through-the-thickness profile for a laminate exposed to a constant humidity and temperature is discussed. Particular attention is also given to the details of environmental testing. Data indicates that absorbed moisture and temperature effects on the neat resin translate directly to matrix dominated properties of the composite and can lead to a change in failure mode, while filament dominated properties show very little environmental sensitivity. Results also indicate that resin glass transition temperature is a function of processing as well absorbed moisture.

Secondary Loss Peaks in Glassy Amorphous Polymers

Abstract: A short description is given of the determination–as a function of frequency and temperature–of secondary mechanical loss peaks in glassy amorphous polymers. The molecular motions causing secondary loss peaks are classified. The dependence on frequency of the temperature of the maximum of the loss peak is discussed. The effects on the secondary loss peaks of structural features such as plasticization, intermoleeularsteric hindrance and polarity, are illustrated by the example of the β-maximum of poly(methyl methacrylate), emphasis being laid on the difference between a glass transition and a secondary loss peak.

The influence of water on the glass transition temperature of cellulose

Abstract: The approach of Kaelbe for calculating the effect of plasticizers on the glass transition temperature of the system polymer-plasticizer is discussed and compared with reported experimental data. By using these equations of Kaelbe, one can calculate the softening effect of water on cellulose. The results are shown to be in reasonable agreement with available data for the glass transition temperature of wet and plasticized cellulose. The glass transition temperature for cellulose containing variable amounts of water is calculated for cellulose material of different degree of crystallinity. Also the softening effect of water in cellulose systems containing hemicellulose and lignin is discussed.

Free volume theories of the glass transition and the special case of metallic glasses

Abstract: Theories based on the concepts of free volume and the existence of holes in liquids are briefly reviewed. Available experimental data on the changes in specific heat and thermal expansion at the glass transition temperature and the temperature dependence of viscosity near transition have been utilized to evaluate the hole formation energy and critical hole size in palladium-, platinum- and gold-based metallic glasses. It has been found that in conformity with theoretical predictions, transport in metallic glasses occurs by the movement of highly ionized atoms. A linear relationship exists between the hole formation energy and glass transition temperature of metallic glasses. It is suggested that a high energy of hole formation is a necessary criterion for easy vitrification of metallic melts. The behaviour of vacancies in crystalline metals is compared with the behaviour of holes in metallic glasses.

Model for the kinematics of polymer dissolution

Abstract: The dissolution of glassy polymers is described by a phenomenological model of the motion of two boundaries: the liquid-gel boundary and the gel-glass boundary. The motion of these boundaries, as well as the concentration profile in the layers of a dissolving polymer, was obtained by numerical solution of the Stefan boundary value problem. Confirmation of the experimental program written to simulate the problem is established by its good agreement with direct observation of the dissolution dynamics of polystyrene in methyl ethyl ketone. A potential application of this model to the study of the dissolution dynamics of other polymer-solvent systems is done by simulating the dissolution of three types of polymer-solvent pairs: 1) swelling of rubber, 2) high glass transition concentration, and 3) low glass transition concentration. Contrasting dissolution characteristics are shown for the effect of different types of polymer-solvent pairs as well as for the effect of different molecular weights for the same type of polymer-solvent pair.

Thermally Stable Polymers.

Abstract: : Electrophilic trisubstituted ethylenes, where X, Y, Z are electro-negative substituents, CN, COOCH3, Cl, and F have been synthesized They do not homopolymerize but readily undergo free radical copolymerization with electron-rich vinyl monomers such as styrene The copolymers, obtained in reasonably high molecular weights, have glass transition temperatures in the 150-225 C range, decomposed in the 225-275 range, and were soluble in many organic solvents (Author)

Viscous liquids and the glass transition. VII. Molecular mechanisms for a thermodynamic second order transition

Abstract: It has been previously shown that the ’’configurational’’ specific heat ΔCp of a liquid above Tg contains significant contributions from nonconfigurational sources such as changes with structural state (fictive temperature) of (1) lattice vibrational frequencies, (2) anharmonicity, and (3) numbers of molecular groups participating in secondary relaxations. Using the potential energy U of the structural state as an order parameter, we calculate the behavior of the liquid specific heat for simple models based on the first two mechanisms, and show that either gives a second order transition of the type generally conjectured as necessary to account for the entropy–temperature relation of the equilibrium supercooled liquid. We give qualitative arguments for believing that the third mechanism acts similarly. We show also that all three mechanisms if acting simultaneously give a single transition. Direct experimental evidence that anharmonicity, the most plausible mechanism on the basis of present scanty evidenc...

On the heat capacity, entropy and ‘glass transition’ of vitreous ice

Abstract: An analysis of the thermal properties of vitreous ice shows that both its heat capacity C p and its entropy above 100 K are partly configurational in origin. The configurational contribution increases with temperature, and the excess C p and entropy near the solid-liquid transformation temperature are 36 7 and 2 1 JK−1 mol−1, respectively. The increase is interpreted as indicating the onset of molecular mobility in vitreous ice. The configurational C p of the melt of vitreous ice at 133–6 K, of ∼ 36 7 JK−1 mol−1, is the same as the configurational Cp of water at 273 K. Thus, the short-range order in the melt differs little from that in water at 273 K. The maximum calorimetrie residual entropy of vitreous ice is 13 4 JK−1 mol−1, which is in fair agreement with the maximum value of 9 2 JK−1 mol−1 anticipated for a tetrahedral random network model with fully disordered positions of H-atoms. Thermodynamic consideration of a glass transition in supercooled liquid water indicates that there is no conti...

Formation of Glasses with High Ionic Conductivity in the System AgI‐Ag2O‐MoO3

Abstract: The glass-forming region and the glass transition temperature (T/sub g/) in the system Agl-Ag/sub 2/O-MoO/sub 3/ are presented.

Viscous liquids and the glass transition. VI. Relaxations in simple molecule glasses in the 4–77 K range

Abstract: Previous dielectric studies from our laboratory have shown that a secondary relaxation β, usually attributed to molecular flexibility, occurs as often, with the same strength, and in the same place (0.75 Tg for 1 kHz measuring frequency) in glasses made of simple rigid molecules. We concluded that the β relaxation is a universal feature of amorphous packing. We have now extended our previous studies on molecular and fused salt glasses to the 4–77 K temperature range, and found relaxations at about 0.2–0.5 Tg in seven of 16 substances examined. These low temperature relaxations appear in some rigid molecule and in some flexible molecule glasses, but not all. We conclude flexibility is not a necessary condition for a low temperature relaxation, but feel the evidence that such relaxations are also characteristic of amorphous packing is weak. Four alcohols studied gave evidence of an additional loss peak below 4 K. We suggest this peak may be related to the well‐known very low temperature anomalies common to ...

Electrical relaxation in amorphous protonic conductors

Abstract: The dc conductivities of seven concentrated aqueous acids, and the electrical relaxation properties of four of these, are reported. At the glass transition temperature Tg, as determined by differential thermal analysis, the dc conductivities increase with Tg for the individual acids, according to an Arrhenius function with an activation energy (at constant structure) of 8.6±0.3 kcal mole−1.A similar activation energy is approached for the conductance of individual acid glasses at temperatures low enough for the structure to be completely ’’frozen,’’ suggesting all systems conduct by the same mechanism. It is believed this is a proton transfer (Grotthuss‐type) mechanism. Under conditions where the structure can change with temperature, the apparent activation energy for conduction is higher, ∼35 kcal mole−1. Electrical relaxation in these media, both supercooled liquid and glass, is found to be characterized by nonexponential response functions. Data have been analyzed using an electrical modulus formalism...

Castor oil based interpenetrating polymer networks, IV. Mechanical behavior

Abstract: The physical and mechanical properties of interpenetrating polymer networks (IPN's) based on castor oil-urethane and polystyrene are detailed in this paper. Dynamic mechanical spectroscopy showed extensive but incomplete molecular mixing of the two polymers. The glass transition temperatures of the IPN's gradually merged from two distinct transitions into one broad transition at an intermediate temperature as the crosslink level of the castor oil component was increased. At low polystyrene contents, the IPN's yielded stress-strain behavior similar to reinforced elastomers, but at high polystyrene contents, they exhibited increased elongation. For example, the latter materials showed well developed yield points. Stress whitening and necking were also observed, suggesting a possible failure mechanism by crazing. Cold drawing was exhibited by the materials under tension. The tensile strength and Young's modules were enhanced as the polymer II (polystyrene) content was increased at constant crosslink level of polymer I (castor oil-urethane). A similar effect was also observed by maintaining the polystyrene content constant but increasing the crosslink level of polymer I. The impact strength of the materials ranged from 32.1 to 53.4 N · m/m, which is approximately two to three times that of homopolymer polystyrene. The best materials were those with compositions in the range of 40-46 percent castor oil-urethane. The materials prepared by using tolylene diisocyanate as crosslinker for the castor oil phase had the best impact properties, especially at an NCO/OH ratio of 0.95.

Calorimetric study of the glassy state. XIII. Thermodynamic properties of normal and deuterated orthoboric acid crystals.

Abstract: Thermal properties of normal and deuterated orthoboric acids were studied through the measurements of heat capacity in the temperature range from 13 to 370 K by using an adiabatic calorimeter and differential thermal analysis curves above room temperature. For both crystals, the heat capacity anomaly was found around 290 K in the heat capacity values dependent upon the thermal history of the specimen; i.e., the endothermic or exothermic enthalpy relaxation was observed in this temperature range. This behavior is of characteristic to the glass transition and is considered to be ascribed to the freezing-in phenomenon of the motion of rearrangement of the protons in hydrogen bondings. The enthalpy relaxation curves were analyzed with the exponential law and the characteristic time constant toward the equilibrium state was longer for enthalpy-excessive side than for enthalpy-deficient side. The glass transition temperature at which the endothermic relaxation time becomes 1 ks is 296.6 K for normal orthoboric ...

Heat Capacity and Structural Relaxation of Enthalpy in As2Se3 Glass

Abstract: The evolution of enthalpy of As2Se3 glass during structural relaxation in the glass transition region was measured via differential scanning calorimetry for two types of time-temperature programs: rate-heating at 10 K/min following a cool at a constant rate (-20 to -0.31 K/min) and isothermal annealing following a temperature step from an equilibrium state. The rate-heating data yield kinetic parameters for the structural relaxation which predict accurately the evolution of the enthalpy during isothermal annealing. The glass heat capacities were independent of cooling rate within experimental precision (≤0.2%). In this respect, As2Se3 is unlike previously studied glasses whose heat capacities are more dependent on thermal history.

Thermodynamics of the glassy state with multiple order parameters

Abstract: Experimental data, both old and more recent, indicate the need for more than a single order parameter for adequate description of glassy materials. The thermodynamics of the glassy state with multiple order parameters, previously discussed by others, has been examined and developed further systematically. The result shows that the Prigogine‐Defay ratio can be defined at every point on the (T,p) plane near the glass transition line and is generally greater than unity. One has to define a number of glass transition lines (Tg versus p), depending on which of the order parameters are assumed frozen‐in at the glass transition. Even for a definition of the glass transition line operationally in terms of the relaxation time, one has to specify not only the time scale of the experiment but also the particular property observed and the mode by which the system is disturbed from equilibrium. A one‐dimensional lattice gas model is utilized to obtain an expression for the Gibbs free energy as a function of T, p, and two order parameters, and this is then used to illustrate and test out the concepts and the thermodynamic relations derived here.