Showing papers on "Glass transition published in 1990"

Random loose packings of uniform spheres and the dilatancy onset

Abstract: The random-loose-packing fraction of uniform spheres at the limit of zero gravitational force is determined to be 0.555\ifmmode\pm\else\textpm\fi{}0.005. This structure corresponds to a sphere packing at its rigidity-percolation threshold. The onset of dilatancy was also measured to be at approximately the same packing fraction. Preliminary evidence also indicates that shear thickening of suspensions with macroscopic spheres and a negative pore liquid pressure develops at the rigidity threshold. These provide further evidence that a hard-sphere glass transition could exist in the packing fraction range of 0.555 to 0.645.

The glass transition as a free volume related kinetic phenomenon

Abstract: The glass transition, observed in glasses during DSC experiments at a constant heating rate, is described as a kinetic phenomenon caused by the continuous approach of free volume towards equilibrium during the warming up. At a certain temperature below the glass temperature T g , the amount of free volume becomes smaller than the equilibrium value at that temperature. The glass transition is caused by a rather sudden increase of free volume towards equilibrium near T g . This can be calculated by making use of the well known kinetics of free volume annilation and production in some metallic glasses. Calculations performed on this basis are in good agreement with available experimental data, both on structural relaxation and the glass transition.

Glass transition temperature versus conversion relationships for thermosetting polymers

Abstract: Owing to enthalpy relaxation, values of the glass transition temperature (Tg) for partially reacted polymers may depend on the thermal history of samples and the heating rate used for measurements. Use of theoretical relations between Tg and the extent of reaction (x) of a thermoset must take this fact into account. The original DiBenedetto equation has been reevaluated as a convenient constitutive equation for expressing Tg versus x. An extension of Couchman's approach for the expression of the compositional variation of Tg enabled us to derive the same functionality as given by the DiBenedetto equation. Thus, the DiBenedetto equation may be regarded as based on entropic considerations applied to a model of the thermosetting polymer consisting of a random mixture of a fully reacted network with the initial monomers in an amount which depends on the particular conversion level. These two equations have been applied with success to different diepoxy-diamine copolymers.

The glass transition temperature (Tg) as an index of chemical conversion for a high‐Tg amine/epoxy system: Chemical and diffusion‐controlled reaction kinetics

Abstract: The glass transition temperature (Tg) is used as a parameter to monitor the isothermal cure of a tetrafunctional aromatic diamine and a difunctional aromatic epoxy system. There is a one-to-one relationship between Tg and conversion that is independent of cure temperature, Tcure. Prior to vitrification (Tg = Tcure), the reaction is only kinetically controlled; after vitrification, the reaction becomes diffusion-controlled. Time-temperature shifts of Tg vs. In (time) data at different cure temperatures to form a master curve for the kinetically controlled reaction at an arbitrary reference temperature yield a single Arrhenius activation energy (15.2 kcal/mole). The master curve and the reaction activation energy are used in calculating iso-Tg contours prior to vitrification and also the vitrification contour in the time-temperature-transformation (TTT) isothermal cure diagram for the system. The chemical kinetics of the reaction is satisfactorily described by an autocatalyzed reaction mechanism. The overall rate constant of the reaction in both kinetically and diffusion-controlled regimes is modeled by a combination, in parallel, of the chemical rate constant and the diffusion rate constant. The chemical rate constant has the usual Arrhenius form, whereas the diffusion rate constant is assumed to be given by a modified form of the WLF equation. Results suggest that both primary and secondary amino hydrogens are equally reactive. A theoretical model for calculating Tg as a function of conversion is presented for a network-forming system with one bond-forming reaction.

Glass Transition

Abstract: Computer simulations of melting, crystallization, glass transition, and annealing for a model system of 864 Lennard-Jones LJ atoms under a periodic boundary condition are carried out using constant-pressure molec ular dynamics techniques with temperature control. When an fcc crystal of LJ atoms is heated, melting occurs; however, an LJ liquid, when quenched slowly, crystallizes into layers with stacking faults. Each layer forms a two-dimensional, close-packed structure with occasional point defects but without dislocations. When the quench rate is high enough, an LJ liquid transforms into a disordered structure without a discontinuous change in volume. The dependence of the glass transi tion on the quench rate is determined by examining macroscopically observable physical features. Several microscopic structure parameters are introduced to ana lyze, at the atomic level, the structures of glasses pro duced by different quench rates. When annealed, a glass made with a low enough quench rate is stable against crystallization. Identifying the number of atoms in the system having local icosahedral symmetry is a prom ising method of characterizing the glass's stability. The microscopic structural changes in the annealing pro cesses are presented in the companion video to this paper.

Positron annihilation in amine‐cured epoxy polymers—pressure dependence

Abstract: Positron annihilation spectroscopy has been used to study free volume in an arnine-cured epoxy as a function of external pressure at temperatures above and below the glass transition temperature. The observed ortho-positronium lifetime τ3 and formation probability I3 decreased with increasing pressure. The decrease in τ3 is interpreted in terms of a corresponding decrease in average free-volume hole size over the range from 0.135 to 0.045 nm3. The fractional free-volume and the free-volume compressibility in the epoxy are calculated as functions of pressure at 100°C.

The Relationship Between the Glass Transition Temperature and Water Vapor Absorption by Poly(vinylpyrrolidone)

Abstract: Water associated with amorphous solids is known to affect significantly the physical and chemical properties of dosage form ingredients. An analysis of water vapor absorption isotherms of poly(vinylpyrrolidone) measured in this and other laboratories, over the range −40 to 60°C, along with the measurement of the glass transition temperature of poly(vinylpyrrolidone) as a function of water content is reported. It is observed that the amount of water vapor absorbed at a particular relative humidity increases with decreasing temperature, along with a significant change in the shape of the isotherm. It is also shown that at any temperature the state of the solid changes from a highly viscous glass to a much less viscous rubber in the region where absorbed water appears to enter into a “solvent-like” state. Further, the apparent “tightly bound” state, observed at low relative humidities, appears to exist when the polymer enters into a very viscous glassy state. It is concluded that the apparent states of water and polymer are interrelated in a dynamic manner and, therefore, that they cannot be uncoupled by simple thermodynamic analyses based only on a water-binding model.

Synthesis, characterization, and properties of a series of osmium- and ruthenium-containing metallopolymers

Abstract: Preparation par reaction de la poly(vinyl pyridine) et de la poly(vinyl imidazole) avec les complexes Os(bipy) 2 Cl 2 et Ru(bipy) 2 Cl 2 . Les polymeres sont caracterises par spectroscopie UV-visible et d'emission et par leurs proprietes electrochimiques. L'irradiation du polymere [Ru(bipy) 2 (PUP) n Cl] depose en surface d'une electrode provoque un echange de ligand

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers

Abstract: A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers has been developed, which takes into account the influences of end groups, branching, and crosslinking, and their functionality distribution. DiBenedetto's theory was found to correctly characterize the influence of crosslinks on the glass temperature. Normalized to constant crosslink functionality, the crosslink constant is a universal parameter suggesting that the entropic theory of glasses is applicable to crosslinked systems. Data on linear polymers and networks from the crosslinking of polymer chains, vinyl/divinyl-copolymers and step-growth polymers, such as polyurethanes, amine-cured epoxies, or inorganic glasses, are presented.

Specific-heat spectroscopy and dielectric susceptibility measurements of salol at the glass transition.

Abstract: We compare specific-heat-spectroscopy and dielectric susceptibility measurements of salol (phenyl salicylate) near the glass transition. The specific-heat-spectroscopy measurements cover 5 decades in frequency; the dielectric susceptibility measurements cover 13 decades. Over their common range, both probes measure the same characteristic relaxation time. In contrast to the viscosity measurements of salol reported in the literature, the characteristic relaxation time measured by these techniques shows non-Arrhenius behavior near ${\mathit{T}}_{\mathit{g}}$. The Vogel-Fulcher divergence temperature is consistent with the Kauzmann temperature. As is seen in other glass formers, the relaxation widths broaden rapidly with decreasing temperature.

Rare-Earth Aluminosilicate Glasses

Abstract: Rare-earth aluminosilicate glasses of the general formula 20R2O3· 20Al2O3· 60SiO2 have been formed for 10 of the 14 possible rare-earth oxides. Two series of “mixed-rare-earth” glasses were also formed (Nd/Er and Nd/Y). These glasses exhibit exceptionally high glass transformation temperatures, moderate thermal expansion coefficients, and refractive indices of approximately 1.65. The glass transformation temperature and thermal expansion coefficients vary linearly with the field strength of the rare-earth ion. No evidence of a “mixed-rare-earth effect” was observed. MAS-NMR indicates that the aluminum ions are tetrahedrally coordinated in at least some of these glasses.

Amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole

Abstract: Amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole with at least one fluoro comonomer selected from one or more of ten defined classes having a recited minimum mole proportion of perfluoro-2,2-dimethyl-1,3-dioxole, in no event less than 65%, have high glass transition temperatures of 140° C. or higher, low indices of refraction, and good physical properties, and low dielectric constants, which make them suitable for cladding optical fibers as well as for many electronics applications, including the manufacture of substrates for circuit boards. They all are soluble at room temperature in perfluoro(2-butyltetrahydrofuran), which makes it practical to apply them as coatings from solution.

High Mechanical Strengths of Mg–Ni–Y and Mg–Cu–Y Amorphous Alloys with Significant Supercooled Liquid Region

Abstract: Etude des intervalles de compositions dans lesquels une phase amorphe se forme dans les alliages Mg-Ni-Y et Mg-Cu-Y elabores par filage a l'etat liquide. Analyse de l'influence de la composition sur la temperature de transition a l'etat amorphe, la temperature de cristallisation et les proprietes mecaniques de ces alliages.

Molecular dynamics simulation of diffusion of simple gas molecules in a short chain polymer

Abstract: Diffusion of simple gas molecules in polymers has been studied using molecular dynamics (MD) computer simulations. Two model systems were used, each composed of short chain molecules having twenty segments and small molecules interacting with the segments. One of the systems is a mimic of polymethylene with oxygen as a solute. For the other system, potential parameters are identical except that the torsional potential of the chains is set to zero to alter the chain flexibility. Self‐diffusion coefficients of the small molecules and relaxation times of internal rotations of the chains were calculated at around the density of amorphous polyethylene and at temperatures of 250 to 300 K above glass transitions of the systems. Behavior of the models obtained within the time scale of usual MD simulations agrees well with experimental observations for amorphous polymers above glass transition temperatures. The free volume theory of simple liquids is found to be applicable to the diffusion as well as the relaxation. The diffusion strongly depends on amount of free volume so that thermal expansion of the system considerably affects the diffusion. Both dynamic properties show Arrhenius‐type temperature dependence regardless of the chain flexibility. Although the amount of free volume is a dominant factor for the diffusion, the distribution and dynamics of free volume, which depend on the chain flexibility, clearly affect the diffusion for a fixed amount of free volume; the high chain flexibility causes large diffusion coefficients and a low apparent activation energy of diffusion.

Thermodynamic aspects of the vitrification of toluene, and xylene isomers, and the fragility of liquid hydrocarbons

Abstract: The three xylenes, 1,2‐, 1,3‐, and 1,4‐dimethyl benzene, provide a well‐characterized family of isomeric molecules within which the melting points vary greatly while fluid‐state properties such as boiling points and viscosities remain very similar. Because all pure isomers crystallize rapidly on cooling, their behavior at the low‐temperature end of the liquid state, i.e., near the glass transition, has not been studied. We report here differential scanning calorimetry studies of phase relations for the three mixed isomer systems, and identify composition regions in which emulsified and also bulk mixtures can be vitrified at ordinary cooling rates. For one of the latter, 70% m‐xylene+ 30% o‐xylene by volume, we determine the change in heat capacity through the glass transition. On the assumption that this is the same for pure m‐xylene, we assess the Kauzmann limit on liquid behavior, TK, for m‐xylene to fall at 104.6 K, Tg/TK to be 1.20, and the excess entropy at Tg to be 15.6 J/mol K or 30.5% of the entro...

Raman spectroscopic study of ion–ion interaction and its temperature dependence in a poly(propylene‐oxide)‐based NaCF3SO3 –polymer electrolyte

Abstract: Raman scattering measurements have been carried out on poly(propylene oxide) complexed with NaCF3SO3 salt of concentration O:M=30:1 (where O:M is the PO:Na ratio) over a temperature range of 186–360 K in order to study ion–ion associations of the dopant salt and their temperature dependence. Splitting of the symmetric stretching mode of the CF3SO−3 anion into a double band was observed and attributed to the existence of different environments of the anions. A two‐component band analysis led to the identification of coexisting dissociated free ions and ion pairs, suggested to be in contact. Below the glass transition temperature, Tg, the intensity of the mode corresponding to the free ions was more or less constant with temperature; the amount of free ions in the glassy state was found to be about 84% of the total salt concentration. Above Tg the amount of dissociated free ions decreased rapidly with temperature in an Arrhenius‐type behavior. The resulting reduction of the number of charge carriers has lit...

Two-dimensional solid-state NMR studies of ultraslow chain motion: glass transition in atactic poly(propylene) versus helical jumps in isotactic poly(propylene)

Abstract: The motional mechanism was found to reflect the different chain conformation and packing: whereas in amorphous aPP in the vicinity of the glass transition a diffusive motion was observed, iPP was shown to perform helical jumps about the helix axis in the crystalline domains at temperatures above 360 K. The temperature variation of the mean correlation times follows the WLF equation over 11 orders of magnitude between 10 −10 and 10 s. The parameters extracted from this fit correspond to text-book values known from macroscopic measurements of the viscoelastic behavior in amorphous polymers

Poly(aryl ether-benzoxazoles)

Abstract: A general method for the preparation of poly(aryl ether-benzoxazoles) has been developed where the generation of an ether linkage is the polymer-forming reaction. The polymers [Ph-C 7 H 3 NO-R 1 -C 7 H 3 NO-PhO-ArO] n (R 1 =-, C(CF 3 ) 2 ; Ar=Ph-C(CF 3 ) 2 Ph, Ph-C(Ph) 2 -Ph, Ph-(uorenylene)-Ph) were prepared by polycondensation of dihalo bisbenzoxazoles with bisphenols in NMP in the presence of K 2 CO 3 . High molecular weight polymers were obtained, with glass transition temperatures ranging from 213 to 300 o C. The resulting polymers were processable from solution or the melt and showed excellent thermal stability and mechanical properties

Structural relaxation and dynamical correlations in a molten state near the liquid–glass transition: A molecular dynamics study

Abstract: Molecular dynamics calculations have been used to study structural relaxation and dynamical correlations near the glass transition in the system [Ca(NO3)2]0.4[KNO3]0.6. As in a typical molten salt, the overall structure is determined by charge ordering. However, the radial distribution function for Ca2+ ions is unusual in that even at high temperatures it shows a split first peak due to specific spatial correlations of the cations with the nitrate anions. Structural relaxation that accompanies cooling of the system has been characterized with the aid of the van Hove real‐space correlation functions Gs (r,t) for the constituent atoms (Ca, K, N, O). The relaxation of the incoherent structure factor Fs (k,t), with a wave vector k near the peak of the static structure factor, has been investigated as a function of temperature. The results clearly reveal both the α and β relaxation processes; the former can be well represented by a master curve with a stretched exponential shape. An analysis of the susceptibil...

Temperature dependence of self‐diffusion in compressed monohydric alcohols

Abstract: The p,T‐dependence of the self‐diffusion coefficient D for methanol, methan(2H)ol and ethanol has been studied between 150 and 450 K at pressures up to 250 MPa. The experiments were performed in strengthened high‐pressure glass cells by the application of the nuclear magnetic resonance (NMR) spin‐echo technique with pulsed magnetic‐field gradients. Upon cooling, molecular mobility is strongly reduced, leading to a pronounced non‐Arrhenius temperature dependence of D. Applying the rough hard‐sphere model (Chandler) to our data, a dramatic decrease of the A‐parameter with falling temperature is observed. This behavior indicates that attractive intermolecular interactions dominate translational mobility. The best description of the data is given by the empirical Vogel–Tammann–Fulcher (VTF) equation, with ideal glass transition temperatures T0, that are in excellent agreement with those obtained from calorimetric studies. The isotope effect for self‐diffusion in methanol and methan(2H)ol increases from ∼5% at...

Diffusion, viscosity and structural slowing down in soft sphere alloys near the kinetic glass transition

Abstract: The kinetic glass transition in a binary alloy, modelled by two species of atoms interacting via an inverse-12 “soft sphere” pair potential, is studied by a series of careful molecular dynamics (MD) simulations. Comparison with previous work shows that the cooling rate of the simulated quenches has practically no influence on the observed dynamical behaviour near the transition. It is shown that the shear viscosity and the self diffusion constants of the two species are reasonably well correlated via the Stokes-Einsteinrelation over a wide range of temperatures in the supercooled liquid. The wavenumber dependence of the order parameters characterizing the (nearly) frozen structure, agrees reasonably well with the result of mode-coupling calculations.

Preparation and properties of new aromatic polyamides from 4,4'-diaminotriphenylamine and aromatic dicarboxylic acids

Abstract: New aromatic polyamides (aramids) having inherent viscosities of 0.5–2.6 dL/g were prepared by the direct polycondensation of 4,4′-diaminotriphenylamine with various aromatic dicarboxylic acids in N-methyl-2-pyrrolidone using triphenyl phosphite and pyridine as condensing agents. These aramids were amorphous and soluble in a variety of solvents such as N,N-dimethyl-acetamide, dimethyl sulfoxide, and pyridine, and afforded transparent and tough films by solvent casting. The glass transition temperatures of these aramids were in the range of 280–310°C, and the 10% weight loss temperatures were above 430°C in air.

Two‐dimensional exchange nuclear magnetic resonance of powder samples. III. Transition to motional averaging and application to the glass transition

Abstract: The two‐dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster motions in powdered solids. The theoretical framework required for the simulation of 2D exchange of the faster motions, and for the evaluation of the experimental data, is developed. Calculations are presented for two standard models: two‐site jump and isotropic rotational diffusion. For discrete jump motion, anisotropic spin‐lattice relaxation during the mixing time is also considered. The resulting, simulated 2D line shapes show new characteristics in the intermediate dynamic range. Experimental data are presented for two‐site exchange in the model compound polycrystalline dimethylsulphone. The technique is then applied to study the chain dynamics of linear polystyrene in the glass transition range. Close to Tg the correlation times extracted from 2D exchange NMR exhibit strong non‐Arrhenius behavior. This data together with correlation times obtained at higher temperatures from conventional T1 data follows the WLF equation over 11 orders of magnitude, from 10−6 to 1000 s. It is shown that 2D exchange NMR and spin‐lattice relaxation probe the α and the β process, respectively, of the chain dynamics in the glass transition region.

The glass transition temperature (tg) as a parameter for monitoring the cure of an amine/epoxy system at constant heating rates

Abstract: : A continuous heating transformation (CHT) diagram displ ays the time and temperature of events that a material encounters during the course of continuous heating at different heating rates. For thermosetting systems, these events include gelation, vitrification and devitrification. Analysis of isothermal data for a diglycidyl-type epoxy/tetrafunctional aliphatic amine system, which is initially liquid at room temperature, indicated that kinetic control applied prior to vitrification. The parameters so obtained (i.e., order of reaction, Arrhenius pre-exponential term, and activation energy) together with a relationship between Tg and extent of reaction were used as a basis for modeling the onset of vitrification and devitrification in the CHT diagram. For heating scans from below Tgo (glass transition temperature of the reactants), the initially glassy material will devitrify when the temperature reaches Tgo, except for very low heating rates for which initial devitrification is not encountered. Keywords: Cure, Vitrification, Glass transition, Kinetics, Time-temperature-transformation cure diagram, Continuous heating, Time-temperature-transformation cure.