Showing papers on "Glass transition published in 1978"

A method for evaluating viscosities of metallic glasses from the rates of thermal transformations

Abstract: The apparent activation energies E(T) for the glass transition and crystallization in Pd77.6Cu6Si16.5 and Pd48Ni32P20 glass are seen to coincide with those for the viscous flow. This implies that both the rates of glass transition and crystallization in metallic glasses scale as the viscosity. Based on this proposition, the viscosity of a Pt45Ni30P25 glass, for example, has been evaluated, by means of thermal measurements, from the glass transition far into the crystallization temperatures. The viscosity η decreases rapidly from 1013 P at 480 K to 107 P at 580 K and is described by a Fulcher's expression as η = 10−3 exp[5950/(T − 320)].

CO2 sorption in poly(ethylene terephthalate) above and below the glass transition

Abstract: High-pressure CO2 sorption data in semicrystalline poly(ethylene terephthalate) (PET) are presented for temperatures ranging from 25 to 115°C. The results are described by Henry's law above the glass-transition temperature of PET, while a dual-mode sorption model comprised of a Henry's law and a Langmuir isotherm applies in the glassy state. The disappearance of the Langmuir capacity of the polymer above Tg presumably results from the elimination of regions of localized lower density which are frozen into the glass upon quenching from the rubbery state. Exposure of PET to a high CO2 pressure produced a systematic variation in the apparent sorption equilibria. Correlation of the Langmuir capacity of PET with the dilatometric parameters of the polymer provides a useful framework for understanding the origin of the Langmuir sorption mode and for interpreting annealing and conditioning effects in glassy polymers.

The influence of structural relaxation on the density and Young’s modulus of metallic glasses

Abstract: The Young’s modulus E of metallic glasses increases by ∼7% while the density ρ increases by only ∼0.5% upon annealing near the glass transition temperature. ρ and E of an ’’ideal’’ glass are calculated to approach the crystalline values within 0.5 and 10%, respectively. The large decrease in E of ∼30% observed upon vitrification of many glass‐forming alloys may be attributed mainly to the frozen‐in excess volume. The rates of increase in E associated with decrease in volume V, −d lnE/d lnV, during structural relaxation, crystallization, and thermal expansion are large and ∼15, 17, and 10, respectively. Structural reconstruction and consequent electron redistribution near the Fermi surface are suggested to be responsible for the large −d lnE/d lnV observed.

A free‐volume interpretation of the influence of the glass transition on diffusion in amorphous polymers

Abstract: A new version of the free-volume theory of diffusion is used to describe polymer–solvent diffusion both above and below the glass transition temperature. Expressions are derived for the temperature dependence of the mutual diffusion coefficient and for the effective activation energy in the limit of zero penetrant concentration. The theory also describes the effect of the glass transition on the diffusion process. Predictions of the theory are compared with available diffusivity data for amorphous polymer–solvent systems.

Diffusion in a Pd‐Cu‐Si metallic glass

Abstract: The diffusivities (D) of a Pd77.5Cu6Si16.5 glass above the glass transition agree within a factor of 3 with diffusivities (Dη) evaluated from viscosity data using the Stokes‐Einstein relationship. Below the glass transition, however, the measured D is critically related to the state of structure and can be many orders of magnitude higher than Dη. The time constant for structural relaxation is suggested to be at least three orders of magnitude longer than that for diffusion processes.

Effect of polymer matrix on the cis-trans isomerization of azobenzene residues in bulk polymers

Abstract: Several acrylates and methacrylates were copolymerized with two 4-aminoazobenzene methacrylic derivatives. The photochemical and thermal cis-trans isomerization about the azolink was investigated in the bulk polymers. All the copolymers show photochromic behaviour without fading; the photostationary states upon irradiation with UV-light of the appropriate wavelengths were similar in bulk and in solution. The thermal cis-trans isomerization follows simple first order kinetics in the rubbery specimens as in solution. Below the glass transition temperature Tg, the isomerization can only be described by two simultaneous first order reactions. Depending on the conditions of irradiation and on the reaction temperature, a portion of the azoaromatic compound reacts anomalously fast. The temperature dependence of the relaxation time of the isomerization reaction in all the polymer films investigated can be represented by a single WLF-type graph; the corresponding Arrhenius plot is curved above Tg. The findings are interpreted by considering the effects of the free volume and of the chain segmental mobility on the cis-trans isomerization. A rotational and a translational relaxation mechanism is discussed for the normal and the anomalously fast reaction below Tg.

The crystallization kinetics of Fe-Ni based metallic glasses

Abstract: Differential scanning calorimetry is used to study the crystallization kinetics of two commerical Fe-Ni metallic galsses near their glass transition point. For 0.01

Relationships on the effect of water on glass transition temperature and young's modulus of nylon 6

Abstract: The glass transition temperature Tg of nylon 6 decreases monotonically toward a finite value Tgl upon increase of the moisture content. The mechanism of this decrease entails the reversible replacement of intercaternary hydrogen bonds in the accessible regions of the polyamide. The limiting glass transition temperature Tgl is approached when the moisture content approaches Wl, which corresponds to the amount of water required for complete interaction with all accessible amide groups. Denoting with Tg0 the glass transition temperature of the dry polymer, the effect of water on Tg is represented by the equation, Tg = (ΔTg)0 exp{−[ln(ΔTg)0]W/τWl} + Tgl, where (ΔTg)0 = Tg0 −Tgl, and τ = W(Tgl+1)/Wl. This equation appears to be generally applicable to hydrophilic polymers, since correspondingly calculated data are also in very good agreement with experimental data for polymers such as nylon 66, poly(vinyl alcohol), and polyN-vinylpyrrolidone. The effect of water of Young's modulus E of nylon 6 is represented by an analogous relationship, and the quantity In[(E−El)/(Tg−Tgl)] is a linear function of the moisture content.

Transient and steady‐state permeation in poly(ethylene terephthlate) above and below the glass transition

Abstract: Transient and steady-state permeation data are reported for CO2 in semicrystalline poly(ethylene terephthalate) for temperatures ranging from 25 to 115°C over the pressure range from 1 to 20 atm. The pressure dependency of the time lag and permeability disappears completely above the glass transition of the polymer, and Fick's law with a concentration-independent diffusion coefficient applies. In the glassy state, a concentration-dependent diffusion coefficient is necessary to describe the data. The form of this concentration dependence is described well by the partial immobilization transport model that attributes a different mobility to each of the two populations of sorbed gas which exist in local equilibrium with each other in glassy polymers. The importance of reporting the pressure used in transport experiments involving glassy polymers is emphasized by comparing the difference in the activation energy of the apparent diffusivity calculated from the measured time lag at 1 and 20 atm. Also, the magnitude of the observed slope discontinuity at Tg in Arrhenius plots of these apparent diffusities is shown to be a function of the upstream pressure used in the experiment. The independently measured time lags are compared with the predicated values calculated from various transport models and found to be described best by the partial immobilization model.

Orientation effects and cooling stresses in amorphous polymers

Abstract: This paper deals with two kinds of internal stresses in amorphous polymers. First, the internal stresses connected with molecular orientation are discussed. A framework is presented that describes the interrelations of the various orientation effects (anisotropy, birefringence, etc.) and their dependence on the thermomechanical history of the material. The second part of the paper deals with the residual thermal stresses generated by rapid inhomogeneous cooling through the glass transition range. These so-called cooling stresses are well-known in glass technology. The theory of the quenching of flat glass plates is described, generalized to objects of arbitrary geometry, and shown to be applicable to polymers. The practical importance of cooling stresses is discussed.

Effect of Moisture on Epoxy Resins and Composites

Abstract: The effect of absorbed moisture on the properties of six neat epoxy resins (3501-5, 3501-6, 3502, 5208, 934, and NMD 2373) and three epoxy composites (AS/3501-5, B/5505, and hybrid B/5505-AS/3501-5) is explored. Excellentagree. ment among the diffusion coefficients, activation energy, and activation entropy is observed for all resins with the exception of NMD 2373. Good correlation exists between neat resins and corresponding composites. The effect of a weekly thermal cycle on the diffusivity and equilibrium moisture content of two resins and three composites is presented. Correlations are developed between preconditioning temperature and relative humidity on the magnitude of the anomalous moisture absorption. In addition, the effect of absorbed water on the glass transition temperature of six neat epoxy resins and two composites using thermomechanical analysis is considered. The water generally behaves as a classical plasticizer of the epoxy resin, lowering the glass transition temperature as predicted from theory. Comparison between neat 3501-5 resin and AS/3501-5 reveals generally good correlation in T g at comparable resin moisture levels, but plasticized hybrid boron-graphite/epoxy laminate exhibits a somewhat lower T g than predicted from theory.

An assessment of Ge-Sb-Se glasses as 8 to 12μm infra-red optical materials

Abstract: This paper presents measured data on glass transition temperature, thermal expansion coefficient, density and hardness of some Ge-Sb-Se glasses. A method of purifying the elements Sb and Se from surface oxides is reported which enables low absorption glass to be synthesized. Suitable compositions for bulk synthesis are identified.

Microstructure and its relationship to deformation processes in amorphous polymer glasses

Abstract: The microdeformation morphology of a number of vinyl polymers with bulky side chains (type I) and arylene polymers with flexible oxygen linkages (type II) was studied by electron microscopy. The polyarylenes crazed only near the glass transition while the polyvinyls exhibited a crazing regime that extended to liquid nitrogen temperatures. In addition significantly less plastic strain was localized in type II glass crazes relative to those in type I glasses. In compatible blends of polystyrene (PS) and 2,6-dimethyl poly(phenylene oxide) (2MPPO), ca. 30% 2MPPO was sufficient to induce a transition from type I to type II crazing behavior. Small amounts of PS suppressed the low-temperature 2MPPO β relaxation but enhanced the intermediate transition of 2MPPO at higher temperatures. Blending increased the conformational energy of the 2MPPO chain and improved interchain packing. The propensity for the polymer glass to form sharp shear bands at the expense of diffuse bands was increased by a decrease in the conformation energy of the polymer chain and an improvement in the glassy state packing.

Sub‐Tg annealing studies of rubber‐modified and unmodified epoxy systems

Abstract: Epoxy network systems based on DGEBA (bisphenol‐A‐diglycidyl ether) and NMA (nadic methyl anhydride) systems modified with the low molecular weight CTBN (carboxyl‐terminated butudiene‐acrylonitrile copolymer) rubbers were prepared and studied. It was found that below the glass transition of the epoxy matrix these materials displayed time‐dependent changes in their mechanical properties; specifically, the strain to break as well as the rate of stress relaxation were observed to decrease in a near‐linear behavior with the logarithm of time at sub‐Tg annealing. Calorimetric methods clearly showed a simultaneous decrease in enthalpy with time that behaved in a similar fashion as the time‐dependent mechanical properties. All the calorimetric and mechanical data are qualitatively related. The importance of this phenomena is considered in view of the widespread use of epoxys. Similar behavior is expected for other network glasses thermally quenched into a nonequilibrium state.

Nature of transient currents in polymers

Abstract: Transient charging and discharging currents were measured in several polymers from room temperature to the glass transition temperature, and the effect of varying parameters such as field strength, electrode material, sample thickness, method of polymer preparation, and addition of impurities was investigated in some of them. Some limited comparative studies of the isochronal characteristics (i.e., current‐temperature plots at fixed times) with the thermally stimulated depolarization and polarization currents were also carried out in order to test possible correlations between the phenomena observed by the different methods. All the results suggested that, at temperatures lower than the glass transition temperature and in low to moderate fields, the transient currents are essentially governed by dipolar mechanisms and are thus critically dependent on the position of the measuring temperature with respect to the temperature ranges of the various molecular relaxation processes characteristic of the material...

Nature of melt rheological transition in a styrene–butadiene–styrene block copolymer

Abstract: Our laboratory previously reported the observation of a high temperature, melt rheological transition in a styrene–butadiene–styrene (S:7 × 103 and B:43 × 103) block copolymer from the highly elastic, nonlinear viscous behavior typical of a multiphase structure to linear viscous behavior with insignificant elasticity typical of a single-phase structure. We have investigated the precise nature of this melt rheological transition in the 7S-43B-7S sample by measuring the dynamic viscoelastic properties at more than 11 temperatures, including several in the transition region. A new procedure was developed for accurately measuring the sample temperature in a Weissenberg rheogoniometer. The transition is found to start at about 140°C and proceed over a narrow transition region from 140 to about 150°C. Data at all temperatures superimpose onto a single master curve only at high reduced frequencies. At low reduced frequencies, two characteristic branches of the master curve are formed. The data at temperatures below the transition region superimpose onto the upper branch where the dynamic viscosity η′(ω) is a strong function of ω, whereas the data at temperatures above the transition region superimpose onto the lower branch where η′(ω) is independent of ω. The data at temperatures within the transition region fall between the upper and lower branches, ordered according to their temperature positions. The apparent flow activation energy is found to be constant at about 22.8 kcal/mole below the transition region, but appears to decrease to about 17.4 kcal/mole above the transition region. The narrowness of the rheological transition far above the glass transition temperature of the polystyrene domains and the limiting linear viscoelastic behavior at low frequencies above the transition suggest an accompanying morphological transition rather than a gradual weakening of the polystyrene domains.

A study of the crystallization of two non-crystalline CuZr alloys

Abstract: Two CuZr alloys, Cu-50 at% Zr and Cu-54 at% Zr, were splat quenched to the non-crystalline state using a piston and anvil device. The glass transition and crystallization temperatures, as well as the enthalpy releases observed during crystallization were measured using differential scanning calorimetry of as-splat specimens and specimens aged below Tg. Transmission electron microscopy and X-ray diffraction experiments were utilized to monitor phase and structural changes in the alloys as they were transformed to the crystalline state. The results of the investigation indicated that the non-crystalline to crystalline transformation of these two alloys in constant heating rate experiments above Tg was a two-step process. The initial step, which is associated with a large exothermic reaction, results in the appearance of crystallites in a matrix of non-crystalline material. The final step, associated with a smaller exothermic reaction, results in the total transformation of glass to the crystalline state and the formation of the equilibium crystalline phases. The effect of aging these splat-quenched non-crystalline alloys at temperatures below Tg was also investigated. It was determined from these experiments that crystallization does occur when the non-crystalline alloys are aged ∼ 15°C below Tg. However, the incubation time for crystalline nucleus formation was found to be substantially greater for the Cu50Zr50 glass. Finally, it was determined that the thermal stability of the aged glass relative to the spontaneous crystallization observed during the constant heating rate experiments above Tg decreases as a function of aging time.

Alloying effect on the viscous flow in metallic glasses

Abstract: The temperature dependence of viscosities near the glass transition is measured from the rates of thermal transformation for metallic glasses PtNiP, PdNiP, NiPBA1 and (Fe, Co)PBA1. Alloying among metallic elements which lowers the glass transition temperature Tg lowers the ideal glass transition temperature T0, but raises the residual configurational entropy Sg and the activation energies for “diffusive” rearrangement, Δμ ∗ , of the alloying glasses, while compositional ordering associated with the addition of metalloids raises the Tg and T0 and lowers the Sg and Δμ ∗ . Results are correlated to the atomic ordering and stability of the glasses. The extracted free volume and the critical diffusive volume are much smaller, by a factor of 4, for metallic glasses than for many other glasses.

Dielectric and Mechanical Relaxations in a Nafion Precursor

Abstract: The dielectric and mechanical properties of a Nafion precursor have been determined from -196 to +80 °C. The material is a partly crystalline perfluorinated polymer containing a SO2F group at the end of each per. fluorinated ether side chain. Four relaxations are observed. labeled 1, fr, IS", and a in order of increasing temperature. The two relaxations can be partially resolved only at very low frequencies (ca. 0.1 Hz). The 7 relaxation is assigned to motion of the SO2F group, fl' to backbone (-(CF 2)"-) motions, d" to side chain motions (-(CF(CF3)0CF 2)5-), and a to the glass transition. The effect of ionic forces on the properties of polymers has received considerable attention in recent years. 28 Despite this effort, however, a number of factors have still not been elucidated completely, either because of the inherent difficulty of the problem (such as clustering of the ionic groups in media of low dielectric constant) or because of the difficulty of ob- taining an ionic polymer of varying ion contents which has a structurally exact nonionic analogue. The latter is needed in order to be able to separate clearly the effects of ionic inter- actions alone from those due to other factors such as molecular weight, sequence distribution, etc. In the case of copolymers containing acidic groups, the ion content can be varied by utilizing different degrees of neutralization of the carboxylic acid groups 26-41 and by esterification. 7 However, the unique properties of the proton in general, and its participation in hydrogen bonding in particular, confer special properties onto the acidic analogues of these ionomers which may invalidate any direct comparison with the salts. This paper concerns itself with an investigation of a non- ionic material which is the chemical precursor of the family of ionic polymers known as Nafions, developed by Du Pont for use primarily as electrolyte separators. It consists of a tetrafluoroethylene backbone, with a perfluoronated ether side chain which is terminated by an SO2F group (I)