Showing papers on "Glass transition published in 1980"

Complete vitrification in pure liquid water and dilute aqueous solutions

Abstract: Pure water can only be vitrified by the very slow condensation of vapour on a metal surface maintained at very low temperatures1,2. Attempts to form vitreous ice by rapid cooling of liquid water invariably lead to formation of ice Ih (ref. 3). (Pryde and Jones4 did report a heat capacity change of rapidly cooled water at 126 K which they attributed to a glass transition, but could not reproduce this result in subsequent experiments.) Dilute aqueous solutions in contrast to concentrated aqueous solutions5 behave similarity to water and separate during freezing, even with the highest cooling rates available, into pure ice and concentrated solute6. We report here that macroscopic parts of samples of pure liquid water and of dilute aqueous solutions can be vitrified completely by jet-freezing of micrometre-sized aqueous droplets distributed in n -heptane as an emulsion—the resulting supercooling effect of ∼40 K being essential for vitrification7.

Sorption and transport in glassy polymers–a review

Abstract: After defining Fickian diffusion in rubbery polymers an overview of transport behavior in polymers is presented. Diffusion and sorption below the glass transition are discussed, followed by a review and classification of the various theoretical models which have been proposed to account for these phenomena. A short list of recommendations for future work is included.

Transition Behavior and Dielectric Properties in Trifluoroethylene and Vinylidene Fluoride Copolymers

Abstract: The transition behavior and dielectric properties of trifluoroethylene (TrFE)–vinylidene fluoride (VDF) copolymers were studied. The DSC and X-ray diffraction measurements indicated that the conformation of the polymers was in a crystalline state throughout the entire polymer composition, and crystallized isomorphically. In the thermal expansion measurement of the 56:44 TrFE–VDF copolymer, two breaks were observed, indicating two glass-to-rubber transitions. These two transitions were also observed in a mechanical relaxation study with peaks at 40°C (designated as β1) and −20°C (designated as β2), measured at 35 Hz. The β1-transition was tentatively assigned to the micro-Brownian motion of the main chain in the amorphous phase rich in TrFE-units; the β2-transition was assigned to the micro-Brownian motion of the main chain in the amorphous phase, rich in VDF-units. It was also found that the dielectric constant e′ of TrFE–VDF copolymer took on a maximum at 55 mol% VDF content. The numerical value of e′ was 15 (at 1 kHz and at 22°C) and about 1.7 times larger than those of poly(trifluoroethylene) and poly(vinylidene fluoride) homopolymers.

Epoxy‐water interactions

Abstract: It has been shown that sorbed moisture plasticizes epoxy resins with a resultant depression in the glass transition temperature of the polymer. The nature of the epoxy-water interaction still requires further investigation. The equilibrium sorption and diffusion of water in a high Tg epoxy is examined. Other analytical techniques including differential scanning calorimetry, infrared spectroscopy, electron microscopy and nuclear magnetic resonance are applied to the system. Experimental results suggest that the sorbed water at low concentrations is strongly localized at specific segments or groups in the polymer. Discussion is given relative to the structure-properties of the epoxy and its possible correlation to the experimental data obtained.

An isothermal–isobaric computer simulation of the supercooled‐liquid/glass transition region: Is the short‐range order in the amorphous solid fcc?

Abstract: It is demonstrated that one can easily distinguish between a supercooled liquid that becomes a glass and a supercooled liquid that nucleates to a defect crystal during a quench using the isothermal–isobaric Monte Carlo method.(AIP)

Glasses and glass ceramics for sealing to aluminum alloys

Abstract: We have developed alkali alkaline earth phosphate glasses which hermetically seal to aluminum alloys. They have coefficients of thermal expansion greater than 200 × 10−7 °C−I and glass transition temperature below 400°C. The relatively low fracture toughness values (KIC) of the glasses, 0.43 to 0.53 MPa (m) 1 2 , (KIC of silica = 0.9 MPa 1 2 ) increase by approximately 25% when the glasses are converted into glass ceramics. Depending upon parent glass compositions and heat treatment, crystal phases of sodium metaphosphate, sodium trimetaphosphate, sodium barium phosphate, or sodium calcium phosphate may develop. DC electrical resistivities of the glasses are 106 to 1010 ohm-cm at 230°C.

Thermal Transitions in Gelatin and Aqueous Gelatin Solutions

Abstract: On the basis of the characteristic enthalpy changes associated with glass transformation, the glass transition (Tg) in gelatin can be detected readily, in addition to the helix-to-coiltransition, by difTerential scanning calorimetry, and the effects of water content on these transitions can be measured directly. The thermal behaviour of aqueous, deionized bone-gelatin systems with up to about 20 wt";' water is typical of amorphous or partially amorphous polymer-diluent systems depending upon the amount of gelatin structural order developed. Extrapolations of the Tg data to the Tg reported for water and of the helix-to-coiltransition data to that reported for aqueous gelatin solutions containing 90 wt% water are reasonable. Further, the Tg data suggest that the equilibrium water contents of aqueous gelatin systems containing gelatin structural order are higher than those of structureless systems at corresponding relative humidities.

Glass transition in a soft‐sphere model

Abstract: A molecular dynamics study of the glass forming properties of the inverse‐twelfth‐power soft‐sphere model is reported. MD results in the equilibrium range for N=4000 are used to parametrize a six‐term virial equation for the fluid and a three term anharmonic perturbation equation‐of‐state for the crystal, and thereby determine the excess entropy of the supercooled fluid. The hypothetical Kauzmann catastrophe temperature (Tk) of the extrapolated supercooled fluid corresponding to zero excess entropy is thus determined. When a 4000‐particle system is cooled at a rate approximately 60 times slower than has previously been accomplished there is no evidence of crystal nucleation. On annealing the supercooled fluid for a very long time nucleation is apparent. The supercooled fluid equation‐of‐state data is fitted to a polynomial and the variation in heat capacity through the glass transition range is obtained. This yields an operational glass transition temperature Tgk, defined by the onset of kinetic retardati...

The time temperature position of the glass-rubber transition of amorphous polymers and the free volume

Abstract: Accurate measurements of the shear creep compliance in the glass rubber transition region have been performed by means of a combined torsional pendulum and torsional creep apparatus over a broad experimental window for commercial PS, PMMA and PVC and for two slightly crosslinked rubbers, NR and PU. For the three plastic materials, specific volume vs. temperature curves were obtained by volume dilatometry under different rates of cooling.

Phase transitions in mesophase macromolecules. I. Novel behavior in the vitrification of poly(ethylene terephthalate‐co‐p‐oxybenzoate)

Abstract: A 40:60 mole ratio poly(ethylene terephthalate-co-p-oxybenzoate) was studied by dynamic scanning calorimetry. Vitrification occurred in two steps, which could be linked by optical microscopy to a two-phase structure. For the lower glass transition, hysteresis was absent on heating and cooling through the transition temperature. Since all prior studied glasses show such hysteresis, this may be linked to the mesophase structure and raises the question whether mesophases always display such glass-transition behavior.

Tin-phosphorus oxyfluoride glasses

Abstract: Glass compositions in the tin-phosphorus-oxyfluoride composition system exhibiting very low glass transition temperatures and capable of being modified by the addition of selected constituents to exhibit excellent stability in a humid environment, are described.

Multiple transitions in isotactic polypropylene around and above the glass transition

Abstract: Gas chromatography, thermal expansion, dynamic mechanical relaxation and thermally stimulated current measurements were used to study multiple transitions in isotactic polypropylene. The crystalline phase relaxation Tαc was decomposed into two components, Tαc1 and Tαc1, in order of decreasing temperature. The activation energy of the Tαc mode was found to be lower than that of the relaxation observed around the glass transition temperature. Thermally stimulated current (TSC) measurements revealed the existence of an apparent double liquid-liquid transition at 39°C and 68 °C in undrawn polypropylene and at 28°C and 55°C in drawn polypropylene. These TSC peaks correspond to a relaxation time following a Vogel equation whose critical temperatures let us predict an apparent double glass transition in polypropylene.

Electrical properties of GeSbSe glasses

Abstract: The dc and ac conductivities of glasses of the system GeSbSe (with the general formula Ge x Sb 10 Se 90− x have been studied. The dc conductivity results indicate a maximum in the glass transition temperature and activation energy and a minimum in conductivity for the composition Ge 25 Sb 10 Se 65 . These results have been explained on the basis of the prevalent structural arrangement wherein structural units of GeSe 2 and Sb 2 Se 3 are dispersed among excess Se or Ge. Based on this picture, a model has been developed which accounts for the observed dependence of conductivity on composition at any temperature. The ac results have been utilized to find the hopping conductivity as a function of composition; the characteristics of lone pair amorphous semiconductors seem to account for the observed features.

Infrared absorption and structure of chlorophosphate glasses

Abstract: The infrared absorption spectra of glasses in the NaPO3ZnCl2 system were studied in the range of 4000−350 cm−1. The results were interpreted using a local mode approach based on existing frequency charts. A continuous structural breakdown of the glass structure occurs upon addition of ZnCl2 to the NaPO3 polymer, with a consequent decrease in its chain length. The actual structure of the chlorophosphate glasses is discussed on the basis of possible anionic groups present. There is evidence for the occurrence of mixed oxychloride coordination shells. The density and glass transition temperature of the glasses were also determined.

Dynamic mechanical analysis of fiber reinforced composites

Abstract: Dynamic mechanical and thermal properties were determined for unidirectional epoxy/glass composites at various fiber orientation angles. Resonant frequency and relative logarithmic decrement were measured as functions of temperature. In low angle and longitudinal specimens a transition was observed above the resin glass transition temperature which was manifested mechanically as an additional damping peak and thermally as a change in the coefficient of thermal expansion. The new transition was attributed to a heterogeneous resin matrix induced by the fiber. The temperature span of the glass-rubber relaxation was found to broaden with decreasing orientation angle, reflecting the growth of fiber contribution and exhibiting behavior similar to that of Young's modulus. The change in resonant frequency through the glass transition was greatest for samples of intermediate fiber angle, demonstrating behavior similar to that of the longitudinal shear modulus.

Sorption and transport of CO2 above and below the glass transition of poly(ethylene terephthalate)

Abstract: At temperatures at least 30°C above the glass transition (Tg) the sorption and transport of carbon dioxide in poly(ethylene terephthalate) (PET) can be described conveniently using Henry's law and Fick's law with a constant diffusion coefficient. Below Tg Fick's law with a concentration- dependent diffusion coefficient, coupled with a sorption isotherm which is concave toward the pressure axis adequately describes the observed sorption and transport data. Physical interpretations of the quantitative deviations from Henry's law and the form of the concentration dependence of the diffusion coefficient is provided by a model which hypothesizes dual modes of sorption and separate non zero mobilities of two populations of sorbed species in local equilibrium. The implications of the observed temperature variations of the phenomenological model parameters are discussed. Dilatometric parameters for PET, polycarbonate, and poly(acrylonitrile) (PAN) are shown to correlate well with a simple. relationship developed to explain the existence of the “extra” mode of sorption responsible for deviations from Henry's law for CO in glassy polymers. In the temperature range from Tg to + 20°C, deviations from Fickian behavior are also apparent. These effects are consistent with a transition in the nature of the polymer from an elastic solid below Tg to a viscous liquid above Tg In the narrow temperature range slightly above T the time scale for chain rearrangements apparently approaches that for the diffusion process. The polymer's viscoelastic response to the probing molecule, therefore, causes deviations from the classical time lag predictions. These deviations disappear 30°C above Tg.

Dissolution dynamics of some polymers: Solvent‐polymer boundaries

Abstract: A critical angle illumination microscopy technique was used to study the in situ dissolution dynamics of polystyrene, poly (α-methylstyrene) and the two tactic forms of poly (methyl methacrylate), (PMMA), in several solvents. The dissolution characteristics; of polymers were found to be greatly influenced by several factors: type of polymer, processing condition of the sample, type of solvent, and tacticity, Polystyrene was found to exhibit extensive swelling in several solvents while atactic PMMA exhibited extensive cracking on dissolution. Isotactic PMMA, which has a glass temperature of about 70°C lower than the atactic PMMA, showed swelling behavior similar to atactic polystyrene, while the α-methylstyrene showed the cracking phenomena exhibited by atactic PMMA.

A kinetic treatment of glass formation: IV. Crystallization on reheating a glass

Abstract: An analysis, based on the approach of crystallization statistics, is presented to describe crystallization on reheating a glass. The analysis treats heating carried out at the same or different rate relative to the cooling rate used to form the glass. The analysis takes account of the finite size of the critical nucleus and provides a description of the state of crystallinity in a body for both short and long times. The temperature of crystallization on reheating a glass is shown to depend strongly on the heating rate and on the barrier to crystal nucleation and less strongly on the cooling rate used to form the glass. For glasses with volume fractions crystallized greater than 10−9, the stability on subsequent heating increases with decreasing fraction crystallized at the glass transition.

The influence of composition on the formation and stability of Ni-Si-B metallic glasses

Abstract: The influence of composition on the formation and stability of Ni-based glassy alloys containing Si and B has been investigated systematically. Depending on the Si∶B ratio certain compositions, with total metalloid contents in the range 17 to 49 at% (a wider range than has hitherto been reported for metal-metalloid glasses), have been vitrified by melt-spinning to an average thickness of 17 μm. For metalloid concentrations greater than 36 to 40 at% the amorphous phase is brittle in the as-quenched state. The highest crystallization temperatures, Tx, occur at 32 to 38 at% metalloid, the actual value of Tx again depending on the Si∶B ratio. It is shown that, over a wide composition range, the glass-forming boundary corresponds closely with a value for the reduced crystallization temperature isometric of 0.52. This value corresponds to a critical cooling rate for glass formation of about 106 K sec−1, predicted from kinetic theories and assuming that Tx is a good estimate of the glass transition temperature, Tg. This agrees quite closely with the cooling rate for approximately 17 μm thick tape predicted from thermal studies of the melt-spinning process. Hence, Tx/Tliq usefully describes the glass-forming ability (GFA) for much of the composition range studied, although preliminary results suggest that near the centre of the glass-forming range the GFA may be over-estimated. Substitution of Si by other metalloid elements from Groups IIIb–Vb in Ni78Si10B12 glassy alloy generally decreases the thermal-stability.

Pressure-volume-temperature relationships of solid and molten poly(ethylene terephthalate)

Abstract: The pressure-volume-temperature (PVT) properties of poly(ethylene terephthalate) (PET) have been studied experimentally and theoretically in the temperature range 30–342 °C and at pressures to 2000 kg/cm2. In the semicrystalline solid state (ρ = 1.409 g/ cm3 at 20°C), two transitions were resolved. One is the glass transition at Tg = 70°C (P = 0), increasing with pressure according to dTg/dP = 0.025 °C/(kg/cm2). A second transition was observed as a break in the zero-pressure isobar and also as a break in the zero-pressure compressibility versus temperature curve at 160°C. It is interpreted as a premelting (αc) transition. The melt data were fitted successfully to both the empirical Tait equation and the Simha-Somcynsky hole theory (with T∗ = 11710°K, V∗ = 0.7408 cm3/g, and P∗ = 11570 kg/cm2). The Simha-Somcynsky theory was also used to establish a semi-empirical equation of state for amorphous PET in the temperature range Tg < T < Tm and to derive a theoretical pressure dependence of Tg in compl...