Showing papers on "Glass transition published in 1998"

Signatures of distinct dynamical regimes in the energy landscape of a glass-forming liquid

Abstract: Most materials attain a glassy state at low temperatures under suitable methods of preparation. This state exhibits the mechanical properties of a solid, but shows microscopic structural disorder1,2. A comprehensive understanding of the glassy state is, however, still lacking3. A widespread assumption is that the non-exponential relaxation processes observed in the dynamics of glasses — and also in protein dynamics, protein folding and population dynamics — are (in common with other manifestations of complex dynamics) strongly influenced by the underlying energy landscape associated with the structural configurations that the system may adopt. But concrete evidence for this in studies of glass formation has been scarce. Here we present such evidence, obtained from computer simulations of a model glass-forming liquid. We demonstrate that the onset of non-exponential relaxation corresponds to a well defined temperature below which the depth of the potential-energy minima explored by the liquid increases with decreasing temperature, and above which it does not. At lower temperatures, we observe a sharp transition when the liquid gets trapped in the deepest accessible energy basin. This transition temperature depends on the cooling rate, in a manner analogous to the experimental glass transition. We also present evidence that the barrier heights separating potential-energy minima sampled by the liquid increase abruptly at a temperature above the glass transition but well below the onset of non-exponential relaxation. This identification of a relationship between static, topographic features of the energy landscape and complex dynamics holds the promise of a clearer, possibly thermodynamic, understanding of the glass transition.

The luminescence and structure of novel transparent oxyfluoride glass-ceramics

Abstract: New transparent oxyfluoride glass-ceramics can provide a low phonon energy fluoride environment for active rare earth ions while maintaining the formability and physical properties of an oxide glass. Transparent glass-ceramics were made by casting crucible melted glass onto a steel plate and subsequently heating the resultant glass patty above the glass transition temperature to nucleate and grow LaF3 crystals. Eu3+ doped glasses emit only red luminescence from the 5D0 level but after heat treatment emit blue, green, and red luminescence indicative of a low phonon energy rare earth environment. TEM micrographs and XRD show no features in the base glass, but do show extensive LaF3 crystallites after heat treatment that are responsible for the novel optical properties of these hybrid materials.

Asymmetric Triaryldiamines as Thermally Stable Hole Transporting Layers for Organic Light-Emitting Devices

Abstract: The synthesis of a series of asymmetric triaryldiamines has provided a number of materials with a wide range of thermal, electrochemical, and spectroscopic properties. The asymmetric materials described herein have two different diarylamine groups bound to a 1,4-phenylene or 4,4‘-biphenylene core, i.e., Ar1Ar2N−C6H4−NAr1‘Ar3 or Ar1Ar2N-biphenyl-NAr1‘Ar3, respectively. The diarylamines studied include diphenylamine, phenyl-m-tolylamine, naphthylphenylamine, iminostilbene, iminodibenzyl, and carbazole. These materials were prepared by copper- and palladium-catalyzed coupling of aryl halides and diarylamines. The asymmetry inherent in these compounds prevents these low molecular mass compounds from crystallizing, thus yielding higher thermal stability over that of the symmetric derivatives. In all cases, the asymmetric diamines form stable glasses, with glass transition temperatures up to 125 °C. HOMO levels for these materials, estimated by cyclic voltammetry, show a broad range of values, with oxidation po...

Measurement of the self-intermediate scattering function of suspensions of hard spherical particles near the glass transition

Abstract: Dynamic light-scattering measurements are reported for suspensions at concentrations in the vicinity of the glass transition. In a mixture of identically sized but optically different particles having hard-sphere-like interactions, we project out the incoherent (or self-) intermediate scattering functions by adjusting the refractive index of the suspending liquid until scattering from the structure is suppressed. Due to polydispersity, crystallization is sufficiently slow so that good estimates of ensemble-averaged quantities can be measured for the metastable fluid states. Crystallization of the suspensions is still exploited, however, to set the volume fraction scale in terms of effective hard spheres and to eliminate (coherent) scattering from the structure. The glass-transition volume fraction is identified by the value where large-scale particle motion ceases. The nonequilibrium nature of the glass state is evidenced by the dependence on the waiting time of the long time decay of the relaxation functions. The self-intermediate scattering functions show negligible deviation from Gaussian behavior up to the onset of large-scale diffusion in the fluid or the onset of waiting time effects in the glass.

Thermodynamics and kinetics of the Mg65Cu25Y10 bulk metallic glass forming liquid

Abstract: The thermodynamics and kinetics of the bulk metallic glass forming Mg65Cu25Y10 liquid were investigated using differential scanning calorimetry and three-point beam bending. The experiments lead to the determination of the thermodynamic functions as well as the viscosity of the supercooled liquid. The viscosity shows a temperature dependence, which is consistent with that of a strong glass similar to Zr–Ti–Cu–Ni–Be bulk metallic glasses or sodium silicate glasses. This contrasts with more fragile conventional metallic glass formers or pure metals. The relatively weak temperature dependence of the thermodynamic functions of the supercooled liquid is related to these sluggish kinetics in the supercooled liquid. Entropy, viscosity, and kinetic glass transition are compared in the frameworks of the fragility concept and the Adam–Gibbs theory. Strong liquid behavior retards the formation of crystals kinetically and thermodynamically.

Crystallization Behavior and Phase Formation in Zr–Al–Cu–Ni Metallic Glass Containing Oxygen

Abstract: Differential scanning calorimetry, X-ray diffraction and transmission electron microscopy were used to study the thermal stability and the phase formation during annealing of (Zr 0.65 Al 0.075 Cu 0.175 Ni 0.10 ) 100-x O x metallic glasses (0.2≤x≤ 0.8) prepared by melt spinning. Increasing oxygen content changes the crystallization mode from a single to a double step process. The glass transition temperature T g increases slightly with increasing oxygen content whereas the crystallization temperature T x decreases, causing a reduction of the undercooled liquid region. The deterioration of the thermal stability is related to oxygen-induced formation of metastable quasicrystalline and fcc NiZr 2 -type phases which coexist with tetragonal Cu(Al, Ni)Zr 2 and hexagonal Zr 6 NiAl 2 for x=0.2 and 0.4. For x=0.8 only the quasicrystalline phase is observed in the first step of crystallization. The details of phase formation and transformation during crystallization depend strongly on the annealing conditions. At elevated temperatures the quasicrystals transform into Cu(Al, Ni) Zr 2 and the fcc phase transforms into the stable Zr 6 NiAl 2 compound. Increasing oxygen content leads to more pronounced metastable phase formation in the first step of crystallization at lower temperatures, and extends the stability regime of these phases to higher temperatures. For annealing temperatures ≥ 873 K the samples exhibit the same crystallization products regardless of the oxygen content of the material.

An empirical model for the solubility of H2O in magmas to 3 kilobars

Abstract: We present 16 new manometric determinations of H2O solubility for a range of natural silicate liquid compositions equilibrated up to 3 kbar of H 2O pressure. As the threshold temperature of dehydration of the quenched glasses during measurements of the H2O content becomes lower as a function both of bulk silicate composition and the dissolved H2O content, we measured the H2O released on heating over a range of temperature intervals. For example, alkali-rich samples having a dissolved H2O content greater than ;6 wt% start to evolve H2O at temperatures less than 150 8C, whereas more mafic samples and silicic samples with less than 6 wt% H2O begin to dehydrate at temperatures greater than 200 8C. This behavior is consistent with the concept that alkali-rich liquids can have their glass transition temperatures lowered substantially by dissolved H 2O and that H2O is released only significantly on heating in the supercooled liquid region, rather than in the glass region. Using these new data, in conjunction with previous data from the literature, we refined and extended the empirical H 2O solubility model of Moore et al. (1995b). The new model works well (2 s56 0.5 wt%) between 700‐1200 8C and 1‐3000 bar and can be applied to any natural silicate liquid in that range. The model may also be used for systems where X , 1 in the vapor phase. HO 2

Thermal and Mechanical Properties of Ti–Ni–Cu–Sn Amorphous Alloys with a Wide Supercooled Liquid Region before Crystallization

Abstract: A wide supercooled liquid region exceeding 50K before crystallization was observed for melt-spun Ti 50 Ni 25-x Cu 25 Sn x (x=3 and 5 at%) amorphous alloys. The temperature interval of the supercooled liquid region defined by the difference between crystallization temperature (T x ) and glass transition temperature (T g ), Δ T x (= T x -T g ) is 40 K for the 0%Sn alloy and increases to 50 K for the 3%Sn alloy and 60 K for the 5%Sn alloy. With increasing Sn content, the ΔT x value decreases significantly. A similar increase in thermal stability of the supercooled liquid was also recognized for the 3 at%Sb-containing alloy in the Ti 50 Ni 25-x Cu 25 Sb x system. The replacement by 3 to 5 at%Sn for Ni also induces an increase of mechanical strength, in addition to the increase in Δ T x . The tensile fracture strength (σ f ), Young's modulus (E) and Vickers hardness (H v ) increase from 1800 MPa, 93 GPa and 530, respectively, for the Ti 50 Ni 25 Cu 25 alloy to 2050 MPa, 102 GPa and 650, respectively, for the Ti 50 Ni 20 Cu 25 Sn 5 alloy. The crystallization takes place through a single exothermic reaction, accompanying the simultaneous precipitation of multiple (CuTi + Cu 4 Ti 3 + NiTi) phases. The crystallization mode requires long-range atomic rearrangements for precipitation of the crystalline phases, leading to the increase in the stability of supercooled liquid against crystallization. The high thermal stability of the supercooled liquid enabled the production of bulk amorphous alloys in the diameter range up to about 6 mm by copper mold casting. There is no appreciable difference in the stability of the supercooled liquid region between the melt-spun and cast bulk amorphous alloys. The first synthesis of the Ti-based amorphous alloys with high glass-forming ability and good mechanical properties allows us to expect the future development of bulk amorphous alloys as a new type of high specific strength material.

Glass Transition of Stereoregular Poly(methyl methacrylate) at Interfaces

Abstract: The glass transition temperature (Tg) of layers of stereoregular poly(methyl methacrylate) (PMMA) spin-cast on silicon and aluminum surfaces has been investigated by ellipsometry. The interfacial specific interactions were highlighted and quantified by infrared reflection absorption spectroscopy. It was found that depending on PMMA tacticity, a strong correlation exists between the density of the polymer/surface interactions and the Tg of that polymer at the interface. Indeed, i-PMMA with a large density of interfacial interactions increases its Tg at the interface whereas s-PMMA with a lower value of bonded segments exhibits a Tg depression. It is suggested that a certain level of interfacial interactions associated with an increase of density of the layer will compensate for the increase of mobility resulting from a reduction of the entanglement density or a segregation of chain ends at an interface.

Viscoelastic and morphological behavior of hybrid styryl-based polyhedral oligomeric silsesquioxane (POSS) copolymers

Abstract: We report on the viscoelastic behavior of linear thermoplastic nonpolar hybrid inorganic-organic polymers. These materials have been synthesized through copolymerization of an oligomeric inorganic macromer with 4-methylstyrene where the inorganic portion of the material is a well-defined polyhedral oligosilsesquioxane (POSS), R7(Si8O12)(CH2CH2C6H4CH|CH2), with R A c-C6H11 or c-C5H9. A series of 4-methyl styrene copolymers with approximately 4, 8, and 16 mol % POSS macromer incorporation were investigated. Rheological measurements show that the polymer dynamics are profoundly affected as the percent of POSS increases. In particular, a high-temperature rubbery plateau develops (where a terminal zone is not observed), despite the fact that the parent poly 4-methylstyrene is unentangled. It is also observed that the thermal properties are influenced as the percent of POSS incorporation in- creases, with increases in the glass and decomposition temperatures. The results sug- gest that interchain interactions between the massive inorganic groups are responsible for the retardation of polymer chain motion, a mechanism similar to the ''sticky repta- tion'' model conceived for hydrogen-bonded elastomers and developed by Leibler et al. (Macromolecules, 24, 4701 (1991)).Control over the interchain interactions would also give rise to the observed increases in glass transition and the establishment of a rubbery plateau. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1857-1872, 1998

Molecular-dynamics simulations of the thermal glass transition in polymer melts: α-relaxation behavior

Abstract: We present molecular-dynamics simulations of the thermal glass transition in a dense model polymer liquid. We performed a comparative study of both constant volume and constant pressure cooling of the polymer melt. Great emphasis was laid on a careful equilibration of the dense polymer melt at all studied temperatures. Our model introduces competing length scales in the interaction to prevent any crystallization tendency. In this first manuscript we analyze the structural properties as a function of temperature and the long time or \ensuremath{\alpha}-relaxation behavior as observed in the dynamic structure factor and the self-diffusion of the polymer chains. The \ensuremath{\alpha} relaxation can be consistently analyzed in terms of the mode coupling theory of the glass transition. The mode coupling critical temperature ${T}_{c},$ and the exponent \ensuremath{\gamma} defining the power law divergence of the \ensuremath{\alpha}-relaxation time scale, both depend on the thermodynamic ensemble employed in the simulation.

Polyamide 6/silica nanocomposites prepared by in situ polymerization

Abstract: A polyamide 6 (PA 6)/silica nanocomposite was obtained through a novel method, in situ polymerization, by first suspending silica particles in ϵ-caproamide under stirring and then polymerizing this mixture at high temperature under a nitrogen atmosphere The silicas were premodified with aminobutyric acid prior to the polymerization The effects of the addition of unmodified and modified silicas on the dispersion, interfacial adhesion, isothermal crystallization, and mechanical properties of PA 6 nanocomposites were investigated by using scanning electron microscopy, dynamic mechanical analysis, differential scanning calorimetry, and mechanical tests, respectively The results show that the silicas dispersed homogeneously in the PA 6 matrix The addition of silicas increases the glass transition temperature and crystallization rate of PA 6 The mechanical properties such as impact strength, tensile strength, and elongation at break of the PA 6/modified silica nanocomposites showed a tendency to increase and decrease with increase of the silica content and have maximum values at 5% silica content, whereas those of the PA 6/unmodified silica system decreased gradually © 1998 John Wiley & Sons, Inc J Appl Polym Sci 69: 355–361, 1998

Heat capacity of poly(trimethylene terephthalate)

Abstract: Thermal analysis of poly(trimethylene terephthalate) (PTT) has been carried out using standard differential scanning calorimetry and temperature-modulated differential scanning calorimetry. Heat capacities of the solid and liquid states of semicrystalline PTT are reported from 190 K to 570 K. The semicrystalline PTT has a glass transition temperature of about 331 K. Between 460 K and 480 K, PTT shows an exothermic ordering. The melting endotherm occurs between 480 K and 505 K with an onset temperature of 489.15 K (216 C). The heat of fusion of typical semicrystalline samples is 13.8 kJ/mol. For 100% crystalline PTT the heat of fusion is estimated to be 28--30 kJ/mol. The heat capacity of solid PTT is linked to an approximate group vibrational spectrum, and the Tarasov equation is used to estimate the skeletal vibrational heat capacity ({Theta}{sub 1} = 542 K and {Theta}{sub 3} = 42 K). A comparison of calculation and experimental heat capacities show agreement of better than {+-}2% between 190--300 K. The experimental heat capacity of liquid PTT can be expressed as a linear function of temperature: C{sub p} {sup L}(exp) = 211.6 + 0.434 T J/(K mol) and compares well with estimations from the ATHAS data bank using groupmore » contributions of other polymers with the same constituent groups ({+-} 0.5%). The change of heat capacity at T{sub g} of amorphous PTT has been estimated from the heat capacities of liquid and solid to be 86.4 J/(K mol). Knowing C{sub p} of the solid, liquid, and the transition parameters, the thermodynamic functions: enthalpy, entropy and Gibbs function were obtained.« less

Structure-property correlations in a combinatorial library of degradable biomaterials.

Abstract: A combinatorial library of degradable polyarylates was prepared. These polymers are A-B-type copolymers consisting of an alternating sequence of a diphenol and a diacid. The library was prepared by copolymerizing, in all possible combinations, 14 different tyrosine-derived diphenols and eight different aliphatic diacids, resulting in 8 x 14 = 112 distinct polymers. This approach (a) increases the number of available polymeric candidate materials for medical applications, and (b) facilitates the identification of correlations between polymer structure and glass transition temperature, air-water contact angle, mechanical properties, and fibroblast proliferation. The pendent chain and backbone structures were systematically varied by (a) simple homologative variations in the number of methylene groups, (b) substitution of oxygen for methylene groups, and (c) introduction of branched and aromatic structures. The polymers contained within the library exhibited incremental variations in Tg (from 2 degrees C to 91 degrees C) and air-water contact angle (from 64 degrees to 101 degrees ). Fibroblast proliferation (in vitro, serum-containing media) ranged from approximating that measured on tissue culture polystyrene to complete absence of proliferation. Generally, decreased proliferation correlated linearly with increased surface hydrophobicity, except in those polymers derived from oxygen-containing diacids in their backbone which were uniformly good growth substrates even if their surfaces were very hydrophobic. In a selected subgroup of polymers, tensile strength of thin solvent cast films ranged from about 6 to 45 MPa, while Young's modulus (stiffness) ranged from about 0.3 to 1.7 GPa. Combinatorial biomaterial libraries such as these tyrosine-derived polyarylates permit the systematic study of material-dependent biological responses and provide the medical device designer with the option to choose a suitable material from a library of related polymers that encompasses a broad range of properties.

The effect of silicon on the glass forming ability of the cu47ti34zr11ni8 bulk metallic glass forming alloy during processing of composites

Abstract: Composites of the Cu47Ti34Zr11Ni8 bulk metallic glass, reinforced with up to 30 vol % SiC particles are synthesized and characterized. Results based on x-ray diffraction, optical microscopy, scanning Auger microscopy, and differential scanning calorimetry (DSC) are presented. During processing of the composites, a TiC layer forms around the SiC particles and Si diffuses into the Cu47Ti34Zr11Ni8 matrix stabilizing the supercooled liquid against crystallization. The small Si addition between 0.5 and 1 at. % increases the attainable maximum thickness of glassy ingots from 4 mm for Cu–Ti–Zr–Ni alloys to 7 mm for Cu–Ti–Zr–Ni–Si alloys. DSC analyses show that neither the thermodynamics nor the kinetics of the alloy are affected significantly by the Si addition. This suggests that Si enhances the glass forming ability by chemically passivating impurities such as oxygen and carbon that cause heterogeneous nucleation in the melt.

Anin situ study of plasticization of polymers by high-pressure gases

Abstract: A high-pressure differential scanning calorimetric technique is described for studying polymer plasticization by compressed gases at pressures to 100 atm. The in situ measurements avoid problems due to gas desorption encountered with conventional DSCs, thus providing an accurate way to determine the change in glass transition temperature, Tg, with pressure, p. The entire Tg–p curve can be established in less than 2 days. The glass transition was observed as a sharp step in the case of 100–200-μm thin samples, whereas thicker samples gave a broad transition; highly reproducible results were obtained for the thin samples. For PS–CO2, the measured Tgs under various pressures were found to be in good agreement with literature values. Results for the systems PS–HFC134a, PVC–CO2, and PC–CO2 are also reported. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 977–982, 1998

On the glass transition in ultrathin polymer films of different molecular architecture

Abstract: Optical waveguide spectroscopy has been employed to monitor the temperature dependence of the refractive index, K = ∂n/∂T, and of the thermal (linear) expansivity, β = (∂d/∂T). d -1 , for thin poly(methyl methacrylate) (PMMA) films of thicknesses d > 0.2 μm independently from each other. A break in the linearized slope of n (and d) was identified as the film glass transition temperature, T g . For thinner samples only surface plasmon resonances could be analyzed, but the kinetic mode, i.e. reflectivity-versus -temperature scans, still gave reliable T g values. The PMMA films of different thicknesses, ranging from d 3 nm to d 800 nm, were prepared by three different techniques: by spin-casting from solution, by the grafting-from approach and by the Langmuir/Blodgett/Kuhn technique. We found that all these films prepared on hydrophobic substrates show the expected decrease of T g for ultrathin samples (d < 100 nm). However, this behavior is independent of the strongly varying intramolecular architecture and organization of the macromolecular chains in the various films.

Influence of water on the crispness of cereal-based foods: acoustic, mechanical, and sensory studies

Abstract: The texture properties of crispy breads were studied as a function of water content using compression tests, acoustic measurements and sensory analysis. The addition of water slowly lowers the characteristics associated with crispness up to 9%, after which they steeply decrease. Dynamic Mechanical Thermal Analysis (DMTA) measurements were carried out, at room temperature and at 5 Hz, on the same bread samples. The evolution of the textural properties of crispy bread are accompanied by an increase of the loss factor prior to the one associated to the glass transition. It was therefore proposed that the effect of water on the brittle character, on crispness and on the intensity of the sound emitted at fracture were due to the onset of molecular motions preceding or accompanying the glass transition.