Showing papers on "Glass transition published in 2000"

Three-Dimensional Direct Imaging of Structural Relaxation Near the Colloidal Glass Transition

Abstract: Confocal microscopy was used to directly observe three-dimensional dynamics of particles in colloidal supercooled fluids and colloidal glasses. The fastest particles moved cooperatively; connected clusters of these mobile particles could be identified; and the cluster size distribution, structure, and dynamics were investigated. The characteristic cluster size grew markedly in the supercooled fluid as the glass transition was approached, in agreement with computer simulations; at the glass transition, however, there was a sudden drop in their size. The clusters of fast-moving particles were largest near the α-relaxation time scale for supercooled colloidal fluids, but were also present, albeit with a markedly different nature, at shorter β-relaxation time scales, in both supercooled fluid and glass colloidal phases.

Surfactant-assisted processing of carbon nanotube/polymer composites

Abstract: Interfacial interaction is one of the most critical issues in carbon nanotube/polymer composites In this paper the role of nonionic surfactant is investigated With the surfactant as the processing aid, the addition of only 1 wt % carbon nanotubes in the composite increases the glass transition temperature from 63 °C to 88 °C The elastic modulus is also increased by more than 30% In contrast, the addition of carbon nanotubes without the surfactant only has moderate effects on the glass transition temperature and on the mechanical properties This work points to the pathways to improve dispersion and to modify interfacial bonding in carbon nanotube/polymer composites

Glass transitions and dynamics in thin polymer films: Dielectric relaxation of thin films of polystyrene

Abstract: The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d

Crystallization Temperature-Dependent Crystal Orientations within Nanoscale Confined Lamellae of a Self-Assembled Crystalline−Amorphous Diblock Copolymer

Abstract: For a lamella-forming poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer (MnPEO = 87K and MnPS = 92K), the glass transition temperature of the PS blocks is 62 °C, and the melting temperature of the PEO crystals is around 51 °C when the sample is crystallized below 40 °C The PEO blocks thus crystallize in a one-dimensionally confined lamellar space of 88 nm, as studied recently by one-dimensional small-angle X-ray scattering (SAXS) and transmission electron microscopy In this report, the crystal orientation (the c-axis of the PEO crystals) within nanoscale confined lamellae has been investigated using combined two-dimensional SAXS and wide-angle X-ray scattering experiments The c-axis orientation in the PEO crystals is observed for the first time to change from random to perpendicular, then to inclined, and finally to parallel to the lamellar surface normal, depending only on the crystallization temperature (Tc) Detailed crystallographic analyses indicate that the c-axis orientati

Solvent mobility and the protein 'glass' transition.

Abstract: Proteins and other biomolecules undergo a dynamic transition near 200 K to a glass-like solid state with small atomic fluctuations. This dynamic transition can inhibit biological function. To provide a deeper understanding of the relative importance of solvent mobility and the intrinsic protein energy surface in the transition, a novel molecular dynamics simulation procedure with the protein and solvent at different temperatures has been used. Solvent mobility is shown to be the dominant factor in determining the atomic fluctuations above 180 K, although intrinsic protein effects become important at lower temperatures. The simulations thus complement experimental studies by demonstrating the essential role of solvent in controlling functionally important protein fluctuations.

Direct observation of molecular cooperativity near the glass transition

Abstract: The increasingly sluggish response of a supercooled liquid as it nears its glass transition (for example, refrigerated honey) is prototypical of glassy dynamics found in proteins, neural networks and superconductors The notion that molecules rearrange cooperatively has long been postulated to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition Recently, cooperativity was observed and analysed in colloid glasses and in simulations of binary liquids well above the glass transition But nanometre-scale studies of cooperativity at the molecular glass transition are lacking Important issues to be resolved include the precise form of the cooperativity and its length scale, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity in an ensemble of such regions Here we describe direct observations of molecular cooperativity near the glass transition in polyvinylacetate (PVAc), using nanometre-scale probing of dielectric fluctuations Molecular clusters switched spontaneously among two to four distinct configurations, producing random telegraph noise Our analysis of these noise signals and their power spectra reveals that individual clusters exhibit transient dynamical heterogeneity and non-exponential kinetics

Thermal Probe Measurements of the Glass Transition Temperature for Ultrathin Polymer Films as a Function of Thickness

Abstract: The glass transition temperature of polystyrene and poly(methyl methacrylate) films on polar and nonpolar substrates was measured as a function of thickness using a thermal probe in contact with a polymer film. Using a technique called local thermal analysis, heat loss into the film was monitored as the temperature of the probe was ramped from ambient temperature to temperatures as high as 200 °C. The glass transition temperature was determined from a change in slope in the heat loss versus temperature plot. The Tg of polystyrene on silicon oxide decreased by as much as 25 °C below the bulk value for films 13 nm thick. The same trend in the glass transition temperature was observed for polystyrene films on silicon oxide treated with hexamethyldisilizane (HMDS). The Tg of poly(methyl methacrylate) on silicon oxide increased by up to 7 °C above the bulk value for films 18 nm thick. For poly(methyl methacrylate) on silicon oxide treated with HMDS, the Tg decreased by 10 °C below the bulk value for films 21 n...

Molecular simulation of ultrathin polymeric films near the glass transition.

Abstract: Properties such as the glass transition temperature ( T(g)) and the diffusion coefficient of ultrathin polymeric films are shown to depend on the dimensions of the system. In this work, a hard-sphere molecular dynamics methodology has been applied to simulate such systems. We investigate the influence that substrates have on the behavior of thin polymer films; we report evidence suggesting that, depending on the strength of substrate-polymer interactions, the glass transition temperature for a thin film can be significantly lower or higher than that of the bulk.

Quantifying glass transition behavior in ultrathin free-standing polymer films

Abstract: We have used Brillouin light scattering to make a detailed study of the behavior of the glass transition temperature T(g) in ultrathin, free-standing polystyrene films. The glass transitions were experimentally identified as near discontinuities in the thermal expansion. The effects of film thickness, molecular weight, and thermal history on the measured T(g) values have been investigated. While the size of the glass transition effects was comparable for all molecular weights, a complicated M(n) dependence suggested a separation of the results into two regimes, each dominated by a different length scale: a low M(n) regime controlled by a length scale intrinsic to the glass transition and a high M(n) region, where polymer chain confinement induced effects take over.

Polymer crystallization in 25-nm spheres

Abstract: Crystallization within the discrete spheres of a block copolymer mesophase was studied by time-resolved x-ray scattering. The cubic packing of microdomains, established by self-assembly in the melt, is preserved throughout crystallization by strong interblock segregation even though the amorphous matrix block is well above its glass transition temperature. Homogeneous nucleation within each sphere yields isothermal crystallizations which follow first-order kinetics, contrasting with the sigmoidal kinetics normally exhibited in the quiescent crystallization of bulk polymers.

Characteristic Length of Dynamic Glass Transition near Tg for a Wide Assortment of Glass-Forming Substances

Abstract: Dynamic heterogeneity is an active field of glass-transition research. The length scale of this heterogeneity is called the characteristic length. It can be calculated from complex heat capacity curves in the equilibrium liquid or from dynamic calorimetry curves corrected with regard to nonequilibrium. No molecular parameters or microscopic models are necessary for obtaining the length. We report the characteristic length near glass temperature for about 30 glass formers including small-molecule liquids, polymers, silicate glasses, a metallic glass, a liquid crystal, and a plastic crystal. The lengths are between 1.0 and 3.5 nm with certain cumulations between 1.0 and 2.0 nm and between 2.5 and 3.5 nm. To try a correlation to other properties, we find that at least two should be included, e.g., Angell's fragility and the distance of Tg from the crossover temperature, Tc.

Shear modulation force microscopy study of near surface glass transition temperatures

Abstract: We report results of glass transition (T(g)) measurements for polymer thin films using atomic force microscopy (AFM). The AFM mode, shear modulation force microscopy (SMFM), involves measuring the temperature-dependent shear force on a tip modulated parallel to the sample surface. Using this method we have measured the surface T(g) of thin (17-500 nm) polymer films and found that T(g) is independent of film thickness (t>17 nm), strength of substrate interactions, or even presence of substrate.

Chain mobility in the amorphous region of nylon 6 observed under active uniaxial deformation

Abstract: A specially designed solid-state deuterium nuclear magnetic resonance probe was used to examine the effect of uniaxial elongation on the chain mobility in the amorphous region of semicrystalline nylon 6. In measurements conducted near the glass transition temperature, there was measurable deformation-induced enhancement of the mobility of the amorphous chains up to the yield point. This enhanced mobility decayed once deformation was stopped. Enhanced mobility was not observed in deformation at room temperature. The mechanics of deformation can be explained by the Robertson model for glassy polymers near the glass transition temperature, which states that applied stress induces liquid-like behavior in the polymer chains.

Size and confinement effects on the glass transition behavior of polystyrene/ o -terphenyl polymer solutions

Abstract: Polystyrene $(\mathrm{PS})/o\ensuremath{-}\mathrm{terphenyl} (o\mathrm{TP})$ solutions confined to nanometer scale pores were studied by differential scanning calorimetry to investigate size and confinement effects on the glass transition. We observed two glass transitions ${T}_{g}$ in all thermograms for materials confined in the controlled pore glasses. One was at a lower temperature than the bulk state ${T}_{g}$ and the other was at a higher temperature. The lower transition temperature decreases with decreasing pore size, which is consistent with previous reports from this laboratory on small molecule glass formers and some other reports in similar systems. Although $o\mathrm{TP}$ and $o\mathrm{T}\mathrm{P}/\mathrm{P}\mathrm{S}$ are not hydrogen bonding materials, we interpret the higher temperature transition as due to the existence of an interacting layer at the pore surface. A two-layer model in which there exists a ``core'' liquid in the center surrounded by the interacting layer at the pore surface is consistent with our observations.

Reduced glass transition temperature and glass forming ability of bulk glass forming alloys

Abstract: Onset temperature (solidus) T m and offset temperature (liquidus) T l of melting of a series of bulk glass forming alloys based on Zr, La, Mg, Pd and rare-earth elements have been measured by studying systematically the melting behaviour of these alloys using DTA or DSC. Bulk metallic glass formation has been found to be most effective at or near their eutectic points and less effective for off-eutectic alloys. Reduced glass transition temperature T rg given by T g / T l is found to show a stronger correlation with critical cooling rate or critical section thickness for glass formation than T rg given by T g / T m .

Thermodynamics of Cu47Ti34Zr11Ni8, Zr52.5Cu17.9Ni14.6Al10Ti5 and Zr57Cu15.4Ni12.6Al10Nb5 bulk metallic glass forming alloys

Abstract: The differences in the thermodynamic functions between the liquid and the crystalline states of three bulk metallic glass forming alloys, Cu47Ti34Zr11Ni8, Zr52.5Cu17.9Ni14.6Al10Ti5, and Zr57Cu15.4Ni12.6Al10Nb5, were calculated. The heat capacity was measured in the crystalline solid, the amorphous solid, the supercooled liquid, and the equilibrium liquid. Using these heat capacity data and the heats of fusion of the alloys, the differences in the thermodynamic functions between the liquid and the crystalline states were determined. The Gibbs free energy difference between the liquid and the crystalline states gives a qualitative measure of the glass forming ability of these alloys. Using the derived entropy difference, the Kauzmann temperatures for these alloys were determined.

Shape memory polyurethane or polyurethane-urea polymers

Abstract: The present invention relates to a shape memory polyurethane or polyurethane-urea polymer including a reaction product of: (A) (a) silicon-based macrodiol, silicon-based macrodiamine and/or polyether of the formula (I): A-[(CH2)m-O]n-(CH2)m-A', wherein A and A´ are endcapping groups; m is an integer of 6 or more; and n is an integer of 1 or greater; (b) a diisocyanate; and (c) a chain extender; or (B) (b) a diisocyanate: and (c) a chain extender, said polymer having a glass transition temperature which enables the polymer to be formed into a first shape at a temperature higher than the glass transition temperature and maintained in said first shape when the polymer is cooled to a temperature lower than the glass transition temperature, said polymer then being capable of resuming its original shape on heating to a temperature higher than the glass transition temperature. The present invention also relates to a shape memory composition which includes a blend of two or more of the shape memory polyurethane of polyurethane-urea polymers defined above or at least one shape memory polyurethane or polyurethane-urea polymer defined above in combination with another material. The present invention further relates to processes for preparing, materials having improved mechanical properties, clarity, processability, biostability and/or degradation resistance and devices or articles containing the shape memory polyurethane or polyurethane-urea polymer and/or composition defined above.

The thermophysical properties of bulk metallic glass-forming liquids

Abstract: Bulk metallic glass-forming liquids are alloys with typically three to five metallic components that have a large atomic-size mismatch and a composition close to a deep eutectic. They are dense liquids with small free volumes and viscosities that are several orders of magnitude higher than in pure metals or previously known alloys. In addition, these melts are energetically closer to the crystalline state than other metallic melts due to their high packing density in conjunction with a tendency to develop short-range order. These factors lead to slow crystallization kinetics and high glass-forming ability. Crystallization kinetics is very complex, especially in the vicinity of the glass transition, due to the influence of phase separation and the decoupling of the diffusion constants of the different species.