Showing papers on "Glass transition published in 2010"

Effects of Thermal Annealing Upon the Morphology of Polymer-Fullerene Blends

Abstract: Grazing incidence X-ray scattering (GIXS) is used to characterize the morphology of poly(3-hexylthiophene) (P3HT)–phenyl-C61-butyric acid methyl ester (PCBM) thin film bulk heterojunction (BHJ) blends as a function of thermal annealing temperature, from room temperature to 220 °C. A custom-built heating chamber for in situ GIXS studies allows for the morphological characterization of thin films at elevated temperatures. Films annealed with a thermal gradient allow for the rapid investigation of the morphology over a range of temperatures that corroborate the results of the in situ experiments. Using these techniques the following are observed: the melting points of each component; an increase in the P3HT coherence length with annealing below the P3HT melting temperature; the formation of well-oriented P3HT crystallites with the (100) plane parallel to the substrate, when cooled from the melt; and the cold crystallization of PCBM associated with the PCBM glass transition temperature. The incorporation of these materials into BHJ blends affects the nature of these transitions as a function of blend ratio. These results provide a deeper understanding of the physics of how thermal annealing affects the morphology of polymer–fullerene BHJ blends and provides tools to manipulate the blend morphology in order to develop high-performance organic solar cell devices.

Bulk Metallic Glasses

Abstract: Introduction Motivation Advanced Materials Rapid Solidification Processing Mechanical Alloying Outline of the Book Metallic Glasses Distinction between Crystals and Glasses Differences between Amorphous Alloys and Metallic Glasses The Concepts of Glass Formation Thermodynamics and Kinetics of Glass Formation Methods to Synthesize Metallic Glasses Bulk Metallic Glasses Potential Resources of Literature on Metallic Glasses Glass-Forming Ability of Alloys Critical Cooling Rate Reduced Glass Transition Temperature Deep Eutectics Topological Models Bulk Metallic Glasses Inoue Criteria Exceptions to the Above Criteria New Criteria Transformation Temperatures of Glasses Thermodynamic Modeling Structural and Topological Parameters Physical Properties Computational Approaches Miscellaneous Criteria Criteria for Glass Formation by Non-Solidification Methods Synthesis of Bulk Metallic Glasses Principles of Rapid Solidification Processing General Techniques to Achieve High Rates of Solidification Melt Spinning Bulk Metallic Glasses Bulk Metallic Glass Casting Methods Bulk Metallic Glass Composites Mechanical Alloying Bulk Metallic Glass Foams Crystallization Behavior Methodology Crystallization Modes in Melt-Spun Ribbons Differences in the Crystallization Behavior between Melt-Spun Ribbons and Bulk Metallic Glasses Thermal Stability of Metallic Glasses Crystallization Temperatures and Their Compositional Dependence Annealing of Bulk Metallic Glasses Effect of Environment Effect of Pressure during Annealing Physical Properties Density Thermal Expansion Diffusion Electrical Resistivity Specific Heat Viscosity Corrosion Behavior Terminology and Methodology Copper-Based Bulk Metallic Glasses Iron-Based Bulk Metallic Glasses Magnesium-Based Bulk Glassy Alloys Nickel-Based Bulk Metallic Glasses Titanium-Based Bulk Metallic Glasses Zirconium-Based Bulk Metallic Glasses Other Bulk Metallic Glassy Alloys Mechanical Behavior Deformation Behavior Deformation Maps Temperature Rise at Shear Bands Strength Ductility Fatigue Yield Behavior BMG Composites Magnetic Properties Soft Magnetic Materials Nanocrystalline Alloys Hard Magnetic Materials Applications Special Characteristics of Bulk Metallic Glasses Structural Applications Chemical Applications Magnetic Applications Miscellaneous Applications Epilogue Size and Shape Mechanical Properties Magnetic Properties Fundamental Properties Index References appear at the end of each chapter.

Single-layer graphene nanosheets with controlled grafting of polymer chains

Abstract: Single-layer graphene nanosheets (SLGNs) are prepared by reduction of well-exfoliated graphite oxide aided by a surfactant (sodium dodecylbenzene sulfonate, SDBS). Grafting density and polystyrene (PS) chain lengths are controlled by modulating the concentrations of diazonium compound and monomer during the grafting reaction of the initiator and the succeeding atomic transfer radical polymerization (ATRP). Atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectra and transmission electron microscopy (TEM) are used to confirm the single-layer structure of graphene sheets, covalent bonding at the interface, and distribution uniformity of grafting PS chains at the SLGN surface. Thermogravimetric analysis (TGA) is performed to assess the control of grafting density and chain length. PS chains grafted on the SLGN surface exhibited remarkably confined relaxation behavior. An increase in the glass transition temperature (Tg) of up to 18 °C is observed for high grafting density, low molecular weight polymer-grafted graphene samples. The low grafting density, high molecular weight sample shows an increase in Tg of ∼9 °C, which is attributed to superior heat conduction efficiency. The measured thermal conductivity for the PS composite film with 2.0 wt% SLGNs increase by a factor of 2.6 compared to that of the pure PS.

Critical-like behaviour of glass-forming liquids

Abstract: As a liquid approaches its glass transition its dynamics slow down and simultaneously the material becomes more heterogeneous. A static structural heterogeneity, now shown to be widely present in glass-forming liquids, is suggested to be the origin of this dynamic heterogeneity that links structural parameters to the glass transition.

Glass Transition Dynamics and Surface Layer Mobility in Unentangled Polystyrene Films

Abstract: Most polymers solidify into a glassy amorphous state, accompanied by a rapid increase in the viscosity when cooled below the glass transition temperature (T(g)). There is an ongoing debate on whether the T(g) changes with decreasing polymer film thickness and on the origin of the changes. We measured the viscosity of unentangled, short-chain polystyrene films on silicon at different temperatures and found that the transition temperature for the viscosity decreases with decreasing film thickness, consistent with the changes in the T(g) of the films observed before. By applying the hydrodynamic equations to the films, the data can be explained by the presence of a highly mobile surface liquid layer, which follows an Arrhenius dynamic and is able to dominate the flow in the thinnest films studied.

Carbon Nanotubes with Temperature-Invariant Viscoelasticity from –196° to 1000°C

Abstract: Viscoelasticity describes the ability of a material to possess both elasticity and viscosity. Viscoelastic materials, such as rubbers, possess a limited operational temperature range (for example, for silicone rubber it is –55° to 300°C), above which the material breaks down and below which the material undergoes a glass transition and hardens. We created a viscoelastic material composed from a random network of long interconnected carbon nanotubes that exhibited an operational temperature range from –196° to 1000°C. The storage and loss moduli, frequency stability, reversible deformation level, and fatigue resistance were invariant from –140° to 600°C. We interpret that the thermal stability stems from energy dissipation through the zipping and unzipping of carbon nanotubes at contacts.

Bulk metallic glass matrix composites

Abstract: A method of forming bulk metallic glass engineering materials, and more particularly a method for forming coarsening microstructures within said engineering materials is provided. Specifically, the method forms ‘designed composites’ by introducing ‘soft’ elastic/plastic inhomogeneities in a metallic glass matrix to initiate local shear banding around the inhomogeneity, and matching of microstructural length scales (for example, L and S) to the characteristic length scale R P (for plastic shielding of an opening crack tip) to limit shear band extension, suppress shear band opening, and avoid crack development.

Glass-based seals for solid oxide fuel and electrolyzer cells - A review

Abstract: High temperature hermetic seal is essential for utilizing the full potentials of planar solid oxide fuel/electrolyzer cells. A seal glass needs to have excellent thermal and chemical stabilities, mechanical integrity, and sealing ability in stringent oxidizing and reducing environments and for hundreds of thermal cycles. Comprehensive analysis and understanding are needed in the design of a seal glass in order to meet the demanding requirements. In this review, seal requirements and the advantages of glass-based seals are first discussed. Different glass compositions are reviewed from thermal, chemical, mechanical, and electrical property point of view. Based on these considerations, glass composition design approaches are provided that aid in search of the best seal glass that can offer all the desired properties and stabilities. Required thermal properties such as thermal expansion coefficient, glass transition temperature, and softening temperature have been achieved in several alkaline earth borosilicate glass systems. Interfacial compatibility with other cell components has also been obtained for several alkaline earth borosilicate glass systems. However, long-term thermal and chemical stabilities are yet to be achieved. Among all the glass systems studied, a boron-free SrO–La2O3–Al2O3–SiO2 seal glass has been specifically discussed because it has met all the thermal and chemical properties along with high thermal and chemical stabilities. For future endeavors, the relationships between seal glass constituents, glass network structures, required thermal, chemical, mechanical, and electrical properties need to be established in order to improve sealing performance while maintaining design flexibility and low fabrication cost.

Glass transition temperature and its relevance in food processing.

Abstract: Amorphous, noncrystalline solids are typical of low water content and frozen foods. Solids in these foods, e.g., confectionary, dehydrated foods, cereal foods, and frozen foods, often form nonequilibrium glass-like structures. The glassy state of the solids forms during food processing in a reversible glass transition. Vitrification can occur in numerous glassy states that exhibit various relaxations around the glass transition. The success of freeze drying, spray drying, and extrusion and the stability of dehydrated foods against flow, collapse, and crystallization is based on the control of the glassy state during the dehydration process and storage. Encapsulation processes often use glass-forming materials to entrap dispersed components or improve retention of volatiles. Plasticization of the noncrystalline structures by temperature or water reduce relaxation times exponentially above the glass transition, which results in rapid deterioration. Critical values for water activity and water content expres...

Characterization of glass solutions of poorly water-soluble drugs produced by melt extrusion with hydrophilic amorphous polymers.

Abstract: Indomethacin, lacidipine, nifedipine and tolbutamide are poorly soluble in water and may show dissolution-related low oral bioavailability. This study describes the formulation and characterization of these drugs as glass solutions with the amorphous polymers polyvinylpyrrolidone (PVP) and polyvinylpyrrolidone-co-vinyl acetate by melt extrusion. The extrudates were compared with physical mixtures of drug and polymer. X-ray powder diffraction, thermal analysis, infrared spectroscopy, scanning electron microscopy, HPLC, moisture analysis and dissolution were used to examine the physicochemical properties and chemical stability of the glass solutions prepared by melt extrusion at a 1:1 drug/polymer ratio. Depending on the temperature used, melt extrusion produced amorphous glass solutions, with markedly improved dissolution rates compared with crystalline drug. A significant physico-chemical interaction between drug and polymer was found for all extrudates. This interaction was caused by hydrogen bonding (H-bonding) between the carbonyl group of the pyrrole ring of the polymer and a H-donor group of the drug. Indomethacin also showed evidence of H-bonding when physical mixtures of amorphous drug and PVP were prepared. After storage of the extrudates for 4-8 weeks at 25 degrees C/75% relative humidity (RH) only indomethacin/polymer (1:1) extrudate remained totally amorphous. All extrudates remained amorphous when stored at 25 degrees C/< 10% RH. Differences in the physical stability of drug/polymer extrudates may be due to differences in H-bonding between the components.

Influence of magnesia on the structure and properties of bioactive glasses

Abstract: Five bioactive glasses based on 49.5SiO2–1.1P2O5–(23.0(1 − x))CaO–xMgO–26.4Na2O mol.% were synthesised where CaO was replaced progressively on a molar basis by MgO (where 0 ⩽ x ⩽ 1). The glasses were characterised by 31P and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, dilatometry, differential thermal analysis (DTA) and density measurements with the aim of gaining a comprehensive understanding of the structure–property relationships. The 31P NMR spectra of the glasses exhibited well-defined resonances at ca. 9–14 ppm, corresponding to a Q0 orthophosphate environment. Full substitution of MgO caused a downfield shift of ca. 5 ppm, implying the preferential association of sodium with the orthophosphate species. The 29Si NMR spectra exhibited a peak at ca. −79 ppm, indicating a Q2 silicon species, along with a shoulder at ca. −90 ppm, corresponding to a Q3 species. On addition of MgO, the Q3 shoulder was seen to progressively increase in magnitude. These results suggest that MgO, rather than depolymerising the silicate network by acting as a network modifier, is behaving in part, as an intermediate oxide. Deconvolution of the 29Si NMR peaks suggest that, although 86% of the magnesium oxide is acting traditionally as a network modifying cation, up to 14% of the magnesium oxide is entering the silicate network as tetrahedral, MgO4, removing network modifying ions for charge compensation and resulting in the observed polymerisation of the silicate network. Correspondingly, both the glass transition temperature and dilatometric softening point values were seen to decrease whilst the thermal expansion coefficient values rose, with increasing MgO content. This was due to the significantly lower bond strength, of Mg–O compared to Si–O, weakening the glass network.

Strain-Induced Molecular Ordering in Polylactide upon Uniaxial Stretching

Abstract: This study deals with the tensile drawing behavior of a polylactide material containing 4% of d-stereoisomer units, in the amorphous state. The draw temperature domain spanned from the glass transition to the onset of thermal crystallization, namely 70−100 °C. The stress−strain curves exhibit a strain-hardening strongly sensitive to the draw temperature regarding both the onset and the slope of the phenomenon. A detailed structural investigation reveals that various strain-induced phase changes take place depending on the draw temperature. For Td = 70 °C, a mesomorphic form develops from the strain-oriented amorphous chains, starting at a strain level e ≈ 130%. In the case Td = 90 °C, a well-defined crystalline phase grows beyond the strain e ≈ 250%. In the midtemperature range, i.e. Td = 80 °C, both the mesomorphic and the crystalline phases are generated in parallel. In all cases, the final weight content of ordered phases at rupture was roughly 30%, irrespective of their form. The observed evolution wi...

New Insights on the strain-induced mesophase of poly(D,L-lactide): in situ WAXS and DSC study of the thermo-mechanical stability

Abstract: This work deals with the study of the mesomorphic form or mesophase induced by tensile drawing from the amorphous state of a polylactide material containing 4 mol % of d-stereoisomer units. Investigations have been carried out over the draw temperature domain 45−90 °C, i.e. an interval spanning roughly ±20 °C about the glass transition temperature. In situ WAXS experiments during drawing, stress relaxation, and/or heating of stretched samples invariably showed the strain-induced occurrence of the mesophase as far as temperature did not exceed 70 °C. This seems to be the upper stability temperature of the mesophase identified in a previous study. DSC traces upon heating of drawn samples exhibit a post glass transition endothermic peak similar to the enthalpy relaxation phenomenon observed for aged polymers. The amplitude of this strain-induced endotherm proved to be strongly dependent on draw temperature and draw ratio. Draw ratio also appeared to strongly influence the temperature domain of cold crystalli...

Transport properties of glass-forming liquids suggest that dynamic crossover temperature is as important as the glass transition temperature

Abstract: It is becoming common practice to partition glass-forming liquids into two classes based on the dependence of the shear viscosity η on temperature T. In an Arrhenius plot, ln η vs 1/T, a strong liquid shows linear behavior whereas a fragile liquid exhibits an upward curvature [super-Arrhenius (SA) behavior], a situation customarily described by using the Vogel–Fulcher–Tammann law. Here we analyze existing data of the transport coefficients of 84 glass-forming liquids. We show the data are consistent, on decreasing temperature, with the onset of a well-defined dynamical crossover η×, where η× has the same value, η× ≈ 103 Poise, for all 84 liquids. The crossover temperature, T×, located well above the calorimetric glass transition temperature Tg, marks significant variations in the system thermodynamics, evidenced by the change of the SA-like T dependence above T× to Arrhenius behavior below T×. We also show that below T× the familiar Stokes–Einstein relation D/T ∼ η-1 breaks down and is replaced by a fractional form D/T ∼ η-ζ, with ζ ≈ 0.85.

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Abstract: Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of −31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of −53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.

Glassy Dynamics and Glass Transition in Nanometric Thin Layers of Polystyrene

Abstract: Broadband dielectric spectroscopy (BDS), spectroscopic vis-ellipsometry (SE), X-ray reflectometry (XRR), and alternating current (ACC) as well as differential scanning calorimetry (DSC) are combined to study glassy dynamics and the glass transition in nanometric thin (≥5 nm) layers of polystyrene (PS) having widely varying molecular weights (27 500−8 090 000 g/mol). For the dielectric measurements two sample geometries are employed, the common technique using evaporated electrodes and a recently developed approach taking advantage of nanostructures as spacers. All applied methods deliver the concurring result that deviations from glassy dynamics and from the glass transition of the bulk do not exceed margins of ±3 K independent of the layer thickness and the molecular weight of the polymer under study. Our findings are discussed in the context of the highly controversial literature and prove that an appropriate sample preparation is of paramount importance.

Dynamical transition of protein-hydration water.

Abstract: Thin layers of water on biomolecular and other nanostructured surfaces can be supercooled to temperatures not accessible with bulk water. Chen et al. [Proc. Natl. Acad. Sci. U.S.A. 103, 9012 (2006)] suggested that anomalies near 220 K observed by quasielastic neutron scattering can be explained by a hidden critical point of bulk water. Based on more sensitive measurements of water on perdeuterated phycocyanin, using the new neutron backscattering spectrometer SPHERES, and an improved data analysis, we present results that show no sign of such a fragile-to-strong transition. The inflection of the elastic intensity at 220 K has a dynamic origin that is compatible with a calorimetric glass transition at 170 K. The temperature dependence of the relaxation times is highly sensitive to data evaluation; it can be brought into perfect agreement with the results of other techniques, without any anomaly.

Glass Fibers with Carbon Nanotube Networks as Multifunctional Sensors

Abstract: A simple approach to deposit multiwalled carbon nanotube (MWNT) networks onto glass fiber surfaces achieving semiconductive MWNT-glass fibers is reported, along with application of fiber/polymer interphases as insitu multifunctional sensors. This approach demonstrates for the first time that the techniques of conducting electrical resistance measurements could be applicable to glass fibers for in situ sensing of strain and damage; the techniques were previously limited to conductive and semiconductive materials. The electrical properties of the single MWNT―glass fiber and the "unidirectional" fiber/epoxy composite show linear or nonlinear stress/ strain, temperature, and relative humidity dependencies, which are capable of detecting piezoresistive effects as well as the local glass transition temperature. The unidirectional composites containing MWNT-glass fibers exhibit ultrahigh anisotropic electrical properties and an ultralow electrical percolation threshold. Based on this approach, the glass fiber—the most widely used reinforcement in composites globally—along with the surface electrical conductivity of MWNTs will stimulate and realize a broad range of multifunctional applications.

Stress-temperature scaling for steady-state flow in metallic glasses.

Abstract: Through computer simulation of steady-state flow in a Zr50Cu40Al10 metallic glass using a set of realistic potentials we find a simple scaling relationship between temperature and stress as they affect viscosity. The scaling relationship provides new insight into the microscopic mechanism of shear flow in the glassy state, in terms of the elastic energy of the applied stress modifying the local energy landscape. The results suggest that the plastic flow and mechanical failure in metallic glasses are consequences of stress-induced glass transition.

Assemblies of Titanium Dioxide-Polystyrene Hybrid Nanoparticles for Dielectric Applications

Abstract: Macroscopic assemblies of hybrid nanoparticles composed of titanium dioxide core surrounded by covalently grafted polystyrene corona have been prepared by a combination of phosphonate coupling and “click” chemistry. The attached polymer chains existed in the brush regime, with grafting density inversely proportional to the degree of polymerization. Solution casting afforded preparation of robust films of the composite material where all the polymer chains were covalently bound to the uniformly distributed inorganic particles. Inorganic content from 60 to 80 wt % (27 to 50 vol %) was obtained by varying the molecular weight of polystyrene as well as by using the mixture of high and low molecular weight polymer for grafting. The TiO2 grafted with 105 g/mol polystyrene had a volume fraction of nanoparticles of 27% and exhibited glass transition at 110 °C and ∼100% extensibility above Tg. Thin films of this material had a dielectric constant of 6.4 and a dielectric loss of 0.04 at 1 kHz.

Strontium containing bioactive glasses: Glass structure and physical properties

Abstract: The influence of substituting strontium for calcium in the following glass series 49.46 SiO 2 –1.07 P 2 O 5 –(23.08-X) CaO–X SrO–26.38 Na 2 O was studied on the physical properties. Solid state nuclear magnetic resonance and vibrational spectroscopy showed that the glasses were predominantly composed of Q 2 silicate chains. Addition of strontium did not result in any structural alteration of the glass network due to the similar role of SrO compared with that of CaO. The density increased with strontium content whilst the oxygen density decreased indicating a more expanded glass network. The glass transition temperature reduced with strontium substitution in a linear fashion and there was no evidence of a mixed alkaline earth effect with a lower than expected glass transition temperature. Dilatometric softening points also reduced with increasing strontium content, whilst the thermal expansion coefficients increased. The results are consistent with a weaker network as a result of the lower charge to size ratio of Sr 2+ compared to Ca 2+ .