Volume relaxation in amorphous and semicrystalline PET

Two types of polyethyleneterephthalate (PET) were investigated, one nearly amorphous and the other highly crystallized. DSC analysis and mercury-in-glass dilatometry were used to determine the effect of crystalline phase content on the thermal behavior of amorphous phase. Increasing portion of crystals caused an increase in glass transition temperature (T _g) and broadening of the transition zone. Thermal expansion coefficient and specific heat decreased. The amount of rigid amorphous fraction, RAF, was calculated to be around 21–26%. Volume relaxation measurements initiated by temperature down-jump from the equilibrium above T _g to several temperatures in the vicinity of T _g showed considerably reduced relaxation rate for semicrystalline PET.     

Relaxation behaviour of the amorphous components of wood

The viscoelastic properties of mod were investigated using dynamic mechanical thermal analysis and differential scanning calorimetry. Under a limited set of conditions, two separate glass transitions (T _g) could be identified with both techniques. These two transitions were identified as arising from the amorphous lignin and hernicellulose matrix in the wood cell wall. Moisture dramatically affected the temperature at which the two dispersions occurred and, consequently, the ability to resolve their independent responses. The relationship betweenT _g and moisture for both components could be modelled with the Kwei equation, which accounts for the presence of secondary interactions. Annealing and specific interactions of a series of organic diluents were wed in an attempt to enhance the resolution of the two components values ofT _g. Time-temperature superposition was shown to be applicable to wood plasticized with ethyl formamide, following Williams-Landel-Ferry behaviour over the temperature rangeT _g toT _g + 85° C. These observations allow certain conclusions to be drawn concerning the applicability of existing models of the wood cell wall's supermolecular morphology. Most notably, models of thein situ morphology of the three wood components can be limited to those which consider the amorphous matrix of lignin and hemicellulose to be immiscible.     

Effect of norbornene content on deformation properties and hot embossing of cyclic olefin copolymers

The dynamic mechanical behaviour of a series of cyclic olefin copolymers (COCs) with varying norbornene content has been examined in the vicinity of the glass transition temperature, T _g. Using dynamic mechanical thermal analysis (DMTA), the temperature of the glass transition in COC increased linearly with increase in % norbornene. Above T _g, the magnitude of the elastic storage modulus, E′, decreased exponentially with rise in temperature for all of the copolymers. The loss modulus, E″, has also sharply decreased at temperatures above the transition with a levelling-off in E″ at ≥20 °C above T _g for all grades. The results of DMTA have been used in the identification of the optimum conditions for hot embossing experiments. Hot embossing of COC at ≥20 °C above the transition temperature in a region of viscous liquid flow has resulted in a full replication of channel depth without cracking or distortion.     

Preparation and superconductivity of NbN powder by a vapour phase reaction

The formation of superconducting NbN powders by the vapour phase reaction of the NbCl₄-NH₃-H₂ system has been investigated. The properties of NbN powders changed with the reaction temperature, mixing temperature of NbCl₄ and NH₃, and gas composition ratio ([NH₃]/[NbCl₄]). The reaction system gave NbN powders consisting of particles with sizes less than 50 nm. The process of particle formation is discussed. The NbN powders produced had vacancies at both the Nb and N sublattices. The vacancies influenced the superconducting transition temperature (T _c) of NbN, the highestT _c observed being 14.1 K.     

Thermoanalytical characterization of epoxy matrix-glass microballoon syntactic foams

Syntactic foams are finding new applications where their thermal stability and high temperature response are important. Therefore, the high temperature response of these advanced composites needs to be characterized and correlated with various material parameters. The present study is aimed at evaluating the effect of microballoon (hollow particle) volume fraction (Φ) and wall thickness (w) on thermoanalytical characteristics of epoxy matrix syntactic foams containing glass microballoons. These composites are characterized to determine the glass transition temperature (T _g), the weight loss, and the char yield. It is observed that T _g decreases and the char yield increases due to the presence of microballoons in the resin. The T _g is increased with an increase in Φ but is not significantly affected by w. The thermal stability is increased by increasing w and is relatively less sensitive to Φ. Understanding the relations between thermal properties of syntactic foams, the microballoon wall thickness, and microballoon volume fraction will help in developing syntactic foams optimized for mechanical as well as thermal characteristics. Due to the increased interest in functionally graded syntactic foams containing a gradient in microballoon volume fraction or wall thickness, the results of the present study are helpful in better tailoring these materials for given applications.     

Storage modulus and glass transition behaviour of CdS/PMMA nanocomposites

The storage modulus and glass transition temperature (T _g) of CdS/PMMA nanocomposites have been evaluated as a function of concentration of CdS nanoparticles. CdS particles have been synthesised via chemical route using cadmium acetate, thiourea and dimethylformamide. The solution-based processing has been used to prepare PMMA composites with CdS nanoparticles at different filler concentration. Size and shape of CdS nanoparticles in PMMA have been determined with the help of small angle X-ray scattering and transmission electron microscope measurements. Dynamical mechanical analysis was carried out on the CdS/PMMA nanocomposites to study storage modulus and tan?δ. It is observed that CdS nanoparticles enhance the storage modulus and T _g for composites. The storage modulus and T _g show the maximum value of 6?wt.% of CdS nanoparticles embedded PMMA composite. The results indicate that the 6?wt.% of CdS nanoparticles in PMMA matrix provides more stability to the composite over the other composites.     

Physical ageing and glass transition in amorphous polymers as revealed by microhardness

Microhardness (MH) data as a function of temperature for two amorphous polymers [poly(methylmethacrylate) and poly(vinylacetate)] and two semicrystalline polymers [poly(ethyleneterephthalate) and poly(arylether ether ketone)] quenched into the amorphous state are presented. It is shown that MH can conveniently detect the glass transition temperature (T _g) for the above mentioned polymers. Molecular rearrangements taking place above and belowT _g, such as physical ageing leading to a more compact molecular packing, and thermal expansion can also be followed by means of MH measurements. Finally, the presence of a crystalline phase in these materials has been shown to shift theT _g value towards higher temperatures.     

Continuous observation of microstructural degradation during tensile loading of particle reinforced aluminium matrix composites

Abstract

The microstructural degradation of aluminium alloy composites by external tensile loading was continuously observed by in situ scanning electron microscopy tensile testing. The composites, which contained spherical Al₂O₃ and angular SiC particles, were prepared by the powder extrusion method. Some microcracks were initiated at small plastic strains after yielding in both composites by inteliace debonding and particle cracking. Angular particles generate microvoids at smaller strains than spherical particles. The microcracks do not propagate with increasing external loading because of the ductility of the matrix, but a number of new microcracks developed just before failure. Most microcracks are due to interface debonding rather than particle cracking. Many dimples on the matrix aluminium alloy are observed on some particles seen in the fracture surface, which proves that there is relatively strong bonding between the matrix and the particles in the composite produced by powder extrusion. However, part of the original surface observed on the debonded particles indicates that an incompletely bonded interface also exists in the composites.

MST /3111     

An approach to prepare defect-free PES/MFI-type zeolite mixed matrix membranes for CO2/N2 separation

Polyethersulfone (PES) and MFI-type zeolite mixed matrix membranes (MMMs) were successfully fabricated by a novel particle suspension impregnating and solution-casting method. The effects of two different membrane preparation methods on the membrane morphology were investigated by scanning electron microscopy; the particle crystal type and distribution were explored by X-ray diffraction and fourier transform attenuated total reflectance infrared spectroscopy. The results showed that the MFI-zeolite particles were successfully synthesized, with particle diameter around 250 nm. Distribution and dispersion of MFI-zeolite particles suspended in the polymer matrix are homogeneous and uniform. The thermal analysis indicates that the glass transition temperature (T _g) of MMMs is around 230 °C and the initial degradation temperature is up to 460 °C in air. The increment of T _g compared to the neat PES confirms that the polymer chain rigidification is induced by zeolite particles. The MMMs have been evaluated by the CO₂/N₂ separation factor and permeability measured as a function of permeation temperature, the maximum separation factor of the PES/10 % MFI-silica zeolite for the pure gas reaches 35 at 25 °C.     

Switching between Crystallization from the Glassy and the Undercooled Liquid Phase in Phase Change Material Ge2Sb2Te5

Controlling crystallization kinetics is key to overcome the temperature–time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 T_m to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge₂Sb₂Te₅ with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature T_g. Moreover, highly unusual for glass‐forming systems, crystallization at conventional heating rates is observed more than 50 °C below T_g, where the atomic mobility should be vanishingly small.     

Ultrasonic velocities and thermal expansion coefficients of amorphous Se80Te20 and Se90Te10 alloys near glass transitions

Precise measurements of the ultrasonic velocities and thermal expansivities of amorphous Se₈₀Te₂₀ and Se₉₀Te₁₀ alloys are reported near the glass transition. The samples are produced by liquid quenching. The longitudinal and transverse velocities are measured at 10 MHz frequency using the McSkimin pulse superposition technique. The thermal expansivities,, are measured using a three-terminal capacitance bridge. The-values show a sharp maximum near the glass transition temperature,T _g. The ultrasonic velocities also show a large temperature derivative, dV/dT nearT _g. The data are discussed in terms of existing theories of the glass transition. The continuous change in shows that the glass transition is not a first-order transition, as suggested by some theories. The samples are found to be deformed by small loads nearT _g. The ultrasonic velocities and dV/dT have contributions arising from this deformation.     

Tensile behaviour of blends of poly(vinylidene fluoride) with poly(methyl methacrylate)

Blends of poly(vinylidene fluoride) (PVF₂) and poly(methyl methacrylate) (PMMA) were prepared over a wide concentration range and tested in tension at the same relative temperature below the glass transition. Testing was performed at strain rates ranging from 10 to 0.01 min^–1 at test temperatures fromT _g-40 toT _g-10. By normalizing the test temperature to fixed increments belowT _g, blends and homopolymers can be compared on the basis of PVF₂ and PMMA composition and crystallinity. In nearly all blends, under conditions favouring disentanglement, (decrease in strain rate, or increase in test temperature), the yield stress and drawing stress decreased while the breaking strain increased. For materials with about the same degree of crystallinity, those with a higher proportion of amorphous PVF₂ exhibited brittle-like behaviour as a result of interlamellar tie molecules. In the semicrystalline blends, yield stress remains high as the test temperature approachesT _g, whereas in the amorphous blends the yield stress falls to zero nearT _g. Results of physical ageing support the role of interlamellar ties which cause semicrystalline blends to exhibit ageing at temperatures aboveT _g.     

Kinetic spraying deposition behavior of bulk amorphous NiTiZrSiSn feedstock

A kinetic spraying process, which is basically a solid-state deposition process, was used for the formation of a fully amorphous coating. By using a pre-heating system for the powder carrier gas and using helium for the process gas, it was possible to form an amorphous coating. The main process parameters evaluated during this study were gas species [N₂ and He] and pre-heating temperature [RT (below T_g) and 550 °C (liquid metallic region)]. Aside from the empirical approach, in-flight particle velocity within the kinetic spraying process was measured using a SprayWatch-2i system. The deposition behavior of a NiTiZrSiSn bulk amorphous powder was observed when it was sprayed using the kinetic spraying process. In order to predict the temperature-dependent deformation behavior of the bulk amorphous material during impact, Vickers microhardness, as an indirect method, was measured at various temperatures.While the bulk amorphous feedstock material was being coated, both the kinetic and thermal energies of the in-flight particles were important. The former affected the deposition of the bulk amorphous coating, while the latter had more effect on the mechanical properties of the coating. Particle deposition behavior was considered from the viewpoint of the environmental effect, such as particle–energy combination, on the deposition behavior. The bonding of the impacting NiTiZrSiSn bulk amorphous particle was primarily caused by temperature-dependent deformation and fracture (local liquid formation) behavior.     

Kinetics of two-stage crystallization in Metglas 2826A

Crystallization kinetics of MSI and MSII stages in Metglas 2826A has been investigated by quantitative transmission electron microscopy. The volume fraction of crystallization against time curve has been found to show a plateau in the lower temperature region of annealing. It has been found that the glass transition temperature, T _g, has no effect on the sequence reversal of the crystallization reaction. It has been shown that the sequence reversal of transformation stages occurs due to the large difference in the activation energy of crystallization of MSI and MSII. In addition, the change in the morphology of the crystals obtained by annealing the amorphous alloy below and above the glass transition temperature, T _g, is reported.     

A magnesium alloy matrix composite reinforced with metallic glass

Novel light-weight materials of advanced performance are now experiencing global interest due to the strong need to reduce energy consumption in land and air transportation sectors. Here we report on a novel magnesium alloy matrix composite material. The reinforcing phase in the magnesium alloy is a fine dispersion of metallic glass particles. The composite is sintered from the powder mixture of the alloy and metallic glass at a temperature slightly above the glass transition T_g of the metallic glass particles that is close to the Mg alloy’s solidus temperature. At the compaction temperature, the metallic glass acts as a soft liquid-like binder but upon cooling it becomes the hard reinforcement component of the composite. Processing, microstructure and mechanical properties of the composite are discussed.     

The Potential of Small-Scale Fusion Experiments and the Gordon-Taylor Equation to Predict the Suitability of Drug/Polymer Blends for Melt Extrusion

The aim of this study was to investigate the use of small-scale fusion experiments and the Gordon-Taylor (GT) equation to predict whether melt extrusion of a drug with an amorphous polymer produces a stable amorphous dispersion with increased drug dissolution. Indomethacin, lacidipine, nifedipine, piroxicam, and tolbutamide were used as poorly soluble drugs. Drug/polyvinylpyrrolidone (PVP) blends were prepared at a 1:1 mass ratio. Small-scale fusion experiments were performed in a differential scanning calorimeter (DSC) and in stainless steel beakers. Extrusion was performed in a Brabender Plasti-corder. The glass transition temperatures T_g were determined by DSC. Taking an average T_g from the DSC melt, beaker melt, and GT equation accurately predicted the extrudate T_g. Physical stability of beaker melt and extrudate samples was tested by X-ray powder diffraction (XRPD) and DSC after storage at 30°C (beaker melt) or 25°C (extrudate) and less than 10%, 60%, and 75% relative humidity (RH). Beaker melts were amorphous, apart from some residual crystallinity. Extrudates were amorphous after preparation. Except for indomethacin/PVP, which remained amorphous, the crystallinity of beaker melts and extrudates increased only at 75% RH. Recrystallization occurred even when the T_g of the sample was well above the storage temperature. Chemical stability of the beaker melts and extrudates was tested by capillary electrophoresis and high-performance liquid chromatography (HPLC). Stability was slightly improved in the extrudate compared to the beaker melt. In general, the order for rate of dissolution was crystalline drug was less than the physical mixture, which was less than the drug/PVP beaker melt, which was approximately equal to the extrudate. The use of beaker melts allows a conservative estimate of the potential to melt extrude a drug. To predict physical stability, analysis of the T_g must be combined with physical stability experiments.     

Dynamic mechanical analysis of carbon/epoxy composites for structural pipeline repair

The viscoelastic behavior of a carbon fiber/epoxy matrix composite material system used for pipeline repair has been evaluated though dynamic mechanical analysis. The effects of the heating rate, frequency, and measurement method on the glass transition temperature (T_g) were studied. The increase in T_g with frequency was related to the activation energy of the glass transition relaxation. The activation energy can be used for prediction of long term performance. The measured tan delta peak T_g’s of room temperature cured and post-cured composite specimens ranged from 60 to 129 °C. Analysis of T_g data at various cure states was used to determine use temperature limits for the composite repair system.     

Dynamic mechanical properties of hydroxyapatite-ethylene vinyl acetate copolymer composites

Dynamic mechanical thermal analysis (DMTA) was carried out to explore the dependence of temperature on the viscoelasticity of composites consisting of synthetic hydroxyapatite (HAP) particulate filled ethylene vinyl acetate copolymer (EVA). Two forms of HAP, differing in their surface characteristics and particle size, were used for the study. The freeze-dried HAP (FDHAP) had a mean particle size of 49.18 μm and an irregular surface morphology. In the contrary the spray dried form of HAP (SDHAP) had a mean particle size of 5.84 (m and a spherical morphology. The dynamic mechanical analysis of the composites showed that both, particle size and morphology, had a significant effect on viscoelastic properties. A tremendous increase of the storage modulus was noted with the addition of HAP. Above the T_g of the polymer matrix, the values were marginally higher for the composites containing FDHAP than for those with SDHAP. The damping (tan δ) was found to decrease with the inclusion of HAP. A marginal upper shift in the value of the glass transition temperature (T_g) was observed for the composites fabricated from SDHAP indicating a slightly strong interaction between this HAP and the EVA matrix.     

Cu-based metallic glass particle additions to significantly improve overall compressive properties of an Al alloy

We report the development of a novel light-weight Al (520) alloy-based composite reinforced with particles of a Cu-based (Cu₅₄Zr₃₆Ti₁₀) metallic glass by mechanical milling followed by induction heated sintering. The consolidation of the composite is performed at a temperature in the super-cooled liquid region of the metallic glass just above its glass-transition temperature (T_g). Metallic glasses are a promising alternative reinforcement material for metal-matrix composites capable of producing significant strengthening along with a «friendly» sintering behavior. The mechanical milling procedures were properly established to allow reduction of the size of the metallic glass particles and their uniform distribution in the matrix. Microstructural observation of the composite did not reveal any porosity. The interface between the glassy particles and the matrix remained free of such defects. The fully dense consolidated composite showed a drastic gain in specific yield strength under compression relative to the matrix alloy and appreciable plasticity at fracture.     

Thermal Transitions in Gelatin and Aqueous Gelatin Solutions

On the basis of the characteristic enthalpy changes associated with glass transformation, the glass transition (T_g) 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 T_g data to the T_g 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 T_g 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.