Thermal investigation of GeSb2Se4 chalcogenide alloy glass

Differential thermal analysis measurements were performed between 115 and 550° on glass samples of GeSb₂Se₄ chalcogenide alloy, and the latent heats of structural transformations were obtained. Heat treatment was found to result in the appearance of an endothermic peak associated with the glass transition.     

Study of non-isothermal crystallization kinetics of Ge20Se70Sn10 chalcogenide glass

Journal of Thermal Analysis and Calorimetry - The glass transition and non-isothermal crystallization behavior of Ge20Se70Sn10 glass prepared by the melt-quenching technique was investigated using...     

The effect of thermal stress on the thermal expansion coefficient and glass transition temperature of glass fiber–polymer composites

The thermal expansion coefficients of glass fiber–polymer composites were calculated applying the solid cylindrical model taking into account the interaction effects among the glass fibers. The stress and displacement in the composite model were determined as functions of the thermal stress. It was found theoretically that the deviation of the thermal expansion coefficient from the linear mixture relationship based on volume additivity appeared at around T_g + 20 K upon cooling. The thermal expansion coefficient of the composite was also found to be markedly dependent on the dispersion state of the glass fibers. An expression for the difference in the T_g of the matrix resin in the composite from that in the unloaded resin was obtained on the assumption that the volume change of the matrix resin caused by mixing was compensated by free volume expansion. The experimental results obtained by differential scanning calorimetry (DSC) measurements were found to agree well with the theoretically predicted ones.     

The thermal stability of chalcogenide glasses

Crystallization processes of Ge_xS_1-x (0.322≤x≤0.44) glasses have been studied by thermal analysis and a new simple method of kinetic analysis is proposed. This method allows the definition of an appropriate model characterizing the crystallization process of glass, as well as calculation of reliable kinetic parameters. Results of kinetic analysis allow definition of a thermal stability criterion which has a general applicability for any glass-forming system. ICTA Young Scientist Award presentation     

Thermal stability and high glass transition temperature of 4-chloromethyl styrene polymers bearing carbazolyl moieties

A new styrene derivative monomer, 4-(N-carbazolyl)methyl styrene (CzMS), was synthesized by reacting 4-chloromethyl styrene with carbazole in the presence of sodium hydride. Then, CzMS was homopolymerized and copolymerized with different monomers such as methyl methacrylate (MMA), ethyl methacrylate (EMA), methyl acrylate (MA), ethyl acrylate (EA) and n-butyl acrylate (BA) by free radical polymerization method in N,N-di-methylformamide (DMF) solution at 70 ± 1 °C using azobisisobutyronitrile initiator to give the copolymers I-V in good yields. The structure of all the resulted polymers was characterized and confirmed by FT-IR, ¹H NMR and ¹³C NMR spectroscopic techniques. The average molecular weight and glass transition temperature of polymers were determined using gel permeation chromatograph (GPC) and differential scanning calorimeter (DSC) instruments, respectively. It was found that these polymers with carbazole moieties have high thermal stability and the presence of bulk carbazole groups in polymer side chains leads to an increase in the rigidity and glass transition temperature of polymers.     

The glass transition temperature of polystyrene

A round robin test was performed to determine the reliability of values for the glass transition temperatureT _g as determined by DTA on polymers. Ten different instruments were involved. The test material was high molecular weight polystyrene. Values forT _g (midpoint) were reported in the range 107°C±2 K. The respective heat flow curves differed considerably in shape. In the literature aT _g of 100°C is often given for polystyrene. The discrepancy between this value and the value of 107°C found in the round robin test is due to three differences: the thermal history of the sample, the evaluation of the heat flow curves, and the effect of finite sample size.     

The effect of neutron and mixed gamma and neutron irradiation on the solar properties of borosilicate glass

Neutron and mixed gamma and neutron irradiation, at different absorbed doses, of borosilicate glass with four different chemical compositions was conducted to investigate the effects on the solar properties of the glass. Irradiation was performed in the tangential beam tube and central thimble of a nuclear reactor. The effect of thermal and epithermal neutrons on such solar properties as secondary heat-transfer factor, solar factor (the total solar energy transmittance), and shading coefficient of the borosilicate glass were investigated to determine the effect on the solar properties of borosilicate glass, because of its neutron absorbance property.     

Influence of thermal treatment on the glass transition temperature of thermosetting epoxy laminate

The influence of accelerated thermal treatment of thermosetting epoxy laminate on its glass transition temperature was studied. Lamplex^® FR-4 glass fibre-reinforced epoxy laminate (used for printed circuit board manufacturing) was used in these experiments. The composite was exposed to thermal treatments at temperatures ranging from 170 °C to 200 °C for times ranging from 10 to 480 h. The glass transition temperature (T_g) was analysed via dynamic mechanical analysis (DMA). It has been proven that the glass transition temperature rapidly decreases in reaction to thermal stress. The obtained T_g data were used for Arrhenius plots for different critical temperatures (T_g-crit. = 105–120 °C). From their slopes (?E_a/R), the activation energy of the thermal degradation process was calculated as 75.5 kJ/mol. In addition to this main relaxation mechanism, DMA also recorded one smaller relaxation process in the most aged samples. Microscopic analysis of the sample structure showed the presence of pronounced small regions of degradation both on the surface and in the inner structure, which are probably the causes of microscopic delamination and the smaller relaxation process.     

The glass transition temperature of polymer melts

We develop an analytic theory to estimate the glass transition temperature T(g) of polymer melts as a function of the relative rigidities of the chain backbone and side groups, the monomer structure, pressure, and polymer mass. Our computations are based on an extension of the semiempirical Lindemann criterion of melting to locate T(g) and on the use of the advanced mean field lattice cluster theory (LCT) for treating the thermodynamics of systems containing structured monomer, semiflexible polymer chains. The Lindemann criterion is translated into a condition for T(g) by expressing this relation in terms of the specific volume, and this free volume condition is used to calculate T(g) from our thermodynamic theory. The mass dependence of T(g) is compared to that of other characteristic temperatures of glass-formation. These additional characteristic temperatures are determined from the temperature variation of the LCT configurational entropy, in conjunction with the Adam-Gibbs model for long wavelength structural relaxation. Our theory explains generally observed trends in the variation of T(g) with polymer microstructure, and we find that T(g) can be tuned either upward or downward by increasing the length of the side chains, depending on the relative rigidities of the side groups and the chain backbone. The elucidation of the molecular origins of T(g) in polymer liquids should be useful in designing and processing new synthetic materials and for understanding the dynamics and controlling the preservation of biological substances.     

The glass transition temperature of natural rubber

In view of the lack of precise experimental data in the literature concerning the determination of a definedTg value for natural rubber (NR) and the differences when such data are given, a reference definition ofTg(Vo) is offered and a procedure for obtaining it described in detail. It is proposed that the value obtained for uncured NR of 200.5 K be used as a low temperature standard in the study of other polymerTg's which occur in this region.     

Influences of temperature and atmosphere on thermal stability of BaCrO4

In this article, the influences of temperature and atmosphere on thermal stability of BaCrO₄ were investigated. BaCrO₄ powders with an orthorhombic structure were synthesized by a facile aqueous solution route. The synthesized BaCrO₄ products were then heat treated at different atmospheres to evaluate their thermal stability by differential thermal analysis–thermogravimetry (DTA–TG), X-ray photoelectron spectroscopy and X-ray diffraction. BaCrO₄ has a good thermal stability and does not decompose in air up to 1,400 °C. However, the decomposition of BaCrO₄ in vacuum depends mainly upon a two-stage chemical reaction. BaCrO₄ is finally decomposed into BaCr₂O₄ with trivalent Cr³⁺ cations and Ba₃(Cr⁶⁺ Cr⁵⁺)₂O_9?x with both pentavalent Cr⁵⁺ and hexavalent Cr⁶⁺ cations after heat treatments in vacuum.     

DSC study of the effect of irradiation upon the glass transition temperature (T g) of polyethylene

The glass transition of an irradiated, ultra-high molecular weight, linear polyethylene was investigated by means of the Perkin-Elmer DSC-2 differential scanning calorimeter. The experimental specific heat data were compared with those of the nonirradiated sample, obtained by DSC and adiabatic calorimetry.     

Boron increases the transition temperature and enhances thermal stability of heme proteins

Transition temperature and thermal stability of proteins were studied in the presence and absence of boron. The observed midpoint of thermal denaturation (T _m) of cytochrome c (Cyt c) at pH 9.2 was 68.8 °C, which in the presence of boron increased to 71.0 °C. For metmyoglobin, T _m increased from 79.7 °C in the absence of boron to 83.5 °C in the presence of boron. Boron caused an increase of 10% in the reversibility of thermal denaturation of cytochrome c when compared with control. Activity measurements of the heat treated proteins and T _m suggest an increased thermal stability toward inactivation and denaturation of heme proteins in the presence of boron.     

The effect of pressure on gas permeation through semicrystalline polymers above the glass transition temperature

A method is proposed to analyze the effect of pressure on permeation of gases through semicrystalline polymers above the glass transition temperature. The method utilizes similarities in molecular diameters of the gases and differences in their solubilities. Two polymers, polyethylene and polypropylene, and a series of gases are chosen for an application of the method, and the effect of pressure on the permeabilities for 10 gases is measured in the pressure range 1–130 atm at 25°C. For polymers, the logarithm of the permeability coefficient is linear in the pressure for each gas, with negative slope for slightly soluble gases (He, Ne, H₂, N₂, O₂, and Ar) and positive slope for highly soluble gases (CH₄, Kr, CO₂, and N₂O). Analyzing these slopes by the method proposed permits contributions of hydrostatic pressure and concentration to the pressure dependence of permeation to be evaluated. On the basis of the results, the mechanism of gas permeation in rubbery films under high pressures is discussed.     

Determination of glass transition temperature,thermal expansion and,shrinkage of epoxy resins

On the basis of measurements of linear thermal expansion of hardened epoxy resins the influence of some modifiers on the thermal expansion of epoxy resin Epidian-5 has been examined. The glass transition temperatures of examined samples were determined.The paper presents also results of the examinations of changes in thermal and cure shrinkage for epoxy resins that occur under the influence of such modifiers as plasticizers and fillers.Five different compositions were examined. A simple and fast measuring method was applied, in which sample elongations vs temperature were determined with a cathetometer. Specific volume changes of liquid resins with temperature were measured with a quartz dilatometer and a cathetometer.     

Effects of cross-links, pressure and temperature on the thermal properties and glass transition behaviour of polybutadiene

The thermal conductivity κ, heat capacity per unit volume ρc(p) and glass transition behaviour under pressure have been established for medium and high vinyl content polybutadiene PB with molecular weights 2600 and 100,000 and their highly cross-linked (ebonite) states obtained purely by high-pressure high-temperature treatments. Cross-linking eliminates the glass transitions and increases κ by as much as 50% at 295 K and 1 atm, and decreases ρc(p) to a limiting level close to that of the glassy state of PB, which is reached before the ultimate cross-link density is achieved. The pressure and temperature behaviours of κ are strongly changed by cross-links, which increases the effect of temperature but decreases the effect of pressure. We attribute these changes to a cross-linked induced permanent densification and consequential increase of phonon velocity simultaneously as conduction along polymer chains is disrupted. The glass transition temperatures for a time scale of 1 s are described to within 0.5 K by: T(g)(p) = 202.5 (1 + 2.94 p)(0.286) and T(g)(p) = 272.3 (1 + 2.57 p)(0.233) (p in GPa and T in K) up to 1 GPa, for PB2600 and PB100000, respectively, and can be estimated for medium and high vinyl content PBs with molecular weights in between by a constant, pressure independent, shift in temperature.     

The use of moisture sorption isotherms and glass transition temperature to assess the stability of powdered baby formulas

The influence of adverse conditions of environment in the case of baby formulas, which are multiple mixtures, should be minimised. Water activity (a _w) and moisture content, correlated through sorption isotherms, and glass transition temperature have been considered relevant parameters to describe food stability. The aim of the study was to analyse water activity and glass transition temperature as the function of water content for samples of baby formulas. Three types of baby formulas (mixture, agglomerate, coated agglomerate) were determined by sorption isotherms, DSC and MDSC. DSC curves of mixture, agglomerate and coated agglomerate did not show differences in shape and course. The glass transition temperature of powders stored at different water activities was measured and it decreased with the increase in moisture content, confirming the strong plasticising effect of water on this property. Critical water activities varied from 0.14 to 0.68 and critical moisture contents varied from 0.032 to 0.062 g g^?1 powder.     

Relationships on the effect of water on glass transition temperature and young's modulus of nylon 6

The glass transition temperature T_g of nylon 6 decreases monotonically toward a finite value T_gl upon increase of the moisture content. The mechanism of this decrease entails the reversible replacement of intercaternary hydrogen bonds in the accessible regions of the polyamide. The limiting glass transition temperature T_gl is approached when the moisture content approaches W_l, which corresponds to the amount of water required for complete interaction with all accessible amide groups. Denoting with T_g0 the glass transition temperature of the dry polymer, the effect of water on T_g is represented by the equation, T_g = (ΔT_g)₀ exp{?[ln(ΔT_g)₀]W/_τW_l} + T_gl, where (ΔT_g)₀ = T_g0 ?T_gl, and τ = W(T_gl+1)/W_l. This equation appears to be generally applicable to hydrophilic polymers, since correspondingly calculated data are also in very good agreement with experimental data for polymers such as nylon 66, poly(vinyl alcohol), and polyN-vinylpyrrolidone. The effect of water of Young's modulus E of nylon 6 is represented by an analogous relationship, and the quantity In[(E?E_l)/(T_g?T_gl)] is a linear function of the moisture content.     

The effect of gamma irradiation on the structural properties of olivine

Journal of Radioanalytical and Nuclear Chemistry - Gamma irradiation studies of (Mg0.905Fe0.095)2SiO4 olivine were performed using X-ray fluorescence method, X-ray diffraction, Raman and...     

Study of negative thermal expansion and shift in phase transition temperature in Ti4+- and Sn4+-substituted ZrW2O8 materials

The negative-thermal-expansion material ZrW(2)O(8) is known to undergo an order-disorder phase transition which affects its expansion behavior. In this study, Ti(4+) and Sn(4+) are examined as possible substituting ions for the Zr(4+) position in ZrW(2)O(8). This substitution leads to a decrease in cell parameters, as the ionic radii of the substituents are smaller than the Zr(4+) ionic radius. A remarkable decrease in transition temperature is noticed. DSC is used to quantify the enthalpy and entropy changes during the phase transition in order to reveal the mechanisms behind this decrease. It is shown that the strength of the M-O bond plays an important role, as it is a partner in the rigid unit mode motion and the order-disorder transition mechanism.