Isothermal cure characterization of fumed silica/epoxy nanocomposites: The glass transition temperature and conversion

The glass transition temperature (T_g) and conversion (α) were measured for a cycloaliphatic epoxy/anhydride system incorporated with different contents of hydrophobic fumed silica. Samples isothermally cured at varying cure temperatures and times were analyzed by differential scanning calorimetry (DSC). T_g vs. ln(time) data at an arbitrary reference temperature were superposed by time–temperature shifts to form a master curve for the kinetically controlled reaction, and the shift factors were used to calculate an Arrhenius activation energy. Influence of the hydrophobic fumed silica was investigated from T_g and α data. For low conversions, the incorporated fumed silica may reduce segmental mobility, giving rise to an increase in T_g. For high conversions, there was a significant depression of T_g which may be associated with the presence of residual hydroxyl groups on the surface of hydrophobic fumed silica. The positive and negative effects of the fumed silica on T_g and α are discussed.     

Thermal characteristics and crystallization kinetics of tellurite glass with small amount of additive As<Subscript>2</Subscript>O<Subscript>3</Subscript>

The crystallization kinetics of the TeO₂/TiO₂/As₂O₃ glassy system was studied under nonisothermal conditions. The method was applied to the experimental data obtained by differential thermal analysis (DTA), using continuous-heating techniques. In addition, two approaches were used to analyze the dependence of glass transition temperature (T _g) on the heating rate (β): One is the empirical linear relationship between (T _g) and (β); The other approach is the use of straight line from the plot of ln( T_\textg² /b ) \textvs . 1/T_\textg \ln \left( {T_{\text{g}}^{2} /\beta } \right)\,{\text{vs}} .\,1/T_{\text{g}} for evaluation of the activation energy for glass transition. The crystallization results are analyzed, and both the activation energy of crystallization process and the crystallization mechanism are characterized.     

Non-isothermal kinetic studies for binary TeO2-P2O5 glass system

A study of TeO₂–P₂O₅ glass system has been carried out by Differential Thermal Analysis (DTA) to elucidate the kinetics of crystallization for these glassy samples. The results of DTA performed at different heating rates are discussed. The values of the glass transition temperature, T _g , as well as the glass crystallization temperature, T _c , are found to be dependent upon the heating rate. From this dependence, the values of activation energy for both the glass transition and crystallization are evaluated and discussed     

Adam-Gibbs Formulation of Enthalpy Relaxation Near the Glass Transition

The entropically based nonlinear Adam-Gibbs equation is discussed in the context of phenomenologies for nonlinear enthalpy relaxation within the glass transition temperature range. In many materials for which adequate data are available, the nonlinear Adam-Gibbs parameters are physically reasonable and agree with those obtained from linear relaxation data and thermodynamic extrapolations. Observed correlations between the traditional Tool-Narayanaswamy-Moynihan parameters are rationalized in terms of the Adam-Gibbs primary activation energy (Δμ) determining how close the kinetic glass transition temperature can get to the thermodynamic Kauzmann temperature. It is shown that increased nonlinearity in the glass transition temperature range is associated with greater fragility in the liquid/rubber state above T_g.     

Crystallization study of Te-Bi-Se glasses

Crystallization studies are carried out under non- isothermal conditions with samples heated at several uniform rates. The dependence of the glass transition temperature (T_g), the crystalline temperature (T_c) and the peak temperature of crystallization (T_p) on the composition and heating rate (β) has been studied. For a memory/switching material, the thermal stability and ease of glass formation are of crucial importance. The glass transition temperature, T_g, increases slightly with the variation of Bi content. From the heating rate dependence of T_g, the activation energy for glass transition (E_t) has been evaluated. The results are discussed on the basis of Kissinger’s approach and are interpreted using the chemically ordered network model (CONM).     

Crystallization kinetics of selenium-tellerium glasses

Bulk glasses of the compositions Se₇₀Te₃₀ and Se₈₀Te₂₀ were prepared by the melt quenching technique. Differential thermal analyses were performed at different heating rates. The values of the glass transition temperature,T _g, the crystallization temperature,T _c, and the peak temperature of crystallization,T _p, were found to depend on the composition and the heating rate. The activation energy for the glass transition,E _i, as well as the activation energy for crystallization,E _c, were evaluated from the heating rate dependence ofT _g andT _p. The crystallization mechanism was examined through analysis of the data under non-isothermal conditions. The results indicated that surface crystallization is dominant for both compositions.     

Kinematical studies of the glass transition in glassy Se80 ? xTe20Sbx

The present paper reports kinematic studies of the glass transition in glassy Se_80-xTe₂₀Sb_x alloys using the differential scanning calorimetric technique. The glass transition temperature (T_g) is found to increase monotonically with the heating rate. T_g also increases with the increase of Sb concentration in ternary Se_80-xTe₂₀Sb_x system. From the heating rate dependence of glass transition temperature, the activation energy ( E_t) for the relaxation time controlling the structural enthalpy, is calculated. The composition dependence of T_g and E_t is discussed in terms of the structure of the Se-Te-Sb glassy system.     

Thermal and mechanical evaluation of cyanate ester composites with low-temperature processability

A low viscosity bisphenol E cyanate ester monomer was used as a reactive diluent in a bisphenol A/novolac cyanate ester to achieve a low viscosity prepolymer with a viscosity suitable for processing at room temperature. Dynamic mechanical analysis was conducted on carbon fiber composite specimens manufactured from two cyanate ester blends with varying catalyst compositions. It was shown that post-curing was needed to produce a polymer matrix with a single glass transition relaxation, but increases in post-cure temperature above 204 °C resulted in slight reductions in T_g and modulus. Also, increasing catalyst composition was shown to have a negative effect on T_g.     

Glass transition evaluation of commercially available epoxy resins used for civil engineering applications

The paper presents a study about the glass transition of commercially available epoxy resins used for structural strengthening of concrete members for instance by means of Carbon Fiber Reinforced Polymer (CFRP) strips. Prior to an experimental investigation with a dynamic mechanical analysis (DMA), an overview on differences between definitions for the glass transition temperature T_g is given. Several testing recommendations are listed in this respect. Subsequently, DMA tests on three commercially available products are presented. A first focus is put on the different evaluation methods for one specific test result. It is visible that considerable differences in the finally adapted glass transition temperature might arise if one or the other procedure is followed. Additional parameters, such as curing procedure, specimen age, temperature history, and ultimate temperature during heating are considered, too. In all the above mentioned cases, differences in the glass transition can be found. Higher specimen age, higher ultimate temperature during testing, accelerated curing, as well as a lower heating rate implicate higher glass transition temperatures, showing that the glass transition temperature is not a fixed material characteristic. In a final step, the relevance for T_g for civil engineering applications is described. The various design code provisions for defining the service temperature in structures related to T_g are presented. The overall aim of the investigation is to show that structural engineers and end users have to be aware of the different influential parameters on the final results regarding the glass transition temperature, which also highlights the need of a potential deeper product investigation in case technical data sheets lack detailed information.     

Composite rheology — I. Elastomer-filler interaction and its effect on viscosity

Experimental results show that the viscosity of elastomer-filler systems cannot be explained as a simple equation depending only on filler loading, such as in the Einstein or the Guth—Simha—Gold models. The viscosity of elastomer-filler systems also depends on the temperature at which it is measured and the glass transition temperature,T _g, of the basic polymer. This finding seems to suggest two facts: (a) polymer melts are not simple Newtonian fluids but relaxational liquids in which the values of rheological data depend on the experimental observation time (or measured frequency) and temperature, (b) composite materials of elastomer-filler systems, will have the sameT _g as the basic polymer if the fillers do not change the main-chain structure of the basic polymer. Therefore, one can conclude that the presence of filler broadens the relaxation spectrum but does not changeT _g, and atT _g the basic polymer and the composite materials will have the same viscosity. A new model for the viscosity of elastomer-filler systems is developed from the concept of molecular structural relaxation and glass transition temperature of polymers.     

The effect of nitrogen on viscosity of La-Si-Mg-O-N glasses by compressive creep and dilatometry

Viscosity as a function of temperature and nitrogen content and the glass transition temperature (T_g) were investigated in 20La-60Si-20Mg-O-N glasses with nitrogen contents varying from 0 to 28 eq.% (e/o) using compressive creep and dilatometric experiments. Although T_g obtained from dilatometry were 6–12°C lower than the lower limit of the transition temperature range from creep, both methods revealed identical and linear dependencies of T_g on N content. The average activation energy was 1184 ± 36 kJ/mol and viscous flow remains the deformation mechanism in all glasses regardless of nitrogen content. Addition of 28 e/o N in oxide glass resulted in an increase of T_g by 94–105°C and an increase in viscosity of around 5 orders of magnitude. These changes are greater than those reported in similar Al-containing glasses. Linear increase of T_g and compactness of the glass with nitrogen content result from enhanced cross-linking of the glass network.     

Correlation between glass transition temperature and melting temperature in metallic glasses

We report that the glass transition temperature (T_g) of a variety of metallic glasses (MGs) correlates with the eutectic or peritectic temperature of two main components corresponding stoichiometric proportion in their binary phase diagram. The correlation suggests that the T_g of MGs is mainly determined by their solvent of two base components, which have composition close to the eutectic and peritectic points in the binary phase diagram and the weakest link in amorphous structure. The results have implications for understanding the structure and glass transition in MGs and for predicting and designing metallic glasses with a desirable T_g.     

Annealing dependence of optical properties of Ga20S75Sb5 and Ga20S40Sb40 thin films

Amorphous Ga₂₀S₇₅Sb₅ and Ga₂₀S₄₀Sb₄₀ thin films were prepared onto glass substrates by using thermal evaporation method. The effect of annealing (under vacuum) at different temperatures on the optical parameters was investigated in the temperature range 373-593 K. The optical absorption coefficient (α) for the as-deposited and annealed films were calculated from the reflectance and transmittance measurements in the range 190-900 nm. X-Ray diffraction indicates that the as-deposited films and those annealed up to the glass transition temperature (T_g) exhibit amorphous state. On annealing above the glass transition temperature these films show a polycrystalline structure. Analysis of the optical absorption data indicates that the optical band gap E_g^opt of these films obeys Tauc's relation for the allowed non-direct transition. It was found that the optical band gap E_g^opt increases with annealing temperature up to T_g, whereas above T_g there is a remarkable decrease. The obtained results were interpreted on the basis of amorphous- crystalline transformation.     

Glass transition and fragility of telluro-vanadate glasses containing antimony oxide

The activation energy (ΔH ^*) of the glass transition and the heating-rate dependence of the glass transition temperature (T _g) of V₂O₅–Sb₂O₃–TeO₂ glasses were determined using differential scanning calorimetry technique. Non-isothermal measurements were performed at different heating rates φ (=3, 6, 9, 10, 13 K/min). The heating rate dependence of T _g was used to investigate the applicability of different theoretical models describing the glass transition. The application of Moynihan and Kissinger et al. models to the present data led to different values of (ΔH ^*) at each different heating-rate regions. This behavior was attributed to the strong heating rate dependence of the activation energy of the process. The fragility parameter (m = ΔH ^*/RT _g) were ≲90, suggesting that these glasses may be classified as strong glasses. The viscosity, η, calculated at a few selected temperatures near the glass transition region increased with increasing Sb₂O₃ content at any given temperature, which is also expected. Also the compositional dependence of T _g and ΔH ^* was investigated.     

Woven hybrid biocomposites: Dynamic mechanical and thermal properties

The dynamic mechanical and thermal analysis of oil palm empty fruit bunch (EFB)/woven jute fibre (J_w) reinforced epoxy hybrid composites were carried out. The storage modulus (E′) was found to decrease with temperature in all cases, and hybrid composites had showed better values of E′ at glass transition temperature (T_g) compared to EFB and epoxy. Loss modulus showed shifts in the T_g of the polymer matrix with the addition of fibre as reinforcing phase, which indicate that fibre plays an important role in case of T_g. The Tan δ peak height was minimum for jute composites and maximum for epoxy matrix. Complex modulus variations and phase behaviour of the hybrid composites was studied by Cole-Cole analysis. Thermal analysis result indicates an increase in thermal stability of EFB composite with the incorporation of woven jute fibres. Hybridization of EFB composite with J_w fibres enhanced the dynamic mechanical and thermal properties.     

Mixed cation effect in xR2O-(40 − x)Na2O-50B2O3-10Bi2O3 (R = Li, K) glasses

Mass density, glass transition temperature and ionic conductivity are measured in xLi₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ and xK₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ glass systems with 0 ≤ x ≤ 40 mol%. The strength of the mixed alkali effect in T_g, dc electrical conductivity and activation energy has been determined in each glass system. The magnitudes of the mixed alkali effect in T_g for the mixed Li/Na glass system are much smaller than those in the mixed K/Na glasses. The impact of mixed alkali effect on dc electrical conductivity in mixed Li/Na glass system is more pronounced than in the K/Na glass system. The results are explained based on dynamic structure model.     

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.     

Surface glass transition temperatures of monodisperse polystyrene films by scanning force microscopy

Surface molecular motion of monodisperse polystyrene (PS) films was examined by scanning viscoelasticity microscopy (SVM) in conjunction with lateral force microscopy (LFM). The dynamic storage modulus, E′, and loss tangent, tan δ, at a PS film surface with a smaller number-average molecular weight, M_n, than 40k were found to be smaller and larger than those for the bulk sample even at room temperature, meaning that the PS surface is in a glass–rubber transition state or a fully rubbery one at this temperature if the M_n, is small. In order to elucidate quantitatively how vigorous the molecular motion at the PS surface is, SVM and LFM measurements were made at various temperatures. The glass transition temperature, T_g, at the surface was discerned to be markedly lower than its bulk T_g, and the discrepancy of T_g between surface and bulk becomes larger with the decreasing M_n. Such an intensive activation of thermal molecular motion at the PS surfaces can be explained in terms of an excess free volume in the vicinity of the film surface induced by the preferential segregation of chain end groups.     

Kinetics of Ge<Subscript>20</Subscript>Se<Subscript>80-x</Subscript>As<Subscript>x</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 5, 10, 15 and 20) in glass transition region

The results of differential scanning calorimetric (DSC) measurements on Ge₂₀Se_{80- x}As_x (x = 0, 5, 10, 15 and 20) system with the specific aim of investigating the effect of heating rate and composition on glass transition temperature have been discussed. The results indicate that the glass transition temperature (T_g) is dependent both on the heating rate and composition. The glass transition activation energy (E_t) and heat absorbed in glass transition region (ΔH) are higher for Ge₂₀Se₆₅As₁₅ as compared to the values of other compositions of arsenic. An effort has also been made to develop an empirical model for the composition dependence of ΔH. A good agreement has been observed between the experimental values and the results of model calculation.     

Tin fluorophosphate nonwovens by melt state centrifugal Forcespinning

This report describes the direct melt processing of inorganic tin fluorophosphate (TFP) glass fibers with average diameters ranging from 2 to 4 µm via centrifugal Forcespinning. This was accomplished by using a TFP glass with low glass transition temperature (T _g) and the melt processing capability of Forcespinning. The thermal behavior of TFP glass fibers was investigated by differential scanning calorimetry and thermogravimetric analysis, while the compositional evolution of the fibers was studied using energy-dispersive spectrometry and Fourier-transform infrared spectroscopy. These fibers exhibited excellent thermal stability after thermal post-treatment at 300 °C. The T _g of the thermally treated fibers increased by 100 °C compared to the bulk material. The fibers were found to undergo dehydration and loss of fluorine during thermal treatment, resulting in a rigid and crosslinked glass network with enhanced thermal stability and increased T _g. The enhanced thermal stability demonstrated the potential of TFP fibers for high temperature catalysis and chemical filtration applications.