Glass Transition Process Research Articles

The evolution of structural and electronic properties during the glass transition process of Al–Ni–Nd alloy

Ab initio molecular dynamic simulations are performed to understand the evolution of structural and electronic properties during the glass transition process of the Al–Ni–Nd alloy based on the density functional theory. The local structural evolution is evaluated by the Honeycutt–Andersen (HA) bond-type index and bond-angle distribution methods. The bond-angle distribution indicates that the close packing of three neighbour atoms and the pentagon configurations become the primary short range order (SRO) as temperature decreases and the HA bond-type result shows that the formation of the crystal structures is suppressed by the increasing pentagonal bi-pyramids ordering during the rapid solidification processes. Our result provides strong evidence for the existence of Al–Ni covalent bonds, where chemical SRO together with the polyhedral local structures play an important role during the glass transition and increase the glass forming ability of the Al–Ni–Nd alloy. We also found that the electrons with an sp character of the Al atoms are more likely to transfer to the d states of the Ni atoms during the glass transition process.

Influence of the Introduction of Copper into Amorphous As2Se3Matrix on Its Thermal and Structural Characteristics

This paper describes the results of the determination of some thermal and structural parameters of glasses from the Cux(As2Se3)100−x system for x = 0, 1, 5, 10 and 15 at.%. Based on the di erential scanning calorimetry curves taken at di erent heating rates, glass transition temperature Tg, onset temperature of crystallization Tonset, and melting temperature Tm of crystalline units formed are determined. The values of activation energy Eg of glass transition process are calculated. These characteristic temperatures served as the basis for the calculation of the parameters of thermal stability of the glasses towards crystallization.

Hybrid clay mineral-carbon nanotube-PLA nanocomposite films. Preparation and photodegradation effect on their mechanical, thermal and electrical properties

Novel hybrid materials, obtained by direct Catalytic Chemical Vapour Deposition (CCVD) growth of carbon nanotubes (CNT) over a clay mineral catalyst, were incorporated into Polylactic acid (PLA) through melt mixing. Nanocomposites in the form of films were exposed to UV irradiation at constant temperature (30°C) and relative humidity (50%). Young's Modulus measurements show a strong mechanical degradation with the UV irradiation time. The decrease was higher in neat PLA compared with nanocomposites and was attributed to the extensive macromolecular chain scission due to the UV irradiation. From thermogravimetric characterization it was found that the degradation temperature decreases with the photoageing. The addition of the hybrid filler causes an initial decrease of this temperature (up to 3%) and a slight increase thereafter. A similar behaviour was found for the enthalpy relaxation during the morphological rearrangement which occurred in the glass transition process. Indeed, the electrical conductivity of the nanocomposites was found to monotonically increase with the UV irradiation time.

Mechanical behaviour of pultruded glass fibre reinforced polymer composites at elevated temperature: Experiments and model assessment

This paper presents an experimental and analytical study about the mechanical response at elevated temperature of glass fibre reinforced polymer (GFRP) pultruded profiles. The paper first describes results of DMA and DSC tests that were used to evaluate the glass transition and decomposition processes of the GFRP pultruded material. The paper then describes an extensive study about the tensile, shear and compressive responses of the GFRP material at temperatures varying from 20°C to 250°C. In these tests the mechanical responses of the GFRP material as a function of temperature were assessed, namely the load–deflection curves, the stiffness, the failure modes and the ultimate strength. Results obtained in these experiments confirmed that the mechanical performance of GFRP is severely deteriorated at moderately high temperatures, particularly when loaded in shear and compression, owing to the glass transition of the resin. The final part of this paper assesses the accuracy of different empirical models and one phenomenological model to estimate the tensile, shear and compressive strengths of GFRP pultruded material as a function of temperature. All empirical models, including a function based on Gompertz statistical distribution suggested in the present paper, provided accurate estimates of tensile, shear and compressive strengths as a function of temperature. The phenomenological model was less accurate and in general provided non conservative estimates of material strength.

Unique evolution of spatial and dynamic heterogeneities on the glass transition behavior of PVPh/PEO blends

Glass transition behavior of hydrogen bonded polymer blends of poly(vinyl phenol) (PVPh) and poly(ethylene oxide) (PEO) is systematically investigated using normal differential scanning calorimetry (DSC) and recently developed multifrequency temperature-modulated DSC (TOPEM), in combination with Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) techniques, focusing on the effect of the PEO molecular weight on the spatial and dynamic heterogeneity. It is found, for the first time, that both the glass transition temperature (T g) and activity energy (E a) of the blends strongly depend on PEO molecular weight, and a common turning point, which separates the rapid and slow increasing regions, can be found. The interchain hydrogen bonding interactions, both determined by FTIR measurements and obtained from the Kwei equation, decrease with increasing PEO molecular weight, indicating a decrease of the componential miscibility. A series of parameters related to the microscopic spatial and dynamic heterogeneity, such as the activity energy, fragility, nonexponential factor and the size of cooperatively rearranging regions, are calculated from frequency dependency complex heat capacity measured using TOPEM. It is found that each of these parameters monotonically changes with increasing the PEO molecular weight during the glass transition process, demonstrating that hydrogen bonding interaction is the key factor in controlling the spatial and dynamic heterogeneity, thus the glass transition. NMR relaxation results reveal the existence of obvious phase separation large than 5 nm, implying that the cooperatively rearranging regions should be closely related to the interphase region between the two components. The above obtained origin and evolution of spatial and dynamic heterogeneity provide a new insight into the glass transition behavior of polymer blends.

Iso-conversional analysis of glass transition and crystallization in as-synthesis high yield of glassy Se98Cd2 nanorods

In the present work, we have prepared high yield of glassy Se98Cd2 nanorods using melt–quench technique. The morphology and micro-structural analysis of as-prepared nanorods are characterized by SEM, XRD and EDX techniques. The glass transition and crystallization kinetics of glassy Se98Cd2 nanorods are studied at different heating rates (5, 10, 15, 20 K/min) using differential scanning calorimetric (DSC) technique. Four iso-conversional methods [Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), Tang and Straink] were used to determine the various kinetic parameters (crystallization temperature Tαc, transition temperature Tαg, activation energy of crystallization Eαc, activation energy of glass transition Eαg, order parameter n) of glass transition and crystallization process in non-isothermal mode. The monotonous decrease in activation energy Eα with the crystallization fraction α confirm the complex kinetic mechanism of the glassy Se98Cd2 nanorods.

Critical analysis of endo-thermal effect in the glass transition process in chalcogenide glasses

Free volume model is widely used as a tool for understanding the nature of glass transition phenomenon and the related consequences. Recently, an analytical derivation of Kissinger's equation has been proposed in literature using free volume model. In this derivation, the glass transition activation energy has been assumed constant throughout the whole glass transition temperature range. The present paper investigates the applicability of free volume model for derivation of Kissinger's relation by checking the constancy of the glass transition activation energy (Eg) throughout the glass transition temperature range. Performing series of experiments, we have legalized the possibility for usage of the Kissinger relation for determination of the glass transition activation energy if the peak value of the endo-thermal effect is taken as temperature of glass transition.

Thermal behavior and phase identification of Valsartan by standard and temperature-modulated differential scanning calorimetry

Thermal behavior of angiotensin II type 1 (AT1) receptor antagonist, Valsartan (VAL), was examined employing thermogravimetric analysis (TGA), standard differential scanning calorimetry (DSC) and temperature-modulated differential scanning calorimetry (TMDSC). The stability of VAL was measured by TGA from 25 to 600°C. Decomposition of Valsartan starts around 160°C. The DSC curve shows two endotherms, occurring around 80°C and 100°C, related to evaporation of water and enthalpy relaxation, respectively. Valsartan was identified by DSC as an amorphous material and it was confirmed by X-ray powder diffraction. The glass transition of fresh Valsartan appears around 76°C (fictive temperature). TMDSC allows separation of the total heat flow rate into reversing and nonreversing parts. The nonreversing curve corresponds to the enthalpy relaxation and the reversing curve shows changes of heat capacity around 94°C. In the second run, TMDSC curve shows the glass transition process occurring at around 74°C. Results from standard DSC and TMDSC of Valsartan were compared over the whole range of temperature.

MOLECULAR DYNAMIC SIMULATIONS OF GLASS TRANSITION TEMPERATURE AND MECHANICAL PROPERTIES IN THE AMORPHOUS REGION OF OIL-IMMERSED TRANSFORMER INSULATION PAPER

The glass transition temperature (Tg) in the amorphous region of an insulation paper is one of the most important characteristics for thermal stability. Molecular dynamic simulations have been performed on three micro-structural models, namely, amorphous pure cellulose, amorphous cellulose with water and amorphous cellulose with oil, to study the microscopic mechanism of the glass transition process for oil-immersed transformer insulation paper. Using the method of specific volume versus temperature curve, the Tg of amorphous pure cellulose, cellulose with water, and cellulose with oil was determined as 448, 418 and 440 K, respectively. The current study may provide some information for thermal aging. The simulation results show that during the glass transition process, both the chain motion and mechanical properties of cellulose changes significantly. Relative to the oil molecules, water molecules immersed in the amorphous region of insulation paper can disrupt hydrogen bonds between cellulose chains. This phenomenon results in a significant reduction in the glass transition temperature and affects the thermal stability of the insulation paper.

Affinity and its derivatives in the glass transition process

The thermodynamic treatment of the glass transition remains an issue of intense debate. When associated with the formalism of non-equilibrium thermodynamics, the lattice-hole theory of liquids can provide new insight in this direction, as has been shown by Schmelzer and Gutzow [J. Chem. Phys. 125, 184511 (2006)], by Möller et al. [J. Chem. Phys. 125, 094505 (2006)], and more recently by Tropin et al. [J. Non-Cryst. Solids 357, 1291 (2011); ibid. 357, 1303 (2011)]. Here, we employ a similar approach. We include pressure as an additional variable, in order to account for the freezing-in of structural degrees of freedom upon pressure increase. Second, we demonstrate that important terms concerning first order derivatives of the affinity-driving-force with respect to temperature and pressure have been previously neglected. We show that these are of crucial importance in the approach. Macroscopic non-equilibrium thermodynamics is used to enlighten these contributions in the derivation of C(p),κ(T), and α(p). The coefficients are calculated as a function of pressure and temperature following different theoretical protocols, revealing classical aspects of vitrification and structural recovery processes. Finally, we demonstrate that a simple minimalist model such as the lattice-hole theory of liquids, when being associated with rigorous use of macroscopic non-equilibrium thermodynamics, is able to account for the primary features of the glass transition phenomenology. Notwithstanding its simplicity and its limits, this approach can be used as a very pedagogical tool to provide a physical understanding on the underlying thermodynamics which governs the glass transition process.

Investigation of the free volume change of Fe41Co7Cr15Mo14C15B6Y2 bulk metallic glass using the cyclic thermal dilatation test

In heating process, the free volume of bulk metallic glass (BMG) changes with temperature. This change, however, is too small to be directly investigated by thermal dilatation (DIL) test. In cyclic heating, the free volume change results in a difference between the thermal expansion during heating and reheating processes, i.e., Δ(δV(T)/V0), which can be obtained by the cyclic DIL test. In this paper, the sample diameter and pre-annealing effect on the free volume change of Fe41Co7Cr15Mo14C15B6Y2 BMG are investigated by the Δ(δV(T)/V0) and ΔδV/V0¯, where ΔδV/V0¯ is the average value of Δ(δV(T)/V0) in the low temperature range and represents the total free volume released in the cyclic DIL test. The as-cast samples with a smaller diameter have a larger ΔδV/V0¯. After pre-annealing below the glass transition temperature for 2hours, a peak that is attributed to the free volume generation in glass transition process occurs in the Δ(δV(T,ta)/V0) curve and becomes sharper as the pre-annealing time ta is prolonged. ΔδV(ta)/V0¯ is fitted by a stretched exponential relaxation function with a characteristic relaxation time τ of 14194±2808s and a Kohlrausch exponent βkww of 0.34±0.04. The βkww of Fe41Co7Cr15Mo14C15B6Y2 BMG is markedly lower than that of Pd- and Zr‐based BMGs, and deserves a further investigation.

Glass Transition and Relaxation Processes of Nanocomposite Polymer Electrolytes

This study focus on the effect of δ-Al(2)O(3) nanofillers on the dc-conductivity, glass transition, and dielectric relaxations in the polymer electrolyte (PEO)(4):LiClO(4). The results show that there are three dielectric relaxation processes, α, β, and γ, in the systems, although the structural α-relaxation is hidden in the strong conductivity contribution and could therefore not be directly observed. However, by comparing an enhanced dc-conductivity, by approximately 2 orders of magnitude with 4 wt % δ-Al(2)O(3) added, with a decrease in calorimetric glass transition temperature, we are able to conclude that the dc-conductivity is directly coupled to the hidden α-relaxation, even in the presence of nanofillers (at least in the case of δ-Al(2)O(3) nanofillers at concentrations up to 4 wt %). This filler induced speeding up of the segmental polymer dynamics, i.e., the α-relaxation, can be explained by the nonattractive nature of the polymer-filler interactions, which enhance the "free volume" and mobility of polymer segments in the vicinity of filler surfaces.

Thermal and optical studies of an amorphous InSe9 alloy produced by mechanical alloying

In this paper we investigated thermal and optical properties of an amorphous alloy of the In–Se system. The amorphous InSe9 alloy was produced by mechanical alloying and it was studied using Differential Scanning Calorimetry and microPhotoluminescence spectroscopy techniques, and from them several properties, such as glass transition and crystallization temperatures and energies and the optical gap energy were determined and compared to the values found in other alloys of the In–Se system. This comparison revealed some differences among our alloy produced by mechanical alloying and alloys produced by other techniques, which is a clear indication of the influence of the fabrication technique in their physical properties. The main differences occur in the activation energies associated with the glass transition and crystallization processes and also in the optical gap energy.

Molecular dynamics simulation for the rapid solidification process of MgO–Al2O3–SiO2 glass–ceramics

A molecular dynamics simulation method was used to study the effects of the microstructure on the solidification process of different cooling rates in the MgO–Al2O3–SiO2 glass–ceramics with cordierite as the main crystalline phase. The reasons for changes in the microstructure during the solidification process were analysed by the radial distribution function curve, the bond angular distribution, the coordination number and the volume changes. The results showed that the cooling rate greatly affected the crystallisation process and the glass transition process. When the cooling rate was too fast, the atoms could not undergo a massive displacement before they were “frozen”, and the ability of atoms to achieve an equilibrium position was limited. Some amorphous phases were formed as a result of the disorder of the atomic arrangement, then some crystalline phase precipitated from the vitreous, and a glass–ceramic material was eventually formed.

Characteristic free volume change of bulk metallic glasses

The free volume change ΔVf(T) of bulk metallic glasses (BMGs) relative to a hypothesized amorphous reference state was measured using the thermal dilatation method. The characteristic free volume change, i.e., the free volume released in structural relaxation ΔVf-sr, was identified quantitatively from the ΔVf(T) curve. For a Fe-based BMG, it was found that ΔVf-sr increases with decreases in the sample diameter and heating rate. ΔVf-sr measured under the same sample diameter and heating rate conditions allowed the convenient comparison of different BMGs. The comparison revealed that the glass-forming ability (GFA) enhancement of each of two Pd-, Mg-, Cu-, Zr-, Ti-, and Fe-based BMGs can be sensitively reflected in the decrease in ΔVf-sr and the narrowing of the difference between the peak temperature of the thermal expansion coefficient and the end temperature of the glass transition process. In addition, for these twelve typical BMGs, there is a good linear relationship between ΔVf-sr and LogDc2 or LogDc, where Dc is the critical diameter. ΔVf-sr is thus sensitive to and has a close correlation with GFA. Furthermore, the ΔVf-sr measurement results are in good agreement with the free volume change measured with the specific heat capacity, room temperature density, and positron annihilation lifetime methods. In the study of the relationship between the structure and properties of BMGs, ΔVf-sr thus plays an important role given its comparability and convenience.

A Thermally Tunable Inverse Opal Photonic Crystal for Monitoring Glass Transition

An optical method was developed to monitor the glass transition of the polymer by taking advantage of reflection spectrum change of the thermally tunable inverse opal photonic crystal. The thermally tunable photonic bands of the polymer inverse opal photonic crystal were traceable to the segmental motion of macromolecules, and the segmental motion was temperature dependent. By observing the reflection spectrum change of the polystyrene inverse opal photonic crystal during thermal treatment, the glass transition temperature of polystyrene was gotten. Both changes of the position and intensity of the reflection peak were observed during the glass transition process of the polystyrene inverse opal photonic crystal. The optical change of inverse opal photonic crystal was so large that the glass transition temperature could even be estimated by naked eyes. The glass transition temperature derived from this method was consistent with the values measured by differential scanning calorimeter.

Low temperature thermal windowing (TW) thermally stimulated depolarization current (TSDC) setup

We report here the design and implementation of a precise and easy to operate thermally stimulated depolarization current (TSDC) measurement setup for temperature range 77–400 K. The sample loading is made simple by sandwiching the sample between two copper disk electrodes using a spring-shaft arrangement. The salient features of the setup are precise thermal windowing (TW) capability and linear heating rate over the entire temperature range. The resolution in the measurement of depolarization current is of the order of 7×10−14 A. This is achieved by means of good electrical insulation of the electrodes from the rest of the setup and utilization of low noise circuitry. Precision of the system is demonstrated by its capability to resolve constituent relaxations present in complex relaxation processes using the TW experiments. Study and detection of glass transition processes in polyethylene terephthalate, four relaxation processes in polymethyl methacrylate, glass and crystallization transitions along with the onset of ferroelectric Curie transitions in polyvinylidene fluoride and characterization of electret state in amino acids l-arginine, phenylalanine, tyrosine, tryptophan, glutamic acid, glutamine and methionine show the versatility of our setup.

OS0902 高分子複合材料の緩和時間の温度依存性解析

Shape memory polymer (SMP) has been attracting attention in recent years, and expected to the application to the deployable structures in the fields of medical and aerospace. Though SMP is often used in the form of composite material to reinforce its mechanical properties, shape recovery properties of SMP are greatly affected by filler reinforcement. Shape recovery is related to the sub-micron behavior of molecules at glass transition process. Therefore, we used a coarse-grained molecular dynamics simulation to analyze the relaxation behavior of polymer composites. We focused on the temperature dependence of the energy relaxation time after the uniaxial tensile test, and evaluated the effects of intermolecular forces, nano filler reinforced composites and existence of void. The simulated results show that relaxation time becomes longer as intermolecular forces are strong. It is because intermolecular forces capture particles and disturb the relaxation. In the case of filler and void, relaxation time becomes long by the effects of filler and short by the effects of void. In addition, at high temperature, we found that the affection of filler on relaxation was weak, while the affection of void on relaxation was strong. These results imply that the glass transition temperature will rise by filler reinforcement and drop by existence of void.

Phase behaviour, transport properties, and interactions in Li-salt doped ionic liquids

We report on the influence of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) doping on the glass transition temperature (Tg), the ionic conductivity, and Li-ion coordination of two dicationic ionic liquids (DILs) based on the TFSI anion. The results are compared to the behaviour of traditional mono-cationic ionic liquids. The cations of the DILs contain two imidazolium rings, connected by a decane hydrocarbon chain. Homogeneous mixtures of these ILs and LiTFSI can be obtained in a large concentration range. With increasing Li-salt concentration the ionic conductivity decreases whereas the glass transition temperature increases in both systems. However, the influence of the salt doping on the ionic conductivity and the glass transition temperature is low compared to typical mono-cationic ionic liquids, based on for example the pyrrolidinium cation and the TFSI anion. This behaviour is mirrored in the average coordination number of TFSI anions around Li-ions, determined by Raman spectroscopy. The coordination number is systematically lower in the DILs, suggesting a connection between the difference in the Li-ion environment and the behaviour of the glass transition and the ionic conductivity. A Tg-scaled Arrhenius plot of the ionic conductivity shows that the ionic conductivity for all LiTFSI concentrations has the same temperature dependence, i.e., the fragility of the liquid is the same. This implies that the conduction process is dominated by the viscous properties of the liquids over the entire concentration range. This provides further support for linking the local environment of the Li-ions to the glass transition and conduction process in the ionic liquid/salt mixtures.

Study on O-Terphenyl/Polystyrene Blends: How Plasticizer Affects the Glass Transition of Polystyrene?

The effect of plasticizer o-terphenyl on the glass transition of polystyrene was investigated by Fourier transform infrared spectroscopy from a new aspect. The peak areas of four conformation insensitive bands as a function of temperature were studied, these being assigned to the vibrational modes of main chain groups and side groups of polystyrene. It was shown that the reorientation relaxation temperature of the main chain around glass transition was lower than that of the side groups when polystyrene was plasticized by o-terphenyl. It was explained on the basis of cohesional entanglements of polystyrene chains. The reorientation relaxation region of the side groups was nearly the same as the macroscopically observable glass transition region of polystyrene, implying that the glass transition process of polystyrene was dominated by the reorientation of side groups.