Investigation of the Phase Diagram of the Na2O–B2O3–SiO2 System by the Small-Angle X-ray Scattering Technique

Sodium borosilicate melts containing 60 mol % SiO₂ are investigated in the temperature range T _g–1100°C by the small-angle X-ray scattering (SAXS) technique. The temperature dependences of the SAXS intensity for the studied melts consist of two linear portions. A change in the slope of these portions is observed at temperatures that coincide, to within the accuracy of the SAXS experiment (±5–10 K), with the liquidus temperatures, which are usually determined to approximately identical accuracy. The change in the slope is associated with the change in the temperature coefficients of the isothermal compressibility upon transition from a liquid state to a supercooled liquid state. Similar results were obtained earlier for binary melts in the sodium borate system and a number of other systems. On this basis, it is assumed that the transition from a liquid state to a supercooled liquid state for melts in any glass-forming system can be revealed from the temperature dependences of the SAXS intensity, and the SAXS technique can be used to determine the liquidus temperatures, primarily for glass-forming melts with a low crystallization ability.     

On the estimation of the Kauzmann temperature from relaxation data

The well known WLF equation describing the relaxation behaviour of glass-forming liquids near the glass transition temperature has been rederived on the basis of Adam and Gibbs' excess entropy model, making use of a novel expression for the entropy of undercooled liquids. It has been shown that C₂ in the WLF equation is a nearly constant fraction of (T_s - T₂), where T_s and T₂ are the reference and the Kauzmann temperatures, respectively. It is demonstrated that the values of T₂ obtained from the relaxation data agree well with those calculated from thermodynamic data. The arguments used provide an explanation for the universality of the WLF constant, C₂.     

Molecular dynamics study on the viscosity of glass-forming systems near and below the glass transition temperature

Temperature-dependent viscosity is critical to decipher two profound questions in condensed matter physics, namely the glass transition and the relaxation of amorphous solids. However, direct measurement of viscosity over a large temperature range is extremely difficult. Here, using classical molecular dynamics (MD) simulations, we report a novel method to calculate the equilibrium viscosity of supercooled liquid both above and below the glass transition temperature (T_g) and to estimate the nonequilibrium viscosity of glass down to room temperature. Based on the shoving model, we derived an analytical formula showing that the shear viscosity in logarithmic scale changes linearly with the shear-induced variation in shear modulus or potential energy of the glass-forming system. The shear viscosity as a function of steady-state potential energy of liquid under different shear strain rates can be directly calculated in MD simulations; together with its equilibrium potential energy, one can extrapolate the zero-strain-rate equilibrium viscosity. We verified the proposed model by reliably calculating equilibrium viscosity near T_g of four glass-forming systems (Kob–Andersen system, silica, Cu_45.5Zr_45.5Al₉, and silicon) with different fragilities. Furthermore, our model can estimate the nonequilibrium viscosity of glass below T_g; the upper-bound nonequilibrium viscosity of amorphous silica and silicon at room temperature are calculated to be ~10³² and 10²⁵ Pa·s, respectively.     

Dilatometric fragility and prediction of the viscosity curve of glass-forming liquids

Viscosity and coefficient of thermal expansion (CTE) are both crucial properties in the design of new glasses for various applications. In this work, we extend the application of dilatometry to measure two important parameters governing the viscosity of glass-forming systems, viz., glass transition temperature and fragility index. We also describe a method to determine the dilatometric fictive temperature (T_f,DIL) and present data for five unique glass compositions covering a range of fragilities spanning 38-96, which are subjected to cooling and reheating rates in the range 1-30 K/min. The results show that the glass transition temperature obtained from the dilatometric method at 10 K/min (T_g,DIL) is consistent with both viscosity-based (T_g,vis) and DSC-based measurements (T_g,DSC). It is shown that the fragility of a liquid (m_vis) can be determined by calibrating the dilatometric fragility (m_DIL) with the same empirical model as in the calorimetric approach. Put together, we have developed a reliable method to measure the fragility and predict the viscosity curves of glass-forming liquids over a wide range (eg, 10¹-10¹⁶ Pa·s) without direct viscosity measurements, while simultaneously obtaining the CTE of the glass. However, this method is not suitable for glasses with a strong tendency toward phase separation or crystallization.     

Nucleation and Crystallization Kinetics in Fragile Glass-Forming Liquids

Isothermal kinetics of crystallization in the "fragile" Ca(NO₃)₂─KNO₃ melts and in AgI─Ag₂SO₄─Ag₂WO₄ melts of intermediate fragility have been investigated using singlestep and multistep calorimetric techniques. Time-temperature-transformation curves for crystal nucleation and growth have been delineated and the temperature for maximum nucleation rate ( T _NN) identified. The results are compared with the kinetics of nucleation observed in other fragile systems, such as fluoride glass melts, and with classical oxide melts (such as Li₂Si₂O₅) which have "strong" liquid characteristics. Reduced-temperature presentations of nucleation-rate data show qualitative correlations between T _NN/ T _L ( T _L is liquidus temperature) and liquid fragility. These correlations show that strong-liquid glass formers survive much larger supercoolings without nucleation than do fragile liquids.     

Fusion bonding of supercooled poly(ethylene terephthalate) between Tg and Tm

Because of its slowly crystallizing nature, poly(ethylene terephthalate) (PET) can be supercooled into an amorphous glass by rapid quenching. Upon reheating between T_g and T_m, the amorphous PET are subjected to two competing processes: rubber softening and crystallization. Fusion bonding of two such crystallizable amorphous polymer sheets in this processing temperature window is thus a complex process, different from fusion of purely amorphous polymer above T_g or semicrystalline polymer above T_m. In this study, the interfacial morphological development during fusion bonding of supercooled PET in the temperature window between T_g and T_m was studied. A unique double‐zone interfacial morphology was observed at the bond. Transcrystals were found to nucleate at the interface and grow inward toward the bulk and appeared to induce nucleation in the bulk to form a second interfacial region. The size and morphology of the two zones were found to be significantly affected by the fusion bonding conditions, particularly the fusion temperature. The fusion bonding strength determined by the peeling test was found to be significantly affected by the state of crystallization and the morphological development at the bonding interface. Based on the interfacial morphology observed and the bonding strength measured, a fusion bonding mechanism of crystallizable amorphous polymer was proposed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Temperature dependences of the density of borate glasses in equilibrium states at temperatures below the glass transition point

The kinetics of density relaxation during isothermal treatment of samples of vitreous boron oxide and sodium borate glasses (containing 15, 20, and 30 mol % Na₂O) cooled from high temperatures and after heating of the samples stabilized at low temperatures is investigated over a wide range of temperatures below the glass transition point T _g . It is established that there exists a temperature T _s below which the density in the equilibrium state does not depend on the temperature, i.e., the temperature at which the supercooled liquid transforms into a new noncrystalline solid state. The distinguishing feature of this state is the absence of structural transformations after a change in the temperature. The temperatures of the transition of the supercooled liquid to the noncrystalline solid state and the ranges of lower temperatures at which the supercooled liquid can reach an equilibrium state within the limits of experimental error coincide with those previously obtained in the study of relaxation processes in these glasses by the small-angle X-ray scattering technique.     

Precipitation of nanosized crystals CuBr and CuCl in potassium aluminoborate glasses

The precipitation of nanosized crystals CuBr and CuCl in potassium aluminoborate glasses containing additives of jointly introduced Cu and Br, as well as Cu and Cl, respectively, has been studied by the methods of small-angle X-ray scattering (SAXS) and X-ray powder diffraction (XRPD) analysis. It has been found that, upon thermal treatments at temperatures close to the glass transition temperature T _g , halide phases precipitate in the form of liquid drops containing CuBr and KBr or CuCl and KCl. The presence of nanocrystals CuBr or CuCl, respectively, has been established in the samples at room temperature by the XRPD method. Liquidus, solidus, and crystallization onset temperatures in the regions of precipitated halide phases in heat treated glass samples have been determined from the temperature dependences of SAXS intensity. The liquid released upon the heat treatment inside a matrix of potassium aluminoborate glass remains in the state of a supercooled liquid at temperatures essentially below the solidus temperature. At the drop sizes of the order of 10 nm, crystallization processes in them start at temperatures of 40?C85°C.     

Thermal behavior of transparent poly(etheretherketone)(PEEK) film

The thermal behavior of poly(etheretherketone)(PEEK) film heated in an open differential scanning calorimetry (DSC) pan at 20°C/min is distorted by relaxation of the strained film. PEEK film in a closed pan or quenched PEEK in open or closed pans shows a glass-transition temperature (T_g) around 144°C, cold crystallization (～22 J/g) at 177°C, melt-temperature (T_m) peaking at 335–340°C, with an enthalpy of fusion of 32–34 J/g, and recrystallization on cooling at 285°C, with a crystallization exotherm of about 40 J/g. The enthalpy of fusion decreases with increasing heating rate from 2–100°C/min and approaches the enthalpy of cold crystallization. With increasing heating rate, further crystallization of PEEK during the DSC scan is suppressed. With increasing cooling rate, PEEK melt crystallizes at larger supercoolings to a lesser extent. Crystallization on cooling the melt was more complete than cold crystallization and annealing on heating.     

Fiber spinnability of glass melts

Fiber spinnability is the ability of a glass-forming melt to be steadily stretched and spun into defect-free fiber filaments. However, its quantification has not been well established owing to many controlling factors such as melt fragility, melt strength, surface tension, liquidus temperature, liquidus viscosity, and crystallization. To understand and quantify the fiber spinnability of a glass melt, we consider two key aspects: fiberizing viscosity window and melt stability. The fiberizing viscosity window is defined by the upper and lower viscosity limits. Fibers rupture above the upper viscosity limit, whereas a stable melt stream cannot form below the lower limit (η_low). We introduce a simple parameter to quantify fiber spinnability, namely, K_fib=η_L/η_low, where η_L is the viscosity at liquidus temperature (T_L). A fiber can only form if K_fib>1. To quantify melt stability we propose the parameter of S=(T_L-T_C)/(T_L-T_g), where T_L and T_C are the liquidus temperature, and the onset temperature of melt crystallization during cooling, respectively. Both parameters (K_fib and S) are important for a rational design of glass fiber compositions, and fiberizing process. We use two basalt melts as examples of this study to demonstrate the high sensitivity of fiber spinnability to a minor variation in chemical composition of melts.     

Constant‐temperature embossing of supercooled polymer films

In conventional hot embossing, a thermoplastic polymer undergoes phase transitions in liquid, semi‐solid, and solid states through cyclic heating and cooling. This paper, in contrast, describes the development of a constant‐temperature embossing process and compares its characteristics against standard hot embossing. The new process utilizes the crystallizing nature of supercooled polymer films to obtain the necessary phase transitions. By softening and crystallizing the supercooled polymer at the same temperature, the embossing and solidification stages can be carried out isothermally without a cooling step. PET, due to its relatively slow crystallizing kinetics, was chosen as a model material for this study. The embossed films with microgroove patterns of different sizes and aspect ratios were characterized for their replication fidelity and accuracy. For supercooled PET films, constant‐temperature embossing with high replication quality and acceptable demolding characteristics was achieved in a large processing temperature window between T_g and T_m of PET. A parametric process study involving changes of the embossing temperature and embossing time was conducted, and the results indicated that the optimal process parameters for constant‐temperature embossing can be derived from the crystallization kinetics of the polymer. The removal of thermal cycling is a major advantage of constant‐temperature embossing over conventional hot embossing and represents an important process characteristic desired in industrial production. POLYM. ENG. SCI., 54:1100–1112, 2014. © 2013 Society of Plastics Engineers     

Mg and Al mixed effects on thermal properties in aluminosilicate glasses

By using the aerodynamic levitation and laser melting technique to well extend the glass-forming region into the Mg-rich and peraluminous regime, a series of magnesium aluminosilicate glasses were prepared to investigate the Mg and Al mixed effects on thermal properties, including glass transition temperature (T_g), crystallization behavior, and thermal stability. With the gradual substitution of Mg by Al, T_g exhibits two types of near-linear rises with different slopes in two compositional regions separated by r = 0.57, where r is equal to the molar ratio of [Al₂O₃]/([Al₂O₃] + [MgO]). Moreover, when it comes to other properties, that is, crystallization behavior and thermal stability, this critical point precisely appears at the same r = 0.57. Compared to the slower increase of T_g in Mg-rich region, the steeper rise of T_g in the peraluminous region is mainly ascribed to the step-by-step formation of oxygen triclusters driven by Pauling's second rule. Moreover, the occurrence of the critical point for T_g rise at r = 0.57 rather than the theoretical 0.5 can be seen as a proof of the role of Mg cations partly as a network former.     

Factors determining glass transition temperature and relaxation time of poly(vinyl chloride)

Glass transition properties were obtained by measurements of the dilatometric glass transition temperature T_g at elevated pressure by isobaric cooling at a constant rate. The properties include the pressure-volume-temperature (P-V-T) relations of the liquid, the specific heat capacity and the dielectric properties for a single sample of poly(vinyl chloride). From the experimental results, various thermodynamic excess quantities such as the free volumes and the configurational entropy and energies were evaluated as factors determining T_g and the relaxation time τ. Some empirical rules concerning T_g, which have been proposed by Simha and Boyer, and Wunderlich, are also examined. The main conclusions are as follows: (1) the free volume is not an essential factor determining T_g and τ; (2) the Adam-Gibbs parameter is slightly superior to the other excess entropy and energies defined as the difference of the entropy and energies between the liquid and the hypothetical crystal whose thermal expansivity and compressibility are the same as those of the corresponding glasses; and (3) the glass transition may not be treated as a quasi-equilibrium thermodynamic transition to which one of the Ehrenfest relations applies.     

Crystallization Behavior and Mechanical Properties of Mg86.33Ni12.67Y1 Amorphous Alloy

Mg_86.33Ni_12.67Y₁ amorphous alloy was obtained by a single-roller melt-spinning technique and some samples were isothermally annealed at different temperatures. The glass-forming ability (GFA), crystallization behavior and the effect of annealing treatment on the mechanical properties of the alloy were studied. The results show that for Mg_86.33Ni_12.67Y₁ amorphous alloy, the reduced glass transformation value T_rg (T_rg = T_g/T_l) is 0.5089 and the width of super-cooled liquid region (ΔT_x) is 40.08 K. When the samples were annealed at 413 K, the degree of short-range order of the amorphous atoms as well as the micro-hardness, increased due to structural relaxation, with increasing time, completing at 10 min. Significant crystallization of the Mg_86.33Ni_12.67Y₁ alloy occurred after annealing above 443 K, and two crystalline phases, Mg₆Ni and Ni₇Y₂ were precipitated into the amorphous matrix. With increasing annealing temperature (range: 573–653 K), Mg₆Ni crystals were gradually transformed into Mg₂Ni and α-Mg. The micro-hardness decreased significantly with temperatures above 413 K. This was accompanied by a decrease in free volume of the matrix during the crystallization of the two phases.     

Development, characterization and stabilization of amorphous form of a low Tg drug

The objective of the present study was to develop a stable amorphous form of model drug carvedilol (CAR). The amorphous material produced by melt quench technique was subjected to physico-chemical characterization. Chemical stability of the drug during preparation of glass was tested by HPLC and IR spectroscopy and presence of amorphous form was confirmed by DSC and XRPD. The rate of dissolution and magnitude of the apparent solubility were found to be significantly higher for amorphous CAR than for crystalline CAR, at 25 °C. However at 37 °C, it was observed that dissolution of the amorphous form did not show a noticeable improvement over pure CAR over the period of 60 min, due to formation of cohesive supercooled liquid state. This observation was supported by enthalpy relaxation study, which indicated increase in enthalpy recovery and structural relaxation of amorphous form towards the supercooled liquid region. This indicated the functional inability of amorphous CAR from stability point of view and suggested the need for elevation of T_g. Hence combination of solid dispersion (SD) and surface adsorption techniques was attempted to overcome the functional limitations of amorphous CAR. SD in the ratio of 1:2:2 parts by weight of CAR, PVP (for elevation of T_g) and Aerosil^® 200 (as adsorbent) respectively presented dramatic improvement in rate and extent of drug dissolution. During accelerated stability studies with SD 1:2:2 the dissolution characteristics were slightly decreased over the period of 3 months and no crystallization events were observed. Thus, to exploit the functional advantage of amorphous form of low T_g drugs, formation of ternary SD system is recommended.     

Ion transport properties of lithium ionic liquids and their ion gels

A new series of lithium ionic liquids were prepared by introducing of two electron-withdrawing trifluoroacetyl groups in borate salts containing two methoxy-oligo(ethylene oxide) groups in the structures. Successive substitution reactions of oligo-ethylene glycol monomethyl ether and trifluroacetic acid from LiBH₄ yielded the lithium salts, which were clear and colorless liquids at room temperature. The fundamental physicochemical properties, such as density, thermal property, viscosity, ionic conductivity, self-diffusion coefficients, and electrochemical stability, were measured. The lithium ionic liquids had self-dissociation ability and conducted ions even in the absence of organic solvents. New polymer electrolytes, named ‘ion gels’, were prepared by radical cross-linking reactions of a poly(ethylene oxide-co-propylene oxide)tri-acrylate macromonomer in the presence the lithium ionic liquid. An increase in the glass transition temperatures (T_g) of the ion gels was very small even with increasing lithium ionic liquid concentration, and the T_g's were lower than that of the ionic liquid itself. The ionic conductivity of the ion gels surpassed that of the lithium ionic liquid in the bulk at certain compositions.     

Isothermal crystallization kinetics in binary miscible blend of poly(ε‐caprolactone)/tetramethyl polycarbonate

The crystallization kinetics of pure poly(ε‐caprolactone) (PCL) and its blends with bisphenol‐A tetramethyl polycarbonate (TMPC) was investigated isothermally as a function of composition and crystallization temperature (T_c) using differential scanning calorimetric (DSC) and polarized optical microscope techniques. Only a single glass‐transition temperature, T_g, was determined for each mixture indicating that this binary blend is miscible over the entire range of composition. The composition dependence of the T_g for this blend was well described by Gordon–Taylor equation with k = 1.8 (higher than unity) indicating strong intermolecular interaction between the two polymer components. The presence of a high T_g amorphous component (TMPC) had a strong influence on the crystallization kinetics of PCL in the blends. A substantial decrease in the crystallization kinetics was observed as the concentration of TMPC rose in the blends. The crystallization half‐time t_0.5 increased monotonically with the crystallization temperature for all composition. At any crystallization temperature (T_c) the t_0.5 of the blends are longer than the corresponding value for pure PCL. This behavior was attributed to the favorable thermodynamics interaction between PCL and TMPC which in turn led to a depression in the equilibrium melting point along with a simultaneous retardation in the crystallization of PC. The isothermal crystallization kinetics was analyzed on the basis of the Avrami equation. Linear behavior was held true for the augmentation of the radii of spherulites with time for all mixtures, regardless of the blend composition. However, the spherulites growth rate decreased exponentially with increasing the concentration of TMPC in the blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3307–3315, 2007     

The polydispersity of ethylene sequence in metallocene ethylene/α-olefin copolymers III: crystallization and melting behavior of short ethylene sequence

Crystallization and melting behavior of short ethylene sequence of metallocene ethylene/α-olefin copolymer with high comonomer content have been studied by standard DSC and modulated-temperature differential scanning calorimetry (M-TDSC) technique. In addition to high temperature endotherm around 120°C, a low temperature endotherm is observed at lower temperatures (40-80°C), depending on time and temperature of isothermal crystallization. The peak position of the low temperature endotherm T_m^low varies linearly with the logarithm of crystallization time and the slope, D, decreases with increasing crystallization temperature T_c. The T_m^low also depends on the thermal history before the crystallization at T_c, and an extrapolation of T_m^low (30.6°C) to a few seconds has been obtained after two step isothermal crystallization before the crystallization at 30°C. The T_m^low is nearly equal to T_c, and it indicates that the initial crystallization at low temperature is nearly reversible. Direct evidence of conformational entropy change of secondary crystallization has been obtained by using M-TDSC technique. Both the M-TDSC result and the activation energy analysis of temperature dependence suggest that crystal perfection process and conformational entropy decreasing in residual amorphous co-exist during secondary crystallization.     

Investigation of the fast relaxation in glass-forming selenium by low-frequency Raman spectroscopy

The low-frequency Raman spectra of liquid and vitreous selenium are investigated. It is demonstrated that the temperature dependence of the intensity of the fast relaxation at the glass transition temperature (T _g = 308 K) exhibits a specific feature. This feature manifests itself in a sharper increase in the intensity at temperatures T > T _g as compared to that observed at lower temperatures. The intensities of the fast relaxation at the critical temperature T _c are evaluated by the extrapolation of the linear dependence to the temperature range T > T _g in the framework of the mode-coupling theory. The new results obtained for selenium are compared with the available data for other glass-forming materials (boron oxide, toluene, arsenic sulfide). It is shown that, for all the glasses under investigation, the parameter describing the contribution of the fast relaxation to the Raman spectrum takes on the same value at the critical temperature T _c and is approximately equal to 0.3.     

Structural,mechanical, and in vitro characterization of freeze-cast scaffolds prepared using a sol-gel-derived bioactive glass from the SiO2–CaO–Na2O–P2O5–K2O–MgO system

This work deals with the preparation of freeze-cast scaffolds using a bioactive glass from the SiO₂–CaO–Na₂O–P₂O₅–K₂O–MgO system. This material could be sintered at lower temperatures (650 °C) than other variations of bioactive glasses, which is an important advantage in terms of energy and cost savings. This behavior represents a great advantage in terms of energy and cost savings. The freeze-casting step was conducted using water as a solvent and liquid nitrogen as a coolant. The prepared samples were examined according to their pore structure, thermal behavior, mechanical stability, and bioactivity. The glass transition temperature (T_g), crystallization onset temperature (T_x), and maximum crystallization temperature (T_c) evaluated for this bioactive glass were about 660 °C, 690 °C, and 705 °C. Consequently, the freeze-cast scaffolds could be sintered at 650 °C for 2–8 h, which favored viscous flow sintering without crystallization. Bioactivity assays were conducted by soaking the scaffolds in simulated body fluid for up to 21 days, showing that these materials present a bioactive behavior, inducing hydroxyapatite formation. These materials' mechanical properties and biocompatibility make them promising candidates for use in trabecular bone repair.