Composition Dependence Of Tg Research Articles

Composition‐dependentglass transition temperature in mixtures: Evaluation of configurational entropy models*

AbstractThe composition dependence of the glass transition temperature (Tg) in mixtures remains an important unsolved problem. Here, it is revisited using three model systems: a series of oligomeric and polymeric cyanurates, blends of oligomeric and polymeric α‐methyl styrene, and molecular mixtures of itraconazole and posaconazole. We evaluate several entropy‐based models to determine the theoreticalTgas a function of molecular composition and compare the results against the experimental data. The assumption that the configurational entropy is invariant at theTgis tested, where the change in configurational entropy is assumed to be given by the integral of ΔCpdlnT, where ΔCpis the temperature‐dependent change in the heat capacity atTg. We find that, although the temperature‐dependent heat capacities in both liquid and glassy states are nearly independent of composition for several of the systems studied (i.e., they are nearly ideal mixtures), the composition dependence ofTgis not well described by simply adding the changes in the mass‐weighted configurational entropy of the components on going from theTgin the pure state to that of the blend. The implication is that either configurational entropy is not invariant atTgor that it cannot be obtained from the integral of ΔCpdlnT.

Prediction of the Synergistic Glass Transition Temperature of Coamorphous Molecular Glasses Using Activity Coefficient Models.

The glass transition temperature (Tg) of a binary miscible mixture of molecular glasses, termed a coamorphous glass, is often synergistically increased over that expected for an athermal mixture due to the strong interactions between the two components. This synergistic interaction is particularly important for the formulation of coamorphous pharmaceuticals since the molecular interactions and resulting Tg strongly impact stability against crystallization, dissolution kinetics, and bioavailability. Current models that describe the composition dependence of Tg for binary systems, including the Gordon-Taylor, Fox, Kwei, and Braun-Kovacs equations, fail to describe the behavior of coamorphous pharmaceuticals using parameters consistent with experimental ΔCP and Δα. Here, we develop a robust thermodynamic approach extending the Couchman and Karasz method through the use of activity coefficient models, including the two-parameter Margules, non-random-two-liquid (NRTL), and three-suffix Redlich-Kister models. We find that the models, using experimental values of ΔCP and fitting parameters related to the binary interactions, successfully describe observed synergistic elevations and inflections in the Tg versus composition response of coamorphous pharmaceuticals. Moreover, the predictions from the NRTL model are improved when the association-NRTL version of that model is used. Results are reported and discussed for four different coamorphous systems: indomethacin-glibenclamide, indomethacin-arginine, acetaminophen-indomethacin, and fenretinide-cholic acid.

Thermal studies of the effect of thallium in ternary Ge-Te-Tl chalcogenide glasses

Alternating differential scanning calorimetric (ADSC) studies have been carried out on bulk Ge17Te83-xTlx glasses over a wide range of compositions (0 ≤ x ≤ 13), to estimate the thermal parameters and also to understand the effect of thallium addition on the non-isothermal crystallization process. It is found that glass transition temperature (Tg) decreases with increasing thallium concentration, indicating decrease in network connectivity with thallium addition. The decrease in Tg is large in the composition range 0 ≤ x ≤ 6 and much less for compositions x ≥ 7. The observed composition dependence of Tg is similar to the variation in switching voltages with composition and is in close agreement with the variation in the average bond energy. Further, two crystallization temperatures (Tc1 and Tc2) have been observed in Ge17Te83-xTlx glasses, in the composition range 2 ≤ x ≤ 10, which indicates that glasses in this range are phase separated. As these glasses are tellurium rich, it is probable that the regions formed by Te- chains crystallize first and the rest of the matrix crystallizes later. The change in enthalpy due to non-reversing part of the heat flow is very small and fairly constant over the entire range of glasses studied. This indicates that structural re-organization is easy in this system which agrees well with the memory type electrical switching observed in these samples.

ESR, physical and structural studies on Mn2+ doped in mixed alkali phosphate glasses

Glass formation and properties of manganese phosphate glasses containing both Na2O and K2O oxides with the general formula 49.95[xNa2O-(1-x)K2O]-0.1MnO2-49.95P2O5 with x = 0–1 mol%, have been investigated. The vitreous samples were prepared by standard melt quenching. To get an insight into their physical properties, the density and the glass transition temperature were procedure determined. The structure of the glass was performed by Infrared, Raman, and electron spin resonance (EPR) spectroscopies. The results obtained by these techniques have allowed us to explain their structure and properties upon the variation of the chemical composition. The variation of the glass transition temperature as a function of the composition presents a minimum for the ratio Na/Na + K = 0.5. Infrared and Raman spectroscopies has identified the presence of different structural grouping units in the glassy-network. It is found that the stretching vibration νs(PO2)− and νs(P–O–P) are more sensitive to the substitution of alkali elements. EPR experiments have shown the presence of Mn2+ centers in the glasses. The variation of the g-factor as a function of the composition is non-linear. The non-linearity behavior of the composition dependence of Tg, vibration bending mode, and the g-factor are a fingerprint of the mixed alkali effect in the glasses under study.

Investigation of Novel Low Melting Phosphate Glasses Inside the Na2O–K2O–ZnO–P2O5 System

Phosphate glasses containing different concentrations of zinc oxide inside the (1 − x)(NaPO3–KPO3)–xZnO system (0 ≤ x ≤ 50 mol%) have been prepared using the conventional melt quenching technique. The prepared glasses were transparent, bubble-free and colourless. Their density, molar volume, glass transition temperature, and structural properties using infrared and Raman spectroscopies are investigated. As the content of ZnO increases, the density increases while the molar volume decreases. The composition dependence of Tg shows a minimum for the glass (x = 20 mol%). Structural approach realized by IR and Raman spectroscopies reveals that zinc ions occupy different sites in the glassy-network, mainly modifier sites and middle phosphate network in low-zinc and high-zinc glasses, respectively. The introduction of ZnO in the network induces some structural rearrangements through the conversion of metaphosphate structural units to pyrophosphate ones. It is also highlighted that the presence of ZnO in the glassy matrix allows the transformation of some P–O–P and P=O bonds to P–O–Zn linkages. From the UV–Visible absorption studies, the values of the optical band gap, Eg, and Urbach energy, ΔE, were evaluated. The optical band gap is found to depend on the glasses composition. Eg and ΔE show a minimum and a maximum respectively, for the glass (x = 20 mol%).

A Study of Thermal Stability And Crystallization Kinetics of SbSeGe Glassy Alloys

Alloys of Sb10Se90-xGex (x = 0, 19, 21, 23, 25, 27) have been prepared using melt quenching technique. Differential thermal analysis has been used to determine the three characteristic temperatures, glass transition (Tg ), glass crystallization (Tc ) and melting temperature (Tm ), at four heating rates 5, 10, 15 and 20 K/min. The thermal stability and ease of glass formation have been evaluated in terms of Hruby parameter and reduced glass transition temperature respectively. The activation energy for glass crystallization (Ec ) has been calculated using Kissinger's method and Marseglia theory. The composition dependence of Tg and Ec has also been discussed.

Isothermal-crystallization kinetics and spherulite growth of aliphatic polyketone/polyamide-6 blends

In this research, the morphologies, isothermal-crystallization kinetics, and spherulite growth of aliphatic polyketone/polyamide-6 blends were studied. A single glass-transition temperature (T g) was determined, and the composition dependence of T g for these blends was well described by the Kwei equation. The strong intermolecular interaction between the two polymer components was confirmed by melting-point depression. The isothermal-crystallization kinetics were analyzed on the basis of the Avrami approach. A linear increase in the radii of the spherulites with time was observed for all compositions. All the spherulites continued to grow at nearly identical growth rates. With increasing polyamide-6 content, the size of the spherulites in the polyketone/polyamide-6 blends gradually decreased, and the number of spherulites in the blends increased.

Local Minimum in Fragility for Trehalose/Glycerol Mixtures: Implications for Biopharmaceutical Stabilization

Approximately a decade ago it was observed that adding a small amount (5 wt %) of glycerol to trehalose could substantially improve the stability of enzymes stored in these glasses even though the final glass transition temperature (Tg) was reduced by ∼20 K. This finding inspired great interest in the fast dynamics of dehydrated trehalose/glycerol mixtures, leading to the observation that suppression of fast dynamics was optimal in the presence of ∼5 wt % of glycerol. It was also recognized that the fast dynamics should, in theory, be related to the fragility of these glass formers, but experimental confirmation of this hypothesis has been lacking for trehalose/glycerol mixtures or any other mixtures of this nature. In the present study a dynamic mechanical analyzer (DMA) was used to determine both the Tg and the kinetic fragility index (m) of trehalose/glycerol mixtures within the mass fraction range of 80-100 wt % of trehalose. It was found that the fragility index correlated with the mass fraction of trehalose in a nonmonotonic manner, with a local minimum between 87.5 and 95 wt % of trehalose, whereas the composition dependence of Tg was found to follow a Gordon-Taylor-like relationship, with no local minimum. The composition of 5-12.5 wt % glycerol in trehalose thus yielded a matrix that maximized the strong glass-forming contribution of glycerol, while minimizing its Tg lowering effect. This quantitative evidence supports speculation about the fragility characteristics of these mixtures that has been ongoing for the past decade. The DMA-based Tg and fragility determination method developed in this study represents a new approach for identifying optimal compositions for preservation of biologics.

A simple method for tuning the glass transition process in inorganic phosphate glasses.

The physical modification of glass transition temperature (Tg) and properties of materials via blending is a common practice in industry and academia and has a large economic advantage. In this context, simple production of hitherto unattainable new inorganic glass blends from already existing glass compositions via blending raises much hope with the potential to provide new glasses with new and improved properties, that cannot be achieved with classical glass synthesis, for a plethora of applications such as computers screens, glass-to-metal seals, and storage materials for nuclear wastes. Here, we demonstrate that blends of the specific glass compositions studied are miscible in all proportions, an unreported phenomenon in hard condensed matter like glass. Interestingly, excellent agreement was found between the obtained data and calculated Tgs from theoretical equations (Supplementary information) for predicting the composition dependence of Tg for miscible blends with weak but significant specific interactions between the blend components. That this blending method is at present not applied to inorganic glasses reflects the fact that water and chemically resistant phosphate glasses with relatively low Tgs have become available only recently.

Effects of blending on the molecular dynamics of highly interacting binary polymer blends of poly(methyl methacrylate) and poly[styrene-co -(maleic anhydride)

The molecular dynamics and miscibility of highly interacting binary polymer blends of poly(methyl methacrylate) (PMMA) and poly[styrene-co-(maleic anhydride)] random copolymer with 8 wt% maleic anhydride content (SMA) were investigated as a function of composition over a wide range of frequency (10−2–106 Hz) at different constant temperatures (30–160 °C). Only one common glass relaxation process (α-process) was detected for all measured blends, and its dynamics and broadness were found to be composition dependent. The existence of only one common α-relaxation process located at a temperature range between those of the pure polymer components indicated the miscibility of the two polymer components over the entire range of composition. The miscibility was also confirmed by measuring the glass transition temperatures of the blends, Tg, using differential scanning calorimetry. The composition dependence of Tg of the blends showed a positive deviation from the linear mixing rule and well described by the Gordon–Taylor–Kwei equation. The relaxation spectrum of the blends was resolved into α- and β-relaxation processes using the Havriliake–Negami (HN) equation and ionic conductivity. The dielectric relaxation parameters obtained from HN analysis, such as broadness of relaxation processes, maximum frequency, fmax, and dielectric strength, Δϵ (for the α- and β-relaxation processes), were found to be blend composition dependent. The kinetics of the α-relaxation process of the blends were well described by the Meander model, while an Arrhenius-type equation was used to evaluate the molecular dynamics of the β-relaxation process. Blending of PMMA and SMA was found to have a considerable effect on the kinetics and broadness of the β-relaxation process of PMMA, indicating that the strong interaction and miscibility between the two polymer components could effectively change the local environment of each component in the blend. © 2013 Society of Chemical Industry

Investigation on Anomalous Calorimetric Behaviors of Polystyrene and Triphenylmethyl Chloride Blends

Polystyrene (PS), with three different molecular weights, and triphenylmethyl chloride blends were used to investigate their “cusp” in Tg-composition dependence by DSC. There exists a cusp in all three systems; with PS molecular weight increasing the critical concentration increases. The fitting results of three classical thermodynamics equations show the blends are not completely miscible. A free volume theory predicts the appearance of the cusp successfully. The dependence of PS content in amorphous phase of the mixtures on total PS concentration shows that when polymer concentration is low, the composition of the amorphous phase matrix is constant. For high polymer content blends, the relation indicates the polymer solution was completely amorphous and no crystalline phase appears, so it could be considered, approximately, as an “ideal solution.” The microphase separation, caused by molecular size asymmetry was proposed to explain the “cusp” in polymer–solvent systems. The investigation of heat capacity confirmed the theory proposed.

Calorimetric study of blend miscibility of polymers confined in ultra-thin films

Miscibility in blends of polystyrene and poly(phenylene oxide) (PS/PPO) confined in thin films (down to 6 nm) was investigated using a recently developed sensitive differential alternating current (AC) chip calorimeter. Comparison of composition dependence of glass transition in thin films with common models should provide information on miscibility. This study focuses on the blend system polystyrene and poly(phenylene oxide) (PS/PPO) because it is thought as a miscible model system in the whole composition range. Furthermore, its local dynamic heterogeneity is already identified by dynamic mechanic thermal analysis (DMTA) and solid state NMR techniques. For this blend, we find that even for the thinnest films (6 nm, corresponding to about half of PPO’s radius of gyration R g) only one glass transition is observed. The composition dependence of T g is well described by the Fox, Couchman or Gordon-Taylor mixing law that are used for the miscible bulk blends. Although there is a contradicting result on whether T g decreases with decreasing film thickness between our calorimetric measurements and Kim’s elipsometric measurements on the same blend (Kim et al. Macromolecules 2002, 35, 311–313), the conclusion that the good miscibility between PS and PPO remains in ultrathin films holds for both studies. Finally, we show that our chip calorimeter is also sensitive enough to study the inter-layer diffusion in ultrathin films. PS chain in a thin PS/PPO double layer that is prepared by spin coating PPO and PS thin film in tandem will gradually diffuse into the PPO layer when heated above T g of PS, forming a PSxPPO100−x blend. However, above the PSxPPO100−x blend, there exists an intractable pure PS like layer (∼30 nm in our case) that does not diffuse into the blend beneath even staying at its liquid state over 10 hours.

Calorimetric studies of the glass transition phenomenon in glassy Se75Te15−xCd10Inx alloys using the non-isothermal DSC technique

Glass transition kinetics was studied in a glassy Se75Te15−xCd10Inx (x=0, 5, 10 and 15) system. Differential scanning calorimetric (DSC) runs were taken at four different heating rates (5, 10, 15 and 20 K min−1) for this purpose. The heating rate dependence of the glass transition temperature (Tg) was studied using different non-isothermal methods. The activation energy of glass transition (Eg) was evaluated using Kissinger's relation and the theory of Moynihan. The composition dependence of Tg and Eg is also discussed.

Hydrogen-bond network formation of water molecules and its effects onthe glass transitions in the ethylene glycol aqueous solutions: failureof the Gordon–Taylor law in the water-rich range and absence of theTg = 115 K rearrangement process in bulk pure water

Enthalpy relaxation processes proceeding in ethylene glycol (EG) aqueous solutions[(EG)x(H2O)1 − x] within silica-gel nanopores were studied by adiabatic calorimetry. While thex = 0.25 solution within pores with diameter of 52 nm showed a glass transition atTg = 139 K, ageing of the solution at 160 K caused a phase separation to reveal glass transitions atTg = 145 and 160 K for EG-rich and water-rich regions, respectively: the water moleculesare understood to form a more developed hydrogen-bond network, andconsequently force the EG molecules in between the water-rich regions. TheTg = 160 K is in goodagreement with the Tg value of the internal (not interfacial) water confined within poreswith thickness of 1.1 nm. The ageing further remarkably diminished theTg = 115 K glass transition. This indicates that, while the molecules responsible for the glasstransition are the mobile water ones forming a lower number of hydrogen bonds than four,the fraction of such water molecules is reduced in association with the development of thenetwork and the glass transition is absent in bulk pure water. When the samex = 0.25 solution was confined within 1.1- and 12 nm pores, the water molecules developed ahydrogen-bond network in the pore centre due to the presence of the pore wall and pushed theEG molecules onto the pore surface even at higher temperatures: the water-rich region gaveTg = 155 K close to 160 K. It is concluded that the hydrogen-bond network inherentto water structure is developed/collapsed remarkably in the range nearx = 0; consequently, the composition dependence ofTg inthe bulk system deviates sharply in the range from the Gordon–Taylor empirical law followed for largex > 0.2.

Calorimetric studies of glass transition phenomenon in glassy Se80−xTe20Snx alloys

Glass transition kinetics has been studied in a glassy Se80−xTe20Snx (0 ⩽x ⩽10) system. Differential scanning calorimetry (DSC) runs were taken at four different heating rates (5, 10, 15 and 20 K min−1) for this purpose. The heating rate dependence of the glass transition temperature (Tg) has been studied using different non-isothermal methods. The activation energy of glass transition (Eg) has been evaluated using Kissinger's relation and the theory of Moynihan. The composition dependence of Tg and Eg has also been discussed.

Glass transition temperature of bulk metallic glasses: A linear connection with the mixing enthalpy

A linear relationship is found between the glass transition temperature Tg and the absolute value of the mixing enthalpy, |ΔHchem|, for bulk metallic glass systems. The increasing (or lowering) of Tg with an admixture of metals or other elements manifests itself in the larger (or smaller) of |ΔHchem| in a given system. The results indicate that the composition dependence of Tg results from the change of excess entropy (Sex) during thermal excitation. The |ΔHchem|, which relates to the strength of interaction among different atoms, corresponds to part of the Sex at Tg [Sex(Tg)]. The glass transition temperatures for Cu-Zr (Hf)-, Zr-Cu-, and La-Al-based glassy alloys are correlated with the interaction intensity between their based binary eutectic compositions, respectively.

Dynamics of Poly(cyclohexyl methacrylate): Neat and in Blends with Poly(α-methylstyrene)

Pressure−volume−temperature measurements, calorimetry, and dielectric spectroscopy at ambient and elevated pressures were carried out on poly(cyclohexyl methacrylate) (PCHMA) and its blend with poly(α-methylstyrene) (PaMS). Both the glass transition temperature and local segmental relaxation times, τα, go through a maximum as a function of blend composition, plausibly due to a negative excess volume. Using a nonlinear function for the composition dependence of Tg, a value of the self-concentration parameter of the Lodge−McLeish model is determined, which is close to that calculated from the PCHMA chain statistics. For both neat PCHMA and the blend, the τα superpose as a function of TVγ. Since the breadth of the local segmental relaxation dispersion is a unique function of τα, this means that the parameter γ defines both the magnitude and distribution of relaxation times. The thermodynamic scaling for the blend was corroborated by calculating the exponent γ from the isochronal thermal expansion coefficient, determined from the pressure coefficient of Tg. γ increases upon blending, consistent with a larger activation volume of PCHMA when mixed with the PaMS. PCHMA exhibits a weak β-relaxation, which lacks correspondence to the segmental dynamics and thus is presumably due to local motion of the carbonyl group. A more intense γ-relaxation and a frequency-insensitive loss at higher frequencies are also observed. Unlike the segmental process, these secondary relaxations are insensitive to blending.

Effect of composition on the width of the calorimetric glass transition in polymer–solvent and solvent–solvent mixtures

AbstractThe calorimetric glass‐transition temperature (Tg) and transition width were measured over the full composition range for solvent–solvent mixtures of o‐terphenyl with tricresyl phosphate and with dibutyl phthalate and for polymer–solvent mixtures of polystyrene with three dialkyl phthalates. Tg shifted smoothly to higher temperatures with the addition of the component with the higher Tg for both sets of solvent–solvent mixtures. The superposition of the differential scanning calorimetry traces showed almost no composition dependence for the width of the transition region. In contrast, the composition dependence of Tg in polymer–solvent mixtures was different at high and low polymer concentrations, and two distinct Tg's were observed at intermediate compositions. These results were interpreted in terms of the local length scale and associated local composition variations affecting Tg. The possible implications of these results for the dynamics of miscible polymer blends were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1155–1163, 2004

Hydrogen‐Bonding Interaction and Crystalline Morphology in the Binary Blends of Poly(ε‐caprolactone) and Polyphenol Catechin

AbstractThe specific intermolecular hydrogen‐bonding interaction between the ester carbonyl groups of poly(ε‐caprolactone) (PCL) and the phenolic hydroxyl groups of catechin has been studied by Fourier‐transform infrared spectroscopy (FT‐IR) and differential scanning calorimetry (DSC). According to quantitative curve‐fitting analysis of the FT‐IR spectra of PCL/catechin blends, it was found that the fraction of hydrogen‐bonded carbonyl groups of PCL increased with catechin content, while that of hydrogen‐bonded hydroxyl groups of catechin decreased. The calculated crystallinity of PCL in the binary blends, based on the curve‐fitting results, suggested that the crystallization of PCL was restrained in the blends with catechin. Only single glass transition temperature, Tg, was observed over the whole range of blend compositions, which was between those of the pure components. The melting point, Tm, depressed and Tg increased, indicating also the existence of strong intermolecular association. The blend composition dependence of Tg could be predicted very well by the Kwei equation with a positive ‘q’ value of 124. With the aid of small angle X‐ray scattering measurement, the segregation of catechin was investigated. It was found that the extent of extra‐lamellar segregation increased with catechin content. It was suggested that the crystal growth rate played the dominant role in the formation of morphology. With decreasing crystal growth rate of PCL component in the blends, enough time has been given to catechin molecules to diffuse into extra‐lamellar region. magnified image

Thermal and structural properties of B2O3–H2O glasses

B2–2xO3–2xH2x glasses were prepared by quenching the melt contained in sealed tubes. The glass-forming range extends from x = 0 to 0.50 (equal to the stoichiometry of metaboric acid, HBO2). The glasses were characterized by differential scanning calorimetry and x-ray scattering. With increasing water content, the glass-transition temperature, Tg, decreases from 553 to 333 K. The specific heat of water-rich samples shows an unusual peak just above Tg. The origin of this peak, which is seen upon heating and cooling, has not been identified. Unlike the composition dependence of Tg, the x-ray structure factors depend for the most part linearly on the composition. In analogy with the crystalline layer compounds α-HBO2 and B(OH)3, the x-ray scattering data show evidence for layering in the medium-range order of water-rich glasses.