Ideal Glass Transition Temperature Research Articles

Theory of glassy dynamics in conformationally anisotropic polymer systems

A mode coupling theory for the ideal glass transition temperature, or crossover temperature to highly activated dynamics in the deeply supercooled regime, T(c), has been developed for anisotropic polymer liquids. A generalization of a simplified mode coupling approach at the coarse-grained segment level is employed which utilizes structural and thermodynamic information from the anisotropic polymer reference interaction site model theory. Conformational alignment or/and coil deformation modifies equilibrium properties and constraining interchain forces thereby inducing anisotropic segmental dynamics. For liquid-crystalline polymers a small suppression of T(c) with increasing nematic or discotic orientational order is predicted. The underlying mechanism is reduction of the degree of coil interpenetration and intermolecular repulsive contacts due to segmental alignment. For rubber networks chain deformation results in an enhanced bulk modulus and a modest elevation of T(c) is predicted. The theory can also be qualitatively applied to systems that undergo nonuniversal local deformation and alignment, such as polymer thin films and grafted brush layers, and large elevations or depressions of T(c) are possible. Extension to treat directionally dependent collective barrier formation and activated hopping is possible.

Distinct Difference in Ionic Transport Behavior in Polymer Electrolytes Depending on the Matrix Polymers and Incorporated Salts

Two different electrolyte salts, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI), were incorporated into network polymers to obtain ion-conductive polymer electrolytes. Network polymers of poly(ethylene oxide-co-propylene oxide) (P(EO/PO)) and poly(methyl methacrylate) (PMMA) were chosen as matrixes for LiTFSI and EMITFSI, respectively. Both of the polymer electrolytes were single-phase materials and were completely amorphous. Ionic conductivity of the polymer electrolytes was measured over a wide temperature range, with the lowest temperatures close to or below the glass transition temperatures (Tg). The Arrhenius plots of the conductivity for both of the systems exhibited positively curved profiles and could be well fit to the Vogel-Tamman-Fulcher (VTF) equation. The conductivity of the PMMA/EMITFSI electrolytes was higher at most by 3 orders of magnitude than that of the LiTFSI/P(EO/ PO) electrolytes at ambient temperature. When the ideal glass transition temperature, T0 (one of the VTF fitting parameters), was compared with the Tg, a difference in the ionic conduction was apparent in these systems. In the P(EO/PO)/LiTFSI electrolytes, the T0 and Tg increased in parallel with salt concentration and the T0 was lower than the Tg by ca. 50 degrees C. On the contrary, the difference between the T0 and the Tg increased with increasing content of PMMA in the PMMA/EMITFSI electrolytes, with the observed difference in the concentration range studied reaching up to ca. 100 degrees C. The conductivity at the Tg, sigma(Tg), for the LiTFSI/P(EO/PO) electrolytes was on the order of 10(-14-)10(-13) S cm(-1) and increased with increasing salt concentration, whereas that for the PMMA/EMITFSI polymer electrolytes reached 10(-7) S cm(-1) when the concentration of PMMA was high. The ion transport mechanism was discussed in terms of the concepts of coupling/decoupling and strong/fragile for the two different polymer electrolytes.

Kinetics of Glass Transition Simulated by Path Probability Method

Kinetics of glass transition is simulated within the Path Probability Method (PPM) which is the natural extension of the Cluster Variation Method (CVM) to time domain. Temperature dependences of order parameter during continuous cooling are calculated as a function of cooling rate. When the temperature dependence of viscosity is introduced in the mobility term in the PPM, the order parameter is frozen as approaching the ideal glass transition temperature. Furthermore, a preliminary calculation which incorporates the cooling rate dependence into the viscosity reproduces the experimental tendency of the glass transition temperature. Together with the previous studies of thermodynamic frameworks of glass transition based on the CVM, it is confirmed that the combination of the CVM and PPM provides a unique theoretical tool to study glass transition in a consistent manner covering thermodynamics and kinetics.

Electrodeposition of cesium at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide room-temperature ionic liquid

The electrochemistry of cesium was investigated at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Bu 3MeN +Tf 2N −) room-temperature ionic liquid (RTIL) by using cyclic staircase voltammetry, rotating disk electrode voltammetry, and chronoamperometry. The reduction of cesium ions at mercury exhibits quasireversible behavior with k 0 = 9.8 × 10 −5 cm s −1 and α = 0.36. The diffusion coefficient of Cs + in this RTIL was 1.04 × 10 −8 cm 2 s −1 at 303 K. Bulk deposition/stripping experiments conducted at a rotating mercury film electrode gave an average recovery of 97% of the electrodeposited Cs. The density, absolute viscosity, and equivalent conductance of Bu 3MeN +Tf 2N − were measured over the range of temperatures from 298 to 353 K. A polynomial equation describing the temperature dependence of the density is presented. Both the viscosity and conductance exhibited the non-Arrhenius temperature dependence typical of glass-forming liquids. The ideal glass transition temperature and the activation energies for viscosity and conductance were obtained by fitting the Vogel–Tammann–Fulcher (VTF) equation to the experimental data for these transport properties.

Dynamical signature of two “ideal glass transitions” in nematic liquid crystals

A temperature scaling analysis using the same mode coupling theory (MCT) scaling relationships employed for supercooled liquids is applied to optical heterodyne detected optical Kerr effect data for four liquid crystals. The data cover a range of times from ∼1 ps to 100 ns and a range of temperatures from ∼50 K above the isotropic to nematic phase transition temperature TNI down to ∼TNI. The slowest exponential component of the data obeys the Landau–de Gennes (LdG) theory for the isotropic phase of liquid crystals. However, it is also found that the liquid crystal data obey MCT scaling relationships, but, instead of a single scaling temperature TC as found for supercooled liquids, in the liquid crystals there are two scaling temperatures TCL (L for low temperature) and TCH (H for high temperature). TCH is very close to T*, which results from LdG scaling, just below the isotropic to nematic phase transition temperature, TNI, but is 30–50 K higher than TCL. The liquid crystal time dependent data have the identical functional form as supercooled liquid data, that is, a fast power law decay with temperature independent exponent, followed by a slower power law decay with temperature independent exponent, and on the longest time scales, an exponential decay with highly temperature dependent decay constant. For each liquid crystal, the amplitudes of the two power laws scale with expressions that involve TCL, but the exponential decay time constant (long time dynamics) scales with an expression that involves TCH. The existence of two scaling temperatures can be interpreted as a signature of two “glass transitions” in liquid crystals. In ideal MCT developed for spheres, TC is the “ideal glass transition temperature,” although it is found experimentally to be ∼20%–30% above the experimental glass transition temperature, Tg. The transition in nematic liquid crystals at TCL corresponds to the conventional ideal MCT glass transition, while the transition at TCH can occur for nonspherical molecules, and may correspond to the freezing in of local nematic order.

Ideal glass transitions in thin films: An energy landscape perspective

We introduce a mean-field model for the potential energy landscape of a thin fluid film confined between parallel substrates. The model predicts how the number of accessible basins on the energy landscape and, consequently, the film’s ideal glass transition temperature depend on bulk pressure, film thickness, and the strength of the fluid–fluid and fluid–substrate interactions. The predictions are in qualitative agreement with the experimental trends for the kinetic glass transition temperature of thin films, suggesting the utility of landscape-based approaches for studying the behavior of confined fluids.

Evaluation of Glass-Forming Ability for Metallic Glasses from Time-Reduced Temperature-Transformation Diagram

An equation expressing the transformation curve for crystallization of metallic glasses has been proposed by using two parameters, viscosity and melting temperature (T m ). The Vogel-Fulcher-Tammann (VFT) equation, η = η 0 exp(B/(T - T 0 )) where η 0 , B and To (ideal glass transition temperature) are empirical parameters, was used for expressing the viscosity. A Time-reduced Temperature-Transformation (T-T r -T) diagram was calculated using the following five quantities: reduced temperature (Tr = T/T m ), three reduced viscosity parameters (η 0r = η 0 /T m , B r = B/T m and T 0r = T 0 /T m ), and reduced critical cooling rate (R cr = R c /T m ) for formation of the glassy phase. The R cr in the T-T r -T diagram was calculated for Ni, metallic glasses and SiO 2 The glass-forming ability (GFA) was estimated from T 0r -R cr (R cr = R c / T m ) diagrams corresponding to T g / T m - R c diagrams obtained experimentally. The metallic glasses with T 0r of 0.55 are calculated to have R cr ranging from 10 -5 to 10 0 s -1 , which agrees with the experimental data that metallic glasses with T g /T m of 0.6 or more have R c of less than 10 3 K/s. The following three points are clarified: (1) the higher GFA of metallic glass is achieved because of higher viscosity, (2) higher viscosity causes both the homogeneous nucleation frequency (I hom v ) and the ratio I N /I max ), at reduced nose temperature against the maximum of I hom v , to decrease, and (3) the R cr is numerically derived from reduced viscosity parameters.

New alkali ionomers: transport mechanism from temperature and pressure conductivity measurements

New ionomers, exclusively conducting by alkali cations (Li +, Na + and K +), are synthesised by grafting the counter anion to a macromolecular polyethylene oxide chain. An experimental set up allows accurate measurement of the conductivity versus pressure P (1–5000 bars) as well as versus temperature T (20–120°C). The variations of the cationic conductivity as a function of these two parameters are interpreted in terms of a free volume mechanism. It is assumed that the available free volume V ̄ f, governing the mobility of charge carriers, obeys the simple state equation, P V ̄ f =R(T−T 0) where T 0 is the ideal glass transition temperature, i.e. the temperature at which the free volume vanishes. Microscopically, it means that the kinetic energy of the charge carriers is proportional to the thermal energy received between temperature T and T 0. This approach justifies correctly the Vogel, Tamman, Fulcher (VTF) behaviour of isobar data and the exponential decrease of the conductivity with pressure. Fitting to experimental data leads to T 0=195 K and values of respectively 7.7, 9.7 and 12.6 cm 3 mol −1 of the critical free volumes V f*, of the Li +, Na + and K + cations solvated in the polymer matrix.

C, T-Dependence of the viscosity and the self-diffusion coefficients in some aqueous carbohydrate solutions

Self-diffusion coefficients for both components are reported for the highly concentrated aqueous solutions of some disaccharides and fructose as a function of temperature and concentration. These data are complemented by viscosity measurements. The disaccharides studied are sucrose, α,α-trehalose, allosucrose, and leucrose. Up to a sugar concentration of ≈30% w/w the viscosity and the self diffusion coefficients of the four disaccharides are identical within experimental error for a given concentration and temperature. Water diffusion shows no differences in the four systems studied under these conditions. At higher concentrations significant differences are observed that become more pronounced with increasing temperature. Analysis of the data by the VTF equation yields the result that at a given concentration the self diffusion coefficients of the sugar D c and the viscosity η are described by identical ideal glass transition temperatures T 0, while the diffusion of the water D W molecule decouples from these properties. T 0 W is always lower than T c, η 0.

Analysis of Thermodynamic Parameters of Glass Forming Polymeric Melts

The temperature dependence of the Gibbs free energy difference (ΔG), enthalpy difference (ΔH) and entropy difference (ΔS) between the undercooled meltand the corresponding equilibrium solid has been analysed for glass forming polymeric materials by calculating ΔG, ΔH and ΔS within the framework of the hole theory of liquids. The study is made for nine samples of glass forming polymeric melts; polypropylene oxide (PPO), polyamid-6 (PA-6), polytetramethylene oxide (PTMO), polyethylene oxide (PEO), polystyrene (PS), polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET) and polybutadiene (PB) and three simple organic liquids: tri-α-naphthyl benzene (tri-α-NB), o-terphenyl (o-ter) and phenyl salicylate (salol) in the entire temperature range Tm (melting temperature) to Tg (glass transition temperature). The ideal glass transition temperature (TK) and the residual entropy (ΔSR) of these samples have also been studied due to their important role in the study of the glass forming ability of materials.

Absolute Viscosity and Density of Trisubstituted Phosphoric Esters

This paper presents measurements on the absolute viscosity (η) and density (ρ) of trisubstituted phosphoric esters which are useful in understanding their flow mechanism necessary for accessing their role as plasticizers. The effect of chain length and branching has been examined on the η and ρ trends. From η data, by using the Vogel−Tammann−Fulchur (VTF) equation, the VTF temperature (To) has been obtained which also represents the ideal glass transition temperature. To is related to the flexibility of the molecules. It is observed that To initially decreases with molecular weight, reaches a minimum, and increases thereafter. The initial decrease in To has been attributed to the enhanced flexibility of the phosphate esters. Reversal of flexibility with relative molar mass beyond 400 is due to the gentle collision of the arms of the trisubstituted phosphoric esters. This has been further corroborated from the molar mass exponent as exhibited in the η−molar mass plot. The isomeric effect on η has also been...

Temperature and Pressure Dependence of the Electrical Conductivity of Two Tetraalkylammonium Tetraalkylborate Salts

Electrical conductivities of molten trimethylpentylammonium triethyloctylborate (N1115B2228) and triethylpentylammonium triethylpentylborate (N2225B2225) were measured as functions of temperature (c. 293 · 15–383 · 15 K) and pressure (from 1 bar to 5 kbar). Analysis of the temperature dependence of the electrical conductivity was made by using the Vogel–Tammann–Fulcher equation, κ = Aexp[ – B/(T – T0)]. The empirical nature of the pressure dependence of the B and T0 parameters has revealed the possibility of obtaining an isothermal equation to explain the pressure dependence of the electrical conductivity. Accordingly, an empirical equation of the form κ = a′exp(b′ P+c′ P2) has been found to describe the pressure dependence of the electrical conductivity. The ideal glass transition temperature, T0, is the key parameter in controlling the pressure dependence of the electrical conductivity for both systems under study.

Stress-induced amorphization at moving crack tips in NiTi

In situ fracture studies have been carried out on thin films of the NiTi intermetallic compound under plane stress, tensile loading conditions in the high-voltage electron microscope. Local stress-induced amorphization of regions directly in front of moving crack tips has been observed. The upper cutoff temperature, TC–Amax, for the stress-induced crystalline-to-amorphous transformation was found to be 600 K, identical to that for heavy ion-induced amorphization of NiTi and for ion-beam mixing-induced amorphization of Ni and Ti multilayer specimens. 600 K is also both the lower cutoff temperature, TA–Cmin, for radiation-induced crystallization of initially-unrelaxed amorphous NiTi and the lowest isothermal annealing temperature, TXmin, at which stress-induced amorphous NiTi crystallizes. Since TXmin should be TK, the ideal glass transition temperature, the discovery that TC–Amax=TA–Cmin=TXmin=TK implies that disorder-driven crystalline-to-amorphous transformations result in the formation of the ideal glass, i.e., the glassy state that has the same entropy as that of the defect-free crystal. As the glassy state with the lowest free energy, its formation can be understood as the most energetically-favored, kinetically-constrained response of crystalline alloys driven far from equilibrium.

Proton NMR Study of Rouse Dynamics and Ideal Glass Transition Temperature of Poly(ethylene oxide) LiCF3SO3 Complexes.

A range of low-molecular-weight linear poly(ethylene oxide) chains with LiCF3SO3 have been examined via the transverse proton NMR relaxation. From the measured relaxation functions it is shown that the chains obey Rouse dynamics. This analysis gives the molecular weight of a Rouse statistical subunit and the fundamental Rouse relaxation time. It was found that varying the temperature and salt concentration had no effect on the size of a Rouse statistical segment and therefore none on the conformation of the chains. The temperature dependence of the fundamental Rouse relaxation time was found to be well modeled by the Vogel-Tamman-Fulcher equation. The NMR-measured correlation times were then successfully used to predict the glass transition temperature as measured by DSC. The main conclusion of this paper is that under the addition of salt the ideal glass transition temperature T0 is found to remain constant or indeed to slightly decrease.

P, T dependence of self-diffusion in 2-fluoroethanol, 2,2 difluoroethanol and 2,2,2-trifluoroethanol

The self-diffusion of ethanols substituted by fluorine at the methyl carbon has been studied by FTNMR using pulsed gradient spin-echo techniques. There is much interest in fluoroethanols, particularly trifluoroethanol (TFE), because of its use in inducing conformational change in peptides and proteins. One of the rationales for looking at the fluoroethanols was to study the effect of changing hydrogen bond structures by fluorine substitution upon the self-diffusion behaviour. One important property of TFE in this context is its ability to form intra-molecular hydrogen bonds. The results for the diffusion measurements showed that the effects of adding fluorine appear to be steric in nature, with little effect of changing the hydroxyl hydrogen bond strength. 2-fluoroethanol has self diffusion coefficients very similar in magnitude and p, T dependence to 1-propanol. These two molecules have similar masses. 2,2-difluoroethanol has a more pronounced temperature dependence than 2-fluoroethanol or 2,2,2-trifluoroethanol. In addition, analysis of the experimental data to a VTF type equation reveals that 2,2-difluoroethanol has a much higher ideal glass transition temperature, T 0, than either 2-fluoroethanol or TFE. These data indicate that steric conformations in difluoroethanol produce more stable hydrogen bonded structures than in the other two liquids. There is little evidence from these self-diffusion data of any effect of intra-molecular hydrogen bonding. This is thought to occur, particularly for 2,2,2-trifluoroethanol.

Viscosity and Molar Volume of Potassium Thiocyanate + Sodium Thiocyanate + Acetamide Melt Systems

Density and viscosity of 0.25[xKSCN + (1 − x)NaSCN] + 0.75CH3CONH2 melt systems were measured as functions of temperature (293.15 ≤ T/K ≤ 343.15) and mole fraction, x = 0.0 to 1.0. The temperature dependence of viscosity has been analyzed by using the Vogel−Tammann−Fulcher (VTF) equation. Molar volumes and intrinsic volume were found to be additive in nature. The existence of the mixed alkali effect (MAE) has been established by the significant negative deviation from the additivity in the ideal glass transition temperature and in the viscosity isotherms. The onset of the MAE in the present system has been explained by the mutual contribution of the polarization effect of SCN- ion on the Na+ ion and the presence of the rigid polymeric type structure, which decrease as the Na+ ions are progressively replaced by the K+ ions.

Analysis of thermodynamic parameters of glass forming metallic melts

The temperature dependence of the Gibbs free energy difference (ΔG) between the undercooled liquid and the corresponding equilibrium solid has been analysed for metallic glass forming systems in the frame of the expression obtained by expanding free energies of the undercooled liquid and solid phases in the form of Taylor's series expansion. The enthalpy difference (ΔH) and the entropy difference (ΔH) between the undercooled liquid and solid phases have also been analysed. The study is made for five different metallic glass forming materials, Au77Ge13.6Si9.4, Au53.2Pb27.5Sb19.3, Au81.4Si18.6, Mg85.5Cu14.5 and Mg81.6Ga18.4 and a very good agreement is found between calculated and experimental values of ΔG. The ideal glass transition temperature (T k) and the residual entropy (ΔS R) of these materials have also been studied due to their important role in assigning the glass formation ability of materials.

Ion transport in high temperature rotator phase solid electrolytes

The ionic conductivity has been investigated within a wide temperature and concentration range for the rotator phase fcc-Li 2SO 4 doped with 0 to 8 mol% MgSO 4. For comparison we also measured the ionic conductivity in the melt. For the solid phase the temperature dependence of the conductivity deviates from the Arrhenius behaviour, but it can be successfully fitted to the Vogel–Tammann–Fulcher (VTF) equation. Also for the bcc high temperature rotator phases of LiNaSO 4 and LiAgSO 4 we found that the VTF equation provides a better fit than the Arrhenius equation. The ideal glass transition temperature obtained from the fit to the VTF equation is (166±20)°C for the fcc phase of Li 2SO 4–MgSO 4, independent of the MgSO 4 concentration. For the bcc-phases of LiNaSO 4 and LiAgSO 4 it is (113±19)°C and (135±49)°C respectively. This indicates that although these materials are crystalline, the ionic conductivity behaves in a similar way to that of an amorphous material above the glass transition temperature. For comparison we also reevaluated ionic conductivity data over a wide temperature range for the archetypical crystalline solid electrolyte, α-AgI. As expected, the Arrhenius equation provides an excellent fit in this case since the ionic transport can be described as ionic hopping between available sites. For the high temperature rotator phases the dynamical disorder, due to the rotational freedom of the sulphate ions, is thought to be a key factor for the ionic transport. It is thus suggested that the free volume model may be used to describe the ionic transport in high temperature rotator phases.

Electrical Conductance and Viscosity of Concentrated H2SO4/H2O Binary Systems at Low Temperatures: Correlation with Phase Transitions

Density, viscosity, and electrical conductance of concentrated H2SO4/H2O systems (above 66 wt % H2SO4) were measured in the subambient temperature region. The lowest temperature of measurement was 198 K. Density varied linearly with temperature. Viscosity and conductance data were fitted to the Vogel−Tammann−Fulcher (VTF) equation and also to the power-law equation, the latter being a better functional form to describe the temperature dependence of transport properties. Concentration dependences of viscosity, conductance, expansivity, and ideal glass transition temperature indicate the occurrence of structural changes in the H2SO4/H2O system in the regions 83−85 and 92−94 wt % H2SO4 which are attributed to the formations of H2SO4·H2O (at 84.48 wt% H2SO4) and the eutectic between H2SO4·H2O and H2O (at 93.8 wt % H2SO4), respectively. Above ∼75 wt % H2SO4 a direct correlation exists between the viscosity−concentration curve and the phase diagram of the H2SO4/H2O system.

Measurements of thermophysical properties by contactless modulation calorimetry

Contactless modulation calorimetry was applied to measure the specific-heat of glass forming eutectic Zr alloys in the stable and undercooled liquid under microgravity conditions during the space shuttle IML-2 mission. The experimental method is described. leading to a quantitative determination of the specific heat from measured temperature modulations. In addition, the enthalpy of Vision of the specimen and the total hemispherical emissivity can be obtained. The data on ZrNi36, permit calculation of the thermodynamic functions in the stable and undercooked melt as well as estimation of the ideal glass transition temperature.