Equilibrium Glass Transition Temperature Research Articles

Thermal stability of amorphous nimesulide: from glass formation to crystal growth and thermal degradation.

Thermally induced physico-chemical transformations in amorphous nimesulide were studied by means of differential scanning calorimetry (DSC), thermogravimetry, and Raman microscopy. The equilibrium glass transition temperature was found to be Tg0 = 10-15 °C, and the relaxation motions were found to be temperature-dependent. Crystal growth from the amorphous phase was found to be crucially dependent on the presence of mechanical defects that serve as centers for heterogeneous nucleation. The large amounts of mechanical defects significantly decrease the activation energy of the macroscopic crystallization; the positions of the crystallization peaks and their asymmetry/shape remain however almost unchanged. At laboratory temperature, powdered nimesulide fully crystallizes within several hours, with an absolute majority of the crystalline phase being formed as the thermodynamically stable form I polymorph. Amorphous nimesulide does not crystallize from the free smooth surface (no trace of formed crystallites was found by optical microscopy after 30 days at laboratory temperature). Nimesulide was found to be very stable at temperatures above its melting point of 147.5 °C; thermal degradation starts to proceed slowly at 200 °C. Mutual correlations between the macroscopic and microscopic crystal growth processes and between the viscous flow and structural relaxation motions were discussed based on the values of the corresponding activation energies. A link between the cooperativity of structural domains, parameters of the Tool-Narayanaswamy-Moynihan relaxation model, and microscopic crystal growth was proposed.

Heterogeneous dynamics of the three-dimensional Coulomb glass out of equilibrium

The nonequilibrium relaxational properties of a three-dimensional Coulomb glass model are investigated by kinetic Monte Carlo simulations. Our results suggest a transition from stationary to nonstationary dynamics at the equilibrium glass transition temperature of the system. Below the transition the dynamic correlation functions lose time translation invariance and electron diffusion is anomalous. Two groups of carriers can be identified at each time scale: electrons whose motion is diffusive within a selected time window and electrons that during the same time interval remain confined in small regions in space. During the relaxation that follows a temperature quench an exchange of electrons between these two groups takes place and the nonequilibrium excess of diffusive electrons initially present decreases logarithmically with time as the system relaxes. This bimodal dynamical heterogeneity persists at higher temperatures when time translation invariance is restored and electron diffusion is normal. The occupancy of the two dynamical modes is then stationary and its temperature dependence reflects a crossover between a low-temperature regime with a high concentration of electrons forming fluctuating dipoles and a high-temperature regime in which the concentration of diffusive electrons is high.

Effect of plasticizers on magnesium-poly(ethyleneoxide) polymer electrolyte

Solid polymer electrolytes (SPEs) of poly(ethyleneoxide) and magnesium triflate, which are plasticized with propylene carbonate (PC), ethylene carbonate (EC) and a mixture of PC and EC, are studied for their conductivity, ac impedance of the Mg ∣ SPE interface, cyclic voltammetry, infrared spectroscopy and differential scanning calorimetry. In the presence of plasticizers, the ionic conductivity ( σ ) increases from a value of 1×10 −8 S cm −1 to about 1×10 −5 S cm −1 at ambient temperature. The σ is found to follow a VTF relationship with temperature. The values of the activation energy, pre-exponential factor and equilibrium glass transition temperature are shown to depend on the concentration of plasticizer. Ac impedance studies indicate lower interfacial impedance of Mg/plasticized SPE than stainless steel/plasticized SPE. The impedance spectra are analyzed using a non-linear least square curve fitting technique and the interfacial resistance of Mg/plasticized SPE is evaluated. The cyclic voltammetric results suggest a quasireversible type of Mg/Mg 2+ couple in plasticized SPE.

Conformation and Ion‐Transport Models for the Structure and Ionic Conductivity in Complexes of Polyethers with Alkali Metal Salts

Two ion‐transport mechanisms are described for ion transport in polyether‐alkali metal salt complexes: an intrahelical jumping process along crystalline (helical) regions of the polymer, and a transport process in the amorphous regions which is dependent on formation of fourfold coordination sites via mutual motion of ether oxygens from two or more polymer chains. The intrahelical jumping process may exhibit Arrhenius behavior, while transport in the amorphous regions should behave like a configurational entropy dominated process, showing a temperature dependence like , where is the equilibrium glass transition temperature. For the highly crystalline poly(ethylene oxide) · complexes, an Arrhenius behavior is observed to dominate, whereas for the amorphous polyether · salt complexes the configurational entropy behavior is observed. Even for the highly crystalline complexes, however, amorphous regions separate the crystalline regions of the polymer, and segmental motion of the polymer chains is postulated to be crucial here as well. The models are consistent with the observed frequency‐dependent ionic conductivity, as well as spectroscopic, x‐ray, thermal, and physical characterization measurements. They provide a reasonable microscopic picture for ion motion and yield testable predictions concerning the dependence of the ionic conductivity on pressure, temperature, and crystallinity.