Calorimetric Tg Research Articles

Molecular mobility of supercooled amorphous indomethacin, determined by dynamic mechanical analysis.

To determine the viscosity and the frequency-dependent shear modulus of supercooled indomethacin as a function of temperature near and above its glass transition temperature and from these data to obtain a quantitative measure of its molecular mobility in the amorphous state. Viscoelastic measurements were carried with a controlled strain rheometer in the frequency domain, at 9 temperatures from 44 degrees to 90 degrees C. The viscosity of supercooled indomethacin shows a strong non-Arrhenius temperature dependence over the temperature range studied, indicative of a fragile amorphous material. Application of the viscosity data to the VTF equation indicates a viscosity of 4.5 x 10(10) Pa.s at the calorimetric Tg of 41 degrees C. and a T0 of -17 degrees C. From the complex shear modulus and the Cole-Davidson equation the shear relaxation behaviour is found to be non-exponential, and the shear relaxation time at Tg is found to be approximately 100 sec. Supercooled indomethacin near and above its Tg exhibits significant molecular mobility, with relaxation times similar to the timescales covered in the handling and storage of pharmaceutical products.

Calorimetric studies of structural relaxation in AgIAgPO3 glasses

Abstract Differential scanning calorimetry data of AgPO3 and AgIAgPO3 glasses have been described in terms of structural relaxation time using the Tool-Narayanaswamy-Moynihan formalism. For a given glass composition, the stretched exponential parameter, β, the activation energy, Δh, the non-linearity parameter, x, and the fitted relaxation time A (as T and Tf → ∞) are independent of the cooling and heating rates. This indicates the adequacy of the formalism for these glasses. Initial addition of AgI increases the parameter β, related to the breadth of the relaxation time distribution, which remains constant on further addition. Δh, x and A decrease systematically with the amount of AgI in the glass. It is suggested that a more appropriate definition of the glass transition temperature, Tg, of a glass can be achieved by determining its relaxation time at different temperatures, rather than the usual methods of tangent intersection. This eliminates the effects of variation of β, Δh and x on the usual calorimetric Tg of different glasses.

Dielectric and conductivity relaxations in poly(hema) and of water in its hydrogel

AbstractThe dielectric permittivity ε′ and loss ε″ of anhydrous poly(2‐hydroxyethyl methacrylate) and its 38.6 w/w% hydrogel have been measured in the frequency range from 12 Hz to 200 kHz and the temperature range from 77 to 273 K. The former has a sub‐Tg relaxation with a half‐width of 4.5 decades for the loss spectra, whose strength increases with temperature, and an activation energy of 62.5 kJ/mol. The dielectric relaxation time of the α process of supercooled water in the hydrogel is 53 s at its calorimetric Tg of 135 K. The half‐width of the relaxation spectrum is 2.85 decades and, in the narrow temperature range, its apparent activation energy is 60.8 kJ/mol. Heating of the hydrogel causes crystallization of water which begins at about 207 K and becomes readily detectable as a second dielectric loss peak at about 230 K. For each temperature between 207 and 267 K, supercooled water in the hydrogel coexists with its crystallized form, with the amount of the crystallized solid increasing with increasing temperature. These results are discussed in terms of “bound” and “free” states of water in the hydrogel.

Thermodynamic aspects of the vitrification of toluene, and xylene isomers, and the fragility of liquid hydrocarbons

The three xylenes, 1,2-, 1,3-, and 1,4-dimethyl benzene, provide a well-characterized family of isomeric molecules within which the melting points vary greatly while fluid-state properties such as boiling points and viscosities remain very similar. Because all pure isomers crystallize rapidly on cooling, their behavior at the low-temperature end of the liquid state, i.e., near the glass transition, has not been studied. We report here differential scanning calorimetry studies of phase relations for the three mixed isomer systems, and identify composition regions in which emulsified and also bulk mixtures can be vitrified at ordinary cooling rates. For one of the latter, 70% m-xylene+ 30% o-xylene by volume, we determine the change in heat capacity through the glass transition. On the assumption that this is the same for pure m-xylene, we assess the Kauzmann limit on liquid behavior, TK, for m-xylene to fall at 104.6 K, Tg/TK to be 1.20, and the excess entropy at Tg to be 15.6 J/mol K or 30.5% of the entropy of fusion. Equivalent data are obtained for the better-known glass former toluene, for which we find Tg/TK=1.16. Comparisons of these results are made with literature data for some saturated analogs. From these comparisons emerges the simple rule that the excess heat capacity of supercooled liquid over crystal can be described as a hyperbolic function of temperature, the one parameter of which varies linearly with the carbon number of the molecule. The plot passes through the origin, and has a greater slope for aromatic than for paraffinic molecules. This is correlated both with the greater ‘‘fragility’’ of the aromatic liquids assessed from viscosity-temperature relations normalized by the calorimetric Tg, and with a simple ‘‘liquid range’’ index Tb/Tg, where Tb is the boiling point.