Molecular relaxation and excess entropy in liquids and their connection to the structure of glass

Fragility and thermodynamics in nonpolymeric glass-forming liquids

For nonpolymeric supercooled liquids, the empirical correlation m = 56Tg DeltaCp(Tg)/DeltaHm provides a reliable means of correlating dynamic and thermodynamic variables. The dynamics are characterized by the fragility or steepness index m and the glass transition temperature Tg, while thermodynamics enter in terms of the heat capacity step DeltaCp at Tg and the melting enthalpy DeltaHm. The combination of the above correlation with the 23 rule for the Tg/Tm ratio yields an expression, m = 40DeltaCp(Tg)/DeltaSm, which was rationalized as the correlation of the thermodynamic and kinetic fragilities. Defining a thermodynamic fragility via DeltaCp(Tg)/DeltaSm also reveals that the slopes in Kauzmann's original DeltaS(T)/DeltaSm versus T/Tm plot reflect the fragility concept [Chem. Rev. 43, 219 (1948)], so long as Tm/Tg = 1.5. For the many liquids whose excess heat capacity is a hyperbolic function of temperature, we deduce that the fragility cannot exceed m = 170, unless the Tg/Tm = 2/3 rule breaks down.

Glass Transition Phenomena in Two-Component Plastic Crystals: Study of Hexasubstituted Benzenes

We have critically examined the relaxation that is known to occur in the crystalline phase of pentachloronitrobenzene (PCNB) and 2,3,4,5,6-pentabromotoluene using dielectric spectroscopy and differential scanning calorimetry (DSC). Within the resolution of our experimental setup, a relaxation process similar to that of the primary (or alpha-) relaxation is found. A slight deviation from Arrhenius behavior is noticed only in the vicinity of the glass transition temperature (T(g)). This deviation and a small steplike change found in the DSC scans at T(g) indicates that the "fragility" of these plastic crystals is rather low. However, in PCNB, the dielectric strength (Deltaepsilon) of the above said alpha-process did not change appreciably with temperature, and, interestingly, a small addition of an impurity such as pentachlorobenzene (PCB) to the molten state of PCNB drastically lowered the dielectric strength and the calorimetric signature of glass transition phenomena in the DSC data at T(g). The room-temperature powder X-ray diffraction measurements in combination with the DSC data in the melting temperature region did not indicate any observable change in the crystalline structure. A residual alpha-process with no significant change in the shape of the dielectric spectrum indicates that the hindrance to the rotational motion of PCNB molecules is caused by the presence of a small number of PCB molecules in the crystalline lattice of PCNB over a certain region. Outside of this region, the original PCNB disordered phase is preserved, which is the origin of the residual alpha-process. With a further increase in PCB concentration, the alpha-process, characteristic of pure PCNB, vanishes, and instead another relaxation develops. This process is explained with the help of a solid-liquid phase diagram of the alpha-process of the plastic phase of 2:1 and 1:2 compound formations, which are stable below 386 +/- 1 and 366 +/- 1 K, respectively.

Thermodynamic−Kinetic Correlations in Supercooled Liquids: A Critical Survey of Experimental Data and Predictions of the Random First-Order Transition Theory of Glasses

Thermodynamics and kinetics are thought to be linked in glass transitions. The quantitative predictions of alpha-relaxation activation barriers provided by the theory of glasses based on random first-order transitions are compared with the experimental results for 44 substances. The agreement found between the predicted activation energies near T(g) and experiment is excellent. These predictions depend on the configurational heat capacity change on vitrification and the entropy of melting the crystal which are experimental inputs.

Sub-Tg relaxations due to dipolar solutes in nonpolar glass-forming solvents

It is well known that rigid dipolar solutes (in smaller quantity) dispersed in a nonpolar glassy matrix exhibit a sub-T(g) (or beta(s)) relaxation due to the solute often designated as Johari-Goldstein (JG) relaxation, which is intermolecular in nature. In this article, we report the results of our study of such a sub-T(g) process in a wide variety of dipolar solutes in different glassy systems using dielectric spectroscopy over a frequency range of 20-10(6) Hz down to a temperature of 77 K. The T(g) of these solutions are determined using differential scanning calorimetry. The solvents used in this study are o-terphenyl (OTP), isopropylbenzene (IPB), and methylcyclohexane. In the case of rigid molecular solutes, like mono-halogen benzenes, the activation energy (DeltaE(beta)) of the beta(s) process is found to increase with decreasing T(g) of the solvent, with a corresponding decrease in the magnitude of the beta(s) process. In the case of more symmetrical molecular solute, for example, tert-butylchloride, the change in DeltaE(beta) is not very appreciable. These results emphasize the importance of the size of the cage of the host matrix in the relaxation of the solute molecules. We have also studied the sub-T(g) relaxation(s) due to some flexible molecular solutes, viz., 1butylbromide, 1hexylbromide, 1butylacetate, and benzylacetate. These solutes in IPB matrix exhibit only one relaxation, whereas in OTP matrix they exhibit an additional sub-T(g) process, which may be identified with a JG type of relaxation. These observations lead us to the conclusion that the beta process observed in the glassy states of these pure solutes is predominantly intramolecular in nature.