Kinetic Fragility

Molecular liquids such as salol whose structural integrity relies primarily on weak van der Waals interactions tend to collapse easily with temperature and typically exhibit the most fragile behaviors [Fig. 2.3). On the other end, covalent networks such as silica remain connected at higher temperatures and exhibit strong behaviors. 

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Ito et al. observed a remarkable similarity between kinetic fragility plot and normalized entropy data, namely, Kauzmann plot, exhibited in a scaled- / , form sheds considerable light into the role of excess entropy... 

As will be discussed in more detail later, the kinetic fragility can be correlated with excess entropy for a wide class of glass-forming liquids... 

An explicit expression relating kinetic fragility to thermodynamic behavior of supercooled liquids was accomplished for the first time by Mohanty and coworkers [55,56] and independently by Speedy [54], These authors derived an expression for the steepness parameter, a measure of kinetic fragility, from the temperature variation of the relation time or viscosity, with the ratio of excess entropy and heat capacity changes at the glass transition temperature [54-56]. A detailed description of this work will be provided later in the review chapter. 

Wang, L.-M., Velikov V., and Angell C. A., Direct determination of kinetic fragility indices of glassforming liquids by differential scanning calorimetry kinetic versus thermod mamic fragilities. /. Chem. Phys., 117,10184-10192 (2002). 

Figure 5. The horizontal axis shows the value of fragility as computed from the thermodynamics by the RFOT theory, and the vertical axis contains the fragility directly measured in kinetics experiments. Here m is the so-called fragility index, defined according to m = [(ilogjoi(7 )/(i(l/7 )]j,. m is somewhat more useful than the fragility D, because deviations from...

An alternative and perhaps more accurate definition of thermodynamic fragility that can be determined experimentally for almost all liquids would be to determine the fraction of entropy of fusion lost by T/Tm = 0.8 [37,88]. In both the definitions, the property of the crystal phase enters this disadvantage is missing in the kinetic measures of fragility [37,88]. 

In parallel with Eq. (2.12), the E 3/ 4 tllermo metric is defined as the value of ( Tg/T) for which Sexc(Tg)/Sexc(T) With this definition, a correlation was observed between kinetic F1/2 and thermodynamic F3/2 thermo fragilities [38]. 

A comparison of Eqs. (2.20) and (2.18) leads to a remarkable expression that relates the fragility index, a kinetic property, with measurable thermodynamic quantities... 

Figure 2.2. A plot of the predicted fragility of several glass-forming polymeric liquids versus average experimental fragility index (adapted from Ref. 56). The solid line denotes the points for which the y-axis and the x-axis are equal, is a guide to the eye. The predicted correlation indicates an intricate connection between thermodynamic and kinetics of supercooled polymeric liquids [56],...

The X-ray penetration power enables the use of cells under a controlled atmosphere. Currently, much development work concerns in situ XRD, a valuable aide in the study of fragile or unstable solids in ambient conditions but also offering possibilities of monitoring reaction kinetics in phase transformations. 

Considering the formation and deposition process of carbon clusters by PMCS, it is reasonable to assume that the carbynoid species are formed in the cluster source prior to deposition. The picture coming out from our observations is characterized by relatively fragile sp chains that are quite surprisingly able to survive deposition at kinetic energies per atom, well above the thermal energy measured to induce the sp rearrangement and sp formation. 

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