Angells Fragility

We shall see throughout this book that different chemical and physical processes occur in the troposphere and stratosphere, and we shall frequently refer to different regions in Fig. 1.1. However, it is important to put the atmosphere in perspective with respect to the size of the earth itself. The earth s average diameter is 12,742 km, yet the average distance from the earth s surface to the top of the stratosphere is only 50 km, less than 0.4% of the earth s diameter The space shuttle orbits outside the atmosphere, but at an altitude of only several hundred miles, which is less than the distance from Los Angeles to San Francisco. Clearly, the atmosphere is a very thin, and as we shall see, fragile shield upon which life as we know it on earth depends. 

C. A. Angell, Strong and Fragile Liquids, in K. L. Ngai and G. B. Wright, (eds.) Relaxation in Complex Systems, Naval Research Laboratory, Washington, 1X7, 1984. 

Equation (3.9) clearly indicates that ionic conductivity could be improved by lowering the Tg of the system. The difference in the temperature dependences of ionic conductivity (and viscosity) for ion-conductive glass-forming materials has been discussed by Angell et al. using fragility parameters [115]. 

Figure 2.3 TgScaled Arrhenius plot showing data for molten salts ZnCl2 and calcium potassium nitrate (CKN), with data for the calcium nitrate hydrate (CaNOs-W ) and the tetrafluoroborates of quaternary ammonium (MOMNM2E, M= methyl, E = ethyl) and 1-n-butyl-3-methyl-imidazolium (BMI) cations, and the bis-oxalatoborate (BOB) of the latter cation, in relation to other liquids of varying fragility (from Xu, Cooper, and Angell [15]).

Figure 4.5 Viscosity versus inverse temperature for glass-forming liquids, showing behavior classified as strong, typified by open tetrahedral networks, to fragile, typical of ionic and molecular liquids. Here Tg is defined by the criterion that nlT ) = 10 P. For most of the liquids, the viscosities seem to extrapolate to a common value of around 10" P at high temperatures, corresponding to a fundamental molecular vibrational frequency of around 10 sec-i. (Reprinted from J. Non-Cryst. -Solids, 73 1, Angell (1985), with kind permission from Elsevier Science—NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Fig. 4. The strong-fragile classification of liquids. This Arrhenius plot differs from Fig. 3 in that the temperature is scaled with Eg. Strong liquids display Arrhenius behavior fragile liquids do not. (From Angell, 1988.)...

Angell, C. A., Structural instability and relaxation in liquid and glassy phases near the fragile liquid limit. J. Non-Cryst Sol. 102,205 (1988). 

Ito, K., Moynihan, C.T.and Angell. C. A.. Thermodynamic determination of fragility in liquids and a fragile-to-strong liquid transition in water. Nature 398,492 (1999). 

The fragility plot of Angell is reproduced in Fig 3.20(a) (In rj vs TgIT). However, plots of similar kind using T as an iso-viscosity... 

Angell, C.A. Liquid fragility and the glass transition in water and aqueous solutions, Chem. Rev., 102, 2627, 2002. 

Angell, C.A. Relaxation in liquids, polymers and plastic crystals — strong/fragile patterns and problems, /. Non-Cry stall. Solids, 131,13,1991. 

Figure 5 shows this plot for 3 different binary mixtures, as well as for a neat system (the epoxy resin PPGE). A good superposition for the a-relaxation frequency is obtained, as it is expected, since the steepness index of Za(T) m (fragility according to Angell definition ) is almost constant under pressure... 

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