Fragility and glass forming liquids

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 

The variation of viscosity close to glass transition temperature itself provides a quantitative measure of fragility. This fragility, F, is given by the reduced activation energy, EjJRTg. 

This representation actually gives the steepness index - a measure of the rapid rise in rj close to Tg. Some typical values of F, determined this way are given in Table 3.6. 

Multiplicity of the energy minima close to Tg in the supercooled state suggested by Angell has not been borne out by experiment and has been discussed by Johari (2000). 

B is from VTF equation and (assumed to be) independent of T. T and T have to be very close in magnitude to give very high fragilities (Table 3.5). Low fragilities are expected when Ep h high. 

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