To calculate the value of the fragility index of glass forming liquids it is necessary to know the value of the glass transition temperature Tg often chosen as the temperature for which the viscosity reaches a value of 1012poises or equivalently the temperature for which the relaxation time is of the order of t = 100 s. In spite of the fact that it is known that some fragile systems exhibit lower values for t , the value t = 100 s is often still used because it is difficult to estimate experimentally the true glass transition. In fact, by coupling data obtained by different ways, we will see that it is possible to get a good estimation of t(Tg) . In the process it will be shown why for fragile systems the fragility indexes are very different for the same sample when obtained by means of different experiments (i.e., dielectric, calorimetric, etc.).
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