Ph.D. thesis (pdf) - dirac

36 What we learn from pressure experiments
3.3 Correlations with fragility
As described in section 2.6 there is a flourishing variety of correlations between
fragility and other properties of the liquids or its corresponding glass. These correlations
hold a lot of empirical information which should be useful as guidelines and
tests in the development of models and theories of the dynamics in viscous liquids.
However in order for this to be possible, it is important to clarify if the correlations
result from, and consequently unveil information on, the effect of density on
the relaxation time, the intrinsic effect of temperature on the relaxation time, (or
a balanced combination of the two). In the following we provide a framework for
analyzing correlations between fragility and other properties of the liquid or glass
by disentangling temperature and density effects [Niss and Alba-Simionesco, 2006;
Niss et al., 2007]. The arguments presented in this section are not relevant for correlations
between fragility and the temperature dependence in the liquid of some other
property. These types of correlations are specifically considered in section 3.4
3.3.1 Pressure and isobaric fragility
The correlations between fragility and other properties that have been suggested in
literature all refer to the standard isobaric fragility. They are obtained empirically
by plotting the property in question as a function of fragility (measured at T g , τ ≈
100 s -1000 s). The plots obtained in this way are mostly very scattered, which is
to be expected, because specific characteristics of the system can affect either the
fragility or the property which is suggested to correlate with fragility.
Assuming that there is nothing special about atmospheric pressure (from the point
of view of the liquid), then a strict correlation to m P should follow this pressure
dependence, where the property correlating to m P is to be evaluated at the same
pressure. Consequently, the simplest way in which pressure can be used to test
this type of correlation is to view pressure as a smooth way to change the isobaric
fragility without changing the chemistry of the system. The correlation between
isobaric fragility and some chosen property can then be scrutinized by measuring
both quantities as a function of pressure.
If the property in question has a pressure dependence which is consistent with the
correlation then the situation would be the one illustrated in figure 3.2. This type
of agreement would suggest that the property in question is sensitive to the same
factors as the isobaric fragility, m P .