Ph.D. thesis (pdf) - dirac

38 What we learn from pressure experiments
pure temperature dependence of the relaxation time as it is measured by m ρ should
then possess the same intrinsic characteristic and be independent of pressure along
the T g -line. This means that the pressure dependence of a property can help clarify
if it is fundamentally correlated to the intrinsic effect of temperature as measured by
m ρ or rather to the pressure dependent combined effect of density and temperature
as measured by m P .
3.3.3 The relative effect of density
In section 3.3.1 we have considered the pressure dependence of a correlation between
a given property and the isobaric fragility without taking the consequences of the
scaling law into account. If we now incorporate the scaling it follows that m ρ is
density independent and that the pressure dependence of m P (P) is due to a pressure
dependence of α P /α τ (equation 3.1.3). This term is a measure of the relative effect
of density on the viscous slowing down. This underlines that if a correlation follows
the pressure dependence of m P then it is because the property in question is not
just related to the effect of temperature but also to the relative effect of density on
the viscous slowing down.
A last situation which should also be considered is that the correlations suggested
between m P and other properties could in fact be a reflection of the effect of density
on the relaxation time. That is they could fundamentally be correlations to α P /α τ .
In general it is found, as mentioned above, that m ρ is the dominating term governing
m P , which makes such a proposition appear unlikely. However, some of the most
archetypal glass formers, glycerol, salol and oTP who cover a range of m P ≈ 50
to m P ≈ 80, have a very similar isochoric fragility ranging only from m ρ ≈ 38
to m ρ ≈ 45 (see appendix B). Hence, the difference in isobaric fragility found
at atmospheric pressure when comparing these three liquids is not related to an
intrinsically different response to temperature but rather to a different effect of the
change in density upon isobaric cooling (just as it is the case for the change of m P (P)
when changing pressure). If a property is related to the relative effect of density on
the viscous slowing down, then it is expected to correlate with the ratio α P /α τ . This
implies that the property in question should also follow the pressure dependence of
α P /α τ , and as a consequence usually decrease with increasing pressure.
3.3.4 How to evaluate pressure dependence in the glass ?
The properties that are correlated to fragility are often considered at T g . This is for
example true for the stretching parameter β KWW and for the boson peak intensity