Ph.D. thesis (pdf) - dirac

Chapter 5
Alpha Relaxation
The two most distinctive features of the slow dynamics of liquids approaching the
glass transition is the non-Arrhenius temperature dependence of the alpha relaxation
time characterized by the fragility (section 2.2) and the non-Debye character of the
alpha relaxation time (section 2.3) [Angell et al., 2000]. It could even be claimed
that “understanding the glass transition” essentially means understanding these two
phenomena [Dyre, 2006]. The hypothesis that they might be correlated, is therefore
appealing. Böhmer et al. [1993] correlates larger (isobaric) fragility with larger
deviation from Debye relaxation, with the latter measured in terms of stretching
and the paper has been cited 703 times, which is one illustration of the impact of
this idea.
In this chapter we present a study of the pressure and temperature dependence
of the dielectric alpha relaxation time and peak shape in two molecular liquids.
We rationalize the temperature and density dependence in terms of the scaling law
presented in section 3.2 and determine isobaric and isochoric fragility.
We combine our data with data from the literature to revisit the proposed correlation
between stretching and fragility. We do this in terms of the ideas developed in
section 3.3.
5.1 Dielectric spectroscopy
5.1.1 Experimentals
The dielectric measurements were performed at the laboratory, LCP, in Orsay on a
homebuilt setup. The pressure environment was developed prior to this work, while
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