Ph.D. thesis (pdf) - dirac

2.6. Fragility and other properties 17
dependence in the glass. This is also true for the glassy dynamics, which as we
shall see in many cases just correspond to the fast dynamics in the liquid measured
at T g . From the phenomenological point of view it is thus found that the major
difference between glassy dynamics and fast dynamics in the liquid is that the former
is virtually temperature independent while the latter can depend on temperature.
Figure 2.3 illustrates how the glassy dynamics correspond to the fast dynamics.
2.5.3 Time scales - the actual phenomenology
In the above we have considered dynamics happening at two different timescales.
The actual phenomenology of glass-forming liquids is somewhat more complicated
and also system dependent. The structural relaxation often bifurcates in two separate
relaxations when the structural relaxation time is lower than approximately
10 −5 s. The process which appears in addition to the alpha process is faster and it
has lower intensity as well as weaker temperature dependence. It is referred to as
the slow beta-relaxation or the Johari Goldstein (JG) beta relaxation. The position
of the bifurcation point differs by several decades from system to system (and from
one experimental probe to another). Moreover, there are also numerous systems
where this separation in two relaxations is not detectable. The JG-beta relaxation
is faster than the alpha relaxation and it also stays active when the glass is formed.
As such it is part of the fast as well as of the glassy dynamics. We shall deal a bit
with the JG-process when discussing the interpretation of dielectric spectroscopy in
chapter 5. However, the fast dynamics studied and discussed in this work occurs on
a still faster time scale, namely in the pico-nanosecond range.
2.6 Fragility and other properties
The fragility introduced in section 2.2 is the central point in our description of slow
dynamics, and the main problem addressed here is to understand which properties
(if any) relate to the fragility. The ultimate goal is to look for causal relations and
to use them to understand what governs the viscous slowing down.
We have already mentioned in section 2.3 that the fragility has been suggested to
correlate to the departure from Debye relaxation. This correlation is just one among
a large body of properties that have been suggested to correlate to the fragility.
The excess in entropy of the liquid as compared to that of the glass has been found
to decrease faster as a function of decreasing temperature in fragile liquids than