Ph.D. thesis (pdf) - dirac

80 Alpha Relaxation
log10(τ α
)
2
1
0
−1
−2
−3
−4
−5
−6
205.5 K
219.3 K
236.3 K
253.9 K
Patm
234 MPa
Sekula
Paluch 295.5 K
Cook 295.5 K
e(ρ)
2.5
2
ρ 2.5
e(ρ)
−7
1.5
−8
1.1 1.2 1.3
ρ [g/cm 3 ]
−9
4 5 6
e(ρ)/T
7 8
x 10 −3
Figure 5.7: The alpha-relaxation times shown in figure 5.5 plotted as a function of
X = e(ρ)/T, with increasing dloge(ρ)/ dlogρ as ρ increases. The inset shows the
density-dependent activation energy e(ρ) (dashed line) used in the scaling variable
dlog e(ρ)
X = e(ρ)/T for collapsing data in (a) (the associated x(ρ) =
dlog ρ
increases from
1.5 to 3.5 in the density range under study). We also display the power law giving
the best scaling, ρ 2.5 , at low density (full line).
2004; Mandanici et al., 2005] and references therein.)
In the inset of figure 5.9 we show the pressure-dependent relaxation time at 216.4 K.
Extrapolating the data to τ α = 100 s leads to P g = 340 ± 10 MPa, corresponding to
dT g /dP = 0.085 K.MPa −1 . This value is, as we also saw it in the case of DBP, about
a factor 2 higher than the dT g /dP = 0.045 K.MPa −1 reported for the calorimetric
glass transition by Alba-Simionesco et al. [1997]. This could suggest that this type
of decoupling is common, however there are also examples where no such decoupling
is found [Chauty-Cailliaux, 2003].
In figure 5.9 we show the alpha-relaxation time as a function of density along the
atmospheric pressure isobar (data from Mandanici et al. [2005]) and the 216.4 K
isotherm (see appendix A regarding the determination of density). The data taken
at atmospheric pressure and the data taken along the 216.4 K isotherm cover two
different ranges in density. It is therefore not possible from this data to verify the
validity of the scaling in X = e(ρ)/T. Moreover, it is seen in figure 5.9 that there
is not complete agreement at ambient pressure between the two experiments. Our
data actually correspond better to the results of Carpentier et al. [2004] than those
of Mandanici et al. [2005]; it would be ideal to measure the high pressure and the