Ph.D. thesis (pdf) - dirac

5.2. Relaxation time 75
Data treatment
The relaxation time is determined from a polynomial fit of the logarithm of the loss
as function of the logarithm of the frequency. The fit is performed over less than a
decade around the top. The further treatment, e.g. evaluation of the fragility and
analysis of the spectra shape, is discussed in detail in the course of the chapter.
5.2 Relaxation time
5.2.1 Dibutyl phtalate
The relaxation time of DBP at atmospheric pressure is shown in figure 5.2 along
with literature results. T g (P atm ) = 177 K, when defined as the temperature where
τ α = 100 s. We also present the data taken at P = 230 MPa in this figure. It is clearly
seen that T g increases with pressure. An extrapolation of the data to τ α = 100 s
gives T g = 200 K for P = 230 MPa, corresponding to dT g /dP = 0.1 K.MPa −1 . This
corresponds well to the pressure dependence of T g (at τ α = 1 s) reported by Sekula
et al. [2004], based on measurements taken at pressures higher than 600 MPa. It
is however a stronger pressure dependence than dT g /dP = 0.06 K.MPa −1 reported
by Fujimori et al. [1997] based on isothermal calorimetry. This indicates that the
calorimetric and the dielectric relaxation may have somewhat different dependence
of density.
In figure 5.3 we illustrate the determination of T g and of the fragility m P for the
atmospheric-pressure data, using the part of the atmospheric-pressure data of figure
5.2 where the relaxation time is longer than a millisecond. Along with the data
we show the VTF fit (equation 2.2.2) from Sekula et al. [2004] extrapolated to low
temperatures, which gives T g = 177.4 K and m P = 84. We have also performed
a new VTF fit restricted to the data in the 10 −6 s−10 2 s region. The result of
this fit yields T g = 176.6 K and m P = 82. Finally, we have made a simple linear
estimate of log 10 τ α as a function of 1/T in the temperature range shown in the
figure. This linear slope fits the data close to T g better than any of the VTF fits.
The corresponding glass transition temperature and fragility are T g = 176 K and
m P = 70. This illustrates that the determination of T g is rather robust while this
is less so for the fragility. This latter depends on how it is obtained, and the use of
extrapolated VTF fits can lead to an overestimation. (Of course, a VTF fit made
over a very narrow range, e.g. 10 −2 s − 10 2 s, will agree with the linear fit, because
the VTF becomes essentially linear over the relevant range.) The fragility of DBP