Ph.D. thesis (pdf) - dirac

6.3. Nonergodicity factor 105
whereas there is no pressure dependence in the glass. Lastly a slight increase in Γ
is observed as temperature increases. The effect of temperature is most pronounced
at ambient pressure at Q=2 nm −1 .
6.3 Nonergodicity factor
6.3.1 PIB
Before looking at the nonergodicity factor (the ratio of inelastic to total intensity)
we take a look at the total intensity itself (figure 6.13). The total intensity at low
Q, is weakly linearly increasing with temperature at 300 MPa while it is strongly
temperature dependent above T g at ambient pressure (figure 6.13). Note that the
absolute values of the different molecular weights cannot be compared directly as
these will depend on the number of scattering centers in the beam. This depends
both on how the cell was filled and on how it was placed in the beam. However, the
experiments at different pressures where performed on the same samples without
moving the cell. It is significant that the total intensity decreases as pressure is
increased, particularly at high temperatures.
14000
12000
10000
Mw 680 Patm
Mw 680 300MPa
Mw 3580 Patm
Mw 3580 300MPa
counts / 60 s
8000
6000
4000
2000
0
0 50 100 150 200 250 300
T [K]
Figure 6.13: The integrated intensity as a function of temperature at Q=2 nm −1 .
Two molecular weights and two pressures. The lines are guides to the eye.
The nonergodicity factor evaluated at Q=2 nm −1 decreases with temperature, but
there is no pressure dependence within error-bars. This is seen in figure 6.14. On the
other hand, it is seen in the figure that there is a strong dependence on the molecular
weight, with the nonergodicity factor increasing with increasing molecular weight at
all pressures.