Ph.D. thesis (pdf) - dirac

106 High Q collective modes
1
0.9
f(Q) , Q=2nm −1
0.8
0.7
Mw 3580 Patm
0.6 Mw 3580 3kbar
Mw 680 Patm
Mw 680 3kbar
0.5
0 50 100 150 200 250 300
T [K]
Figure 6.14: The nonergodicity factor as a function of temperature. Two molecular
weights and two pressures.
The wave-vector dependence of the nonergodicity factor follows the expected oscillation
with the S(Q). That is, it is Q-independent in the low Q-region and increases
when approaching the structure factor maximum (figure 6.15).
6.3.2 Cumene
Unlike the case of PIB, there is a pressure dependence of the non ergodicity factor
of cumene. It is moreover non-trivial in the sense that it is different in the glass
as compared to the melt. The nonergodicity factor at Q=2 nm −1 increases with
increasing pressure in the melt while the effect is opposite in the glass (figure 6.16).
The latter effect is weak and maybe not significant compared to the error-bars.
The pressure dependence of the nonergodicity factor at Q=4 nm −1 is qualitatively
the same at all temperatures with an increase in f Q with increasing pressure. The
effect is most pronounced at high temperatures, while the difference between the
two temperatures essentially disappears in the glass.
6.3.3 Interpretation in terms of compressibility
In this section we rationalize the pressure dependence of the inelastic and the total
intensities of PIB3580 in terms of compressibilities. The nonergodicity factor is determined
from the ratio between the elastic intensity (total intensity minus inelastic
intensity) over the total intensity. The considerations presented here are therefore
directly relevant for understanding the nonergodicity factor, particularly its pressure
dependence is of interest.