Ph.D. thesis (pdf) - dirac

7.3. Relation to alpha relaxation 127
Glycerol is known to be a case where the temperature dependence of the mean
square displacement passes smoothly through T g at atmospheric pressure. This is
seen in figure 7.6, where we also display the mean square displacement at 300 MPa.
The slope changes rapidly at about 1.25T g in both cases (figure 7.7).
0.25
0.2
P=0.1 MPa
300 MPa
Tg
Tg @ 300 MPa
[A 2 ]
0.15
0.1
0.05
0
0 50 100 150 200
T [K]
Figure 7.6: The 〈u 2 〉 of glycerol at ambient pressure and at 300 MPa.
0.5
0.45
0.4
0.35
P=0.1 MPa
300 MPa
Tg
Tg @ 300 MPa
[A 2 ]
0.3
0.25
0.2
0.15
0.1
0.05
0
0 50 100 150 200 250 300
T [K]
Figure 7.7: The 〈u 2 〉 of glycerol at ambient pressure and at 300 MPa.
7.3 Relation to alpha relaxation
From the data shown in the previous section it is seen that the mean square displacement
at the nanosecond timescale often has a change in its temperature dependence
in the vicinity of T g , which is governed by the dynamics on the time scale of hundreds
of seconds. This is a general observation and it is probably the most clear example
of a coupling between fast and slow dynamics [Buchenau and Zorn, 1992; Angell,
1995; Casalini and Ngai, 2001; Ngai, 2000]. The origin of this coupling is considered