Ph.D. thesis (pdf) - dirac

100 High Q collective modes
25
20
Patm
3kbar
S(Q) [arb. unit]
15
10
5
0
0 5 10 15 20
Q [nm −1 ]
Figure 6.4: The static structure factor of PIB680 at room temperature at atmospheric
pressure and 300 MPa.s, measured on ID28.
sound speed decreases when temperature is increased above T g . This is illustrated
by the data of PIB680 in figure 6.5. This change in temperature dependence of
the high frequency sound speed is a signature of the transition from glassy to high
frequency equilibrium dynamics according to the definitions given in section 2.5.
4000
3500
Patm
3kbar
v l
[m/s]
3000
2500
2000
0 100 200 300
T [K]
Figure 6.5: The sound speed of PIB680 at room pressure and at 300 MPa as a
function of temperature (calculated from the excitation at Q=2nm −1 ). The sound
speed in the glass is temperature independent within error-bars, while the sound
speed decreases when temperature is increased above T g . An equivalent behavior is
found for PIB3850 (not shown).
The sound speed increases with increasing molecular weight at ambient pressure
and room temperature, with a molecular weight dependence that levels off around
Mw=10000 g/mol. However, when comparing the (temperature independent) sound
speeds in the glass we see no molecular weight dependence (figure 6.6).