Ph.D. thesis (pdf) - dirac

Chapter 6
High Q collective modes
In this chapter we present a study of temperature, pressure and molecular weight
dependence of the coherent dynamical structure factor measured by IXS in cumene
and PIB. The study of the molecular weight dependences allows to change macroscopic
characteristics of the sample while the relaxing entity is kept the same. It
is in this sense analogous to pressure studies. The IXS spectra give information on
the collective modes in the Q-range 1-10 nm −1 which gives 1/Q ∼ 0.1-1 nm. This
means that we are studying “sound” at wavelengths so small that they are comparable
to the distance between the molecules (or the monomers of a polymer). The
modes in this Q region have frequencies of the same order of magnitude as the boson
peak frequency, and the information on their behavior is therefore relevant in order
to understand the meaning of the boson peak. Some of the results of this chapter
therefore play a central role in chapter 8 where we analyze the pressure dependence
of the boson peak in polyisobutylene.
The main focus of the analysis and discussion in this current chapter is the pressure
and temperature dependence of the nonergodicity factor as measured by IXS and
particularly to scrutinize the proposed correlation to fragility [Scopigno et al., 2003].
The correlation was briefly introduced in section 2.7.1 and more details will be given
in section 6.4.1.
6.1 Inelastic X-ray scattering
6.1.1 The experimental technique
The IXS experiment on cumene was performed on the IXS beamline ID16 at the
ESRF, while the experiment on PIB was performed on its sister beamline ID28. In
95