University of Fribourg, Department of Physics
“A study of cluster formation and phase transitions in suspensions of
platelet-like clay particles using scattering techniques”
Andrey Shalkevich
This thesis explores the physical properties of plate-like particle suspensions. We focus here on clay
particles, which are studied both in their natural state as well as chemically (surface) modified. As a
model system to investigate the plate-like particle dispersions, we use suspensions of commercially
available montmorillonite particles. We systematically investigate the phase diagram of clay particles in
water at fixed pH and variable ionic strength. We find five different well-defined liquid- and solid-like
states. The peculiarities of the different states are discussed. We determine the static structure factor S(q)
of the glassy state phase using different scattering techniques such as small-angle light, x-ray and neutron
scattering and propose a new cluster model for this phase. Combining screened electrostatic repulsion
with a “house of card” structure, this cluster glass model allows for a consistent interpretation of the
scattering data at all lengths. From the phase diagram we also observe a phase separation region with
transient gel formation at sufficiently high clay content.
Finally, we investigate the behavior of clay particles in polymer nanocomposite formation since they
play a major role as filler material. For the best composite performance clay layers need to be exfoliated
into single sheets. In most cases, this requires surface modification of the clay particles. Characterization
of intercalation and exfoliation of the clay particles in a polymer matrix or in an organic solvent is
required in order to find better modifiers for organoclay synthesis and optimizing processing conditions.
We study exfoliation of the different organoclays in different organic solvents as first step towards a claypolymer nanocomposite as well as in a real polymer matrix. Small-angle neutron and x-ray scattering
experiments are applied to observe organoclays in paraffin oil, hexadecane and high-density polyethylene
at different temperatures. Careful data modeling provides detailed information on the distribution of the
stacks (relative concentrations and fraction of individual layers) and the characteristics of the platelet
stacks (mean number of layers per stack, layer repeat distance and fractional stack disorder). Finally, a
simple examination of the deviations between the applied scattering model and the experimental data
provide us with detailed information on the clay surface layer structure in the stacks. Here we use the
contrast variation technique in neutron scattering, which allows us to gain information on the surface
layer structure of the modifier molecules and on the correlations between the state of the surface modifier
and the exfoliation behavior.
Jury:
Prof. Peter Schurtenberger (Thesis Director)
Prof. Claude-Pierre Aebischer (External Expert)
Prof. Frank Scheffold (Expert)
Prof. Jean-Claude Dousse (President of the Jury)