Propriétés optiques et volumiques en équilibre et hors-équilibre près de la transition de ségrégation de solutions aqueuses de PNIPAM par réfractométrie modulée en température

Self-assembly is particularly important for soft condensed matter. It may lead to
manifold equilibrium or non-equilibrium structures and macroscopic properties of soft
materials. The self-organization may be induced by changes in environmental
conditions such as temperature, pH and irradiation by light. Among the different
polymer families, the stimuli-responsive polymers are particularly sensitive to external
stimuli. A structural instability, which is usually related to a demixing phase transition,
commonly occurs for stimuli-responsive polymer systems. For such aqueous polymer
solutions and hydrogels, a demixing transition of the Lower Critical Solution
Temperature (LCST) type often happens, this means that the segregation occurs
upon heating. Its macroscopic order parameter is attributed to the difference in mass
concentration within the segregated state. Since the variation in mass density is a
closely related quantity, the transition is also called a volume phase transition. In
case of hydrogels, the order parameter also is the macroscopic deformation. In
consequence the related susceptibility is the isothermal compressibility and the
demixing transition is denoted as being ferroelastic.
In this thesis, basic aspects of the static, kinetic and dynamical nature of the LCSTtype
demixing transition are analyzed. Aqueous solutions of the thermo-responsive
model homopolymer poly(N-isopropylacrylamide) (PNIPAM) are studied. Part of the
phase diagram is probed by varying the polymer concentration in solution and the
temperature. In part unconventional experimental techniques were used to access
the macroscopic order parameter susceptibilities, like the dynamical volume
expansion coefficient, and linear and nonlinear elastic properties at GHz frequencies.
The major experimental technique is Temperature Modulated Optical Refractometry
(TMOR), which has recently been developed in collaboration with the company Anton
Paar (Seelze, Germany) and patented in 2012 by the University of Luxembourg.
First studies by Brillouin spectroscopy were carried out on the segregation of
aqueous PNIPAM solutions. The linear and in particular nonlinear elastic moduli are
much more affected than the thermal expansion coefficient by this phase transition.
Furthermore, it could be shown that the specific refractivity is astonishingly sensitive,
probably to the changes in molecular interactions, during segregation. This allows for
a novel perspective on the microscopic order parameters of the transition. The key
investigations of this thesis focus on kinetics and thermo-mechanical relaxation
processes related to the segregation, studying optical and mechanical quantities by
TMOR. Sinusoidal temperature perturbations were therefore applied around the
demixing temperature of different PNIPAM solutions. First direct indications for the
location of the binodal and spinodal lines could be