Structure and dynamics of complex liquids. The phase behaviour of silica-PNIPAm nanogels

In this thesis, the structure and dynamics of highly concentrated core-shell nanogel particles composed of a silica core and a poly(N-isoproylacrylamide) (PNIPAm) shell suspended in water are studied. The particles were synthesised with dierent core-to-shell size ratios and prepared at various concentrations with volume fractions ${\Phi }_{eff}$ between approximately 0.004 and 2. X-ray photon correlation spectroscopy (XPCS) enables to follow the dynamical, small-angle X-ray scattering (SAXS) the structural changes of the thermo-responsive nanogel particles which undergo a volume phase transition at a so-called lower critical solution temperature (LCST) of 32°C. All samples were studied in a broad temperature range around the LCST with approximately 15°C < T < 45°C and the main focus is on densely-packed systems. For a system with ${\Phi }_{eff}$ = 0.55, the structural relaxation time ${\tau }_c$ was found to decrease with increasing temperature. Above the LCST, the structural relaxation time initially continues to decrease. The effect is accompanied by a transition from stretched to compressed exponential behaviour of the intensity autocorrelation functions, which could be determined by analysing the intensity autocorrelation functions. Upon further heating a sudden slowing down at a critical temperature T${}_c$ $\approx$ 37°C. In parallel, the q-dependence of the relaxation time shows an anomalous change from ${\tau }_c$ $$/propto$q$^{-3}$ to ${\tau }_c$ $/propto$q$^{-1}$. The results reveal a temperature-induced transition from swollen repulsive particles with polymer driven dynamics at lower temperatures T < T${}_c$ towards a colloidal gel composed of collapsed attractive particles. The transition of structure and dynamics shows a distinct dependency on the effective particle volume fraction. Below a critical concentration no transition upon heating was found. The de-swelling behaviour of the PNIPam nanogel was inuenced with the addition of Trimethylamine N-oxide (TMAO) to the concentrated particle suspension. With increasing TMAO concentration, a shift of the LCST towards lower temperatures and a preferential hydrophobic state could be determined. With all results a full phase diagram of a silica-PNIPAm core-shell system is presented as a function of temperature and concentration. It spans from dilute suspensions to an effective volume fraction of more than ninety percent covering two liquid phases, a structural rearrangement phase and a colloidal gel phase.