(232q) Gelation of Polymer-Grafted Silica Nanoparticles Studied with X-Ray Photon Correlation Spectroscopy (XPCS) and Rheology

We report a combined XPCS study of particle dynamics and rheological study of moderately concentrated suspensions of silica colloids and their binary mixtures that form a gel on cooling. The suspensions are comprised of silica colloids (32nm, 86nm and 185 nm) coated with octadecyl-hydrocarbon chains and suspended in decalin at colloidal volume fractions (f) ranging from 0.2 to 0.4. Different scaling relationships are explored to predict the elastic modulus and the limit of linearity. Gel temperatures of the different size particles and the elastic moduli, though different, collapse onto a single curve when scaled as Gâ??D³/fkT and plotted versus (1/T â?? 1/T_gel), where Gâ?? is the elastic modulus and T_gel is the gel temperature. This scaling is in line with predictions of mode coupling theory which emphasizes the importance of local structure (localization length) over longer-range correlations in determining the dynamical and mechanical properties of such gels. The dynamics of nanoparticles during the gelation process is measured using X-ray Photon Correlation Spectroscopy (XPCS) at Argonne National Labs. Using a newly developed ultrafast frame rate (11.8 kHz) pixel-array-detector, we have, for the first time, captured the complete transition of the dynamics of the suspension from tens of seconds in the gel state to hundreds of µs in the liquid state. The transition is triggered when the gel is slowly heated. Our nanoscale measurements of the colloid dynamics as a function of temperature and particle size can be compared to macroscopic viscoelastic properties probed by rheology under the same conditions and suggest that smaller colloids form stronger networks when they gel. This is in line with the measurements of the gel boundary with smaller particles gelling at higher temperatures. Efforts to extend the scaling relationships of elastic modulus to smaller particle sizes and their mixtures and limitations of the theories in predicting the mechanical properties will also be discussed.