8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Wed 711:10-14:00P5.175Effect of cross-link density on reentrant melting ofmicrogel colloidsMalte Wiemann, 1 Norbert Willenbacher, 2 Jan Sudaporn Vesaratchanon, 2 OttilieThorwarth, 1 and Eckhard Bartsch 11 Universität Freiburg, Stefan-Meier-Str. 31, 79104, Freiburg, Germany2 Karlsruher Institut für Technologie (KIT), Karlsruhe, GermanyColloidal dispersions are widely used, their technical application usually requiring their fluidity.With an increase of concentration colloidal dispersions encounter an arrest transition at a volumefraction φ g of around 0.58. Earlier research showed that fluid states of colloidal dispersions atvolume fractions above the glass transition can be accessed by using the depletion effect - aphenomenon addressed as re-entrant melting or re-entry effect predicted by theory [1]. DLSmeasurementsrevealed a fluidization up to a volume fraction φ = 0.69 when linear non-adsorbingpolymer (polystyrene up to a concentration of more than the overlap concentration c*) is addedto polystyrene microgel particles (cross-linked 1:50; good organic solvent). Recent researchconfirmed these results and demonstrated that DLS captures the same physics as monitored byrheological measurements [2]. However, the high packing fraction up to which fluid states couldbe observed raised the question whether the magnitude of this effect is a specialty of the microgelsystem or due to non-ideality of the free polymer. Therefore we prepared a microgel system witha much higher crosslinking density (1:10) which can be expected to be a good approximationof hard sphere colloids. The reentry effect was studied by adding the same free polymer as inthe 1:50 case. It was found that fluid states could again be prepared above φ g , however, upto a significantly smaller packing fraction. The amount of free polymer needed to fluidize thedispersion was much lower than the overlap concentration. Thus, the existence of fluid states upto unexpectedly high volume fraction for the 1:50 crosslinked particles is - at least partially - amicrogel effect which is at present not fully understood and possible origins will be discussed.[1] T. Eckert, E. Bartsch, J. Phys. : Cond. Matter 16, 4937 (2004).[2] N. Willenbacher, J. S. Vesaratchanon, O. Thorwarth, E. Bartsch, Soft Matter (2011), in print.175