The effect of free polymer on the glass transition dynamics of microgel colloids

From the phase behavior we establish that 1:10 crosslinked polystyrene microgel colloids in a good solvent can be considered as hard spheres whereas 1:50 crosslinked particles are slightly softer. Nevertheless, the glass transition dynamics of 1:10 microgels and a binary mixtures of 1:50 microgels can be described in both cases within the mode coupling theory for hard spheres. The only difference is that the increase of the polydispersity from about No to about 13% when going from the one-component system to the mixture is accompanied by a shift of the glass transition from phi(g) approximate to 0.56 to phi(g) approximate to 0.595. On addition of linear polystyrene to the 1:50 binary mixture we find a tremendous acceleration of the density fluctuations and the melting of a colloidal glass. This is accompanied by a significant decrease of the first peak of the static structure factor and its shift to higher scattering vectors. The presence of free polymer in colloidal dispersions is known to act as an effective attraction via the 'depletion effect'. Thus, our observations are consistent with the predictions of recent extensions of mode coupling theory [Phys. Rev. E 63 (2001) 1401] where the inclusion of short-ranged attractions has been found to shift the glass transition line to significantly higher volume fractions. (C) 2002 Elsevier Science B.V. All rights reserved.