Influence of the polymer size on depletion attraction—induced gel and glass transitions of microgel colloids

Abstract

In previous work of our group on the re-entrant melting of colloidal glasses we observed an unexpectedly huge re-entry region for a binary system of 1:50 crosslinked polystyrene (PS) microgel spheres when a depletion attraction range ξ=Rg,polymer/Rcolloid=0.08 was induced by non-adsorbing (free) PS chains. To investigate whether this fluid region can even be increased by reducing polymer size and therefore the attraction range as predicted by mode coupling theory (MCT), we studied an almost identical system. It consists of a binary 1:50 PS microgel mixture (Rh,L=105.8nm, Rh,S=82.9nm; Γ=Rh,S/Rh,L=0.85) with a reduced attraction range of ξ=0.059 by use of an appropriate free PS polymer (Mw=79,600g/mol, Rg,polymer=9.18nm). Here the glass transition is shifted from ϕg=0.577 to higher values, allowing for fluid states up to volume fractions of ϕ=0.715. This is significantly higher than the maximum fluid state of ϕ=0.69 reached for ξ=0.08. Tentatively correcting for osmotic deswelling these volume fractions reduce to 0.673 and 0.66, respectively. This is in qualitative agreement with MCT, but in quantitative disagreement because the theory predicts a shift of only a few percent. Such fluid states beyond the maximum amorphous packing of hard spheres (ϕrcp∼0.64) do not occur for PMMA colloids and 1:10 crosslinked PS microgel spheres that can both be considered as hard spheres. Thus, we tentatively attribute the observation of fluid states at such high volume fractions to a combination of the slight softness of the 1:50 microgels, leading to osmotic deswelling and particle deformability. In addition, polydispersity and polymer non-ideality effects should be taken into account.