Stability of non-aqueous microgel dispersions in the presence of free polymer

Abstract

A series of cross-linked polystyrene microgel particles have been prepared and characterised in terms of their surface carboxyl-group concentration and their unswollen (in water) and swollen (in ethyl benzene) diameters. From the values for these two diameters the average segment volume fraction in the swollen microgel particles has been calculated (values range from 0.10 to 0.63). The effect on adding homopolymer polystyrene samples covering a wide molecular-weight range (1800 to 2 × 10⁶) has been studied. Critical polymer/solvent volume fractions (ϕ⁺_ps) have been established, at different microgel volume fractions, below which the microgel dispersion is stable, but above which weak, reversible flocculation is observed. For a fixed microgel volume fraction, ϕ⁺_ps decreased with increasing polymer molecular weight. For a given molecular weight, ϕ⁺_ps increased with increasing average segment concentration in the swollen microgel particles. Both of these trends are predicted by the recent theory of Vincent, Luckham and Waite, which analyses the effect of free polymer on the stability of sterically stabilised dispersions. By a suitable choice of the form of the segment concentration distribution in the swollen microgel particles, good agreement between experimental and theoretical ϕ⁺_ps values may be obtained.