Soft water-soluble microgel dispersions: Structure and rheology

The size and structural characteristics of polyacrylamide-based water-soluble microgel dispersions were investigated by optical and rheological methods. Microgel hydrodynamic radii R h were measured by light scattering and derived from intrinsic shear viscosity [ η ] 0 . The variations of R h 3 and [ η ] 0 with the crosslink density N x , follow the scaling law R h 3 ≅ N x − α with α close to 0.63, in good agreement with the simple structural model proposed in this paper showing how the exact value of α depends on inner structural details of the microgel. The plateau viscosity versus particle apparent volume fraction shows a monotonous change from hard sphere dispersions (high crosslink density of microgels) to flexible linear polymer solutions. Measurements of the first normal stress difference N 1 show that increasing the microgel crosslink density affects the system viscosity more than its elasticity. Under oscillatory shear flow, loss and storage moduli undergo both qualitative and quantitative changes with crosslink density. At moderate concentrations, the elastic modulus is the most affected and its slope in low frequency regime decreases from two to less than one as N x increases. We discuss the experimental results within the frame of knowledge on linear, branched polymer solutions and soft microgel suspensions. We investigate the effects of change of crosslink density on the structural and rheological properties of well-characterized microgel dispersions. A wide range of crosslink density was explored, from linear polymer to highly crosslinked microgels.