Rheology of Concentrated Dispersions of Sterically Stabilized Polydisperse Lamellar Droplets

The rheological behavior of concentrated dispersions of sterically stabilized lamellar droplets has been studied as a function of the molecular weight and of the amount of adsorbed hydrophobically endcapped poly(sodium acrylate)s. The chemical compositions of the samples are identical to those described before. Despite the polydispersity of the sample, scaling laws and equations that are well established in the rheology of monodisperse colloidal suspensions can be successfully applied. Although the amount of added stabilizing polymer at constant molecular weight hardly influences the elastic modulus (G‘) as a function of (core) volume fraction of lamellar droplets (φlam), increasing the polymer molecular weight at constant grafting density results in a pronounced increase of the elastic modulus. The ratio of particle radius to adsorbed layer thickness (R/Δ) decreases with molecular weight, thereby increasing the effective volume fraction. A peculiar effect occurs if the polymer molecular weight drops below 1000. Polymer molecules penetrate into the lamellar droplets, and φlam is increased (at constant surfactant concentration). This so-called “building-in” effect can be used to thicken lamellar dispersions. Thickening can also be induced by addition of “bridging polymers”, which carry multiple hydrophobic anchors, linking several droplets. Concomitant adverse effects of bridging flocculation can be counteracted by admixing of hydrophobically endcapped polymers; the resulting dispersions are characterized by enhanced shear-thinning behavior and good physical stability.