Rheology of concentrated dispersions of sterically stabilized polydisperselamellar droplets

The rheological behavior of concentrated dispersions of sterically stabiliz ed lamellar droplets has been studied as a function of the molecular weight and of the amount of adsorbed hydrophobically endcapped poly(sodium acryla te)s. The chemical compositions of the samples are identical to those descr ibed before. Despite the polydispersity of the sample, scaling laws and equ ations that are well established in the rheology of monodisperse colloidal suspensions can be successfully applied. Although the amount of added stabi lizing polymer at constant molecular weight hardly influences the elastic m odulus (G') as a function of (core) volume fraction of lamellar droplets (p hi(lam)), increasing the polymer molecular weight at constant grafting dens ity results in a pronounced increase of the elastic modulus. The ratio of p article radius to adsorbed layer thickness (R/Delta) decreases with molecul ar weight, thereby increasing the effective volume fraction. A peculiar eff ect occurs if the polymer molecular weight drops below 1000. Polymer molecu les penetrate into the lamellar droplets, and phi(lam) is increased (at con stant surfactant concentration). This so-called "building-in" effect can be used to thicken lamellar dispersions. Thickening can also be induced by ad dition of "bridging polymers", which carry multiple hydrophobic anchors, li nking several droplets. Concomitant adverse effects of bridging flocculatio n can be counteracted by admixing of hydrophobically endcapped polymers; th e resulting dispersions are characterized by enhanced shear-thinning behavi or and good physical stability.