Viscoelastic properties of polystyrene and poly(methyl methacrylate) dispersions sterically stabilized by...

Recently, steric repulsive forces induced by a new graft copolymer surfactant, which is based in inulin (polyfructose), have been described. Previous investigations by atomic force microscopy between solid surfaces covered with adsorbed surfactant indicated strong repulsive forces even at high electrolyte concentration, due to the steric repulsion produced by the surfactant hydration. In the present paper, the colloidal stabilization provided by this surfactant is studied by rheology. The measurements were carried out on sterically stabilized polystyrene (PS) and poly(methyl methacrylate) (PMMA) containing adsorbed surfactant (INUTEC SP1). Steady-state shear stress as a function of shear rate curves was established at various latex volume fractions. The viscosity volume fraction curves were compared with those calculated using the Doughtry-Krieger equation for hard sphere dispersions. From the experimental eta r-phi curves the effective volume fraction of the latex dispersions could be calculated and this was used to determine the adsorbed layer thickness Delta. The value obtained was 9.6 nm, which is in good agreement with that obtained using atomic force microscopy (AFM). Viscoelastic measurements of the various latex dispersions were carried out as a function of applied stress (to obtain the linear viscoelastic region) and frequency. The results showed a change from predominantly viscous to predominantly elastic response at a critical volume fraction (phi c). The effective critical volume fraction, phi eff, was calculated using the adsorbed layer thickness (Delta) obtained from steady-state measurements. For PS latex dispersions phi eff was found to be equal to 0.24 whereas for PMMA phi eff=0.12. These results indicated a much softer interaction between the latex dispersions containing hydrated polyfructose loops and tails when compared with latices containing poly(ethylene oxide) (PEO) layers. The difference could be attributed to the stronger hydration of the polyfructose loops and tails when compared with PEO. This clearly shows the much stronger steric interaction between particles stabilized using hydrophobically modified inulin.