Rheo-optical properties of silicone oil emulsions in the presence of polymer emulsifiers

Oil in water emulsions prepared by dispersion of silicone oils into an aqueous solution of hydroxylpropyl methyl cellulose (HPMC) or poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers were characterized by measurements of steady-state shear viscosities, dynamic moduli, and stress-strain sweep curves coupled with optical microscopic observation. The emulsions prepared with HPMC showed solidlike viscoelastic responses and a clear yield stress, whereas the emulsions emulsified by PEO-PPO-PEO block copolymers indicated liquidlike viscoelastic behavior. The difference should be attributed to stronger protective colloidal effects, that is, the former emulsifiers form a more viscoelastic polymer layer adsorbed on the oil droplets than the latter ones. Moreover, the simultaneous optical microscopic observation showed that the emulsions stabilized by HPMC do not flow below the yield stress and beyond the yield stress the movements of oil droplets occur first.