Rheological properties of silicone oil emulsions stabilized by poly(N-isopropyl acryl amide)s

Oil in water emulsions were prepared by dispersion of silicone oil into aqueous solutions of poly(N-isopropyl acryl amide) (PNIPAM) as a function of concentration for three different molecular weights of PNIPAM. The resulting emulsions were characterized by the measurements of visual observation, microscopic observation, adsorbed amounts of PNIPAM, and rheological properties, such as stress–strain sweep curves, shear viscosities, and dynamic moduli. Added silicone oil was almost emulsified by PNIPAM above an overlapping polymer concentration C*, leading to a volume fraction of silicone oil in the emulsion higher than 0.74, irrespective of the molecular weight, and the emulsions showed no coalescence and they were stable. The emulsions prepared at twice C* showed solid-like viscoelastic responses and stronger shear-thinning flow, and moreover the emulsion prepared by the highest molecular weight of PNIPAM only indicated a clear yield stress. On the other hand, below C* all added oil was not emulsified by PNIPAM and the volume fraction of the emulsified oil was decreased with a decrease in the concentration of PNIPAM. The emulsions prepared at C = 0.5 g/100 mL, which is lower than C* for two lower molecular weight PNIPAM samples indicated viscoelastic responses and weak shear-thinning flow.