Temperature-induced gelation of emulsions stabilised by responsive copolymers: A rheological study

Abstract

The steady-state and dynamic rheological properties of 1-bromohexadecane-in-water emulsions stabilised by responsive poly(N-isopropylacrylamide)-co-poly(ethyleneglycol methacrylate) (poly(NIPAM-co-PEGMa)) copolymer have been investigated. The data were compared to measurements performed using dilute and concentrated poly(NIPAM-co-PEGMa) copolymer solutions. These solutions exhibit viscosities that decrease and increase, respectively, with increasing temperature. The increase in viscosity for the concentrated solution is attributed to transient network formation. The presence of dispersed oil droplets (volume fraction = 0.30) in the emulsion causes temperature-induced gelation even though the copolymer concentration in the aqueous phase is such that transient network formation due to overlapping copolymer chains throughout the aqueous phase does not occur. Dynamic rheological data confirmed gelation for the emulsions at elevated temperatures. Emulsion gelation is attributed to flocculation between neighbouring droplets. The adhesive droplets form a network that entraps the aqueous phase. The strength of the network increased with increasing temperature. To our knowledge these data represent the first comprehensive rheological investigation of reversible, temperature-induced gelation for an oil-in-water emulsion.