Phase inversion emulsification using block copolymer surfactants

This research aims at understanding the mechanism of emulsification through phase inversion using a block copolymer as surfactant. Phase inversion emulsification technique is effective in producing concentrated aqueous emulsions of high viscosity component, with small droplet size and a narrow distribution. The oil-continuous mixture undergoes a dramatic water-in-oil to oil-in-water transformation. Many factors (thermodynamic and hydrodynamic) are involved in such nonequilibrium, dynamic process, such as material characteristics and composition, mixing condition, temperature, etc. This dissertation seeks comprehensive fundamental understanding of the inversion mechanism when block copolymers are used to emulsify a high viscosity oil.; An epoxy oil phase and poly(ethyleneoxide)-b-poly(propyleneoxide) triblock copolymer (Pluronic) surfactants that allow systematic variation of properties are used. Stable aqueous emulsions of high volume fraction are prepared, and the droplet size tuned by appropriate selection of surfactant structure, concentration and temperature. Phase inversion can be divided into two major categories with separate mechanisms. The hydrodynamic driven (droplets breakup and coalescence) inversion results in large droplet size and broad distribution, in which the surfactant influence is secondary. The thermodynamics controlled inversion, where submicron droplet size and narrow distribution results, has the equilibrium phase behavior and the curvature constraint (spontaneous emulsification) as the main driving force, while the mechanical mixing serves to speed up material redistribution. Comparison of transitional and catastrophic inversion in the thermodynamic controlled regime reveals a strong similarity between the two processes, and suggests the possibility that they may be governed by a common mechanism in the spontaneous emulsification domain.; Measurements of conductivity and rheology reveal microstructural transitions in the pre-inversion mixture. A discrete peak in conductance and elasticity prior to inversion is observed for certain conditions. Flow video microscopy provides evidence for the creation of a network via percolation of water droplets. The collapse of the network structure (antipercolation) is closely related to local inversion, and the fast propagation of the inverted domains by spontaneous emulsification at the resin/water interface, eventually leads to global inversion. The relationship between the percolation network formation under hydrodynamic condition prior to the inversion point and the production of fine emulsion through spontaneous emulsification is discussed.