On the stability and chemorheology of a urea choline chloride deep-eutectic solvent as an internal phase in acrylic high internal phase emulsions

Abstract

High internal phase emulsions are an interesting emulsion subset attainable by surpassing the critical volume of uniform spherical arrangement resulting in a new metastable polyhedral motif. By profiting from their stability time frame and through the introduction of a polymerizable phase, these unique structures can be “locked” in place, affording interconnected hierarchically porous polymers. Rheological exploration has establishing several emulsion stability key parameters including surfactant concentration, internal phase volume fraction (ϕ), interfacial tension (σ), phase polarity, and temperature. Because the majority of HIPEs studied are aqueous, additional parameters such as internal phase viscosity have not been studied in detail. Deep-eutectic solvents (DES) are a new generation of green solvents sharing several ionic liquid properties. DES provide an ideal opportunity to study nonaqueous polar internal phases of increased viscosity while expanding on the conditions for polymerization to potential scale up applications. This study presents the first detailed investigation on the DES-non ionic surfactant HIPE systems. Shear stability of non-aqueous HIPEs was evaluated taking into account continuous phase viscosity as well as monomer and surfactant molecular nature (i.e. chain length and functionality). HIPE polymerization was evaluated through isothermal oscillatory time-sweep experiments. Longer tail methacrylic monomers presented preferential stability over acrylic or short tail monomers. Furthermore, emulsions showed improved stability and elasticity compared to aqueous HIPEs due to their high internal phase viscosity. Internal phase viscosity mediated by hydrogen bonding increased activation energy as estimated by complex viscosity plots.