(514d) Controlling Viscoelasticity of Nanoemulsions Using Polymer-Surfactant Self-Assembly

Nanoemulsions have been considered for a range of advanced applications where fine control over their rheological properties is desired, independent of their colloidal stability. To address this issue, we have developed a scheme for imparting thermoreversible viscoelasticity to model oil-in-water nanoemulsions. Specifically, we have shown that polymer-surfactant interactions in the aqueous phase cause thermoreversible gel-like viscoelasticity at low temperatures, distinct from their emulsion-free counterparts. Interestingly, these gels exhibit Maxwell-like viscoelasticity that follows time-temperature superposition (TTS) over a wide temperature range - the first time such behavior has been observed in emulsions. Small angle neutron scattering (SANS) shows that the low-temperature nanoemulsion gels exhibit temperature-insensitive microstructure, confirming that the gel-like viscoelasticity arises entirely from the dynamics of the system. We hypothesize that this low-temperature gelation arises from a temporary polymer network mediated by polymer-surfactant interactions at the droplet interface. To test this hypothesis, we use TTS to probe the relation of viscoelastic properties and material parameters and compare them with temporary network theory. The results show that the viscoelasticity is critically linked to the energetics of polymer-surfactant complexation, as confirmed by isothermal titration calorimetry (ITC). We have also studied the steady shear rheology of these nanoemulsion gels, which exhibit characteristics of extreme shear thinning and shear banding. Rheo-SANS measurements indicate the formation of shear-induced structure of the droplet phase, which we believe is linked to the macroscopic rheology by non-linear deformation of the gel network. Overall, these studies demonstrate polymer-surfactant complexation as a highly generic mechanism to control both the linear and non-linear rheology of nanoemulsions, which could be exploited to design new materials with dynamically controlled rheology.