Interdroplet Interactions and Rheology of Concentrated Nanoemulsions for Templating Porous Polymers

In the current study, we investigate the colloidal behavior of nanoemulsions over a wide range of oil volume fractions (φ) from dilute to concentrated regime. The dilute system contains 25% silicone oil dispersed in the aqueous phase consisting of poly(ethylene glycol)-diacrylate (PEGDA) and sodium dodecyl sulfate (SDS), which is concentrated through evaporation of water at two different rates at ambient temperature. The rheological studies show that the liquid-like nanoemulsions transform into viscoelastic gels at a volume fraction of ∼30%. The plateau storage modulus of the nanoemulsions increases in the semidilute systems (φ below 45%) and then decreases steadily with increasing φ up to 60%. Dependency of the modulus on the evaporation rate can be observed in the rheological results. According to the rheological results and the overall pairwise interactions estimated between droplets, we propose two regimes of colloidal interactions. In the semidilute regime, the attractive gelation occurs due to considerable short-range attractive depletion induced by the PEGDA oligomer and SDS micelles. In the concentrated regime, the gel weakens by increasing φ mainly due to the structural stabilization barrier from a high concentration of micelles. The PEGDA in the continuous phase of the nanoemulsions can be crosslinked through photopolymerization, resulting in nanoporous PEGDA hydrogels upon removal of oil droplets. We study the water uptake of the nanoporous hydrogels prepared from the nanoemulsion templates at φ = 60%. The hydrogel obtained from the nanoemulsion with fast evaporation rate shows higher water uptake than that obtained from the slowly concentrated nanoemulsion. The tunable viscoelastic behavior of concentrated nanoemulsions as well as the resulting nanoporous hydrogels offers a new platform to design the soft materials for a wide range of applications.