(686c) Re-Entrant Viscoelastic Behavior in Thermoresponsive Nanoemulsion Organohydrogels

Recently, we reported the discovery of novel gelling suspensions based on oil-in-water (O/W) nanoemulsions that thermoreversibly form “organohydrogels” – solid-like gels comprised of up to 90% oil and water – at high temperature due to the presence of a thermoresponsive polymer [1]. Here, we report on similar O/W nanoemulsions which exhibit re-entrant viscoelastic behavior, whereby gel-like phases are observed at both low and high temperatures, with an intermediate excursion into a low-viscosity liquid phase. Using both linear and non-linear rheological probes to fingerprint the mechanical properties at both temperature extremes reveals significantly different viscoelasticity driven by relatively mild changes in polymer solution behavior. At low temperatures the material forms a transient gel where the droplets act as a physical cross-linker for the polymer, whereas at high temperature it forms a percolated network of droplets. Coupling rheological techniques with in situ scattering measurements allows for detailed characterization of the microstructure in the respective phases, and in particular how the structure responds to large-scale deformation to give rise to a number of nonlinear rheological phenomena, including yielding and shear banding. Ultimately, we find that the nature and behavior of both the low and high-temperature phases is critically linked to the strength and the range of inter-droplet attractions. This key result serves as a general guideline for the design of viscoelastic phases in colloid-polymer mixtures, and using it we show how the re-entrant viscoelastic and non-linear behavior can be programmed based on several key parameters, including the nanoemulsion droplet size as well as polymer and surfactant chemistry.

[1] M.E. Helgeson, S.E. Moran, and P.S. Doyle, “Thermogelling, photocrosslinkable nanoemulsions for the formation of tunable mesoporous organohydrogels”, Nature Materials, 2012, 11(4): 344-352.