Peculiarities of Emulsions Stabilized by Stimuli-Responsive\nInterpenetrating Polymeric Network Microgels

Abstract

Emulsions have become a crucial product\nform in various industries\nin modern times. Expanding the class of substances used to stabilize\nemulsions can improve their stability or introduce new properties.\nParticularly, the use of stimuli-responsive microgels makes it possible\nto create “smart” emulsions whose stability can be controlled\nby changing any of the specified stimuli. Thus, finding new ways to\nstabilize emulsions may broaden their application. In this work, for\nthe first time, we applied microgels based on interpenetrating polymeric\nnetworks (IPNs) of poly(<i>N</i>-isopropylacrylamide) (PNIPAM)\nand poly(acrylic acid) (PAA) as stabilizing agents for “oil-in-water”\nemulsions. We have demonstrated that emulsions stabilized by such\nsoft particles can remain colloidally stable for an extended period,\neven after being heated up to 40 °C, which is above the lower\ncritical solution temperature (LCST) of PNIPAM. On the contrary, the\nemulsions stabilized by PNIPAM homopolymer microgels were broken upon\nheating. To understand the stabilization mechanism of the emulsions,\nmesoscopic computer simulations were performed to study the IPN microgels\nat the liquid–liquid interface. The simulations demonstrated\nthat when the first subnetwork (PNIPAM) collapses, the particle adopts\na flattened core–shell morphology with a highly swollen PAA-rich\nshell and a collapsed PNIPAM-rich core. Unlike its PNIPAM homopolymer\ncounterpart, the IPN microgel maintains its three-dimensional shape,\nwhich provides stability to the microgel-based emulsions over a wide\nrange of temperatures. Our combined findings could be useful in developing\nnew approaches to emulsions’ storage, biphasic catalysis, and\nlubrication of mechanisms in various operating and climatic conditions.