(92a) Responsive Nanoparticulate Polymer Brushes: Emulsion Stabilization and Interfacial Behaviors

Nanoparticulate polymer brushes, either in the form of polymers grafted at high density from solid nanoparticles or of crosslinked star or miktoarm star copolymers prepared by the arm-first polymerization method, are extremely effective emulsifiers.  Similar to Pickering emulsions prepared with conventional particulate emulsifiers, emulsions stabilized by nanoparticulate polymer brushes often contain very large volume fractions of the dispersed liquid phase in the emulsion and are highly stable against coalescence, often requiring more than a year for the emulsion to begin showing signs of spontaneous breaking.  Compared to conventional particulate emulsifiers, nanoparticulate polymer brushes are more efficient emulsifiers. We observe with a variety of different polymer chemistries and architectures, under preferred emulsion type conditions of 1:1 overall oil:water ratio, that they typically require on the order of 0.01 to 0.1 wt% emulsifier to stabilize high volume fraction emulsions for many months.  Nanoparticulate polymer brushes also differ from conventional particulate emulsifiers in their ability to produce large surface pressures by spontaneous adsorption from suspension, on the order of 20 to 30 mN/m, compared to surface pressures of no more than a few mN/m normally attained by spontaneous adsorption of bare particles.   A variety of stimulus responsive polymer brushes were prepared as emulsifiers for this study. These include the pH- and thermally responsive polycation poly(2-(dimethylamino)ethyl methacrylate) grafted from silica nanoparticles (Si-PDMAEMA), thermally responsive poly(oligoethylene oxide methacrylate) grafted from silica nanoparticles (Si-POEOMA), as well as thermally responsive poly(ethylene oxide)methacrylate star polymers with divinylbenzene crosslinked cores and pH-responsive miktoarm star copolymers consisting of poly(butyl acrylate) and poly(acrylic acid) with cores crosslinked by divinylbenzene (PBA-PAA) and ethylene glycol dimethacrylate-crosslinked PDMAEMA star polymers.  Each produced stable oil-in-water emulsions.  Some displayed stimulus-responsive emulsion breaking.  Emulsions stabilized by PBA-PAA miktoarm stars were pH-responsive, breaking rapidly upon acidification.  Emulsions prepared with Si-PDMAEMA were thermally responsive, breaking rapidly upon heating to the critical flocculation temperature (i.e., cloud point) of the particles in aqueous suspension.  Emulsions stabilized by PEO stars were also thermally responsive, but in a different manner.  They did not break upon heating to the cloud point, but heating to the cloud point and subsequent cooling back to room temperature caused extensive droplet flocculation that in turn produced a significant increase in the viscous and elastic moduli of the emulsions.  In a similar manner, the rheological properties of Si-PDMAEMA stabilized emulsions could be modified by controlling the degree of droplet flocculation, which depended on whether the nanoparticulate brushes were initially dispersed in the aqueous phase or the oil phase before emulsification. Differences in the emulsification behaviors were correlated with differences in static and dynamic interfacial tension and effective Gibbs elasticity for the various nanoparticulate brushes adsorbing at the oil/water interface.