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(285f) Microstructural Characterization of Pickering Emulsions Stabilized By Polymer Brush Nanoparticles Via Small-Angle Neutron Scattering

Authors: 
Riley, J. K., Carnegie Mellon University
Tilton, R. D., Carnegie Mellon University
Butler, P., National Institute of Standards and Technology
Star polymers and polymer-grafted nanoparticles with multiple arms emanating from a central core behave as “soft colloids” in solution and their properties reflect both the dynamic flexibility of polymer chains and the generally spherical nanoscale conformation of solid particles. The combination of polymeric and colloidal behavior is particularly beneficial when designing soft colloids as functional emulsifiers – irreversible adsorption driven by multi-segment contact of the polymer arms provides strong surface tension reduction between oil and water while the particle-like conformation of the star polymer imparts long-term Pickering-like stability against coalescence. Both star polymers and polymer-grafted nanoparticles have been shown to produce significantly more stable emulsions that their constituent linear polymers while requiring 1 to 2 orders of magnitude lower mass concentrations to produce stable emulsions than solid-particle Pickering emulsions (~ 0.005 wt% for stars vs. ~ 0.1 wt% for silica particles). In addition, emulsions stabilized by star polymers with pH-responsive polyelectrolyte arms or thermally-responsive LCST-type polymer arms have tunable emulsion stability, flocculation, and coalescence behavior via external triggers. While much is understood about the properties of these polymer brush nanoparticle emulsifiers via surface tension measurements and stability jar tests, ultimately the emulsification ability of a soft colloid with complex molecular architecture is tied to its microstructure, concentration, and interactions both in solution and at the droplet fluid-fluid interface. In this talk, we will discuss our recent efforts to characterize Pickering emulsions stabilized by star polymers and polymer-grafted nanoparticles with small-angle neutron scattering (SANS) and ultra small-angle neutron scattering (USANS). We will discuss the core-shell microstructure of the particles in bulk solution and compare with resolvable changes in their conformation upon adsorption to a contrast-matched oil/water droplet interface. Contrast variation is used to isolate polymer arm effects from their nanoscale cores. The combined SANS/USANS data will be discussed in the context of a recently developed model for small-angle scattering from Pickering emulsions.