Coexistence and Phase Behavior of Solvent-Polystyrene-Grafted Gold Nanoparticle Systems

Polymer-grafted nanoparticles (PGNs) are widely used as additives or as single-component assemblies in numerous technologies, spanning from composites and coatings to membranes, optical elements, and printable electronics. How the design modularity of PGNs relates to their dispersibility and morphology in thermal or poor solvents, however, is not well established. Herein, we provide experimental volume fraction (phi)-temperature (T) coexistence curves (phi CE and TCE) for polystyrene-grafted gold nanoparticles (PS-AuNPs) in solvents of various qualities (cyclohexane, cyclopentane, and ethyl acetate) using a combination of UV-vis spectroscopy and small-angle X-ray scattering. These systems exhibit upper critical solution temperatures, with coexistence curves that are broader and shifted to lower temperatures relative to their polymer analogs, consistent with prior reports of the impact of macromolecular branching. The coexistence curves span over 6 orders of magnitude in concentration, exhibit solvent-rich and solvent-poor arms that are phi PGN-dependent, display a reduction in critical temperature (TC*) as graft molecular weight decreases, exhibit an overall narrowing of the solvent-poor arm at higher graft molecular weights, and reveal a minimal influence of core nanoparticle volume (900-4200 nm3). Additionally, the structure within the solvent swollen PS-AuNPs aggregates is disordered or face-centered cubic and depends on the aggregation temperature relative to TC* than on solvent characteristics. Such quantification of PGN phase behavior in thermal or poor solvents is used to demonstrate thermal fraction for purification and will inform future synthesis methods, self and directed assembly techniques, film and fiber processing, and formulation of inks, adhesives, thermosets, and coatings.