Fabrication of Amphiphilic Nanoparticles via Mixed Homopolymer Brushes and NMR Characterization of Surface Phase Separation

Abstract

“Patchy particles”, where the surface is anisotropically patterned through variation in the surface composition, can assemble into different colloidal crystal structures as well as act as interface stabilizers, heterogeneous reaction catalysts, and targeted drug delivery agents. Patchy nanoparticles (NPs) can be formed by adsorbing two chemically different polymer chains that will spontaneously phase separate. Although there is growing interest in polymer-based patchy nanoparticles, the majority of the studies have been theoretical rather than experimental due to difficulties in preparing significant quantities of nanoparticles with controlled polymer ratios. Likewise, characterization of the phase separation on the nanoparticle surface is challenging. Here we simultaneously overcome the synthesis and characterization hurdles by developing a facile, versatile protocol to produce sufficient quantities of patchy NPs for quantitative solid-state NMR measurements of the patch fractions, degree of phase separation, and morphology. Monodisperse 3.5 nm ZrO2 nanocrystals with polystyrene (PS) and poly(ethylene oxide) (PEO) ligands, covering the entire possible composition range, were reproducibly prepared through a simple exchange process. This approach has the advantage of well-defined polymer molecular weights and NP sizes, allowing experimental validation of theoretical predictions for nanophase separation in NPs with mixed homopolymer brushes. Upon exposure to a nonselective solvent, the nanoparticles assemble into different morphologies, namely micelles and vesicles, as a function of the PEO:PS ratios.