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.

Characterizing Polymer-Grafted Nanoparticles: From Basic Defining Parameters to Behavior in Solvents and Self-Assembled Structures

“…Multicomponent polymer brush-grafted NPs have received considerable interest in recent years, owing to their abilities to undergo chain reorganization within the brush layer in response to environmental changes, exhibiting distinct nanostructures under different conditions, and to self-assemble into hierarchical structures such as vesicles and tubules. ,− ,− ,− Examples of such HNPs include binary mixed polymer- and diblock copolymer-grafted NPs as well as bicomponent brush Janus NPs. The most important analytical tool for characterizing the self-assembled structures of multicomponent brush NPs is electron microscopy, including scanning electron microscopy (SEM), TEM, and TEM electron tomography (ET) .…”

Phase Behavior of Mixed Polymer Brushes Grown from Ultrathin Coatings

“…To overcome this challenge, a “Y-shaped” initiator was developed to grow high grafting density mixed brushes with equal number of chains by atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP) methods. ,− Microphase separation of poly­( tert -butyl acrylate)/polystyrene (PtBA/PS) mixed brushes on nanoparticles into ripple phases after solvent annealing was reported using the Y-shaped initiator. Other approaches to synthesize mixed brushes such as sequential initiation from the same initiator, − orthogonal polymerization from mixed self-assembled monolayers (SAMs), − and “grafting to” have also been reported. , These methods often encounter similar issues of low grafting density, nonuniform grafting points, and unpredictable chain lengths. Although in response to solvent exposures ripple and cylinder morphologies were observed in these reports, there is no report of successful phase separation of mixed brushes using thermal annealing so far.…”

Nanostructuring Single-Molecule Polymeric Nanoparticles via Macromolecular Architecture

“…Very recently, solid-state NMR measurements were used to determine the patch fractions, the degree of phase separation, and the morphology of well-defined mixed brush-grafted nanoparticles consisting of ZrO 2 nanocrystals with polystyrene (PS) and poly(ethylene oxide) (PEO) ligands. 28 Macromolecular architecture may serve also as an important tool with which to obtain complex multicompartment nanostructured all-polymer particles such as Janus 29−31 and patchy 32,33 particles. Experimentally, the challenge is the synthesis of monodispersed multicompartment building blocks with programmable shape and spatial compositional anisotropy (e.g., patches).…”

“…Depending on the polymer chain lengths, the Flory interaction strength, the grafting density, and the size of the solid particle, the self-consistent mean field theory and the fluctuating dynamic mean field theory have predicted the creation of a variety of surface patterns from Janus to multi-patchy ones. − Such surface morphologies may be used as a direct way to empower specific interactions between nanoparticles and, consequently, direct their self-assembly behavior as well as the mesoscopic morphology of the resulting material. Very recently, solid-state NMR measurements were used to determine the patch fractions, the degree of phase separation, and the morphology of well-defined mixed brush-grafted nanoparticles consisting of ZrO 2 nanocrystals with polystyrene (PS) and poly­(ethylene oxide) (PEO) ligands …”