Nanoparticle Brushes: Macromolecular Ligands for Materials Synthesis

Abstract

Conspectus Colloidal nanoparticles have unique attributes that can be used to synthesize materials with exotic properties, but leveraging these properties requires fine control over the particles’ interactions with one another and their surrounding environment. Small molecules adsorbed on a nanoparticle’s surface have traditionally served as ligands to govern these interactions, providing a means of ensuring colloidal stability and dictating the particles’ assembly behavior. Alternatively, nanoscience is increasingly interested in instead using macromolecular ligands that form well-defined polymer brushes, as these brushes provide a much more tailorable surface ligand with significantly greater versatility in both composition and ligand size. While initial research in this area is promising, synthesizing macromolecules that can appropriately form brush architectures remains a barrier to their more widespread use and limits understanding of the fundamental chemical and physical principles that influence brush-grafted particles’ ability to form functional materials. Therefore, enhancing the capabilities of polymer-grafted nanoparticles as tools for materials synthesis requires a multidisciplinary effort, with specific focus on both developing new synthetic routes to polymer-brush-coated nanoparticles and investigating the structure–property relationships the brush enables. In this Account, we describe our recent work in developing polymer brush coatings for nanoparticles, which we use to modulate particle behavior on demand, select specific nanoscopic architectures to form, and bolster traditional bulk polymers to form stronger materials by design. Distinguished by the polymer type and capabilities, three classes of nanoparticles are discussed here: nanocomposite tectons (NCTs), which use synthetic polymers end-functionalized with supramolecular recognition groups capable of directing their assembly; programmable atom equivalents (PAEs) containing brushes of synthetic DNA that employ Watson–Crick base pairing to encode particle binding interactions; and cross-linkable nanoparticles (XNPs) that can both stabilize nanoparticles in solution and polymer matrices and subsequently form multivalent cross-links to strengthen polymer composites. We describe the formation of these brushes through “grafting-from” and “grafting-to” strategies and illustrate aspects that are important for future advancement. We also examine the new capabilities brushes provide, looking closely at dynamic polymer processes that provide control over the assembly state of particles. Finally, we provide a brief overview of the technological applications of nanoparticles with polymer brushes, focusing on the integration of nanoparticles into traditional materials and the processing of nanoparticles into bulk solids.

Polymer Brushes and Surface Nanostructures: Molecular Design, Precise Synthesis, and Self-Assembly

“…As shown in Figure , two methods, “grafting to” and “grafting from”, are employed in the synthesis of polymer brushes. − In the “grafting to” method, preprepared polymers are chemically or physically anchored to a substrate at chain ends. Reactive groups that are capable of making reactions with the substrates are presented at the chain ends. − However, this method makes it difficult to prepare polymer brushes at high grafting densities, due to the steric hindrance .…”

Synthesis of Raspberry-like Nanoparticles via Surface Grafting of Positively Charged Polyelectrolyte Brushes: Colloidal Stability and Surface Properties

“…Stability of colloidal dispersions has been researched widely for a multitude of applications, including drug therapy, bioimaging, oil recovery, and geothermal energy production . A common strategy to counterbalance the van der Waals attraction leading to particle aggregation is to introduce electrostatic or steric motifs that shield and, thus, prevent the particles from approaching close together. , Electrostatic stabilization results because of an electrical double layer comprising an immobile Stern plane (counterions with a charge opposite to the particle surface charge) and a diffuse layer (a layer of mobile ions), which can be easily disrupted in various electrolytes (i.e., in the presence of charge-screening ions) . Hence, electrostatic stabilization strategies that work well in salt-free environments become less effective in the presence of even low ionic strength (IS) electrolytes.…”

Surface-Initiated Zerovalent Metal-Mediated Controlled Radical Polymerization (SI-Mt0CRP) for Brush Engineering

“…Surface-tethered polymer brushes have been a versatile approach to tailoring the chemical and physical properties of surfaces and interfaces. − Growing demands for generating diverse functional polymer brushes advance the development of innovative and effective polymerization methods. − One of the most prevailing strategies is surface-initiated controlled radical polymerization (SI-CRP), also denoted as surface-initiated reversible-deactivation radical polymerization (SI-RDRP), − which is generally performed on substrates modified with a self-assembled monolayer (SAM) of initiators, tethering polymer chains to the substrate by one end , and thus endowing the arbitrary surfaces with the desired placement of groups and targeted functions. , …”