Polymer-Grafted Nanoparticles

Polymer nanocomposites are an important class of materials.^1–5 They combine the many desirable physical properties of polymers, such as their mechanical properties and ease of processing, with functionalities provided by a nanoparticle component. The nanoparticles can provide mechanical reinforcement, as well as impart unique optical responses, enhanced electrical conductivities, decreased gas permeability, and so on. To obtain these enhanced physical properties from the nanoparticle component, the spatial dispersion of the nanoparticles (NPs) must be controlled—this fact has now been well-appreciated and the focus of community work in the last few decades.^1–5 The limiting step in this context is that inorganic NPs (typically hydrophilic) are immiscible with non-polar, hydrocarbon-based polymers. A common approach to resolve this difficulty is to attach polymer chains from the nanoparticle surface—leading to “polymer-grafted” nanoparticles (PGNPs)—which have surfactant-like behavior because hydrophilic and hydrophobic components are chemically tethered to each other.⁶ However, PGNPs have uses that go beyond traditional polymer nanocomposites, and this class of nanoparticles can be broadly defined to include micellar particles, single chain nanoparticles, and many others that are covered in this Polymer-Grafted Nanoparticles Special Topic.^2,5 In this Guest Editorial, we will briefly review key fundamental concepts related to PGNP properties and introduce the articles that comprise the Special Topic. These articles cover (i) grafting strategies to enhance dispersion, (ii) environmental tuning of PGNPs, (iii) advances in PGNP characterization, and (iv) modeling efforts.

There is a wealth of information regarding PGNP properties in the literature, including many reviews and perspectives. Here, we briefly review some of the fundamental concepts common to different types of PGNPs.

PGNPs include those with an inorganic core from which polymer chains can be grown or attached. The inorganic core may be SiO₂, Ag, Au, CdSe, Fe₃O₄, graphene, clay, or many other materials. Using polymerization methods such as surface-initiated atom transfer radical polymerization (SI-ATRP) or surface-initiated reversible addition-fragmentation chain transfer polymerization (SI-RAFT),^7,8 these “grafting-from” techniques have the ability to produce dense polymer brushes on the surface and can generate complex brush configurations, such as ones containing a bimodal distribution of molecular weights.⁹ An alternative approach to “grafting-from” is to graft polymers to the surface of a nanoparticle by taking advantage of polymer chains with terminal functional groups. A common example is using thiol-terminated polymers to graft to the surface of Au nanoparticles.^10,11 The “grafting-to” approach may be easier to accomplish experimentally but typically results in lower grafting densities, especially for high molecular weight polymers. Other types of PGNPs include those synthesized via polymerization-induced self-assembly (PISA) of block copolymers.¹² In this process, polymers self-assemble in solution as polymerization proceeds, leading to an array of possible structures, including spheres, vesicles, and worm-like particles. The particles are not unlike those formed by surfactants in solution; in fact, polymer chains can self-assemble in solution to form micellar structures that behave like PGNPs in many ways.¹³ Recently, virus-like particles have been synthesized and functionalized with a polymer brush.¹⁴ Finally, nanoparticles can be synthesized by crosslinking single chains into small, dense nanoparticles. Those particles may contain a corona on the exterior—resembling a PGNP—or may appear more solid-like, depending on the synthesis conditions.¹⁵ 

The structures of polymer brushes that result from grafting chains to a surface depend on a range of factors, including grafting density, polymer molecular weight, and interactions between the chains and their environments.¹⁶ At low grafting densities, a brush may not form and the conformation of the polymer chains is similar to that of free chains. As grafting density increases, excluded volume effects lead to stretched conformations.⁴ Polymer micelles exhibit similar conformations in their coronas. Stretched chains may exhibit slower relaxation dynamics, which can impact the mechanical properties of nanocomposite materials.^17–20 The brush also influences nanoparticle organization within a material. Autophobic dewetting is an entropic effect observed in systems in which the chemical composition of the grafted chains is the same as free, matrix chains. It affects nanoparticle dispersion, polymer diffusion, and for plasmonic nanoparticles, optical properties of nanocomposite materials.^2,3,21 As demonstrated in the Polymer-Grafted Nanoparticles Special Topic, as well as several studies in the literature, if there are favorable interactions between a brush and its environment, it becomes possible to significantly enhance dispersion within a material or to spatially confine particles, such as in one domain of an ordered block copolymer.²² 

Finally, numerous techniques exist for studying PGNPs on both the experimental and theoretical ends of the spectrum. These techniques include small-angle scattering for determining brush structure,^13,20,23,24 quasi-elastic scattering for following brush and particle dynamics,^19,20,25,26 and electron tomography for real space imaging of grafting polymers when there is sufficient electron contrast.^14,27 On the theoretical side, Monte Carlo simulations, polymer field theories, and molecular dynamics techniques have proven to be very successful, although many open challenges remain.²⁸ The Polymer-Grafted Nanoparticles Special Topic contains several tutorials and research articles that expand on these methods.

Utilizing SI-ATRP as platform, Zhang et al.²⁹ design poly(methyl methacrylate) grafted-silicon nanoparticles and probe the impact of reaction conditions on grafting architecture. Penaloza and Seery describe a surface-initiated approach to the design of polymer-tethered clay nanocomposites.³⁰ By controlling and tuning the architecture of the tethered polymer brushes on the clay surface, the authors highlight a robust strategy to enhance dispersion in polymer nanocomposites. Extending grafting approaches to understudied immiscible systems, Hickey et al.³¹ detail an innovative in situ technique that balances enthalpic and entropic components in the quest for well-dispersed polymer nanocomposites. To apply these concepts to biomedicine, Ray and colleagues devise a reverse micelle process to fabricate poly(acrylate) tethered-reduced graphene oxide nanocomposites.³² This approach enables water solubility and enhances paramagnetic response. Aggregation of matrix-free polymer grafted gold nanoparticles is examined as a function of nanoparticle sphericity controlled via grafting density and polymer chain molecular weight, and connections are drawn to its influence on plasmonic behavior.³³ Gogoi and Chowdhury utilize nanocomposite technology to combine polymer functionalization with layered nanohybrid materials to manufacture devices with enhanced resistive memory performance due to synergistic behavior.³⁴ Utilizing functionalized silica nanoparticles, Rishi et al.³⁵ investigate the dispersion and specific interactions of elastomeric nanoparticles via spectroscopic, scattering, and modeling studies.

Block copolymer structures serve as scaffolds to tune the assembled architecture of confined, polymer-grafted nanoparticles in a self-consistent field theory investigation, revealing a rich variety of PGNP morphologies.³⁶ Solvent quality is also a handle to control nanostructure in functionalized nanoparticles, leading to a facile process to tailor morphology derived from a single starting material.³⁷ Perilla and co-workers³⁸ describe zwitterionic grafted silica nanoparticles with changes in hydrodynamic radius dictated by pH, temperature, and ionic concentration, showcasing their potential as therapeutic delivery vehicles. Domhoff and Davis explore the interplay of substrate interactions on morphology and ion transport in solvent-cast, silica/ionomer nanocomposite membranes as a potential handle for flow battery design.³⁹ An ice-templating approach is used to fabricate microporous nanocomposite materials with mechanical response tuned by particle shape and pH-responsive particle-matrix interactions.⁴⁰ 

An in-depth tutorial of the use of quasi-elastic neutron scattering spectroscopy to probe local and macroscopic dynamics in polymer nanocomposites with a focus on spatial and temporal considerations is included.⁴¹ Rose et al.⁴² present a tutorial on the fundamentals of high resolution optical microscopy applied to particle tracking to correlate nanoscale dynamics in soft materials. Epoxy curing provides a model system to showcase x-ray photon correlation spectroscopy as a technique to explore curing kinetics via nanoscale dynamics of filler particles.⁴³ Colmenero et al.⁴⁴ highlight neutron scattering techniques as a viable strategy to investigate structural heterogeneities in melts of intramolecularly crosslinked, single chain nanoparticles, focusing on relaxation events at various length scales. Rostom and Dadmum introduce a unique strategy to measure nanoparticle diffusion in polymer nanocomposites, utilizing all-particle nanocomposites with tunability in nanoparticle softness to probe motion within the matrix.⁴⁵ Geethu and colleagues track percolation in and stability of microemulsions as a function of hydrophobic chain length via dielectric spectroscopy and small-angle neutron scattering.⁴⁶ 

Molecular dynamics simulation enables the examination of different regimes of surface switching of mixed polyelectrolyte brushes as a function of length and charge fraction of the arms, strength of electrostatic interactions, and environmental conditions.⁴⁷ Langevin dynamics simulations are also employed to probe polymer-grafted nanoparticles with constrained or unconstrained soft polymer shells.⁴⁸ The impact of polymer-grafted nanoparticles with asymmetry in glass transition temperatures is examined using molecular simulations to understand the reinforcing effect in nanocomposites as it relates to dynamics.⁴⁹ 

The guest editors thank all of the authors who contributed articles to this Special Topic of the Journal of Applied Physics.