Dynamic coassembly of nanodiamonds and block copolymers in organic solvents

Abstract

Nonaqueous suspensions of nanoparticles have potential value in a wide range of applications, from coatings to environmental remediation. However, organic solvents often lead to nanoparticle agglomeration, and few methods developed to counteract this phenomenon. We present a novel approach to achieve stable nanoparticle suspensions in both nonpolar and polar aprotic solvents. Our method utilizes nanodiamonds (NDs) as a model system, due to the rich nature of their surface chemistry. This technique involves three key steps: (1) ultrasonication of air-oxidized NDs (OXNDs) in dimethylacetamide (DMAc), (2) the introduction of amphiphilic dispersants primarily composed of PVA-b-PMMA block copolymers (BCPs), and (3) exchanging DMAc with organic solvents to promote composite micelle formation, where BCP forms the micelle corona encapsulating OXND as the micelle core. Our method yields stable suspensions with particle sizes strongly influenced by thermokinetic interactions between OXND, BCP, and the solvent. Dynamic light scattering measurements of OXND–PVA-b-PMMA suspensions demonstrate that stabilization in toluene is governed by steric hindrance, while in THF or CHCl3 both electrostatic repulsion and steric hindrance contribute to particle stabilization. Characterization through electron microscopy and AFM reveals the efficacy of our coassembly method for producing uniformly-sized composite micelles. We identify a critical BCP:OXND weight ratio, analogous to the critical micelle concentration in surfactant-stabilized suspensions, crucial for effective particle stabilization. By analyzing the evolution of particle size distribution with varying solvent composition, we propose a dynamic coassembly mechanism between OXND and BCP, wherein OXND particles and encapsulating BCP chains dynamically rearrange to reach an equilibrium structure as the thermodynamic quantities of the surrounding medium change. Furthermore, BCP micellization is shown to play a vital role in preventing OXND aggregation during solvent exchange, resulting in uniformly sized particles. This enhanced understanding of the OXND–BCP coassembly mechanism in organic solvents holds promise for the development of novel functional materials. Our study provides insights into fabricating stable nanoparticle suspensions with diverse applications to coatings, lubricants, drug delivery, nanocomposite materials, biosensing, medical imaging, and environmental remediation.