Self-assembly of polymer-coated ferromagnetic nanoparticles into mesoscopic polymer chains

The self-assembly of dispersed polymer-coated ferromagnetic nanoparticles into micron-sized one-dimensional mesostructures at a liquid-liquid interface was reported. When polystyrene-coated Co nanoparticles (19 nm) are driven to an oil/water interface under zero-field conditions, long (≈ 5 μm) chain-like assemblies spontaneously form because of dipolar associations between the ferromagnetic nano-particles. Direct imaging of the magnetic assembly process was achieved using a recently developed platform consisting of a biphasic oil/water system in which the oil phase was flash-cured within 1 s upon ultraviolet light exposure. The nanoparticle assemblies embedded in the crosslinked phase were then imaged using atomic force microscopy. The effects of time, temperature, and colloid concentration on the self-assembly process of dipolar nanoparticles were then investigated. Variation of either assembly time t or temperature T was found to be an interchangeable effect in the 1D organization process. Because of the dependence of chain length on the assembly conditions, we observed striking similarities between 1D nanoparticle self-assembly and polymerization of small molecule monomers. This is the first in-depth study of the parameters affecting the self-assembly of dispersed, dipolar nanoparticles into extended mesostructures in the absence of a magnetic field.