Self-Assembly of Core-Shell Hybrid Nanoparticles by Directional Crystallization of Grafted Polymers

Nanoparticle self-assembly is an efficient bottom-up strategy for the creation of nanostructures. In the standard approach, ligands are grafted on the surfaces of nanoparticles to keep them separated and control interparticle interactions. Ligands then remain secondary and usually are not expected to order significantly during superstructure formation. Here, we investigate how ligands can play a more primary role in the formation of inorganic-organic hybrid materials. We graft poly(2-iso-propyl-2-oxazoline) (PiPrOx) as a crystallizable shell onto SiO$_2$ nanoparticles. By varying the PiPrOx grafting density, solution stability, and nanoparticle aggregation behavior can be controlled. Upon prolonged heating, anisotropic nanostructures form in conjunction with the crystallization of the ligands. Self-assembly of hybrid PiPrOx@SiO$_2$ (shell@core) nanoparticles proceeds in two steps: First, rapid formation of amorphous aggregates via gelation, mediated by the interaction between nanoparticles through grafted polymers; second, slow radial growth of fibers via directional crystallization, governed by the incorporation of crystalline ribbons formed from unbound polymers coupling to the grafted polymer shell. Our work reveals how crystallization-driven self-assembly of ligands can create intricate hybrid nanostructures.