Controlled grafted brushes of polystyrene on magnetic γ-Fe2O3 nanoparticlesvianitroxide-mediated polymerization

Abstract

We present a new multi-step efficient “grafting from” method to obtain well defined magnetic γ-Fe₂O₃ nanoparticles grafted by polystyrene (PS) chains, from the synthesis of the native nanoparticles up to the refined characterization of the final grafted objects in different organic solvents, using a combination of small angle scattering (SAXS and SANS) and deuteration methods. The polymerization technique, based on a two-step Nitroxide-Mediated Polymerization (NMP), has been adapted for maghemite particles from a protocol previously established for negatively charged silica nanoparticles in dimethylacetamide (DMAc), a polar organic solvent. Such a method requires an initial inversion of the charge surface of the maghemite particles after their synthesis in an aqueous solvent. The colloidal stability of the system has been kept and controlled at every stage of the process to avoid aggregation of the particles to optimize the synthesis parameters in terms of grafting density, conversion rates, and reproducibility. The grafted objects, redispersed in different solvents, have been studied by SAXS (to characterize their γ-Fe₂O₃ core) and SANS (to characterize their grafted PS corona, either hydrogenated or deuterated) at different concentrations to extract both the form factor of the object and the structure factor of the suspension. The experimental scattered curves were modelled by a Pedersen model. The parameters extracted from the model can be directly compared with the masses of the degrafted chains. The grafted objects, better dispersed in DMAc than in toluene, are organized as linear fractal aggregates of 3–4 native particles grafted with PS chains whose behavior is in agreement with scaling laws derived for brushes in a theta solvent.