Hydrophilization of magnetic nanoparticles with an amphiphilic polymer revisited: Roles of nanoparticle capping density and polymer structure

Abstract

Amphiphilic comb polymers, like poly(maleic anhydride-alt-1-octadecene) (PMAO), are frequently used to transfer hydrophobic magnetic nanoparticles (NPs) into aqueous media. While it is accepted that the hydrophilization is based on the interdigitation of the hydrocarbon chains from the polymer and the NP capping, the factors that determine the success of such process are still not fully understood. In this paper, some key aspects of the formation of water-soluble NP-PMAO assemblies from oleate-capped iron oxide NPs, not considered in previous works, are discussed thoroughly. We have found that the process is notably hindered when the chemisorbed oleates coverage decreases from 2.9 to 1.5 molecules/nm2, revealing the importance of the capping packing density in the assembly mechanism. Combined results from small-angle x-ray scattering and electron microscopy show that, for polymer concentrations ranging from 10 to 200 monomer units/nm2, the NPs are individually covered by polymer chains, although forming rather planar NP-PMAO assemblies which contain several NPs. Both the minimum polymer concentration needed for a successful hydrophilization (0.7 gPMAO/L) and the geometry of the assemblies suggest that intermolecular structures typical of comb polymers act as templates in the assembly process. Finally, we show that the magnetic properties are modulated by the polymer loading.