Synthesis of a thermoresponsive crosslinked MEO₂MA polymer coating on microclusters of iron oxide nanoparticles

Encapsulation of magnetic nanoparticles (MNPs) of iron (II, III) oxide (Fe₃O₄) with a thermopolymeric shell of a crosslinked poly(2-(2-methoxyethoxy)ethyl methacrylate) P(MEO₂MA) is successfully developed. Magnetic aggregates of large size, around 150-200 nm are obtained during the functionalization of the iron oxide NPs with vinyl groups by using 3-butenoic acid in the presence of a water soluble azo-initiator and a surfactant, at 70 °C. These polymerizable groups provide a covalent attachment of the P(MEO₂MA) shell on the surface of the MNPs while a crosslinked network is achieved by including tetraethylene glycol dimethacrylate in the precipitation polymerization synthesis. Temperature control is used to modulate the swelling-to-collapse transition volume until a maximum of around 21:1 ratio between the expanded: shrunk states (from 364 to 144 nm in diameter) between 9 and 49 °C. The hybrid Fe₃O₄@P(MEO₂MA) microgel exhibits a lower critical solution temperature of 21.9 °C below the corresponding value for P(MEO₂MA) (bulk, 26 °C). The MEO₂MA coating performance in the hybrid microgel is characterized by dynamic light scattering and transmission electron microscopy. The content of preformed MNPs [up to 30.2 (wt%) vs. microgel] was established by thermogravimetric analysis while magnetic properties by vibrating sample magnetometry.