Synthesis of Polymer-Encapsulated Metal Oxide Nanoparticles via RAFT Polymerization in Dispersed Media for MRI and Thermal Interface Applications

Abstract

This work addresses the challenge of metallic oxide colloidal stability via encapsulation within polymer shells. RAFT emulsion/dispersion polymerization was employed to prepare polymer-encapsulated metallic oxide nanoparticles, focusing on iron oxide and aluminium oxide nanoparticles for MRI in aqueous media and thermal interface applications in oil-based systems, respectively.

In the first study, amphiphilic random copolymer macro-RAFT agents [p(AMPS-co-BA)] were used to synthesize polymer nanoparticles via RAFT emulsion polymerization. A systematic investigation of macro-RAFT agent compositions and charge density on the resulting latexes was carried out. It was found that increasing hydrophobicity produced smaller polymeric nanoparticles. This approach provides a simple and efficient strategy for producing well-defined polymer nanoparticles without the need for preformed seed latexes or complex block copolymers. Building on this, a two-step strategy was used to prepare macro-RAFT coated iron oxide nanoparticles, thereby eliminating pre-encapsulation aggregation. A variety of magnetic latexes with different sizes and degrees of clustering were synthesized depending on the stabilizer used and the system ph. The versatility of this encapsulation technique was demonstrated by the first successful encapsulation of iron oxide nanoparticles within a pH-responsive shell. In vitro cytotoxicity and relaxometric studies on the magnetic latexes revealed high biocompatibility and potential as MRI contrast agents.

Finally, the scope of this encapsulation strategy was extended beyond aqueous media by encapsulating aluminium oxide nanoparticles via RAFT dispersion polymerization in oil. Various oil-soluble macro-RAFT agents containing polar (methacrylic acid) and apolar (lauryl methacrylate) segments were synthesized and used as both coupling agents and stabilizers. The aluminium oxide nanoparticles were sterically stabilized by macro-RAFT agents and subsequently encapsulated. The effect of parameters such as macro-RAFT agent concentrations and the shell-forming monomer on nanoparticle encapsulation was explored. Under optimum conditions, the resulting hybrid latexes showed excellent dispersion stability and enhanced thermal conductivity as confirmed by thermal conductivity measurements.