Controlled synthesis and characterization of templated, magneto-responsive nanoparticle structures

Abstract

Magnetic fluids are colloidal dispersions of magnetic nanoparticles that are stable with respect to gravitational and moderate magnetic fields because of their small particle size, and to unbounded aggregation due to their surface coatings. The interaction between individual magnetic nanoparticles in a suspension is negligible even under applied magnetic fields. However, when they are incorporated into composite structures they act in concert to provide the desired magnetic response. The dynamic response of such composite structures can be exploited in a wide range of applications including high energy absorption scenarios. The goal of this thesis was to use magnetic nanoparticles as building blocks to create 3D magneto-responsive nanostructures and manipulate their behavior in the presence of an external magnetic field for various applications. Two approaches were followed to create composite structures. In the first approach, rigid magnetic chains composed of magnetic nanoparticles were synthesized. The layer-by-layer technique was used to coat polystyrene beads with magnetic nanoparticles to create novel core-shell structures. The behavior of these structures under an applied magnetic field was modeled and the results were verified experimentally.

(cont.) These magnetic polystyrene beads were then aligned within a microchannel by an external magnetic field and linked together using sol gel chemistry to yield rigid superparamagnetic chains. Linking the magnetically aligned beads with a flexible linker yielded flexible superparamagnetic chains. These permanently-linked magnetic chains can be used as micro-mixers in a microfluidic channel under a rotating magnetic field. The reorientation dynamics of these chains under an external magnetic field was modeled. Microcontact printing was employed to tether the flexible chains in a desired pattern on a glass surface. Tethered flexible magnetic chains have potential applications in microfluidics and separations. Rings and icosahedra shaped electrostatically charged templates were generated from the self-assembly of mixtures of surfactants in an aqueous solution and were investigated for their application in the synthesis of non-spherical magnetic structures. The magnetic response of the magnetic rings was modeled and the results were verified experimentally. "Templateless" aggregation of magnetic nanoparticles using radiation crosslinking was also investigated.

(cont.) Aqueous magnetic nanoparticles stabilized with a radiation crosslinkable polymer resulted in magnetic gels at high dosage amount of the ionizing radiation. Magnetic gels can have potential applications in biological areas. Different size monodisperse magnetic nanoparticles were synthesized via an organic synthesis route, and the effect of size on the Nel relaxation behavior of the fixed magnetic nanoparticles was investigated. Theoretical analysis suggested that incorporation of magnetic nanoparticles with high relaxation times in a matrix can be used to absorb energy. The energy penalty associated with the deflection of the magnetic dipole against the field should result in the stiffening of the matrix. This was demonstrated both experimentally and theoretically. Drop ball impact test was performed on foam embedded with infinite Nel relaxation nanoparticles and the deflection profile of the foam was monitored both in the presence and in the absence of a magnetic field. The deflection of the foam by the ball was modeled to calculate the strain profile developed by the foam, which was then converted into the equivalent amount of energy absorbed by the foam and the magnetic nanoparticles.

(cont.) A method of electrospinning was used to encapsulate magnetic nanoparticles in a polymeric matrix to create field responsive nanofibers for various applications. The magnetization properties of the nanofibers were also characterized and their behavior under an applied magnetic field was modeled.