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fbtwitterlinkedinvimeoflicker grey 14rssslideshare1
Zhang, Qian (2007)
Publisher: Virginia Tech
Types: Doctoral thesis
Subjects: nanoparticle, DLVO, magnetite, stabilization, poly(ethylene oxide), stability
Superparamagnetic Magnetite (Fe3O4) nanoparticles were synthesized and complexed with carboxylate-functionalized block copolymers, and aqueous dispersions of the complexes were investigated as functions of their chemical and morphological structures. The block copolymer dispersants possessed either poly(ethylene oxide), poly(ethylene oxide-co-propylene oxide), or poly(ethylene oxide-b-propylene oxide) outer blocks, and all contained a polyurethane center block with pendant carboxylate functional groups. The complexes were formed through interactions of the carboxylates with the surfaces of the magnetite nanoparticles. Initial efforts utilized an aqueous coprecipitation method for the synthesis of magnetite nanoparticles, which yielded polydisperse magnetite nanoparticles. The nanoparticle complexes were characterized with a range of solution- and solid-state techniques including TGA, XPS, TEM, VSM, DLS and zeta potential measurements. DLVO calculation methods, which sum the contributions from van der Waals, steric, electrostatic and magnetic forces were utilized to examine the interparticle potentials in the presence and absence of external magnetic fields. Compositions were identified wherein a shallow, attractive interparticle potential minimum appears once the magnetic term is applied. This suggested the possibility of tuning the structures of superparamagnetic nanoparticle shells to allow discrete dispersions without a field, yet permit weak flocculation upon exposure to a field. This property has important implications for biomedical applications where movement of particles with an external magnetic field is desirable. In a second study, well-defined, narrow size dispersity magnetite nanoparticles were synthesized via the thermolysis of an iron (III) acetylacetonate (Fe(acac)3) precursor in the presence of benzyl alcohol. The magnetite nanoparticles were coated with triblock and pentablock copolymers possessing poly(ethylene oxide) and poly(propylene oxide-b-ethylene oxide) tailblocks and the carboxylate-functional anchor block. DLVO calculations were applied to the new magnetite particles and diagrams of potential energy versus interparticle distance indicated the predominant effect of steric and magnetic interactions on the particle stability. Exposure of the pentablock copolymer-magnetite complexes in phosphate buffered saline to a 1500 Oe magnetic field with concomitant DLS measurements indicated flocculation of the magnetic nanoparticles. DLS measurements showed increased hydrodynamic radii and scattering intensities with time.
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