Browsing by Author "Vadala, Michael Lawrence"

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    Complexation of Block Copolysiloxanes with Cobalt Nanoparticles
    Vadala, Michael Lawrence (Virginia Tech, 2003-04-12)
    Poly(dimethylsiloxane-b-methylvinylsiloxane) (PDMS-b-PMVS) diblock copolymers were synthesized via anionic living polymerization with controlled molecular weights and narrow molecular weight distributions. Targeted molecular weights agreed well with experimental values determined by 1H NMR, 29Si NMR, and GPC. Morphologies were investigated by DSC to analyze glass transition temperatures. Only one Tg was observed for each PDMS-b-PMVS block copolymer suggesting that the blocks were miscible in bulk. Tg's ranged from approximately -126 to -128 °C and were between the Tg's of the PDMS (-123 °C) and PMVS (-137 °C) homopolymers. The PMVS blocks were functionalized with trimethoxysilethyl or triethoxysilethyl pendent groups via hydrosilations to yield poly(dimethylsiloxane-b-[poly(methylvinyl)-co-(methyl-(2-trimethoxysilethyl)siloxane)] (PDMS-b-[PMVS-co-PMTMS]) or poly(dimethylsiloxane-b-[poly(methylvinyl)-co-(methyl-(2-triethoxysilethyl)siloxane)] (PDMS-b-[PMVS-co-PMTES]) copolymers, respectively. The PMVS blocks were either derivatized with the functional groups or half of the repeat units were functionalized. The fully hydrosilated materials were diblock copolymers, and the materials that were 50% hydrosilated had a random sequence of methylvinylsiloxy units and methyl-(trialkoxysilethyl)siloxy units. The PDMS-b-[PMVS-co-PMTES] block copolymers had Tg's ranging from -124 to -126 °C and only one Tg was observed. Surface tension measurements suggested that PDMS-b-[PMVS-co-PMTES] copolymers formed aggregates in toluene. Stable suspensions of superparamagnetic cobalt nanoparticles were prepared in toluene in the presence of PDMS-b-[PMVS-co-PMTMS] or PDMS-b-[PMVS-co-PMTES] copolymers via thermolysis of Co2(CO)8. It is hypothesized that the block copolymers functioned as micellar templates for the cobalt nanoparticles. TEM micrographs showed non-aggregated cobalt nanoparticles coated with copolymers that had mean particle diameters ranging from ≥10-15 nm. Specific saturation magnetizations of these cobalt-copolymer complexes ranged from 90-110 emu g-1 Co, comparable to literature values for this particle size.
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    Preparation and Functionalization of Macromolecule-Metal and Metal Oxide Nanocomplexes for Biomedical Applications
    Vadala, Michael Lawrence (Virginia Tech, 2006-04-18)
    Copolymer-cobalt complexes have been formed by thermolysis of dicobalt octacarbonyl in solutions of copolysiloxanes. The copolysiloxane-cobalt complexes formed from toluene solutions of PDMS-b-[PMVS-co-PMTMS] block copolymers were annealed at 600-700 °C under nitrogen to form protective siliceous shells around the nanoparticles. Magnetic measurements after aging for several months in both air and in water suggest that the ceramic coatings do protect the cobalt against oxidation. However, after mechanical grinding, oxidation occurs. The specific saturation magnetization of the siliceous-cobalt nanoparticles increased substantially as a function of annealing temperature, and they have high magnetic moments for particles of this size of 60 emu g⁻¹ Co after heat-treatment at temperatures above 600 °C. The siliceous-cobalt nanoparticles can be re-functionalized with aminopropyltrimethoxysilane by condensing the coupling agent onto the nanoparticle surfaces in anhydrous, refluxing toluene. The concentration of primary amine obtained on the surfaces is in reasonable agreement with the charged concentrations. The surface amine groups can initiate L-lactide and the biodegradable polymer, poly(L-lactide), can be polymerized directly from the surface. The protected cobalt surface can also be re-functionalized with poly(dimethylsiloxane) and poly(ethylene oxide-co-propylene oxide) providing increased versatility for reacting polymers and functional groups onto the siliceous-cobalt nanoparticles.Phthalonitrile containing graft copolysiloxanes were synthesized and investigated as enhanced oxygen impermeable shell precursors for cobalt nanoparticles. The siloxane provided a silica precursor whereas the phthalonitrile provided a graphitic precursor. After pyrolysis, the surfaces were silicon rich and the complexes exhibited a substantial increase in Ms. Early aging data suggests that these complexes are oxidatively stable in air after mechanical grinding. Aqueous dispersions of macromolecule-magnetite complexes are desirable for biomedical applications. A series of vinylsilylpropanol initiators, where the vinyl groups vary from one to three, were prepared and utilized for the synthesis of heterobifunctional poly(ethylene oxide) oligomers with a free hydroxy group on one end and one to three vinylsilyl groups on the other end. The oligomers were further modified with carboxylic acids via ene-thiol addition reactions while preserving the hydroxyl functionality at the opposite terminus. The resulting carboxylic acid heterobifunctional PEO are currently being investigated as possible dispersion stabilizers for magnetite in aqueous media.