abstract / Chemistry / Doctoral / Doctor of Philosophy

Carbon nanotubes (CNTs) have attracted significant attention as novel one-dimensional nanomaterials due to their unique structures and properties. Aggregate properties of CNTs such as high surface area, length, or chemical composition are further tailored to enhance their potential application in nanomedicine, through post synthesis chemical modification procedures. These modifications simultaneously alter their aggregate physiochemical properties and this has a direct impact on cytotoxicity of CNTs in cells. A lot of research has been done towards the toxicity of CNTs, however, there is need for results that are consistent and standardized if the application of CNTs in nanomedicine is to be a reality. Indeed the toxicology study of CNTs has been compromised by conflicting toxicity results due to lack of physiochemical characterization, regulation of the synthesis and standardized cytotoxicity assays. Herein, the effects of the physiochemical characteristics of riluzole loaded CNTs on their toxicity in neuronal cells is evaluated to elucidate a better understanding of CNTs toxicity. Furthermore the cellular uptake and overall efficacy of riluzole loaded CNTs is evaluated. As prepared multiwalled carbon nanotubes (MWCNTs) synthesized by the Catalytic Chemical Vapor Deposition (CCVD) method were initially acid oxidized using strong acids at different temperature and reaction time so as to remove impurities whilst introducing carboxylic groups on to the surface. The drug riluzole was then conjugated to the oxidized MWCNTs via carbodiimide activated amidation. The purification and functionalization led to the isolation of physicochemical properties as characterized by the Transmission Electron Microscopy (TEM), Raman spectroscopy, BET surface area analysis and Thermogravimetric Analysis (TGA). These physiochemical properties i.e. length, surface area, degree of fictionalization and amount of chemical impurities were key determinants of the drug loaded MWCNTs’ cytotoxicity. The data from this study supports the hypothesis that physiochemical modifications of MWCNTs that occur due to the functionalization of the drug to its surfaces alter their toxicity in neuronal systems. The riluzole loaded MWCNTs with <15% metallic residue, 500-2000nm length, and high surface area (30-76 m2/g) were found to cross the cell membrane without causing toxic effects as all the cells were viable compared to the untreated cells control. Covalently linking riluzole to MWCNTs and the consequent changes in the physiochemical properties did not lead to the generation of toxic effects in cells. Furthermore chemically binding riluzole to the MWCNTs did not deactivate the drug and reduce its ability to be antiglutamate. The identification of specific physiochemical properties governing CNTs toxicity presents the opportunity for carbon nanotube based drug delivery system designs or applications that reduce human and environmental impacts.

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 2009. / Thesis Advisor(s): Kwon, Young W. Second Reader: Pollak, Randal D. "December 2009." Description based on title screen as viewed on January 29, 2010. Author(s) subject terms: Carbon nanotubes, CNTs, MWNTs, nanocomposites, carbon fiber composite, adhesive strength, crack propagation, Mode II, functionalized carbon nanotubes, carboxyl. Includes bibliographical references (p. 38-40). Also available in print.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 91-92). Also available in electronic version. Access restricted to campus users.

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2002. / On t.p. "4̳" is subscript. Includes bibliographical references (leaves 84-86). Also available in electronic version. Access restricted to campus users.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2002. / Includes bibliographical references (leaves 55). Also available in electronic version. Access restricted to campus users.

The discovery of nanotubes with unique mechanical, electrical, and thermal properties has led to their use in the development of the next generation of composite materials. However, their poor solubility and dispersion properties in any organic and aqueous solvents limits their potential applications. In order to improve their solubility, single-walled carbon nanotubes (SWNTs) were functionalized along their sidewalls with phenol groups using a 1,3-di^polar cycloaddition reaction. These phenols could be further derivatized with 2-bromoisobutyryl bromide, resulting in the attachment of atom transfer radical polymerization initiators to the sidewalls of the nanotubes. These initiators were found to be active in the polymerization of methyl methacrylate and t-butyl acrylate from the surface of the nanotubes. However, the polymerizations were not controlled, leading to the production of high molecular weight polymeric grafts with relatively large polydispersities. The resulting polymer carrying nanotubes were analyzed by IR, Raman spectroscopy, solid-state NMR, DSC, TEM, and AFM. The nanotubes functionalized with poly(methyl methacrylate) were found to be insoluble in organic solvents, such as THF and CH2CI2. However, the dispersion property of SWNTs in the polymer matrix was improved dramatically. Another monomer t-butyl acrylate (t-BuA) was also polymerized using the same macroinitiators. It was found that the SWNTs functionalized with t-BuA iii were soluble in a variety of organic solvents. The t-butyl groups of these appended polymers could also be removed to produce nanotubes functionalized with poly (acrylic acid), resulting in nanocomposites that are soluble in aqueous solutions. In addition, polystyrene (PS) which was prepared by stable free radical polymerization, was used to functionalize SWNTs through the radical coupling reaction. IR, NMR, TEM, and AFM confirmed that this polystyrene was covalently bonded to the SWNTs. It was also found that the resulting PS-SWNTs composites were quite soluble in organic solvents, such as THF and CH2C12. / Thesis / Master of Science (MSc)

This study focuses on the processing, structure, and properties of gel spun polyacrylonitrile (PAN) and polyacrylonitrile/carbon nanotube (PAN/CNT) carbon fibers. Gel spun PAN based carbon fibers are manufactured beginning with a study of PAN dissolution in an organic solvent (dimethylformamide, DMF). Homogeneity of the PAN/DMF solution is determined through dynamic shear rheology, and the slope of the Han Plot (log G’ vs log G’’). Solutions were then extruded into gel spun fibers using a 100 filament fiber spinning apparatus in a class 1000 cleanroom. Fibers were then subjected to fiber drawing, stabilization, and carbonization, to convert the PAN precursor fiber into carbon fiber. Carbon fiber tensile strength was shown to scale with the homogeneity of the PAN/DMF solution, as determined by the slope of the log G’ vs log G’’ plot. After the development of the understanding between the homogeneity of the PAN/DMF solutions on the gel spun PAN based carbon fiber tensile properties, the effect of altering the fiber spinning processing conditions on the gel spun PAN based carbon fiber structure and properties is pursued. Cross-sectional shape of the gel spun PAN precursor fiber, characterized with a stereomicroscope, was found to become more circular in cross-section as the gelation bath temperature was increased, the amount of solvent in the gelation bath was increased, and when the solvent was switched from DMF to dimethylacetamide (DMAc). Gel spun fibers were then subjected to fiber drawing, stabilization, and carbonization to manufacture the carbon fiber. Carbon fibers were characterized to determine single filament tensile properties and fiber structure using wide-angle x-ray diffraction (WAXD) and high resolution transmission electron microscopy (HRTEM). It was found that the carbon fiber tensile properties decreased as the carbon fiber circularity increased, as a result of the differences in microstructure of the carbon fiber that result from differences in fiber spinning conditions. In the second half of this study, the addition of CNT into the PAN precursor and carbon fiber is investigated. CNT addition occurs during the solution processing phase, prior to gel spinning. As a first study, Raman spectroscopy is employed to investigate the bundling behavior of the CNT after gel spinning and drawing of the PAN/CNT fibers. By monitoring the peak intensity of the (12,1) chirality in the as-received CNT powder, and in differently processed PAN/CNT fibers, the quality of CNT dispersion can be quickly monitored. PAN/CNT fibers were then subject to single filament straining, with Raman spectra collected as a function of PAN/CNT filament strain. As a result of the PAN/CNT strain, stress induced G’ Raman band shifts were observed in the CNT, indicating successful stress transfer from the surrounding PAN matrix to the dispersed CNT. Utilization of the shear lag theory allows for the calculation of the interfacial shear strength between the PAN and incorporated CNT, which is found to increase as the quality of CNT (higher aspect ratio, increased graphitic perfection, and reduced impurity content), quality of CNT dispersion, and fiber drawing increase. PAN/CNT fibers were then subjected to stabilization and carbonization for the manufacture of gel spun PAN/CNT based carbon fibers. These fibers were then characterized to investigate the effect of CNT incorporation on the structure and properties of the carbonized fibers. The gel spun PAN/CNT based carbon fibers were compared to commercially produced T300 (Toray) and IM7 (Hexcel) carbon fibers, and gel spun PAN based carbon fiber. Fiber structure was determined from WAXD and HRTEM. Carbon fibers properties investigated include tensile properties, and electrical and thermal conductivity. PAN/CNT based carbon fibers exhibited a 103% increase in room temperature thermal conductivity as compared to commercially available IM7, and a 24% increase in electrical conductivity as compared to IM7. These studies provide a further understanding of the processing, structure, property relationships in PAN and PAN/CNT based carbon fibers, beginning at the solution processing phase. Through the manufacture of more homogeneous PAN/DMF solutions and investigations of the fiber spinning process, gel spun PAN based carbon fibers with a tensile strength and modulus of 5.8 GPa and 375 GPa, respectively, were successfully manufactured in a continuous carbonization facility. Gel spun PAN/CNT based carbon fibers exhibit room temperature electrical and thermal conductivities as high as 74.2 kS/m and 33.5 W/m-K.

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