Functionalized single-walled carbon nanotubes for composite applications and radiation studies
Pulikkathara, Merlyn Xavier
Wilson, Lon J.
Doctor of Philosophy thesis
This dissertation offers a hopeful advance towards our future in space. Materials for space applications need to be strong, lightweight, and be multifunctional. Single-walled carbon nanotubes (SWNTs) have various properties that fit these criteria as a novel material for space and biomedical applications. To utilize SWNTs for this purpose, new chemical functionalization schemes need to be developed and studied. Fluorinated single-walled carbon nanotubes (F-SWNTs) with fluorine atom substitutions on the sidewalls can serve as the building blocks for the new functionalization schemes. In this work, F-SWNTs have been derivatized with urea, guanidine, thiourea, and alkylated chains that have also been perfluorinated. In addition to these materials, F-SWNTs have been modified with amino acids and silanes of various lengths. Finally, the F-SWNTs, Urea-F-SWNTs, and alkylated and perfluorinated SWNTs have been further employed in new composite materials and in radiation damage studies of the types that might be encountered in space. The Urea-F-SWNTs have also been derivatized with nanodiamonds (NT-ND) and coated onto glass as a potential new coating for space applications. The polymers in the composite studies were medium density polyethylene (MDPE) and high density polyethylene (HDPE) and nylon fibers. The MDPE polymer nanocomposites with the perfluorinated nanotubes showed an increase of 51% in tensile strength. The nylon fiber study showed the effect of a reactive vs. non-reactive nano filer with SWNTs and F-SWNTs, on mechanical and thermal properties and alignment through Raman spectroscopy studies. Among these functionalized materials, F-SWNTs, Urea-F-SWNTs, alkylated (F-SWNT-C11H23), and perfluorinated (F-SWNT-C11 FxHy) nanotubes in both powder and as fillers in thermoplastic composites were subjected to radiation damage studies. The radiation damage studies involved various ionizing radiation particles and photons, such as protons of different energies and fluences, gamma rays, neutrons and other ions, and focused on the damage done to the functionalized materials. Protons with an energy of 30 MeV and fluences ranging from 1x109 to 5x1010 protons/cm2 had a de-functionalizing effect on F-SWNT as evidenced by the increased G:D ratio in the Raman spectra from 0.76 for an unradiated sample compared to 1.98 for an irradiated one. XPS studies also indicated a decrease of fluorine atom substitution from 40.3 to 36.5%fluorine. Sensor studies, however, showed a slight decrease in the G:D ratio which indicated that the fluorinated nanotubes were further disordered on the sidewall due to proton irradiation damage. F-SWNT-C 11H23 and F-SWNT-C11FxHy were also placed on a sensor chip, with both types of materials showing sensitivity in terms of increased resistance to 10 and 30 MeV protons with fluences of 1x1011 and lx107protons/cm2 respectively. Both had shown recovery to baseline resistance when the radiation was discontinued. Non-functionalized SWNTs had shown no significant change in resistance after irradiation. In general, this sensor work showed that functionalized SWNTs serve as better radiation detector platform then their pristine counterparts. The HDPE nanocomposite studies showed that various radiation environments increased or decreased the melting temperature compared to a control. The effects of protons on unfunctionalized SWNTs were also studied to gauge whether SWNTs could serve as a potential radiation shielding material against neutrons, and it was determined that composites with SWNTs performed as well as other material candidates such as carbon black and neat HDPE, and functional nanomaterials could be considered for further experiments. Overall, this work has produced a variety of new SWNT derivatives for biomedical, sensor and structural applications. The new alkylated and perfluorinated nanotube materials increased the tensile strength of polyethylene and could be used as a potential reinforcing structural material. Finally, the radiation studies of this work showed that various particles and photons have a measurable effect on the stability of SWNT covalently-bonded sidewall substituents and on the internal diameter of the SWNTs themselves. This pattern of radiation damage to the various SWNT materials suggest potential applications of these materials for advanced sensor and shielding devices for space exploration.
Organic chemistry; Physics