The Role of Specific Interactions on the Dispersion and Properties of Carbon Nanotube-Polymer Nanocomposites

This dissertation presents work that exammes the role of specific chemical interactions in enhancing the dispersion of carbon nanotubes in a polymer matrix. Carbon nanotubes (CNT) possess unique mechanical, electrical and thermal properties hence can have a number of potential applications including high strength, light weight composites. Due to strong interaction among nanotubes, they stay as aggregates and hence their full potential for application is severely limited. Utilization of specific chemical interactions to induce/improve miscibility in polymer blends is well known. In this thesis, this approach is applied to carbon nanotubes polymer composites to enhance the dispersion of nanotubes in a copolymer of styrene and vinyl phenol with the idea that oxygenated functional groups on the nanotubes surface may potentially interact with the vinyl phenol groups on the polymer chain via hydrogen bonding. The first part of this thesis compares methods to oxidize carbon nanofibers by using various oxidizing agents, such as 6M HNO3, KMnO4, RuO4, and a mixture of concentrated H2SO4/HNO3. The efficacy of an oxidizing agent is discussed in terms of the yield of oxidized nanofibers and the amount of oxygenated functional groups generated on the nanofiber surface. Next, the dispersion of single walled carbon nanotubes (SWNT) in the matrix of a copolymer of styrene and vinyl phenol containing 0, 10, 20, 40 and 100% vinyl phenol, is examined. This study provides a method to control the dispersion of nanotubes in the polymer matrix by utilizing specific chemical interactions. The dispersion of the nanotubes was observed by optical microscopy. The dispersion of the SWNT in the polymer matrix is also quantified by optical microscopy and Raman spectroscopy. Raman spectroscopy is also used to investigate preferred interactions between the SWNTs and the copolymers via the shift in the characteristic Raman peak of the SWNTs in the composites. All composites show regions of SWNT aggregates, however the aggregate size varies with composition of the PSVPh copolymer and the amount of SWNT oxidation. Optimal dispersion of the SWNT is observed in PSVPh with 20% vinyl phenol and oxidized nanotubes, which correlates with spectroscopic evidence that indicates that this system also incorporates the most interactions between SWNT and polymer matrix. Polymer nanocomposite films containing 5 wt% single-walled carbon nanotubes (SWNT) or 5 wt% multi-walled carbon nanotubes (MWNT) with PSVPh copolymers were processed from dimethyl formamide solutions. The vinyl phenol mole ratio in the copolymers was 0, 10, 20, 30, and 40%. FTIR analysis indicates that the composites containing the copolymer with 20% vinyl phenol exhibit the maximum intermolecular interactions (hydrogen bonding) between the hydroxyl group of the vinyl phenol and the carbon nanotube functional groups. Tensile properties and electrical conductivity also are the highest in the samples containing the copolymer with 20% vinyl phenol. Thus, these results show that the optimization of the extent of intermolecular interactions between a polymer chain and a carbon nanotube results in an optimal increase in macroscopic properties. Moreover, the extent of intermolecular hydrogen bonding can be improved by optimizing the accessibility of the functional groups to participate in the non-covalent interaction. In this system, this optimization is realized by control of the amount of vinyl phenol in the copolymer, i.e., the copolymer composition. Finally, the approach of utilizing intermolecular interaction to enhance dispersion and properties was applied to composites of carbon nanofibers. Dynamic mechanical analysis (OMA) and differential scanning calorimetry (DSC) indicate that the composites prepared from oxidized nanofibers exhibit improved thermal and structural properties relative to those prepared from unoxidized nanofibers. The optimum enhancement in the mechanical and thermal properties was observed for the composite formed from oxidized nanofiber and the copolymer containing 20% vinyl phenol