Fabrication of functional composites based on carbon nanomaterials and their applications

Abstract

Carbon nanomaterials have received growing attention over the past decades since the successive dis-covery of fullerene, carbon nanotube, and graphene. Intensive studies have been directed toward potential applications on account of their outstanding physical, electrical, and thermal properties originating from the sp2-hybridized carbon network. In this regard, a variety of hybrid materials and functional composites have been proposed not only to realize synergistic combination for target properties, but also to open a new prospect in the field of nanotechnology. This dissertation will not be limited to consideration of a specific subject, but will seek for wide utilization of carbon nanotube and graphene. In chapter 2, rheological percolation of carbon nanotubes (CNTs) in microstructured polymer matrices were investigated. Polymer/CNT composites were fabricated from polycarbonates with different molecular weights to diversify the microstructures, which vary with the polymer radius of gyration and entanglements. A model for the dispersion of CNTs in polymer matrices was proposed to explain the electrical and rheological properties. The percolation theory represented by a power-law relation could not account for the rheological percolation of CNTs for specific cases. Therefore, we investigated the crossover points to provide a quantitative analysis of the rheological percolation threshold of nanofillers in polymer matrices. For the first time, the rheological percolation threshold was experimentally determined with this definition. In chapter 3, graphene nanoplatelets (GNPs) and water-soluble polyvinyl alcohol were assembled into thin films on flexible plastic substrates by a facile and cost-effective layer-by-layer (LbL) technique. UV-visible light absorption, film thickness, and mass per unit area increased linearly with the number of bilayers, indicating that almost the same amount of components were deposited in each assembling step resulting in the linear growth of films. It was found that the assembled thin films provided not only good electrical conductivity but oxygen barrier property for gas molecules. The films were treated with vaporized $HNO_3$ to im-prove electrical conductivity further. As a result, $HNO_3$-treated films showed much improved conductivity about an order of magnitude higher than as-assembled films. Raman and XPS analysis proved that the effect of $HNO_3$ treatment originated from the removal of polymeric components rather than chemical doping of graphene. Meanwhile, oxygen barrier properties were maintained constant in spite of $HNO_3$ treatment, implying that the treatment did not disrupt the inner structure of assembled films. In the last chapter, ultrathin graphene/graphene oxide (GO)/silver nanowire (AgNW) hybrid films were proposed for efficient electromagnetic interference (EMI) shielding. GNP/GO/AgNW hybrid films with the thickness of $3 ~ 13 \mu m$ were fabricated by facile vacuum filtration process. The films were composed of GNP with a small amount of GO to improve film durability as well as dispersion stability. It was demonstrated that ultrathin GNP/GO films exhibited excellent shielding effectiveness. Moreover, the incorporation of AgNWs into films enhanced the shielding performance further and reached higher than 30 dB at 5.0 GHz (> 99.9% shielding of incident electromagnetic energy). These ultrathin hybrid films go beyond conventional polymer composites to achieve excellent EMI shielding performance which can be applicable to electronic mobile devices operated at GHz frequency.