Maryam Majidian
Synthesis and characterization of polymer-based composites containing carbon nanostructures is the focus of this dissertation. The main polymer which has been used for this research is SU8. The motivation of the work is to overcome the drawbacks of SU8 e.g. (high electrical and thermal resistance, brittleness) by addition of the carbon-based nanofillers. In this thesis, three classes of carbon nanostructures have been used as composite fillers: 2 dimentional graphene, 1 dimentional carbon nanotubes and 0 dimentional onion like carbons. The idea behind this material selection is to study the influence of dimensionality of the fillers on the electrical transport properties of the composites. In addition, we have performed comprehensive characterizations of graphene composites and addressed some questions about carbon nanotubes and nano onions composites. SU8 graphene composites were prepared using solution mixing method. The microstructure analysis of the composites showed a homogeneous dispersion of the graphene flakes. The study of electrical properties of the composite as function of the filler loading exhibited superior electrical conductivity compared to other graphene-base polymer composites. The study of the viscoelastic behavior of the composites showed that the rheological percolation is very close to zero, which we attribute to the polymer chain restriction due to high aspect ratio graphene fillers. The mechanical properties were evaluated with nanoindentation technique. 67 percent enhancement for Youngâs modulus and 75 percent enhancement for hardness were acquired. The possibility of the linkage between the filler and the matrix was investigated by spectroscopy techniques including Photoluminescence, Raman and Fourier Transform Infrared spectroscopy. Our findings suggest that covalent bonds are formed between SU8 and the functional groups on the surface of the graphene flakes. SU8-CNT composites were synthesized using both randomly dispersed and well-aligned tubes. For composites with randomly dispersed CNTs, the effect of nanotube length and polydispersity was investigated with experimental approach, for the first time. We have shown that the conductivity in such composites is proportional to mean length of CNTs, Ln, rather than weighted average length, Lw, which is predicted by theory. For case of aligned CNTs, we have measured the thermal conductivity, kappa, parallel and perpendicular to the orientation of the tubes, which exhibited an anisotropy close to 20 . The study was motivated by thermal management applications. From the same composite, lamellas with thickness ranged 20-100 nm were prepared using ultramicrotomy technique, for proton channeling applications. The successful sample preparation and pioneering channeling experiments give an encouraging outlook for future investigations in this field. Composites containing Poly methyl methacrylate as matrix and onion-like carbon as fillers were prepared for transport studies. The temperature and pressure dependence of the conductivity were measured. Due to the complexities associated to non homogeneous structure of these composites, we do not have a unified model to describe the dependence of conductivity upon concentration, temperature and pressure, and this question has remained open.
EPFL2016
