AbstractsEngineering

The main objective of this work is to study the surface modification and the mechanics of interfaces in nanocomposites. Investigation of this topic is motivated by the exceptional mechanical properties that have been demonstrated in many nanomaterials. A combined modeling and experimental approach is developed. The systems studied mainly include polystyrene polymers that are reinforced by carbon nanotubes subjected to different surface modifications. In the experimental part of this work, the surfaces of three different types of carbon nanotubes are modified by plasma polymerization technique using a custom-built RF plasma system. Characterization indicates that the surface properties of carbon nanotubes are modified by the process and a thickness of 2-7 nm deposited polymer film is achieved. Results from DSA and TGA measurements provide the evidences for such polymer film existence and uniformity is examined based on above measurements. The thickness of deposited polymer film estimated from TGA measurements matches the thickness observed from HRTEM very well. SIMS and Raman measurement further confirmed the existence of the deposited film and its uniformity. It is found that the chemical structure of carbon nanotube surface can be tailored by changing the monomer in plasma polymerization processing. The deposited polymer films not only significantly improve the dispersion of carbon nanotube in solvent and polymeric matrix, but also significantly improve the interfacial properties between carbon nanotube and polymer matrix. Mechanical testing also indicates that the plasma treatment can improve the mechanical properties of nanocomposite significantly. When the concentration of MWCNT is 3%, the mechanical property of nanocomposite reaches the maximum point. The experimentally observed relation between the mechanical properties and interface motivates the modeling and simulation study. Molecular mechanics and molecular dynamics are two of the primary tools used for this study. The interactions among the atoms in the system are governed by the empirical potentials. More specifically, the interactions of the atoms in the polymer are modeled by the DL_Poly force field. The interactions of the carbon atoms in carbon nanotube are modeled by the Brenner potential. The interaction between the carbon nanotube and the polymer is modeled by the Lennard-Jones potential. To directly probe the interfacial mechanics, nanotube pull-out test is simulated using molecular mechanics. The interfacial properties between carbon nanotube and polystyrene matrix are evaluated from the numerical experiments. The numerical results show that both interfacial energy and interfacial shear stress are improved by introducing functional group on the surface of carbon nanotube, which are consistent with the experimental observations. The interaction between carbon nanotube and polymer matrix is also studied by using molecular dynamics technique to simulate the vibration behavior. The effects of CNT length, deformation degree…