Analysis of damping characteristics of polymeric composites containing carbon nanotubes

In recent years, there has been an increasing interest in exploring the damping characteristics of carbon nanotube (CNT) based materials, such as densely packed CNT thin films and polymeric composites with dispersed CNT fillers. The unique features that make carbon nanotubes ideal fillers for high performance damping composites are their large surface area, large aspect ratio, high stiffness, low density, and high thermal conductivity characteristics. Some theoretical study on the damping characteristics of CNT-based composites has been initiated but not yet explored in depth. This thesis aims to advance the state-of-art of the damping characteristics of polymeric composites containing well dispersed, aligned or randomly oriented carbon nanotubes.;First, a micromechanical damping model is proposed to address the damping properties of composites containing dilute, aligned or randomly oriented nanotubes. The composite is modeled as a three-phase system composed of a resin, a resin sheath, and MWNTs or SWNT ropes. The resin is described as a viscoelastic material using the three-element standard solid model. The concept of stick-slip motion caused by interfacial friction is applied to the interfaces of the nanotubes and the resin as well as between nanotubes. The overall loss factor, which combines the stick-slip motion and the contribution of viscoelastic material, is calculated through cycles of harmonic loading. Numerical analysis on composites with aligned nanoropes shows that the damping ability of the composites increases with a small addition of nanotubes. If the magnitude of the applied stress is large enough to cause complete sliding at the interface, compared to the viscoelastic material, the stick-slip friction is the main contribution for the total loss factor of the CNT-based composites. The loss factor from stick-slip motion and the total loss factor are both stress-dependent. The inter-tube and SWNT/sheath shear strengths play an important role on the onset and completion of the debonding at the interface. The volume fraction of the CNT ropes and the aspect ratio of the CNT have significant effects on the maximum loss factor over the entire stress range and the applied stress magnitude when the maximum value is achieved.;To advance the state of the art and derive an effective tool that can be used to synthesize any polymeric damping material with carbon nanotubes, a multiscale approach is proposed. It can characterize the CNT interfacial strength at the molecular/atomic level, the stick-slip phenomena at the microscopic level and the material damping feature for macroscopic applications. In this sequential multiscale model, the interfacial shear strengths between the nanotube ropes and between the nanotube and the resin are first calculated from a pull-out test with molecular dynamics simulation. The calculated shear strength values are then applied to the micromechanical model to predict the damping ability of the composites. Both soft polyethylene matrix and hard epoxy resin are investigated. It is observed that with only van der Waals force, the interaction between the nanotube and the resin in both matrices is weaker than that between nanotube ropes.;The effect of carbon nanotube functionalization on the interfacial shear strength as well as the damping feature of the composites is also explored. Analysis results indicate significant increase of the interfacial shear strength by only adding functional groups to less than 1% carbon atoms in the CNT. The optimal damping properties of the composites vary with different combinations of the interfacial shear strength and operational stress range.;Some guidelines for design optimal polymeric damping material with carbon nanotubes are provided with this study. The choice of the polymer matrix, type of CNT (functionalized or non-functionalized), aspect ratio and volume fraction of the CNT are suggested.