Investigating Mechanical Properties of Single-Walled Carbon Nanotubes Nanocomposites
|關鍵字:||奈米碳管;奈米複合材料;分子動力學模擬;機械性質;負荷傳遞效率;carbon nanotube;nanocomposites;molecular dynamic simulation;mechanical properties;load transfer efficiency|
The aim of thesis is to characterize the mechanical properties of single-walled carbon nanotubes (SWCNTs) reinforced with polyimide nanocomposites using a multi-scale simulation approach, and to investigate the stress distribution of SWCNTs embedded within polyimide nanocomposites under applied loading using molecular dynamics (MD) simulation. The hollow cylindrical molecular structures of SWCNTs were modeled as transversely isotropic solids, and the equivalent elastic properties of which were determined from the molecular mechanics calculations in conjunction with the energy equivalent concept. The molecular structures of the SWCNT/polyimide nanocomposites were established through MD simulation, from which the non-bonded gap as well as the non-bonded energy between the SWCNTs and the surrounding polyimide, were evaluated. The normalized non-bonded energy (non-bonded energy divided by surface area of the SWCNTs) was postulated to be correlated with the extent of interfacial interaction. An effective interphase was introduced between the SWCNTs and polyimide polymer to characterize the degree of non-bonded interaction. The dimension of the interphase was assumed to be equal to the non-bonded gap, and the corresponding elastic stiffness was calculated from the normalized non-bonded energy. The elastic properties of the SWCNT nanocomposites were predicted by a three-phase micromechanical model, in which the equivalent solid cylinder of SWCNTs, polyimide matrix, and the effective interphase were included. Results indicated that the longitudinal moduli of the nanocomposites based on the three-phase model were in good agreement with those generated from MD simulation. Moreover, they are consistent with the conventional rule-of-mixtures predictions. In the transverse direction, the three-phase model is superior to the conventional micromechanical model because it is capable of predicting the dependence of the transverse modulus on the radii of nanotubes. The stress distribution of SWCNTs embedded within polyimide nanocomposites subjected to applied loading was investigated using the MD simulation. The purpose of evaluating the stress distribution of SWCNTs is to characterize the loading transfer efficiency between the nano-reinforcement and surrounding polyimide matrix, which is an essential factor controlling the mechanical properties of nanocomposites. Three different interfacial adhesions between the SWCNTs and polyimide molecular were considered: the van der Waals (vdW) interaction, SWCNTs with surface modifications, and covalent bonds. The stress distribution of the SWCNTs was calculated using the atomic level stress formulation and by taking the derivative of the potential functions. The results revealed that when the SWCNTs surface was modified, a higher load transfer efficiency from the polyimide to the SWCNTs was observed, resulting in a higher modulus of the nanocomposites. Without surface modification on SWCNTs, the load transfer efficiency, which depends on the intensities of the vdW interactions, is relatively low. As a result, the surface modification on SWCNTs is an effective method to improve the load transfer efficiency as well as the modulus of the nanocomposites, and should be suggested in the fabrication of SWCNTs nanocomposites.
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