探討單壁奈米碳管複合材料之機械性質

Abstract

本研究利用多尺度模擬方法探討以單壁奈米碳管為加強材之聚亞醯胺奈米複合材料之機械性質，並且利用分子動力學探討奈米碳管嵌入聚亞醯胺分子所構成之奈米複合材料，在其遭受負荷時奈米碳管上所產生的應力分佈。奈米碳管之分子結構為一中空圓柱，利用一橫向等向性之實心圓柱結構模擬奈米碳管，而等效實心圓柱結構之機械性質可利用能量等效之概念獲得，藉由分子動力學建立奈米碳管/聚醯亞胺奈米複合材料，進而計算出奈米碳管與週遭聚醯亞胺之間的非鍵結間距以及非鍵結能。假設正交化非鍵結能與界面層作用力相關，則可在奈米碳管以及高分子之間考慮一等效界面層，其中尺寸為非鍵結間距，而彈性勁度則可藉由正交化非鍵結能得到。最後利用三層界面層微觀力學模型，分別考慮等效實心圓柱，高分子基材以及等效界面層可預測奈米複合材料的機械性質。結果可發現利用三界面層微觀力學模型預測奈米複合材料的軸向機械模數與分子動力學結果一致，且都與傳統之混合原則結果相符。而在橫向部分，三層界面層微觀力學模型比傳統微觀力學模型更能夠預測出奈米複合材料的橫向模數隨著奈米碳管半徑增加而下降之現象。 利用分子動力學探討奈米碳管嵌入聚亞醯胺分子所構成之奈米複合材料，在其遭受負荷時奈米碳管上所產生的應力分佈。基本上，負荷傳遞效率在奈米複合材料的機械性質上扮演著重要角色。藉由奈米碳管上的應力分佈情形，可以了解奈米加強材及其周遭聚亞醯胺基材之間的負荷傳遞效率。考慮奈米碳管及聚亞醯胺基材之間三種不同的原子作用力，分別為凡得瓦作用力、表面改質之奈米碳管以及共價鍵結合之界面層。當奈米複合材料遭受負荷時，可利用原子等級應力公式以及能量公式推導計算嵌入其內之奈米碳管的應力分佈，進而可定義負荷傳遞至奈米碳管的效率。結果發現表面改質過的奈米碳管可以使負荷更容易由聚亞醯胺分子傳遞至奈米碳管，使得奈米複合材料有較好的機械性質。此外，若奈米碳管的表面沒改質劑時，界面層僅存在凡得瓦作用力，因而導致較差的負荷傳遞效率。因此，在製作以奈米碳管為加強材之奈米複合材料時，表面改質的奈米碳管可以有效的改善奈米複合材料的負荷傳遞效率以及機械性質。

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.