| Carbon nanotubes (CNTs) with unique structure and properties are in the vanguard of technological development of the time, and are predicted to be the best candidate as the multifunctional component of the next generation of composite materials.To fully utilize the unique mechanical and physical properties of carbon nanotubes, it is essential to assemble them into macroscopic structures. Polyimide, as the most well-known high temperature resistance polymer, is widely utilized in aerospace applications. Therefore, the combination of these two materials will be promising for multifunctional application. However, previous studies have confined to mix staple carbon nanotube with polymer solutions and fabricate composites with comparatively low volume fraction.In composite design, the morphology and content of reinforcement, and the interface between carbon nanotube and polymer are the main factors to determine the composite property. Thus, this dissertation aims to solve the CNTs dispersion, alignment and interface problems in carbon nanotube reinforced composite by demonstrating the interfacial property using molecular dynamics simulation and plasma treatment method, fabricating CNT composites with ideal structural features and excellent mechanical,thermal and electrical performances, and increasing the understanding of the structure-property relationship in these materials. The detailed studies are described below:(1)Atmospheric plasma treatment is applied on carbon nanotube, in the form of bucky paper, to improve the interfacial interaction between carbon nanotube and polyimdie. The fabrication of bucky paper is consisted of dissolve, filtration and drying processes, following the plasma treatment and fabrication of composite. The SEM, XPS and static contact angle analysis are conducted to discover the morphology and hydrophilicty change before and after plasma treatment. It is shown that the plasma treatment can effectively "cut off’ the heads of CNTs without altering the bulk properties of composite, which increase the nano-mechanical interlocking between CNTs and polymer. Besides, the number of oxygen-containing groups increases and contact angle transfers from hydrophobicity to hydrophilicity. Based on the two mechanisms, the mechanical property of composite is improved distinctly. The tensile fracture surface also shows that less pull-out happens after plasma treatment. And high temperature resistance is retained even after the plasma treatment.(2) In order to reveal the structure-related property, we adopt a new fabrication method named spray-winding to make carbon nanotube reinforced polyimide composite. Due to the strong Van der Waals between CNTs, the CNT can be pulled out in the form of continuous ribbons, in which CNTs keep oriented and aligned along the longitudinal direction. The unique structure helps the electron and phonon transmission, thus improving the electrical and thermal properties. Compared with the traditional carbon nanotube reinforced composite, this continuous structure not only solves the dispersion and alignment problems, but improve volume fraction to a high level.We further use pre-stretch treatment to improve the alignment and compactness of CNT prepregs, thus obtaining better mechanical, electrical and thermal properties.(3) To discover the interaction mechanism between carbon nanotube and polyimide, especially in the molecular scale, we need a special tool—molecular dynamics simulation to achieve this goal. Compared with experimental techniques, this computational method is more effective and time-saving. Single chain simulation is firstly to employ to find that the interaction energy between carbon nanotube and polyimide is very high, and this phenomon is also verified in pull-out simulation where co-movement occurs. Limiting pulling force is found in pull-out simualtions, above which the pull-out process can not be achieved. For SWNT and DWNT, the limiting pulling forces are the same value, but for TWNT, it is higher than the SWNT and DWNT. This is due to the inter-tube interactions during pull-out. The interfacial shear strength is calculated to be around5～30MPa which are in the reasonable value among previous experimental and simulation values.(4) The volume fraction of carbon nanotube is an important factor influencing the properties of composite. How the volume faction affects the polymer chain structure configuration is meaningful in composite design, but no previous study has been done. Therefore, we use molecular dynamics simulation to build composite and polyimide structures separately and invesitigate the polymer configuration change with the increasing of volume fraction. We modify the traditional volume fraction formula and regard the interface as the voids in calculation. Meanwhile, we make "fully crystallized" and "fully amorphous" polyimide structure in simulation and calculate the crystallinity using the rule of mixture. The simulation results are in the agreement with assumption that carbon nanotubes act as the nucleation site in composite.To sum up, this paper discusses the effective methods to improve the propertyies of carbon nanotube based composite by improving the interface and changing carbon nanotube structure. The molecular dynamics simulation is applied to investigate the micro-scale mechanism of carbon nanotube and polyimide interactions, thus providing valid methods and evidence for composite design and opening up a new vision for nano-reinforced composite studies. |
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