Analysis of Carbon Nanotube Pull-out from a Polymer Matrix
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Analysis of Carbon Nanotube Pull-out from a Polymer Matrix* S. J. V. Frankland and V. M. Harik ICASE, M/S 132C NASA Langley Research Center Hampton, VA 23681-2199 ABSTRACT Molecular dynamics (MD) simulations of carbon nanotube (NT) pull-out from a polymer matrix are carried out. As the NT pull-out develops in the simulation, variations in the displacement and velocities of the NT are monitored. The existence of a carbon-ring-based period in NT sliding during pull-out is identified. Linear trends in the NT velocity-force relation are observed and used to estimate an effective viscosity coefficient for interfacial sliding at the NT/polymer interface. As a result, the entire process of NT pull-out is characterized by an interfacial friction model that is based on a critical pull-out force, and an analog of Newton’s friction law used to describe the NT/polymer interfacial sliding. INTRODUCTION Polymer nanocomposites with carbon nanotubes (NTs) as fillers have been fabricated and mechanically tested to explore their potential as strong, lightweight materials [1, 2]. While some reinforcement of the polymer matrix has been observed in the form of enhanced moduli relative to the polymer matrix alone, a detailed understanding of the NT/polymer interfacial region remains fundamental to the optimization of the mechanical behavior of these composites. The interface plays a significant role in the stress transfer and the consequent improvements in the nanocomposite stiffness and strength. Characterization of the NT/polymer interfacial interactions during pull-out will be the focus of this study. For traditional fiber-reinforced polymer matrix composites, the interfacial shear strength is typically evaluated by employing fiber pull-out [3] or fiber push-out tests [4]. The fiber decohesion process is characterized by the critical shear stress required to debond the fiber. The value of the shear stress is estimated from the critical axial load using the shear-lag model. For nanocomposites, a similar procedure can be utilized both experimentally [5] and theoretically [6]. For a NT embedded in a polymer matrix, the critical shear stress at the interface has been evaluated via molecular dynamics (MD) simulations and an analog of the shear-lag model [6]. A comparison was made between nanocomposites with and without the nanotube chemically bonded to the polymer indicating that chemical bonds reinforce the interface [6]. This study extends the previous work for the case of a non-bonded NT to refine the analysis of interfacial sliding between the NT and polymer nanostructured surfaces. In the system simulated here, the matrix is crystalline polyethylene, which interacts with the NT via van der Waals forces represented by the Lennard-Jones potential. MD simulations are used to confirm the critical pull-out force and investigate interfacial sliding as a NT is pulled through the surrounding polymer. NT velocity and displacement are traced in order to *
This research was supported by the National Aeronautics and Space Administration under NA
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