Simulation of Carbon Nanotube Pull-outWhen Bonded to a Polymer Matrix
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Simulation of Carbon Nanotube Pull-out When Bonded to a Polymer Matrix S. J. V. Frankland1 and V. M. Harik2 ICASE, M/S 132C, NASA Langley Research Center, Hampton, VA 23681-2199 2 Swales Aerospace, M/S 186A, NASA Langley Research Center, Hampton, VA 23681-2199
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ABSTRACT A carbon nanotube pulling through a polyethylene matrix was simulated using molecular dynamics. The interfacial sliding was characterized in terms of a nanoscale friction model, which is parametrized from the molecular dynamics simulation, and involves determining the critical pull-out force on the nanotube and the effective viscosity at the nanotube/polymer interface. Comparison was made of the pull-out behavior of non-bonded and functionalized nanotube composites. Chemical bonds between the polymer and the nanotube increased the critical pullout force, the resistance to interfacial sliding, and the interfacial viscosity.
INTRODUCTION In polymer-nanotube composites, the strength of the interface between the polymer and the nanotube (NT) may be enhanced by chemically bonding the NTs to the polymer. Chemical functionalization of NTs remains a challenge [1, 2], but in some polymer-NT composites, where significant load transfer between the polymer and the nanotube has been observed, chemical bonding with the NT may occur [3]. In traditional fiber-reinforced composites, the fiber-matrix interfacial shear strength is often characterized by fiber pull-out experiments [4]. Analogous nanoscale pull-out experiments have been carried out with atomic force microscopy for the sliding of NTs in multi-walled NTs [5] and in bundles of single-walled NTs [6]. A simulated pull-through test has also been devised for polymer-NT composites via molecular dynamics (MD) to determine the interfacial shear strength while a force is applied to the NT. These simulations have indicated an increase in the interfacial shear strength with the addition of chemical bonds between the polymer and a NT [7]. Furthermore, the simulations have been extended to study the interfacial sliding of the NT during pull-out in non-bonded composites, and to characterize the effective interfacial viscosity due to the van der Waals interactions of the polymer and the NT [8, 9]. In this work, the interfacial sliding during pull-out of a NT chemically bonded to a crystalline polymer matrix was modeled with MD simulations. The objective was to show some of the mechanisms which may serve to enhance its ability to transfer mechanical load within the composite. First, a description of the composite structure will be given with details of the MD simulations. Then stages encountered by the NT during interfacial sliding will be analyzed. Finally, the sliding velocity will be interpreted by using the interfacial viscosity model [8, 9]. MOLECULAR DYNAMICS SIMULATION The simulated polymer-NT composite consisted of a (10,10) carbon nanotube embedded in a crystalline polyethylene (PE) matrix (Figure 1a). The NT was chemically bonded at three
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locations to one of the nearby polymer chains (Fig
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