Local Elastic Constants For Epoxy-Nanotube Composites From Molecular Dynamics Simulation
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1056-HH08-27
Local Elastic Constants For Epoxy-Nanotube Composites From Molecular Dynamics Simulation S. J. V. Frankland1, and T. S. Gates2 1 National Institute of Aerospace, Hampton, VA, 23666 2 NASA Langley Research Center, Hampton, VA, 23681 ABSTRACT A method from molecular dynamics simulation is developed for determining local elastic constants of an epoxy/nanotube composite. The local values of C11, C33, K12, and K13 elastic constants are calculated for an epoxy/nanotube composite as a function of radial distance from the nanotube. While the results possess a significant amount of statistical uncertainty resulting from both the numerical analysis and the molecular fluctuations during the simulation, the following observations can be made. If the size of the region around the nanotube is increased from shells of 1Å to 6Å in thickness, then the scatter in the data reduces enough to observe trends. All the elastic constants determined are at a minimum 20Å from the center of the nanotube. The C11, C33, and K12 follow similar trends as a function of radial distance from the nanotube. The K13 decreases greater distances from the nanotube and becomes negative which may be a symptom of the statistical averaging. INTRODUCTION The inclusion of carbon nanotubes in structural composites for aerospace applications is limited to date, not only by production and processing issues, but also by their lower than expected mechanical performance in polymeric materials. In a composite, the weak interaction between the nanotube and the polymer may be improved by chemical modifications (functionalization) to the carbon nanotube that improve the interface with the polymer. One way to probe the interfacial region is to examine the changes in elastic constants as a function of distance from the carbon nanotube. Elastic constants of polymer nanocomposites can then be determined for representative volume elements (RVE) of the molecular structure by using molecular dynamics simulation to calculate configurational energy changes under mechanical deformation. It is anticipated that the changes observed in the elastic constants are representative of the changes in molecular structure of the polymer in the vicinity of the nanotube, and therefore a measure of the size of the interfacial region. To understand how the nanotube affects the elastic constants of the polymer at the polymer nanotube interface, a broad range of spatial definition is required. Several theoretical studies have probed the ‘interfacial region’ of the polymer surrounding the nanotubes to elucidate the effect of the polymer-nanotube interaction on the mechanical behavior. Molecular dynamics (MD) simulations of nanotube pull-out and frictional behavior of nanotubes in polyethylene have demonstrated the slippage of pristine nanotubes in polymer, and the increased resistance to motion when axial load is applied to the nanotube for nanotubes covalently bonded into the polymer [1, 2]. MD simulations have also been used to calculate the stress-strain behavior of nanotubes in poly
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