Computational Study of Polymerization in Carbon Nanotubes
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Computational Study of Polymerization in Carbon Nanotubes Steven J. Stuart, Brad M. Dickson, Bobby G. Sumpter1, and Donald W. Noid1 Department of Chemistry, Clemson University, Clemson, SC 29634-0973, USA. 1 Chemical and Analytical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA. ABSTRACT Molecular dynamics simulations of ethylene polymerization have been performed using a chemically realistic, reactive potential. These simulations have been performed in the bulk liquid and in the interior of both (10,10) and (7,7) nanotubes as a means of investigating the effects of nanoscale confinement on the polymerization reaction. The structure of the resulting polymer was found to be similar in the bulk and in the (10,10) tube at the elevated temperatures investigated, while only very small oligomers were formed in the (7,7) tube. The reaction rate was substantially reduced in the nanotubes, when compared to the bulk, primarily as a result of spatial interference due to reaction products. These simulations have implications for the possible use of nanotubes as synthetic reaction vessels, as well as for the general understanding of association reactions in confined spaces. INTRODUCTION Carbon nanotubes are under investigation for a number of their remarkable properties. Among these is their ability to be filled via capillary forces in order to create nanoparticles, nanowires, and other novel structures.[1-5] Although most applications involve filling nanotubes with aqueous solutions, molten salts, or molten metals, previous work using mesoporous silica fibers as a synthetic support for polyethylene[6] suggests that polymerization in nanoscale geometries will also generate unique structures. In addition to the likelihood of forming novel polymeric structures, there is also the interesting possibility of observing fractal kinetics for association reactions in spaces of reduced dimensionality.[7] For these reasons, the polymerization of ethylene was studied in the confined geometry of nanotubes of two different diameters, and compared to comparable simulations in the bulk liquid phase. MODEL AND COMPUTATIONS Because the reactivity of the hydrocarbons was of critical importance, it was crucial to perform the simulations a model that is capable of accurately modeling dissociation and formation reactions in hydrocarbons. For this reason, the AIREBO (adaptive intermolecular reactive empirical bond-order) potential[8] was selected. This is a bond-order potential based on Brenner’s well-known REBO potential for hydrocarbons[9,10]. The AIREBO potential preserves the treatment of C—C, C—H, and H—H covalent bonding interactions that has been validated in numerous studies with the REBO potential, while introducing terms corresponding to dispersion, torsional, and exchange repulsion interactions in a way that does not interfere with the covalent bonding potential. The focus in this study is on the geometric effects of confinement within a carbon nanotube, rather than any potential reactions with the nanotube walls.
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