Chenyu Wei

Nanomechanics of carbon nanotubes and composites

Computer simulation and modeling results for the nanomechanics of carbon nanotubes and carbon nanotube-polyethylene composite materials are described and compared with experimental observations. Young’s modulus of individual single-wall nanotubes is found to be in the range of 1 TPa within the elastic limit. At room temperature and experimentally realizable strain rates, the tubes typically yield at about 5–10% axial strain; bending and torsional stiffness and different mechanisms of plastic yielding of individual single-wall nanotubes are discussed in detail. For nanotube-polyethylene composites, we find that thermal expansion and diffusion coefficients increase significantly, over their bulk polyethylene values, above glass transition temperature, and Young’s modulus of the composite is found to increase through van der Waals interaction. This review article cites 54 references. @DOI: 10.1115/1.1538625#

Tensile strength of carbon nanotubes under realistic temperature and strain rate

A transition state theory based predictive model is developed for the tensile failure of carbon nanotubes (CNTs). We show that the tensile yield strain has linear dependence on the activation energy and the temperature, and has a logarithmic dependence on the strain rate. Based on the parameters fitted from strain rate and temperature-dependent simulations within a wide range of molecular-dynamics time scales, the model predicts that a defect-free micrometer long single-wall nanotube at 300 K, stretched with a strain rate of 1%/h, yields at about

Nanomechanics of carbon nanofibers: Structural and elastic properties

9\ifmmode\pm\else\textpm\fi{}1%

Radius and Chirality Dependent Conformation of Polymer Molecule at Nanotube Interface

tensile strain for small diameter CNTs, and about 2\char21{}3 % higher for larger diameter CNTs. This is in good agreement with recent experimental findings.

Adhesion and reinforcement in carbon nanotube polymer composite

A general analytic expression for the Young’s modulus of a range of carbon nanofibers (CNFs) with single or multishell nanocone or cone stacked structures has been developed from continuum elastic theory. The Young’s modulus of a single-shell nanocone is found to be cos4θ of that of an equivalent single-wall carbon nanotube (CNT). The CNFs of short lengths and small tilting angles have very large Young’s modulus comparable to that of single or multiwall CNTs, whereas the inverse is true for the CNFs with long lengths and large tilting angles. The dependence of the stiffness of CNFs on various structural parameters has been predicted by the model, validated through full-scale molecular dynamics simulations, and categorized for scanning probe tip or reinforcing fiber type applications.

Tensile yielding of multiwall carbon nanotubes

Temperature-dependent conformations of linear polymer molecules adsorbed at carbon nanotube (CNT) interfaces are investigated through molecule dynamics simulations. Model polyethylene (PE) molecules are shown to have selective conformations on the CNT surface, controlled by atomic structures of the CNT lattice and geometric coiling energy. PE molecules form entropy driven assembly domains, and their preferred wrapping angles around large radius CNT (40, 40) reflect the molecule configurations with energy minimums on a graphite plane. While PE molecules prefer 0 degrees wrapping on small radius armchair CNT (5, 5) predominantly at low temperatures, their configurations are shifted to larger wrapping angle ones on a similar radius zigzag CNT (10, 0). A nematic transformation around 280 K is identified through the Landau-de Gennes theory, with molecule aligning along tube axis in extended conformations.

MOLECULAR TRANSPORT AND FLUIDICS IN CARBON NANOTUBE

Temperature dependent adhesion behavior and reinforcement in carbon nanotube (CNT)-polymer (polyethylene) composite is studied through molecular dynamics simulations. The interfacial shear stress through van der Waals interactions is found to increase linearly with applied tensile strains along the nanotube axis direction, until the noncovalent bonds between CNTs and molecules break successively. A lower bound value about 46MPa is found for the shear strength at low temperatures. Direct stress-strain calculations show significant reinforcements in the composite in a wide temperature range, with ∼200% increase in the Young’s modulus when adding 6.5% volume ratio of short CNTs, and comparisons with the Halpin–Tsai formula are discussed.

Thermal Expansion and Diffusion Coefficients of Carbon Nanotube-Polymer Composites

The tensile yielding of multiwall carbon nanotubes (MWCNTs) has been studied using molecular-dynamics simulations and a transition state theory based model. We find a strong dependence of the yielding on the strain rate. A critical strain rate has been predicted above/below which yielding strain of a MWCNT is larger/smaller than that of the corresponding single-wall carbon nanotubes (CNTs). At an experimentally feasible strain rate of 1%/h and T=300 K, the yield strain of a MWCNT is estimated to be about 3%–4% higher than that of an equivalent single-wall CNT. This is in good agreement with recent experimental observations.

Theory of Transport of Long Polymer Molecules through Carbon Nanotube Channels

In this article we review recent developments in molecular transport and fluidics in carbon nanotube (CNT)-based nanochannels. Atomic molecular dynamics simulations and theoretical studies based on Fokker–Planck diffusion equation on the transport of large and long polymer molecules in CNTs are the focus of the article. Fast translocation and diffusion processes of large molecules in CNTs are reviewed and discussed, considering the effects of interfacial interactions and molecular conformations and structures at interface. The transport features for multiple molecules diffusing through CNTs are also discussed.