Molecular Dynamics Simulations of Single-Walled Carbon Nanotubes Wrapped by Various Polymers
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Date
2009-08-07
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Abstract
Carbon nanotubes (CNTs) possess superior electrical and mechanical properties and thus are excellent candidates for nanostructured materials. Due to the very high length-to-diameter ratio of CNTs, they are ideal reinforcements for polymer nanocomposites. Engineering of the polymer-CNT interface through noncovalent modifications is necessary to achieve the desired mechanical properties and yet preserve the inherent properties of the CNTs. However, the effects of chemical composition and backbone stiffness on the adsorption characteristics of polymers are not well understood. Molecular dynamics simulations in vacuum were used to study the interaction between a (10,0) zig-zag type single-walled carbon nanotube (SWCNT) and a series of polymers. These simulations investigate whether the polymers prefer to wrap the SWCNT, what the molecular details of that interface are, and how the interfacial interaction is affected by the chemical composition and structure of the polymer. The simulations indicate that polymers with both flexible and stiff backbones tend to wrap around the SWCNT, although in different conformations. Flexible backbones like nylon6 (N6) and poly(lactide) (PLA) wrap in a random conformation along both the longitudinal axis and the diameter of the SWCNT. One flexible polymer, poly(acrylnitrile) (PAN), preferred to extend along the longitudinal axis rather than wrap the diameter of the SWCNT as a means of optimizing pi-pi overlap between the cyano side chain and the SWCNT; PAN was the only flexible backbone polymer that exhibited preferential orientation of chemical groups along the SWCNT surface. Flexible polymers with bulky and aromatic side groups such as poly(methylmethacrylate) (PMMA) and poly(styrene) (PS) prefer intra-chain coiling rather than wrapping the SWCNT. Poly(ethylene terephthalate) (PET), the only polymer with a semi-flexible backbone in this study, exhibited a partial wrap in an S-conformation along the side of the SWCNT. Polymers with stiff backbones such as poly(acetylene) (PA), poly(p-phenylene vinylene) (PPV), poly(pyrrole) (PPy), and poly(arylene ethynylene) (PPE) exhibit distinct conformations upon adsorption. Helical-like wrapping conformations were only obtained for PPV and PPE. Aromatic groups along the backbone tend to dictate the adsorption conformation due to pi-pi interactions with the SWCNT, although the presence of bulky aliphatic side chains can have a slight impact on this interaction. Plots of the rotational moment of inertia of each polymer about the SWCNT longitudinal axis as a function of time quantify the interplay between intra-chain coiling and adsorption to the SWCNT surface. These plots indicate that the adsorption of polymers with stiff backbones tends to be a two-step process, whereas flexible backbones tend to exhibit a multi-step wrapping mechanism, especially those that have a preference for intra-chain coiling. To quantify the correlation between the chemical composition of the repeat unit and the conformational limitations of long polymer chains, MD simulations were also performed with small molecules that correspond to the repeat units of a subset of the polymers. These simulations indicate that the individual molecules have more conformational freedom, yet still exhibit some orientation characteristics similar to the polymers, such as adsorption of both aromatic rings and aliphatic hydrocarbons along the SWCNT surface.
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Keywords
carbon nanotubes, polymers, molecular dynamics, interfacial interactions, adsorption, nanocomposite, wrapping, polymer wrapping, computational modeling, single walled nanotube,
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Degree
MS
Discipline
Textile Engineering
