Density functional theory study of epoxy polymer chains adsorbing onto single-walled carbon nanotubes: electronic and mechanical properties

We performed first principles calculations based on density functional theory (DFT) to investigate the effect of epoxy monomer content on the electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs). Our calculation results reveal that interfacial interaction increases with increasing numbers of epoxy monomers on the surface of SWCNTs. Furthermore, density of states (DOS) results showed no orbital hybridization between the epoxy monomers and nanotubes. Mulliken charge analysis shows that the epoxy polymer carries a positive charge that is directly proportional to the number of monomers. The Young’s modulus of the nanotubes was also studied as a function of monomer content. It was found that, with increasing number of monomers on the nanotubes, the Young’s modulus first decreases and then approaches a constant value. The results of a SWCNT pullout simulation suggest that the interfacial shear stress of the epoxy/SWCNT complex is approximately 68 MPa. These results agreed well with experimental results, thus proving that the simulation methods used in this study are viable.     

Molecular dynamics analysis on tensile properties of carbon nanotubes with different cracks

Molecular dynamics simulation is employed for the axial tension of single-walled carbon nanotubes (SWCNTs) with different cracks. The cracks of SWCNTs in this study actually are the crack-like defects. AIREBO potential is used to simulate the interactions among carbon atoms. The effects of the crack length, temperature, strain rate and tube diameter on the mechanical properties of SWCNTs are studied. It is found that the failure stress and failure strain decrease with the increase of crack length. And the results show that the failure stress and failure strain are related to the applied strain rate and affected by temperature especially by lower temperature. It is also revealed that the failure stress increases with the increase in tube diameter. The deformation behaviours of SWCNTs are also obtained.     

Resonance frequency distribution of cantilevered (5,5)(10,10) double-walled carbon nanotube with different intertube lengths

Analysis of ultrahigh frequency nanomechanical resonators, which are based on double-walled carbon nanotubes (DWCNTs) with various wall lengths, was carried out via classical molecular dynamics simulations. In the case of the inner wall entirely encapsulated inside the outer wall, the outer wall vibration has a significant effect on the vibration of the DWCNT; while in the case of the inner wall longer than the outer wall, the vibration of the extruded inner wall has a substantially stronger effect on the DWCNT vibration. It is shown that variations of the DWCNT resonance frequency with different wall lengths can be well fitted by Pearson VII and Gauss distribution functions. This result is potentially useful for developing design guidelines for making very fine tuners using DWCNT resonators of various wall lengths.     

On the elastic properties of single-walled carbon nanotubes/poly(ethylene oxide) nanocomposites using molecular dynamics simulations

Molecular dynamics simulations are used to study the physical and mechanical properties of single-walled carbon nanotubes/poly(ethylene oxide) nanocomposites. The effects of nanotube atomic structure, diameter, and volume fraction on the polymer density distribution, polymer atom distribution, stress–strain curves of nanocomposites and Young’s, and shear moduli of single-walled carbon nanotubes/poly(ethylene oxide) nanocomposites are explored. It is shown that the density of polymer, surrounding the nanotube surface, has a peak near the nanotube surface. However, increasing distance leads to dropping it to the value near the density of pure polymer. It is seen that for armchair nanotubes, the average polymer atoms distances from the single-walled carbon nanotubes are larger than the polymer atom distance from zigzag nanotubes. It further is shown that zigzag nanotubes are better candidates to reinforce poly (ethylene oxide) than their armchair counterparts.     

BENDING STIFFNESS OF CYLINDRICAL PLANT ORGANS WITH A ‘CORE-RIND’ CONSTRUCTION: EVIDENCE FROM JUNCUS EFFUSUS LEAVES

The mechanical behavior of leaves of Juncus effusus L. in bending was investigated in terms of a closed-form analytical solution derived to predict the bending stiffness of a cylindrical sandwich beam consisting of an outer ‘rind’ (sclerenchyma and chlorenchyma) and an inner ‘core’ (aerenchyma). The elastic moduli (E_TOTAL) of intact leaves was measured by means of multiple resonance frequency spectra and compared to that of leaves for which the aerenchymatous core was surgically destroyed. Based on ten leaves, E_TOTAL = 22.33 × 10⁴ ± 5.37 ± 10⁴ kg · cm^–2 while the elastic modulus of the ‘rind’ was 22.29 × 10⁴ ± 5.69 × 10⁴ kg · cm^–2. The elastic modulus of the ‘core’ was estimated at 3.12 × 10⁴ ± 1.42 × 10⁴ kg · cm^–2. Load-deflection curves for three leaf segments indicated leaves were linearly elastic within the range of loading and could be predicted with considerable accuracy based on the closed-form solution. The aerenchymatous core was found to contribute very little to the bending stiffness of leaves, although its contribution appeared to increase as leaf diameter decreased. Leaves mechanically failed by Brazier buckling when excessively loaded and were best considered to mechanically operate as hollow tubes. Nonetheless, the analytical solution for bending stiffness could be applied and, in theory, can be used to predict the behavior of other plant organs with a ‘corerind’ construction.     

Trubs,but no trianas: filled and empty regions of angiosperm stem length‐diameter‐mechanics space

Discrete plant habit categories such as ‘tree’, ‘shrub’, and ‘liana’ belie continuous variation in nature. To study the evolution of this continuous variation, we gathered data on stem length, diameter and tissue mechanical stiffness across a highly morphologically diverse highland xerophytic scrub on a lava flow in central Mexico. With stem allometric and mechanical data from 1216 segments from 50 species, we examined relationships between stem length–diameter proportions and tissue mechanical stiffness using linear mixed‐effects models. Rather than a series of discrete clouds in stem length–diameter–tissue stiffness space, corresponding to traditional habit categories, the plants of this xerophytic scrub formed a single continuous one. Within this cloud, self‐supporting plants had stems that became predictably longer and tissues that became stiffer for a given diameter increase, and there was no paucity of intermediates between trees and shrubs (‘trubs’). Non self‐supporting plants had a steeper stem length–diameter slope and their tissues did not increase in stiffness with stem size. The area between self‐ and non self‐supporting plants was sparsely occupied as stem size increased. We predict that this ‘empty’ space between lianas and trees is developmentally accessible but of low fitness, meaning that there should be few ‘trianas’ in nature. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 361–373.     

Structural stability and buckling analysis of a series of carbon nanotorus using molecular dynamics simulations

Based on molecular dynamics (MD) simulations, the buckling analysis of a perfect carbon nanotorus is presented herein. First of all, the minimum length of single-walled carbon nanotubes (SWCNTs) with different radii is determined at which perfect toroidal CNTs can be formed without any ripple at the inner side of the rings. According to the results, by increasing the radius of SWCNT (r), the radius of its corresponding perfect nanotorus (R) increases. Also, for SWCNTs with various lengths, it is found that the buckling force and strain of related carbon nanotoruses increase by increasing R/r. In addition, as the perfect toroidal CNTs are arranged vertically in a column form in accordance with two different schemes, the effects of increasing the radius (R) and the number of carbon nanotoruses (the height of the column made by nanotoruses) on the buckling force and strain are investigated. Based on the results, as a fixed number of carbon nanotoruses with the same radius are arranged vertically in the column form, the buckling force and strain increase by increasing R/r. By contrast, increasing the height of the column made by carbon nanotoruses with similar radius leads to the reduction of buckling force and strain.     

Stability of the thin partitioned carbon nanotubes

We report on the research of the stability of partitioned (bamboo-like) carbon nanotubes with different diameters. The stability of the partitioned carbon nanotubes of the smallest diameter were determined by the tight-binding method. For the prediction of the destruction regions of the bamboo-like nanotubes atomic framework subjected to strain the new original method of the calculation of the local stress of atomic network was developed. Using this method it was shown that partitioned carbon nanotubes with a diameter of 2.02 nm are stable. These partitioned carbon nanotubes with chirality (15,15) are the most stable partitioned carbon nanotubes with the smallest diameter.     

Probing the Diameter Limit of Single Walled Carbon Nanotubes in SWCNT: Fullerene Solar Cells

In this work, for the first time, the diameter limit of surfactant wrapped single walled carbon nanotubes (SWCNTs) in SWCNT:C₆₀ solar cells is determined through preparation of monochiral small and large diameter nanotube devices as well as those from polychiral mixtures. Through assignment of the different nanotube chiralities by photoluminescence and optical density measurements a diameter limit yielding 0% internal quantum efficiency (IQE) is determined. This work provides insights into the required net driving energy for SWCNT exciton dissociation onto C₆₀ and establishes a family of (n,m) species which can efficiently be utilized in polymer‐free SWCNT:C₆₀ solar cells. Using this approach the largest diameter nanotube with an IQE > 0% is found to be (8,6) with a diameter of 0.95 nm. Possible strategies to extend this diameter limit are then discussed.     

Carbon nanotube prevents the secondary structure formation of amyloid-β trimers: an all-atom molecular dynamics study

Abstract

Increasing evidence shows that the formation of misfolded aggregates amyloid-β (Aβ) peptide is associated with the Alzheimer’s disease (AD). Recent experiments reveal a significant correlation of the amount of trimer species bound to neurons with increasing neuro-toxicity. Our previous computational study demonstrated that carbon nanotubes (CNT) can inhibit effectively the formation of β-sheet-rich oligomers of Aβ(16-22) – a hydrophobic peptide essential for Aβ fibrillization. However, the influence of CNT on the oligomers formed by full-length Aβ remains elusive. In this study, we have investigated the conformational dynamics of Aβ(1-42) trimer, built from an NMR structure of α-helical monomer, in the absence and presence of a single-walled carbon nanotube (SWCNT). Our simulations demonstrate that SWCNT can significantly hinder the trimerisation and prevents the secondary structure formation of Aβ(1-42) peptide. Hydrophobic and aromatic stacking interactions between SWCNT and Aβ play important roles in the secondary structure formation of the Aβ trimer. This study reveals a complete picture of the detailed preventable mechanism of full-length Aβ(1-42) by SWCNT, providing theoretical insights into the development of drug candidates of AD.     

Cyclic Bending Contributes to High Stress in a Human Coronary Atherosclerotic Plaque and Rupture Risk: In Vitro Experimental Modeling and Ex Vivo MRI-Based Computational Modeling Approach

Many acute cardiovascular syndromes such as heart attack and stroke are caused by atherosclerotic plaque ruptures which often happen without warning. MRI-based models with fluid-structure interactions (FSI) have been introduced to perform flow and stress/strain analysis for atherosclerotic plaques and identify possible mechanical and morphological indices for accurate plaque vulnerability assessment. In this paper, cyclic bending was added to 3D FSI coronary plaque models for more accurate mechanical predictions. Curvature variation was prescribed using the data of a human left anterior descending (LAD) coronary artery. Five computational models were constructed based on ex vivo MRI human coronary plaque data to assess the effects of cyclic bending, pulsating pressure, plaque structure, and axial stretch on plaque stress/strain distributions. In vitro experiments using a hydrogel stenosis model with cyclical bending were performed to observe effect of cyclical bending on flow conditions. Our results indicate that cyclical bending may cause more than 100% or even up to more than 1000% increase in maximum principal stress values at locations where the plaque is bent most. Stress increase is higher when bending is coupled with axial stretch, non-smooth plaque structure, or resonant pressure conditions (zero phase angle shift). Effects of cyclic bending on flow behaviors are more modest (21.6% decrease in maximum velocity, 10.8% decrease in flow rate, maximum flow shear stress changes were < 5%). Computational FSI models including cyclic bending, plaque components and structure, axial stretch, accurate in vivo measurements of pressure, curvature, and material properties should lead to significant improvement on stress-based plaque mechanical analysis and more accurate coronary plaque vulnerability assessment.     

DFT studies of COOH tip-functionalized zigzag and armchair single wall carbon nanotubes

Structure and energy calculations of pristine and COOH-modified model single wall carbon nanotubes (SWCNTs) of different length were performed at B3LYP/6-31G* level of theory. From 1 to 9 COOH groups were added at the end of the nanotube. The differences in structure and energetics of partially and fully functionalized SWCNTs at one end of the nanotube are observed. Up to nine COOH groups could be added at one end of (9,0) zigzag SWCNT in case of full functionalization. However, for (5,5) armchair SWCNT, the full functionalization was impossible due to steric crowding and rim deformation. The dependence of substituent attachment energy on the number of substituents at the carbon nanotube rim was observed.     

Carbon nanotube reinforced Bombyx mori silk nanofibers by the electrospinning process

Nanocomposite fibers of Bombyx mori silk and single wall carbon nanotubes (SWNT) were produced by the electrospinning process. Regenerated silk fibroin dissolved in a dispersion of carbon nanotubes in formic acid was electrospun into nanofibers. The morphology, structure, and mechanical properties of the electrospun nanofibers were examined by field emission environmental scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and microtensile testing. TEM of the reinforced fibers shows that the single wall carbon nanotubes are embedded in the fibers. The mechanical properties of the SWNT reinforced fiber show an increase in Young's modulus up to 460% in comparison with the un-reinforced aligned fiber, but at the expense of the strength and strain to failure.     

Characterizing gait induced normal strains in a murine tibia cortical bone defect model

The critical role that mechanical stimuli serve in mediating bone repair is recognized but incompletely understood. Further, previous attempts to understand this role have utilized application of externally applied mechanical loads to study the tissue’s response. In this project, we have therefore endeavored to capitalize on bone’s own consistently diverse loading environment to develop a novel model that would enable assessment of the influence of physiologically engendered mechanical stimuli on cortical defect repair. We used an inverse dynamics approach with finite element analysis (FEA) to first quantify normal strain distributions generated in mouse tibia during locomotion. The strain environment of the tibia, as previously reported for other long bones, was found to arise primarily due to bending and was consistent in orientation through the stance phase of gait. Based on these data, we identified three regions within a transverse cross-section of the mid-diaphysis as uniform locations of either peak tension, peak compression, or the neutral axis of bending (i.e. minimal strain magnitude). We then used FEA to quantify the altered strain environment that would be produced by a 0.6 mm diameter cylindrical cortical bone defect at each diaphyseal site and, in an in situ study confirmed our ability to accurately place defects at the desired diaphyseal locations. The resulting model will enable the exploration of cortical bone healing within the context of physiologically engendered mechanical strain.     

First-principles investigation of the interaction of gold and palladium with armchair carbon nanotube

In this paper, we investigate the adsorption mechanisms at the interface between carbon nanotubes and metal electrodes that can influence the Schottky barrier (SB). We developed a theoretical model based on the first-principles density functional theory for the interaction of an armchair single-wall carbon nanotube (SWNT) with either Au(111) or Pd(111) surface. We considered the side-wall contact by modelling the full SWNT as well as the end-contact geometry using the graphene ribbon model to mimic the contact with very large diameter nanotubes. Strong interaction has been found for the Pd–SWNT interface where the partial density of states (DOS) shows that d-orbitals of palladium are dominant at the Fermi energy so that the hybrid Pd-orbitals have the correct symmetry to overlap with π-electrons and form covalent bonds. The SWNT can only be physisorbed on the gold surface for which the contribution to the DOS of the d-orbitals is very low. Moreover, the filling of antibonding states makes the Au–SWNT bond unstable. The average and ‘atom to atom’ energy barriers at the interface have been evaluated. The matching of open-edge carbon dimers with metal lattice in the end-contact geometry is more likely for large diameter SWNTs and this makes lower the SB at the interface.     

Effect of balloon pre-dilation on performance of self-expandable nitinol stent in femoropopliteal artery

Balloon pre-dilation is usually performed before implantation of a nitinol stent in a femoropopliteal artery in a case of severe blockage or calcified plaque. However, its effect on performance of the nitinol stent in a diseased femoropopliteal artery has not been studied yet. This study compares the outcomes of stenting with pre-dilation and without it by modelling the entire processes of stent deployment. Fatigue deformation of the implanted stent is also modelled under diastolic–systolic blood pressure, repetitive bending, torsion, axial compression and their combination. Reduced level of stress in the stent occurs after stenting with pre-dilation, but causing the increased damage in the media layer, i.e. the middle layer of the arterial wall. Generally, pre-dilation increases the risk of nitinol stent’s fatigue failure. Additionally, the development of in-stent restenosis is predicted based on the stenting-induced tissue damage in the media layer, and no severe mechanical irritation is induced to the media layer by pre-dilation, stent deployment or fatigue loading.

     

Inhalation vs. aspiration of single-walled carbon nanotubes in C57BL/6 mice: inflammation, fibrosis, oxidative stress, and mutagenesis

Nanomaterials are frontier technological products used in different manufactured goods. Because of their unique physicochemical, electrical, mechanical, and thermal properties, single-walled carbon nanotubes (SWCNT) are finding numerous applications in electronics, aerospace devices, computers, and chemical, polymer, and pharmaceutical industries. SWCNT are relatively recently discovered members of the carbon allotropes that are similar in structure to fullerenes and graphite. Previously, we (47) have reported that pharyngeal aspiration of purified SWCNT by C57BL/6 mice caused dose-dependent granulomatous pneumonia, oxidative stress, acute inflammatory/cytokine responses, fibrosis, and decrease in pulmonary function. To avoid potential artifactual effects due to instillation/agglomeration associated with SWCNT, we conducted inhalation exposures using stable and uniform SWCNT dispersions obtained by a newly developed aerosolization technique (2). The inhalation of nonpurified SWCNT (iron content of 17.7% by weight) at 5 mg/m(3), 5 h/day for 4 days was compared with pharyngeal aspiration of varying doses (5-20 microg per mouse) of the same SWCNT. The chain of pathological events in both exposure routes was realized through synergized interactions of early inflammatory response and oxidative stress culminating in the development of multifocal granulomatous pneumonia and interstitial fibrosis. SWCNT inhalation was more effective than aspiration in causing inflammatory response, oxidative stress, collagen deposition, and fibrosis as well as mutations of K-ras gene locus in the lung of C57BL/6 mice.     

Simultaneous Synthesis of Single-walled Carbon Nanotubes and Graphene in a Magnetically-enhanced Arc Plasma

Carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene attract a deluge of interest of scholars nowadays due to their very promising application for molecular sensors, field effect transistor and super thin and flexible electronic devices^1-4. Anodic arc discharge supported by the erosion of the anode material is one of the most practical and efficient methods, which can provide specific non-equilibrium processes and a high influx of carbon material to the developing structures at relatively higher temperature, and consequently the as-synthesized products have few structural defects and better crystallinity.To further improve the controllability and flexibility of the synthesis of carbon nanostructures in arc discharge, magnetic fields can be applied during the synthesis process according to the strong magnetic responses of arc plasmas. It was demonstrated that the magnetically-enhanced arc discharge can increase the average length of SWCNT ⁵, narrow the diameter distribution of metallic catalyst particles and carbon nanotubes ⁶, and change the ratio of metallic and semiconducting carbon nanotubes ⁷, as well as lead to graphene synthesis ⁸. Furthermore, it is worthwhile to remark that when we introduce a non-uniform magnetic field with the component normal to the current in arc, the Lorentz force along the J×B direction can generate the plasmas jet and make effective delivery of carbon ion particles and heat flux to samples. As a result, large-scale graphene flakes and high-purity single-walled carbon nanotubes were simultaneously generated by such new magnetically-enhanced anodic arc method. Arc imaging, scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman spectroscopy were employed to analyze the characterization of carbon nanostructures. These findings indicate a wide spectrum of opportunities to manipulate with the properties of nanostructures produced in plasmas by means of controlling the arc conditions.     

Molecular dynamics simulation of the folding of single alkane chains with different lengths on single-walled carbon nanotubes and graphene

Chain folding is an important step during polymer crystallization. In order to study the effects of the surface on chain folding, molecular dynamics simulations of the folding of different alkane chains on three kinds of single-walled carbon nanotubes (SWCNTs) and graphene were performed. The folding behaviors of the single alkane chains on these surfaces were found to be different from their folding behaviors in vacuum. The end-to-end distances of the chains were calculated to explore the chain folding. An increasing tendency to fold into two or more stems with increasing alkane chain length was observed. This result indicates that the occurrence and the stability of chain folding are related to the surface curvature, the diameter of the SWCNT, and surface texture. In addition, the angle between the direction of the alkane chain segment and the direction of the surface texture was measured on different surfaces.     

Structural, mechanical and electronic properties of nano-fibriform silica and its organic functionalization by dimethyl silane: a SCC-DFTB approach

Self-consistent-charge density-functional tight-binding (SCC-DFTB) approximated method was employed to investigate the structural, mechanical and electronic properties of the zigzag and armchair nano-fibriform silica (SNTs) and their outer surface organic modified derivatives (MSNTs) with internal radii in the range of 8 to 36 Å. The strain energy curves showed that the nanotubes structures are energetically more stable compared to the respective sheet structures. External hydroxyl dihedral angles in silica nanotubes have small influence, about 0.5 meV.atom^?1, in the strain energy curve tendency of those materials favoring the zigzag chirality. The chemical modification of outer surface of SNTs by dimethyl silane group affects their relative stability favoring the armchair chirality in approximately 2 meV.atom^?1. MSNTs have axial elastic constants, Young’s moduli, determined at the harmonic approximation, around 100 GPa smaller than the respective SNTs. The Young’s moduli of zigzag and armchair SNTs are in the range of 150–195 GPa and 232–260 GPa, respectively. And for the zigzag and armchair MSNTs these values are in the range of 77–89 and 110–140 GPa, respectively. The SNTs and MSNTs were characterized as insulators with band gaps around 8–10 eV.