Experimental and Computational Investigations of the Properties of Fluorinated Single‐Walled Carbon Nanotubes

Fluorination of single‐walled carbon nanotubes by reaction with elemental fluorine at elevated temperatures provides fluorinated single‐walled carbon nanotubes (F‐SWNT), which have the highest degree of functionalization (up to F/C=1/2) of any derivatized carbon‐nanotube material reported to date. Also, F‐SWNTs have received more scrutiny than any other functionalized carbon nanotubes. This Minireview covers experimental and computational investigations of F‐SWNTs with a focus on the nature and the strength of the C–F linkage.     

Dispersion of Carbon Nanotubes in Solutions of Oxyethylated Isononylphenols

Nonionic surfactants added in concentrations significantly exceeding the critical micelle concentration in water increase by a factor of 1.5–2 the content of nanotubes in the dispersion volume upon ultrasonic dispersion of carbon nanotubes and enhance the stability of the system. Homologs with the mean degree of oxyethylation n = 10–12 exhibit the strongest dispersing and stabilizing properties. The electrokinetic properties of carbon nanotube dispersions are influenced by the concentration and degree of oxyethylation of the surfactants. The dispersions obtained can be used for modification of butadiene–styrene latexes and vulcanized rubbers based on them.     

The effect of carbon nanotubes on epoxy matrix nanocomposites

The paper concerns thermal properties of epoxy/nanotubes composites for aircraft application. In this work, influence of carbon nanotubes on thermal stability, thermal conductivity, and crosslinking density of epoxy matrix was determined. Three kinds of nanotubes were used: non-modified with 1- and 1.5-μm length, and 1-μm length modified with amino groups. Scanning electron microscopy observations were done for examining dispersion of nanotubes in the epoxy matrix. Glass transition temperature (T _g) was readout from differential scanning calorimetry. From dynamic mechanical analysis, crosslinking density was calculated for epoxy and its composites. Also, thermogravimetric analysis was done to determine influence of nanotubes addition on thermal stability and decomposition process of composites. Activation energy was calculated from TGA curves by Flynn–Wall–Ozawa method. Thermal diffusivity was also measured. SEM images proved the uniform dispersion of carbon nanotubes without any agglomerates. It was found that nanotubes modified with amino groups lead to the increase of epoxy matrix crosslinking density. The significant increase in T _g was also observed. On the other hand, addition of carbon nanotubes leads to the decrease of thermal stability of polymer due to the increase of thermal diffusivity.     

Coupled thermogravimetry, mass spectrometry, and infrared spectroscopy for quantification of surface functionality on single-walled carbon nanotubes

We have successfully applied coupled thermogravimetry, mass spectrometry, and infrared spectroscopy to the quantification of surface functional groups on single-walled carbon nanotubes. A high-purity single-walled carbon nanotube sample was subjected to a rapid functionalization reaction that attached butyric acid moieties to the nanotube sidewalls. This sample was then subjected to thermal analysis under inert desorption conditions. Resultant infrared and mass spectrometric data were easily utilized to identify the desorption of the butyric acid groups across a narrow temperature range and we were able to calculate the degree of substitution of the attached acid groups within the nanotube backbone as 1.7 carbon atoms per hundred, in very good agreement with independent analytical measurements made by inductively coupled plasma optical emission spectrometry (ICP-OES). The thermal analysis technique was also able to discern the presence of secondary functional moieties on the nanotube samples that were not accessible by ICP-OES. This work demonstrates the potential of this technique for assessing the presence of multiple and diverse functional addends on the nanotube sidewalls, beyond just the principal groups targeted by the specific functionalization reaction.      

Modification of the surface of carbon fibers with multi-walled carbon nanotubes and its effect on mechanical characteristics of composites with epoxy resin

Effect of the catalyst composition on the structure of nanotubes layers obtained on the surface of carbon nanofibers was studied. We found the preliminary functionalization of the surface of carbon fibers to affect the coating uniformity and the thickness of synthesized nanotube layer. We determined the optimal surface concentration of the catalyst (Fe–Co) which provides uniform layer of nanotubes on the surface of carbon fibers. The effect of modification of the surface of carbon fibers with multi-walled carbon nanotubes on the mechanical properties of carbon fiber–epoxy resin composites was examined. The modification of the carbon fibers with multi-walled carbon nanotubes were shown to increase the flexural modulus and the flexural strength.     

Overcoming the insolubility of carbon nanotubes through high degrees of sidewall functionalization

The use of carbon nanotubes in materials applications has been slowed due to nanotube insolubility and their incompatibility with polymers. We recently developed two protocols to overcome the insoluble nature of carbon nanotubes by affixing large amounts of addends to the nanotube sidewalls. Both processes involve reactions with aryl diazonium species. First, solvent-free functionalization techniques remove the need for any solvent during the functionalization step. This delivers functionalized carbon nanotubes with increased solubility in organic solvents and processibility in polymeric blends. Additionally, the solvent-free functionalization process can be done on large scales, thereby paving the way for use in bulk applications such as in structural materials development. The second methodology involves the functionalization of carbon nanotubes that are first dispersed as individual tubes in surfactants within aqueous media. The functionalization then ensues to afford heavily functionalized nanotubes that do not re-rope. They remain as individuals in organic solvents giving enormous increases in solubility. This protocol yields the highest degree of functionalization we have obtained thus far-up to one in nine carbon atoms on the nanotube has an organic addend. The proper characterization and solubility determinations on nanotubes are critical; therefore, this topic is discussed in detail.     

The role of the medium in electrochemical functionalization and dispersion of carbon nanotubes

An electrochemical method for dispersion of single-walled carbon nanotubes (SWNTs) is described. The technique is based on grafting of oxygen-containing functional groups to the nanotube surface during electrolysis in aqueous and nonaqueous potassium bromide solutions. A dependence of the degree of functionalization of nanotubes on the solvent was revealed experimentally. Nanotubes treated in DMSO have about 14 carbon atoms per oxygen atom from functional groups (cf. nearly four C atoms per oxygen atom in the nanotubes treated in aqueous solutions). The corresponding maximum specific capacities of the electrodes are nearly 10 and 60 F g⁻¹. The samples treated in solutions of KBr in DMSO have about 300 carbon atoms per bromine atom on the nanotube surface (cf. only 30 carbon atoms in the samples treated in aqueous solution). A mechanism of electrochemical modification of SWNTs is proposed. Its key step is production of atomic oxygen that oxidizes the nanotube surface with the formation of functional groups.     

Effects of noncovalently functionalized multiwalled carbon nanotube with hyperbranched polyesters on mechanical properties of epoxy composites

In order to improve the dispersibility and interface properties of multi-walled carbon nanotubes (MWCNTs) in epoxy resin (EP), aromatic hyperbranched polyesters with terminal carboxyl (HBP) and aromatic hyperbranched polyesters with terminal amino groups (HBPN) were used for noncovalent functionalization of MWCNTs. Epoxy composites reinforced by different types of MWCNT were prepared. The effects of noncovalent functionalization of MWCNTs on the dispersibility, wettability, interface properties and mechanical properties of epoxy composites were investigated. The results show that the dispersibility and wettability of MWCNTs are significantly improved after noncovalent functionalization. A large number of terminal primary amines (NH₂) on noncovalently functionalized MWCNT with HBPN (HBPN-MWCNT) form covalent bonds with EP matrix, and thus the interfacial adhesion is enhanced significantly, resulting in high load transfer efficiency and substantial increase in mechanical properties. The interface with covalent bonding formed between the flexible hyperbranched polyester layer on the surface of HBPN-MWCNT and the EP matrix promotes plastic deformation of the surrounding EP matrix. The toughening mechanisms of HBPN-MWCNT are MWCNT pull-out and a large amount of plastic deformation of the surrounding EP matrix.     

Synthesis of Filamentary Carbon Material on a Self-Organizing Ni–Pt Catalyst in the Course of 1,2-Dichloroethane Decomposition

A series of model microdisperse Ni_1–xPt _x alloys (x =0–0.05) was synthesized by a coprecipitation method with the subsequent sintering of the precipitate in an atmosphere of H₂ at 800°C. Their chemical and phase compositions were determined (by AAS and XRD analysis, respectively), and it was found that the synthesis method proposed afforded Ni–Pt solid solutions based on the face-centered nickel lattice. The kinetic features of the carbon erosion of Ni_1–xPt _x alloys in their contact with 1,2-dichloroethane vapor in a temperature range of 550–700°C were studied. It was found that the presence of Pt in the alloy increased the rate of accumulation of carbon product by a factor of ~1.5 regardless of the concentration of Pt. The catalyst did not undergo deactivation for 5 h of reaction to ensure a high yield of carbon material (103 g/g_Cat). With the use of electron microscopy (SEM and TEM techniques), it was found that the carbon product consisted of carbon fibers with a segmented structure. An increase in the concentration of Pt in the alloy to 4.3 wt % sharply changed the disintegration of the alloy to cause the formation of carbon product with a bimodal fiber diameter distribution (d_av = 0.4 and 1.2 μm).     

十八醇亲核取代反应修饰溴代多壁碳纳米管

Octadecanol modified multiple-walled carbon nanotubes, with octadecanol covalently bound to the nanotube surface, have been synthesized by bromination of the carbon nanotubes followed by nucleophilic substitution using octadecanol and sodium hydride. Scanning electron microscopy and transmission electron microscopy images show that the morphologies of the nanotubes are largely intact after functionalization. The brominated carbon nanotubes and octadecanol modified carbon nanotubes were characterized using energy-dispersive X-ray spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. The mechanism of nucleophilic substitution was discussed, and it is believed that the reaction occurs with an SN1 mechanism.     

Modification of mixed conducting Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Co<Subscript>0.8</Subscript>Fe<Subscript>0.2</Subscript>O<Subscript>3–δ</Subscript> by partial substitution of cobalt with tungsten

The Ba_0.5Sr_0.5Co_0.8–xW_xFe_0.2O_3–δ (х = 0–0.1) materials prepaMIECred by partial substitution of cobalt in BSCF with tungsten were studied. The tungsten solubility limit in the structure of cubic perovskite BSCF was shown to be ~2%. The doping with the highly charged W⁶⁺ (2%) cation improved the functional properties of BSCF: it increased the oxygen permeability and membrane stability in the CO₂-containing atmosphere and suppressed the cubic–hexagonal perovskite polymorphic transition. This stabilizes high oxygen fluxes during long-term stability tests.     

In Situ polymerization functionalization of single‐walled carbon nanotubes with polystyrene

The in situ polymerization functionalization of single‐walled carbon nanotubes (SWNT) with polystyrene (PS) is demonstrated utilizing stabilized nanotubes reduced by dissolution of excess lithium in ammonia. Short PS chains are tethered to SWNT sidewalls to facilitate a robust compatibilization strategy for nanotube dispersion. To augment extents of functionalization, while maintaining in situ dispersion stability, the effects of multiple monomer addition steps and varied carbon to lithium ratio are studied. The developed functionalization scheme is also effective for the reductive alkylation of SWNT with dodecyl surface groups. By studying the dodecylated SWNT, the molecular weight of grafted PS chains is estimated. The discovery of a general experimental artifact has implications for all functionalization routes utilizing reduction with lithium in ammonia. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3716–3725     

Poly(lactic acid) composites with poly(lactic acid)‐modified carbon nanotubes

This work reports the study of the effect of chemical functionalization of carbon nanotubes on their dispersion in poly(lactic acid). The nanotubes were functionalized by the 1,3‐dipolar cycloaddition reaction, generating pyrrolidine groups at the nanotube surface. Further reaction of the pyrrolidine groups with poly(lactic acid) was studied in solution and in the polymer melt. The former involved refluxing the nanotubes in a dimethylformamide/polymer solution; the latter was carried out by direct melt mixing in a microcompounder. The carbon nanotubes collected after each process were characterized by thermogravimetry and by X‐ray photoelectron spectroscopy, showing evidence of polymer bonded to the nanotube surface only when the reaction was carried out in the polymer melt. The composites with polymer modified nanotubes present smaller average agglomerate area and a narrower agglomerate area distribution. In addition, they show improved tensile properties at low CNT concentration and present lower electrical resistivity. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3740–3750     

Selective adsorption of proteins on single-wall carbon nanotubes by using a protective surfactant

The dispersion of highly hydrophobic carbon materials such as carbon nanotubes in biological media is a challenging issue. Indeed, the nonspecific adsorption of proteins occurs readily when the nanotubes are introduced in biological media; therefore, a methodology to control adsorption is in high demand. To address this issue, we developed a bifunctional linker derived from pyrene that selectively enables or prevents the adsorption of proteins on single-wall carbon nanotubes (SWNTs). We demonstrated that it is possible to decrease or completely suppress the adsorption of proteins on the nanotube sidewall by using proper functionalization (either covalent or noncovalent). By subsequently activating the functional groups on the nanotube derivatives, protein adsorption can be recovered and, therefore, controlled. Our approach is simple, straightforward, and potentially suitable for other biomolecules that contain thio or amino groups available for coupling.     

Polymer/carbon nanotube nanocomposites: Influence of carbon nanotubes on EVA photodegradation

The influence of carbon nanotubes on the photodegradation of EVA/carbon nanotube nanocomposites was studied by irradiation under photooxidative conditions (at λ > 300 nm, at 60 °C and in the presence of oxygen). The influence of the nanotubes on both the photooxidation mechanism of EVA and the rates of oxidation of the matrix was characterized on the basis of infrared analysis. On one hand, it was shown that the carbon nanotubes act as inner filters and antioxidants, which contribute to reduction in the rate of photooxidation of the polymeric matrix. On the other hand, it was shown that light absorption could provoke an increase in the local temperature and then induce the photooxidation of the polymer. The competition between these three effects determines the global rate of photooxidation of the polymeric matrix. Several factors are involved, the concentration of the carbon nanotubes, the morphology of the nanotubes and the functionalization of the nanotube surface.     

Chemical functionalization of carbon nanotubes by carboxyl groups on stone-wales defects: a density functional theory study

Chemical functionalization of carbon nanotubes with Stone-Wales (SW) defects by carboxyl (COOH) groups is investigated by density functional calculations. Due to the localized donor states induced by the SW defect, the binding of the COOH group with the defective carbon nanotube is stronger than that with the perfect one. A quasi-tetrahedral bonding configuration of carbon atoms, indicating sp3 hybrid bonding, is formed in the adsorption site. The charge distribution analysis shows that, in comparison with benzoic acid, the localized or delocalized pi states on the nanotube would affect the polarities of chemical bonds of the COOH group without losing the acidity. Furthermore, it is found that the double-adsorption system (two COOH groups are respectively adsorbed on two individual carbon atoms of the SW defect) is more energetically favorable than the monoadsorption one. The adsorption of COOH groups leads to a significant change of the electronic states around the Fermi level, which is advantageous for the electrical conductivity. The functionalization by introducing functional groups on the topological defects provides a pathway for applications of carbon nanotubes in chemical sensors and nanobioelectronics.     

Multi-walled carbon nanotubes functionalized with triethylenetetramine as fillers to enhance epoxy dimensional thermal stability

Multi-walled carbon nanotubes (MWCNT) have been used as fillers to improve thermal properties such as glass transition temperature (T _g) of epoxy materials. In this work, nanocomposites based on diglycidyl ether of bisphenol A resin and triethylenetetramine (TETA) were prepared by a three-roll mill process with TETA-functionalized (MWCNT–COTETA) and neat MWCNT. Thermogravimetric analysis of the nanofillers showed that in the case of MWCNT–COTETA, there is a 15 % mass loss that can be attributed to –COTETA and residual oxygen-containing functional groups. The influence of chemical modification on the behavior of the glass T _g was evaluated by dynamic scanning calorimetry. The MWCNT–COTETA allowed a ~20 °C reproducible increase of T _g in concentrations in the range of 0.5–1.0 mass%. Furthermore, images obtained by scanning electron microscopy were used to investigate the morphology of the polymer matrix and its interfaces. The quality of the dispersion and interaction of the nanotubes in the epoxy matrix was assessed from the images. Both the neat epoxy and the nanocomposite with MWCNT showed low thermal shrinkage upon curing.     

Hydrogen physisorption energies for bumpy,saturated, nitrogen-doped single-walled carbon nanotubes

Finite saturated regular carbon nanotubes (CNTs) are predicted to exhibit higher capacity as hydrogen storage media compared to unsaturated regular CNTs. In the present study, molecular hydrogen physisorption energies (MHPEs) for finite saturated and unsaturated bumpy defected CNTs were calculated by density functional theory (DFT-D3) methods at the B3LYP/6-31G(d) theory level, with rigorous inclusion of van der Waals interactions. The calculated MHPEs for both regular and bumpy defected armchair, chiral and zigzag CNTs with similar diameters and lengths, with and without nitrogen doping, were compared in terms of Eph/H₂, defined as the MHPE per hydrogen molecule adsorbed inside the nanotube. For all studied systems, Eph/H₂ increased with the number of physisorbed hydrogen molecules. Nitrogen doping of regular and bumpy CNTs resulted in an increase in the Eph/H₂ values, with the exception of bumpy chiral nanotubes. The results of this study demonstrate that bumpy defects are important nanotube structural features whose effects depend on nanotube chirality. For instance, bumpy defects were beneficial for undoped and doped zigzag nanotubes, resulting in a decrease in Eph/H₂ values for regular structures from 0.5 and 0.74 to 0.26 and 0.42 eV, respectively. By contrast, for doped armchair regular structures with an Eph/H₂ value of 0.38 eV, bumpy defects increased Eph/H₂ to 0.45 eV. These Eph/H₂ values for bumpy doped armchair and the zigzag nanotubes are all within the range of 0.1–0.5 eV/H₂ reported as ideal for reversible hydrogen storage under environmental conditions.     

Electronic properties of n-type carbon nanotubes prepared by CF4 plasma fluorination and amino functionalization

Single-walled carbon nanotubes (SWNTs) have been fluorinated by CF4 plasma exposure and further functionalized with 1,2-diaminoethane. The degree of amino functionalization is dependent on the degree of initial fluorination rather than oxygen or carbon defects. Reaction at both ends of 1,2-diaminoethane was observed to increase with fluorine content. Back-gated SWNT devices have shown p-type semiconducting behavior for CF4-functionalized SWNTs and n-type semiconducting behavior for amino-functionalized SWNTs. The degree of n-type behavior increases with the amount of nitrogen attached to the SWNTs.     

1,3-Dioxa-2-silacyclohexane in nanotubes: Conformational transformations and structural features

Using the hybrid DFT method PBE/3ζ conformational transformations of 1,3-dioxa-2-silacyclohexane in the model one-layer carbon nanotubes were investigated. It was found that the effect of the nanotube force field results in shortening of the Si–O bond distances, decrease in the OSiO bond angle and increase in the interconversion barrier between the chair or semi-chair forms, as well as in appearing of the negative charge on the encapsulated molecule.