UV-assisted grafting of polymers: A method towards biocompatible carbon nanotubes

A strategy for covalent grafting of biocompatible polymers onto sidewalls of multi-walled carbon nanotubes (MWNTs) via UV-initiated free-radical polymerization is presented. The effects of the irradiation doze(time) and monomer/MWNTs ratio on the stability of the corresponding aqueous dispersions were investigated. It was found that stable dispersions of MWNTs modified with polyacrylamide, poly(N-isopropylacrylamide), poly[poly(ethylene glycol) methacrylate] and poly(sodium methacrylate) can be obtained by irradiation with UV light for at least 5 min at an irradiation dose rate of 5.7 J/cm² min at a minimum monomer/CNTs ratio of 200:1. Biocompatibility of polymer-modified MWNTs was assessed using the standard MTT-dye reduction assay and compared to pristine MWNTs. As a rule, all polymer-functionalized nanotubes examined in this study were non-cytotoxic up to concentration 150 μg/mL and, remarkably, MWNTs-g-PNIPAAm did not exhibit cytotoxicity even at the highest concentration studied (300 μg/mL). MWNTs modified with stimuli-sensitive polymers underwent a reversible transition from well-dispersed nanotubes in water to precipitate triggered by changes in temperature or pH.     

Synthesis and characterization of pH-responsive single-walled carbon nanotubes with a large number of carboxy groups

A new type of pH-responsive single-walled carbon nanotubes (SWNTs) with a large number of carboxy groups was prepared by in situ grafting polymerization. Through exfoliating the SWNT bundles with sodium dodecylbenzene sulfonate (SDBS), individual SWNTs have been obtained. Due to the existence of SDBS, the individual SWNTs could be readily dispersed in water, then forming stable dispersions. Grafting polymerization of acrylonitrile was performed in micelles of SWNTs to produce polyacrylonitrile funtionalized SWNTs (PAN-SWNTs). Experimental results showed that adsorbing acrylonitrile on the SWNT surfaces plays a key role in the grafting process. After hydrolyzing PAN, polyacrylic acid functionalized SWNTs (PAA-SWNTs) were obtained. The amount of PAA grafted could be controlled by changing the feed ratio of initiator to monomer, and the maximum grafting amount could reach 40 wt%. The solubility of PAA-SWNTs in water could be adjusted by pH, with better solubility at higher pH. The large number of carboxy groups on the SWNT surfaces lends the systems convenient for further modification via amidation or esterfication.     

Amino-functionalized carbon nanotubes for effectively improving the mechanical properties of pre-impregnated epoxy resin/carbon fiber

Pre-impregnated carbon fiber/epoxy resin (CF/epoxy prepreg) gained its popularity for significant stress applications, especially in the aerospace industry, owing to its excellent resistance and low specific mass. However, these CF/epoxy prepregs have a tendency to crack propagation. A solution for the prepregs fragility is the addition of carbon nanotubes (CNTs), especially those functionalized with amino groups, reinforcing the material due to its exceptional mechanical properties. In this work, the influence of the carbon chain length of two different amino-functionalized CNTs from diverse backgrounds (commercial and laboratory growth CNTs) is studied. The nanofillers were added in CF/epoxy prepregs by dry spraying without solvent aid. CNTs' samples were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, and thermogravimetric analysis (TGA), while the composites were analyzed by TGA, dynamic-mechanical analysis, and field emission scanning electron microscopy. The various surface treatment occurred at different levels according to the CNTs background, and all samples exhibited a distinct behavior. These differences were also observed in the composites' thermomechanical performance: CNTs functionalized with larger carbon chain amine presented the best results, with an increase of almost 100% in the storage moduli (E'), confirming the efficiency of amino-functionalized CNTs in the reinforcement of CF/epoxy prepregs.     

Polymer adsorption in the grafting reactions of hydroxyl terminal polymers with multi-walled carbon nanotubes

Melt stirring of non-functional polymers such as poly(ethylene oxide) dimethylether (PEO-Me) and polystyrene (PS-H) with multiwalled carbon nanotubes (MWNTs) in the absence of solvent for 48 h induced a substantial amount of polymer adsorption on the MWNTs. The chloroform extraction of the reaction products using centrifugation yielded black colored solutions exhibiting UV absorbance corresponding to the presence of MWNTs. The adsorption of polymer was confirmed on the surfaces of solvent washed residual and recovered MWNTs from the reactions using thermogravimetric analysis (TGA) and FT-IR spectroscopy. Covalent grafting reactions carried out using hydroxyl-terminated PEO-OH and PS-OH with acid chloride containing MWNTs under identical melt stirring condition produced similar results. The presence of polymer on the residual and recovered MWNTs irrespective of the nature of the terminal groups indicates that the adsorption of polymers poses a problem in accurately determining the grafting efficiency. FT IR spectra of the PEO-g-MWNTs shows a substantial shift in CH stretching vibrations indicating a plausible weak intermolecular interaction with π electrons of the MWNTs.     

Nitrogen-doped carbon nanotubes synthesized with carbon nanotubes as catalyst

Nitrogen-doped carbon (CN_x) nanotubes were synthesized with carbon nanotubes (CNTs) as catalyst by detonation-assisted chemical vapor deposition. CN_x nanotubes exhibited compartmentalized bamboo-like structure. Electron energy loss spectroscopy and elemental mapping studies indicated that the synthesized tubes contained high concentration of nitrogen (ca. 17.3 at.%), inhomogeneously distributed with an enrichment of nitrogen within the compartments. X-ray photoelectron spectroscopy analysis revealed the presence of pyridine-like N and graphitic N incorporated into the graphitic network. The catalytic activity of CNTs for CN_x nanotube growth was ascribed to the nanocurvature and opening edges of CNT tips, which adsorbed C_n/CN species and assembled them into CN_x nanotubes.     

Covalent functionalization of multi-walled carbon nanotube surfaces by conjugated polyfluorenes

The outer surface of multi-walled carbon nanotubes (MWCNTs) was successfully modified with 7-bromo-9,9-dioctylfluorene-2-carbaldehyde by using 1,3-dipolar cycloaddition of azomethine reaction to introduce bromo functional groups. The peripheral bromo functional groups can be used to further react with AB-type monomers through Suzuki polycondensation to afford the PF-functionalized MWCNTs, which are of a cable-like structure. Through covalent connection to MWCNTs, the fluorescence of polyfluorenes was completely quenched by the MWCNTs, indicating a fast photo-induced electron transfer from polyfluorenes to MWCNTs.     

The density of water in carbon nanotubes

In this paper, the density of water confined in carbon nanotubes of different sizes and chirality is calculated. Molecular dynamics is used to simulate the spontaneous filling of the nanotube with water molecules coming from an external bath. Three H₂O filling modes are found and a correlation, which relates the density with the nanotube diameter, is proposed.     

A comparative study on electrochemical co-deposition and capacitance of composite films of conducting polymers and carbon nanotubes

Composite films of carbon nanotubes (CNTs) with polyaniline (PANI), polypyrrole (PPY) or poly[3,4-ethylenedioxythiophene] (PEDOT) were prepared via electrochemical co-deposition from solutions containing acid treated CNTs and the corresponding monomer. In the cases of PPY and PEDOT, CNTs served as the charge carriers during electro-deposition, and also acted as both the backbone of a three-dimensional micro- and nano-porous structure and the effective charge-balancing dopant within the polymer. All the composites showed improved mechanical integrity, higher electronic and ionic conductivity (even when the polymer was reduced), and exhibited larger electrode specific capacitance than the polymer alone. Under similar conditions, the capacitance was enhanced significantly in as-prepared PPY-CNT and PEDOT-CNT films. However, the fresh PANI-CNT film was electrochemically similar to PANI, but PPY-CNT and PEDOT-CNT differed noticeably from the respective polymers alone. In continuous potential cycling tests, unlike the pure polymer and other composite films, PANI-CNT performed much better in retaining the capacitance of the as-prepared film, and the possible cause is analysed.     

A facile gemini surfactant-improved dispersion of carbon nanotubes in polystyrene

Debundling and dispersion of carbon nanotubes (CNTs) are very important for preparation of polymer/CNT nanocomposites. In the present study, a self-prepared gemini surfactant, 6,6′-(butane-1,4-diylbis(oxy))bis(3-nonylbenzenesulfonic acid), is employed to achieve homogeneous and stable dispersion of multi-walled carbon nanotubes (MWNTs) in organic solvent and subsequent polystyrene (PS)/MWNT nanocomposite. Sedimentation, optical microscopy and transmission electron microscopy studies demonstrate that the gemini surfactant can greatly improve the dispersion and stabilization of MWNTs in toluene. Scanning electron microscopic images clearly confirm the homogenous dispersion of individual MWNTs in PS. In addition, desired enhanced electrical conductivity and thermal stability of the nanocomposite relative to those of the neat PS are obtained.     

超支化聚合物共价修饰碳纳米管的制备及应用研究

碳纳米管(CNTs)具有很多优良性能,但由于在聚合物基体分散性差,限制了其广泛应用。超支化聚合物是高度支化的大分子,具有低黏度、高溶解性及含有大量末端基团等特点。利用超支化聚合物共价修饰CNTs,不仅可以提高CNTs在聚合物基体中的分散性,还能使CNTs拥有新的功能。介绍了超支化聚合物共价修饰CNTs的方法,探讨了其在生物医学、纳米材料等方面的应用前景,最后对超支化聚合物共价修饰CNTs的发展提出了建议。     

Easy synthesis of carbon nanotubes with polypyrrole nanotubes as the carbon precursor

An easy synthesis route for carbon nanotubes with polypyrrole nanotubes as a carbon precursor has been developed. Polypyrrole nanotubes were fabricated via a reactive self-degraded template method. Carbon nanotubes were further obtained by pyrolysis of the polypyrrole nanotube at 900 °C under a nitrogen atmosphere. The resultant carbon nanotube structure was found to be amorphous carbon on the basis of XRD, Raman spectra and high-resolution transmission electron microscopy (HRTEM) studies.     

Oxygen functionalization of multiwall carbon nanotubes by Ar/H2O plasma treatment

To increase the applicability of multiwall carbon nanotubes (MWCNTs), oxygen-containing functional groups were introduced on the surfaces of MWCNTs by using microwave-excited Ar/H₂O surface-wave plasma. X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine dependencies of Ar/H₂O gas partial pressure, treatment time and microwave power. The oxygen functionalization of MWCNTs by plasma can be achieved very rapidly, about 10 min. The C-O and O-C═O fractions firstly increase and then decrease with increasing Ar partial pressure. The C-O and O-C═O fractions increase with increasing microwave power from 400 W to 700 W. A slight increase of the R (I_D/I_G ratio) value for the treated MWCNTs indicated disordering in the surface microstructure of MWCNTs coincident with the introduction of surface oxygen. The oxygen-containing groups introduced on the surfaces of MWCNTs by plasma treatment are hydrophilic. The dispersion of plasma treated MWCNTs is therefore improved.     

Dry coating polymer powder particles with deagglomerated carbon nanotubes to improve their dispersion in nanocomposites

Deagglomerated multi-walled carbon nanotubes (MWCNTs) were dry coated onto the surfaces of polyethylene oxide powder particles by magnetic assisted impact coater (MAIC). The deagglomeration of the tightly agglomerated MWCNTs was carried out using two procedures: rapid expansion of supercritical suspension (RESS) of MWCNTs and CO₂, and high-intensity ultrasonic agitation of a suspension of MWCNTs in acetone, using an ultrasonic probe. FESEM images show that the high-intensity ultrasonic probe was more effective in deagglomerating the agglomerated MWCNTs than the RESS method. Furthermore, it was found that the extent of PEO particle surface coverage by MWCNTs greatly depends on the extent of deagglomeration.     

Polymer-grafted multi-walled carbon nanotubes through surface-initiated ring-opening polymerization and click reaction

Multi-walled carbon nanotubes (MWNTs) are modified to possess the hydroxyl groups and are used as coinitiators to polymerize poly(ε-caprolactone) (PCL) or poly(α-chloro-ε-caprolactone) (PαClCL) by surface-initiated ring-opening polymerization. Pendent chlorides were converted into azides by the reaction with sodium azides. Finally, various types of terminal alkynes were reacted with pendent azides by copper-catalyzed Huisgen’s 1,3-dipolar cycloaddition (click reaction). Chemical structure of resulting product and the quantities of grafted polymer were determined by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy (HR-TEM) images clearly indicate that the nanotubes were coated with a polymer layer. The MWNT-g-PCLs and MWNT-g-(PαN₃CL-g-alkyne)s are well dispersed in the organic solvent. The dispersability of MWNTs with grafted organic moieties is easier in CHCl₃ than in THF. The average thickness of the enwrapped polymer layer is approximately 8-10 nm for MWNT-g-PCL and 3 nm for MWNT-g-(PαN₃CL-g-PBA).     

The fine dispersion of functionalized carbon nanotubes in acrylic latex coatings

Nanocomposites of a polymer and carbon nanotubes exhibit high electrical and thermal conductivity and enhanced mechanical properties in comparison to the polymer alone. Film formation from latex dispersions is an ideal way to create nanocomposite coatings with the advantages of solvent-free processing and a high uniformity of dispersion. It is shown here that carbon nanotubes functionalised with poly(vinyl alcohol) (PVA) can be blended with two types of acrylic latex to create stable colloidal dispersions without the need for added surfactant or emulsifier. Waterborne nanocomposite films with optical transparency can be formed. Microscopic analysis shows that the PVA-functionalized nanotubes are finely and uniformly dispersed in the polymer matrix.     

Decoration of carbon nanotubes with chitosan

In this letter, a non-destroyable surface decoration of carbon nanotubes with biopolymer chitsoan via a controlled surface-deposition and crosslinking process is described. The method utilizes the emulsifying capacity of chitosan, the completely different water-solubility of chitosan in acidic and basic solutions, and the crosslinking reaction among chitosan polymers. As the pristine structures of the carbon nanotubes are not recomposed under those treatments, the unique properties of the pristine carbon nanotubes have not been compromised. Combining the properties of carbon nanotubes and the versatility and biocompatibility of chitosan, these chitosan surface-decorated carbon nanotubes could find potential applications in biosensing, gene and drug delivering as well as other chemical and biological applications.     

Multi-step purification of carbon nanotubes

An efficient purification procedure for multi-walled carbon nanotubes (MWNTs) synthesized by the floating catalyst method was discussed. The process involves ultra-sonication, heat treatment in hot water, bromination, oxidation and acid treatment. Most of amorphous carbon, multishell carbon nanocapsules as well as metal particles were successfully removed from the MWNT product. With this procedure, MWNTs with purity of more than 94% were obtained and the yield could approach 50%. It was found that bromination took an important role in the purification of MWNTs. Transmission electron microscopy, XPS and thermo-gravimetric analysis were used to evaluate the purification process of MWNTs. The mechanism of bromination on purification of the MWNTs was also discussed.     

The use of noncovalently modified carbon nanotubes for preparation of hybrid polymeric composite materials with electrically conductive and lightning resistant properties

In this work, approach to use of noncovalently modified carbon nanotubes is given for preparation of functional hybrid polymeric composite materials (HPCM) based on epoxy resin. Conductive glass‐fiber plastics with resistivity in transverse and lengthwise direction 9.0·× 10² and 30–50 Ohm cm, respectively, were obtained. The tetrafluoroethylene telomer and fluorocontaining organosilicon copolymer with amino groups were used as modifiers for carbon nanotubes. Thermal, electrical, and mechanical properties of the obtained materials were studied. The mechanism of the effect of noncovalent modification of carbon nanotubes on functional properties of HPCM was discussed. It was found, that type of modifier significantly affects the level of functional properties. The use of fluorocontaining organosilicon copolymer is more optimal in comparison with tetrafluoroethylene telomer. Thus, HPCM with carbon‐fiber filler and this modifier has higher electrical conductivity and lightning strike resistance in comparison with nonmodified HPCM. This approach is promising to impart antistatic properties for glass‐fiber plastics and increase lightning resistance of carbon‐fiber plastics. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46108.     

Modification of carbon nanotubes with well-controlled fluorescent styrene-based polymers using the Diels-Alder reaction

A simple and single-step “grafting to” approach based on the Diels-Alder (DA) reaction is described to functionalize multi-wall carbon nanotubes (MWCNTs) with polystyrene (PSt). Thus, several fluorescent and furfuryl functionalized PSts, synthesized by the atom transfer radical polymerization (ATRP) of styrene, furfuryl methacrylate and low proportion of a fluorescent monomer, were covalently attached onto the pristine nanotubes. Furfuryl (diene) groups allowed the chemical attachment of the PSt polymer onto the as-synthesized MWCNTs by a DA reaction. On the other hand, the incorporation of fluorescent groups in the polymer has two main advantages. It permits to determine the attachment of the polymer onto the nanotubes and, in further applications, it will allow to follow the dispersion of these modified MWCNTs in a matrix. The efficiency of the functionalization was verified by FTIR, Raman spectroscopy, TEM, AFM and fluorescence techniques.