Adsorption of single-ringed N- and S-heterocyclic aromatics on carbon nanotubes

Adsorption of single-ringed N- and S-heterocyclic aromatics on single-walled carbon nanotubes (SWCNTs) was examined to explore the potential of using carbon nanotubes (CNTs) as drug carriers and environmental adsorbents. Adsorbates included pyrimidine, 2-aminopyrimidine, 4,6-diaminopyrimidine, thiophene, benzene and aniline. Adsorbents included pristine SWCNTs, oxidized SWCNTs, and nonporous graphite. Adsorption of N- and S-heterocyclic aromatics was significantly enhanced by non-hydrophobic interactions. Particularly, the -NH₂-substituted compounds exhibited much stronger (up to 2 orders of magnitude) adsorption affinities to oxidized SWCNTs than benzene, even though they are much less hydrophobic. The π-π coupling or electron donor-acceptor (EDA) interactions are likely adsorption-enhancement mechanisms for all six compounds. The lone-pair electrons of the N heteroatoms and the -NH₂ group can enable n-π EDA interactions with SWCNT surfaces. Lewis acid-base interactions are another significant adsorption-enhancement mechanism for the -NH₂-substituted compounds (and possibly for pyrimidine) on SWCNTs. For the N-heterocyclic aromatics, adsorption affinity is highly dependent on the O-functionality of the SWCNT surface and on solution pH, due to the speciation reactions of both adsorbates and SWCNT surface O-functional groups, indicating that selective adsorption of N-heterocyclic aromatics is possible by combining the surface functionality of CNTs and solution chemistry.     

Evidence of sidewall covalent functionalization of single-walled carbon nanotubes and its advantages for composite processing

Single-walled carbon nanotubes (SWCNTs) were functionalized in a three-step procedure. The first step is a radical reaction creating a covalent bond between the carbon nanotube surface and grafted p-methoxyphenyl functional groups. In a second step, a deprotection of the methoxy functions generates free alcohol groups and in the final step an esterification is done in order to install a double bond for further polymerization. Evidence that functionalization has actually occurred on the SWCNT sidewalls is furnished through investigations involving several complementary techniques (visual dispersion tests, transmission electron microscopy, thermal gravimetric analysis and adsorption volumetry). We show that surface properties of SWCNTs are changed throughout the chemical treatments and that the obtained level of functionalization is low. Incorporation of functionalized SWCNTs in a polymer (poly(methyl methacrylate)) matrix was done through an in situ polymerization process. Observations of the obtained composites using scanning and transmission electron microscopy illustrate that interactions between the SWCNT surface and the polymer matrix are improved.     

Effect of surface properties of silica nanoparticles on their cytotoxicity and cellular distribution in murine macrophages

In this study, complexes composed of poly-l-tyrosine (pLT) and single-walled carbon nanotubes (SWCNTs) were produced and the dispersibility of the pLT/SWCNT complexes in water by measuring the ζ potential of the complexes and the turbidity of the solution were investigated. It is found that the absolute value of the ζ potential of the pLT/SWCNT complexes is as high as that of SWCNTs modified with double-stranded DNA (dsDNA) and that the complexes remain stably dispersed in the water at least for two weeks. Thermogravimetry analysis (TGA) and visualization of the surface structures of pLT/SWCNT complexes using an atomic force microscope (AFM) were also carried out.     

Synthesis and Characterization of the Nitrogen‐Rich Hyperbranched Polymers – Poly([1,2,3]‐Triazole‐[1,3,5]‐Triazine)s

Novel hyperbranched poly([1,2,3]‐triazole‐[1,3,5]‐triazine)s (HBP TT) were synthesized by a 1,3‐dipolar cycloaddition reaction from AB₂ monomer – 2‐azido‐4,6‐bis‐prop‐2‐yn‐1‐yloxy‐ [1,3,5]‐triazine (ABPOT). The monomer contains one azide group A and two terminal alkyne units B. Thermal polymerization of ABPOT in bulk or in DMF solution leads to hyperbranched polymers containing both 1,4‐ and 1,5‐disubstituted [1,2,3]‐triazoles. The monomer was also polymerized catalytically in the presence of Cu(I) salts under mild reaction conditions in DMSO solution and in bulk affording hyperbranched poly‐[1,2,3]‐triazoles 1,4‐disubstituted only. The reactions lead to the products soluble in aprotic polar solvents like DMSO or DMF. Side reactions can proceed in a few cases, particularly: (i) homocoupling of alkyne groups, leading to the formation of insoluble products as a result of cross‐linking, (ii) isomerization of propynyloxytriazine fragments to propynyl‐ or propadienyltriazinone ones, and (iii) hydrolysis of triple bonds without the loss of solubility. Heats of formation of monomer and synthesized polymers were calculated from their combustion heats. All products were characterized by NMR‐, IR‐spectroscopy, and size exclusion chromatography (SEC) data. The obtained results open the prospect for the use of HBP TT as the high‐enthalpy modifiers for energetic and non‐energetic binders.     

Advanced carbon nanotube/polymer composite infrared sensors

We demonstrate that both single-walled carbon nanotube (SWCNT) types and nanotube-matrix polymer-nanotube (CNT-P-CNT) junctions have profound impact on electro-optical properties of SWCNT/polymer composites. Composite IR sensors based on CoMoCAT^®-produced SWCNTs (SWCNTs_CoMoCAT) significantly outperform those based on HiPco^®-produced SWCNTs (SWCNTs_HiPco). Higher semiconducting nanotube concentration in a SWCNT material is critical to enhance the photo effect of IR light on SWCNT/polymer nanocomposites, whereas CNT-P-CNT junctions play a dominant role in the thermal effect of IR light on supported SWCNT/polymer composite films.     

The influence of single-walled carbon nanotube functionalization on the electronic properties of their polyaniline composites

The “in situ” preparation and characterization of composites of polyaniline (PANI) and single-walled carbon nanotubes (SWCNTs) are reported. To improve the dispersion and compatibility with the polymer matrix the raw SWCNTs were modified following different routes. SWCNTs oxidized by chemical or thermal treatments (nitric acid and air oxidation, respectively) were subjected to covalent functionalization with octadecylamine (ODA). SWCNT/PANI composites were prepared either from just oxidized SWCNTs, or from ODA functionalized SWCNTs. Temperature-programmed desorption, elemental analyses, ultraviolet-visible (UV-vis), UV-vis with near infrared and Raman spectroscopy, X-ray diffraction, scanning and transmission electron microscopy and conductivity measurements were used to characterize the functionalized SWCNT materials, dispersions and composites. The PANI composite prepared from air oxidized SWCNTs showed the best electrical conductivity indicating a better interaction with polyaniline than ODA functionalised SWCNTs. The improvement of conductivity is attributed to the doping effect or charge transfer of quinoide rings from PANI to SWCNTs.     

Fabrication of high strength PVA/SWCNT composite fibers by gel spinning

High-strength composite fibers were prepared from polyvinyl alcohol (PVA) (Degree of polymerization: 1500) reinforced by single-walled carbon nanotubes (SWCNTs) containing few defects. The SWCNTs were dispersed in a 10 wt.% PVA/dimethylsulfoxide solution using a mechanical homogenizer that reduced the size of SWCNT aggregations to smaller bundles. The macroscopically homogeneous dispersion was extruded into cold methanol to form fibers by gel spinning followed by a hot-drawing. The tensile strength of the well-oriented composite fibers with 0.3 wt.% SWCNTs was 2.2 GPa which is extremely high value among PVA composite fibers ever reported using a commercial grade PVA. The strength of neat PVA fibers prepared by the same procedure was 1.7 GPa. Structural analysis showed that the PVA component in the composite fibers possessed almost the same structure as that of neat PVA fibers. Hence a small amount of SWCNTs straightforward enhanced by 0.5 GPa the tensile strength of PVA fibers. The results of mechanical properties and Raman spectra for the SWCNT composites suggest the relatively good interfacial adhesion of the nanotubes and PVA that improves the load transfer from the polymer matrix to the reinforcing phase.     

Surfactant-free assembling of functionalized single-walled carbon nanotube buckypapers

The electrical and textural properties of single-walled carbon nanotube buckypapers were tunned through chemical functionalization processes. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized with three different chemical groups: Carboxylic acids (-COOH), benzylamine (-Ph-CH₂-NH₂), and perfluorooctylaniline (-Ph-(CF₂)₇-CF₃). Functionalized SWCNTs were dispersed in water or dimethylformamide (DMF) by sonication treatments without the addition of surfactants or polymers. Carbon nanotube sheets (buckypapers) were prepared by vacuum filtration of the functionalized SWCNT dispersions. The electrical conductivity, textural properties, and processability of the functionalized buckypapers were studied in terms of SWCNT purity, functionalization, and assembling conditions. Carboxylated buckypapers demonstrated very low specific surface areas (< 1 m²/g) and roughness factor (R_a = 14 nm), while aminated and fluorinated buckypapers exhibited roughness factors of around 70 nm and specific surface areas of 160-180 m²/g. Electrical conductivity for carboxylated buckypapers was higher than for as-grown SWCNTs, but for aminated and fluorinated SWCNTs it was lower than for as-grown SWCNTs. This could be interpreted as a chemical inhibition of metallic SWCNTs due to the specificity of the diazonium salts reaction used to prepare the aminated and fluorinated SWCNTs. The utilization of high purity as-grown SWCNTs positively influenced the mechanical characteristics and the electrical conductivity of functionalized buckypapers.     

Increasing the thermoelectric power generated by composite films using chemically functionalized single-walled carbon nanotubes

We prepared and characterized flexible thermoelectric (TE) materials based on thin films of single-walled carbon nanotube (SWCNT) composites with polyvinylalcohol. While pristine SWCNTs incorporated in a polymer matrix generated a p-type TE material, chemical functionalization of SWCNTs by using polyethyleneimine produced an n-type TE material. TE modules made of both p- and n-type composite were fabricated to demonstrate TE voltage and power generation. A single p–n junction made of two composite strips containing 20 wt.% of SWCNTs generated a high TE voltage of 92 μV per 1 K temperature gradient (ΔT). By combining five electrically connected p–n junctions an output voltage of 25 mV was obtained upon the applying ΔT = 50 K. Furthermore, this module generated a power of 4.5 nW when a load resistance matched the internal module resistance of 30 kΩ. These promising results show the potential of TE energy conversion provided by the SWCNT composite films connected in scalable modules for applications that require light weight and mechanical flexibility.     

Preparation of single-walled carbon nanotube/TiO2 hybrid atmospheric gas sensor operated at ambient temperature

Single-walled carbon nanotubes (SWCNTs)/TiO₂ hybrid gas sensors operated at a room temperature were fabricated. SWCNTs were stabilized on a Si substrate with interdigitated Pt-electrodes to prepare a gas sensor. Sensing properties of the gas sensor were measured in various concentrations of NO gas. Resistance of the prepared SWCNT based gas sensor decreased with increase of NO gas concentration due to electron transfer from p-type SWCNTs to NO molecules. The SWCNT gas sensor showed high sensitivity and rapid response to the test gas. The hybrid gas sensor using SWCNTs doped with anatase TiO₂ nano-particles was developed, which could work at room temperature under UV-LED (λ = 377 nm) irradiation. It showed rapid recovery to the initial state and higher sensitivity than the SWCNT gas sensor due to TiO₂ photocatalytic effect.     

Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes

A new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with well-dispersed SWCNTs. For example, a SWCNT/PS composite with 0.5 wt% nanotubes made by this coated particle process (CPP) has an electrical conductivity of ∼3 × 10⁻⁴ S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ∼10⁻⁸ S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that the composites made by this new CPP are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common approach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT/polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymers and nanoparticles.     

Morphology of composite particles of single wall carbon nanotubes/biodegradable polyhydroxyalkanoates prepared by spray drying

Spray drying was investigated as a strategy for producing single wall carbon nanotube (SWCNT)/polymer composites. The spray-drying method produced SWCNT/poly(3-hydroxybutyrate) and SWCNT/poly(3-hydroxyoctanoate) composite particles in which the SWCNTs have been trapped in a well-dispersed state throughout the polymer matrix. Increasing SWCNT content in the composite led to a change in particle morphology from spherical and smooth to rosette shape with angular distortions. The technique shows potential for bulk carbon composite fabrication.     

Orientation of carbon nanotubes in polypropylene melt

The correlation between shear stress and the orientation of single‐walled carbon nanotubes (SWCNTs) in an SWCNT/polypropylene composite during the melt process was investigated. Highly oriented composite fibers were produced by extruding the polypropylene melt using a capillary rheometer. The experimental range of shear rates covered those of common polymer melt‐shaping processes. The effect of functionalization of the SWCNTs on orientation was also investigated. Polarized Raman spectroscopy was used to analyze the orientation of the SWCNTs. A high degree of SWCNT orientation was observed under high shear stress, and the functionalized SWCNTs induced a higher degree of orientation than did pristine SWCNTs. The existence of a critical shear stress was observed for the orientation of the SWCNTs, and their orientation was found to occur more efficiently above this critical shear stress. The crystallization temperature and heat of fusion were characterized using a differential scanning calorimeter, and both parameters were observed to increase with the incorporation of SWCNTs. © 2012 Society of Chemical Industry     

Assembly of untreated single-walled carbon nanotubes at a liquid-liquid interface

Untreated single-walled carbon nanotubes (SWCNTs) were assembled at a liquid-liquid interface to form an ultrathin film. The SWCNTs were dispersed into water using sodium dodecyl sulfate (SDS) as a solubilizing agent. Then, hexane was added to the dispersion to create a liquid-liquid interface. The SWCNTs were assembled at the interface to form a smooth ultrathin film when ethanol was added to the SWCNT water dispersion/hexane solution. The assembly mechanism was considered to be caused by the decreased wettability of SDS-coated SWCNT during the addition of ethanol because of desorption of SDS from the SWCNT surface. The assembly was remarkably robust and easily transferable to substrates. An AFM image of the film transferred onto a silicon substrate shows a closely packed uniform film of 3-8 nm thickness. The SWCNT ultrathin film showed high transparency of ca. 97% with an electrical conductivity of 71.4 S/cm. Fabrication processing was carried out in ambient conditions, thereby making it an attractive application for use in flexible electric devices.     

Phenolic Hyperbranched Polymers as Additives in Cationic Photopolymerization of Epoxy Systems

Summary: A phenolic group containing hyperbranched polyester (HBP) was synthesized and employed as chain transfer agent in cationic photopolymerization of a biscycloaliphatic epoxy monomer ( CE ). The epoxy group conversion increases by increasing the amount of HBP in the photocurable resin, due to a chain transfer reaction involving the phenolic‐OH groups. HBP acts as a plasticizer inducing decrease of the T_g values together with an increase of the toughness properties. Meanwhile gel content increases together with the E′ values. By increasing the amount of HBP in the photocurable resin an increase of the density is evident indicating a decrease of free volume. Therefore an improvement of the gas barrier properties might be expected; at the same time an increase of the thermal stability is evident.

     

Effects of preparation method on thermoelectric properties of poly(aniline-co-3,4-ethylenedioxythiophene)/SWCNT composites

Composites of conducting polymer and single-walled carbon nanotubes (SWCNTs) are attracting great attentions in harvesting low-grade waste heat. Prefabricated SWCNTs film used as the working electrode was placed at the liquid interface between the inorganic phase (dilute sulfuric acid solution) and the organic phase consisting of dichloromethane (DCM), aniline (ANI), and 3,4-ethylenedioxythiophene (EDOT), together with a platinum wire (the counter electrode) and a silver chloride (AgCl/Ag) electrode (the reference electrode), to perform electrochemical polymerization of ANI and EDOT at the liquid interface. Thermoelectric (TE) composites of poly(ANI-co-EDOT) and SWCNTs were produced. Compared with composites from ultrasonic mixing and coating methods, the 10 wt% SWCNTs-composites in situ formed in electrochemical polymerization have the highest power factor (PF) of 41.56 ± 3.58 μW m⁻¹ K⁻², higher than the PF values of the composites formed by other two methods. The work indicates that the TE properties of ANI-EDOT copolymer/SWCNT (poly[ANI-co-EDOT]/SWCNT) composites prepared by electrochemical polymerization were better than those of the composites obtained by physical mixing the electrochemically synthesized poly(ANI-co-EDOT) with SWCNTs. Moreover, SWCNTs treated with sodium dodecylbenzene sulfonate (SDBS) could further improve the TE properties of the composites.     

3D microfabrication of single-wall carbon nanotube/polymer composites by two-photon polymerization lithography

We present a method to develop single-wall carbon nanotube (SWCNT)/polymer composites into arbitrary three-dimensional micro/nano structures. Our approach, based on two-photon polymerization lithography, allows one to fabricate three-dimensional SWCNT/polymer composites with a minimum spatial resolution of a few hundreds nm. A near-infrared femtosecond pulsed laser beam was focused onto a SWCNT-dispersed photo resin, and the laser light solidified a nanometric volume of the resin. The focus spot was three-dimensionally scanned, resulting in the fabrication of arbitrary shapes of SWCNT/polymer composites. SWCNTs were uniformly distributed throughout the whole structures, even in a few hundreds nm thick nanowires. Furthermore, we also found an intriguing phenomenon that SWCNTs were self-aligned in polymer nanostructures, promising improvements in mechanical and electrical properties. Our method has great potential to open up a wide range of applications such as micro- and nanoelectromechanical systems, micro/nano actuators, sensors, and photonics devices based on CNTs.     

Doping single-walled carbon nanotubes with surfactant peptides containing electron-donor substituents and nitrogen heterocycles

We examined the potential of a series of aromatic moieties with different electron-donating ability to alter SWCNT electronic properties. The selected aromatic moieties included p-amino-phenylalanine, 3-[4-(dimethylamino)phenyl]propanoic acid, tyrosine, tryptophan, and 4-pyridylalanine. These moieties contained exocyclic electron-donor substituents, such as amine, dimethylamine, and hydroxyl groups, as well as nitrogen-containing heterocycles, including indole and pyridine. We attached these aromatic molecules to the N-terminus of an amphiphilic surfactant peptide and obtained stable aqueous peptide/SWCNT dispersions. Atomic force microscopy images and ultraviolet–visible–near-infrared spectra revealed that, under the conditions used, all peptides, except for the one with 4-pyridylalanine, disperse SWCNTs well in solution. The local electron density of states of the peptide-coated SWCNTs was examined using scanning tunneling spectroscopy dI/dV plots. All spectra showed an additional peak in the conduction band side very close to the Fermi level, indicating n-type doping of the SWCNTs. The doping behavior was further verified by Raman spectroscopy G-band peak downshifts. Additionally, we found that the location of the heteroatom contributes significantly to the interaction between the aromatic moieties and the SWCNTs.     

Chirality fingerprinting and geometrical determination of single-walled carbon nanotubes: Analysis of fine structure of X-ray diffraction pattern

Single-walled carbon nanotubes (SWCNTs) are seamless cylindrical tubes consisting of carbon atoms with diameters ranging from less than one nanometer to a few nanometers. The arrangement of carbon atoms in a SWCNT is uniquely specified by using a pair of integers (n, m) referred to as the chiral indices. While the detailed structures, such as a carbon–carbon bond length, should be important, they have not been fully clarified yet. In this work, we examine the possibility of powder X-ray diffraction (XRD) method to characterize structures of SWCNTs. It is found that the XRD is a useful tool to “fingerprint” the chiral indices of bulk SWCNT samples. Besides, we find that information on the detailed structure within a SWCNT can be obtained from the XRD pattern. The application to a highly concentrated SWCNTs clarifies that the (6,5) SWCNT is expanded along the radial direction compared to that of ideal rolling up structure of graphene, with a negligible change along the tube axis.     

Poly(vinyl chloride)/single wall carbon nanotubes composites: Investigation of mechanical and thermal characteristics

Composites of polyvinylchloride (PVC) with single wall carbon nanotubes (SWCNTs) were prepared by plastisol curing. Scanning electron microscopy (SEM) observations revealed that appropriate dispersion of the nanotubes was achieved. The mechanical properties showed that SWCNT improved the Young's modulus and tensile strength of the PVC. The composites have higher elongation at break and toughness as well. By comparing the mechanical properties of the composites, it is found that there is a critical SWCNT loading (about 1 wt%) below which the tensile properties increase with increasing nanofiller concentration. For the composites containing 0.25–0.75 wt% of SWCNT, this situation was observed, whereas for a sample with 1 wt% SWCNT, the mechanical properties decreased due to the agglomeration of the nanotubes. Thermogravimetric analysis indicated that the SWCNT increased T_5%, T_10%, T_50%, T_onset, and T_max and decreased weight loss in the degradation process of the PVC. In addition, by adding SWCNT to the polymer, residual mass at 600°C increased significantly. These results are advantages for the applications of the polymer in which high mechanical properties, including high tensile modulus and toughness, and good thermal properties are needed. J. VINYL ADDIT. TECHNOL., 22:128–133, 2016. © 2014 Society of Plastics Engineers