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.     

Characterization of single-walled carbon nanotube mats and their performance as electromechanical actuators

Carbon nanotube mats (buckypapers) were prepared from three commercial grades of single-walled carbon nanotubes (SWCNTs) and by two processing variants (i.e. filtration of centrifuged and uncentrifuged dispersions). Material properties such as Young’s modulus, tensile strength, electrical conductivity, electrical capacitance, specific surface area and morphology were investigated and put in relation to the in-plane actuation performance in an aqueous electrolyte (1 M NaCl). A dynamic mechanical analyzer was adapted for actuation strain measurements on the samples under various tensile prestress levels. High SWCNT purity and dispersibility were found to be crucial for preparing dense and strong cohesive mats with good actuation performance.     

Multiple “optimum” conditions for Co-Mo catalyzed growth of vertically aligned single-walled carbon nanotube forests

Carbon nanotubes (CNTs) were grown directly on substrates by alcohol catalytic chemical vapor deposition using a Co-Mo binary catalyst. Optimum catalytic and reaction conditions were investigated using a combinatorial catalyst library. High catalytic activity areas on the substrate were identified by mapping the CNT yield against the orthogonal gradient thickness profiles of Co and Mo. The location of these areas shifted with changes in reaction temperature, ethanol pressure and ethanol flow rate. Vertically aligned single-walled CNT (SWCNT) forests grew in several areas to a maximum height of ca. 30 μm in 10 min. A pure Co catalyst yielded a vertically aligned SWCNT forest with a bimodal diameter distribution. The effects of Mo on the formation of catalyst nanoparticles and on the diameter distribution of SWCNTs are discussed and Mo as thin as a monolayer or thinner was found to suppress the broadening of SWCNT diameter distributions.     

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.     

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.     

The production of large scale ultrathin aligned CNT films by combining AC electric field with liquid flow

We report a simple and versatile technique combining the use of an AC electric field with a liquid shear force to prepare ultrathin aligned CNT films on solid substrates. Multiwalled carbon nanotubes (MWCNTs), which were synthesized by a template method and acid-treated single walled carbon nanotubes (SWCNTs) were dispersed in water and used for the ultrathin film fabrication. A solid substrate was immersed in the CNT dispersions and withdrawn at constant speed under AC electric field. SEM images of the substrate showed that CNTs were aligned with the AC electric field and the withdrawal direction and formed uniform films with a thickness around 10 nm for SWCNTs and 90 nm for MWCNTs. Repeating the deposition process increases the density and size of the film while also maintaining nanometer-scale thickness. Unidirectional alignment of CNTs was also confirmed by Raman spectra and electric conductivity measurements. It was found that ultrathin films of aligned SWCNTs exhibited very high anisotropic electrical conductivity with conductivity measured parallel to the alignment direction 3.3 × 10⁵ times higher than that measured in the perpendicular direction. We demonstrate that use of the aligned ultrathin SWCNT film for a unidirectional alignment of liquid crystal.     

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.     

Influence of carbon nanotubes on the thermal, electrical and mechanical properties of poly(ether ether ketone)/glass fiber laminates

Novel poly(ether ether ketone) (PEEK)/single-walled carbon nanotube (SWCNT)/glass fiber laminates incorporating polysulfone as a compatibilizing agent were fabricated by melt-blending and hot-press processing. Their morphology, mechanical, thermal and electrical properties were investigated and compared with the behavior of similar non-compatibilized composites. Scanning electron micrographs demonstrated better SWCNT dispersion for samples with polysulfone. Thermogravimetric analysis indicated a remarkable improvement in the thermal stability of PEEK/glass fiber by the incorporation of SWCNTs wrapped in the compatibilizer, ascribed to a significant thermal conductivity enhancement. Differential scanning calorimetry showed a decrease in the crystallization temperature and crystallinity of the polymer with the addition of both wrapped and non-wrapped SWCNTs. The laminates exhibit anisotropic electrical behavior; their conductivity out-of-plane is lower than that in-plane. Dynamic mechanical studies revealed an increase in the storage modulus and glass transition temperature in the presence of polysulfone. Mechanical tests demonstrated significant enhancements in stiffness, strength and toughness by the incorporation of wrapped nanofillers, whilst the mechanical properties of non-compatibilized composites only improved marginally. Samples with laser-grown SWCNTs exhibit enhanced overall performance. This investigation confirms that SWCNT-reinforced PEEK/glass fiber compatibilized composites possess excellent potential to be used as multifunctional engineering materials in industrial applications.     

Development and characterization of PEEK/carbon nanotube composites

New poly(ether ether ketone) (PEEK) based composites have been fabricated by the incorporation of single-walled carbon nanotubes (SWCNTs) using melt processing. Their structure, morphology, thermal and mechanical properties have been investigated. Scanning electron microscopy observations demonstrated a more uniform distribution of the CNTs for samples prepared following a processing route based on polymer ball milling and CNT dispersion in ethanol media. Thermogravimetric analysis indicated a remarkable improvement in the thermal stability of the matrix by the incorporation of SWCNTs. Differential scanning calorimetry showed a decrease in the crystallization temperature with increasing SWCNT content, whilst no significant changes were observed in the melting of the composites. The crystallite size determined by X-ray diffraction decreased at high SWCNT loading, which is attributed to the spatial limitations on crystal growth by confinement within the CNT network. Dynamic mechanical analysis revealed an increase in the storage moduli, hence in the rigidity of the systems, with increasing SWCNT content. Their addition shifts the glass transition peak to higher temperatures due to the restriction in chain mobility imposed by the CNTs. Higher thermal stability and mechanical strength were found for composites with improved dispersion of the SWCNTs.     

Low electrical conductivity threshold and crystalline morphology of single-walled carbon nanotubes - high density polyethylene nanocomposites characterized by SEM, Raman spectroscopy and AFM

The electrical conductivities (σ) of nanocomposites of single-walled carbon nanotubes (SWCNTs) and high density polyethylene (HDPE) have been studied for a large number of nanocomposites prepared in a SWCNT concentration range between 0.02 and 8 wt%. The values of σ obey a percolation power law with an SWCNT concentration threshold, p_c = 0.13 wt%, the lowest yet obtained for any kind of carbon-polyethylene nanocomposites. Improved electrical conductivities attest to an effective dispersion of SWCNT in the polyethylene matrix, enabled by the fast quenching crystallization process used in the preparation of these nanocomposites. Characterization by scanning electron microscopy (SEM) and Raman spectroscopy consistently points to a uniform dispersion of separate small SWCNT bundles at concentrations near p_c and increased nanotube clustering at higher concentrations. Near p_c, high activation energies and geometries of long isolated rods suggest that electron transport occurs by activated electron hopping between nanotubes that are close to each other but still geometrically separate. The degree of SWCNT clustering given by Raman spectroscopy and the barrier energy for electrical conductivity are highly correlated. The nanotubes act as nucleants in the crystallization of the polyethylene matrix, and change the type of supermolecular aggregates from spherulites to axialitic-like objects. The size of crystal aggregates decreases with SWCNT loading, however, in reference to the unfilled polyethylene, the three-dimensional growth geometry extracted from the Avrami exponents remains unchanged up to 2 wt%. Consistency between SEM, Raman and electrical transport behavior suggests that the electrical conductivity is dominated by dispersion and the geometry of the SWCNT in the nanocomposites and not by changes or lack thereof in the HDPE semicrystalline structure.     

Spectroscopic studies and electrical properties of transparent conductive films fabricated by using surfactant-stabilized single-walled carbon nanotube suspensions

This study evaluates the effect of anionic and cationic surfactants on the dispersion of purified SWCNTs in water in terms of dispersibility and on electrical conductivity of TCFs and electronic band structures of SWCNTs. The dispersibility of surfactants in an aqueous SWCNT suspension is assessed with the amount of SWCNTs dispersed, the content of surfactants required to suspend SWCNTs, and the long-term stability of dispersion. Sodium dodecylbenzene sulfonate (SDBS) shows better dispersibility and electrical conductivity of SWCNTs than sodium dodecyl sulfate, sodium cholate, and cetyltrimethyl ammonium bromide. Electronic band structures of SWCNTs vary with surfactants and nitric acid treatment, investigated by using UV–Vis–NIR and Raman spectroscopy. Metallic and semiconducting SWCNTs and surfactants make electrostatic charge interactions between them, which occur in different manners according to the electronic types of tubes and the natures of surfactants. TCFs are fabricated by using the SWCNT suspension dispersed with SDBS, which reveal a low percolation threshold with the two dimensional percolation behavior. The highest ratio of dc to optical conductivity (σ_dc/σ_op) is observed to be ∼23.1, corresponding to sheet resistance of 69 Ω/sq at the 550-nm optical transmission of 80%, upon nitric acid treatment of the SWCNT films.     

Enhanced figure of merit of poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) and SWCNT thermoelectric composites by doping with FeCl3

Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT) generally has a large Seebeck coefficient, and single-walled carbon nanotubes (SWCNTs) have high electrical conductivity. In this work, we prepared F8BT/SWCNT composites to combine the good Seebeck coefficient of the polymer and the excellent electrical conductivity of SWCNTs to achieve enhanced thermoelectric properties. For the composite materials, the maximum power factor of 1 μW mK⁻² was achieved when the SWCNT content was 60%, with the maximum ZT value of 4.6 × 10⁻⁴. After ferric chloride was employed as the oxidative dopant for the composites, the electrical conductivity of the composites improved significantly. The maximum value of power factor (1.7 μW mK⁻²) was achieved when the SWCNT content was 60%, and the ZT value of 7.1 × 10⁻⁴ was about 1.5 times as high as that of the composites with undoped F8BT. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47011.     

Aluminum nitride‐single walled carbon nanotube nanocomposite with superior electrical and thermal conductivities

Development of aluminum nitride (AlN)‐single walled carbon nanotube (SWCNT) ceramic‐matrix composite containing 1‐6 vol% SWCNT by hot pressing has been reported in this article. The composites containing 6 vol% SWCNT are dense (~99% relative density) and show high dc electrical conductivity (200 Sm^?1) and thermal conductivity (62 Wm^?1K^?1) at room temperature. SWCNTs contain mostly metallic variety tubes obtained by controlled processing of the pristine tubes before incorporation into the ceramic matrix. Raman spectroscopy and field emission scanning electron microscopy (FESEM) of the fracture surface of the samples show the excellent survivability of the SWCNTs even after high‐temperature hot pressing. The results indicate the possibility of preparation of AlN nanocomposite for use in plasma devices and electromagnetic shielding.     

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.     

High performance PEEK/carbon nanotube composites compatibilized with polysulfones-II. Mechanical and electrical properties

The mechanical and electrical properties of poly(ether ether ketone) (PEEK)/single-walled carbon nanotube (SWCNT) composites with polysulfones as compatibilizers have been analyzed. Dynamic mechanical studies reveal that these composites exhibit considerably higher storage modulus and glass transition temperature than non-compatibilized samples. Tensile tests indicate a non-linear growth in the Young’s modulus and strength with increasing SWCNT loading, related to changes in the degree of crystallinity of the composites. The moduli of samples with very low CNT content exceed the predictions by the rule of mixtures, whilst at higher concentrations fall slightly below the theoretical values. The addition of the polysulfones increases the stiffness and toughness of the composites, attributed to an improved filler dispersion and stronger matrix-reinforcement interfacial adhesion. Fractography analyses suggest that these compatibilizers favor the ductile deformation of the matrix. The electrical and thermal conductivities of the composites decrease slightly in the presence of the polysulfones, albeit are well above the values of pure PEEK. Enhanced properties are found for samples including wrapped laser-grown SWCNTs. The overall mechanical performance of the compatibilized composites is suitable for use in lightweight structural applications, particularly for the aeronautic industry.     

Synthesis of epoxy composites with high carbon nanotube loading and effects of tubular and wavy morphology on composite strength and modulus

Single-walled carbon nanotube (SWCNT)/epoxy composites with a high nanotube loading up to ∼39.1 wt% are fabricated by combining the mixed-curing-agent assisted layer-by-layer method and the hot press process. This combined method is an efficient and effective approach for making composite sheets in practical applications since the hot press process makes it possible and easy to readily prepare thick and large composites. The tensile and dynamic properties are greatly improved by the incorporation of SWCNTs. At ∼39.1 wt% SWCNTs, compared with those of the neat epoxy, the tensile strength and Young’s modulus are increased by 183% and 408%, respectively. The storage modulus is also significantly increased by 406% at room temperature due to this high loading of SWCNTs. Moreover, the loss factor in the temperature range −100-200 °C shows dramatic improvements by the introduction of high loading SWCNTs. Finally, analytical modeling is proposed to predict the strength and modulus of the CNT/epoxy composites by considering the effects of the tubular and wavy morphology of the CNTs. Both composite strength and composite modulus are found to decrease substantially with increasing waviness ratio. The predicted results show reasonable agreement with experimental data.     

Scalable,solvent-less de-bundling of single-wall carbon nanotube into elastomers for high conductive functionality

Composite fabrication techniques predominantly involve wet-synthetic protocols with organic solvents. While the resulting composite exhibits good electrical properties, their mass-production have been severely hindered due to use of excessive organic solvents. In contrast, dry-compounding methods are well-suited for industrialization but result in composites with lower electrical properties. This mutually exclusivity between (a) the fabrication process, (b) the composite properties and (c) the industrial scalability has been a major road-block for their commercialization. Addressing this obstacle, we report an electrically conductive polymer composite with long single-wall carbon nanotubes (SWCNT) as conductive fillers. The SWCNT/polymer composite possesses superior electrical properties to those achieved previously with other fillers or CNTs, obtained through dry-processes. The method involved efficient loosening of long SWCNT bundles through a biaxial shear force and subsequent kneading into the rubber matrix. The structural damage to SWCNTs was thereby minimized, as indicated by Raman spectroscopy and optical microscopy. Consequently, we achieved a SWCNT/polymer composite exhibiting ∼200 fold higher electrical conductivity than composite materials made by conventional dry-compounding methods. Finally, we demonstrate the industrial scalability of the process through the continuous, batch-production of the SWCNT-polyurethane composite sheet (12 m long and 60 mm wide) with uniform electrical conductivity (1.5 S/cm).     

Thermodynamic approach to enhanced dispersion and physical properties in a carbon nanotube/polypeptide nanocomposite

A high molecular weight synthetic polypeptide has been designed which exhibits favorable interactions with single wall carbon nanotubes (SWCNTs). The enthalpic and entropic penalties of mixing between these two molecules are reduced due to the polypeptide's aromatic sidechains and helical secondary structure, respectively. These enhanced interactions result in a well dispersed SWCNT/Poly (l-Leucine-ran-l-Phenylalanine) nanocomposite with enhanced mechanical and electrical properties using only shear mixing and sonication. At 0.5 wt% loading of SWCNT filler, the nanocomposite exhibits simultaneous increases in the Young's modulus, failure strain, and toughness of 8%, 120%, and 144%, respectively. At 1 kHz, the same nanotube loading level also enhances the dielectric constant from 2.95 to 22.81, while increasing the conductivity by four orders of magnitude.     

Carbon nanotube/epoxy composites fabricated by resin transfer molding

Carbon nanotube (CNT)/epoxy composites with controllable alignment of CNTs were fabricated by a resin transfer molding process. CNTs with loading up to 16.5 wt.% were homogenously dispersed and highly aligned in the epoxy matrix. Both mechanical and electrical properties of the CNT/epoxy composites were dramatically improved with the addition of the CNTs. The Young’s modulus and tensile strength of the composites reach 20.4 GPa and 231.5 MPa, corresponding to 716% and 160% improvement compared to pure epoxy. The electrical conductivity of the composites along the direction of the CNT alignment reaches over 1 × 10⁴ S/m.     

Electronic transport in composites of graphite oxide with carbon nanotubes

We show that the presence of electrically insulating graphite oxide (GO) within a single wall carbon nanotube (SWCNT) network strongly enhances electrical conductivity, whereas reduced graphite oxide, even though electrically conductive, suppresses electrical conductivity within a composite network with SWCNTs. Measurements of Young’s modulus and of Raman spectra strongly support our interpretation of the “indirect” role of the oxide groups, present in GO within the SWCNT-GO composite, through electronic doping of metallic SWCNTs.