Dispersion of pristine single-walled carbon nanotubes using pyrene-capped polystyrene and its application for preparation of polystyrene matrix composites

A pyrene-capped polystyrene (PyPS) is synthesized by an anionic polymerization method and acts as dispersant for dispersion of pristine single-walled carbon nanotubes (SWCNTs). Through a well-known π-stacking interaction confirmed qualitatively by proton nuclear magnetic resonance and fluoroscopic analyses, PyPS is strongly but noncovalently adsorbed onto the nanotube surface, affording highly uniform and stable SWCNT dispersion in chloroform with the nanotube content as high as 250 ± 30 mg L⁻¹. Since no direct chemical reaction takes place on the nanotubes, their intrinsic electronic structure is maintained, thus ensuring them as functional fillers for application in conductive polymer composites. The so-obtained dispersion is subsequently used to prepare polystyrene matrix composites. A solution-based process adopted here preserves the good nanotube dispersing state in dispersion into the composites. Hence, the resultant composites show good optical transmittance and a low electrical percolation threshold of 0.095 wt.% SWCNTs. In comparison, the composites with absence of PyPS prepared by the same process have a relatively high percolation threshold of 0.28 wt.% SWCNTs.     

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).     

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 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.     

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.     

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.     

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.     

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.     

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.     

Enhanced conductivity in polybenzoxazoles doped with carboxylated multi-walled carbon nanotubes

Two representative polybenzazoles, poly(p-phenylene benzobisoxazole) (PBO) and poly(2,5-benzoxazole) (ABPBO), have been used as matrix materials for fabricating electrically conducting nanocomposite films. In this strategy, pristine multi-walled carbon nanotubes (MWCNTs) were first treated with nitric acid to form carboxylated multi-walled carbon nanotubes (MWCNTs-COOH). Subsequently, MWCNTs-COOH were dispersed efficiently in the methanesulfonic acid (MSA) solution of polybenzazole, sonicated, and then processed into thin films. MWCNTs-COOH in MSA formed an isotropic regime at the concentration of ∼0.1 wt.%. Nanotubes could form net like structures and conductive channels in the polymer matrix to improve electrical conductivity, mechanical properties, and thermal stability. At the MWCNT-COOH composition of 5 wt.%, polybenzazole/MWCNT-COOH composite films exhibited a dramatic enhancement in electrical conductivity by 8 orders of magnitude from ∼10⁻¹² to 1.6 × 10⁻⁴ S cm⁻¹ without significantly sacrificing optical transparency.     

Influence of shear deformation on carbon nanotube networks in polycarbonate melts: Interplay between build-up and destruction of agglomerates

Shear-induced destruction and formation of conductive and mechanical filler networks formed by multi-wall carbon nanotubes in polycarbonate melts were investigated by simultaneous time-resolved measurements of electrical conductivity and rheological properties under steady shear and in the quiescent melt. The steady shear experiments were performed at shear rates between 0.02 and 1 rad/s and for nanotube concentrations ranging from 0.5 to 1.5 wt%. The influence of thermo-mechanical history on the state of nanotube dispersion and agglomeration was studied in detail.For melts with well-dispersed nanotubes a shear-induced insulator-conductor transition was observed, which is explained by the agglomeration of nanotubes under steady shear and the formation of an electrical conductive network of interconnected agglomerates. Simultaneously, a drastic decrease of the shear modulus (G^∗ = G′ + iG″) during steady shear was observed, which can be related to a reduction of mechanical reinforcement due to agglomeration of dispersed nanotubes. These findings indicate a substantial difference in the nature of “electrical” and “mechanical” network and contradict earlier assumptions that steady (or transient) shear is always destructive for the conductive filler network in highly viscous polymer composites.It was also shown that after a certain time of steady shear the filler network asymptotically reaches its steady state characterized by the constant electrical conductivity and shear modulus of the composite melt. Such asymptotic behaviour of composite properties was experimentally shown to be related to the interplay of the destructive and build-up effects of steady shear. For modelling of the electrical conductivity in presence of steady shear a kinetic equation was proposed for filler agglomeration with shear-dependent destruction and build-up terms. This equation was coupled to the generalized effective medium (GEM) approximation for insulator-conductor transition.     

Optical and electrical characterization of conducting polymer-single walled carbon nanotube composite films

The optical and electrical properties of the conducting polymer poly(3-hexylthiophene) and single walled carbon nanotube (SWCNT) composites have been investigated. The composites were prepared by dispersing carbon nanotubes in the polymer matrix already dissolved in 1,2-dichlorobenzene. The optical absorption, photoluminescence (PL) and electrical conductivity of the composite was studied as a function of SWCNT concentration in the solution. The absorption coefficient of the polymer was found to be unaffected upto a SWCNT concentration 5% w/w. However a minor decrease in the absorption in visible region was observed for higher SWCNT concentrations. The intensity of PL emission from the composite was measured and was found to decrease with the increase in SWCNT concentration. For a SWCNT concentration of 30% w/w, ∼90% of the PL was quenched, indicating an ultra fast transfer of photoinduced charges from donor polymer to acceptor SWCNT. Direct current conductivity of the composite film was found to increase rapidly with the increase in SWCNT concentration and an increase of ∼5 orders of magnitude was observed for a 30% w/w concentration. The enhancement in conductivity is explained in terms of percolation theory with an estimated percolation threshold of 2% w/w.     

Separation of single-walled carbon nanotubes from graphite by centrifugation in a surfactant or in polymer solutions

Electric arc single-walled carbon nanotubes (SWCNTs) can be separated from their graphitic impurities by a single centrifugation process in a surfactant or in polymer solutions. The purity of SWCNT dispersions, evaluated from near infrared (NIR) spectroscopy measurements, substantially increased after centrifugation at a moderate speed. The supernatant NIR purity was affected by the surfactant choice, following the sequence: sodium cholate ∼ Pluronic F68 > sodium dodecylbenzene sulfonate > Pluronic F127 > sodium dodecyl sulfate. NIR purity was also influenced by the centrifugation speed and the pristine SWCNT concentration in the starting dispersion, but not by the surfactant concentration. SWCNT enrichment was not observed in a pure organic solvent (N,N′-dimethylformamide) under identical centrifugation conditions. X-ray diffraction analysis demonstrated that graphitic impurities were mostly eliminated from SWCNTs during the centrifugation process in a surfactant or in polymer solutions. Thermogravimetric analysis under CO₂ showed that metallic impurities were substantially reduced during the centrifugation process.     

Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique

A new method was developed to disperse carbon nanotubes (CNTs) in a matrix polymer and then to prepare composites by melt processing technique. Due to high surface energy and strong adsorptive states of nano-materials, single-walled carbon nanotubes (SWNTs) were adsorbed onto the surface of polymer powders by spraying SWNT aqueous suspected solution onto fine high density polyethylene (HDPE) powders. The dried SWNTs/powders were blended in a twin-screw mixture, and the resulting composites exhibited a uniformly dispersion of SWNTs in the matrix polymer. The electrical conductivity and the rheological behavior of these composites were investigated. At low frequencies, complex viscosities become almost independent of the frequency as nanotubes loading being more than 1.5 wt%, suggesting an onset of solid-like behavior and hence a rheological percolation threshold at the loading level. However, the electrical percolation threshold is ∼4 wt% of nanotube loading. This difference in the percolation thresholds is understood in terms of the smaller nanotube-nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. The measurements of mechanical properties indicate that this processing method can obviously improve the tensile strength and the modulus of the composites.     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

Electrical conductivity of graphite/polystyrene composites made from potassium intercalated graphite

Composites between graphite and polystyrene have been synthesized starting from potassium intercalated graphite and styrene vapor. This in situ polymerization process can be used to make electrically conductive composites containing well-dispersed thin graphite sheets. The conductivities of the composites increase as the number of ordered carbon layers increases. With only 10% graphite in a polystyrene matrix, an electrical conductivity up to 1.3 × 10⁻¹ S/cm can be obtained. The key is synthesizing a material with at least four ordered graphite layers (a stage IV complex) separated by polystyrene. This composite shows an improvement in conductivity over a control composite made by radical polymerization of styrene containing the same amount of dispersed graphite which had a conductivity of 5.0 × 10⁻³ S/cm. Characterization of the complexes by powder X-ray diffraction, scanning electron microscopy and electrical conductivity is presented.     

Electrical and mechanical properties of polyimide-carbon nanotubes composites fabricated by in situ polymerization

Polyimide (PI)-carbon nanotubes composites were fabricated by in situ polymerization using multi wall carbon nanotubes (MWNT) as fillers. It suggested that in situ polymerization is an ideal technique to make a perfect dispersion of carbon nanotubes into matrixes. Besides it, the pre-treatment of carbon nanotubes in solvent to make the networks untied enough and to let solvent percolated into the networks is very important for forming uniform entanglements between carbon nanotubes and polymer molecular chains. The results imply that the percolation threshold for the electric conductivity of the resultant PI-MWNT composites was ca. 0.15 vol%. The electrical conductivity has been increased by more than 11 orders of magnitude to 10⁻⁴ S/cm at the percolation threshold. The mechanical properties of the polyimide composite were not improved significantly by addition of carbon nanotubes.     

Electrical conductivity of well-exfoliated single-walled carbon nanotubes

Aqueous suspensions of single-walled carbon nanotubes (SWCNTs) with controlled degree of exfoliation were used to prepare conductive thin films. Controlled exfoliation was achieved by physical separation of SWCNT bundles using our previously established nanoplatelet-dispersion method. Thin film networks of individual SWCNTs produced with this approach exhibit universal conduction behavior indicative of an isotropic network of random resistors with nearly monodisperse bond conductance distribution. Networks made of partially exfoliated SWCNTs experience a significant shift in percolation threshold because of effective local alignment of individual SWCNTs into bundles. Bundling increases the conductivity of the SWCNTs at higher concentration because of low contact resistance electron transport between metallic SWCNTs. The most significant impact of bundling is the development of non-universal electrical scaling. These findings suggest that while individually exfoliated SWCNTs should be of substantial importance for electrical devices requiring small increases in electrical conductivity at low concentration, adequate control of bundling may enable or enhance performance for applications requiring higher conductivity.     

High electrical conductivity and n-type thermopower from double-/single-wall carbon nanotubes by manipulating charge interactions between nanotubes and organic/inorganic nanomaterials

Single- and double-wall carbon nanotubes were decorated with organic or inorganic nanomaterials in order to obtain desired electrical transport properties such as a high electrical conductivity or an n-type thermopower. For instance, the electrical conductivity of double-wall carbon nanotubes (DWCNTs) decorated with tetrafluoro-tetracyanoquinodimethane (F₄TCNQ) was increased up to 5.9 × 10⁵ S/m, and single-wall carbon nanotubes (SWCNTs) were converted from p-type to n-type with a large thermopower (−58 μV/K) by using polyethyleneimine without vacuum or controlled environment. When inorganic nanoparticles made of Fe and Cu were used for decorating nanotubes, the electrical conductance of the nanotube films was decreased with an enlarged thermopower. On the other hand, Au decorations yielded higher electrical conductances with lower thermopowers. The thermoelectric power factors were improved by ∼180% with F₄TCNQ on DWCNTs and ∼140% with Fe on SWCNTs. We believe these transport property changes can be attributed to charge interactions resulted from the difference between the work functions/reduction potentials of nanotubes and nanomaterials. This study shows a first step toward the synthesis of both n-type and p-type conductors with carbon nanotubes, which are essential to thermoelectric energy conversion applications.