Synthesis of glass fiber‐multiwall carbon nanotube hybrid structures for high‐performance conductive composites

The conductive polyamide 66 (PA66)/carbon nanotube (CNT) composites reinforced with glass fiber‐multiwall CNT (GF‐MWCNT) hybrids were prepared by melt mixing. Electrostactic adsorption was utilized for the deposition of MWCNTs on the surfaces of glass fibers (GFs) to construct hybrid reinforcement with high‐electrical conductivity. The fabricated PA66/CNT composites reinforced with GF‐MWCNT hybrids showed enhanced electrical conductivity and mechanical properties as compared to those of PA66/CNT or PA66/GF/CNT composites. A significant reduction in percolation threshold was found for PA66/GF‐MWCNT/CNT composite (only 0.70 vol%). The morphological investigation demonstrated that MWCNT coating on the surfaces of the GFs improved load transfer between the GFs and the matrix. The presence of MWCNTs in the matrix‐rich interfacial regions enhanced the tensile modulus of the composite by about 10% than that of PA66/GF/CNT composite at the same CNT loading, which shows a promising route to build up high‐performance conductive composites. POLYM. COMPOS. 34:1313–1320, 2013. © 2013 Society of Plastics Engineers     

Chain conformation,crystallization behavior,electrical and mechanical properties of electrospun polymer-carbon nanotube hybrid nanofibers with different orientations

An improved electrospinning technique was used to produce poly(ethylene oxide) (PEO) and PEO-multi-walled carbon nanotube (MWCNT) hybrid nanofibers. By this technique, both the orientation of MWCNTs in the electrospun PEO nanofibers and the orientation of electrospun PEO–MWCNT hybrid nanofibers can be controlled. The morphologies of the as-spun PEO–MWCNT hybrid nanofibers and the dispersion and orientation of MWCNTs in the fiber matrix were observed by scanning and transmission electron microscopy. The effect of electrospinning process and the incorporation of MWCNTs on the chain conformation and semicrystalline framework of PEO were examined by Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, and differential scanning calorimetry, and compared with pure PEO and PEO–MWCNT films prepared by casting. Finally, to investigate how the fiber assemblies affect the mechanical and electrical properties of the hybrid materials, tensile testing and impedance analysis were performed on randomly oriented, uniaxially and biaxially oriented PEO–MWCNT hybrid nanofiber mats. The results indicated that both the uniaxially and biaxially oriented assembled hybrid materials have better tensile strength, modulus, and electrical conductivity compared with random nanofibers.     

Effects of carbon nanotubes aspect ratio on the qualitative and quantitative aspects of frequency response of electrical conductivity and dielectric permittivity in the carbon nanotube/polymer composites

To study the effect of carbon nanotube aspect ratio (AR) on the frequency response of the electrical properties, the alternating current (AC) electrical conductivity and dielectric permittivity of different AR multi-wall carbon nanotubes (MWCNTs)/thermoplastic elastomer (TPE) composites were studied in the AC frequency range of 100 Hz to 10  MHz. Qualitatively, the effect of frequency on the electrical properties of the composites was the same for all AR MWCNTs and shared many typical features of electrically percolative composites. Quantitatively, the frequency responses of electrical properties were found to be independent of nominal AR, concentration, percolation threshold, and the diameter of the MWCNT. Instead, frequency response of electrical properties was dependent on the MWCNT length and initial electrical conductivity of the composites. With the same initial conductivity of the MWNT composites, frequency-conductivity sensitivity varied inversely with the nominal length of the MWCNTs. Composites with MWCNTs of the same nominal length and similar electrical conductivity values, regardless of whether the MWCNT concentration was below or above the percolation threshold, exhibited quantitatively similar frequency-conductivity sensitivity. The frequency-dielectric sensitivity at the percolation threshold was a reflection of frequency-conductivity sensitivity and was also found to be dependent on the initial conductivity of the composites.     

Dispersion and aspect ratio of carbon nanotubes in aqueous suspension and their relationship with electrical resistivity of carbon nanotube filled polymer composites

Multiwalled carbon nanotube (MWCNT)/acrylonitrile butadiene styrene (ABS) composites were prepared by a processing method using solvent–nonsolvent precipitation. Size distributions of MWCNT agglomerates in aqueous suspension were investigated in order to predict aspect ratio of nanotubes by evaluating the effects of sonication time, MWCNT content, and surfactant. Aspect ratios of MWCNTs were predicted on the basis of the size distribution measurements for MWCNT agglomerates. Sonication time or applied sonic energy has a strong effect on the size distribution of MWCNT agglomerates. Compared with simple shear mixing method, it was shown that this processing method is more suitable for the MWCNT/ABS composites. An electrical percolation threshold was observed for the weight fraction of MWCNTs in the range of about 0.5–1.0 wt.%. Shorter MWCNTs are more suitable to induce fine dispersion, but lead to higher percolation threshold weight fraction. It was illustrated that fine dispersion can overcome the handicap of short length or low aspect ratio of MWCNTs.     

Fabrication and Characterization of Highly Oriented Composite Nanofibers with Excellent Mechanical Strength and Thermal Stability

Here, highly‐oriented poly(m‐phenylene isophthalamide)/polyacrylonitrile multi‐walled carbon nanotube (PMIA/PAN‐MWCNT) composite nanofiber membranes with excellent mechanical strength and thermal stability are successfully produced using electrospinning. It is demonstrated that the cooperation of multi‐walled carbon nanotubes (MWCNT) and high‐speed rotating collection is beneficial to the acquisition of highly oriented fibers and effectively improves the mechanical strength of the membrane along the orientation direction. Specifically, the tensile stress of poly(m‐phenylene isophthalamide)/polyacrylonitrile (PMIA/PAN) membrane is enhanced significantly from 10.6 to 20.7 MPa, benefiting from the highly oriented alignment of the fibers as well as the reinforcing effect of MWCNTs on the fibers. Furthermore, the stressing process of single fiber and fiber aggregates is carefully simulated, and the influence of MWCNTs on the mechanical properties of PMIA/PAN‐MWCNT membranes is analyzed comprehensively, providing a meaningful auxiliary means for the study of mechanical properties. In addition, the composite nanofiber membrane has the advantages of both PMIA and PAN, possessing high temperature resistance, flame‐retardancy, and chemical stability, for an ideal high‐temperature material. In short, the as‐prepared PMIA/PAN‐MWCNT composite membrane with excellent comprehensive property emerges a promising application in many fields, especially in high‐tech.     

Comparison of electrical properties between multi-walled carbon nanotube and graphene nanosheet/high density polyethylene composites with a segregated network structure

Multi-walled carbon nanotube (MWCNT)/high density polyethylene (HDPE) and graphene nanosheets (GNS)/HDPE composites with a segregated network structure were prepared by alcohol-assisted dispersion and hot-pressing. Instead of uniform dispersion in polymer matrix, MWCNTs and GNSs distributed along specific paths and formed a segregated conductive network, which results in a low electrical percolation threshold of the composites. The electrical properties of the GNS/HDPE and MWCNT/HDPE composites were comparatively studied, it was found that the percolation threshold of the GNS/HDPE composites (1 vol.%) was much higher than that of the MWCNT/HDPE composites (0.15 vol.%), and the MWCNT/HDPE composite shows higher electrical conductivity than GNS/HDPE composite at the same filler content. According to the values of critical exponent, t, the two composites may have different electrical conduction mechanisms: MWCNT/HDPE composite represents a three-dimensional conductive system, while the GNS/HDPE composite represents a two-dimensional conductive system. The improving effect of GNSs as conducting fillers on the electrical conductivity of their composites is far lower than theoretically expected.     

Electrical conductivity of poly(vinylidene fluoride)/carbon nanotube composites with a spherical substructure

Poly(vinylidene fluoride) (PVDF)/multiwalled carbon nanotube (MWCNT) conducting composites were prepared with a percolation threshold as low as 0.07 wt%. The MWCNTs in a PVDF solution can lead to the formation of spherical PVDF/MWCNT composite particles by sonication. The MWCNTs coated on the surfaces of the spherical particles form a conduction network when the spheres coalesce to form a solid composite. The existence of the spherical particles with a substructure results in the reduction in MWCNT content and improves the electrical conductivity of the composites.     

Electrical and piezoresistive properties of multi-walled carbon nanotube/polymer composite films aligned by an electric field

Aligned multi-walled carbon nanotube (MWCNT)/polymer composite films are prepared by solution casting in the presence of an alternating electric field. Application of 7 kV/m at a frequency of 60 Hz to the polymer composite melt induces MWCNT alignment in the direction of the applied field, which is maintained after polymer crystallization. The electrical conductivity and piezoresistive response of electric-field-aligned and randomly oriented 0.1–0.75 wt% MWCNT/polysulfone films are evaluated. Electrical conductivity is 3–5 orders of magnitude higher for composites with electric-field-aligned MWCNTs than for randomly oriented composites. MWCNT alignment inside the polymer matrix also increases the film piezoresistive sensitivity, enhancing the strain sensing capabilities of the composite film.     

Improved electrical conductivity of very long multi-walled carbon nanotube bundle/poly (methyl methacrylate) composites

Very long and highly dispersible multi-walled carbon nanotube (MWCNT) bundles were synthesized in large quantity by catalytic chemical vapor deposition, and their structural and electrical properties were characterized. It was found that the MWCNTs could be synthesized with either bundled (long-aligned) or short-entangled structure depending on the catalyst system. The aligned MWCNTs were found to be more conductive and more dispersible than the entangled ones. The MWCNT/poly (methyl methacrylate) composites were prepared using both entangled and aligned MWCNTs. The aligned MWCNTs were found to give the composite higher electrical conductivity, which might be attributed to long length and high dispersibility. It was further found that the longer the MWCNT bundle, the higher electrical conductivity of the composite.     

Lignin‐based polyurethane doped with carbon nanotubes for sensor applications

Modified eucalyptus kraft lignin doped with multiwall carbon nanotubes (MWCNTs) was used as a macromonomer in step‐growth polymerization with tolylene 2,4‐diisocyanate terminated poly(propylene glycol) with the aim of producing a conductive copolymer for all‐solid‐state potentiometric chemical sensor applications. The crosslinked elastomeric polyurethane obtained was characterized by Fourier transform infrared attenuated total reflection spectroscopy, scanning electron microscopy, tunnelling electron microscopy and atomic force microscopy. Doping of lignin‐based polyurethane with MWCNTs produced a significant enhancement of its electrical conductivity without deterioration of thermal and viscoelastic properties. The polymer composite displayed a low percolation threshold at an MWCNT concentration of 0.18% (w/w), which was explained by the oriented distribution of MWCNTs along lignin clusters. All lignin‐based polyurethanes doped with MWCNTs at concentrations above the percolation threshold are suitable for sensor applications. Copyright © 2012 Society of Chemical Industry     

Effect of covalent bonds on the mechanical properties of a multi‐walled carbon nanotube/cellulose composite

A multi‐walled carbon nanotube (MWCNT)/cellulose composite was synthesized to improve the mechanical strength of regenerated cellulose film. N,N‐carbonyldiimidazole was mixed with functionalized MWCNTs and sonicated at 60 °C for 12 h. The resulting MWCNT‐imidazolide was mixed with cellulose solution, and reacted at various temperatures for various times. The occurrence of covalent bonds between MWCNTs and cellulose was investigated using Fourier transform infrared spectroscopy and Raman spectroscopy. According to mechanical tensile tests, Young's modulus of the MWCNT/cellulose composite was found to be 11.2 GPa, an increase of approximately 110% with respect to regenerated cellulose film. Copyright © 2010 Society of Chemical Industry     

The electrical conductivity of carbon nanotube/carbon black/polypropylene composites prepared through multistage stretching extrusion

We reported the design of a grape-cluster-like conductive network in a polypropylene (PP) matrix, where oriented multi-walled carbon nanotubes (MWCNTs) served as branches and provided charge transport over large distances while grape-like carbon black (CB) aggregates enriched around MWCNTs and linked these conductive tubes through charge transport over small distances. The key for construction of this grape-cluster-like conductive network was the extension and orientation of MWCNTs, which was achieved in this work by multistage stretching extrusion with an assembly of laminating-multiplying elements (LMEs, which divide and recombine polymer melts). The highest efficient grape-cluster-like conductive network was obtained at a CB:MWCNT weight ratio of 6. The experimental results showed that this novel grape-cluster-like conductive network provided a low percolation threshold for PP/CB/MWCNT composites due to the synergistic effect of CB and oriented MWCNTs. When the combined CB and MWCNT content was about 6.9 vol%, the electrical resistivity of PP/CB/MWCNT composites prepared by multistage stretching extrusion with 6 LMEs decreased to only 0.63 Ω cm.     

Graphene nanoplate and multiwall carbon nanotube–embedded polycarbonate hybrid composites: High electromagnetic interference shielding with low percolation threshold

This study has reported the preparation of polycarbonate (PC)/graphene nanoplate (GNP)/multiwall carbon nanotube (MWCNT) hybrid composite by simple melt mixing method of PC with GNP and MWCNT at 330°C above the processing temperature of the PC (processing temperature is 280°C) followed by compression molding. Through optimizing the ratio of (GNP/MWCNT) in the composites, high electromagnetic interference shielding effectiveness (EMI SE) value (∼21.6 dB) was achieved at low (4 wt%) loading of (GNP/MWCNT) and electrical conductivity of ≈6.84 × 10⁻⁵ S.cm⁻¹ was achieved at 0.3 wt% (GNP/MWCNT) loading with low percolation threshold (≈0.072 wt%). The high temperature melt mixing of PC with nanofillers lowers the melt viscosity of the PC that has helped for better dispersion of the GNPs and MWCNTs in the PC matrix and plays a key factor for achieving high EMI shielding value and high electrical conductivity with low percolation threshold than ever reported in PC/MWCNT or PC/graphene composites. With this method, the formation of continuous conducting interconnected GNP‐CNT‐GNP or CNT‐GNP‐CNT network structure in the matrix polymer and strong π–π interaction between the electron rich phenyl rings and oxygen atom of PC chain, GNP, and MWCNT could be possible throughout the composites. POLYM. COMPOS., 37:2058–2069, 2016. © 2015 Society of Plastics Engineers     

High dispersion ability of fluorene‐based polyester as a polymer matrix for carbon nanotubes

The high compatibility of fluorene‐based polyester (FBP‐HX) as a polymer matrix for multiwalled carbon nanotubes (MWCNTs) is discussed. A low surface resistivity due to the fine dispersion of MWCNTs in FBP‐HX and polycarbonate (PC) is reported. With a solution‐casting method, a percolation threshold with the addition of between 0.5 and 1.0 wt % MWCNTs was observed in the MWCNT/PC and MWCNT/FBP‐HX composites. Because of the coverage of FBP‐HX on the MWCNTs, a higher surface resistivity and a higher percolation ratio of the MWCNT/FBP‐HX composites were achieved compared with the values for the MWCNT/PC composites. In the MWCNT/FBP‐HX composites, MWCNTs covered with FBP‐HX were observed by scanning electronic microscopy. Because of the coverage of FBP‐HX on the MWCNTs, FBP‐HX interfered with the electrical pathway between the MWCNTs. The MWCNTs in FBP‐HX were covered with a 5‐nm layer of FBP‐HX, but the MWCNTs in the MWCNT/PC composites were in their naked state. MWCNT/PC sheets demonstrated the specific Raman absorption of the MWCNTs only with the addition of MWCNTs of 1 wt % or above because of the coverage of the surface of the composite sheet by naked MWCNTs. In contrast, MWCNT/FBP‐HX retained the behavior of the matrix resin until a 3 wt % addition of MWCNTs was reached because of the coverage of MWCNTs by the FBP‐HX resin, induced by its high wettability. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

High‐performance electromagnetic interference shielding material based on an effective mixing protocol

A facile and economic method is developed for the fabrication of new lightweight materials with high electromagnetic interference (EMI) shielding performance, good mechanical properties and low electrical percolation threshold through melt mixing. Electrical properties, DC conductivity, EMI shielding performance and mechanical properties of poly(trimethylene terephthalate) (PTT)/multiwalled carbon nanotube (MWCNT) nanocomposites with varying filler loading of MWCNTs were investigated. High‐resolution transmission electron microscopy was used to determine the distribution of MWCNTs in the PTT matrix. The newly developed nanocomposites show excellent dielectric and EMI shielding properties. Theoretical electrical percolation threshold was achieved at 0.21 wt% loading of MWCNTs, due to the high aspect ratio and the three‐dimensional network formation of MWCNTs. Experimental DC conductivity values were compared with those of theoretical models such as the Voet, Bueche and Scarisbrick models, which showed good agreement. The PTT/3% MWCNT composite showed an EMI shielding value of ～38 dB (99.99% attenuation) with a sample thickness of 2 mm. Power balance was used to determine the actual contribution of reflection, absorption and transmission loss to the total EMI shielding value. The nanocomposites showed good tensile and impact properties and the composite with 2% MWCNTs exhibited an improvement in tensile strength of as much as 96%. © 2018 Society of Chemical Industry     

Supramolecular bionanocomposites 3: Effects of surface functionality on electrical and mechanical percolation

Percolation phenomena in bionanocomposites composed of the bioplastic polylactide and surface decorated supramolecular multiwalled carbon nanotubes (MWCNTs) are investigated. MWCNTs with three distinct surface chemistries—a native surface, a hydroxyl functionalized surface, and a short hydrocarbon functionalized surface—are prepared and studied. Two experimental methods are used to determine percolation thresholds; melt rheology provides a measure of the mechanical percolation threshold and electrical impedance spectroscopy provides values of the electrical percolation threshold. The MWCNT‐loading level required to obtain mechanical percolation is systematically found to be lower than the loading level needed to achieve electrical percolation. Hydroxylated MWCNTs have the highest percolation thresholds of 1.8 (mechanical) and 6.7 wt % (electrical), which is attributed to aggregation caused by hydrogen bonding. Alkane‐grafted MWCNTs have lower percolation thresholds of 0.76 (mechanical) and 2.8 wt % (electrical). However, untreated MWCNTs have the lowest thresholds of 0.16 (mechanical) and 1.5 wt % (electrical). The reduced percolation threshold observed upon conversion of the hydroxylated surface to the alkylated surface is attributed to morphological differences—functionalization disrupts hydrogen bonding and leads to better nanotube dispersion. The combination of higher inherent conductivity and more favorable nanotube‐polylactide interactions implies that unmodified MWCNTs are preferred to produce electrically conductive bionanocomposites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Multi-walled carbon nanotube/polyimide composite film fabricated through electrophoretic deposition

Multi-walled carbon nanotube (MWCNT)/polyimide composite films were fabricated through electrophoretic deposition (EPD) of MWCNT-polyamic acid colloidal suspension which was derived from carboxylated-MWCNTs and poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PMDA-ODA). Under electric field, both negatively charged MWCNTs and PMDA-ODA colloid particles migrate onto a positively charged anode simultaneously, and are converted to a coherent MWCNT/polyimide composite film in the ensuing imidization reaction. Uniform dispersion of MWCNTs in the composite film was observed using transmission electron microscopy. The thickness of the prepared composite film can be tuned by varying processing conditions such as deposition time and anode conductivity. The electrical conductivity of the composite film increased with increasing the concentration of MWCNTs in EPD suspension. The mechanical reinforcement of polyimide using MWCNTs was evaluated by tensile testing and nanoindentation testing.     

The origin of the electric and dielectric behavior of expanded graphite–carbon nanotube/cyanate ester composites with very high dielectric constant and low dielectric loss

High performance expanded graphite (EG)–multiwalled carbon nanotube (MWCNT)/cyanate ester (CE) composites with very high dielectric constant, low dielectric loss and low percolation threshold were developed. In order to understand the electric and dielectric behavior of EG–MWCNT/CE composites, EG/CE and MWCNT/CE binary composites were also prepared for comparison. Results show that the ternary composites have greatly different electric and dielectric properties from the binary composites. The percolation threshold of the EG–MWCNT/CE composite is much lower than that of either the EG/CE or MWCNT/CE composite. With the same content of conductive fillers, the EG–MWCNT/CE composite shows a much higher dielectric constant than EG/CE and MWCNT/CE composites. In addition, to obtain the same dielectric constant, the dielectric loss of the EG–MWCNT/CE composite is lower than that of either binary composite. The difference is attributed to the synergistic effect between EG and MWCNT. The addition of EG not only improves the dispersion of MWCNTs in the resin matrix, but also helps to form conductive networks. An equivalent circuit model is proposed.     

Detecting structural orientation in isoprene rubber/multiwall carbon nanotube nanocomposites at different scales during uniaxial deformation

Enhanced strain‐induced crystallization (SIC) behavior in isoprene rubber/multiwall carbon nanotube (IR/MWCNT) nanocomposites was analyzed in terms of structural orientation during uniaxial deformation. In situ synchrotron wide‐angle X‐ray diffraction and small‐angle X‐ray scattering (SAXS) reveal the molecular orientation in IR/MWCNT composites at different scales. The inclusion of MWCNTs leads to a decrease in the molecular orientation at small strain due to the promotion of SIC. Meanwhile, the presence of MWCNTs induces a large‐scale orientation within the vulcanized rubber network based on SAXS results. Considering the heterogeneous nature of the vulcanized network, the nucleation process during SIC is discussed from the viewpoint of thermodynamics. The oriented large‐scale structure in IR/MWCNT composites is composed of local rubber chains stretched up MWCNTs, from which the additional nuclei are induced. By forming a bound rubber layer around MWCNTs through attractive interactions, MWCNTs can amplify the local strain of rubber segments and form a highly oriented large‐scale structure, but without altering the overall molecular orientation level. The evolution of detailed structural orientation in MWCNT‐filled rubber composites during deformation is revealed for the first time. © 2017 Society of Chemical Industry     

Electrical and rheological percolation in poly(vinylidene fluoride)/multi‐walled carbon nanotube nanocomposites

Nanocomposites of poly(vinylidene fluoride) (PVDF) and multi‐walled carbon nanotubes (MWCNTs) were prepared through melt blending in a batch mixer (torque rheometer equipped with a mixing chamber). The morphology, rheological behavior and electrical conductivity were investigated through transmission electron microscopy, dynamic oscillatory rheometry and the two‐probe method. The nanocomposite with 0.5 wt% MWCNT content presented a uniform dispersion through the PVDF matrix, whereas that with 1 wt% started to present a percolated network. For the nanocomposites with 2 and 5 wt% MWCNTs the formation of this nanotube network was clearly evident. The electrical percolation threshold at room temperature found for this system was about 1.2 wt% MWCNTs. The rheological percolation threshold fitted from viscosity was about 1 wt%, while the threshold fitted from storage modulus was 0.9 wt%. Thus fewer nanotubes are needed to approach the rheological percolation threshold than the electrical percolation threshold. Copyright © 2010 Society of Chemical Industry