Polyethylene multiwalled carbon nanotube composites

Polyethylene (PE) multiwalled carbon nanotubes (MWCNTs) with weight fractions ranging from 0.1 to 10 wt% were prepared by melt blending using a mini-twin screw extruder. The morphology and degree of dispersion of the MWCNTs in the PE matrix at different length scales was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). Both individual and agglomerations of MWCNTs were evident. An up-shift of 17 cm⁻¹ for the G band and the evolution of a shoulder to this peak were obtained in the Raman spectra of the nanocomposites, probably due to compressive forces exerted on the MWCNTs by PE chains and indicating intercalation of PE into the MWCNT bundles. The electrical conductivity and linear viscoelastic behaviour of these nanocomposites were investigated. A percolation threshold of about 7.5 wt% was obtained and the electrical conductivity of PE was increased significantly, by 16 orders of magnitude, from 10⁻²⁰ to 10⁻⁴ S/cm. The storage modulus (G′) versus frequency curves approached a plateau above the percolation threshold with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behaviour. The ultimate tensile strength and elongation at break of the nanocomposites decreased with addition of MWCNTs. The diminution of mechanical properties of the nanocomposites, though concomitant with a significant increase in electrical conductivity, implies the mechanism for mechanical reinforcement for PE/MWCNT composites is filler-matrix interfacial interactions and not filler percolation. The temperature of crystallisation (T_c) and fraction of PE that was crystalline (F_c) were modified by incorporating MWCNTs. The thermal decomposition temperature of PE was enhanced by 20 K on addition of 10 wt% MWCNT.     

Enhanced irradiation-resistance of polymer-derived SiC by incorporation of multiwalled carbon nanotubes

In this work, multiwalled carbon nanotubes were introduced into polycarbosilane to fabricate carbon nanotubes reinforced nano-SiC composites (MWCNTs/ PCS-nano-SiC). Radiation effects on MWCNTs/ PCS-nano-SiC were studied by irradiation with 2 MeV Au²⁺ ions at room temperature and doses ranging from 2 × 10¹⁴ to 8 × 10¹⁴ ions/cm². The sample pyrolyzed at 1400 °C and containing 3 wt% carbon nanotubes exhibited excellent overall performances. The multiple graphite layers in the MWCNTs provided "absorption traps" for defects, improving the efficiency of defect recombination. Tightly combined MWCNTs/ PDC interface is required to enable the two phases to protect each other during irradiation. The critical amorphization dose was increased upon structural optimization, and the hardness degradation was significantly reduced. The rise of Young's modulus at a high damage dose was discovered because of the "interface-driven shrinkage" of nano-SiC. The present study provides insight into SiC_f/ SiC design for an advanced nuclear system.     

Location-selective incorporation of multiwalled carbon nanotubes in polycarbonate microspheres

In this study, we describe a process to produce polycarbonate (PC) microspheres and incorporate multiwalled carbon nanotubes (MWCNTs) within them location-selectively. The process, an oil-in-water (O/W) emulsion method, entails the formation of an aqueous phase with poly(vinylpyrrolidone) (PVP) and a methylene chloride phase that dissolves PC. Uniform PC microspheres were prepared by forming a stable emulsion using a homogenizer and a steric stabilizer, PVP. With this process, MWCNTs can be location-selectively incorporated in PC microspheres to afford the required properties such as electrical conductivity and thermal stability. When MWCNTs were dispersed in an aqueous phase using PVP, they were incorporated only on the surface of the PC microspheres. However, PC microspheres incorporating MWCNTs inside them were fabricated when the MWCNTs were dispersed in methylene chloride by functionalizing them via alkylation.     

Polyelectrolyte-functionalized multiwalled carbon nanotubes: preparation, characterization and layer-by-layer self-assembly

Two kinds of polyelectrolyte: polyacrylic acid (PAA) and poly(sodium 4-styrenesulfonate) (PSS), were grafted onto the convex surfaces of multiwalled carbon nanotubes (MWNTs) by surface-initiating ATRP (atom transfer radical polymerization) from the initiating sites previously anchored onto the convex surfaces of MWNTs. The grafted polyelectrolyte can be efficiently quantified by the feed ratio of monomer to MWNT-based macroinitiator, and the maximum amount of grafted polymer is higher than 55 wt%. The polyelectrolyte-coated MWNTs resembled core-shell structures justified by the TEM images of the samples obtained, which provided direct evidence for the covalent modification of MWNT. FTIR, ¹H NMR and TGA were used to determine the chemical structure of the resulting products. Comparison of UV-Vis spectra demonstrated that the products were water-soluble, and that PSS was more effective for improving the water solubility of carbon nanotubes. Using the polyelectrolyte- and carboxylic acid-functionalized MWNTs as templates, and poly(2-(N,N-dimethylaminoethyl) methacrylate (PDMAEMA)/hyperbranched polysulfone amine (HPSA) and PSS as polycation and polyanion, respectively, layer-by-layer (LbL) electrostatic self-assembly was conducted in order to explore the application of the functionalized nanotubes. It was found that the functionalized MWNTs have a high efficiency for loading polyelectrolytes by the LbL approach (the adsorbed polymer quantity is higher than 10 wt% in one assembling step). TEM observations showed that the assembled polymer shell on the MWNT surfaces was very even and flat.     

Electrical properties of annealed MPCVD grown vertically aligned carbon nanotube films

Vertically aligned multiwalled carbon nanotubes (MWNTs) were grown on silicon substrate at a low temperature (<520 °C) using microwave plasma-enhanced chemical vapor deposition (MPCVD). From the Raman spectra, it was found that the I_D/I_G ratio of MWNTs decreased after annealing, indicating that more graphenes were formed by the annealing process. Nevertheless, a strong Si signal was found in Raman spectra after annealing at a high-temperature (600 °C). From X-ray photoelectron spectroscopy (XPS) analysis it was observed that the ratio of the oxygen to carbon (O/C) signal intensity was from 0.15 to 1.88 for the increasing annealed temperatures of MWNTs, and a Si signal was found nearby the surface of MWNTs after annealing at 600 °C. Moreover, from the I–V measurement, the less symmetric I–V characteristic was found for the metal/MWNTs/metal (MIM) sandwich structure of unannealed MWNTs. After 300 °C annealing process, the positive current was increase and the negative current was decrease. It was conjectured that the MWNTs could obtain more graphenes structure by the 300 °C annealing process. Moreover, the I–V trace of the sample annealed by 600 °C exhibited rapid current descent, indicating the oxygenated and partly silicided phenomena might cover outer graphite layer of MWNTs. The equivalent circuit for the MIM sandwich structure could be represented as two Schottky barrier diodes in a back-to-back configuration. From the data fitting, it was found that the Schottky barrier height (_B0) decreased and the current density (J) increased from unannealing to 300 °C annealing temperature. However, the Schottky barrier height (_B0) was increased from 300 to 600 °C annealing temperature. Comparison with the XPS, this may due to the oxygenated and partly silicided phenomenon on the surface of the MWNTs.     

Dielectric spectroscopy on melt processed polycarbonate—multiwalled carbon nanotube composites

Complex permittivity and related AC conductivity measurements in the frequency range between 10⁻⁴ and 10⁷ Hz are presented for composites of polycarbonate (PC) filled with different amounts of multiwalled carbon nanotubes (MWNT) varying in the range between 0.5 and 5 wt%. The composites were obtained by diluting a PC based masterbatch containing 15 wt% MWNT by melt mixing using a Micro Compounder. From DC conductivity measurements it was found that for samples processed at a mixing screw speed of 150 rpm for 5 min, the percolation occurs at a threshold concentration (p_c) between 1.0 and 1.5 wt% MWNT. For concentrations of MWNT near the percolation threshold, the processing conditions (screw speed and mixing time) were varied. The differences in the dispersion of the MWNT in the PC matrix could be detected in the complex permittivity and AC conductivity spectra, and have been explained by changes in p_c. The AC conductivity and permittivity spectra are discussed in terms of charge carrier diffusion on percolation clusters and resistor-capacitor composites.     

Multiwalled carbon nanotube cryogels with aligned and non-aligned porous structures

Multiwalled carbon nanotube (MWCNT) cryogels were fabricated with aligned and non-aligned porous structures. The MWCNT cryogels contained a major fraction of MWCNTs with a minor fraction of silk fibroin as the structure binder. The morphology of the porous structures was controlled using a sol-gel process of silk fibroin to form the network structures. Microchannel structures were formed by ice-templating. The MWCNT cryogels contained mesopores and formed as monoliths. The MWCNT cryogels with aligned porous structures showed better thermal stability and electrical conductivity than the MWCNT cryogels with non-aligned porous structures due to the advantageous MWCNT interconnections. The morphology of the porous structures was examined by field emission scanning electron microscopy and transmission electron microscopy. The structure-property relationships of the MWCNT cryogels and the performance of the MWCNT cryogels as electrodes were investigated.     

Multiwalled carbon nanotube/polybenzoxazine nanocomposites: Preparation, characterization and properties

A novel nanocomposite composed of polybenzoxazine (PBZ) and multiwalled carbon nanotubes (MWNT) was prepared successfully. The surface modification of MWNT, including nitric acid modification followed by toluene-2,4-diisocyanate (TDI) treatment, introduced hydroxyl, carboxyl, and isocyanate groups on the MWNT surface. The surface carboxyl groups catalyzed the ring-opening reaction of benzoxazine and thus decreased the curing temperature of the system. The isocyanate groups reacted with the phenolic hydroxyl groups generated by the ring opening of benzoxazine resulting in the significant improvement of the adhesion between PBZ and MWNT. Dynamic mechanical analyses indicated the increase of storage modulus as well as T_g by the addition of MWNT into PBZ. A well dispersed modified-MWNT on nanoscale level inside PBZ matrix was observed by TEM and SEM.     

In situ polymerization approach to multiwalled carbon nanotubes-reinforced nylon 1010 composites: Mechanical properties and crystallization behavior

A series of polyamide 1010 (PA1010 or nylon 1010) and multiwalled carbon nanotubes (MWNTs) composites were prepared by in situ polymerization of carboxylic acid-functionalized MWNTs (MWNT-COOH) and nylon monomer salts. Mechanical tensile tests and dynamic mechanical analysis (DMA) show that the Young modulus increases as the content of the nanotubes increases. Compared with pure PA1010, the Young's modulus and the storage modulus of MWNTs/PA1010 in situ composites are significantly improved by ca. 87.3% and 197% (at 0 °C), respectively, when the content of MWNTs is 30.0 wt%. The elongation at break of MWNTs/PA1010 composites decreases with increasing proportion of MWNTs. For the composites containing 1.0 wt% MWNTs, the Young modulus increases by ca. 27.4%, while the elongation at break only decreases by ca. 5.4% as compared with pure PA1010 prepared under the same experimental conditions. Compared with mechanical blending of MWNTs with pure PA1010, the in situ-prepared composites exhibit a much higher Young's modulus, indicating that the in situ polycondensation method improves mechanical strength of nanocomposites. Scanning electron microscopy (SEM) imaging showed that MWNTs on the fractured surfaces of the composites are uniformly dispersed and exhibit strong interfacial adhesion with the polymer matrix. Moreover, unique crystallization and melting behaviors for MWNTs/PA1010 in situ composites are observed using a combination of differential scanning calorimetry (DSC) and X-ray diffraction methods. It was shown that only the α-form crystals are observed in our MWNTs/PA1010 in situ composites. This result is quite different from PA1010/montmorillonite and PA6-clay composites, where both of α- and γ-form crystals were found.     

Dispersion, agglomeration, and network formation of multiwalled carbon nanotubes in polycarbonate melts

Three different industrially available multiwalled carbon nanotube (MWNT) materials were directly incorporated into polycarbonate by melt mixing using a small-scale compounder. Despite of similar aspect ratios the electrical percolation behaviour was different. TEM investigations reveal significant differences in the nanotube dispersion which can be attributed to different dispersability of the raw MWNT materials. It is shown that the investigation of the sedimentation behaviour of aqueous MWNT dispersions is a simple method to estimate the nanotube dispersability.The relationships between melt processing conditions and MWNT dispersion and distribution were studied on polycarbonate samples containing 0.875 wt% MWNT prepared by masterbatch dilution. During melt mixing only high shear forces can provide suitable MWNT dispersion because firstly the MWNT disentanglement is facilitated and secondly secondary agglomeration is prevented. At low shear agglomeration of formerly well dispersed MWNT could be observed. During hot pressing the network or MWNT arrangement and the resulting electrical conductivity can be manipulated by the processing conditions like melt temperature and pressing speed. A certain nanotube agglomeration can enhance the development of an electrical percolated network as shown by dielectric spectroscopy.     

Analysis of agglomerate dispersion mechanisms of multiwalled carbon nanotubes during melt mixing in polycarbonate

Dispersion of primary nanotube agglomerates in polymer melts is one of the difficult tasks when applying melt mixing for nanocomposite preparation. Hence, there is a need for a better understanding of the ongoing processes. Filler agglomerates generally undergo dispersion by rupture and erosion mechanisms, which usually occur simultaneously. To analyse these mechanisms and their corresponding dispersion kinetics 1 wt% multiwalled carbon nanotubes (MWNT) were incorporated into polycarbonate using a microcompounder. Different mixing speeds at constant melt temperature were applied thereby changing the applied stress. The states of MWNT agglomerate dispersion at different mixing times were assessed by quantifying the agglomerate area ratio and particle size distribution using image analysis of optical transmission micrographs. A model is proposed to estimate the fractions of rupture and erosion mechanisms during agglomerate dispersion. At low mixing speeds, the dispersion was found to be governed by both mechanisms, whereas rupture dominance increases with increasing mixing speed. Further, the relationship between electrical resistivity and dispersion was studied indicating a critical behaviour. A dependency on the amount of dispersed nanotubes was found only in a certain range of state of dispersion.     

Preparation of polystyrene-multiwalled carbon nanotube composites with individual-dispersed nanotubes and strong interfacial adhesion

Multiwalled carbon nanotubes (MWCNTs) were modified via polymerization of styrene under microwave irradiation, and polystyrene (PS)-MWCNT composites with individual-dispersed nanotubes were prepared by melt-mixing using industrial extruder and injection moulding machine. The microscopic morphologies of modified MWCNTs (m-MWCNTs) and composites were studied through transmission electronic microscopy and scanning electron microscopy. Result showed that a PS coat layer was introduced on the m-MWCNT surfaces, improving the compatibility of nanotubes with poor polar materials such as tetrahydrofuran, toluene, and PS matrix. Contrary to the composite prepared directly using purified MWCNTs (PS-p-MWCNT composite) without further modification, the composite prepared using m-MWCNTs (PS-m-MWCNT composite) exhibited a PS middle layer between nanotubes and matrix, leading to a strong interfacial adhesion. Thereby, although the nanotubes are individually dispersed in both PS-p-MWCNT and PS-m-MWCNT composites, the mechanical property of the latter is better than that of the former. When the nanotube content is 0.32 wt%, the PS-m-MWCNT composite had a 250% increase of impact strength as compared to pure PS, but the PS-p-MWCNT composite had only a 150% increase. Furthermore, the tensile strength of PS-p-MWCNT composite descended slightly with the addition of nanotube content, whereas, that of the PS-m-MWCNT composite ascended slightly.     

Study of nimesulide and its determination using multiwalled carbon nanotubes modified glassy carbon electrodes

A new method for the determination of nimesulide was established based on the multiwalled carbon nanotubes (MWCNTs) modified glassy carbon electrode (MWCNTs/GCE). In 0.2 M PBS (pH 6.6) buffer solution, the MWCNTs/GCE showed a remarkable catalytic and enhancement effect on reduction of the nimesulide. The reduction peak potential of nimesulide shifted positively from −0.665 V at bare GCE to −0.553 V at MWCNTs/GCE, and the sensitivity increased ca. 7 times. A linear dynamic range of 3.2 × 10⁻⁷-6.5 × 10⁻⁵ M (R = 0.9992) with a detection limit of 1.6 × 10⁻⁷ M was obtained. The electrochemical behaviors of nimesulide were studied and electron-transfer coefficient (α = 0.45), proton number (X = 1) and electron-transfer number (n = 2) have been determined. This method has been used to determine the content of nimesulide in medical tablets. The recovery was determined to be 93.2-106.2% by means of standard addition method. Compared with UV-vis spectrometry, the method was not remarkable difference.     

Improved fracture toughness of immiscible polypropylene/ethylene-co-vinyl acetate blends with multiwalled carbon nanotubes

Functionalized multiwalled carbon nanotubes (f-MWCNTs) have been introduced into immiscible polypropylene/ethylene-co-vinyl acetate (PP/EVA) blends. Two different compositions, one (PP/EVA = 80/20) exhibits the typical sea-island morphology and the other (PP/EVA = 60/40) exhibits the cocontinuous morphology, have been prepared with different contents of f-MWCNTs. The impact measurement shows that f-MWCNTs induce the great improvement of fracture toughness of cocontinuous PP/EVA blends. The results based on the morphologies and the rheological properties of the composites suggest that, a local “single-network structure” of f-MWCNTs exists in PP/EVA (80/20) system whereas a “dual-network structure” of f-MWCNTs and EVA phase exists in PP/EVA (60/40) system, and the latter structure accounts for the largely improved fracture toughness of the composites.     

Potentiometric stripping analysis of bismuth based on carbon paste electrode modified with cryptand [2.2.1] and multiwalled carbon nanotubes

An electrochemical method based on potentiometric stripping analysis (PSA) employing a cryptand [2.2.1] (CRY) and carbon nanotube (CNT) modified paste electrode (CRY-CNT-PE) has been proposed for the subnanomolar determination of bismuth. The characterization of the electrode surface has been carried out by means of scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronocoulometry (CC). It was observed that by employing CRY-CNT-PE, a 9-fold enhancement in the PSA signal (dt/dE) was observed as compared to plain carbon paste electrode (PCPE). Under the optimized conditions, dt/dE (s/V) was proportional to the Bi(III) concentration in the range of 5.55 × 10⁻⁸ to 9.79 × 10⁻¹¹ M (r = 0.9990) with the detection limit (S/N = 3) of 3.17 × 10⁻¹¹ M. The practical analytical utilities of the modified electrode were demonstrated by the determination of bismuth in pharmaceutical formulations, human hair, sea water, urine and blood serum samples. The prepared modified electrode showed several advantages, such as a simple preparation method, high sensitivity, very low detection limits and excellent reproducibility. Moreover, the results obtained for bismuth analysis in commercial and real samples using CRY-CNT-PE and those obtained by atomic absorption spectroscopy (AAS) are in agreement at the 95% confidence level.     

Deformation processes of ultrahigh porous multiwalled carbon nanotubes/polycarbonate composite fibers prepared by electrospinning

Mechanical deformation processes of electrospun composite fibers based on polycarbonate with multiwalled carbon nanotubes (MWCNTs) were investigated by in situ tensile tests under transmission electron microscope (TEM) depending on morphology. Using chloroform as solvent and optimizing process conditions, uniform nanoporous composite fibers were generated by electrospinning process. TEM images indicate that the MWCNTs were embedded in the fibers as individual elements, highly aligned parallel to one another along the fiber axis, which makes the mechanical load transfer from the polymer matrix to the MWCNT more favorable. Due to the slippage of individual MWCNTs within the fibers the strain at break of composite fibers is significantly enhanced. In addition, the nanopores on the fiber surface provide the effective sites for stress concentration for the plastic deformation to form nanonecking of fibers under tensile load. Combination of these unique features makes the electrospun composite fibers extremely strong and tough. The results from present work may provide a feasible consideration of such electrospun composite fibers for use as the reinforcing elements in a polymer based composite of a new kind.     

Using multiwalled carbon nanotube modified electrodes for the adsorptive striping voltammetric determination of hesperidin

Hesperidin, a flavone glycoside found in the skins and juices of citrus fruits, can be detected using multiwalled carbon nanotube (MWCNT)-modified electrodes using the technique of adsorptive stripping voltammetry (AdSV) with accumulation at open circuit potential. This is relevant because hesperidin can be used as an indication of the citrus fruit juice's freshness. The oxidation mechanism to explain the observed voltammetry corresponds to the redox chemistry of the guaiacol sub-unit within the hesperidin molecular structure. Hesperidin could be detected over a linear range up to 30 μM, and with a detection limit of 0.61 μM and 7 nM, with less than 5% variation between different electrodes, using cyclic voltammetric or square wave adsorptive stripping techniques respectively. This methodology was extended to MWCNT-modified screen-printed electrodes (MWCNT-SPEs), allowing the development of a cheap, mass produced, disposable sensor that we show is capable of measuring the concentration of hesperidin in real orange juice samples, and be applied within the citrus fruit industry.     

Well-dispersed single-walled carbon nanotube/polyaniline composite films

Single-walled carbon nanotube (SWNT)/polyaniline (PANI) composite films with good uniformity and dispersion were prepared by electrochemical polymerization of aniline containing well-dissolved SWNTs. The results of atomic force microscopy (AFM) and UV-Vis adsorption spectroscopy show that aniline can be used to solubilize SWNTs via formation of donor-acceptor complexes. The electrochemical deposition of SWNT-aniline solutions have been investigated by cyclic voltammetry. The results show that SWNT-based aniline solutions exhibit a drastic increase in peak current within the potential scanning region. The doping effect of SWNTs on PANI films was investigated by electrochemistry and FTIR spectroscopy. The results indicate that the enhanced electroactivity and conductivity of the SWNT/PANI composite films may be due to the strong interaction between SWNTs and PANI, which facilitates the effective degree of electron delocalization.     

The electrical conduction mechanism of multiwalled carbon nanotubes film synthesized by microwave plasma chemical vapor deposition

Multiwalled carbon nanotubes (MWNTs) were synthesized by microwave plasma chemical vapor deposition (MPCVD) on Ti/Si substrate with Ni catalyst. Then MWNT films were annealed at 300 °C for 10 min. Micro-Raman spectroscopy was performed to analyze the micro structure of the samples. It was noted from the Raman spectra that the ratio of I_D/I_G decreased after the annealing process, indicating more MWNTs were transformed to graphite structure at the temperature of 300 °C. In this research, the electrical properties of MWNTs film were investigated before and after the annealing process, different from the past research that focused on single or bundle of MWNTs. It was found that the I–V traces of the unannealed MWNTs film exhibited abrupt current increase and decrease phenomena by sweeping up and down voltages, respectively, and with different traces. Moreover, the threshold voltages for the saturation region increased after some successive measurements during sweeping up, but the similar effect was not observed during sweeping down. Similar phenomena were observed for the annealed MWNTs film; however, the threshold voltages, compared to that of the unannealed MWNTs film, spread out more during sweeping up and became smaller during sweeping down. It suggests that the nanotubes have thermally activated defects in rectifying conduction contacts between tungsten probes and MWNTs at the higher temperature induced by high applied voltages. Therefore, the breakdown voltages of the rectified contacts would change due to various thermally activated defects at higher temperatures. The equivalent circuit for the W-probe/MWNT-films/W-probe structure will be further presented in this study.     

Crystallization and orientation studies in polypropylene/single wall carbon nanotube composite

Crystallization behavior of melt-blended polypropylene (PP)/single wall carbon nanotube (SWNT) composites has been studied using optical microscopy and differential scanning calorimetry. Polypropylene containing 0.8 wt% SWNT exhibits faster crystallization rate as compared to pure polypropylene. PP/SWNT fibers have been spun using typical polypropylene melt spinning conditions. The PP crystallite orientation and the SWNT alignment in the fibers have been studied using X-ray diffraction and polarized Raman spectroscopy, respectively.