Aggregation Of Multi-walled Carbon Nanotubes Research Articles

Properties of carbon nanotube reinforced linear low density polyethylene nanocomposites fabricated by cryogenic ball‐milling

AbstractA cryogenic ball‐milling process to produce polymer/CNT nanocomposites was investigated. Linear low density polyethylene was used as the matrix material and 1 wt % of multiwalled carbon nanotubes (MWCNT) was used as reinforcement. The influence of the milling time and balls size was evaluated. The morphology of the nanocomposite and the degree of dispersion of the MWCNTs were studied using scanning electron microscopy (SEM), visual inspection, and light transmission microscopy; ropes as well as aggregates of MWCNTs were observed, and there was evidence of wetting of the nanotubes by the matrix polymer. An increase of up to 28% in the elastic modulus (determined by tensile testing) with respect to the matrix was obtained. Differential scanning calorimetry (DSC) analysis showed evidence of increase in the degree of crystallization, a result of the nucleating capability of the carbon nanotubes in the matrix. The degradation temperature of the nanocomposites does not show significant variations with respect to the unfilled polymer. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

Electric-Field-Induced Dispersion of Multiwalled Carbon Nanotubes in Nematic Liquid Crystal

Effect of electric field and frequency on multiwalled carbon nanotubes (MWCNTs) dispersion in liquid crystal (LC) medium has been investigated. At zero field, large MWCNT aggregates are found due to insufficient dispersion throughout the cell under optical microscopy. As the vertical field strength increases gradually, the cluster sizes become smaller, and at above critical field strength, the clusters disappear and are dispersed finally into LC medium. The dispersion of CNT clusters remained stable even after the field is removed. Strong correlation of cluster sizes with field strength was also observed. The dispersion of clusters at above critical field was also confirmed by the light absorption. This robustness of field-induced dispersion of CNTs provides a new method of improving solubility of CNTs in LC medium, which can be advantageous in numerous CNT-LC optoelectronic devices.

Core-sheath polyurethane-carbon nanotube nanofibers prepared by electrospinning

The core-sheath nanofibers consisting of polyurethane (PU) core and PU composites sheath with multi-walled carbon nanotubes (MWNTs) were prepared by electrospinning. At low MWNT concentration, MWNTs appeared highly aligned along the fiber axis with some curving in nanotubes, whereas in case of high concentration, some aggregation of MWNTs appeared due to difficulty in full dispersion of nanotubes. In comparison of the single component nanofiber webs, the core-sheath nanofiber webs showed much better mechanical properties of modulus and breaking stress, including an exceptional elongation-at-break. It indicates that the CNT-incorporated core-sheath structure is very effective for enhancing the mechanical properties of nanofiber webs. In addition, the core-sheath nanofibers demonstrated the fast shape recovery, compared with one component fibers of pure shape memory PU and PU/MWNTs, which provides the possibility of fabricating more sensitive intelligent materials.

Mechanical Properties of Porous Al<SUB>2</SUB>O<SUB>3</SUB> Composite with Surface Modified Multi-Walled Carbon Nanotubes (MWCNTs)

Multi-walled carbon nanotubes (MWCNTs) have been reinforced in alumina (Al2O3) matrix to overcome the inherent brittleness of the Al2O3 matrix. In this work, MWCNTs were treated by acid to provide hydrophilicity to hydrophobic MWCNTs, inducing the homogeneous dispersion of MWCNTs in an aqueous solution. Aluminum hydroxide (Al(OH)3) as a Al2O3 precursor was added in the solution with the modified MWCNTs, and then this mixture solution was filtered at room temperature. The prepared powders were calcinated at 800-1000 degrees C to reduce the gas pocket in the matrix by decomposition of Al(OH)3. Then the calcinated powders were formed, and heat-treated. The porous MWCNTs-Al2O3 composites show higher mechanical properties in flexure strength and hardness than the porous Al2O3 without the reinforcement phase, which is attributed to the high mechanical properties of MWCNTs. However, higher MWCNTs contents in the composites decrease the mechanical properties due to the aggregation of MWCNTs in the composites. Therefore, control of the MWCNTs content and its dispersibility in the matrix are key factors to be considered for the fabrication of the porous MWCNT-Al2O3 composites.

Electrical Properties of Isotactic Polypropylene/Multiwalled Carbon Nanotubes Composites Prepared by Vibration Injection Molding

To study the effect of vibration field on the electrical conductivity properties of nanocomposites, isotactic polypropylene (iPP)/multiwalled carbon nanotubes (MWCNT) composites were prepared by conventional injection molding and vibration injection molding. Results showed that the electrical conductivity of iPP/MWCNT composites was significantly promoted by vibration injection molding. Vibration injection molded samples had a percolation threshold of about 2.7 wt% compared with the threshold of about 4.5 wt% for conventional injection molded samples. The effects of test locations and vibration frequency on the electrical conductivity of composites were investigated. The samples exhibited an inhomogeneity along the injection direction. The electrical conductivity of the samples was different at different test locations and increased with increasing vibration frequency. Polarized light microscopy (PLM) results indicated that vibration injection molding can induce MWCNT aggregates to be stretched and oriented along the flow direction, which could form conductive networks and greatly enhance the electrical conductivity of iPP/MWCNT composites.

Decoration of multiwall carbon nanotubes with nickel nanoparticles: effect of deposition strategy on metal dispersion and performance in the hydrogenation of p-chloroacetophenone

The sonochemical deposition of nickel nanoparticles onto the multiwall carbon nanotube aggregates leads to a uniformly distributed metal phase at high loadings and an average nickel crystal size of 4–8 nm at a 25–50 wt% nickel content. Its application enhances the catalytic activity of Ni/multiwall carbon nanotube material in the selective hydrogenation of chloroacetophenone by factors of 2–18, as compared with that prepared by traditional decoration methods.

Effective functionalization of multiwalled carbon nanotube with amphiphilic poly(propyleneimine) dendrimer carrying silver nanoparticles for better dispersability and antimicrobial activity

Two multiwalled carbon nanotube hybrids have been prepared: (a) multiwalled carbon nanotubes (MWCNTs) functionalized with amphiphilic poly(propyleneimine) dendrimer (APPI), viz. MWCNTs-APPI, and (b) silver nanoparticles (AgNPs)-deposited multiwalled carbon nanotubes functionalized with an amphiphilic poly(propyleneimine) dendrimer (MWCNTs-APPI-AgNPs). The degree of covalent functionalization of APPI in MWCNTs and deposition of AgNPs in MWCNTs-APPI were examined by Fourier transform infrared spectroscopy, zeta potential, scanning and high-resolution transmission electron microscopy, energy-dispersive spectroscopy, thermogravimetric analysis, and Raman spectroscopy. The amount of APPI functionalized on MWCNTs determined by thermal gravimetric analysis was about 67% which enables an effective dispersability in aqueous and organic solvents without sonication and these solutions were stable for 6months without undergoing aggregation of MWCNTs. The electronic properties of the hybrid materials were not altered drastically as verified by the Raman studies. The antimicrobial activities of MWCNTs-APPI and MWCNTs-APPI-AgNPs against three different bacteria, viz. Bacillus subtilis, Staphylococcus aureus, and Escheriachia coli illustrated excellent activity.

Non-destructive functionalization of multi-walled carbon nanotubes with naphthalene-containing polymer for high performance Nylon66/multi-walled carbon nanotube composites

A new compatibilizer, poly(vinyl benzyloxy methyl naphthalene)- g-poly( t-butyl methacrylate- co-methacrylic acid), was synthesized for Nylon 66 (N66)/multi-walled carbon nanotube (MWCNT) composites. It has been shown that the naphthalene unit in the main chain of the compatibilizer interacts with MWCNTs by π–π interaction and that the carboxylic acid unit in the graft chain of the compatibilizer interacts with the amide group of N66. The use of the compatibilizer produces well-dispersed MWCNTs in N66 matrix, which results in improved mechanical and electrical properties of the composites, while the simple mixture of N66/MWNCTs without the compatibilizer exhibits poor mechanical and electrical properties due to severe aggregation of MWCNTs. It is also found that the compatibilizer with a small amount of carboxylic acids is more effective for improving the mechanical and electrical properties of N66/MWCNT composites.

Lipophilic carbon nanotubes and their phase-separation in SBS

Highly lipophilic multi-walled carbon nanotubes (MWNTs) were successfully prepared by grafting PAA onto the surface of MWNT, followed by replacing carboxyl groups of PAA with octadecylamine through amidation. The improved dispersion of functionalized MWNTs and enhanced mechanical properties of nanotube/SBS composites were observed. The composite thin films with different MWNT loading prepared via spin-coating were investigated by means of atomic force microscopy (AFM) in order to study the dispersion of MWNTs in the polymer matrix. The evolution of MWNT/SBS composite thin film surface morphology with the variation of MWNT concentration is presented. When the MWNT concentration is lower than 1 wt%, the composite thin film is homogeneous. With increase of MWNT concentration, phase separation can be clearly observed. It is found that the state of aggregation of MWNTs in the polymer matrix determine the mechanical properties of the composites.

Increasing the thermal conductivity of palmitic acid by the addition of carbon nanotubes

Four different methods, acid oxidation, mechanochemical reaction, ball milling, and grafting following acid oxidation, were used to treat multi-walled carbon nanotubes (MWCNTs). During treatment, hydroxyl groups, carboxylic groups, and amidocyanogen were introduced onto the surfaces of the MWCNTs. The MWCNTs were dispersed into palmitic acid (PA) to prepare phase change composites with high thermal conductivity. Both chemical treatment and ball milling help to break the MWCNT aggregates and to enhance their dispersibility. Measurements show that the thermal conductivity increase of the composites is highly dependent on the MWCNT pretreatment process. We propose that the difference in the interfacial thermal resistance between the MWCNTs and the matrix is due to the difference of the MWCNT surface state caused by different treatment processes. In all the MWCNT/PA composites, the one containing MWCNTs with hydroxyl groups, treated by a mechanochemical reaction, has the highest thermal conductivity increase, which, at room temperature, is up to 51.6% for a MWCNT addition of 1.0%.

Exposure of murine macrophages to carbon nanoparticles promotes inflammation and inhibits phagocytosis (130.5)

Abstract Exposure to respirable ultrafine particles (2.5 μM) can adversely affect human health and have been implicated with episodes of increased respiratory diseases such as asthma and allergies; the same is predicted for nanoparticles, which can reach deeper regions of the lung. The objective of this investigation was to examine the effects of carbon nanoparticles on alveolar macrophages. Macrophages (MΦ) function as the first line of defense against invading pathogens and are likely to be amongst the first cells affected by nanoparticulates. We focused on two manufactured nanoparticles: multiwalled carbon nanotube aggregates and black-carbon. The two were tested against MΦ in a chronic contact model. We hypothesized that exposure to nanoparticles would decrease MΦ ability to effectively respond to immunological challenge. Production of interleukin (IL)-12 and nitric oxide (NO), as well as phagocytosis, and MΦ activation were examined in response to lipopolysaccharide (LPS) following a 144 hr exposure to the nanoparticles. Data demonstrated an increase in NO and IL-12 production, a decrease in phagocytosis, and a decrease in the expression levels of activation markers. The data suggests that carbon nanoparticle exposure alters MΦ responses to LPS marked by increased inflammation while potentially limiting clearance and ability to interact with effector T-cells. Thus, physiological exposure to carbon nanoparticles could potentially lead to ineffective pulmonary immunity.

Dual-stimuli sensitive composites based on multi-walled carbon nanotubes and poly( N,N-diethylacrylamide-co-acrylic acid) hydrogels

A strategy to prepare dual-stimuli sensitive poly( N,N-diethylacrylamide-co-acrylic acid) composite hydrogels has been demonstrated. Multi-walled carbon nanotubes (MWCNTs) functionalized with vinyl groups were copolymerized with N,N-diethylacrylamide (DEA), acrylic acid (AA) and N,N′-methylenebisacrylamide (NNMBA) to obtain the composites (NCG). As expected, the compression properties of the NCG were greatly improved without noticeably reducing the response rates to temperature changes. Field emission scanning electron microscopy showed that the surface of the NCG was denser, which affords the NCG better compression properties, while there was obvious MWCNTs aggregation on the surface of the hybrid gel sample by physical mixing of the MWCNTs (HYG).

Crystallization and morphology of iPP/MWCNT prepared by compounding iPP melt with MWCNT aqueous suspension

In this work, isotactic polypropylene (iPP) composites filled with multiwalled carbon nanotubes (MWCNTs) were prepared by compounding iPP melt with MWCNT aqueous suspension using a corotating twin-screw extruder, and the morphology and crystallization behavior of the composites were investigated. Scanning electron microscopy micrographs showed that MWCNTs dispersed individually at nanoscale in the iPP matrix when the MWCNTs concentration was low, though MWCNTs aggregates were detected when the filler concentration increased. The results of differential scanning calorimetry, wide-angle X-ray diffraction, and polarized light microscopy indicated that the β-form crystal of iPP was induced by MWCNTs at the concentration of 0.1 wt.% which was dispersed individually in the iPP matrix. At higher content, however, MWCNTs acted as α-nucleating agent, and the crystals in the iPP/MWCNT composites showed higher degree of perfection than that of pure iPP though smaller in dimension. Crystallization rate of iPP increased significantly with increasing MWCNT content.

Effects of Copper, Lead, and Cadmium on the Sorption and Desorption of Atrazine onto and from Carbon Nanotubes

There are currently few studies on the dual effects of metal ions on the sorption of atrazine and conversely of atrazine on metal adsorption on multiwalled carbon nanotubes (MWCNTs). While a number of sorption models were considered to describe the sorption of atrazine on MWCNTs, the Polanyi-Manes model (PMM) fit the sorption isotherms well with the lowest mean weighted square errors. Atrazine was mainly adsorbed onto the surface and micropores of MWCNTs bundles or aggregates. Hydrogen bonding between azo and amino nitrogen of atrazine and functional groups on MWCNTs also occurred. Oxygenated functionalities, mainly carboxylic groups on MWCNTs surface, decreased the sorption of atrazine. Metal cations Cu2+, Pb2+, and Cd2+ diminished the sorption of atrazine depending on the oxygenated functionalities densities. The mechanisms ascribed were due to the formation of surface or inner-sphere complexes of Cu2+, Pb2+, and Cd2+ through carboxylic groups and hydration, which may occupy part of the surface of MWCNTs-O. The large hydration shell of metal cations may intrude or shield the hydrophobic and hydrophilic sites and indirectly compete with atrazine for surface sites, leading to the inhibition of atrazine adsorption around the metal-complexed moieties.

Aggregation Kinetics of Multiwalled Carbon Nanotubes in Aquatic Systems: Measurements and Environmental Implications

The initial aggregation kinetics of multiwalled carbon nanotubes (MWNTs) were examined through time-resolved dynamic light scattering. Aggregation of MWNTs was evaluated by varying solution pH and the concentration of monovalent (NaCl) and divalent (CaCl2 and MgCl2) salts. Suwannee River humic acid (SRHA) was used to study the effect of background natural organic matter on MWNT aggregation kinetics, Increasing salt concentration and addition of divalent calcium and magnesium ions induced MWNT aggregation by suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The critical coagulation concentration (CCC) values for MWNTs were estimated as 25 mM NaCI, 2.6 mM CaCl2, and 1.5 mM MgCl2. An increase in solution pH from acidic (pH 3) to basic (pH 11) conditions resulted in a substantial (over 2 orders of magnitude) decrease in MWNT aggregation kinetics, suggesting the presence of ionizable functional groups on the MWNT carbon scaffold. The presence of humic acid in solution markedly enhanced the colloidal stability of MWNTs, reducing the aggregation rate by nearly 2 orders of magnitude. The enhanced MWNT stability in the presence of humic acid is attributable to steric repulsion imparted by adsorbed humic acid macromolecules. Our results suggest that MWNTs are relatively stable at solution pH and electrolyte conditions typical of aquatic environments.

Improvement of tensile properties of poly(methyl methacrylate) by dispersing multi-walled carbon nanotubes functionalized with poly(3-hexylthiophene)- graft-poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA)/multi-walled carbon nanotube (MWCNT) composites are prepared using poly(3-hexylthiophene) (P3HT)- graft-PMMA as a compatibilizer. Fluorescence emission and Raman spectra indicate that the P3HT part of the compatibilizer strongly interacts with the surface of MWCNT. Addition of the compatibilizer results in well-dispersed PMMA/MWCNT composite with enhanced mechanical properties, whereas PMMA/MWCNT composite prepared without addition of the compatibilizer shows an aggregation of MWCNT in PMMA matrix. Particularly, PMMA/MWCNT composites prepared in the presence of the compatibilizer do not exhibit the trade-off of mechanical properties, whereas PMMA/MWCNT composites without the compatibilizer exhibit the trade-off behavior.

Pulmonary and Systemic Immune Response to Inhaled Multiwalled Carbon Nanotubes

Inhalation of multiwalled carbon nanotubes (MWCNTs) at particle concentrations ranging from 0.3 to 5 mg/m3 did not result in significant lung inflammation or tissue damage, but caused systemic immune function alterations. C57BL/6 adult (10- to 12-week) male mice were exposed by whole-body inhalation to control air or 0.3, 1, or 5 mg/m3 respirable aggregates of MWCNTs for 7 or 14 days (6 h/day). Histopathology of lungs from exposed animals showed alveolar macrophages containing black particles; however, there was no inflammation or tissue damage observed. Bronchial alveolar lavage fluid also demonstrated particle-laden macrophages; however, white blood cell counts were not increased compared to controls. MWCNT exposures to 0.3 mg/m3 and higher particle concentrations caused nonmonotonic systemic immunosuppression after 14 days but not after 7 days. Immunosuppression was characterized by reduced T-cell-dependent antibody response to sheep erythrocytes as well as T-cell proliferative ability in presence of mitogen, Concanavalin A. Assessment of nonspecific natural killer (NK) cell activity showed that animals exposed to 1 mg/m(3) had decreased NK cell function. Gene expression analysis of selected cytokines and an indicator of oxidative stress were assessed in lung tissue and spleen. No changes in gene expression were observed in lung; however, interleukin-10 (IL-10) and NAD(P)H oxidoreductase 1 mRNA levels were increased in spleen.

Synthesis of Polythiophene-graft-PMMA and Its Role as Compatibilizer for Poly(styrene-co-acrylonitrile)/MWCNT Nanocomposites

A new compatibilizer, polythiophene-graft-poly(methyl methacrylate) (PMMA), for poly(styrene-co-acrylonitrile) (SAN)/multiwalled carbon nanotube (MWCNT) composite is synthesized. Fluorescence emission spectra and Raman spectra indicate that the polythiophene part of the compatibilizer strongly interacts with the surface of MWCNT, while differential scanning calorimeter (DSC) thermograms show that the PMMA graft of the compatibilizer is miscible with the SAN matrix. The use of new compatibilizer yields well-dispersed SAN/MWCNT nanocomposites with enhanced mechanical properties, whereas the simple mixture of SAN/MWCNT prepared without the addition of the compatibilizer shows an aggregation of MWCNT in SAN matrix. Particularly, SAN/MWCNT nanocomposites in the presence of the compatibilizer show dramatically improved mechanical properties even with the addition of a very small amount (0.01 wt %) of MWCNT.

Experimental observation on the flow-induced assembly of Carbon nanotube suspensions to form helical bands

This paper describes optical observations on the way aggregates of multi-walled carbon nanotubes (CNTs) in an epoxy matrix can form unusual helical bands (HBs) that are aligned perpendicular to the shear flow. By applying specific flow conditions, HBs were formed from isotropic aggregates of CNTs suspended in an essentially Newtonian epoxy matrix. Both optical and matching rheological data are presented together with a schematic model of the way HBs are formed. It was discovered that the steady shear rheology of the CNT suspension did not substantially change during the observed micro-structure change. The HB structure may have relevance in terms of potential applications for CNTs.

Cytotoxic effects of aggregated nanomaterials

This study deals with cytotoxicity assays performed on an array of commercially manufactured inorganic nanoparticulate materials, including Ag, TiO 2, Fe 2O 3, Al 2O 3, ZrO 2, Si 3N 4, naturally occurring mineral chrysotile asbestos and carbonaceous nanoparticulate materials such as multiwall carbon nanotube aggregates and black carbon aggregates. The nanomaterials were characterized by TEM, as the primary particles, aggregates or long fiber dimensions ranged from 2 nm to 20 μm. Cytotoxicological assays of these nanomaterials were performed utilizing a murine alveolar macrophage cell line and human macrophage and epithelial lung cell lines as comparators. The nanoparticulate materials exhibited varying degrees of cytoxicity for all cell lines and the general trends were similar for both the murine and human macrophage cell lines. These findings suggest that representative cytotoxic responses for humans might be obtained by nanoparticulate exposures to simple murine macrophage cell line assays. Moreover, these results illustrate the utility in performing rapid in vitro assays for cytotoxicity assessments of nanoparticulate materials as a general inquiry of potential respiratory health risks in humans.