Improving dispersion of multiwalled carbon nanotubes in polyamide 6 composites through amino‐functionalization

The focus of this study is to investigate the state of dispersion of different treated multiwalled carbon nanotubes (MWNTs) in polyamide 6 (PA6). The MWNTs used in composites were grafted by 1,6‐hexamethylenediamine (HMD) via acid‐thionyl chloride to improve their compatibility with PA6 matrix. A microstructure transformation of MWNTs is found during the treatment process. Acidification makes the MWNTs compact and grafting HMD promotes the compact structure loose again. The MWNTs after different treatment were used to fabricate MWNTs/PA6 composites through melt blending. The dispersion of different MWNTs in PA6 was observed by a combination of scanning electron microscopy, optical microscopy, and transmission electron microscopy. The results show that the amino‐functionalized MWNTs are dispersed more homogeneously in PA6 than the purified MWNTs, and the poorest dispersion is achieved for acid treated MWNTs. It is indicated that the loose structure and functionalized surface of MWNTs benefit the dispersion of MWNTs in PA6. In addition, the amino‐functionalization of MWNTs improves the compatibility between the MWNTs and PA6, resulting in stronger interfacial adhesion. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Deposition of silver nanoparticles onto human serum albumin‐functionalised multi‐walled carbon nanotubes

To facilitate the design of carbon nanotube (CNT)‐based hybrid materials, a strategic approach for nanotube dispersion in aqueous media is required. This means that the reactants must exhibit certain selectivity towards both the nanotubes and the solvent medium. The main goal of this study was to prepare new bionanomaterials based on human serum albumin (HSA) and multi‐walled CNTs (MWCNTs), under mild conditions and without covalent modification of the nanotubes. Silver nanoparticles (AgNPs) of a controlled particle size, about 2‐nm diameter, were prepared and directly deposited onto the HSA‐functionalised MWCNTs. The characterisation of AgNP/HSA‐MWCNT hybrids prepared was carried out by scanning tunneling microscopy and transmission electron microscopy (TEM). The presence of the AgNPs was corroborated by elemental chemical analysis using TEM‐coupled energy‐dispersive X‐ray spectroscopy. The density of AgNPs coverage is discussed as a function of HSA concentration, the strength of the reducing agent, and the nature of protein employed (HSA vs. bovine serum albumin and rotavirus nucleocapsid protein VP6). © 2012 Canadian Society for Chemical Engineering     

Crystallization and melting behavior of multi‐walled carbon nanotube‐reinforced nylon‐6 composites

The crystallization and melting behavior of neat nylon‐6 (PA6) and multi‐walled carbon nanotubes (MWNTs)/PA6 composites prepared by simple melt‐compounding was comparatively studied. Differential scanning calorimetry (DSC) results show two crystallization exotherms (T_{CC, 1} and T_{CC, 2}) for PA6/MWNTs composites instead of a single exotherm (T_{CC, 1}) for the neat matrix. The formation of the higher‐temperature exotherm T_{CC, 2} is closely related to the addition of MWNTs. X‐ray diffraction (XRD) results indicate that only the α‐phase crystalline structure is formed upon incorporating MWNTs into PA6 matrix, independently of the cooling rate and annealing conditions. These observations are significantly different from those for PA6 matrix, where the increase in cooling rate or decrease in annealing temperature results in the crystal transformation from α‐phase to γ‐phase. The crystallization behavior of PA6/MWNTs composites is also significantly different from those reported in PA6/nanoclay systems, probably due to the difference in nanofiller geometry between one‐dimensional MWNTs and two‐dimensional nanoclay platelets. The nucleation sites provided by carbon nanotubes seem to be favorable to the formation of thermodynamically stable α‐phase crystals of PA6. The dominant α‐phase crystals in PA6/MWNTs composites may play an important role in the remarkable enhancement of mechanical properties. Copyright © 2005 Society of Chemical Industry     

Properties of polyamide 6/multiwalled carbon nanotubes composites: The influence of processing methods

The objective of this work was to elucidate the influence of shear rates on the properties of polyamide 6/multiwalled carbon nanotube (PA6/CNT) composites which was realized by adopting different types of processing methods that feature different orders of magnitude in shear rates, such as compression molding (CM, ~0 s⁻¹), conventional injection molding (CIM, ~10² s⁻¹) and microinjection molding (μIM, ~10⁵ s⁻¹). Electrical conductivity (σ) results indicated that the prevailing high shearing conditions in injection molding was unfavorable for the formation of intact filler network, thereby resulting in a much lower σ than CM counterparts. Moreover, the σ of PA6/CNT microparts was higher than that of CIM macroparts when the filler content was less than 5 wt%, otherwise the σ of CIM macroparts prevailed over that of μIM counterparts. A better filler distribution was observed when PA6/CNT composites were processed under higher shearing conditions, as corroborated by SEM. In addition, CNTs were preferentially aligned along flow direction and a higher degree of CNT orientation was expected with increasing shear rates, as confirmed by Raman spectral analysis. The tensile strength of injection molded PA6/CNT samples increased with increasing filler concentrations, and the more preferential orientation and better distribution of CNT were considered to be the contributing factors. The comparative study of the properties of PA6/CNT composites that processed using different methods was important for their practical applications in industrial sectors.     

Efficient dispersion of multi‐walled carbon nanotubes by in situ polymerization

Multi‐walled carbon nanotube (MWNT)‐reinforced polyimide nanocomposites were synthesized by in situ polymerization of monomers in the presence of acylated MWNTs. The acyl groups associated with the MWNTs participated in the reaction through the formation of amide bonds. This process enabled uniform dispersion of MWNT bundles in the polymer matrix. The resultant MWNT–polyimide nanocomposite films were optically transparent with significant mechanical enhancement at a very low loading (0.5 wt%). Evidence has been obtained for improved interactions between the nanotubes and the matrix polymer. Copyright © 2006 Society of Chemical Industry     

Preparation and characterization of multi‐walled carbon nanotubes/polymer gradient composite films

BACKGROUND: Recently, much work has focused on the efficient dispersion of carbon nanotubes (CNTs) throughout a polymer matrix for mechanical and/or electrical enhancement. However, there are still only few reports about gradient distribution of CNTs in polymer matrices. In the work reported here, CNTs embedded in a polymer film with a gradient distribution were successfully obtained and studied. RESULTS: For composite films with gradient distributions of CNTs, the upper surface behaves as an intrinsic insulator, while the lower one behaves as a semiconductor, or even as a conductor. It is also found that with an increase of 1 wt% CNTs, the resistance of the bottom surface decreases by 2–3 orders of magnitude, as compared with pure polyarylene ether nitrile; furthermore, when the proportion of CNTs increases up to 5 wt%, the resistance of the bottom surface shows only very little change. As a result, sufficient matrix conductivity of the bottom surface could be achieved at a lower filler concentration with CNTs in a gradient distribution. Meanwhile, the thermal stability, glass transition temperature and tensile properties of the matrix are maintained. CONCLUSION: There is considerable interest in such gradient composite films, which could be applied in the electrical engineering, electronics and aerospace fields, for their excellent mechanical properties, thermal stability and novel electrical properties. Copyright © 2008 Society of Chemical Industry     

Mechanical and surface properties of polyurethane/fluorinated multi‐walled carbon nanotubes composites

To improve the mechanical and surface properties of poly(etherurethane) (PEU), multi‐walled carbon nanotubes (MWCNTs) were surface grafted by 3,3,4,4, 5,5,6,6,7,7,8,8,8‐tridecafluoro‐1‐octanol (TDFOL) (MWCNT‐TDFOL) and used as reinforcing agent for PEU. Fourier‐transform infrared spectroscopy revealed the successful grafting of MWCNTs. PEU filled with MWCNT‐TDFOL could be well dispersed in tetrahydrofuran solution, and tensile stress–strain results and dynamic mechanical analysis showed a remarkable increase in mechanical properties of PEU by adding a small amount of MWCNT‐TDFOL. Contact angle testing displayed a limited improvement (just 9°) in the hydrophobicity of PEU surface by solution blending with MWCNT‐TDFOL. However, a large improvement of surface hydrophobicity was observed by directly depositing MWCNT‐TDFOL powder on PEU surface, and the water contact angle was increased from 80° to 138°. Our work demonstrated a new way for the modification of carbon nanotubes and for the property improvement of PEU. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Assessment of adequate sodium hypochlorite concentration for pre‐oxidization of multi‐walled carbon nanotubes

BACKGROUND: Inadequate surface oxidization can potentially destroy the structure and chemical characteristics of the carbon nanotubes (CNTs) and result in loss of sorption capacity and weight. It is necessary to carefully oxidize CNTs using an adequate NaOCl concentration before using it as a sorbent. RESULTS: FE‐SEM images showed that the structure of CNTs oxidized with 7% NaOCl concentration was significantly destroyed and agglomerated as larger carbon particles instead of nanotubes. The surface acidities of CNTs oxidized with 3% and 5% NaOCl concentration were almost the same, with maximum values 6.20 and 6.25 mmol g^?1, respectively, in all studied cases. Conversely, increasing NaOCl concentration to 7% decreased the acidity from 6.25 to 5.0 mmol g^?1, indicating that 7% NaOCl concentration is not suitable for oxidization of CNTs. Assessing the factors (CNTs mass, contact time, pH, and ionic strength), that influence adsorption performance showed that CNTs oxidized with 5% NaOCl concentration performed better than those with 1%, 3% and 7% concentrations. CONCLUSION: Considering simultaneously the percentage recovery, adsorption performance and isotherms of CNTs oxidized by NaOCl solutions at four different concentrations, an optimum NaOCl concentration of 5% is suggested by this study. Copyright © 2010 Society of Chemical Industry     

Polymer nanocomposites reinforced with multi‐walled carbon nanotubes for semiconducting layers of high‐voltage power cables

Polymer nanocomposites reinforced with multi‐walled carbon nanotubes (MWCNTs) have been newly introduced for semiconducting layers of high‐voltage electrical power cables. Homogeneity of the MWCNT‐reinforced polymer nanocomposites was achieved by solution mixing, and their mechanical, thermal and electrical properties were investigated depending on the type of polymer. By changing the polymer matrix, the volume resistance of the MWCNT‐reinforced polymer nanocomposites could be varied by more than four orders of magnitude. Through systematic experiments and analysis, two possible factors affecting the volume resistance were found. One is the degree of crystallinity of the polymer used and the other is the change of MWCNT morphology under strain. By increasing the degree of crystallinity above a certain level, the volume resistance linearly increased. The MWCNTs embedded in the nanocomposites gradually protruded through the surface on stretching the sample and reversibly returned back to the original positions at a relatively small strain (below 20%). Based on the criteria of tensile properties and volume resistance, a poly[ethylene‐co‐(ethyl acrylate)]/MWCNT nanocomposite was selected as the best candidate for the semiconducting layers of high‐voltage electrical power cables. Copyright © 2009 Society of Chemical Industry     

Enhanced dielectric properties of polyamide 11/multi‐walled carbon nanotubes composites

Composites with multi‐walled carbon nanotubes (MWNTs) involved in polyamide 11 (PA11) were prepared via a conventional melt blending method. The structure, morphology, crystallization behavior, electrical, and dielectric properties of composites were investigated. The results demonstrated that the dispersed uniformly MWNTs favored the formation of α crystal of PA11 when the composites were quenched from melt. The dielectric constant of composites was dependent on the electric field frequency and MWNTs content, and the highest value of dielectric constant was as high as 350 for the composite with 1.21 vol % MWNTs at 10³ Hz, accompanied by a low dielectric loss. The enhanced dielectric properties could be interpreted by the formation of abundant nanocapacitors within the composites and the interfacial polarization effect resulting from accumulation of charge carriers at the internal interfaces between MWNTs and PA11. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42642.     

Functionalized multi‐wall carbon nanotubes/silicone rubber composite as capacitive humidity sensor

Functionalized multi‐wall carbon nanotubes (MWCNTs) treated by mixed acids have been used to develop a capacitive humidity sensor based on MWCNTs/silicone rubber (SR) composite film. The MWCNTs/SR composites were prepared through conventional solution processed method. The micrographs of MWCNTs/SR composites were observed by transmission electron microscopy (TEM) and scanning electron microscope. The FT‐IR spectra demonstrated the successfully grafting of ? OH groups on the treated MWCNTs. The sensing properties of the composite at different relative humidity (RH) and frequency were characterized and linear sensing responses of the MWCNTs/SR composites to RH were observed. The treated MWCNTs/SR composite film (Tr‐film) had higher sensitivity than that of the untreated MWCNTs/SR composite film (Un‐film). Experimental data indicate that the Tr‐film exhibits an excellent long‐term stability, small hysteresis, and fine reproducibility. The response and recovery time of the Tr‐film were 30 and 27 s, respectively. Thereby, such Tr‐film had potential applications as humidity sensors. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40342.     

Functionalization of multi‐walled carbon nanotubes with poly(ε‐caprolactone) using click chemistry

Multi‐walled carbon nanotubes (MWNTs) were covalently functionalized with poly(ε‐caprolactone) (PCL) using click chemistry. First, chlorine moiety‐containing PCL was synthesized by the copolymerization of α‐chloro‐ε‐caprolactone with ε‐caprolactone monomer using ring opening polymerization, and further converted to azide moiety‐containing PCL. The alkyne‐functionalized MWNTs were prepared with the treatment of p‐amino propargyl ether using a solvent free diazotization procedure. The covalent functionalization of alkyne‐derived MWNTs with azide moiety‐containing PCL was accomplished using Cu(I)‐catalyzed [3+2] Huisgen dipolar cycloaddition click chemistry. The PCL‐functionalization of MWNTs was confirmed by the measurements of Fourier transform infra‐red, NMR, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Ionomer covalent functionalization of single‐walled carbon nanotubes by radical polymerization of zirconium acrylate

A facile and efficient covalent functionalization of single‐walled carbon nanotubes (SWCNTs) via peroxide‐mediated free radical covalent attachment and polymerization of zirconium acrylate is reported. The resulting covalently functionalized SWCNTs exhibit improved solubility in organic solvents. The covalently functionalized SWCNTs are characterized by cross polarization magic angle spinning ¹³C NMR, differential scanning calorimetry, thermogravimetric analysis, x‐ray diffraction, Raman, and infrared spectroscopy. Infrared spectroscopy reveals that carboxylate groups of covalently attached ionomers chelate with zirconium ions and the participating carboxylate groups may be from different ionomer chains leading to cross‐linking the chains. The SWCNT topology, ionic clustering, and π‐electron clouds were explored by transmission electron microscopy. © 2014 American Institute of Chemical Engineers AIChE J, 60: 820–828, 2014     

Preparation of biocompatible multi‐walled carbon nanotubes as potential tracers for sentinel lymph nodes

Carbon nanotubes (CNTs) are capable of traversing cellular membranes by endocytosis and are therefore promising materials for use in imaging and drug delivery. Unfortunately, pristine CNTs are practically insoluble and tend to accumulate inside cells, organs and tissues. To overcome the poor dispersibility and toxicity of pristine CNTs, hydrophilic functionalization of CNTs has been intensively investigated. Water‐soluble multi‐walled carbon nanotubes (MWCNTs) were prepared by in situ polymerization of acrylic acid in a poor solvent for poly(acrylic acid) (PAA). The solvent type influenced the grafted density and chain length of PAA. MWCNTs with a high grafted density of PAA (22 wt%) could be well dispersed in water, NaCl aqueous solution (0.9 wt%) and cell culture media. The in vitro cytotoxicity of these MWCNTs for endothelial cells is reasonably low even at high concentration of PAA‐g‐MWCNT (70 µg mL^?1). The experimental results show that the biocompatibility of these MWCNTs is sufficient for biological applications. PAA‐g‐MWCNTs were successfully utilized for lymph node tracing. Experimental results suggest that PAA‐g‐MWCNTs have potential to be used as a vital staining dye, which may simplify the identification of lymph nodes during surgery. Copyright © 2009 Society of Chemical Industry     

Curing kinetics of a novolac resin modified with oxidized multi‐walled carbon nanotubes

The influence of oxidized multi‐walled carbon nanotubes (o‐MWCNTs) on the curing kinetics of a novolac resin was studied by means of non‐isothermal differential scanning calorimetry. Regarding the kinetics issues, the high concentration of hydroxyl groups on the o‐MWCNTs slightly modified the curing reaction of the novolac resin, shifting the differential scanning calorimetry exothermic peak to higher temperatures. The effective activation energy of the curing reaction was calculated by the isoconversional Kissinger‐Akahira‐Sunose method and increased by the presence of o‐MWCNTs with respect to neat novolac. This change was attributed to the increase of the material viscosity. In addition, thermogravimetric analysis revealed that nanocomposites samples containing 0.4 and 1.0 wt% o‐MWCNTs presented increased char yield values, indicating an improvement of flame retardancy.     

Multi‐walled carbon nanotubes as unexpected accelerators of chemical reactions

The influence of carbon nanotubes on various chemical reactions has been examined. These reactions include epoxy crosslinking, in situ polymerization of copolymers in the presence of carbon nanotubes and thermal degradation of a thermoplastic matrix. The various characterizations were based on different types of analyses, including differential scanning calorimetry, proton nuclear magnetic resonance, size exclusion chromatography and thermogravimetric analysis. In each case, we show that the presence of multi‐walled carbon nanotubes accelerates the process. © 2012 Society of Chemical Industry     

New piezoelectric damping composites of poly(vinylidene fluoride) blended with clay and multi‐walled carbon nanotubes

Damping materials are used to control mechanical vibrations, and piezoelectric damping composite is a very promising material due to its unique mechanism. In this study, a potential piezoelectric damping composite was developed by simply melt mixing poly(vinylidene fluoride) (PVDF) with small amounts of organic modified montmorillonite (OMMT) and multi‐walled carbon nanotubes (MWCNTs). The piezoelectric, mechanical and electrical properties were investigated using a dynamic mechanical analyser, direct current electrical resistivity measurements, X‐ray diffraction, Fourier transform infrared spectroscopy and the direct quasi‐static d₃₃ piezoelectric coefficient method. It was found that the damping property of PVDF can be greatly improved by adding both MWCNTs and OMMT, and the composite containing 1.9 wt% of MWCNTs and 3 wt% of OMMT showed the best damping property. A model and an approximate calculation were applied to explain the improved damping property. Moreover, similar mechanical properties of PVDF composites were observed in the tensile testing and dynamic mechanical analyser measurements. Copyright © 2012 Society of Chemical Industry     

Effect of multi‐walled carbon nanotubes dispersity on the light transmittancy of multi‐walled carbon nanotubes/epoxy composites

Two types of multi‐walled carbon nanotubes (MWCNTs), chemically modified and unmodified, were dispersed in epoxy resin with ultrasonication. The light transmittance characteristics of epoxy composites with different ratios of MWCNTs to epoxy resin were measured at wavelengths ranging from 200 to 1100 nm. Results showed that composites with modified MWCNTs had a much higher light transmittance than those with unmodified MWCNTs. This was presumably due to a more uniform dispersion of modified MWCNTs in the epoxy matrix, as indicated by both transmission electron microscopy and optic microscopy. The wavelength dependency of light transmittance of the composites was expressed empirically as a function of weight fraction (f_w) of MWCNTs and the light wavelength (λ). POLYM. ENG. SCI. 46:635–642, 2006. © 2006 Society of Plastics Engineers.