Structure-property relationship of SELF-sustained homogeneous ternary nanocomposites: Key issues to evaluate properties of rrP3HT wrapped MWNT dispersed in TPU

An interesting correlation between nature of wrapping, wrapping thickness and crystallinity of regioregular poly(3-hexyl thiophene) (rrP3HT) wrapped multi-walled nanotube (MWNT) arises due to different loading of rrP3HT and their combined effect on the properties of a ternary system prepared by uniform dispersion of wrapped CNT into thermoplastic polyurethane (TPU) are highlighted in the article. Data accumulated through different techniques demonstrate that 2.5 wt.% of rrP3HT with 0.5 wt.% of MWNT can be the ideal ratio of filler to achieve highest properties in these stable self-sustained homogeneous composites. Wrapping of rrP3HT on the wall of CNT through π-π and/or CH-π interaction is ascertained from shifting in peak position and I_asym/I_sym ratio of CC bond of rrP3HT in FTIR spectroscopy. Strong quenching of fluorescence intensity of rrP3HT in composite further support π-π interaction between rrP3HT and CNTs. SEM micrograph of rrP3HT/TPU blends suggest uniform globular dispersion of polythiophene into TPU matrix without any separate phase domain and addition of CNTs considerably reduce globule size. Single Tg(∼−40 °C, DMA, DSC, TMA) clearly ascertain the miscibility of composite. An ‘order to order transition’ through coil to rod transformation leads to strong, sharp red shifting (∼150 nm shift compared to pristine rrP3HT) in emission peaks of rr-poly (3-hexylthiophene) in blends. Further red shifting and highest quenching is observed in case of 2.5% rrP3HT loaded ternary system whereas blue shifting and quenching in case of 0.5 wt.% (non-uniform wrapping) and 5 wt.% (agglomerates) rrP3HT loading.     

Controlling bubble density in MWNT/polymer nanocomposite foams by MWNT surface modification

Polymer nanocomposite foams are promising substitutes for polymeric foams. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Creating polymer foams with controlled foam morphology is of great importance for controlling foam properties. The foam morphology is influenced by the foaming conditions and filler properties. For carbon nanotube/polymer composite foams, dispersion state and aspect ratio of the carbon nanotubes have been shown to influence the bubble density and bubble size. In the current study, the influence of carbon nanotube surface chemistry on the bubble density of multi-walled carbon nanotube/poly(methyl methacrylate), MWNT/PMMA, nanocomposite foams was investigated. The surface of the MWNTs with controlled aspect ratio was covalently modified with glycidyl phenyl ether (GPE). Surface modified MWNT/PMMA nanocomposite foams were produced using a supercritical carbon dioxide foaming process. At constant MWNT concentration, the bubble density of polymer nanocomposite foams filled with GPE surface modified MWNT was found to be several times higher than that of polymer nanocomposite foams filled with nitric acid treated MWNT. After the MWNTs were modified with GPE, the surface chemistry of the MWNT became the dominant factor in determining the bubble density while the MWNT aspect ratio became less influential.     

Fabrication of homogeneous titania/MWNT composite materials

MWNT/titania nanocomposites were prepared by an impregnation method and subsequent heat treatment at 400 °C. Precursor compounds such as titanium (IV) propoxide and titanium (IV) ethoxide were used to cover the surface of CNTs under solution conditions. Electron microscopy and X-ray diffraction techniques were carried out to characterize the as-prepared titania layers.     

Preparation of a crack-free rough titania coating on stainless steel mesh by electrophoretic deposition

TiO₂ (anatase) coating was prepared on stainless mesh by electrophoretic (EPD) process utilizing an isopropyl alcohol (IPA)-based suspension with submicron TiO₂ powder. When the deposition time was 30 s, a smooth thin coating was obtained. It remained crack-free even after sintering. Coating surface morphology was roughened by UV pre-illumination of the suspension. Photocatalytic decomposition of IPA to acetone and resultant electrochemical reaction at cathode during EPD provides heterogeneous deposition.     

Surface coating of multi-walled carbon nanotube nanopaper on shape-memory polymer for multifunctionalization

We are presenting a method of synthesizing three-dimensional self-assembled multi-walled carbon nanotube (MWCNT) nanopaper on hydrophilic polycarbonate membrane. The process is based on the very well-defined dispersion of nanotube and controlled pressure vacuum deposition procedure. The morphology and structure of the nanopaper are characterized with scanning electronic microscopy (SEM) over a wide range of scale sizes. A continuous and compact network observed from the microscopic images indicates that the MWCNT nanopaper could have highly conductive property. As a consequence, the sensing properties of conductive MWCNT nanopaper are characterized by functions of temperature and water content. Meanwhile, in combination with shape-memory polymer (SMP), the conductive MWCNT nanopaper facilitates the actuation in SMP nanocomposite induced by electrically resistive heating. Furthermore, the actuating capability of SMP nanocomposite is utilized to drive up a 5-gram mass from 0 to 30 mm in height.     

Preparation of hydroxyapatite-containing titania coating on titanium substrate by micro-arc oxidation

Hydroxyapatite-containing titania coatings on titanium substrates were formed by micro-arc oxidation (MAO) in electrolyte containing calcium acetate monohydrate (CH₃COO)₂Ca·H₂O) and sodium phosphate monobasic dihydrate (NaH₂PO₄·2H₂O) using a pulse power supply. Scanning electron microscopy (SEM) with Energy dispersive X-ray spectrometer (EDX) and X-ray diffraction (XRD) were employed to characterize the microstructure, elemental composition and phase components of the coatings. The coatings were rough and porous, without apparent interface to the titanium substrates. All the oxidized coatings contained Ca and P as well as Ti and O, and the porous coatings were made up of anatase, rutile and hydroxyapatite. Such MAO films are expected to have significant applications as artificial bone joints and dental implants.     

Effects of gamma-irradiation on UHMWPE/MWNT nanocomposites

Ultra high molecular weight polyethylene (UHMWPE) is a polymer that is widely used in industrial and orthopaedic applications. In this work, pristine multiwalled carbon nanotubes (MWCNTs) were incorporated into UHMWPE in different concentrations (1, 3 and 5 wt.%) using a ball milling process. UHMWPE/MWCNT nanocomposites were gamma irradiated at 90 kGy to improve the interaction between MWCNTs and the polymer matrix. Structural, thermal and mechanical characterizations were conducted by means of transmission electron microscopy (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and uniaxial tensile techniques. Gamma irradiation produced an increase in the melting temperature, crystallinity and temperature of maximum decomposition rate. The irradiation produced a 38% decrease in the toughness of neat UHMWPE. The incorporation of MWCNTs did not significantly affect the melting point of the neat UHMWPE but decreased the degree of crystallinity of the raw UHMWPE, which was related to a reduction in the UHMWPE lamellar density. An increase in thermal stability was also observed for the nanocomposites compared to neat UHMWPE. The tensile tests showed a 38% increase in the Young’s modulus in the reinforced nanocomposites and a small decrease in toughness (5%). Gamma irradiation of the nanocomposites increased crystallinity, which was related to an increased lamellar thickness, and also improved their thermal stability. The Young’s modulus increased by up to 71% for irradiated nanocomposites and their toughness showed no significant changes in comparison with the non-irradiated nanocomposites. The incorporation of MWCNTs reduced the negative effects of irradiation and compensated for the reduction in toughness. This fact might be attributed to the radical scavenger behaviour of the MWNT as was proved by Electron Spin Resonance (ESR) detection of the radiation-induced radicals.     

Correlation between electrokinetic potential, dispersibility, surface chemistry and energy of carbon nanotubes

This paper proposes the correlation between the electrokinetic potential, dispersibility in solvents, surface energy and oxygen content of carbon nanotubes (CNTs) affected by functionalization. Colloidal systems consisting of CNTs with varying degrees of dispersion are prepared and characterized to evaluate CNT dispersibility and suspension stability in solvents with different polarities. The results show that an absolute value of zeta potential at about 25 mV is closely related to the micro- and macroscopic dispersion of CNTs, whereas a high absolute value of 40 mV is regarded as an indication of high quality CNT dispersion with much enhanced suspension stability in solvents. The absolute zeta potential value increases consistently with increasing degree of CNT functionality, the increase being most pronounced in a hydrophilic liquid such as water. A linear correlation is established between the surface energy of a CNT film and the oxygen to carbon ratio of CNT surface. The CNT dispersibility in a liquid is determined not only by their physical states, but also by the hydrophilicity and surface functionality of CNTs, all of which are reflected by zeta potential.     

Aligned carbon nanotube coating on polyethylene surface formed by microwave radiation

Multifunctional carbon nanotube (CNT) architectures have been created on polyethylene (PE) surface by a microwave welding process. The continuous and aligned CNT films drawn from super-aligned CNT arrays can significantly absorb microwave energy and act as a network of nanosized thermal sources to locally melt the PE substrate beneath, leading to polymer wrapping around individual nanotubes. Uniform and highly conductive CNT/PE nanocomposite layer was formed without undermining the original alignment of the CNTs. CNT patterns have also been precisely fabricated on PE samples. The PE/CNT/PE bonds showed high interfacial strengths, which were affected by the duration of microwave radiation. With ultra-low content of CNTs introduced as antistatic agents, the dissipation of surface charges on PE substrate has been tremendously improved.     

Effect of multi-walled carbon nanotubes on the crystallization and hydrolytic degradation of biodegradable poly(l-lactide)

Biodegradable poly(l-lactide) (PLLA)/carboxyl-functionalized multi-walled carbon nanotubes (f-MWNTs) nanocomposites were prepared via solution blending. Scanning electron microscopy observations reveal a fine dispersion of f-MWNTs in the PLLA matrix. The presence of f-MWNTs enhances the crystallization of PLLA in the nanocomposites compared with that of neat PLLA; moreover, the overall crystallization rate of PLLA increases with increasing the f-MWNTs content in the PLLA matrix. The incorporation of f-MWNTs improves the storage modulus of the PLLA/f-MWNTs nanocomposites, with this effect being more pronounced at lower f-MWNTs content. The exciting aspect of this research is the enhanced hydrolytic degradation of PLLA after nanocomposites preparation with f-MWNTs, which may be of great interest for its wide practical application.     

Measurement of the critical aspect ratio and interfacial shear strength in MWNT/polymer composites

This paper presents a bulk composite method for determining the critical aspect ratio and relative interfacial shear stress (ISS) for multiwalled carbon nanotube (MWNT)/polymer composites. Through a modified pullout test and fragmentation test, it was found that the critical aspect ratio was 300 and decreased by a factor of 3 due to surface modification, and that MWNTs at an angle of greater than 60° to the loading direction failed in bending instead of pulling out of the matrix. Finite element analysis was used to determine the critical bending shear strength and MWNT modulus. The obtained bending shear strength was used in a mechanics model developed to provide bounds for the ISS in the experimental composite system. The calculated ISS for as-received nanotube falls between 4.8 and 13.7 MPa, and for surface treated nanotube falls in the range of 11.1 and 38.3 MPa. These values are consistent with the ISS reported for carbon fiber/polymer composites and also show that the ISS almost triples due to chemical modification of the MWNT surface.     

Melt mixed nano composites of PA12 with MWNTs: Influence of MWNT and matrix properties on macrodispersion and electrical properties

Nanocomposites containing four different polyamide 12 (PA12) types and three grades of multiwalled carbon nanotubes (MWNTs) were prepared via small-scale melt processing to study the effect of different MWNTs and the influence of polymer properties on the dispersion of the fillers and the electrical properties of the composites. Under the selected mixing conditions the lowest electrical percolation threshold of 0.7 wt.% was found for Nanocyl™ NC7000 in low viscous PA12. Moreover, big influences of the end group functionality (acid or amine excess) and the melt viscosity of the matrix were found. Composites of PA12 with acid excess showed lower percolation thresholds than those based on amine terminated materials. At constant end group ratio low viscous matrices resulted in lower percolation thresholds than high viscous materials. The best MWNT dispersion was obtained in both high viscous PA12 composites. In these systems the mixing speed was varied indicating an optimum concerning electrical conductivity at 150 rpm as compared to 50 and 250 rpm.     

Tribological behaviour and wear of carbon nanotubes grafted on carbon fibres

Carbon nanotubes (CNTs) grafted on fibres are widely used to reinforce composites in order to improve their mechanical properties. This study concerned the tribological properties of CNTs grafted on carbon fibres by the flame method. The aim of this study was to determine whether CNTs on fibres suffer damage under stress, similar to those applied during composite manufacturing, which can damage composite properties, particularly fibre/matrix adhesion. For this purpose, reciprocating friction tests were performed to examine the resistance of CNTs and highlight a wear mechanism. The results showed that the presence of CNTs increased the coefficient of friction in the first friction cycles and then decreased it to close to the COF of the fibre without CNTs. The wear mechanism showed that after a small number of friction cycles, the CNTs were flattened out and formed a transfer film.     

The impact of fluorinated MWCNT additives on the enhanced dynamic mechanical properties of e-beam-cured epoxy

Multi-walled carbon nanotubes were embedded into e-beam-cured epoxy resin to improve the mechanical properties of epoxy resin. The surfaces of these carbon nanotubes were modified using a fluorination treatment to improve their dispersion and adhesion in epoxy resin. The dynamic mechanical properties of epoxy/carbon nanotube composites were investigated at various heating rates and frequencies. As an effect of fluorination treatment, the semi-ionic bond of C–F on the surface of multi-walled carbon nanotubes played an important role in the improved dispersion and adhesion of carbon nanotubes into the epoxy resin. The storage modulus and loss modulus of the composites increased with higher applied frequency. The activation energy of the composites was increased by the effects of a higher heating rate due to the slow heat transfer in the epoxy/carbon nanotube composites. Eventually, the dynamic mechanical properties of the investigated epoxy were significantly improved by the carbon nanotubes dispersed therein via the fluorination treatment.     

Effect of phenoxy-based coating resin for reinforcing pitch carbon fibers on the interlaminar shear strength of PA6 composites

Carbon fiber-reinforced thermoplastic composites have not been considered as constituent materials for structural parts due to the poor interfacial adhesion between the fiber and the thermoplastic matrix. In this work, polyamide 6 (PA6) composites with pitch carbon fibers (pCF) were fabricated by alternatively stacking PA6 films and pCF fabrics followed by being pressed. In order to improve the interfacial adhesion, phenoxy resin-based materials were coated on the surface of the fiber. The surface analyses of the fiber were carried out by XPS, TGA and dynamic contact angle method. Interlaminar shear strength (ILSS) of the composites was measured to evaluate the effect of the coating materials. The results showed that the composites with the coated pCF had higher ILSS than that with neat pCF by more than 20%. This indicated that a proper coating material can improve mechanical properties of the PA6 composites, which can be applied to the structural parts.     

Fabrication of a nitrided coating on a novel Ni-Cr nanocomposite with increased surface hardness

A novel type of Ni-Cr nanocomposite that had a nanocrystalline Ni matrix dispersing Cr nanoparticles was developed by electrodeposition. During plasma nitriding at 560 °C for 10 h, the nanocomposite formed a surface hard coating in which the Cr nanoparticles were internally converted into CrN. The coating decreased in thickness but increased in hardness with the increase in the Cr content from 10.8 to 30 (by wt.%). Moreover, the Cr content increase caused a shift of the hardest area of the nitrided coating from the innermost to the topmost surface, as a result of the change in the nitridation mechanism of the composite from the internal to external.     

Fracture mechanisms of epoxy filled with ozone functionalized multi-wall carbon nanotubes

This work focused on the fracture mechanisms and reinforcing effects of ozone-treated multi-walled carbon nanotubes (MWCNTs) in epoxy matrix. Ozone functionalization of MWCNTs was found to be of help for a better dispersion and stronger interfacial bonding with epoxy matrix, which in turn improve the strength and fracture toughness of the resin. The MWCNT/epoxy composites showed complicated failure modes than the conventional fibrous composites, which have been quantitatively investigated and correlated with the fracture toughness of the nanocomposites studied.     

Effect of surface oxidation on thermal shock resistance of the ZrB2-SiC-ZrC ceramic

The isothermal oxidation of the ZrB₂-SiC-ZrC ceramic was carried out in static air at a constant temperature of 1000 ± 15 °C, 1200 ± 15 °C and 1400 ± 15 °C for 30 min, respectively. Compared with the original strength of 580 MPa, the strength for the specimen oxidized at 1000 °C, 1200 °C and 1400 °C for 30 min increased to 609 MPa, 656 MPa and 660 MPa, respectively, because the flaws in the surface of the specimen were sealed by the oxide layer. The thermal shock resistance of the specimens before and after the oxidation was measured by the water quenching. The measured ΔT_crit for the specimen oxidized at 1000 °C, 1200 °C and 1400 °C were 352 °C, 453 °C and 623 °C, respectively, which was obviously higher than 270 °C for the unoxidized specimen. The improvement in the thermal shock resistance was attributed to the formation the oxide layer on the surface of the specimen. The results here pointed to a promising method for improving strength and thermal shock resistance of ZrB₂-based ceramics.     

MWNT reinforced polyurethane foam: Processing, characterization and modelling of mechanical properties

The mechanical properties of a foam material changes when the foam is reinforced with nanoparticles. In this paper it is investigated how the addition of multi-walled carbon nanotubes (MWNTs) influences the effective properties of polyurethane foam. Both pure and nano-reinforced foams containing different amounts of MWNT are produced and both pristine and functionalized MWNT are used as reinforcement. The MWNT are dispersed in the polyol using high-shear mixing with various mixing times to examine how that influences the properties of the produced foams. SEM is used to characterize the microstructure of the produced foams and these examinations reveals that the foam changes from a completely closed cell material for the pure PU foam to a partly open celled foam when adding MWNT. Compressive tests are performed in order to determine the strength and stiffness of the produced foams and the increase in these properties are very dependent on both the wt.% of MWNT and the mixing time used to disperse them in the polyol. Furthermore, the effective properties of the reinforced foams are determined using the Mori-Tanaka (MT) method and generally the correlation between the experimentally and numerically determined properties improves when the mixing time used increases for a constant wt.% of MWNT.     

Effect of surfactant treatment on thermal stability and mechanical properties of CNT/polybenzoxazine nanocomposites

The mechanical and thermo-mechanical properties of polybenzoxazine nanocomposites containing multi-walled carbon nanotubes (MWCNTs) functionalized with surfactant are studied. The results are specifically compared with the corresponding properties of epoxy-based nanocomposites. The CNTs bring about significant improvements in flexural strength, flexural modulus, storage modulus and glass transition temperature, T_g, of CNT/polybenzoxazine nanocomposites at the expense of impact fracture toughness. The surfactant treatment has a beneficial effect on the improvement of these properties, except the impact toughness, through enhanced CNT dispersion and interfacial interaction. The former four properties are in general higher for the CNT/polybenzoxazine nanocomposites than the epoxy counterparts, and vice versa for the impact toughness. The addition of CNTs has an ameliorating effect of lowering the coefficient of thermal expansion (CTE) of polybenzoxazine nanocomposites in both the regions below and above T_g, whereas the reverse is true for the epoxy nanocomposites. This observation has a particular implication of exploiting the CNT/polybenzoxazine nanocomposites in applications requiring low shrinkage and accurate dimensional control.