Dispersion and thermal conductivity of carbon nanotube composites

A mechanical method was used to shorten carbon nanotubes (CNTs) for improving dispersion without reducing their thermal conductivity. Single walled carbon nanotubes (SWCNTs) were mechanically cut to produce short and open-ended fullerene pipes. These shortened SWCNTs were then used in polymer composites. Both atomic force microscopy and scanning electron microscopy characterizations suggested that nanotube shortening significantly improved CNT dispersion. Thermal conductivity of composites containing short CNTs were found to be much better than those containing pristine CNTs.     

A comparison of the effects of multi-wall and single-wall carbon nanotube additions on the properties of zirconia toughened alumina composites

The use of multi-wall carbon nanotubes (MWCNTs) or single-wall carbon nanotubes (SWCNTs) as filler in ceramic matrices could create composites with exceptional mechanical properties. We have prepared dense monolithic alumina (Al₂O₃) and zirconia-toughened alumina (ZTA) composites with additions of 0.01 wt% of MWCNTs or 0.01 wt% of SWCNTs by conventional sintering and have demonstrated that the mechanical properties depend on (a) the distribution of CNTs in the matrix and (b) the interaction between the ceramic phases and CNTs. The fracture toughness of Al₂O₃ ceramics reinforced with SWCNTs was significantly better than those reinforced with MWCNTs. However, fracture toughness in MWCNT-reinforced ZTA increased 41% over ZTA free of the toughening agent and 44% over ZTA reinforced with SWCNTs. A well dispersed and small amount of MWCNTs was enough to produce an increase of fracture toughness in ZTA composites.     

Development and characterization of PEEK/carbon nanotube composites

New poly(ether ether ketone) (PEEK) based composites have been fabricated by the incorporation of single-walled carbon nanotubes (SWCNTs) using melt processing. Their structure, morphology, thermal and mechanical properties have been investigated. Scanning electron microscopy observations demonstrated a more uniform distribution of the CNTs for samples prepared following a processing route based on polymer ball milling and CNT dispersion in ethanol media. Thermogravimetric analysis indicated a remarkable improvement in the thermal stability of the matrix by the incorporation of SWCNTs. Differential scanning calorimetry showed a decrease in the crystallization temperature with increasing SWCNT content, whilst no significant changes were observed in the melting of the composites. The crystallite size determined by X-ray diffraction decreased at high SWCNT loading, which is attributed to the spatial limitations on crystal growth by confinement within the CNT network. Dynamic mechanical analysis revealed an increase in the storage moduli, hence in the rigidity of the systems, with increasing SWCNT content. Their addition shifts the glass transition peak to higher temperatures due to the restriction in chain mobility imposed by the CNTs. Higher thermal stability and mechanical strength were found for composites with improved dispersion of the SWCNTs.     

Supercritical carbon dioxide assisted preparation of conductive polypyrrole/cellulose diacetate composites

Supercritical carbon dioxide (SC‐CO₂) has been used to assist the preparation of conductive polypyrrole/cellulose diacetate (PPy/CDa) composites by in situ chemical oxidative polymerization. The morphology and conductivity of resulted composites were investigated with scanning electron microscopy and four‐probe method, respectively. With the assistance of strong swelling effect of SC‐CO₂, composite films were obtained with a macroscopically homogeneous structure and conductivity up to 10^?1 S cm^?1 order of magnitude. Increasing the pressure of SC‐CO₂ increased conductivity, while increasing the temperature decreased conductivity. For comparison, PPy/CDa composite was also prepared with conventional oxidative method in aqueous solution. From the viewpoint of conductivity and environmental protection, the SC‐CO₂ method showed its superiority over the conventional one. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4575–4580, 2006     

Characterization of PP/Mg(OH)2 and PP/Nanoclay Composites with Supercritical CO2 (scCO2)

In this article, supercritical carbon dioxide (scCO₂) is used to form a high density microcellular foam structure to reduce the polymer use and facilitate dispersion of Mg(OH)₂ and Nanoclay fillers. A twin-screw extruder system was used to predistribute the inorganic filler from the PP polymer, resulting composite PP/filler pellets. This followed by the use of a single-screw extruder wherein supercritical carbon dioxide is introduced in the formulation. Finally the resulting foam PP/filler/CO₂ pellets are injection molded into test samples. The structure and properties of the composites are characterized using a scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), and density measurements. Furthermore, PP/Clay/Mg(OH)₂ polymer composites are subjected to examinations to obtain their yield and tensile strengths, elasticity modulus, % elongation, Izod impact strength, hardness, Heat deflection temperature (HDT), Vicat softening point and Melt flow index (MFI).     

Synthesis of sea urchin-like particles of carbon nanotubes directly grown on stainless steel cores and their effect on the mechanical properties of polymer composites

Multi-walled carbon nanotubes (CNTs) were directly grown on spherical stainless steel (SS) particles of average diameter of 100 nm to produce a structure that resembled sea urchins. Micro-structural observations by scanning and transmission electron microscopy revealed that the CNTs were densely deposited and adhered well to the SS particles. Using these particles as a filler in polymer composites produced a high tensile strength as results of their effective dispersion and good interfacial bonding to the polymer matrix.     

Tailored dielectric and mechanical properties of noncovalently functionalized carbon nanotube/poly(styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene) nanocomposites

The preparation of high‐dielectric poly(styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene) (SEBS) composites containing functionalized single‐walled carbon nanotubes (f‐SWCNTs) noncovalently appended with dibutyltindilaurate are reported herein. Transmission electron microscopy and X‐ray photoelectron and Raman spectroscopy confirmed the noncovalent functionalization of the SWCNTs. The SEBS‐f‐SWCNT composites exhibited enhanced mechanical properties as well as a stable and high dielectric constant of approximately 1000 at 1 Hz with rather low dielectric loss at 2 wt% filler content. The significantly enhanced dielectric property originates from the noncovalent functionalization of the SWCNTs that ensures good dispersion of the f‐SWCNTs in the polymer matrix. The f‐SWCNTs also acted as a reinforcing filler, thereby enhancing the mechanical properties of the composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermal conductivity of poly vinylidene fluoride composites filled with expanded graphite and carbon nanotubes

The composites composed of Poly (vinylidene fluoride), expanded graphite (EG), and carbon nanotubes (CNTs) have been prepared by solution mixing, followed by compression. The structure of the composites was examined with scanning electron microscope and their electrical and thermal properties were investigated. About 1.2 wt % content of CNTs could present a percolated network in the polymer matrix, characterized by the electrical conductivity. The incorporation of EG and CNTs in the polymer caused an enhancement in thermal conductivity for the composites. However, a hybrid of EG and CNTs as filler of the polymer yielded a further improvement in thermal conductivity as compared to single component filler. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Grafting of an aminated poly(phenylene sulphide) derivative to functionalized single-walled carbon nanotubes

An aminated poly(phenylene sulphide) derivative (PPS-NH₂) has been covalently anchored to the surface of epoxy and acid-functionalized single-walled carbon nanotubes (SWCNTs). The characterisation through Fourier transform infrared spectroscopy, nuclear magnetic resonance, thermogravimetric analysis and Kaiser test corroborated the success of the grafting reactions, and allowed the identification and quantification of the covalent moieties. Scanning and transmission electron microscopy indicated an increase in the bundle diameter of the SWCNTs upon anchoring of the polymer chains. The results showed that the storage modulus, glass transition temperature and electrical conductivity of the polymer were exceptionally enhanced by the attachment to the SWCNTs. In contrast, the crystallization and melting temperature, degree of crystallinity and crystal size considerably decreased, as revealed by differential scanning calorimetry and X-ray diffraction experiments, due to the inactive nucleating role of these SWCNTs and the intense restrictions on chain mobility imposed by the SWCNT–polymer interactions. Acid-functionalized SWCNTs were more effective for reinforcing PPS-NH₂ than epoxy-functionalized SWCNTs, attributed to the formation of a larger number of covalent bonds, albeit led to a smaller increase in the electrical conductivity of the polymer. The results herein offer useful insights into the development of multifunctional CNT-reinforced thermoplastic composites for a wide variety of applications.     

Using supercritical carbon dioxide in preparing carbon nanotube nanocomposite: Improved dispersion and mechanical properties

Improvements in carbon nanotube (CNT) dispersion and subsequent mechanical properties of CNT/poly(phenylsulfone) (PPSF) composites were obtained by applying the supercritical CO₂ (scCO₂)‐aided melt‐blending technique that has been used in our laboratory for nanoclay/polymer composite preparation. The preparation process relied on rapid expansion of the CNTs followed by melt blending using a single‐screw extruder. Scanning electronic microscopy results revealed that the CNTs exposed to scCO₂ at certain pressures, temperatures, exposure time, and depressurization rates have a more dispersed structure. Microscopy results showed improved CNT dispersion in the polymer matrix and more uniform networks formed with the use of scCO₂, which indicated that CO₂‐expanded CNTs are easier to disperse into the polymer matrix during the blending procedure. The CNT/PPSF composites prepared with scCO₂‐aided melt blending and conventional melt blending showed similar tensile strength and elongation at break. The Young's modulus of the composite prepared by means of conventional direct melt blending failed to increase beyond the addition of 1 wt% CNT, but the scCO₂‐aided melt‐blending method provided continuous improvements in Young's modulus up to the addition of 7 wt% CNT. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Extrusion with Carbon Dioxide (CO2) and Characterization of ABS/Mg (OH)2/Nanoclay Composites

In this paper, carbon dioxide (CO₂) is used to form a high-density microcellular thermoplastic foam structure in order to reduce polymer consumption and facilitate dispersion of Mg (OH)₂ and nanoclay fillers. A twin-screw extruder system was used to predistribute inorganic fillers into the ABS polymer, resulting in composite ABS/filler pellets. This is followed by the use of a single-screw extruder wherein supercritical carbon dioxide is introduced into the formulation. Finally, the resulting foam ABS/filler/CO₂ pellets are injection- molded into test samples. The structure and properties of the composites are characterized using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Furthermore, ABS/Mg(OH)₂/nanoclay polymer composite samples are tested to obtain their yield and tensile strengths, elastic moduli, yield and tensile elongations, izod impact strengths, hardness values, heat deflection temperatures (HDT), Vicat softening points, and melt flow indices (MFI). These tests reveal that for the overall reduction in the amount of polymer in the samples, material properties did not generally deteriorate and even showed improvements in some areas. Moreover, resulting injection-molded samples have been shown to possess dimensional integrity due to the continued expansion of CO₂ during the molding operation.     

Synthesis and field emission properties of carbon nanotubes/nanofibers grown on pyrolyzed polyaniline–SiO2 substrates

Pyrolyzed polyaniline–SiO₂ substrates with the rough surface containing some holes were prepared by the pyrolysis of polyaniline–SiO₂ composites at temperature of 900 °C. Carbon nanotubes/nanofibers (CNTs/CNFs) were grown on the rough surface and inside the holes using a CVD method with a xylene–ferrocene mixture as a carbon and catalyst precursor source. The structural and morphological properties of CNTs were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that the SiO₂ content of the substrates was responsible to the diameter and electron field emission properties of CNTs.     

Preparation and properties of MoSi2 based composites reinforced by carbon nanotubes

Molybdenum disilicide (MoSi₂) based composites with various contents of carbon nanotubes (CNTs) were made by sintering in vacuum at 1500 °C for 1 h. Mechanical properties of these composites at room temperature revealed the addition of CNTs to have good hardening and toughening effect on the matrix. Especially when adding 6.0% CNTs by volume, the hardness and fracture toughness were improved respectively by about 25.3% and 45.7% compared to pure MoSi₂. Phase identification and microstructure of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HTEM). Multi-walled CNTs were found in the powders synthesized by self-propagating high temperature synthesis (SHS) and SiC phase existed in the sintering samples. Fine grain and the favorable effect of dispersed SiC particles resulted in a high hardness of the CNTs/MoSi₂ composite. The toughening mechanisms for the CNTs/MoSi₂ composites included crack deflection, crack micro-bridging, crack branching, crack bowing and fine-grain pullout.     

Crystallinity of biodegradable polymers reinforced with functionalized carbon nanotubes

Well-dispersed multiwall carbon nanotubes (MWCNTs) were prepared by grafting poly(L-lactide-co-ε-caprolactone) (PLACL) biodegradable copolymer onto the sidewall of hydroxylated MWCNTs using oligomeric L-lactide (LA) and ε-caprolactone (CL). After preparation of MWCNT/PLACL composites, the effect of functionalized MWCNTs on crystallinity of PLACL was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and polarized light optical microscopy (POM). The surface functionalization effectively improved the dispersion and adhesion of MWCNTs which acted as reinforcing filler in the PLACL polymer matrix and hence improved the physical and thermomechanical properties of the nanocomposites. The glass transition temperature (T _g) and the crystallinity of nanocomposites decreased in comparison with those of neat PLACL when the concentration of functionalized MWCNTs in nanocomposites was 0.5 wt%. With further increment in concentration of functionalized MWCNTs, the T _g of composites increased until the T _g of neat PLACL, and also the crystallinity of composites increased. The functionalized MWCNTs have no significant effect on the melting point of nanocomposites. The MWCNTs acted as heterogeneous nucleation points and increased the lamella size and therefore the crystallinity of PLACL. Furthermore, the larger agglomerated clusters of both kinds of MWCNTs (i.e., MWCNT-grafted-PLACL and pristine MWCNTs) are more effective than small clusters as nucleation points for growing the spherulites.     

Different filler effect of carbon nanotube and graphene nanoplatelet in the poly(arylene ether nitrile) matrix

The different filler effects of identical nitrile‐functionalized carbon nanotubes (CNTs) and graphene nanoplatelets (GNs) in a poly(arylene ether nitrile) (PEEN) matrix were investigated. PEEN/CNT and PEEN/GN composites were prepared by a facile solution‐casting method and systematically investigated for their differences in morphological, thermal and rheological properties. In the PEEN matrix GNs contact one another in a plane‐to‐plane manner, while CNTs are separated. Compared with PEEN/CNT composites, PEEN/GN composites below 2 wt% filler content exhibited higher thermal stability. Rheological properties of the resulting composites indicated that PEEN/GN composites were more sensitive to strain and exhibited higher η*, G′ and G″ than PEEN/CNT composites. The rheological percolation for CNTs is over 2 wt%, higher than that for GNs (around 1 wt%). All these differences originate from the different dimensions and structures of CNTs and GNs: GNs with a flake‐like structure and larger surface area can have stronger physical and interfacial interactions with the polymer matrix. This work gives a comparative view of the different filler effects that functionalized CNTs and GNs can have in the polymer host. With identical processing technology, GNs can show a stronger filler effect than CNTs. © 2012 Society of Chemical Industry     

Evidence of sidewall covalent functionalization of single-walled carbon nanotubes and its advantages for composite processing

Single-walled carbon nanotubes (SWCNTs) were functionalized in a three-step procedure. The first step is a radical reaction creating a covalent bond between the carbon nanotube surface and grafted p-methoxyphenyl functional groups. In a second step, a deprotection of the methoxy functions generates free alcohol groups and in the final step an esterification is done in order to install a double bond for further polymerization. Evidence that functionalization has actually occurred on the SWCNT sidewalls is furnished through investigations involving several complementary techniques (visual dispersion tests, transmission electron microscopy, thermal gravimetric analysis and adsorption volumetry). We show that surface properties of SWCNTs are changed throughout the chemical treatments and that the obtained level of functionalization is low. Incorporation of functionalized SWCNTs in a polymer (poly(methyl methacrylate)) matrix was done through an in situ polymerization process. Observations of the obtained composites using scanning and transmission electron microscopy illustrate that interactions between the SWCNT surface and the polymer matrix are improved.     

Coating carbon nanotubes with metal oxides in a supercritical carbon dioxide-ethanol solution

Metal oxide films, including Ce₂O₃ and/or CeO₂, Al₂O₃, La₂O₃, were deposited on the outer surfaces of carbon nanotubes (CNTs) through the decomposition of metal nitrate precursors in supercritical CO₂ modified with ethanol. Transmission electron microscopy showed that CNTs could be coated with metal oxide layers that were nominally complete and uniform. The thickness of the coating could be readily tailored by tuning the ratio of the initial mass of precursors to CNTs. The as-prepared CeO₂-CNT composites showed high sensitivity and selectivity to acetone on the basis of chemiluminescence detection.     

A direct synthesis of mesoporous carbon supported MgO sorbent for CO2 capture

Ordered mesoporous carbon supported MgO (Mg-OMC) materials were synthesized by the carbonization of sulfuric-acid-treated silica/triblock copolymer/sucrose/Mg(NO₃)₂ composites. In the current approach, triblock copolymer P123 and sucrose were employed as both structure-directing agents for the self-assembly of rice husk ash silica solution and carbon precursor. Sulfuric acid was used to cross-link P123 and sucrose in the as-synthesized composites in order to improve the carbon yield. The synthesized Mg-OMC was characterized by X-ray diffraction, N₂ adsorption-desorption isotherm method, X-ray photoelectron spectroscopy, scanning electron microscope equipped with energy dispersive X-ray analysis and transmission electron microscopy. The thermal stability of Mg-OMC was verified by CO₂-temperature programmed desorption, which confirmed the chemisorption of CO₂ on MgO. The CO₂ adsorption capacity of Mg-OMC-1 was observed to be 92 mg/g of sorbent which is comparable with that of the well established CO₂ sorbents.     

Hard and tough carbon nanotube-reinforced zirconia-toughened alumina composites prepared by spark plasma sintering

It is demonstrated that 0.1 wt% of multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) added to zirconia toughened alumina (ZTA) composites is enough to obtain high hardness and fracture toughness at indentation loads of 1, 5, and 10 kg. ZTA composites with 0.01 and 0.1 wt% of MWCNTs or SWCNTs were densified by spark plasma sintering (SPS) at 1520 °C resulting in a higher hardness and comparable fracture toughness to the ZTA matrix material. The observed toughening mechanisms include crack deflection, pullout of CNTs as well as bridged cracks leading to improved fracture toughness without evidence of transformation toughening of the ZrO₂ phase. Scanning electron microscopy showed that MWCNTs rupture by a sword-in-sheath mechanism in the tensile direction contributing to an additional increase in fracture toughness.     

Sub-micron calcium carbonate isolated carbon nanotubes/polyethylene composites with controllable electrical conductivity

The dispersion and distribution of carbon nanotubes (CNTs) on/in the polymer composites are greatly affected by the molding technology progress, which results in different electrical conductivity. The uncontrollable electrical conductivity has limited the application of conductive polymer composites, for example, sensor components. In this work, to enhance the dispersion stability of CNTs in polyethylene (PE) matrix, sub-micron calcium carbonate isolated CNTs (smCaCO₃@CNTs) were selected based on the fact that smCaCO₃ is much easier to disperse in polymer in comparison with CNTs. This good distribution of CNTs in smCaCO₃@CNTs/PE was characterized by transmission electron microscope and Raman mapping. The electrical performance test results show that when 0.5 wt% of CNTs filled in smCaCO₃@CNTs/PE, the percolation network begins to form; when CNTs filled increases to 1-2 wt%, the surface resistance of smCaCO₃@CNTs/PE ranges from 10⁶ to 10⁹Ω almost not affected by the molding technology process (compression molding or injection molding). The possible reason is that the isolated CNTs by smCaCO₃ in polymer matrix are favorable for the formation of the stable conductive network.