Rheological behavior of multiwalled carbon nanotube/polycarbonate composites

The rheological behavior of compression molded mixtures of polycarbonate containing between 0.5 and 15 wt% carbon nanotubes was investigated using oscillatory rheometry at 260 °C. The nanotubes have diameters between 10 and 15 nm and lengths ranging from 1 to 10 μm. The composites were obtained by diluting a masterbatch containing 15 wt% nanotubes using a twin-screw extruder. The increase in viscosity associated with the addition of nanotubes is much higher than viscosity changes reported for carbon nanofibers having larger diameters and for carbon black composites; this can be explained by the higher aspect ratio of the nanotubes. The viscosity increase is accompanied by an increase in the elastic melt properties, represented by the storage modulus G′, which is much higher than the increase in the loss modulus G″. The viscosity curves above 2 wt% nanotubes exhibit a larger decrease with frequency than samples containing lower nanotube loadings. Composites containing more than 2 wt% nanotubes exhibit non-Newtonian behavior at lower frequencies. A step increase at approximately 2 wt% nanotubes was observed in the viscosity-composition curves at low frequencies. This step change may be regarded as a rheological threshold. Ultimately, the rheological threshold coincides with the electrical conductivity percolation threshold which was found to be between 1 and 2 wt% nanotubes.     

Molecular dynamics simulations of the interactions and dispersion of carbon nanotubes in polyethylene oxide/water systems

Molecular dynamics simulations were carried out to investigate carbon nanotube (CNT) interactions and dispersion in a polyethylene oxide (PEO)/water solution. The potential of mean forces (PMF) which embodies the entropic and enthalpic contributions by the solvent and the polymer molecules were computed. The relative enthalpic and entropic contributions to the PMF were studied in order to understand the CNT interaction mechanisms in solution. An adaptive biasing force (ABF) method was used to speed up the PMF calculations. The simulation results provide detailed atomic arrangements and atomic interactions between the CNTs and surrounding molecules (PEO and water). This molecular level computational study provides insights into the CNT’s interactions with PEO polymer/water systems.     

Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites

Effects of different dispersion states of carbon nanotubes (CNTs) on rheological, mechanical, electrical, and thermal properties of the epoxy nanocomposites were studied. The dispersion states were altered depending upon whether a solvent was employed or not. To characterize dispersion of the CNTs, field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) were used. It was found that the nanocomposites containing poorly dispersed CNTs exhibited higher storage modulus, loss modulus, and complex viscosity than ones with well dispersed CNTs. It means that the poorly dispersed CNTs/epoxy composites have, from a rheological point of view, a more solid-like behavior. Tensile strength and elongation at break of the nanocomposites with different dispersion of CNTs were measured. Both of the well and the poorly dispersed CNTs composites showed a percolation threshold of electrical conductivity at less than 0.5 wt.% CNTs loading and the former had higher electrical and thermal conductivities than the latter. Effects of the CNTs content on the physical properties were also examined experimentally. As loading of the CNTs increased, improved results were obtained. From the morphological observation by FESEM and TEM, it was found that when the solvent was not used in the CNTs dispersion process, aggregates of pristine CNTs remained in the nanocomposites.     

High-shear processing induced homogenous dispersion of pristine multiwalled carbon nanotubes in a thermoplastic elastomer

We report a high-shear processing technology that allows the homogenous dispersion of unmodified multiwalled carbon nanotubes (UMWNTs) in a thermoplastic elastomer, poly(styrene-b-butadiene-co-butylene-b-styrene) (SBBS). We demonstrated that the dispersion of UMWNTs in a polymer matrix depends greatly on the shear stress exerted during melt processing. Mechanical tests showed that the tensile modulus, tensile strength and elasticity of the composites with fine nanotube dispersion processed at a high-shear rate are much higher than those of the composites processed at a low shear rate. The results indicated that high-shear processing is an effective method of improving the dispersion of unmodified carbon nanotubes in a polymer matrix.     

Titania-doped multi-walled carbon nanotubes epoxy composites: Enhanced dispersion and synergistic effects in multiphase nanocomposites

Multi-walled carbon nanotube-epoxy composites are modified with titania nanoparticles in order to obtain multiphase nanocomposites with an enhanced dispersion of carbon nanotubes. The dispersion is monitored using rheological and electrical conductivity measurements. An increase in dispersion quality can be correlated to an increased storage shear modulus of the uncured suspensions and to a decreased electrical conductivity in the bulk nanocomposite. The newly formed microstructure is revealed using transmission electron and optical microscopies. Due to chemical interactions between both types of nanoparticles an attractive potential is generated leading to a significant rearrangement in the particle network structure. Besides an enhanced dispersion, the hybrid structure leads to synergistic effects in terms of the glass transition of the nanocomposites. Although a decrease of the glass transition temperature (T_g) is observed for the nanocomposites containing only one type of filler, the combination of titania and carbon nanotubes into a hybrid structure reduces the decrease of T_g, thus demonstrating the potential of such hybrid structures as fillers for multi-functional epoxy nanocomposites.     

Effects of Multi-Walled Carbon Nanotube (MWCNT) Content on Physical Properties and Cell Structure in Ethylene Vinyl Acetate Copolymer (EVA)/MWCNT Nanocomposite Foams

Melt compounding is used to mix ethylene-vinyl acetate copolymer (EVA) and multi-walled carbon nanotube (MWCNT). Then the obtained EVA/MWCNT mixtures were foamed using a chemical blowing agent. Without any modification of MWCNT, a significant improvement of the tensile properties was observed for the EVA/MWCNT foams. With increasing the MWCNT content to 10 phr, the average cell size decreases to 36 µm due to the higher melt viscosity and the average cell density increases to 10.5 × 10⁶ cell/cm³ due the heterogeneous nucleation. To investigate the possible applications for static dissipative purpose, the surface resistivity of EVA/MWCNT foams was also investigated.     

Poly(3-hydroxyalkanoate)-grafted carbon nanotube nanofillers as reinforcing agent for PHAs-based electrospun mats

Poly(3-hydroxyalkanoate)s, PHAs, have been covalently grafted onto the surface of multi-walled carbon nanotubes, MWCNTs, providing nanofillers (MWCNT-graft-PHAs) with enhanced compatibility and reinforcement effect towards PHAs. MWCNTs were first modified by in-situ generated diazonium cations obtained from a hydroxyl-containing aniline derivative, yielding MWCNTs with reactive hydroxyl surface groups, MWCNT-OH. Then, MWCNT-graft-PHAs were obtained by direct, i.e. without using any catalyst, transesterification approach. The successful chemical modification of MWCNTs surface was evidenced by Raman spectroscopy and XPS analysis confirming the covalent grafting of PHA on MWCNT. 3-Dimension mats were further produced through electrospinning of a PHA/MWCNT-graft-PHA solution providing nanocomposites with well-defined nanofibrous morphology. No aggregation of the MWCNTs was evidenced by SEM attesting that the grafting of PHA onto MWCNT improved their dispersion within the PHA matrix and consequently, the properties of the corresponding nanomaterials. Indeed, mechanical analysis results have shown that nanofibers loaded with MWCNT-graft-PHA (3 wt%) displayed excellent properties with an increase (+41%) of the tensile strain at break without any decrease of the high elastic modulus as compared to pristine PHA (131 MPa).     

Aggregate structure of hydroxyproline-rich glycoprotein (HRGP) and HRGP assisted dispersion of carbon nanotubes

Hydroxyproline-rich glycoproteins (HRGP) comprise a super-family of extracellular structural glycoproteins whose precise roles in plant cell wall assembly and functioning remain to be elucidated. However, their extended structure and repetitive block co-polymer character of HRGPs may mediate their self-assembly as wall scaffolds by like-with-like alignment of their hydrophobic peptide and hydrophilic glycopeptide modules. Intermolecular crosslinking further stabilizes the scaffold. Thus the design of HRGP-based scaffolds may have practical applications in bionanotechnology and medicine. As a first step, we have used single-molecule or single-aggregate atomic force microscopy (AFM) to visualize the structure of YK20, an amphiphilic HRGP comprised entirely of 20 tandem repeats of: Ser-Hyp₄-Ser-Hyp-Ser-Hyp₄-Tyr-Tyr-Tyr-Lys. YK20 formed tightly aggregated coils at low ionic strength, but networks of entangled chains with a porosity of ~0.5–3 μm at higher ionic strength. As a second step we have begun to design HRGP-carbon nanotube composites. Single-walled carbon nanotubes (SWNTs) can be considered as seamless cylinders rolled up from graphene sheets. These unique all-carbon structures have extraordinary aromatic and hydrophobic properties and form aggregated bundles due to strong inter-tube van der Waals interactions. Sonicating aggregated SWNT bundles with aqueous YK20 solubilized them presumably by interaction with the repetitive, hydrophobic, Tyr-rich peptide modules of YK20 with retention of the extended polyproline-II character. This may allow YK20 to form extended structures that could potentially be used as scaffolds for site-directed assembly of nanomaterials.     

Nanocomposites of poly(ether ether ketone) with carbon nanofibers: Effects of dispersion and thermo-oxidative degradation on development of linear viscoelasticity and crystallinity

Poly(ether ether ketone), PEEK, is a widely used engineering plastic that is especially suitable for high temperature applications. Compounding of PEEK with carbon nanofibers, CNF, has the potential of enhancing its mechanical and thermal properties further, even at relatively low CNF concentrations. However, such enhancements can be compromised by myriad factors, some of which are elucidated in this study. Considering that the dispersion of the CNF into any high molecular weight polymer is a challenge, two different processing methods, i.e., melt and solution processing were used to prepare PEEK nanocomposites with low aspect ratio carbon nanofibers. The linear viscoelastic material functions of PEEK nanocomposites in the solid and molten states were characterized as indirect indicators of the dispersion state of the nanofibers and suggested that the dispersion of nanofibers into PEEK becomes difficult at increasing CNF concentrations for both solution and melt processing methods. Furthermore, the time-dependence of the linear viscoelastic material functions of the PEEK/CNF nanocomposites at 360-400 °C indicated that PEEK undergoes thermo-oxidative cross-linking under typical melt processing conditions, thus preventing better dispersion by progressive increases of the mixing time and specific energy input during melt processing. The crystallization behavior of PEEK is also affected by the presence of CNF and degree of cross-linking, with the rate of crystallization decreasing with increasing degree of cross-linking and upon the incorporation of CNFs both for the solution and melt processed PEEK nanocomposites.     

Structure and crystallization behavior of Nylon 66/multi-walled carbon nanotube nanocomposites at low carbon nanotube contents

Multi-walled carbon nanotubes (MWNTs) were modified with poly(hexamethylene adipamide) (also known as Nylon 66) via a controlled polymer solution crystallization method. A “nanohybrid shish kebab” (NHSK) structure was found wherein the MWNT resembled the shish while Nylon 66 lamellar crystals formed the kebabs. These Nylon 66-functionalized MWNTs were used as precursors to prepare polymer/MWNT nanocomposites. Excellent dispersion was revealed by optical and electron microscopies. Nitric acid etching of the nanocomposites showed that MWNT formed a robust network in Nylon 66. Non-isothermal DSC results showed multiple melting peaks, which can be attributed to lamellar thickness changes upon heating. The crystallite sizes L₁₀₀ and L₀₁₀ of Nylon 66, determined by WAXD, decreased with increasing MWNT contents. Isothermal DSC results showed that crystallization kinetics increased first and then decreased with increasing MWNT contents in Nylon 66. This study showed that the effect of MWNTs on Nylon 66 crystallization is twofold: MWNTs provide heterogeneous nucleation sites for Nylon 66 crystallization while the tube network structure hinders large crystal growth.     

Surfactant functionalization of carbon nanotubes (CNTs) for layer-by-layer assembling of CNT multi-layer films and fabrication of gold nanoparticle/CNT nanohybrid

This paper describes a new strategy through noncovalent functionalization of multi-walled carbon nanotube (MWNTs) with supramolecular surfactant for layer-by-layer (LbL) assembling MWNT multi-layer film onto indium tin oxide (ITO)-coated glass plate and for attaching gold nanoparticles (GNPs) onto the MWNTs to fabricate GNP/MWNT nanohybrid. Surfactant (i.e., sodium dodecyl sulfate, SDS) can interact with the MWNTs through hydrophobic interaction between the hydrophobic chain of SDS and the sidewall of the MWNTs. Such an interaction essentially leads to noncovalent adsorption of SDS onto the MWNTs, resulting in an enhanced solubilization of the MWNTs in distilled water and providing some negative charges on the tube surface. Both properties make it possible to assemble MWNT multi-layer films onto the ITO plate through an alternative adsorption of oppositely charged SDS-functionalized MWNTs and polyelectrolyte [i.e., poly(diallyldimethylammonium chloride), PDDA] as revealed by scanning electron microscopy (SEM), ultraviolet-visible-near-infrared spectroscopy (UV-vis-NIR), quartz crystal microbalance (QCM), and cyclic voltammetry (CV). The same properties of the SDS-functionalized MWNTs are demonstrated to be useful for mediating the attachment of GNPs onto the tube surfaces to form GNP/MWNT nanohybrid as verified with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemistry.     

Rheological behavior of crystallizing palm oil

The static isothermal crystallization of palm oil was studied by oscillatory rheology. The phase angle, complex modulus, storage modulus and loss modulus were followed as a function of the crystallization time. Various crystallization temperatures were applied, and the results obtained by oscillatory rheology were compared with crystallization data obtained by more classical techniques like differential scanning calorimetry (DSC) and pulsed nuclear magnetic resonance (pNMR). It was shown that oscillatory rheology is a valuable complementary method to DSC and pNMR to evaluate primary crystallization. Like DSC and pNMR, oscillatory rheology is capable of differentiating whether crystallization occurs in a two‐stage or a single‐stage process. In addition, oscillatory measurements also allow the evaluation of aggregation, network formation and post‐hardening events like sintering and thus provide information on the crystal network and the final macroscopic properties of the crystallized sample.     

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) multi-bonded carbon nanotube (CNT): Preparation and formation of PPO/CNT nanocomposites

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) multi-bonded carbon nanotube (CNT) (CNT-PPO) was prepared using brominated PPO under the condition of atom transfer radical polymerization. The structure and properties of CNT-PPO were characterized with FTIR, Raman spectroscopy and thermal analyzer. The PPO layer in a thickness of about 4.5 nm was observed covering on the side wall of CNT with a high-resolution TEM. The PPO modification warrants the good dispersion of CNTs in PPO in the formation of PPO/CNT nanocomposites, which demonstrated enhanced mechanical properties and increases in electrical conductivity. The developed approach of CNT modification with engineering plastics can be applied to other polymers and preparation of functional polymer/CNT nanocomposites.     

The density control of carbon nanotubes using spin-coated nanoparticle and its application to the electron emitter with triode structure

The density-controlled carbon nanotubes (CNTs) were grown on the iron nanoparticles by using the freeze–dry method. The iron-acetate [Fe(II)(CH₃COO)₂] solution was used for the preparation of the catalytic iron nanoparticles. The density of CNTs was controlled in order to achieve the enhancement in the field emission process. Furthermore, the patterning of the iron nanoparticle catalyst layer for the fabrication of electronic devices was simply achieved by using an alkaline solution, TMAH (tetramethylammonium hydroxide). We applied this patterning process of catalyst layer to the formation of the electron emitter with under-gate type triode structure.     

Al2O3-MgO·1.35Al2O3复合浆料的流变特性研究

测试了高纯板状刚玉粉(Al2O3)、富铝尖晶石粉(MgO@1.35Al2O3)及其复合粉料(Al2O3-MgO@1.35Al2O3)在水溶液中不同pH值下的ζ电位,三者的等电点分别为7.4、～3和5.3.研究了分散剂三聚磷酸钠对复合浆料的ζ电位和流动性的影响,结果表明外加1 wt%溶剂量的三聚磷酸钠可降低复合浆料的ζ电位约40 mV;制备Al2O3-MgO@1.35Al2O3复合浆料的理想pH值范围为9～11;复合浆料中MgO@1.35Al2O3对流变特性的影响比Al2O3的大.     

Gold nanoparticle/carbon nanotube hybrids as an enhanced material for sensitive amperometric determination of tryptophan

In this work, a highly sensitive electrochemical sensor for the determination of tryptophan (Trp) was fabricate by electrodeposition of gold nanoparticles (AuNPs) onto carbon nanotube (CNT) films pre-cast on a glassy carbon electrode (GCE), forming an AuNP-CNT composite-modified GCE (AuNP-CNT/GCE). Scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used for the surface analysis of the electrode. The results indicate that the hybrid nanomaterials induced a substantial decrease in the overpotential of the Trp oxidation reaction and exhibited a remarkable synergistic effect on the electrocatalytic activity toward the oxidation of Trp. In phosphate buffer solution (pH 7.4), the modified electrode showed excellent analytical performance for the amperometric determination of Trp. The peak currents possess a linear relationship with the concentration of Trp in the range of 30 nM to 2.5 μM, and the detection limit is 10 nM (S/N = 3). In addition, the modified electrode was used to determine Trp concentration in pharmaceutical samples with satisfactory results.     

Rheological characterization of polystyrene-clay nanocomposites to compare the degree of exfoliation and dispersion

Polymer-clay nanocomposites are of great interest due to their improvement in certain material properties relative to virgin polymer or conventional composites. For example, compared to conventional materials, Nylon 6/montmorillonite nanocomposites demonstrated significant improvements, including high strength, high modulus and high heat distortion temperature. Because viscoelastic measurements are highly sensitive to the nanoscale and mesoscale structure of polymeric materials, when combined with X-ray scattering, electron microscopy, thermal analysis, and mechanical property measurements, they will provide fundamental understanding of the state and mechanism of exfoliation of the layered silicate (clay) in a polymer matrix. In addition, understanding rheological properties of polymer nanocomposites is crucial for application development and understanding polymer processability.The objective of this research is to develop a rheological technique to analyze the clay morphology in nanocomposite. Previous work has demonstrated the utility of the rheological technique to differentiate (qualify) the degree of exfoliation/dispersion. This report utilizes findings from the earlier work to further map out the structure-rheological response of polystyrene nanocomposites with various composition, clay types, and dispersion; and to quantify the key parameter that dominates the characteristic rheological response. This report explored a series of polystyrene (PS)-clay nanocomposites with 1,2-dimethyl-3-n-hexadecyl imidazolium (DMHDI) organically modified clays. These PS nanocomposites investigated here demonstrated a change of pattern in dynamic mechanical spectrum, as a function of the degree of exfoliation, from typical polymer response to a terminal response of [G′∼ω, G″∼ω], then to a pattern with double crossover frequencies, and finally to a solid-like response with G′>G″ in all frequency ranges. We showed that the number of particles per unit volume is a key factor determining the characteristic response of nanocomposites.In addition, the rheological response of PS-clays nanocomposite made from DMHDI modified clay combined with high-energy sonication (characterized as exfoliated by XRD and TEM) was compared with that of nanocomposites made by dimethyl, benzyl hydrogenated tallow (2MBHT) modified clay. We found that PS nanocomposites made by DMHDI-modified clay with high-energy sonication are better dispersed than the nanocomposites made previously using 2MBHT-modified clay. We also showed that the glass transition temperatures were not very sensitive to the degree of dispersion.The key finding of this research is that rheological measurements are complimentary to traditional polymer nanocomposite analysis techniques, and they may also serve as an analytical tool by itself (under appropriate conditions), now that some fundamental behavior has been identified.     

Rheological behavior of new melt compounded copolyamide nanocomposites

In this paper the rheological behavior of new polyamide-based nanocomposites produced by melt compounding using three different silicate loadings and screw speeds was investigated. The thermoplastic matrices selected were a polyamide 6 and its statistical copolymer having partially aromatic structure, whereas the clay was a commercial organo-modified montmorillonite. Hybrid systems were prepared by means of a laboratory-scale twin screw extruder and were submitted to rheological and structural investigations. The rheological experiments (dynamic frequency sweep, steady rate sweep and stress relaxation tests) were performed to evaluate the effect of both system composition (kind of matrix and clay content) and extrusion rate on the flow behavior of the nanocomposites. Rheology, that is highly sensitive to the nanoscale structure of the materials, put out a pseudo-solid like flow behavior at long times in the hybrids with silicate content higher than 6 wt% and produced with high extrusion rate; this response was related to the formation of an extended structural network across the polymer matrix due to strong polymer-silicate interactions that slow the relaxation times of the macromolecules. Corresponding to this behavior, TEM micrographs have shown a quite uniform dispersion of clay particles on micron-scale and a fair level of silicate exfoliation on nanoscale with a macroscopic preferential orientation of the layers in samples. The rheological measurements also reveal that this flow response is more marked for nanocomposites based on the copolyamide matrix, suggesting that this resin may have a higher silicate affinity respect to polyamide 6 homopolymer.     

The biocompatibility of carbon nanotubes

Carbon nanotubes (CNT) are well-ordered, high aspect ratio allotropes of carbon. The two main variants, single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) both possess a high tensile strength, are ultra-light weight, and have excellent chemical and thermal stability. They also possess semi- and metallic-conductive properties. This startling array of features has led to many proposed applications in the biomedical field, including biosensors, drug and vaccine delivery and the preparation of unique biomaterials such as reinforced and/or conductive polymer nanocomposites. Despite an explosion of research into potential devices and applications, it is only recently that information on toxicity and biocompatibility has become available. This review presents a summary of the performance of existing carbon biomaterials and gives an outline of the emerging field of nanotoxicology, before reviewing the available and often conflicting investigations into the cytotoxicity and biocompatibility of CNT. Finally, future areas of investigation and possible solutions to current problems are proposed.     

Rheological properties and irreversible dispersion changes in carbon nanotube/epoxy systems

We report on the rheological properties of semidilute carbon nanotube (CNT)/epoxy suspensions with varying filler content prepared with different dispersion methods. We show that dispersion techniques can be distinguished by means of the rheological response. It was found that the suspensions undergo a lasting dispersion changes during the first shear sweep measurement. Shear‐induced changes within the suspension can also be visualized optically. Our results demonstrate that the rheological properties are strongly influenced by the measurement procedure, which has to be known in order to guarantee reproducible and comparable results. Finally, it does not seem possible to completely reset the sample history by shear treatment. POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers