Tensile properties,thermal and morphological analysis of thermoplastic polyurethane films reinforced with multiwalled carbon nanotubes

The paper reports on thermal, tensile and morphological properties of thermoplastic polyurethane (TPU) based films obtained by melt-compounding and chill-roll extrusion. Composite films containing up to 1 wt% of multiwalled carbon nanotubes (MWNTs) are characterized in terms of thermal properties, tensile behavior and morphological issues taking the neat TPU film as the reference material.     

Surface structure and adsorption properties of multiwalled carbon nanotubes

The pore structure, sorption parameters, and chemical composition of the surface of multiwalled carbon nanotubes synthesized by catalytic pyrolysis were determined. The dependences of the amount of cholic acid adsorbed by the nanotube surface on time, pH, and concentration of an equilibrium solution were studied. Physical adsorption of cholic acid is mainly the outcome of nonspecific interactions between the acid and the surface of the nanotubes. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1712–1715, October, 2006.     

Space-resolved thermal properties of thermoplastics reinforced with carbon nanotubes

Composites comprising biobased poly(lactic acid) (PLA) and polyethylene (Bio-PE) were reinforced with multi-walled carbon nanotubes (MWCNTs). These nanocomposites were analyzed using space-resolved thermal analysis (TA) integrated with atomic force microscopy. The deflection temperature, which indicates thermal-induced expansion and thermal transitions of the composite, was monitored by nanoscale TA (nanoTA) utilizing the displacement of a cantilever in contact with the material. Results were compared to bulk electrical, mechanical and thermal properties. Electrical conductivity was detected at lower MWCNT loadings for PLA than for Bio-PE (at 2.5 vs. 5 mass%). Maximal electrical conductivity of 27 S m^?1 for PLA and 0.7 S m^?1 for Bio-PE-based samples was reached at 10 mass% MWCNT loading. Tensile behavior combined with thermogravimetric analysis indicated strong MWCNT–Bio-PE interactions, in contrast to PLA. The glass transition and melting temperature measured by differential scanning calorimetry (DSC) were not changed by the increase in MWCNT loading. Increased deflection temperature was registered by bulk heat deflection measurements on Bio-PE, but not for PLA. The thermal transitions obtained by nanoTA at the nanoscale were in the same temperature range as the first transitions observed upon temperature ramp in DSC (e.g., glass transition and melt temperatures of PLA and Bio-PE, respectively). Remarkably, thermal expansion was detected by nanoTA for PLA- and Bio-PE-based composites below electrical percolation threshold as well as an increase in PLA softening temperature. Space-resolved nanothermal analysis revealed thermal phenomena that are otherwise overlooked when bulk methods are applied.     

Enzymatic degradation of multiwalled carbon nanotubes

Because of their unique properties, carbon nanotubes and, in particular, multiwalled carbon nanotubes (MWNTs) have been used for the development of advanced composite and catalyst materials. Despite their growing commercial applications and increased production, the potential environmental and toxicological impacts of MWNTs are not fully understood; however, many reports suggest that they may be toxic. Therefore, a need exists to develop protocols for effective and safe degradation of MWNTs. In this article, we investigated the effect of chemical functionalization of MWNTs on their enzymatic degradation with horseradish peroxidase (HRP) and hydrogen peroxide (H(2)O(2)). We investigated HRP/H(2)O(2) degradation of purified, oxidized, and nitrogen-doped MWNTs and proposed a layer-by-layer degradation mechanism of nanotubes facilitated by side wall defects. These results provide a better understanding of the interaction between HRP and carbon nanotubes and suggest an eco-friendly way of mitigating the environmental impact of nanotubes.     

Electrochemical behavior and capacitance properties of carbon xerogel/multiwalled carbon nanotubes composites

The electrochemical behavior of carbon xerogel/multiwalled carbon nanotubes composite in a 6 M KOH solution has been investigated. Three different mixtures of teflonized carbons with varying nanotube content were prepared. The electrodes containing multiwalled carbon nanotubes were found to provide enhanced capacities compared with those prepared with only carbon xerogel. Cyclic voltammetry and charge–discharge experiments reveal the presence of a strong resistive component, which decreases as the amount of nanotubes increases. Electrochemical impedance spectroscopy results analyzed in terms of an adequate physicochemical model of the porous electrode, show that an increasing amount of nanotubes enhances both the effective solid-phase conductivity and the effective liquid-phase conductivity, linked to the porosity of the electrodes.     

Electro-oxidation of amino-functionalized multiwalled carbon nanotubes

We report the electrochemistry of amino-functionalized multiwalled carbon nanotubes (MWCNTs-NH₂) in the pH range from 0.3 to 6.4 using quantitative cyclic voltammetry (CV) and single entity electrochemistry measurements, making comparison with non-functionalized MWCNTs. CV showed the latter to both catalyze the solvent (water) decomposition and to undergo irreversible electro-oxidation forming oxygen containing surface functionality. The MWCNTs-NH₂ additionally undergo an irreversible oxidation to an extent which is dependent on the pH of the solution, reflecting the variable amount of deprotonated amino groups present as a function of pH. Nano-impact experiments conducted at the single particle level confirmed the oxidation of both types of MWCNTs, showing agreement with the CV. The pK_a of the amino groups in MWCNTs was determined via both electrochemical methods giving consistent values of ca. 2.5.

A new and generic approach to the study of the oxidation of different forms of CNTs is found by using quantitative single entity and ensemble electrochemistry measurements.     

Sonochemical oxidation of multiwalled carbon nanotubes

Functionalization of carbon nanotubes (CNTs) is important for enhancing deposition of metal nanoparticles in the fabrication of supported catalysts. A facile approach for oxidizing CNTs is presented using a sonochemical method to promote the density of surface functional groups. This was successfully employed in a previous study [J. Phys. Chem. B 2004, 108, 19255] to prepare highly dispersed, high-loading Pt nanoparticles on CNTs as fuel cell catalysts. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, cyclic voltammetry, and settling speeds were used to characterize the degree of surface functionalization and coverage. The sonochemical method effectively functionalized the CNTs. A mixture of -C-O-/-C=O and -COO- was observed along with evidence for weakly bound CO at longer treatment times. The integrated XPS C 1s core level peak area ratios of the oxidized-to-graphitic C oxidation states, as well as the atom % oxygen from the O 1s level, showed an increase in peak intensity (attributed to -CO(x)()) with increased sonication times from 1 to 8 h; the increase in C surface oxidation correlated well with the measured atom %. Most of the CNT surface oxidation occurred between 1 and 2 h. The sonochemically treated CNTs were also studied by cyclic voltammetry and settling experiments, and the results were consistent with the XPS observations.     

Water-soluble multiwalled carbon nanotubes functionalized with sulfonated polyaniline

Multiwalled carbon nanotubes (MWNTs) functionalized with a water-soluble conducting polymer, sulfonated polyaniline (SPAN), were prepared by in situ polymerization of aniline followed by sulfonation with chlorosulfonic acid in an inert solvent and by hydrolysis in water. Electron microscopy, laser Raman spectroscopy, X-ray photoelectron spectroscopy, and UV-vis absorption spectroscopy were employed to characterize the morphology and chemical structure of the resulting product. The results show that the quinonoid structure of SPAN preferentially interacts with the nanotubes and is stabilized by strong pi-pi interaction between two components. The structure of MWNTs was not perturbed by the incorporation of SPAN, since the pi-pi interaction between MWNTs and SPAN is much weaker in comparison to that of the carbon covalent bond. The SPAN functionalized MWNTs are highly dispersible in water, thus opening new possibilities for their prospective technological applications.     

Tunable thermoresponsive water-dispersed multiwalled carbon nanotubes

Polymers containing poly(ethylene glycol) methacrylate and 2-(2-methoxyethoxy)ethyl methacrylate have been synthesized by Cu(0)-mediated radical polymerisation for use as thermoresponsive water-dispersants for carbon nanotubes.     

Controlled modification of multiwalled carbon nanotubes with Zno nanostructures

Multiwalled carbon nanotubes (MWNTs) have been successfully modified with ZnO nanostructures by zinc-ammonitum complex ion covalently attached to the MWNTs through the C-N bonds. Flower-like ZnO on the tips of MWNTs and ZnO nanoparticles on the surface of MWNTs have been obtained, respectively, via adjusting the reaction time. The modified MWNTs have been characterized with X-ray diffraction, scanning electron and transmission electron microscopy. A growth mechanism has been proposed in which the soaking time plays a key role in controlling the size, morphology, and site of ZnO nanostructures. Photoluminescence properties of the as-synthesized products have also been investigated.     

Preparation and properties of natural rubber composites reinforced with pretreated carbon nanotubes

In order to achieve dramatic improvements in the performance of rubber materials, the development of carbon nanotube (CNT)‐reinforced rubber composites was attempted. The CNT/natural rubber (NR) nanocomposite was prepared through solvent mixing on the basis of pretreatment of CNTs. Thermal properties, vulcanization characteristics, and physical and mechanical properties of the CNT/NR nanocomposites were characterized in contrast to the carbon black (CB)/NR composite. Through the addition of the CNTs treated using acid bath followed by ball milling with HRH (hydrated silica, resorcinol, and hexamethylene tetramine) bonding systems, the crystallization melting peak in differential scanning calorimetry (DSC) curves of NR weakened and the curing rate of NR slightly decreased. Meanwhile, the over‐curing reversion of CNT/NR nanocomposites was alleviated. The dispersion of the treated CNTs in the rubber matrix and interfacial bonding between them were rather good. The mechanical properties of the CNT‐reinforced NR showed a considerable increase compared to the neat NR and traditional CB/NR composite. At the same time, the CNT/NR nanocomposites exhibited better rebound resilience and dynamic compression properties. The storage modulus of the CNT/NR nanocomposites greatly exceeds that of neat NR and CB/NR composites under all temperature regions. The thermal stability of NR was also obviously improved with the addition of the treated CNTs. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthetic gecko foot-hairs from multiwalled carbon nanotubes

We report a fabrication process for constructing polymer surfaces with multiwalled carbon nanotube hairs, with strong nanometer-level adhesion forces that are 200 times higher than those observed for gecko foot-hairs.     

Determination of catecolamines on electrodes modified with multiwalled carbon nanotubes

The electrochemical behavior of several biologically active catecolamines in studied on a glassy carbon electrode with the surface preliminarily modified by a composite film containing preliminarily carboxylated multi-walled carbon nanotubes (MWNT). The coatings are characterized by the methods of cyclic voltammetry and scanning microscopy. It is shown that the use of hybrid composites prepared by immobilizing MWNT and gold nanoparticles into the film of poly(isonicotinic) acid provides the high diffusion permeability of the surface layers and the efficiency of the electron transfer as regards catecolamines. The possibility of using these electrodes for selective determination of these substances in drugs widely used in the modern practical medicine is demonstrated.     

Fabrication of a highly sensitive electrochemiluminescence lactate biosensor using ZnO nanoparticles decorated multiwalled carbon nanotubes

ZnO nanoparticles (nanoZnO) were decorated on multiwalled carbon nanotubes (MWCNTs) and then the prepared nano-hybrids, nanoZnO-MWCNTs, were immobilized on the surface of a glassy carbon electrode (GCE) to fabricate nanoZnO-MWCNTs modified GCE. The prepared electrode, GCE/nanoZnO-MWCNTs, showed excellent electrocatalytic activity towards luminol electrochemiluminescence (ECL) reaction. The electrode was then further modified with lactate oxidase and Nafion to fabricate a highly sensitive ECL lactate biosensor. Two linear dynamic ranges of 0.01-10 μmol L⁻¹ and 10-200 μmol L⁻¹ were obtained for lactate with the correlation coefficient better than 0.9996. The detection limit (S/N = 3) was 4 nmol L⁻¹ lactate. The relative standard deviation for repetitive measurements (n = 6) of 10 μmol L⁻¹ lactate was 1.5%. The fabrication reproducibility for five biosensors prepared and used in different days was 7.4%. The proposed ECL lactate biosensor was used for determination of lactate in human blood plasma samples with satisfactory results.     

Electroactive films of heme protein-coated multiwalled carbon nanotubes

A novel method for fabricating protein-MWNT films on pyrolytic graphite (PG) electrodes was described. Positively charged hemoglobin (Hb) or myoglobin (Mb) in buffers at pH 5.5 or 5.0 was first adsorbed on the surface of acid-pretreated, negatively charged multiwalled carbon nanotubes (MWNTs) mainly by electrostatic interaction, forming a core-shell structure. The aqueous dispersion of protein-coated MWNTs was then cast on PG electrodes, forming protein-MWNT films after evaporation of solvent. The protein-MWNT films exhibited a pair of well-defined, quasi-reversible cyclic voltammetric peaks, characteristic of heme Fe(III)/Fe(II) redox couples. The protein films were characterized by voltammetry, UV-vis spectroscopy, and scanning electron microscopy (SEM). This approach for assembly of protein-MWNT films showed higher surface concentration of electroactive proteins than the simple cast method, and the amount of proteins in the films could be controlled more precisely compared with the dipping method. Furthermore, the film assembly using this method was more stable than that using simple cast method. The proteins in MWNT films retained their near-native structure, and electrochemically catalyzed reduction of oxygen and hydrogen peroxide, suggesting the potential applicability of the films as the new type of biosensors or bioreactors based on direct electrochemistry of enzymes.     

Microwave-assisted covalent sidewall functionalization of multiwalled carbon nanotubes

Thermal cycloaddition of 1,3-dipolar azomethine ylides to the sidewalls of multiwalled carbon nanotubes (MWNTs) has been used to prepare MWNTs that contain 2-methylenethiol-4-(4-octadecyloxyphenyl) (4), N-octyl-2-(4-octadecyloxyphenyl) (5) or 2-(4-octadecyloxyphenyl)pyrrolidine (6) units. All these contain the 4-octadecyloxyphenyl substituent that acts as a solubilizing group. Microwave (MiW)-assisted heating was found to be highly efficient for soluble MWNTs, for which the amount of added groups after only 2 h of MiW heating at 200 degrees C, determined by using thermogravimetric analysis, was found to be in the same range as that obtained after 100-120 h of conventional heating of soluble and insoluble MWNTs. Solubility is a key feature for a successful MiW-heated reaction; MWNTs insoluble in the reaction medium yielded considerably less addends in the MiW-heated reactions than in the conventionally heated reaction. The location and even distribution of the pyrrolidine units over the outermost layer of the MWNTs was verified by transmission electron microscopy analysis of 4 that had been treated with gold nanoparticles and thoroughly washed to remove gold particles adsorbed on nonfunctionalized parts of the MWNTs.     

Crystallization,rheological, and mechanical properties of PLLA/PEG blend with multiwalled carbon nanotubes

Functionalized multiwalled carbon nanotubes (FMWCNTs) were introduced into poly(L‐lactide)/polyethylene glycol (PLLA/PEG) blend and the effects of FMWCNTs on crystallization behaviors, rheological, and mechanical properties of PLLA/PEG/FMWCNTs were investigated. The results show that FMWCNTs exhibit good distribution in the nanocomposites and absorb some PEG to agglomerate around them. The crystallization behavior of PLLA in the nanocomposites is greatly dependent on the content of FMWCNTs. At low content of FMWCNTs, the addition of FMWCNTs improves the crystallization behavior of PLLA by enhancing the crystallization temperature and accelerating the crystallization rate, whereas at high content of FMWCNTs, the crystallization of PLLA is restricted to a certain degree. Rheological properties show the formation of the network structure of FMWCNTs at high content, which is the main reason for the retarded crystallization behavior of PLLA due to the network structure providing restriction to mobility and diffusion of PLLA chains to crystal growth fronts. The mechanical properties show that FMWCNTs exhibit reinforcement effect for plasticized PLLA. Copyright © 2010 John Wiley & Sons, Ltd.