Enhanced Polymer Grafting from Multiwalled Carbon Nanotubes through Living Anionic Surface-Initiated Polymerization

Anionic surface-initiated polymerization of ethylene oxide and styrene has been performed using multiwalled carbon nanotubes (MWNTs) functionalized with anionic initiators. The surface of MWNTs was modified via covalent attachment of precursor anions such as 4-hydroxyethyl benzocyclobutene (BCBEO) and 1-benzocyclobutene-1′-phenylethylene (BCB-PE) through Diels-Alder cycloaddition at 235 °C. Surface-functionalized MWNTs-g-(BCB-EO) n and MWNTs-g-(BCB-PE) n with 23 and 54 wt % precursor initiators, respectively, were used for the polymerizations. Alkoxide anion on the surface of MWNTs-g-(BCB-EO) n was generated through reaction with potassium triphenylmethane for the polymerization of ethylene oxide in tetrahydrofuran and phenyl substituted alkyllithium was generated from the surface of MWNTs-g-(BCB-PE) n using sec-butyllithium for the polymerization of styrene in benzene. In both cases, the initiation was found to be very slow because of the heterogeneous reaction medium. However, the MWNTs gradually dispersed in the reaction medium during the polymerization. A pale green color was noticed in the case of ethylene oxide polymerization and the color of initiator as well as the propagating anions was not discernible visually in styrene polymerization. Polymer grafted nanocomposites, MWNTs-g-(BCB-PEO) n and MWNTs-g-(BCB-PS) n containing a very high percentage of hairy polymer with a small fraction of MWNTs (<1 wt %) were obtained. The conversion of ethylene oxide and the weight percent of PEO on the surface of the MWNTs increased with increasing reaction time indicating a controlled polymerization. The polymer-grafted MWNTs were characterized using FTIR, 1H NMR, Raman spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and transmission electron microscopy (TEM). Size exclusion chromatography of the polymer grafted MWNTs revealed broad molecular weight distributions (1.3 < Mw/Mn < 1.8) indicating the presence of different sizes of polymer nanocomposites. The TEM images showed the presence of thick layers of polymer up to 30 nm around the MWNTs. The living nature of the growing polystyryllithium was used to produce diblock copolymer grafts using sequential polymerization of isoprene on the surface of MWNTs.

Controlled Covalent Functionalization of Multiwalled Carbon Nanotubes using [4 + 2] Cycloaddition of Benzocyclobutenes

Surface modification of carbon nanotubes (CNTs) through covalent functionalization is vital for the development of high-performance composite materials, chemosensors, nanoelectronics, photovoltaic devices, as well as for a range of biomedical applications. Several methods have been developed to functionalize CNTs. The introduction of acid groups by acid digestion disrupts the structural integrity of CNTs. Apart from shortening the tubes, oxidatively generated acid groups are inhomogenously located at the tips of broken CNTs and, hence, functionalization using acid groups as precursors does not give a statistical distribution of functional groups throughout the surface of the CNTs.

Hydrogen peroxide monitoring in Fenton reaction by using a ruthenium oxide hexacyanoferrate/multiwalled carbon nanotubes modified electrode

A novel amperometric sensor based on the incorporation of ruthenium oxide hexacyanoferrate (RuOHCF) into multiwalled carbon nanotubes (MWCNTs) immobilized on a glassy carbon electrode is described. Cyclic voltammetry experiments indicated that the cathodic reduction of hydrogen peroxide at the RuOHCF/MWCNTs100/GC modified electrode is facilitated, occurring at 0.0 V vs. Ag/AgCl/KCl(sat). Following the optimization of the experimental conditions, the proposed sensor presented excellent analytical properties for hydrogen peroxide determination, with a low limit of detection (4.7 mu mol L-1), a large dynamic concentration range (0.1-10 mmol L-1) and a sensitivity of 1280 mu A mmol(-1) L cm(-2). The usefulness of the RuOHCF/MWCNTs100/GC electrochemical sensor was confirmed by monitoring the consumption of hydrogen peroxide during the degradation of phenol by the Fenton reaction. (C) 2012 Elsevier B.V. All rights reserved.

EVALUATION OF THE POTENTIALITIES OF A CARBON NANOTUBES/SILICONE RUBBER COMPOSITE ELECTRODE IN THE DETERMINATION OF HYDROCHLOROTHIAZIDE

A multiwall carbon nanotube/silicone rubber (MWCNT/SR) composite electrode has been used for the determination of hydrochlorothiazide (HCTZ) in pharmaceutical formulations by differential pulse voltammetry (DPV). The electro-oxidation process was evaluated by cyclic voltammetry, from which it was observed that HCTZ presents an irreversible oxidation peak at 0.82 V vs. saturated calomel electrode (SCE) in the potential range from 0.5 to 1.1 V, in Britton-Robinson buffer pH 7.0 at MWCNT/SR. HCTZ was determined by DPV using a MWCNT/SR 70% (MWCNT, m/m) composite electrode after the optimization of the experimental parameters. The linear range was from 5.0 to 70.0 mu mol L-1, with a limit of detection (LOD) of 2.6 mu mol L-1. The HCTZ was determined in pharmaceutical formulations using the proposed composite electrode and the results agreed with those from the official high performance liquid chromatography (HPLC) method within 95% confidence level, according to the t-Student test.

Photophysics of Carbon Nanotubes: from Dispersion to Supramolecular Systems

The aim of this PhD thesis is the investigation of the photophysical properties of materials that can be exploited in solar energy conversion. In this context, my research was mainly focused on carbon nanotube-based materials and ruthenium complexes. The first part of the thesis is devoted to carbon nanotubes (CNT), which have unique physical and chemical properties, whose rational control is of substantial interest to widen their application perspectives in many fields. Our goals were (i) to develop novel procedures for supramolecular dispersion, using amphiphilic block copolymers, (ii) to investigate the photophysics of CNT-based multicomponent hybrids and understand the nature of photoinduced interactions between CNT and selected molecular systems such as porphyrins, fullerenes and oligo (p-phynylenevinylenes). We established a new protocol for the dispersion of SWCNTs in aqueous media via non-covalent interactions and demonstrated that some CNT-based hybrids are suitable for testing in PV devices. The second part of the work is focussed on the study of homoleptic and heteroleptic Ru(II) complexes with bipyridine and extended phenanthroline ligands. Our studies demonstrated that these compounds are potentially useful as light harvesting systems for solar energy conversion. Both CNT materials and Ru(II) complexes have turned out to be remarkable examples of photoactive systems. The morphological and photophysical characterization of CNT-based multicomponent systems allowed a satisfactory rationalization of the photoinduced interactions between the individual units, despite several hurdles related to the intrinsic properties of CNTs that prevent, for instance, the utilization of laser spectroscopic techniques. Overall, this work may prompt the design and development of new functional materials for photovoltaic devices.

A comparison of acute and long-term effects of industrial multiwalled carbon nanotubes on human lung and immune cells in vitro

The close resemblance of carbon nanotubes to asbestos fibers regarding their high aspect ratio, biopersistence and reactivity increases public concerns on the widespread use of these materials. The purpose of this study was not only to address the acute adverse effects of industrially produced multiwalled carbon nanotubes (MWCNTs) on human lung and immune cells in vitro but also to further understand if their accumulation and biopersistence leads to long-term consequences or induces adaptive changes in these cells. In contrast to asbestos fibers, pristine MWCNTs did not induce overt cell death in A549 lung epithelial cells and Jurkat T lymphocytes after acute exposure to high doses of this material (up to 30 g/ml). Nevertheless, very high levels of reactive oxygen species (ROS) and decreased metabolic activity were observed which might affect long-term viability of these cells. However, the continuous presence of low amounts of MWCNTs (0.5 g/ml) for 6 months did not have major adverse long-term effects although large amounts of nanotubes accumulated at least in A549 cells. Moreover, MWCNTs did not appear to induce adaptive mechanisms against particle stress in long-term treated A549 cells. Our study demonstrates that despite the high potential for ROS formation, pristine MWCNTs can accumulate and persist within cells without having major long-term consequences or inducing adaptive mechanisms.

Relating the physicochemical characteristics and dispersion of multiwalled carbon nanotubes in different suspension media to their oxidative reactivity in vitro and inflammation in vivo

Reactive oxygen species (ROS) production is important in the toxicity of pathogenic particles such as fibres. We examined the oxidative potential of straight (50 microm and 10 microm) and tangled carbon nanotubes in a cell free assay, in vitro and in vivo using different dispersants. The cell free oxidative potential of tangled nanotubes was higher than for the straight fibres. In cultured macrophages tangled tubes exhibited significantly more ROS at 30 min, while straight tubes increased ROS at 4 h. ROS was significantly higher in bronchoalveolar lavage cells of animals instilled with tangled and 10 mum straight fibres, whereas the number of neutrophils increased only in animals treated with the long tubes. Addition of dispersants in the suspension media lead to enhanced ROS detection by entangled tubes in the cell-free system. Tangled fibres generated more ROS in a cell-free system and in cultured cells, while straight fibres generated a slower but more prolonged effect in animals.

Characterization of thermal and mechanical properties of polypropylene-based composites for fuel cell bipolar plates and development of educational tools in hydrogen and fuel cell technologies

In this project we developed conductive thermoplastic resins by adding varying amounts of three different carbon fillers: carbon black (CB), synthetic graphite (SG) and multi-walled carbon nanotubes (CNT) to a polypropylene matrix for application as fuel cell bipolar plates. This component of fuel cells provides mechanical support to the stack, circulates the gases that participate in the electrochemical reaction within the fuel cell and allows for removal of the excess heat from the system. The materials fabricated in this work were tested to determine their mechanical and thermal properties. These materials were produced by adding varying amounts of single carbon fillers to a polypropylene matrix (2.5 to 15 wt.% Ketjenblack EC-600 JD carbon black, 10 to 80 wt.% Asbury Carbon's Thermocarb TC-300 synthetic graphite, and 2.5 to 15 wt.% of Hyperion Catalysis International's FIBRILTM multi-walled carbon nanotubes) In addition, composite materials containing combinations of these three fillers were produced. The thermal conductivity results showed an increase in both through-plane and in-plane thermal conductivities, with the largest increase observed for synthetic graphite. The Department of Energy (DOE) had previously set a thermal conductivity goal of 20 W/m·K, which was surpassed by formulations containing 75 wt.% and 80 wt.% SG, yielding in-plane thermal conductivity values of 24.4 W/m·K and 33.6 W/m·K, respectively. In addition, composites containing 2.5 wt.% CB, 65 wt.% SG, and 6 wt.% CNT in PP had an in–plane thermal conductivity of 37 W/m·K. Flexural and tensile tests were conducted. All composite formulations exceeded the flexural strength target of 25 MPa set by DOE. The tensile and flexural modulus of the composites increased with higher concentration of carbon fillers. Carbon black and synthetic graphite caused a decrease in the tensile and flexural strengths of the composites. However, carbon nanotubes increased the composite tensile and flexural strengths. Mathematical models were applied to estimate through-plane and in-plane thermal conductivities of single and multiple filler formulations, and tensile modulus of single-filler formulations. For thermal conductivity, Nielsen's model yielded accurate thermal conductivity values when compared to experimental results obtained through the Flash method. For prediction of tensile modulus Nielsen's model yielded the smallest error between the predicted and experimental values. The second part of this project consisted of the development of a curriculum in Fuel Cell and Hydrogen Technologies to address different educational barriers identified by the Department of Energy. By the creation of new courses and enterprise programs in the areas of fuel cells and the use of hydrogen as an energy carrier, we introduced engineering students to the new technologies, policies and challenges present with this alternative energy. Feedback provided by students participating in these courses and enterprise programs indicate positive acceptance of the different educational tools. Results obtained from a survey applied to students after participating in these courses showed an increase in the knowledge and awareness of energy fundamentals, which indicates the modules developed in this project are effective in introducing students to alternative energy sources.

FUNCTIONALIZATION OF CARBON NANOTUBES FOR MESENCHYMAL STEM CELL-ASSISTED PHOTOTHERMAL THERAPY

Carbon nanotubes were first cut and functionalized with a newly developed reaction involving autoclaving and sonication in hydrogen peroxide. The functionalized nanotubes were characterized and evaluated for aqueous solubility. Studies which relate reaction conditions to final carbon nanotube length were conducted. Hydroxyl groups present on the carbon nanotubes served as a platform for a series of addition reactions, with the objective of conjugating streptavidin and fluorescent markers onto the carbon nanotubes. The modified nanotubes were attached onto the surface of biotinylated mesenchymal stem cells, creating a novel, tumor-homing delivery system for photothermal anticancer agents.

Hierarchical and hybrid polymer nanocomposites based on carbon nanotubes and inorganic fullerene type nanoparticles

The influence of singlewalled carbon nanotubes (SWCNT) and inorganic fullerenelike tungsten disulfide nanoparticles (IFWS2) on the morphology and thermal, mechanical and electrical performance of multifunctional fibrereinforced polymer composites has been investigated. Significant improvements were observed in stiffness, strength and toughness in poly (ether ether ketone) (PEEK) / (SWCNT) / glass fibre (GF) laminates when a compatibilizer was used for wrapping the CNTs. Hybrid poly(phenylene sulphide) (PPS)/IFWS2/ carbon fibre (CF) reinforced polymer composites showed improved mechanical and tribological properties attributed to a synergetic effect between the IF nanoparticles and CF.

Multiscale-fiber-reinforced thermoplastic composites incorporating carbon nanotubes: A review

This article reviews recent literature on hierarchical thermoplastic-based composites that simultaneously incorporate carbon nanotubes (CNTs) and conventional microscale fibers, and discusses the structure?property relationships of the resulting hybrids. The mixing of multiple and multiscale constituents enables the preparation of materials with new or improved properties due to synergistic effects. By exploiting the outstanding mechanical, thermal and electrical properties of CNTs, a new generation of multifunctional high-performance composites suitable for a wide variety of applications can be developed.

The Environmental Health and Safety of Carbon Nanotubes: Risk, Perception, and Policy

The production and use of carbon nanotubes (CNTs) can negatively impact human health and the environment through occupational, environmental, and product life-cycle exposures. Research is underway to evaluate the known, potential, and perceived hazards associated with CNTs. Recent research and policy analyses regarding CNTs were reviewed extensively. A facility engaged in research, development, and manufacture of CNTs was observed handling CNTs and associated individuals were informally interviewed. The combined investigation characterizes the current state of the art of our understanding and implementation of policy needed to address the impacts of CNTs to human health and the environment. A gap analysis is performed of regulations, policy, and CNT control methods; conclusions and recommendations are made from the results of this analysis.