Metal Mimics: Lightweight, Strong, and Tough Nanocomposites and Nanomaterial Assemblies.

The ideal structural material would have high strength and stiffness with a tough ductile failure, all with a low density. Historically, no such material exists, and materials engineers have had to sacrifice a desired property during materials selection, with metals (high density), fiber composites (brittle failure), and polymers (low stiffness) having fundamental limitations on at least one front. The ongoing revolution of nanomaterials provides a potential route to build on the potential of fiber-reinforced composites, matching their strength while integrating toughening behaviors akin to metal deformations, all while using low-weight constituents. Here, the challenges, approaches, and recent developments of nanomaterials for structural applications are discussed, with an emphasis on improving toughening mechanisms, which is often the neglected factor in a field that chases strength and stiffness.

Real-time mechanistic study of carbon nanotube anion functionalisation through open circuit voltammetry.

The mechanism of the functionalisation of reduced single walled carbon nanotubes with organobromides was monitored by open circuit voltammetry throughout the reaction and further elucidated through a series of comparative reactions. The degree of functionalisation was mapped against the reagent reduction potential, degree of electron donation of substituents (Hammett parameter), and energies calculated, ab initio, for dissociation and heterolytic cleavage of the C-Br bond. In contrast to the previously assumed reduction/homolytic cleavage mechanism, the reaction was shown to consist of a rapid association of carbon-halide bond to the reduced nanotube as a complex, displacing surface-condensed countercations, leading to an initial increase in the net nanotube surface negative charge. The complex subsequently slowly degrades through charge transfer from the reduced single-walled carbon nanotube to the organobromide, utilizing charge, and the carbon-halide bond breaks heterolytically. Electron density on the C-Br bond in the initial reagent is the best predictor for degree of functionalisation, with more electron donating substituents increasing the degree of functionalisation. Both the mechanism and the new application of OCV to study such reactions are potentially relevant to a wide range of related systems.

Applying a potential difference to minimise damage to carbon fibres during carbon nanotube grafting by chemical vapour deposition.

The application of an in situ potential difference between carbon fibres and a graphite foil counter electrode (300 V, generating an electric field ca 0.3-0.7 V µm-1), during the chemical vapour deposition synthesis of carbon nanotube (CNT) grafted carbon fibres, significantly improves the uniformity of growth without reducing the tensile properties of the underlying carbon fibres. Grafted CNTs with diameters 55 nm ± 36 nm and lengths around 10 µm were well attached to the carbon fibre surface, and were grown without the requirement for protective barrier coatings. The grafted CNTs increased the surface area to 185 m2 g-1 compared to the as-received sized carbon fibre 0.24 m2 g-1. The approach is not restricted to batch systems and has the potential to improve CNT grafted carbon fibre production for continuous processing.

Property and Shape Modulation of Carbon Fibers Using Lasers.

An exciting challenge is to create unduloid-reinforcing fibers with tailored dimensions to produce synthetic composites with improved toughness and increased ductility. Continuous carbon fibers, the state-of-the-art reinforcement for structural composites, were modified via controlled laser irradiation to result in expanded outwardly tapered regions, as well as fibers with Q-tip (cotton-bud) end shapes. A pulsed laser treatment was used to introduce damage at the single carbon fiber level, creating expanded regions at predetermined points along the lengths of continuous carbon fibers, while maintaining much of their stiffness. The range of produced shapes was quantified and correlated to single fiber tensile properties. Mapped Raman spectroscopy was used to elucidate the local compositional and structural changes. Irradiation conditions were adjusted to create a swollen weakened region, such that fiber failure occurred in the laser treated region producing two fiber ends with outwardly tapered ends. Loading the tapered fibers allows for viscoelastic energy dissipation during fiber pull-out by enhanced friction as the fibers plough through a matrix. In these tapered fibers, diameters were locally increased up to 53%, forming outward taper angles of up to 1.8°. The tensile strength and strain to failure of the modified fibers were significantly reduced, by 75% and 55%, respectively, ensuring localization of the break in the expanded region; however, the fiber stiffness was only reduced by 17%. Using harsher irradiation conditions, carbon fibers were completely cut, resulting in cotton-bud fiber end shapes. Single fiber pull-out tests performed using these fibers revealed a 6.75-fold increase in work of pull-out compared to pristine carbon fibers. Controlled laser irradiation is a route to modify the shape of continuous carbon fibers along their lengths, as well as to cut them into controlled lengths leaving tapered or cotton-bud shapes.

Probing the charging mechanisms of carbon nanomaterial polyelectrolytes.

Chemical charging of single-walled carbon nanotubes (SWCNTs) and graphenes to generate soluble salts shows great promise as a processing route for electronic applications, but raises fundamental questions. The reduction potentials of highly-charged nanocarbon polyelectrolyte ions were investigated by considering their chemical reactivity towards metal salts/complexes in forming metal nanoparticles. The redox activity, degree of functionalisation and charge utilisation were quantified via the relative metal nanoparticle content, established using thermogravimetric analysis (TGA), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS). The fundamental relationship between the intrinsic nanocarbon electronic density of states and Coulombic effects during charging is highlighted as an important area for future research.

Clinical and Immunological Spectrum of Common Variable Immunodeficiency (CVID).

We have analysed data from 150 patients initially classified as having CVID. About 10% had laboratory abnormalities suggesting known single gene disorders (eg: hyper-IgM syndrome), and in a few a genetic defect has been confirmed. We have attempted to sub-classify the remaining patients by analysis of their circulating lymphocytes. B lymphocyte markers have been used to estimate the numbers of circulating immature and class switched B cells; there is an association between the presence of high relative numbers of immature circulating B cells, splenomegaly and autoimmune disease. About 25% of CVID patients have a moderate CD4+ T lymphopenia, sometimes with a relative expansion of CD8+ T cells. About 30% of CVID patients have persistent relatively high levels of circulating CD8+ T cells binding immunogenic peptides from EBV or CMV. Many of these patients also have high relative numbers of circulating CD8+ perforin positive T cells, and there is evidence that these cells may be responsible for neutropenia or inflammatory bowel disease in some patients. The clinical spectrum of CVID is diverse, with some patients suffering from few infections, and over 50% have evidence of structural lung damage. About 25% of UK patients have chronic inflammation in various organs, particularly the lungs, liver and spleen, often with granulomatous changes. Steroids are used to treat many of the patients with chronic inflammatory complications, although trials are in progress with anti-TNF agents. The incidence of these inflammatory complications is different between countries, being rare in Sweden. Attempts to correlate clinical phenotypes with the laboratory abnormalities described above have been disappointing, suggesting that unknown genetic factors unrelated to the cause of the immunodeficiency determine the complications; attempts to identify some of these factors will be discussed. Finally a provisional scheme to sub classify CVID patients according to lymphocyte abnormalities will be presented, the purpose being to focus the screening of candidate genes causing CVID to specific subsets of patients.