Alan H. Windle

High-Performance Carbon Nanotube Fiber

With their impressive individual properties, carbon nanotubes should form high-performance fibers. We explored the roles of nanotube length and structure, fiber density, and nanotube orientation in achieving optimum mechanical properties. We found that carbon nanotube fiber, spun directly and continuously from gas phase as an aerogel, combines high strength and high stiffness (axial elastic modulus), with an energy to breakage (toughness) considerably greater than that of any commercial high-strength fiber. Different levels of carbon nanotube orientation, fiber density, and mechanical properties can be achieved by drawing the aerogel at various winding rates. The mechanical data obtained demonstrate the considerable potential of carbon nanotube assemblies in the quest for maximal mechanical performance. The statistical aspects of the mechanical data reveal the deleterious effect of defects and indicate strategies for future work.

Dispersion and packing of carbon nanotubes

Suitable oxidative treatment of catalytically grown carbon nanotubes introduces oxygen containing surface groups. Infrared and titration studies indicate that they are predominately phenolic, carboxylic and lactonic groups. These groups stabilize dispersions of nanotubes at much higher concentrations than are possible with the raw material. Plots of the viscosity of dispersions as a function of their concentration show a dramatic increase in gradient above a critical concentration, leading to the formation of viscoelastic gels. During continued drying, the solvent mediates the formation of dense assemblies of nanotubes which then bond together through the surface groups. If the nanotubes are deposited from a dilute suspension by filtration they are able to maximize the number of intertube contacts by packing into a locally parallelized structure, reminiscent of liquid crystalline polymers.

A theory of case II diffusion

Abstract A theory is proposed to explain the transport behaviour of organic penetrants in glassy polymers in terms of two basic parameters: the diffusivity of the penetrant, D, and the viscous flow rate of the glassy polymer, 1 η 0 . The rate controlling process for transport in these systems is considered to be diffusion of solvent down an activity gradient coupled with time-dependent mechanical deformation of the polymer glass in response to the swelling stress. The theory combines these two factors and is able to predict a wide range of observed transport phenomena from Fickian diffusion kinetics at one extreme to so-called Case II and Super-Case II behaviour at the other. The existence of a sharp front separating swollen and unpenetrated polymer is shown to result from the concentration dependence of the viscous flow rate.

Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method

High purity, aligned multi-wall carbon nanotube films were grown on quartz substrates by injecting a solution of ferrocene in toluene into a suitable reaction furnace. The injection CVD method allows excellent control of the catalyst to carbon ratio. The detailed study presented here demonstrates how such a system can be used to control the nanotube diameter, length, alignment and yield by manipulating the experimental parameters. Primary growth was found to occur via a base growth mechanism, although overgrowths of single wall carbon nanotubes were obtained under certain conditions. Such a method also allows nanotubes of various packing densities to be produced which may be useful for specific applications such as electrodes.

True solutions of single-walled carbon nanotubes for assembly into macroscopic materials

Translating the unique characteristics of individual single-walled carbon nanotubes into macroscopic materials such as fibres and sheets has been hindered by ineffective assembly. Fluid-phase assembly is particularly attractive, but the ability to dissolve nanotubes in solvents has eluded researchers for over a decade. Here, we show that single-walled nanotubes form true thermodynamic solutions in superacids, and report the full phase diagram, allowing the rational design of fluid-phase assembly processes. Single-walled nanotubes dissolve spontaneously in chlorosulphonic acid at weight concentrations of up to 0.5 wt%, 1,000 times higher than previously reported in other acids. At higher concentrations, they form liquid-crystal phases that can be readily processed into fibres and sheets of controlled morphology. These results lay the foundation for bottom-up assembly of nanotubes and nanorods into functional materials.

Carbon-nanofibre-reinforced poly(ether ether ketone) composites

Poly(ether ether ketone) nanocomposites containing vapour-grown carbon nanofibres (CNF) were produced using standard polymer processing techniques. Evaluation of the mechanical composite properties revealed a linear increase in tensile stiffness and strength with nanofibre loading fractions up to 15 wt% while matrix ductility was maintained up to 10 wt%. Electron microscopy confirmed the homogeneous dispersion and alignment of nanofibres. An interpretation of the composite performance by short-fibre theory resulted in rather low intrinsic stiffness properties of the vapour-grown CNF. Differential scanning calorimetry showed that an interaction between matrix and the nanoscale filler could occur during processing. Such changes in polymer morphology due to the presence of a nanoscale filler need to be considered when evaluating the mechanical properties of such nanocomposites.

EFFECTS OF CELL IRREGULARITY ON THE ELASTIC PROPERTIES OF OPEN-CELL FOAMS

Foams are more and more widely used because of their high mechanical properties relative to their low density. Most available mechanical models are based on idealised unit-cell structures. A significant disadvantage of the unit-cell modelling approach is that it does not account for the natural variations in microstructure that are typical for most foam structures. Our objective has been to investigate how the cell irregularity affects the elastic properties of open-cell foams. We generated periodic, three-dimensional (3D), random samples with different degrees of irregularity, and used finite element analysis (FEA) to determine the effective elastic properties. The geometrical properties were investigated for 3D random open-cell foams and related to the elastic properties. The results indicate that the more irregular the foams, the larger will be their effective Youngs modulus and shear modulus at constant overall relative density. On the other hand, the bulk modulus reduces with increasing degree of cell irregularity, while the Poissons ratio is largely independent.

Cause of neural death in neurodegenerative diseases attributable to expansion of glutamine repeats

Neurodegenerative diseases resulting from expanded repeat sequences of glutamine residues are associated with the formation of protein aggregates in the cell nuclei of the affected neurons, but whether these are pathogenic is controversial. Recent observations indicate that the ages of onset of these diseases are exponential functions of the repeat lengths and that the probability of neural death is constant with time. The only process known to us that could give rise to such behaviour is nucleation of the aggregates.

Transport of methanol in poly(methyl methacrylate)

Abstract The transport kinetics of liquid methanol in PMMA sheet (Perspex) have been studied over the temperature range 23° to 63°C. At the lower temperatures the transport is typical Case II; the methanol penetrating the polymer behind a sharp front which moves at constant velocity. For higher temperatures the concentration of methanol at the front and the front velocity both decrease with increasing penetration. These factors, which are no doubt related, combine to give mass absorption kinetics in which the exponent of time is no longer unity (Case II) but approaches 0.5, the value typical of Fickian diffusion. Iodine has been added to the methanol to make the positions of the penetration fronts readily visible. The iodine does not affect either the rate of penetration or the equilibrium absorption. Measurements of colour density profiles on thin cross-sections using a microdensitometer give a reliable indication of the true methanol profile. The rate at which the methanol penetrates the glassy polymer is proportional to its concentration at the advancing fronts. The apparent activation energy for methanol penetration for constant concentration at the fronts is 25 kcal/mol.

A deformation model for Case II diffusion

Abstract Case II diffusion in glassy polymers is considered in terms of a deformation model. The process is controlled by the mechanical response of the glass just ahead of the sharp front to an osmostic swelling stress. The important factor governing the establishment of the sharp solvent front is taken to be the concentration dependence of the creep response time. These ideas are set in the context of existing models for diffusion in glassy polymers, and experimental evidence is presented to support the deformation mechanism.