A chemically reactive spinning dope for significant improvements in wet spun carbon nanotube fibres.

Single-walled carbon nanotubes can be spun in a polyvinyl alcohol stream to produce nanocomposite fibres. We use a facile ester linking between both elements to create improved fibres which exhibit outstanding enhancements in the absence of post-processing stages, providing a promising alternative based on a chemical method.

Sensitivity of carbon nanotubes to the storage of stress in polymers.

Residual stress in polymers arises from the freezing of unstable molecular conformations. Residual stress is critical because its relaxation can cause shrinkage, defects, and fractures of polymer materials. The storage of stress is purposely enhanced to develop shape memory materials. Unfortunately, the storage of mechanical stress is still poorly controlled and understood. An approach to sense the storage of stress based on the spectroscopic response of carbon nanotubes is explored. The Raman response of nanotubes exhibits a variable sensitivity to strain when embedded in polymers that have experienced different thermal and mechanical treatments. This unique feature opens up new possibilities for the use of carbon nanotubes as mechanical nanosensors.

Conductivity anisotropy of assembled and oriented carbon nanotubes.

An assembly of packed and oriented rodlike particles exhibit anisotropic physical properties. We investigate in the present work the anisotropic conductivity of films made of intrinsically conducting rods. These films are obtained from more or less ordered carbon nanotube liquid crystals. Their orientational order parameter is measured by polarized Raman spectroscopy. A relationship between the anisotropy of surface conductivity and orientational order parameter is determined. The experimental results are accounted for by a model that takes into account the number of intertube contacts and density of conductive pathways in different directions, as introduced by J. Fischer et al. for magnetically aligned nanotubes. We find that a good agreement, without any fitting parameter, of the proposed model and experiments is obtained when we consider a two-dimensional (2D) Gaussian distribution of the nanotube orientation. The conductivities parallel and perpendicular to the nematic director differ by almost an order of magnitude. This anisotropy is much greater than that of conventional dielectric liquid crystals, where the behavior is governed by the mobility anisotropy of ionic current carriers. The present results do not depend on the intrinsic properties of the nanotubes and are expected to be relevant for other assemblies of conducting rodlike particles, such as metallic or semi-conducting nanowires and ribbons.

Engineering hybrid nanotube wires for high-power biofuel cells.

Poor electron transfer and slow mass transport of substrates are significant rate-limiting steps in electrochemical systems. It is especially true in biological media, in which the concentrations and diffusion coefficients of substrates are low, hindering the development of power systems for miniaturized biomedical devices. In this study, we show that the newly engineered porous microwires comprised of assembled and oriented carbon nanotubes (CNTs) overcome the limitations of small dimensions and large specific surface area. Their improved performances are shown by comparing the electroreduction of oxygen to water in saline buffer on carbon and CNT fibres. Under air, and after several hours of operation, we show that CNT microwires exhibit more than tenfold higher performances than conventional carbon fibres. Consequently, under physiological conditions, the maximum power density of a miniature membraneless glucose/oxygen CNT biofuel cell exceeds by far the power density obtained for the current state of art carbon fibre biofuel cells.

Dispersion and film-forming properties of poly(acrylic acid)-stabilized carbon nanotubes.

We present a detailed study of the influence of pH on the dispersion and film-forming properties of poly(acrylic acid)-stabilized carbon nanotubes. Poly(acrylic acid) (PAA) is a weak polyelectrolyte, with a pH-responsive behavior in aqueous solution. We obtain quantitative UV-visible measurements to show that the amount of polyelectrolyte in optimal pH conditions is weak, showing a good efficiency of the polymer as a carbon nanotube dispersing agent. The best dispersion conditions are achieved at pH 5, a value close to the pK(a) of PAA. Apart from this tenuous pH value, the PAA is not efficient at stabilizing nanotubes and atomic force microscopy allows us to explain the delicate balance between the PAA adsorption and the suspension stability. This study finally permits optimal conditions for making homogeneous and conductive composite films to be determined.

Anisotropic thin films of single-wall carbon nanotubes from aligned lyotropic nematic suspensions.

Lyotropic nematic aqueous suspensions of single-wall carbon nanotubes can be uniformly aligned in thin cells by shearing. Homogeneous anisotropic thin films of nanotubes can be prepared by drying the nematic. Optical transmission between parallel or crossed polarizers is measured and described in order to estimate the dichroic ratio. The order parameter is measured using polarized Raman spectroscopy and found to be quite weak due to entanglement of the nanotubes and/or to an intrinsic viscoelastic behavior of the nanotube suspensions.

Shape and temperature memory of nanocomposites with broadened glass transition.

Shape-memory polymers can revert to their original shape when they are reheated. The stress generated by shape recovery is a growing function of the energy absorbed during deformation at a high temperature; thus, high energy to failure is a necessary condition for strong shape-memory materials. We report on the properties of composite nanotube fibers that exhibit this particular feature. We observed that these composites can generate a stress upon shape recovery up to two orders of magnitude greater than that generated by conventional polymers. In addition, the nanoparticles induce a broadening of the glass transition and a temperature memory with a peak of recovery stress at the temperature of their initial deformation.

Liquid crystal behavior of single-walled carbon nanotubes dispersed in biological hyaluronic acid solutions.

We report the spontaneous liquid crystal phase separation of nanotubes (single-walled carbon nanotubes, SWNTs) stabilized in aqueous biological (hyaluronic acid, HA) solutions. Sonication of SWNTs in solutions of HA produced well-dispersed single-phase isotropic dispersions which, over time, phase separated into dispersions containing birefringent nematic domains in equilibrium with an isotropic phase. The time required for phase separation to occur was shown to depend on the concentration of SWNT and HA, with the attractive interactions between the SWNT and HA shifting the onset of the phase separation toward lower concentration. This phase separation is accompanied by an increase in the dispersion viscosity with this increase qualitatively matching the degree of phase separation. The formation of ordered phases in biological media can offer wide opportunities for processing conducting biomaterials with aligned and oriented domains.

Particle growth of hybrid materials followed by dynamic light scattering.

The hydrolysis of distannylated compounds in which the tin atoms are linked by an organic spacer has been studied under microemulsion conditions using dynamic light scattering and infrared spectroscopy. The experiments provided evidence that the growth of hybrid material particles occurs in the aqueous phase, outside the organic phase of the microemulsion. The growth rates of the particles were found to be strongly dependent on the nature of the spacers, a polymethylene chain inducing the fastest process. This different behavior was explained by a slower condensation process rather than a slower hydrolysis. The high surface areas measured for the hybrid materials could be explained by a possible coating of the hybrid particles by surfactant molecules, thus preventing either their growth or their aggregation.

In situ measurements of nanotube dimensions in suspensions by depolarized dynamic light scattering.

We show that the dimensions of carbon nanotubes (CNTs) in suspension can be characterized by depolarized dynamic light scattering. Taking advantages of this in situ technique, we investigate in detail the influence of sonication procedures on the length and diameter of CNTs in surfactant solutions. Sonication power is shown to be particularly efficient at unbundling nanotubes, whereas a long sonication time at low power can be sufficient to cut the bundles with limited unbundling. We finally demonstrate the influence of CNT dimensions on the electrical properties of CNT fibers. Slightly varying the sonication conditions, and thereby the suspended nanotube dimensions, can affect the fibers conductivity by almost 2 orders of magnitude.

Self-assembled tin-based bridged hybrid materials.

Hybrid materials where layers of tin oxide alternate with layers of hydrophobic organic chains were prepared by the hydrolysis of distannylated compounds containing an organic chain alpha,omega-disubstituted by tripropynylstannyl groups. In the case of an aliphatic chain, hydrolysis under microemulsion conditions led to the organization of the corresponding hybrid. These hydrolysis conditions also induced a high surface area and a defined mesoporosity in the hybrid. When a mixed aromatic-aliphatic spacer was used, weak hydrophobic interactions between the spacers were sufficient to generate the same type of organization in the corresponding material.