Changes of morphology and properties of block copolymers induced by carbon nanotubes

Carbon nanotubes have been extensively used in isotropic polymer media as mechanical reinforcements or as conductive fillers. New phenomena arise when the polymer matrix is made of an ordered block copolymer. Dispersions of nanotubes stabilized by block copolymers in selective solvents can be used to cast composite films in which the nanotubes are segregated in microdomains of the structured polymer. This concept is here investigated for the case of carbon nanotubes in a poly(styrene)-b-poly(butadiene)-b-poly(methylmethacrylate) terpolymer (SBM). It is observed that casting films of SBM from different solvents in the presence or absence of nanotubes can lead to different morphologies with distinct mechanical and electrical properties. In particular it was found that neat SBM cast from a mixture of cyclohexane and acetone exhibits a cylindrical microstructure. This metastable form is mechanically weak and brittle. But the polymer adopts a stable lamellar morphology in the presence of nanotubes. This phase exhibits much better mechanical properties. The capability of nanotubes to alter the morphology of a block copolymer and to lead to large improvements of mechanical properties appears therefore as a new mechanism of mechanical reinforcement beyond the already reported mechanisms of direct reinforcement and network formation

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

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 pKa 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

Diblock copolymer stabilization of multi-wall carbon nanotubes in organic solvents and their use in composites

A versatile method for the preparation of dispersed nanotubes using polystyrene-b-polyisoprene diblock copolymers in different selective organic solvents is presented. Stable dispersions have been obtained in polar (DMF) and apolar (heptane) media depending on the selectivity of the diblock copolymers. They have been characterized by means of optical microscopy, TEM imaging and dynamic light scattering, showing the first demonstration of multiwall carbon nanotubes (MWCNTs) solutions with in situ characterization of diblock copolymer stabilization. The most effectively stabilized dispersions have been used to make nanotube/polystyrene composites. We find that the coating of the nanotubes by the diblock polymer does not prevent electrical transport, so that the system can exhibit a relatively high surface conductivity above the percolation threshold. The low percolation threshold experimentally determined is presumably due to weak attractive interactions between the nanotubes as the composites are dried

Carbon nanotube-based aligned lyotropic nematic suspensions and anisotropic thin films

Carbon nanotubes exhibit exceptional physical properties at the individual scale. In order to optimize these properties at the macroscopic scale, a good control of both the dispersion and alignment of the nanotubes is required. In this work we study the dispersion and alignment of single-wall carbon nanotubes in lyotropic nematic suspensions and we use these suspensions for the elaboration of anisotropic thin films. Individual nanotubes are stabilized by denaturated DNA in aqueous suspensions. Lyotropic nematic phases are prepared by concentrating nanotubes/DNA aqueous suspensions and the nematic is shear-aligned in thin cells. At each step of the preparation, the dispersion and alignment are studied by polarized optical microscopy, photoluminescence and Raman measurements. We show that thin homogeneous anisotropic films can be prepared from wet aligned suspensions. In a first series of experiments, the nematic and the corresponding films exhibited a quite weak order parameter (S about 0.2) [1], which was assigned to some entanglements of the nanotube network. In this talk, we will show that the order parameter of the nematic and the corresponding films can be significantly improved by controling the aspect ratio and/or the bundling of the nanotubes, for example using fractionation through controled ultra-centrifugation during the nematic phase formation. [1] C. Zamora-Ledezma, C. Blanc, M. Maugey, C. Zakri, P. Poulin, E. Anglaret. Nanoletters 8 (12), 4103-4107, (2008)

Raman Response of Carbon Nanotube/PVA Fibers under Strain

We study the strain-induced shift of the D* Raman band of single-wall carbon nanotubes in polyvinyl alcohol-nanotube composite fibers. If embedded in structural components, such strain-sensitive fibers may be considered for potential applications as strain or stress sensors. Due to improved interfacial adhesion, stronger shifts of the D* Raman band are observed when carboxylic functional groups are present at the nanotube surface. This indicates that nanotube carboxylation would yield better efficacy for future sensing applications. However, we also observe that the improvements of interfacial adhesion do not lead to substantially better mechanical properties of the fibers. This effect is discussed by considering possible degradation of nanotubes during surface functionalization

Scalable Process for the Spinning of PDV-CArbon Nanotube composite Fibers

We report a water-based spinning process to produce polyvinyl alcohol (PVA)-carbon nanotube composite fibers that contain a large fraction of nanotubes. The process differs from previous methods to achieve related materials because the spinning solution is injected in a static coagulation bath instead of being circulated in conflowing streams. The resultant wet spinning process in reminiscent of processes industrially developed for neat PVA fibers. Considering its robustness, the process in therefore expected to be easily scalable for greater production. The present method is based on the stabilization of nanotubes by appropriate surfactant molecules that allow the nanotubes to remain homogeneously dispersed in aqueous solutions of PVA. The obtained fibers are homogeneous, uniform in diameter, can be spun indefinitely. They are electrically conducting textile applications. The present process being based on the colloidal stability of the particles in PVA solutions, it is believed that it could be extended to several other types of composite PVA fibers provided that the particles are stabilized by similar surfactants

Dispersion State and Fiber Toughness: Antibacterial Lysozyme-Single Walled Carbon Nanotubes

Novel multicomponent fi bers that include single-walled carbon nanotubes (SWNT) and lysozyme (LSZ) are reported. These fi bers exhibit antibacterial and mechanical properties suitable for fabrics, clothing and technical textiles in medical environments. The challenging combination of several components in a single fi ber material is achieved via fundamental studies on the phase behavior of aqueous LSZ-SWNT dispersions. The addition of molecular cationic surfactants proved to be critical to achieving stable liquid mixtures that can be spun into fi bers. In the absence of the cationic surfactant tetradecyl trimethylammonium bromide (TTAB), depletion effects result in large aggregates at relatively low SWNT concentration. However, the addition of TTAB increases the concentration at which demixing occurrs by approximately one order of magnitude. Dry-spun fi bers with signifi cant antibacterial activity and toughness are obtained from LSZ-TTAB-SWNT dispersions combined with a polyvinyl alcohol (PVA) solution. Toughness is strongly affected by the initial dispersion state. The most remarkable fi bers are produced from concentrated LSZ-TTAB-SWNT supernatants; they both have four times the toughness of spider silk and 70% of the native LSZ activity

Dispersion and alignment of individual Single Wall Carbon Nanotubes in a chromonic liquid crystal

Single Wall Carbon Nanotubes (SWNTs) display remarkable anisotropic features (mechanical, optical and conductivity properties). Exploiting them at a macroscopic scale requires both a good dispersion of individual tubes and a control of their orientational order at a large scale. The use of a liquid crystal as a structured solvent for aligning the tubes is attractive and several studies have already examined the dispersion of SWNTs in thermotropic or lyotropic liquid crystals. In this work, we used Disodium Chromoglycate (DSCG), a chromonic liquid crystal (LCLC) to disperse a large SWNT concentration (more than 0.1 %) in an aqueous nematic phase. The doped nematics and their orientation were studied by polarized microscopy, polarized Raman and photoluminescence spectroscopies (from individual semiconducting SWNT only). A quantitative approach [1-3] allowed us to determine accurately the order parameter of the tubes, which was found to be in the range 0.9-1