Nanotubes Dispersants

Dierking, 1., Scalia, G. and Morales, P. (2005) Liquid crystal-carbon nanotube dispersions. J. Appl. Phys., 97. 044309-1-044309-5. 

Alpatova, A.L. et al. (2010) Single-walled carbon nanotubes dispersed in aqueous media via noncovalent functionalization effect of dispersant on the stability, cytotoxicity, and epigenetic toxicity of nanotube suspensions. Water Research, 44 (2), 505-520. 

Graff RA, Swanson JP, Barone PW, BaikS, Heller DA, Strano MS. Achieving individual-nanotube dispersion at high loading in single-walled carbon nanotube composites. Advanced Materials 2005, 17, 980-984. 

Aqueous solution of SWNTs was suspended with different types of surfactants a) anionic (SDS) (CH3(CH2)ii(S04) Na+), b) cationic (hexadecyltrimethylammonium bromide (CTAB) (CH3(CH2)i5N+(CH3)3Br) and c) and non-ionic (Tween-80) H(Et-0)n0(C4H602CH0HCH20 CO(Ci8H23). Surfactants SDS and CTAB were purchased from Serva , Tween-80 from Shuchard (Germany). 0.05 mg/mL nanotube dispersion with surfactant was mixed and then the suspension was sonicated for 40 minutes. A concentration of surfactants in water solution was 1%. Water was prepared by distillation and then passed through Multi-Q system. The deionized water has resistance 18 MO. 

Basic studies on diazonium-CNT chemistry led to two very efficient techniques for SWCNT derivatization solvent-free functionalization [176] and functionalization of individual (unbundled) nanotubes [175], With the solvent-free functionalization (Scheme 1.26), heavily functionalized and soluble material is obtained and the nanotubes disperse in polymer more efficiently than pristine SWCNTs [176], With the second method, aryldiazonium salts react efficiently with the individual (unbundled) HiPCO produced and sodium dodecyl sulfate (SDS)-coated SWCNTs in water. The resulting functionalized tubes (one addend in nine tube carbons) remained unbundled throughout their entire lengths and were incapable of reroping. [175],... 

Kashiwagi, T., Du, F., Winey, K.I., Groth, K.M., Shields, J.R., Bellayer, S., Kim, S., and Douglas, J.F. 2005. Flammability properties of polymer nanocomposites with single-walled carbon nanotubes Effects of nanotube dispersion and concentration. Polymer 46(2) 471 181. 

Despite the results presented above, near-field microscopy has not been extensively used to characterize polymer/nanotube composites However, it can be noticed that AFM is a useful technique to locally probe the mechanical properties of the composites (at the polymer-nanotube interface for example). One possible reason for the small amount of studies by AFM and STM could be that observing the surface only does not permit to obtain much information on the nanotube dispersion state. For that kind of characterization, transmission electron microscopy is a key technique owing to the small nanotube diameter. 

Once the nanotubes have been characterised and polymer/ nanotubes elaborated, their microstructures have to be precisely determined to understand the relations between the process and the nanocomposites macroscopic properties. It is expected that the microstructural parameters that will play major roles (in addition to the filler geometry) are the nanotube dispersion and orientation... 

Obtaining a homogeneous nanotube dispersion state in a polymer matrix is still a major issue. From a general point of view, the dispersion state is influenced from the following parameters (53) ... 

As a consequence, the nanotube dispersion state has to be characterised at several pertinent scales, including that covered by TEM. 

In some studies, a statistical description of the nanotube dispersion state was obtained from TEM images. For example, Uchida et al. (56) measured the diameter distribution of SWNT bundles in poly(acrylonitrile), with and without a purification treatment involving sonication in methanol. The different bundle diameter distributions (especially the mean diameter) could explain the different composite tensile moduli. Fornes et al. (57) also determined the diameter distribution of SWNTs bundles in a polymer matrix (namely polycarbonate). To improve the contrast in the bright-field TEM images and better measure the bundle diameter, they dissolved the polymer in chloroform and studied the remaining SWNT network. 

At the steps before the elaboration of carbon nanotube nanocom-posites, wet-STEM can be used for the characterization of nanotubes dispersed in a liquid (see Figure 3.18), and for polymer latex/ nanotubes mixing (before evaporation or freeze-drying to elaborate polymer/carbon nanotube nanocomposites). 

TEM remains certainly the most powerful technique to get bulk information, but due to the low sample thickness required for observation, in most cases, CNT are cut and it is almost impossible to observe them surrounded by their neighbours and so, to analyse their mutual interactions. However, in situ spectroscopy leads to more and more precise data on CNT - matrix interface, which one of the key-point of macroscopic behaviour. It can be noticed that SEM can be performed in transmission, leading to images similar to what can be obtained by TEM. However, in the magnification range covered by both techniques, SEM provides images of thicker samples with a higher contrast, which should provide reliable results on the nanotube dispersion state. However, TEM remains unavoidable to locally characterise the nanotube-matrix interface and the nanotube-nanotube contacts. 

Fabrication methods have overwhelmingly focused on improving nanotube dispersion because better nanotube dispersion in polyurethane matrix has been found to improve the properties of the nanocomposites. The dispersion extent of CNTs in the polyurethane matrix plays an important role in the properties of the polymer nanocomposites. Similar to the case of nanotube/solvent suspensions, pristine nanotubes have not yet been shown to be soluble in polymers, illustrating the extreme difficulty of overcoming the inherent thermodynamic drive of nanotubes to bundle. Therefore, CNTs need to be surface modified before the composite fabrication process to improve the load transfer from the polyurethane matrix to the nanotubes. Usually, the polyurethane/CNT nanocomposites can be fabricated by using four techniques melt-mixing (15), solution casting (16-18), in-situ polymerization (19-21), and sol gel process (22). 

Most of PVA/CNT composites are processed under the form of films. Generally, films are casted and dried from water-based PVA and nanotube dispersions. Different types of water-based dispersions have been used. Carbon nanotubes come from various production sources and can be covalently functionalized. The PVA molecular weight and hydrolysis rate can also be varied as well as the nanotube fraction. This is why comparisons between all the contributions in the literature can sometimes be difficult. Nevertheless some general and important features can still be deduced from all the studies reported on this topic. 

Davis et al. [89] reported an important work regarding the immobilization of metalloproteins and enzymes on oxidized, purified and vacuum-annealed SWCNTs in aqueous solution. AFM experiments showed that the immobilization is mainly physical, without need for covalent activation or electrostatic interaction. In fact, cytochrome c at pHs below the isoelectric point and ferritin at pHs above the isoelectric point showed an important adsorption obtained just by stirring the nanotubes dispersion (0.03 mg/mL) in dilute protein solutions (50-100 pg/mL) for a given time (2-20 h). GOx could be also adsorbed in a very efficient... 

Ultrasonication has been used for the purification of CNTs. The low-molecular weight impurities are removed from carbon nanotube dispersion in methanol by filtration under a continuous sonication. The carbon nanotubes are... 

Meyer F, Raquez JM, Dubios P et til (2010) Imidazolium end-functionalized poly(L-lactide) for efficient carbon nanotube dispersion. Chem Commun 46 5527-5529... 

Itoh E, Suzuki 1, Miyairi K (2005) Field emission from carbon-nanotube-dispersed conducting polymer thin film and its application to photovoltaic devices. Jpn J Appl Phys 44 636... 

Figure 13.17 compares inorganic nanoplatelets with single-wall carbon nanotubes, dispersed in a typical thermoplastic polymer melt at given and Af values, including the effects of shear rate and particle flexibility. The fact that the nanotube dispersion shows much higher viscosity than the nanoplatelet dispersion at low shear rates is a result of the superposition of two effects. Firstly, there is the effect of the difference between the geometrical characteristics of fibers and platelets, as was also shown in Figure 13.15. Secondly, there is the effect of the much greater stiffness of single-wall carbon nanotubes (E=5000 GPa [45]) compared to nanoplatelets (E-100 GPa), which results in the effects of particle flexibility becoming very small for the nanotubes.

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