Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W4: Carbon Nanotube Dispersions |
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Sponsoring Units: DPOLY Chair: Ramanan Krishnamoorti, University of Houston Room: Baltimore Convention Center 308 |
Thursday, March 16, 2006 2:30PM - 3:06PM |
W4.00001: From Networks to Nematics -- Carbon Nanotubes as Soft Matter Invited Speaker: Carbon nanotubes represent just one example of an emerging paradigm in condensed matter physics and materials science: traditionally ``hard'' materials appearing in new ``soft'' applications and environments. In part, this trend being is being fueled by the desire to exploit solution and fluid-based approaches, such as self assembly and flow processing, in an effort to streamline the engineering and commercialization of new materials and applications. In this talk I will review our recent work on dispersing, aligning and manipulating carbon nanotubes in complex fluids and polymer melts. Due to the large aspect ratios and strong attractive interaction potentials intrinsic to such materials, a number of scientific and technical challenges become immediately apparent. In particular, I will focus on the subtle interplay of rheological influences, such as externally applied shear and elongation stresses, with the inherent ``stickiness'' of carbon nanotube suspensions and melts, where the latter typically favors the formation of disordered networks or ``gels'' over the more desirable liquid-crystalline order. For simple shear, the strength of the applied stress is found to be a critical factor in dictating carbon nanotube morphology, which varies from a quiescent network to macroscopic aggregates to a fully dispersed, flow-aligned (para)nematic state. Although we find remarkably low loading thresholds for elastic percolation, our results highlight a fundamental dilemma for the engineering of conducting carbon nanotube polymer composites; dispersion stability will often be achieved at the expense of electrical conductivity. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:42PM |
W4.00002: Single-walled carbon nanotubes in strong acids: controlling solubility and the liquid crystal phase. Invited Speaker: Single Walled Nanotubes (SWNTs have remarkable electrical, thermal, and mechanical properties. Neat, well-aligned SWNT fibers and sheets could be the ultimate building blocks of strong, ultra-light multifunctional materials for aerospace applications, and could yield electromechanical actuators and sensors with unprecedented performance. After the achievement of scalable production of SWNTs, the difficulty of processing pristine SWNTs by liquid-phase methods has been the single most important roadblock to manufacturing macroscopic materials composed solely of SWNTs. Here we show that SWNTs dissolve at high concentration in acids; the SWNTs are stabilized because acids protonate their sidewalls, balancing wall-wall van der Waals forces. Acid strength controls the phase behaviour. At low concentration, SWNTs in acids dissolve as individual tubes which behave as Brownian rods. At higher concentration, SWNTs form a highly unusual nematic liquid phase consisting of spaghetti-like self assembled supermolecular strands of mobile, solvated tubes in equilibrium with a dilute isotropic phase. At even higher concentration, the spaghetti strands self-assemble into a polydomain nematic liquid crystal, where the domains are entangled with each other. Under anhydrous condition, the liquid crystalline phase can be processed into continuous highly aligned fibers of pure SWNTs without the aid of surfactants or polymers. By using a new fluorescent staining technique, we measure the rotational diffusivity and persistence length of SWNTs suspended in water with the aid of surfactants, and show that SWNTs behave as Brownian rods. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 4:18PM |
W4.00003: Phase Behavior of Carbon Nanotube Suspensions Invited Speaker: We study the phase behavior of nanotube suspensions stabilized by surfactants or amphiphilic polymers. The control of the composition of the solutions allows the interaction potential between the nanotubes to be finely tuned. As a consequence, it is possible to quantitatively analyze important phenomena such as percolation or liquid crystalline phase transitions. In particular, we describe how the percolation of rod-like particles is quantitatively decreased in the presence of attractive interactions (1). We show that rod-like particles respond much more strongly than spheres to attractive interactions; strengthening thereby the technological interest of carbon nanotubes to achieve low percolation thresholds for electrostatic dissipation or electromagnetic shielding. By contrast, carbon nanotubes which experience repulsive interactions can spontaneously order and form liquid crystalline solutions (2). Aligning and packing nanotubes is a major challenge to obtain macroscopic materials with improved properties. We will briefly discuss at the end of the presentation, our latest results concerning the fabrication of fibers aligned nanotubes (3). In particular, we will present new treatments of these fibers which lead to unusual mechanical properties and shape memory effects with giant stress recovery (4). \begin{enumerate} \item B. Vigolo, C. Coulon, M. Maugey, C. Zakri, P. Poulin, \textbf{Science} \textbf{2005}. \item S. Badaire, C. Zakri, M. Maugey, A. Derr\'{e}, J. Barisci, G. Wallace, P. Poulin, \textbf{Adv. Mat.} \textbf{2005}. \item P. Miaudet, M. Maugey, A. Derr\'{e}, V. Pichot, P. Launois, P. Poulin, C. Zakri, \textbf{Nanoletters} \textbf{2005}. \item P. Miaudet, A. Derr\'{e}, M. Maugey, C. Zakri, P. Poulin, in preparation. \end{enumerate} [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:54PM |
W4.00004: Carbon Nanotube Suspensions: some underlying issues Invited Speaker: Entropy of mixing of rigid particles in a suspending medium is determined on a per-particle basis and thus, for a given weight fraction will decrease with increasing particle size. In the case of carbon nanotubes, the entropy contribution to mixing will thus be small compared with the interparticle forces which comprise the enthalpic energy contribution to any thermodynamic equilibrium. These forces will generally be short range with the exception of electrostatic forces in the cases that the particles carry a charge. The ability to form carbon nanotube suspensions depends on both the chemical affinity between the medium and the tubes and, it appears, the size of the medium molecules. Surface treatments of the nanotubes have been developed both using covalently attached functional groups and surfactants, and each strategy has been successfully applied to both multi and single wall CNTs. Because carbon nanotubes are long, thin, rigid and comparatively straight, they have been shown to self assemble into liquid crystalline phases showing all the attributes of conventional systems. The relationship between such CNT systems and the conventional ‘Flory’ phase diagram will be described, as will the exploitation of these phase equilibria to fractionate nanotubes on the basis of mesogenicity. The use of liquid crystalline phases as a basis for the processing of carbon nanotubes into aligned assemblies such as fibres will also be outlined. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:30PM |
W4.00005: Processes for Dispersing Single Wall Carbon Nanotubes in Polymers and How to Determine Their Spatial and Alignment Distributions. Invited Speaker: We are currently designing and making polymer nanocomposites with single wall carbon nanotubes (SWNT) to obtain improved mechanical properties, electrical conductivity, flammability, and thermal conductivity. Our coagulation method is broadly applicable to nanocomposites using readily-soluble thermoplastics such as polystyrene and poly (methyl methacrylate). A variation of this method has been developed for nanotube/polymer composites based on high-density polyethylene. Nylon-based composites are prepared using our adaptation of an \textit{in situ }interfacial polymerization method. Epoxy-based SWNT composites are prepared using either shear mixing with multi-step thermal treatments or an infiltration method using a freestanding SWNT network. Recent studies have explored the use of solid-state shear pulverization to disperse SWNT. Rheology is employed to evaluate the spatial distribution of SWNT based on the onset of solid-like behavior associated with the formation of a SWNT network. Morphological tools including various microscopy methods, x-ray scattering and Raman imaging, are used to evaluate both dispersion and alignment. The resulting spatial and alignment distributions of SWNT depend on the composite fabrication method and any subsequent processing, such as melt fiber spinning to effectively align SWNT. Examples will be given detailing the importance of (1) SWNT dispersion on flammability and thermal conductivity and (2) SWNT alignment on electrical conductivity. [Preview Abstract] |
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