A latex-based concept for making carbon nanotube/polymer nanocomposites

Several methods have been developed over the last few years to achieve the incorporation of carbon nanotubes (CNTs) into a polymer matrix in order to obtain electrically conductive nanocomposites. The key factors for producing such composites with low CNT loadings comprise the quality of the wetting between the filler and the polymer matrix, as well as the state of dispersion of the CNTs throughout the matrix. The final target is to manufacture easilyprocessable, low density conductive plastics, that in the future would be able to replace metals in applications for which these are still preferred. Nevertheless, as produced CNTs are either stuck together in thick bundles in the case of singlewall carbon nanotubes (SWCNTs), or are highly entangled in the case of multi-wall carbon nanotubes (MWCNTs). As a consequence, one of the main bottlenecks for the production of high performance CNT/polymer nanocomposites remains the resistance of CNTs against individualization. The work presented in this Ph.D. thesis focuses on the study of: - the various steps of a process based on the application of latex technology, which is utilized to prepare CNT/polymer nanocomposites. The key step of this process is the mixing of two colloidal solutions, being a dispersion of mainly individual CNTs covered by surfactant molecules, and a polymer latex, i.e. an aqueous dispersion of submicron polymer particles. The latter are also covered with surfactant molecules. - the nanocomposites produced in this way, in particular their electrical properties, which are governed by the formation of a percolating network of CNTs, and are therefore strongly dependent on the concentration of conductive fillers dispersed in the matrix. The first step of the process consists of debundling CNTs in an aqueous surfactant solution (typically sodium dodecyl sulfate, SDS) in order to obtain a stable dispersion of CNTs covered by surfactant molecules. The achievement of this first step is crucial: in order to get conductive films with filler loadings as low as possible, it is imperative to achieve a good dispersion of the CNTs in the polymer matrix. Consequently, it is very important to control and to monitor the CNT debundling during the first step of the process. However, CNT debundling does not guarantee a proper dispersion of the CNTs in the final nanocomposite. It was demonstrated that this sonication-driven step can be monitored by UV-Vis spectroscopy, see Chapter 3. This method is based on the fact that individual CNTs absorb light in the wavelength region between 204 200 and 1200 nm. The debundling of SWCNTs or MWCNTs results in an increase of the concentration of individual CNTs, and finally in an increase of the UV-Vis signal. A leveling off of the UV-Vis absorbance, recorded as a function of time and/or the total amount of ultrasonic energy supplied to the system, indicates that the maximum degree of exfoliation has been achieved and that, accordingly, further energy input can be stopped in order to prevent unnecessary damage to the CNTs. These results were confirmed with cryo-Transmission Electron Microscopy (cryo-TEM) and Scanning Electron Microscopy (SEM). In addition, four different experimental techniques, based on UV-Vis spectroscopy, surface tension measurements, thermogravimetry and a modified version of the Maron’s titration, have been developed in order to determine the lowest amount of surfactant necessary to reach the highest degree of exfoliation of the CNTs (Chapter 4). The results obtained enabled us to estimate a lower limit of the specific surface area of exfoliated SWCNTs, as well as an estimation of the specific surface covered by one surfactant molecule when adsorbed on the SWCNT surface. It is worth mentioning that these procedures are in principle applicable to a large range of surfactant-particle systems. In a second step, the stable aqueous SDS-CNT dispersion is mixed with a polymer latex. The mixture obtained is freeze dried and subsequently compression-molded. Before melt processing, the system typically consists of closely-packed latex particles (polystyrene latex particles for our model system) mixed with CNTs, which are confined in the interstitial space between the polymer particles. Since flow of the polymer occurs during the compression molding step, CNTs can move through the polymer melt. As a result, the processing conditions have a large influence on the conductivity, as well as on the percolation threshold of the nanocomposite, see Chapter 5. According to SEM analysis, in the nanocomposite films obtained, the CNTs are homogeneously dispersed in the polymer matrix, and form a network of preponderantly individualized CNTs. In particular, electrically conductive nanocomposites with a percolation threshold of about 0.3 wt% (resp. 0.9 wt%) of SWCNTs (resp. MWCNTs) dispersed in a high molecular weight polystyrene produced by free radical emulsion polymerization can be obtained in this way. This latex-based process is extremely versatile since it enables us to disperse SWCNTs and MWCNTs into most of the polymers produced by emulsion polymerization or polymers which can be artificially brought into a latex form. Thanks to this method, CNTs were successfully dispersed in another amorphous polymer than polystyrene, viz. poly(methyl methacrylate), a semi-crystalline polymer, i.e. polypropylene, or in a polymer blend, namely a poly(2,6-dimethyl- 1,4-phenylene ether)/polystyrene blend (see Chapter 7). Additionally, several procedures were explored in order to improve the properties of the composites, i.e. to lower the percolation threshold and increase the conductivity. We demonstrated that tuning the characteristics of the polymer matrix (in particular its molecular weight distribution, see Chapter 5) is a promising way to optimize these properties. It was also shown that the choice of the polymer matrix itself is a relevant parameter (see Chapter 7). Also the characteristics of the CNTs themselves (among others: their type (SWCNTs or MWCNTs), their intrinsic conductivity, their degree of purity, their diameter, and their aspect ratio) are of importance. Summarizing, we can state that the latex-based production of electrically conductive nanocomposites leads to well-defined materials with preponderantly individual CNTs homogeneously dispersed in a polymer matrix. Interestingly, the electrical properties of these materials are strongly influenced by the characteristics of the polymer matrix and the CNTs chosen, as well as by the process parameters

Static and Dry Friction due to Multiscale Surface Roughness

It is shown on the basis of scaling arguments that a disordered interface between two elastic solids will quite generally exhibit static and "dry friction" (i.e., kinetic friction which does not vanish as the sliding velocity approaches zero), because of Tomlinson model instabilities that occur for small length scale asperities. This provides a possible explanation for why static and "dry" friction are virtually always observed, and superlubricity almost never occurs

A versatile latex-based route furnishing polymer/carbon nanofiller composites

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First results from the NA60 experiment at CERN

Since 1986, several heavy ion experiments have studied some signatures of the formation of the quark-gluon plasma and a few exciting results have been found. However, some important questions are still unanswered and require new measurements. The NA60 experiment, with a new detector concept that vastly improves dimuon detection in proton-nucleus and heavy-ion collisions, studies several of those open questions, including the production of open charm. This paper presents the experiment and some first results from data collected in 2002.Comment: Paper presented at the XXXVIII Rencontres de Moriond, QCD and High Energy Hadronic Interactions, Les Arcs, March 22-29, 2003. 4 pages, 6 figure

First results from NA60 on low mass muon pair production in In-In collisions at 158 GeV/nucleon

The NA60 experiment at the CERN SPS studies dimuon production in proton-nucleus and nucleus-nucleus collisions. The combined information from a novel vertex telescope made of radiation-tolerant silicon pixel detectors and from the muon spectrometer previously used in NA50 allows for a precise measurement of the muon vertex and a much improved dimuon mass resolution. We report on first results from the data taken for Indium-Indium collisions at 158 AGeV/nucleon in 2003, concentrating on a subsample of about 370 000 muon pairs in the mass range $<1.2$ GeV/$c^{2}$. The light vector mesons $\omega$ and $\phi$ are completely resolved, with a mass resolution of about 23 MeV/$c^{2}$ at the $\phi$. The transverse momentum spectra of the $\phi$ are measured over the continuous range $0<p_{\rm T}<2.5$ GeV/c; the inverse slope parameter of the spectra is found to increase with centrality, with an average value of $T=252\pm3$ MeV.Comment: 9 pages, 6 figures. Plenary talk, SQM2004 conference, Cape Town, South Africa 15-20 September, 2004. To be published in Journal of Physics G: Nuclear and Particle Physic

The dependence of the anomalous J/psi suppression on the number of participant nucleons

The observation of an anomalous J/psi suppression in Pb-Pb collisions by the NA50 Collaboration can be considered as the most striking indication for the deconfinement of quarks and gluons at SPS energies. In this Letter, we determine the J/psi suppression pattern as a function of the forward hadronic energy E-ZDC measured in a Zero Degree Calorimeter (ZDC). The direct connection between EZDC and the geometry of the collision allows us to calculate, within a Glauber approach, the precise relation between the number of participant nucleons N-part and E-ZDC. Then, we check if the experimental data can be better explained by a sudden or a smooth onset of the anomalous J/psi suppression as a function of the number of participants. (C) 2001 Elsevier Science B.V. All rights reserved.info:eu-repo/semantics/publishedVersio

Bottomonium and Drell-Yan production in p-A collisions at 450 GeV

The NA50 Collaboration has measured heavy-quarkonium production in p-A collisions at 450 GeV incident energy (sqrt(s) = 29.1 GeV). We report here results on the production of the Upsilon states and of high-mass Drell-Yan muon pairs (m > 6 GeV). The cross-section at midrapidity and the A-dependence of the measured yields are determined and compared with the results of other fixed-target experiments and with the available theoretical estimates. Finally, we also address some issues concerning the transverse momentum distributions of the measured dimuons.Comment: 18 pages, 9 figures, to be published in Phys. Lett.

Centrality Behaviour of J/ψ\psi Production in Na50

The J/$\psi$ production in 158 A GeV Pb-Pb interactions is studied, in the dimuon decay channel, as a function of centrality, as measured with the electromagnetic or with the very forward calorimeters. After a first sharp variation at mid centrality, both patterns continue to fall down and exhibit a curvature change at high centrality values. This trend excludes any conventional hadronic model and is in agreement with a deconfined quark-gluon phase scenario. We report also preliminary results on the measured charged multiplicity, as given by a dedicated detector.Comment: 5 pages, 7 figures (in eps) talk given at XXXI International Symposium on Multiparticle Dynamics, Sep. 1-7, 2001, Datong China URL http://ismd31.ccnu.edu.cn

Two-pion Bose-Einstein correlations in central Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 2.76 TeV

The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV at the Large Hadron Collider is presented. We observe a growing trend with energy now not only for the longitudinal and the outward but also for the sideward pion source radius. The pion homogeneity volume and the decoupling time are significantly larger than those measured at RHIC.Comment: 17 pages, 5 captioned figures, 1 table, authors from page 12, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/388