| 1. |
- Kehoe, Laura, et al.
(författare)
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Make EU trade with Brazil sustainable
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 364:6438, s. 341-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 2. |
- Abreu, Barbara, et al.
(författare)
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Gemini surfactants as efficient dispersants of multiwalled carbon nanotubes : Interplay of molecular parameters on nanotube dispersibility and debundling
- 2019
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Ingår i: Journal of Colloid and Interface Science. - : ACADEMIC PRESS INC ELSEVIER SCIENCE. - 0021-9797 .- 1095-7103. ; 547, s. 69-77
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Tidskriftsartikel (refereegranskat)abstract
- Surfactants have been widely employed to debundle, disperse and stabilize carbon nanotubes in aqueous solvents. Yet, a thorough understanding of the dispersing mechanisms at molecular level is still warranted. Herein, we investigated the influence of the molecular structure of gemini surfactants on the dispersibility of multiwalled carbon nanotubes (MWNTs). We used dicationic n-s-n gemini surfactants, varying n and s, the number of alkyl tail and alkyl spacer carbons, respectively; for comparisons, single-tailed surfactant homologues were also studied. Detailed curves of dispersed MWNT concentration vs. surfactant concentration were obtained through a stringently controlled experimental procedure, allowing for molecular insight. The gemini are found to be much more efficient dispersants than their single-tailed homologues, i.e. lower surfactant concentration is needed to attain the maximum dispersed MWNT concentration. In general, the spacer length has a comparatively higher influence on the dispersing efficiency than the tail length. Further, scanning electron microscopy imaging shows a sizeable degree of MWNT debundling by the gemini surfactants in the obtained dispersions. Our observations also point to an adsorption process that does not entail the formation of micelle-like aggregates on the nanotube surface, but rather coverage by individual molecules, among which the ones that seem to be able to adapt best to the nanotube surface provide the highest efficiency. These studies are relevant for the rational design and choice of optimal dispersants for carbon nanomaterials and other similarly water-insoluble materials.
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| 3. |
- Dai, Jing, et al.
(författare)
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Dispersing Carbon Nanotubes in Water with Amphiphiles : Dispersant Adsorption, Kinetics, and Bundle Size Distribution as Defining Factors
- 2018
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:42, s. 24386-24393
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Tidskriftsartikel (refereegranskat)abstract
- Debundling and dispersing single-walled carbon nanotubes (SWNTs) is essential for applications, but the process is not well understood. In this work, aqueous SWNT dispersions were produced by sonicating pristine SWNT powder in the presence of an amphiphilic triblock copolymer (Pluronic F127) as dispersant. Upon centrifugation, one obtains a supernatant with suspended individual tubes and thin bundles and a precipitate with large bundles (and impurities). In the supernatant, that constitutes the final dispersion, we determined the dispersed SWNT concentration by thermogravi-metric analysis (TGA) and UV-vis spectroscopy, and the dispersant concentration by NMR The fraction of dispersant adsorbed at the SWNT surface was obtained by H-1 diffusion NMR Sigmoidal dispersion curves recording the concentration of dispersed SWNTs as a function of supernatant dispersant concentration were obtained at different SWNT loadings and sonication times. As SWNT bundles are debundled into smaller and smaller ones, the essential role of the dispersant is to sufficiently quickly cover the freshly exposed surfaces created by shear forces induced during sonication. Primarily kinetic reasons are behind the need for dispersant concentrations required to reach a substantial SWNT concentration. Centrifugation sets the size threshold below which SWNT particles are retained in the dispersion and consequently determines the SWNT concentration as a function of sonication time.
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| 4. |
- Dai, Jing, et al.
(författare)
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The dispersion process of carbon nanotubes sonicated in aqueous solutions of a dispersant
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Single-walled carbon nanotube (SWNT) dispersions are created by sonicating pristine SWNT powders added to aqueous solutions of the dispersant block copolymer Pluronic F127. In those dispersions, the amount of the dispersed SWNT is determined by the combination of TGA and UV-Vis methods, while the dispersant concentration is estimated by 1H NMR spectroscopy. In addition, the amount of dispersant adsorbed at the SWNT surface is obtained by 1H NMR diffusion experiments. A part of the dispersant is taken up by non-dispersed and precipitated particles. Dispersion curves recording the amount of the dispersed SWNT as a function of either the initial dispersant concentration or the final dispersant concentration are obtained at different initial SWNT loadings and sonication times. The results show in detail the way the original SWNT particles are divided into smaller and smaller sizes thereby increasing the available SWNT surface to be covered by dispersant. Centrifugation sets the size-threshold above which SWNT particles are retained in the dispersion which determined the SWNT content as a function of sonication time.
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| 5. |
- Ferreira Fernandes, Ricardo Manuel, 1983-, et al.
(författare)
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Block copolymers adsorbed on single-walled carbon nanotubes. Block polydispersity and the modes of surface attachment
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 1H NMR peak intensities, 1H NMR diffusion measurements and TGA experiments were used to clarify the fate of the dispersing molecules, block copolymer Pluronic F127, during preparation of single-walled carbon nanotube (CNT) dispersions and their state in the created dispersions. In the dispersions, a fraction of the F127 molecules is adsorbed to the CNT surface. The mode of adsorption is the attachment and significant immobilization of the hydrophobic polypropylene oxide (PPO) block to the CNT surface and, as a result, the 1H NMR signal from the attached PPO blocks is lost. On the other hand, the hydrophilic polyethylene oxide (PEO) blocks remain highly mobile and thereby detectable by NMR. The F127 is revealed to exhibit significant block polydispersity. Molecules with large PPO blocks become enriched upon the surface of that fraction of the initial CNT powder that does not become dispersed. The molecular motions involved in creating the observed NMR features are clarified.
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| 6. |
- Ferreira Fernandes, Ricardo Manuel, 1983-, et al.
(författare)
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Block Copolymers as Dispersants for Single-Walled Carbon Nanotubes : Modes of Surface Attachment and Role of Block Polydispersity
- 2018
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Ingår i: Langmuir. - : AMER CHEMICAL SOC. - 0743-7463 .- 1520-5827. ; 34:45, s. 13672-13679
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Tidskriftsartikel (refereegranskat)abstract
- When using amphiphilic polymers to exfoliate and disperse carbon nanotubes in water, the balance between the hydrophobic and hydrophilic moieties is critical and nontrivial. Here, we investigate the mode of surface attachment of a triblock copolymer, Pluronics F127, composed of a central hydrophobic polypropylene oxide block flanked by hydrophilic polyethylene oxide blocks, onto single-walled carbon nanotubes (SWNTs). Crucially, we analyze the composition in dispersant of both the as-obtained dispersion (the supernatant) and the precipitate-containing undispersed materials. For this, we combine the carefully obtained data from H-1 NMR peak intensities and self-diffusion and thermogravimetric analysis. The molecular motions behind the observed NMR features are clarified. We find that the hydrophobic blocks attach to the dispersed SWNT surface and remain significantly immobilized leading to H-1 NMR signal loss. On the other hand, the hydrophilic blocks remain highly mobile and thus readily detectable by NMR. The dispersant is shown to possess significant block polydispersity that has a large effect on dispersibility. Polymers with large hydrophobic blocks adsorb on the surface of the carbonaceous particles that precipitate, indicating that although a larger hydrophobic block is good for enhancing adsorption, it may be less effective in dispersing the tubes. A model is also proposed that consistently explains our observations in SWNT dispersions and some contradicting findings obtained previously in carbon nanohorn dispersions. Overall, our findings help elucidating the molecular picture of the dispersion process for SWNTs and are of interest when looking for more effective (i.e., well-balanced) polymeric dispersants.
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| 7. |
- Ferreira Fernandes, Ricardo Manuel, et al.
(författare)
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Lateral Diffusion of Dispersing Molecules On Nanotubes As Probed by NMR
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:1, s. 582-589
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Tidskriftsartikel (refereegranskat)abstract
- Noncovalent dispersion of carbon nanotubes is essential to most applications but still poorly understood at the molecular level. The interaction of the dispersing molecule with the nanotube, wrapping or nonwrapping, still awaits consensus. Herein, we have studied by H-1 NMR diffusometry some features of molecular dynamics in the system of carbon nanotubes dispersed by triblock copolymer Pluronics F127 in water. The diffusional decays obtained at different diffusion times, Delta, are not single-exponential and have a complex Delta-dependent profile, ultimately implying that the polymer is observed in two states: free (in unimeric form) and nanotube-bound. Fitting a two-site exchange model to the data indicates that at any instant, only a small fraction of polymers are adsorbed on the nanotubes, with polydisperse residence times in the range of 100-400 ms. Most significantly, we further provide an estimate of D = (3-8) x 10(-12) m(2) s(-1) the coefficient of lateral diffusion of the polymer along the nanotube surface, which is an order of magnitude slower than the corresponding self-diffusion coefficient in water. The emerging picture is that of a nonwrapping mode for the polymer-nanotube interaction.
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| 8. |
- Mohino-Herranz, Inma, et al.
(författare)
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Assessment of Mental, Emotional and Physical Stress through Analysis of Physiological Signals Using Smartphones
- 2015
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Ingår i: Sensors. - : MDPI AG. - 1424-8220. ; 15:10, s. 25607-25627
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Tidskriftsartikel (refereegranskat)abstract
- Determining the stress level of a subject in real time could be of special interest in certain professional activities to allow the monitoring of soldiers, pilots, emergency personnel and other professionals responsible for human lives. Assessment of current mental fitness for executing a task at hand might avoid unnecessary risks. To obtain this knowledge, two physiological measurements were recorded in this work using customized non-invasive wearable instrumentation that measures electrocardiogram (ECG) and thoracic electrical bioimpedance (TEB) signals. The relevant information from each measurement is extracted via evaluation of a reduced set of selected features. These features are primarily obtained from filtered and processed versions of the raw time measurements with calculations of certain statistical and descriptive parameters. Selection of the reduced set of features was performed using genetic algorithms, thus constraining the computational cost of the real-time implementation. Different classification approaches have been studied, but neural networks were chosen for this investigation because they represent a good tradeoff between the intelligence of the solution and computational complexity. Three different application scenarios were considered. In the first scenario, the proposed system is capable of distinguishing among different types of activity with a 21.2% probability error, for activities coded as neutral, emotional, mental and physical. In the second scenario, the proposed solution distinguishes among the three different emotional states of neutral, sadness and disgust, with a probability error of 4.8%. In the third scenario, the system is able to distinguish between low mental load and mental overload with a probability error of 32.3%. The computational cost was calculated, and the solution was implemented in commercially available Android-based smartphones. The results indicate that execution of such a monitoring solution is negligible compared to the nominal computational load of current smartphones.
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| 9. |
- Nadiv, Roey, et al.
(författare)
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Polymer nanocomposites : insights on rheology, percolation, jamming and molecular mobility
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Carbon nanotubes (CNTs) loading in a polymer matrix strongly affect the rheological behavior and in turn hamper the overall performance of the composite. The majority of the research in this topic has focused on bulk rheological properties, while here we employ NMR diffusion experiments to explore the mobility (diffusivity) of epoxy molecules when loaded with CNTs. Rheology and light microscopy indicate percolation, or jamming events of CNT aggregates, caging a substantial amount of epoxy molecules while small angle X-ray scattering indicates rearrangements of epoxy molecules in the vicinity of the nanotubes. NMR diffusion experiments distinguish between the diffusion of the caged molecules and that of the free ones, and relate the fraction of the former to the system viscosity. These findings demonstrate the utility of NMR diffusion experiments as an additional method applied to the rheological behavior of polymer mixtures.
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