| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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