The Experts below are selected from a list of 285 Experts worldwide ranked by ideXlab platform
Emmanuel P Giannelis - One of the best experts on this subject based on the ideXlab platform.
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On The Origins of Silicate Dispersion in Polysiloxane/Layered‐Silicate Nanocomposites
Advanced Functional Materials, 2006Co-Authors: Daniel Schmidt, Florence Clement, Emmanuel P GiannelisAbstract:We report the first multi-system study of a layered-Silicate dispersion in polysiloxane/layered-Silicate nanocomposites. A variety of layered Silicates (montmorillonite, synthetic fluoromica, laponite, and fluorohectorite) and cationic modifiers (single-, twin-, and triple-tailed surfactants with tails of varying lengths and both primary and quaternary head-groups) are combined to form organically modified layered Silicates, which are then screened for compatibility with low-molecular-weight silanol-terminated poly(dimethylsiloxane) (PDMS). Promising combinations are then selected and studied in greater depth with respect to both molecular weight and polysiloxane end-group and substituent chemistry. We find that the PDMS backbone is generally incompatible with the layered Silicates, regardless of modification type, and that dispersion in PDMS systems results from the presence of polar end-groups, a result unprecedented in the field of polymer nanocomposites. We go on to quantify the substituent effect, not only with respect to end-group chemistry, but taking into account changes in the polysiloxane backbone itself. For instance, in the absence of polar end-groups we observe dispersion in the case of poly(methylphenylsiloxane) but not poly(3,3.3-trifluoropropylmethylsiloxane). Finally, we apply a new epoxy/amine PDMS curing chemistry to PDMS-nanocomposite production and show higher levels of layered-Silicate dispersion than observed in comparable silanol-terminated PDMS-based systems. Our findings serve as an indication of what is necessary to achieve a layered-Silicate dispersion in polysiloxane/ layered-Silicate nanocomposites, and may indicate a more general approach for improving dispersion in systems where the polymer backbone is otherwise incompatible with the layered Silicate.
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Polymer layered Silicate nanocomposites
Advanced materials, 1996Co-Authors: Emmanuel P GiannelisAbstract:Polymer nanocomposites with layered Silicates as the inorganic phase (reinforcement) are discussed. The materials design and synthesis rely on the ability of layered Silicates to intercalate in the galleries between their layers a wide range of monomers and polymers. Special emphasis is placed on a new, versatile and environmentally benign synthesis approach by polymer melt intercalation. I0 contrast to in-situ polymerization and solution intercalation, melt intercalation involves mixing the layered Silicate with the polymer and heating the mixture above the softening point of the polymer. Compatibility with various polymers is accomplished by derivatizing the Silicates with alkyl ammonium cations via an ion exchange reaction. By hetuning the surface characteristics nanodispersion (i.e. intercalation or delamination) can be accomplished. The resulting polymer layered Silicate (PLS) nanocomposites exhibit properties dramatically different from their more conventional counterparts. For example, PLS nanocomposites can attain a particular degree of stiffoesr, strength and barrier properties with far less inorganic content than comparable glass- or mineral reinforced polymers and, therefore, they are far lighter in weight. In addition, PLS nanocomposites exhibit significant increase in thermal stability as well as self-extinguishing characteristics. The combination of improved properties, convenient processing and low cost has already led to a few commercial applications with more currently under development.
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kinetics of polymer melt intercalation
Macromolecules, 1995Co-Authors: Richard A Vaia, Klaus D. Jandt, Edward J Kramer, Emmanuel P GiannelisAbstract:the kinetics of polystyrene melt intercalation in organically modified mica-type Silicates were studied using X-ray diffraction and transmission electron microscopy. By monitoring the change in the integrated intensity of the basal reflection of the Silicate host, the rate of conversion from unintercalated to intercalated Silicate was determined at various temperatures and for various molecular weights of polystyrene. Hybrid formation is limited by mass transport into the primary particles of the host Silicate and not specifically by diffusion of the polymer chains within the Silicate galleries. The activation energy of hybrid formation is similar to that previously measured for polystyrene self-diffusion in the melt, implying that the mobility of the polymer chains within the host galleries is at least comparable to that in the melt.
Choon-hong Tan - One of the best experts on this subject based on the ideXlab platform.
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Enantioselective Addition-Alkylation of α,β-Unsaturated Carbonyls via Bisguanidinium Silicate Ion Pair Catalysis.
Journal of the American Chemical Society, 2020Co-Authors: Wenchao Chen, Esther Cai Xia Ang, Siu Min Tan, Zhijie Chua, Jingyun Ren, Ziqi Yang, Bo Teng, Richmond Lee, Choon-hong TanAbstract:Silicon hydrides, alkynylsilanes, and alkoxylsilanes were activated by fluoride in the presence of bisguanidinium catalyst to form hypervalent Silicate ion pairs. These activated Silicates undergo 1,4-additions with chromones, coumarins, and α-cyanocinnamic esters generating enolSilicate intermediates, for a consequent stereoselective alkylation reaction. The reduction-alkylation reaction proceeded under mild conditions using polymethylhydrosiloxane, a cheap and environmentally friendly hydride source. The addition-alkylation reactions with alkynylsilanes and alkoxylsilanes resulted in the construction of two vicinal chiral carbon centers with excellent enantioselectivities and diastereoselectivities (up to 99% ee, >99:1 dr). Density functional theory calculations and experimental NMR studies revealed that penta-coordinated Silicates are crucial intermediates.
Wenchao Chen - One of the best experts on this subject based on the ideXlab platform.
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Enantioselective Addition-Alkylation of α,β-Unsaturated Carbonyls via Bisguanidinium Silicate Ion Pair Catalysis.
Journal of the American Chemical Society, 2020Co-Authors: Wenchao Chen, Esther Cai Xia Ang, Siu Min Tan, Zhijie Chua, Jingyun Ren, Ziqi Yang, Bo Teng, Richmond Lee, Choon-hong TanAbstract:Silicon hydrides, alkynylsilanes, and alkoxylsilanes were activated by fluoride in the presence of bisguanidinium catalyst to form hypervalent Silicate ion pairs. These activated Silicates undergo 1,4-additions with chromones, coumarins, and α-cyanocinnamic esters generating enolSilicate intermediates, for a consequent stereoselective alkylation reaction. The reduction-alkylation reaction proceeded under mild conditions using polymethylhydrosiloxane, a cheap and environmentally friendly hydride source. The addition-alkylation reactions with alkynylsilanes and alkoxylsilanes resulted in the construction of two vicinal chiral carbon centers with excellent enantioselectivities and diastereoselectivities (up to 99% ee, >99:1 dr). Density functional theory calculations and experimental NMR studies revealed that penta-coordinated Silicates are crucial intermediates.
Arnaud Poulesquen - One of the best experts on this subject based on the ideXlab platform.
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dissociation mechanisms of dissolved alkali Silicates in sodium hydroxide
Journal of Physical Chemistry C, 2020Co-Authors: Romain Dupuis, Jean Baptiste Champenois, Roland J.-m. Pellenq, Arnaud PoulesquenAbstract:Recent accelerated simulations of the decondensation of Silicates by sodium hydroxide open a window on understanding complex mechanisms of depolymerization of Silicate chains. Herein, complex mecha...
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Interplay between Silicate and hydroxide ions during geopolymerization
Cement and Concrete Research, 2019Co-Authors: J. Aupoil, Jean Baptiste Champenois, J.-b. D'espinose De Lacaillerie, Arnaud PoulesquenAbstract:Two sets of activating solutions with increasing sodium hydroxide contents were prepared either with or without Silicates. Their buffer capacities, i.e. their ability to resist changes in pH, were determined and compared using the Hammett acidity function, a measure of acidity appropriate for concentrated solutions. This is the first time the Hammett acidity function of sodium Silicate solutions has been measured. The effects of the buffer capacity and of the initial Hammett acidity function on the reactivity of metakaolin-based pastes were assessed using isothermal conduction micro-calorimetry. The reactivity of metakaolin in sodium hydroxide solutions is shown to be directly related to the initial Hammett acidity function, whereas for sodium Silicate mixtures, the buffer capacity is a more pertinent parameter. The mechanism deduced for the role of hydroxide ions during geopolymerization also highlights the role of Silicate species as a hydroxide reservoir that nurtures the dissolution process.
Bo Teng - One of the best experts on this subject based on the ideXlab platform.
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Enantioselective Addition-Alkylation of α,β-Unsaturated Carbonyls via Bisguanidinium Silicate Ion Pair Catalysis.
Journal of the American Chemical Society, 2020Co-Authors: Wenchao Chen, Esther Cai Xia Ang, Siu Min Tan, Zhijie Chua, Jingyun Ren, Ziqi Yang, Bo Teng, Richmond Lee, Choon-hong TanAbstract:Silicon hydrides, alkynylsilanes, and alkoxylsilanes were activated by fluoride in the presence of bisguanidinium catalyst to form hypervalent Silicate ion pairs. These activated Silicates undergo 1,4-additions with chromones, coumarins, and α-cyanocinnamic esters generating enolSilicate intermediates, for a consequent stereoselective alkylation reaction. The reduction-alkylation reaction proceeded under mild conditions using polymethylhydrosiloxane, a cheap and environmentally friendly hydride source. The addition-alkylation reactions with alkynylsilanes and alkoxylsilanes resulted in the construction of two vicinal chiral carbon centers with excellent enantioselectivities and diastereoselectivities (up to 99% ee, >99:1 dr). Density functional theory calculations and experimental NMR studies revealed that penta-coordinated Silicates are crucial intermediates.
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Enantioselective Addition-Alkylation of α,β-Unsaturated Carbonyls via Bisguanidinium Silicate Ion Pair Catalysis
'Sociological Research Online', 2020Co-Authors: Chen Wenchao, Bo Teng, Ang Esther, Tan Siu, Chua Zhijie, Ren Jingyun, Yang Ziqi, Lee Richmond, Lu Haihua, Tan ChoonAbstract:2020 American Chemical Society. Silicon hydrides, alkynylsilanes, and alkoxylsilanes were activated by fluoride in the presence of bisguanidinium catalyst to form hypervalent Silicate ion pairs. These activated Silicates undergo 1,4-additions with chromones, coumarins, and α-cyanocinnamic esters generating enolSilicate intermediates, for a consequent stereoselective alkylation reaction. The reduction-alkylation reaction proceeded under mild conditions using polymethylhydrosiloxane, a cheap and environmentally friendly hydride source. The addition-alkylation reactions with alkynylsilanes and alkoxylsilanes resulted in the construction of two vicinal chiral carbon centers with excellent enantioselectivities and diastereoselectivities (up to 99% ee, \u3e99:1 dr). Density functional theory calculations and experimental NMR studies revealed that penta-coordinated Silicates are crucial intermediates
