The Experts below are selected from a list of 105 Experts worldwide ranked by ideXlab platform
Lewis J. Fetters - One of the best experts on this subject based on the ideXlab platform.
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Surface exchange of polymer amphiphiles
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994Co-Authors: Eugenia Kumacheva, Jacob Klein, P Pincus, Lewis J. FettersAbstract:Abstract It has been shown by surface force studies that the properties of grafted polymer layers formed from functionalized polystyrene (polystyrene terminated by a Zwitterionic Group at the end) can be changed dramatically by surface exchange: the replacement of longer molecules attached to the surface via the end Group by the shorter amphiphile terminated with the same Group — behaviour opposite to that shown by adsorbed homopolymers. This exchange requires a certain minimum concentration of the shorter polymer in the solution, but the substantial replacement of long chains by short chains can occur even when the monomer concentration of the shorter polymer is relatively very small, i.e. only 1/500 that of the longer polymers.
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Lubrication forces between surfaces bearing polymer brushes
Macromolecules, 1993Co-Authors: Jacob Klein, Yoichiro Kamiyama, Hisae Yoshizawa, Jacob N Israelachvili, Glenn H. Fredrickson, P Pincus, Lewis J. FettersAbstract:We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a Zwitterionic Group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes
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Lubrication Forces between Surfaces Bearing Polymer Brushes
Macromolecules, 1993Co-Authors: Jacob Klein, Yoichiro Kamiyama, Hisae Yoshizawa, Jacob N Israelachvili, Glenn H. Fredrickson, P Pincus, Lewis J. FettersAbstract:We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a Zwitterionic Group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes. Dynamic measurements of the surface forces, where the surface separation D is varied sinusoidally in time, show two regimes: at D > 2.5L, the hydrodynamic forces are characteristic of Newtonian flow of liquid with the viscosity of bulk toluene, but with a shear plane shifted a distance L(H) from each mica surface (qualitatively similar to earlier observations with adsorbed homopolymer layers). We find that L almost-equal-to L(H). When the layers are compressed (D < 2L), the hydrodynamic forces are dominated by flow of solvent through the confined polymer layers in the gap. They may be described by an ''effective viscosity'' which increases as D decreases, varying in fair quantitative accord with a recent model. At the highest compressions (D < L/2) the confined polymer layers behave in a solid-like manner, suggesting that the confinement between the surfaces greatly increases molecular relaxation times.
Miguel A Sierra - One of the best experts on this subject based on the ideXlab platform.
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an entry to mixed nhc fischer carbene complexes and Zwitterionic Group 6 metal alkenyls
Chemistry: A European Journal, 2016Co-Authors: Miguel A Sierra, Mar Gomezgallego, Alba D Merinero, Elena A Giner, Carmen Ramirez De ArellanoAbstract:: The addition of NHCs to α,β-unsaturated Cr(0) and W(0) (Fischer) carbene complexes is strongly dependent on the electrophilicity of the carbene carbon. Electrophilic alkoxy-carbene complexes quantitatively react with NHCs to yield stable Zwitterionic (racemic) Cr(0) - and W(0) -alkenyls with total regio- and E-stereoselectivity. Less electrophilic aminocarbenes react with NHCs to promote the displacement of a CO ligand and yield "mixed" NHC/Fischer biscarbenes in a process that is unprecedented in Group 6 metal-carbene chemistry. In fact, the compounds prepared, are some of the scarce examples of Fischer bisylidenes reported in the literature. The electrochemistry of the Zwitterionic Cr(0) - and W(0) -alkenylcomplexes made, show that these compounds have a strong reductor character, which is demonstrated in their reactions towards [Ph3 C][PF6 ]. The oxidation processes lead to new types of cationic Fischer mono- and biscarbene complexes having a charged NHC fragment in their structures, in a new example of the use of electron-transfer reactions as a method to prepare novel Group 6 (Fischer) carbene complexes.
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structure and photoreactivity of stable Zwitterionic Group 6 metal allenyls
Chemical Communications, 2013Co-Authors: Alicia Santiago, Mar Gomezgallego, Carmen Ramirez De Arellano, Miguel A SierraAbstract:The synthesis and mechanistic study of the unprecedented reactivity of a series of Zwitterionic η1-metal allenyls are reported.
Jacob Klein - One of the best experts on this subject based on the ideXlab platform.
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Effect of end-Group sticking energy on the properties of polymer brushes: comparing experiment and theory.
Journal of Chemical Physics, 2004Co-Authors: Simon Titmuss, Wuge H. Briscoe, Iain E. Dunlop, Giorgos Sakellariou, Nikos Hadjichristidis, Jacob KleinAbstract:Using surface force balance measurements we have established that polystyrene chains bearing three Zwitterionic Groups have a higher end-Group sticking energy than equivalent chains bearing a single Zwitterionic Group. In a good solvent, polystyrene chains end-functionalized with three Zwitterionic Groups form brushes of a higher surface coverage than those bearing a single zwitterion. The increase in surface coverage is slow compared with the initial formation of the brush. Measurements of the refractive index allow us to directly quantify the variation of surface coverage, permitting comparison with models for the kinetics of brush formation based on scaling theory and an analytical self-consistent field. We find qualitative support for associating the kinetic barrier with the energy required for an incoming chain to stretch as it penetrates the existing brush.
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Interactions between surfaces bearing highly extended polymer melt brushes. 1. Adhesion and spontaneous thinning
Macromolecules, 2002Co-Authors: Larisa Tsarkova, Jacob Klein, Xueyan Zhang, Nikos HadjichristidisAbstract:A surface force balance (SFB) was used to characterize the behavior of highly stretched polymer brush melts. Langmuir-Blodgett monolayers of polyisoprene (M w = 29.9 kg/mol) end-functionalized with a Zwitterionic Group were deposited onto freshly cleaved mica (areal density 1 chain/(170 A 2 )), and two identical brush monolayers were brought into adhesive contact in the SFB. The changes in film thickness as well as the topography of the contact could be continuously monitored. We observed spontaneous film thinning of the brush-melt bilayer, attributed to the outward lateral motion of the anchoring end Groups resulting from the contact-induced pressure on the confined brushes, and a detailed model for this is presented. Refractive index measurements of the confined PI-X melt brushes did not reveal any significant deviation from the bulk value for polyisoprenes, suggesting that possible effects (if any) on the optical properties due to chain orientation were below our detection limit. The behavior of the two opposing brush melts was compared with that where only one brush monolayer was confined between mica surfaces.
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Surface exchange of polymer amphiphiles
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994Co-Authors: Eugenia Kumacheva, Jacob Klein, P Pincus, Lewis J. FettersAbstract:Abstract It has been shown by surface force studies that the properties of grafted polymer layers formed from functionalized polystyrene (polystyrene terminated by a Zwitterionic Group at the end) can be changed dramatically by surface exchange: the replacement of longer molecules attached to the surface via the end Group by the shorter amphiphile terminated with the same Group — behaviour opposite to that shown by adsorbed homopolymers. This exchange requires a certain minimum concentration of the shorter polymer in the solution, but the substantial replacement of long chains by short chains can occur even when the monomer concentration of the shorter polymer is relatively very small, i.e. only 1/500 that of the longer polymers.
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Lubrication forces between surfaces bearing polymer brushes
Macromolecules, 1993Co-Authors: Jacob Klein, Yoichiro Kamiyama, Hisae Yoshizawa, Jacob N Israelachvili, Glenn H. Fredrickson, P Pincus, Lewis J. FettersAbstract:We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a Zwitterionic Group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes
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Lubrication Forces between Surfaces Bearing Polymer Brushes
Macromolecules, 1993Co-Authors: Jacob Klein, Yoichiro Kamiyama, Hisae Yoshizawa, Jacob N Israelachvili, Glenn H. Fredrickson, P Pincus, Lewis J. FettersAbstract:We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a Zwitterionic Group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes. Dynamic measurements of the surface forces, where the surface separation D is varied sinusoidally in time, show two regimes: at D > 2.5L, the hydrodynamic forces are characteristic of Newtonian flow of liquid with the viscosity of bulk toluene, but with a shear plane shifted a distance L(H) from each mica surface (qualitatively similar to earlier observations with adsorbed homopolymer layers). We find that L almost-equal-to L(H). When the layers are compressed (D < 2L), the hydrodynamic forces are dominated by flow of solvent through the confined polymer layers in the gap. They may be described by an ''effective viscosity'' which increases as D decreases, varying in fair quantitative accord with a recent model. At the highest compressions (D < L/2) the confined polymer layers behave in a solid-like manner, suggesting that the confinement between the surfaces greatly increases molecular relaxation times.
Carmen Ramirez De Arellano - One of the best experts on this subject based on the ideXlab platform.
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an entry to mixed nhc fischer carbene complexes and Zwitterionic Group 6 metal alkenyls
Chemistry: A European Journal, 2016Co-Authors: Miguel A Sierra, Mar Gomezgallego, Alba D Merinero, Elena A Giner, Carmen Ramirez De ArellanoAbstract:: The addition of NHCs to α,β-unsaturated Cr(0) and W(0) (Fischer) carbene complexes is strongly dependent on the electrophilicity of the carbene carbon. Electrophilic alkoxy-carbene complexes quantitatively react with NHCs to yield stable Zwitterionic (racemic) Cr(0) - and W(0) -alkenyls with total regio- and E-stereoselectivity. Less electrophilic aminocarbenes react with NHCs to promote the displacement of a CO ligand and yield "mixed" NHC/Fischer biscarbenes in a process that is unprecedented in Group 6 metal-carbene chemistry. In fact, the compounds prepared, are some of the scarce examples of Fischer bisylidenes reported in the literature. The electrochemistry of the Zwitterionic Cr(0) - and W(0) -alkenylcomplexes made, show that these compounds have a strong reductor character, which is demonstrated in their reactions towards [Ph3 C][PF6 ]. The oxidation processes lead to new types of cationic Fischer mono- and biscarbene complexes having a charged NHC fragment in their structures, in a new example of the use of electron-transfer reactions as a method to prepare novel Group 6 (Fischer) carbene complexes.
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structure and photoreactivity of stable Zwitterionic Group 6 metal allenyls
Chemical Communications, 2013Co-Authors: Alicia Santiago, Mar Gomezgallego, Carmen Ramirez De Arellano, Miguel A SierraAbstract:The synthesis and mechanistic study of the unprecedented reactivity of a series of Zwitterionic η1-metal allenyls are reported.
Mar Gomezgallego - One of the best experts on this subject based on the ideXlab platform.
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an entry to mixed nhc fischer carbene complexes and Zwitterionic Group 6 metal alkenyls
Chemistry: A European Journal, 2016Co-Authors: Miguel A Sierra, Mar Gomezgallego, Alba D Merinero, Elena A Giner, Carmen Ramirez De ArellanoAbstract:: The addition of NHCs to α,β-unsaturated Cr(0) and W(0) (Fischer) carbene complexes is strongly dependent on the electrophilicity of the carbene carbon. Electrophilic alkoxy-carbene complexes quantitatively react with NHCs to yield stable Zwitterionic (racemic) Cr(0) - and W(0) -alkenyls with total regio- and E-stereoselectivity. Less electrophilic aminocarbenes react with NHCs to promote the displacement of a CO ligand and yield "mixed" NHC/Fischer biscarbenes in a process that is unprecedented in Group 6 metal-carbene chemistry. In fact, the compounds prepared, are some of the scarce examples of Fischer bisylidenes reported in the literature. The electrochemistry of the Zwitterionic Cr(0) - and W(0) -alkenylcomplexes made, show that these compounds have a strong reductor character, which is demonstrated in their reactions towards [Ph3 C][PF6 ]. The oxidation processes lead to new types of cationic Fischer mono- and biscarbene complexes having a charged NHC fragment in their structures, in a new example of the use of electron-transfer reactions as a method to prepare novel Group 6 (Fischer) carbene complexes.
-
structure and photoreactivity of stable Zwitterionic Group 6 metal allenyls
Chemical Communications, 2013Co-Authors: Alicia Santiago, Mar Gomezgallego, Carmen Ramirez De Arellano, Miguel A SierraAbstract:The synthesis and mechanistic study of the unprecedented reactivity of a series of Zwitterionic η1-metal allenyls are reported.
