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Selective Solvent (selective + solvent)
Selected AbstractsSphere-to-Rod Transition of Micelles formed by the Semicrystalline Polybutadiene- block -Poly(ethylene oxide) Block Copolymer in a Selective SolventMACROMOLECULAR RAPID COMMUNICATIONS, Issue 5 2010Adriana M. Mihut Abstract We present a morphological study of the micellization of an asymmetric semicrystalline block copolymer, poly(butadiene)- block -poly(ethylene oxide), in the selective solvent n -heptane. The molecular weights of the poly(butadiene) (PB) and poly(ethylene oxide) (PEO) blocks are 26 and 3.5,kg,·,mol,1, respectively. In this solvent, micellization into a liquid PEO-core and a corona of PB-chains takes place at room temperature. Through a thermally controlled crystallization of the PEO core at ,30,°C, spherical micelles with a crystalline PEO core and a PB corona are obtained. However, crystallization at much lower temperatures (,196,°C; liquid nitrogen) leads to the transition from spherical to rod-like micelles. With time these rod-like micelles aggregate and form long needles. Concomitantly, the degree of crystallinity of the PEO-cores of the rod-like micelles increases. The transition from a spherical to a rod-like morphology can be explained by a decrease of solvent power of the solvent n -heptane for the PB-corona chains: n -Heptane becomes a poor solvent at very low temperatures leading to a shrinking of the coronar chains. This favors the transition from spheres to a morphology with a smaller mean curvature, that is, to a cylindrical morphology. [source] Synthesis of poly(n -hexyl isocyanate- b - N -vinylpyrrolidone) block copolymers by the combination of anionic and nitroxide-mediated radical polymerizations: Micellization properties in aqueous solutionsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 19 2006Panayiotis Bilalis Abstract A combination of anionic and nitroxide-mediated radical polymerizations (dual initiator) was employed for the synthesis of poly(n -hexyl isocyanate- b - N -vinylpyrrolidone) (PHIC- b -PNVP) block copolymers. The samples were characterized with a size exclusion chromatograph equipped with refractive-index and light scattering detectors as well as 1H NMR spectroscopy. Relatively good control over the molecular weights was achieved. However, rather broad molecular weight distributions were obtained. The micellar properties of the PHIC- b -PNVP block copolymers were studied in water, which is a selective solvent for the poly(N -vinylpyrrolidone) blocks. Static and dynamic light scattering revealed the presence of equilibrium between the micelles and clusters. The clusters partially deaggregated with increasing temperature. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5719,5728, 2006 [source] Supercritical fluid extraction of cynaropicrin and 20-hydroxyecdysone from Leuzea carthamoides DCJOURNAL OF SEPARATION SCIENCE, JSS, Issue 8 2008Helena Sovová Abstract Leuzea carthamoides is an adaptogenic plant containing biologically active compounds as ecdysteroids and guaianolide-type sesquiterpene lactones, conventionally extracted from the plant with ethanol. It may be a potential source of the mentioned natural compounds. Ethanol-modified near-critical CO2 was used as selective solvent with the aim to increase the level of 20-hydroxyecdysone in the extract from L. carthamoides roots and to remove selectively cynaropicrin, a sesquiterpene lactone of bitter taste, from the leaves. The extraction conditions were varied (pressure 20,28 MPa, temperature 40,60°C, ethanol concentration in the solvent 0,7.1%) and the extraction yield and extract composition were compared with the results of ethanolic extraction. The supercritical fluid extraction (SFE) from finely powdered plant was controlled by phase equilibrium. Cynaropicrin was quantitatively removed from the leaves where 89% of 20-hydroxyecdysone was retained. The extraction yield of 20-hydroxyecdysone from roots with ethanol-modified CO2 was lower by 30% than with ethanol but its concentration in the extract was higher by 67%. [source] One-Pot Synthesis of Micelles with a Cross-Linked Poly(acrylic acid) CoreMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 2 2006Genhua Zheng Abstract Summary: Stable micelles with polystyrene (PS) as a shell and cross-linked poly[(acrylic acid)- co -(ethylene glycol diacrylate)] as a core have been successfully prepared by reversible addition fragmentation chain transfer (RAFT) copolymerization of acrylic acid and ethylene glycol diacrylate in a selective solvent with PS-SC(S)Ph as a RAFT agent. For the preparation of stable micelles, the RAFT polymerizations are carried out in different solvents: benzene, cyclohexane, and mixtures of tetrahydrofuran and cyclohexane. The monomer/PS-SC(S)Ph molar ratio and molecular weight of the macro-RAFT agent, PS-SC(S)Ph, influence the RAFT polymerization and the formation of micelles. Block copolymerization in selective solvent with the RAFT agent. [source] Block Copolymers Under Shear FlowMACROMOLECULAR THEORY AND SIMULATIONS, Issue 4 2005Igor Rychkov Abstract Summary: Microphase separation transition in block copolymer melts and solutions in equilibrium and under shear flow is reviewed. The non-equilibrium molecular dynamics (NEMD) computer simulation methodology is presented in detail including the derivation of the SLLOD equations of motion, Gaussian thermostat, and operator-splitting symplectic integrators. Results of our recent NEMD computer simulation studies of diblock copolymers in a selective solvent under shear flow are presented. Shear-dependent structural, rheological, and microscopical properties are described. New phase transitions are discovered. The parallel-perpendicular orientational transition in a weak-strong flow is revealed. Theoretical approaches are reviewed including the Edwards Hamiltonian, Landau-Ginzburg model, self-consistent mean field theory, field-theoretic simulation, as well as the time-dependent Landau-Ginzburg framework and its application to the studies of complex fluids. [source] Hydrogen bond-mediated self-assembly and supramolecular structures of diblock copolymer mixturesPOLYMER INTERNATIONAL, Issue 5 2009Shiao-Wei Kuo Abstract This review summarizes recent advances in the preparation of hydrogen bonding block copolymer mixtures and the supramolecular structures they form through multiple hydrogen bonding interactions. Hydrogen bonding in block copolymer mixtures that form nanostructures and have unusual electronic, photonic and magnetic properties is a topic of great interest in polymer science. Combining the self-assembly of block copolymers with supramolecular structures offers unique possibilities to create new materials with tunable and responsive properties. The self-assembly of structures from diblock copolymer mixtures in the bulk state is readily controlled by varying the weight fraction of the block copolymer mixture and the copolymer composition; in solution, the morphologies are dependent on the copolymer composition, the copolymer concentration, the nature of the common solvent, the amount of the selective solvent and, most importantly, the hydrogen bonding strength. Copyright © 2008 Society of Chemical Industry [source] Nanostructured copolymer gels for dsDNA separation by CEELECTROPHORESIS, Issue 23 2008Fen Wan Abstract Pluronics are triblock copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) that are able to form many different ordered nanostructures at appropriate polymer concentrations and temperatures in selective solvents. These nanostructured "gels" showed desirable criteria when used as DNA separation media, especially in microchip electrophoresis, including dynamic coating and viscosity switching. A ternary system of F127 (E99P69E99)/TBE buffer/1-butanol was selected as a model system to test the sieving performance of different nanostructures in separating dsDNA by CE. The nanostructures and their lattice constants were determined by small-angle X-ray scattering. Viscosity measurements showed the sol,gel transition phenomena. In addition to the cubic structure, successful electrophoretic separation of dsDNA in 2-D hexagonally packed cylinders was achieved. Results showed that without further optimization, ,X174 DNA,Hae III digest was well separated within 15,min in a 7-cm separation channel, by using F127/TBE/1-butanol gel with a 2-D hexagonal structure. A mechanism for DNA separations by those gels with both hydrophilic and hydrophobic domains is discussed. [source] Application of Nitroxide-Terminated Polymers Prepared by Sonochemical Degradation in the Synthesis of Block CopolymersMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 2 2006Martina Bartsch Abstract Summary: The ultrasonic irradiation of a polymer solution results in the breakage of macromolecular CC bonds. In the presence of radical scavengers the formed macroradicals are prevented from termination reactions as combination or disproportionation. Using nitroxides as trapping agents the polymer is transformed into a macroinitiator, which can be used in controlled free-radical polymerization to synthesize block copolymers. In this work several polymers were exposed to sonochemical degradation and terminated with various nitroxides, e.g. OH-TEMPO and TIPNO. In a second reaction step the prepared polymer-nitroxide-adducts were applied as macroinitiators in controlled free-radical polymerizations with styrene. The obtained products were mixtures of block copolymer and the corresponding homopolymers. The visco-elastic properties were investigated by rheological analysis. A special separation technique with selective solvents was applied to determine the content of block copolymer. Synthesis of block copolymers with sonochemically prepared macroinitiators. [source] | ||||||||||