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At least 955 records · Page 53Linked to original sources
PubMed2006

Self-assembly of polystyrene-block-poly(ethylene oxide) copolymers at the air-water interface: is dewetting the genesis of surface aggregate formation?

Block copolymer self-assembly at the air-water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution and in the bulk; however, the present analysis of atomic force microscopy (AFM) and isotherm data at different spreading concentrations suggests a nonequilibrium mechanism for the formation of various polystyrene-b-poly(ethylene oxide) (PS-b-PEO) aggregates (spaghetti, dots, rings, and chainlike aggregates) at the air-water interface starting with an initial dewetting of the copolymer spreading solution from the water surface. We show that different spreading concentrations provide kinetic snapshots of various stages of self-assembly at the air-water interface as a result of different degrees of PS chain entanglements in the spreading solution. Two block copolymers are investigated: MW = 141k (11.4 wt % PEO) and MW = 185k (18.9 wt % PEO). Langmuir compression isotherms for the 185k sample deposited from a range of spreading concentrations (0.1-2.0 mg/mL) indicate less dense packing of copolymer chains within aggregate cores formed at lower spreading concentrations due to a competition between the interfacial adsorption of PEO blocks and the kinetic restrictions of PS chain entanglements. From AFM analysis of the transferred Langmuir-Blodgett films, it is clear that PS chain entanglements in the spreading solution also affect the morphological evolution of surface aggregates for both samples, with earlier structures being trapped at higher concentrations. At the highest spreading concentration for the 141k copolymer, the coexistence of long spaghetti aggregates with cellular arrays of holes, along with various transition structures, indicates that various surface aggregates evolve from networks of rims formed as a result of dewetting of the evaporating spreading solution from the water surface.

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PubMed2007

Ion distribution around electrostatically stabilized polystyrene latex particles studied by ellipsometric light scattering.

The ion distribution around electrostatically stabilized polystyrene latex spheres for different ionic strengths is investigated by ellipsometric light scattering. This method is sensitive to the refractive index profile around colloidal particles, which is affected by the local salt content. At an average salt concentration of c* = 10(-4) mol L(-1), the ion concentration at the particle interface increases discontinuously, and a layer of high salt content with 20-30 nm thickness is built up. The observation cannot be explained within the framework of the Poisson-Boltzmann equation; it rather resembles a prewetting transition. Interactions that could possibly lead to a stabilization of the observed layer of high salt content are discussed.

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PubMed2007

Structure and dewetting behavior of polyhedral oligomeric silsesquioxane-filled polystyrene thin films.

Polyhedral oligomeric silsesquioxane (POSS) meets increasing interest as a building unit for inorganic-organic hybrid materials. The incorporation of cyclopentyl-substituted POSS (CpPOSS) into polystyrene (PS) thin films led to an inhibition of dewetting. In this paper, the dispersion state of CpPOSS in the CpPOSS/PS hybrid films and, furthermore, the relationships between the structure and dewetting inhibition effect are discussed. Structural analysis of the hybrid films revealed that CpPOSS segregated to the film surface and crystallized. The segregation of CpPOSS to the surface changes the surface free energy and spreading coefficient of the film. Interfacial structure was also roughened by the segregation of CpPOSS, which can contribute to the inhibition of dewetting by pinning the contact line of the PS film with the substrate. The inhibition of dewetting can be attributed to the modification of the film surface and interface by the segregation of CpPOSS.

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PubMed2005

Novel carbon nanotube-polystyrene foam composites for electromagnetic interference shielding.

A novel carbon nanotube-polystyrene foam composite has been fabricated successfully. The electromagnetic interference (EMI) shielding effectiveness measurements indicated that such foam composites can be used as very effective, lightweight shielding materials. The correlation between the shielding effectiveness and electrical conductivity and the EMI shielding mechanism of such foam composites are also discussed.

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PubMed2000

Free radical allyl transfers utilizing soluble non-cross-linked polystyrene and carbohydrate scaffold supports

Free radical allylations were studied using (1) soluble non-cross-linked polystyrene supports, (2) carbohydrate scaffolds, and (3) a combination of both synthetic motifs. Allylations on these custom designer supports provide easily purified products, free of tin residues. A D-xylose carbohydrate scaffold bearing a bromoester was used for a diastereoselective allyl tin transfer thermally at 80 degrees C and with Lewis acids. This is the first example of a diastereoselective radical reaction directed by a removable polymer-supported carbohydrate auxiliary.

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PubMed2001

Catalytic asymmetric dihydroxylation using phenoxyethoxymethyl-polystyrene (PEM)-based novel microencapsulated osmium tetroxide (PEM-MC OsO4).

[reaction: see text]. A phenoxyethoxymethyl-polystyrene (PEM)-based novel polymer-supported osmium catalyst has been developed. The catalyst was readily prepared from PEM polymer based on a microencapsulation technique, and asymmetric dihydroxylation of olefins has been successfully performed using (DHQD)2PHAL as a chiral ligand and K3Fe(CN)6 as a cooxidant in H2O/acetone. The catalyst was recovered quantitatively by simple filtration and reused without loss of activity several times.

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PubMed2004

Use of polystyrene-supported DBU in the synthesis and alpha-selective glycosylation study of the unstable Schmidt donor of L-kedarosamine.

In the alpha-glycosylation study of the unusual, 2-deoxy amino sugar of kedarcidin, polystyrene-supported DBU (PDBU) was found to be invaluable to the clean preparation of the highly labile Schmidt donor of l-kedarosamine. By further recognition that the C4-alcohol should be left free for favorable acceptor reactivity, we could for the first time successfully assemble the C13-O-alpha-glycoside of the ansamacrolide substructure of the kedarcidin chromophore. [reaction: see text]

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PubMed2006

Polystyrene-supported hydroxyproline: an insoluble, recyclable organocatalyst for the asymmetric aldol reaction in water.

4-Hhydroxyproline has been anchored to a polystyrene resin through click chemistry, and the resulting catalyst has been successfully applied to the direct aldol reaction in water. The high hydrophobicity of the resin and the presence of water are key to ensuring high stereoselectivity, whereas yield can be increased by using catalytic amounts of DiMePEG. This effect has been further demonstrated by the inefficiency of a homogeneous, more polar analogue.

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PubMed1999

An allylstannane reagent on non-cross-linked polystyrene support.

[reaction: see text] A new allylstannane reagent on non-cross-linked polystyrene was developed for the first time. This support differs markedly from standard cross-linked polymers because it is completely soluble in organic solvents; moreover, the reactions can be conveniently monitored directly by standard 1H NMR methods. The allylstannane underwent a free radical reaction with an alkyl halide to form a new allyl appendage. Tin byproducts can be easily recovered from cold methanol as white crystalline solids.

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PubMed2002

H2O2 production by cells on titanium and polystyrene surfaces using an in vivo model of exudate and surface related cell function.

The determination of secreted levels of reactive oxygen species by implant-adherent cells in vivo is required for understanding of the role(s) of such reactive oxygen species for the tissue response around medical devices. A model with subcutaneous implants of c.p. titanium (Ti) or polystyrene (PS) (cell culture grade) inserted on the back of rats were used. Implants and associated cells were retrieved and assayed after 1, 3, 5, 7, 14, 21 and 28 days. Morphological analysis of exudate cells showed that polymorphonuclear leukocytes (PMN) predominated after one day whereas macrophages were predominant after three days. The number of implant-adherent cells, as reflected by measurement of DNA, decreased with time. Ultrastructural observations showed that macrophages were predominant cells in contact with the implant surface. Measurement of hydrogen peroxide (H(2)O(2)) secretion by implant-adherent cells during 40 min incubation ex vivo revealed a constitutive generation of 40-400 pmol H(2)O(2)/microg DNA, depending on implantation time. Stimulation with protein kinase C agonist phorbol myristate acetate (PMA) caused an increased H(2)O(2) generation by adherent cells at early (up to five days) but not later (7-28 days) time periods. No major differences between Ti and PS were observed. Taken together, these findings show that Ti and PS implant-adherent cells secrete H(2)O(2) under in vivo conditions. Further, a reduced capacity to mount an enhanced H(2)O(2) secretion upon stimulation was demonstrated at late time periods. The role of this mediator for biocompatibility remains to be established.

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PubMed2000

The ion chromatographic separation of high valence metal cations using a neutral polystyrene resin dynamically modified with dipicolinic acid.

A neutral polystyrene resin column, dynamically loaded with dipicolinic acid at a concentration of 0.1 mM in 1 M potassium nitrate eluent, was investigated for the separation characteristics of a number of high valence metal cations over the pH range 0-3. The metal species studied were Th(IV), U(VI), Zr(IV), Hf(IV), Ti(IV), Sn(IV), V(IV) and V(V), Fe(III) and Bi(III), of which Ti(IV), Sn(IV), V(IV) and Fe(III) did not show any retention. For the remaining metal ions, significant retention was obtained with good peak shapes, except for Th(IV), which moved only slightly from the solvent front with some tailing. The retention order at pH 0.3 was Th(IV) < V(V) < Bi(III) < U(VI) < Hf(IV) < Zr(IV). A notable feature of this separation system was the high selectivity shown for uranium, zirconium and hafnium, the last two being nearly resolved in 15 min on the relatively short 10 cm column.

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PubMed2002

Polystyrene grafted multi-walled carbon nanotubes.

Oxidised, multi-walled, carbon nanotubes can be grafted with polystyrene molecules using an situ radical polymerisation reaction, thereby dramatically modifying their solubility and their suitability for nanocomposite applications.

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PubMed2003

Hydrophobic, low-loading and alkylated polystyrene-supported sulfonic acid for several organic reactions in water: remarkable effects of both the polymer structures and loading levels of sulfonic acids.

A hydrophobic, low-loading and alkylated polystyrene-supported sulfonic acid (LL-ALPS-SO3H) has been developed for several organic reactions such as the hydrolysis of thioesters, the deprotection of acetals and an acetonide, and the hydration of an epoxide and an alkyne in pure water on the basis of remarkable effects of both the polymer structures and loading levels of the sulfonic acid catalysts.

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