Chemically controlled pattern formation in phase-separating materials

Summary The role of chemical reactions in the selection of patterns in phase-separating mixtures is presented. Linearized theory and computer simulation show that the initial long-wavelength instability characteristic of spinodal decomposition is suppressed by chemical reactions, which restrict domain growth to intermediate length scales even in the late stages of phase separation. Our findings suggest that chemical reactions may provide a novel way to stabilize and tune the steady-state morphology of phase-separating materials. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Über asymptotische Entwicklungen von Lösungen linearer Differentialgleichungen 2. Ordnung

Ohne Zusammenfassung     

Formation, isolation and characterization of an AB-biaryl atropisomer of oritavancin

Oritavancin is a semi-synthetic glycopeptide antibiotic which is structurally related to vancomycin. When oritavancin bisphosphate is dried in vacuo with heat, a new compound forms. This new compound is stable only in the solid state and reverts to oritavancin in solution. Highly enriched samples of this compound were obtained by preparative HPLC and the structure of this compound was elucidated by using one and two-dimensional (¹H and ¹³C) NMR spectroscopy in conjunction with computer-assisted molecular modeling. It has been determined that oritavancin adopts a conformation similar to that of vancomycin in solution, while the new compound is the unnatural R-AB-biaryl atropisomer of oritavancin. This is the first observation and isolation of an AB-biaryl atropisomer in an intact member of the vancomycin family of glycopeptide antibiotics.     

Divinylsiloxane‐bisbenzocyclobutene‐based polymer modified with polystyrene–polybutadiene–polystyrene triblock copolymers

Divinylsiloxane‐bisbenzocyclobutene (DVS‐bisBCB) polymer has very low dielectric constant and dissipation factor, good thermal stability, and high chemical resistance. The fracture toughness of the thermoset polymer is moderate due to its high crosslink density. A thermoplastic elastomer, polystyrene–polybutadiene–polystyrene triblock copolymer, was incorporated into the matrix to enhance its toughness. The cured thermoset matrix showed different morphology when the elastomer was added to the B‐staged prepolymer or when the elastomer was B‐staged with the DVS‐bisBCB monomer. Small and uniformly distributed elastomer domains were detected by transmission electron micrographs (TEM) in the former case, but TEM did not detect a separate domain in the latter case. A high percentage of the polystyrene–polybutadiene–polystyrene triblock copolymer could be incorporated into the DVS‐bisBCB thermoset matrix by B‐staging the triblock copolymer with the BCB monomer. The elastomer increased the fracture toughness of DVS‐bisBCB polymer as indicated by enhanced elongation at break and increased K1c values obtained by the modified edge‐lift‐off test. Elastomer modified DVS‐bisBCB maintained excellent electrical properties, high T_g and good thermal stability, but showed higher coefficient of linear thermal expansion values. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1591–1599, 2006     

Flexible sorption and transformation behavior in a microporous metal-organic framework

Crystals of the metal-organic framework material Ni(2)(4,4'-bipyridine)(3)(NO(3))(4) (A) have been grown by reaction of Ni(NO(3))(2).6H(2)O and 4,4'-bipyridine in methanol solution. Single-crystal X-ray diffraction experiments show that the ladder structure of the framework is maintained after desolvation of the material, resulting in the production of a porous solid stable to 215(4) degrees C. Powder X-ray diffraction has been employed to confirm the bulk purity and temperature stability of this material. The crystal structure indicates that the pore window has an area of 12.3 A(2). However, sorption experiments show these windows will admit toluene, which has a minimum cross-sectional area of 26.6 A(2), with no significant change in the structure. Monte Carlo docking calculations show that toluene can be accommodated within the large pores of the structure. Exposure of the related microporous material Ni(2)(4,4'-bipyridine)(3)(NO(3))(4).2C(2)H(5)OH (B) to methanol vapor causes a guest-driven solid-state transformation to A which is observed using powder X-ray diffraction. This structural rearrangement proceeds directly from crystalline B to crystalline A and is complete in less than 1 day. Mechanisms for the transformation are proposed which require breaking of at least one in six of the covalent bonds that confer rigidity on the framework.     

Determination of total carbon and total organic carbon from volatile air pollutants. Part I

Summary A new method of collection of representative air samples for the determination of TC and TOC from volatile air pollutants has been developed. The organic substances undergo combustion and only the carbon dioxide produced is concentrated on molecular sieve 5 A at ambient temperature. The CO₂ is subsequently liberated by thermal desorption at ca. 380°C in a stream of purified gas. Atmospheric CO₂ must be removed first and this can be done (at the sample flow rate of 700 ml/min) by a layer of Ascarite heated at 90° C. Organic compounds passing through the layer are then combusted by a dynamic method utilizing Körbl catalyst and the CO₂ formed is concentrated on a molecular sieve 5 A, followed by thermal desorption and final determination. Organic acids are also retained by the first CO₂ absorber. A diffusion cell for preparation of the mixtures of purified air with vapours of organic compounds is described.

---

Zusammenfassung Eine neue Anreicherungsmethode zur Gewinnung representativer Luftproben zur Bestimmung des Gesamtkohlenstoffgehaltes und des Gehaltes an organischem Kohlenstoff (TOC) aus flüchtigen Luftverunreinigungen wurde entwickelt. Während der Probenahme werden die flüchtigen Verunreinigungen verbrannt und nur das entstandene Kohlendioxid bei Zimmertemperatur am Molekularsieb 5A angereichert. Anschließend wird CO₂ bei ca. 380° C im gereinigten Gasstrom thermisch desorbiert. Bei der TOC-Bestimmung muß atmosphärisches CO₂ vorangehend aus dem Luftstrom entfernt werden und wird an einer auf 90° C erwärmten Ascariteschicht gebunden. Die durchfließenden organischen Verbindungen werden dann im dynamischen System am Körblkatalysator oxydiert, das entstandene CO₂ am Molekularsieb 5A angereichert, thermisch desorbiert und einer Endbestimmung zugeführt. Organische Säuren werden gleichfalls durch den ersten CO₂-Absorber gebunden. Eine Diffusionszelle zur Herstellung von Gemischen gereinigter Luft mit Dämpfen organischer Verbindungen wurde beschrieben.