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  • Chemistry  (9,609)
  • Engineering General
  • 1995-1999  (9,609)
  • 1997  (9,609)
  • 1
    Electronic Resource
    Electronic Resource
    Turbulent mixing of reactive gases in the convective boundary layer (1997)
    Springer
    Boundary layer meteorology 85 (1997), S. 197-222 
    Details
    ISSN: 1573-1472
    Keywords: Turbulence ; Chemistry ; Closure ; Convective boundary layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We study the interactions of chemistry and turbulent mixing of tracersin the convective boundary layer with a second-order closure model,including higher order chemistry terms. In order to limit the number of predictive equations we prescribe the profiles for ¯w¯Θ, ¯w¯θ ¯θ and the lengthscale l. However, for model validation we treat temperature and humidity asinert tracers, and compare the results with profiles observed during theAir Mass Transformation Experiment, and with similarity expressions for thesurface layer. We find good agreement of the mean profiles, but the (co-)variances are slightly underpredicted. Furthermore, the model usesdiagnostic equations expressing third moments of concentration in terms ofsecond moments and their vertical derivatives. They are compared withlarge-eddy model results, showing good agreement and, therefore, thesimplifications are justified. The model is applied to the transport of two gases subject to one bimolecular reaction. The importance of concentration correlations on themean transformation rate is studied. For two gases diffusing in oppositedirections we find for moderate and fast chemistry a 50% and90% decreased transformation rate due to the negatively correlatedconcentrations. These values are similar to large-eddy results of Schumannand Sykes et al. For two bottom-up tracers we find that the covariance ofboth reactive species is either positive or negative, increasing or reducingthe effective transformation rate depending on the Damköhler number (the ratio of the turbulent and the chemistry timescale). A significantdirect influence of chemistry on the flux divergence is found in bothcases. According to the model the effective transport to mid-levels of theboundary layer is increased when two reactive tracers diffuse in oppositedirections, and decreased in the case of two bottom-up tracers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1000414710372
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  • 2
    Electronic Resource
    Electronic Resource
    Synthesis and characterization of biodegradable homopolymers and block copolymers based on adipic anhydride (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 183-192 
    Details
    ISSN: 0887-624X
    Keywords: polyanhydride ; adipic anhydride ; ring-opening polymerization ; block copolymers ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Homopolymers of adipic anhydride (AA) and block copolymers of ∊-caprolactone (∊-CL) and AA have been synthesized with aluminum triisopropoxide as an initiator. Homopolymerization was studied at 20°C in toluene and methylene chloride (CH2Cl2). The end-group analysis agrees with a coordination insertion mechanism based on the acyl-oxygen cleavage of the AA ring. Living poly(∊-caprolactone) (PCL) chains are very efficient macro-initiators for the polymerization of AA, with formation of diblock copolymers of a narrow molecular weight distribution. At our best knowledge, low molecular weight ω-aluminum alkoxide PCL macroinitiators (Mn 〈 1000) allow the first valuable synthesis of PAA with a molecular weight as high as 58,000 and a quite narrow polydispersity (Mw/Mn = 1.2). Size-exclusion chromatography (SEC) and 13C NMR confirm the blocky structure of the copolymers, in agreement with DSC that shows two melting endotherms and two glass transitions characteristic of the crystalline and amorphous phases of PCL and PAA, respectively. Block copolymers of ∊-CL and AA are also sensitive to hydrolysis, which makes them possible candidates for biomedical applications. Initiation of the AA polymerization in bulk with aluminum triisopropoxide in the presence of various ligands is also discussed. © 1997 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    Binary frontal polymerization: A new method to produce simultaneous interpenetrating polymer networks (SINs) (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 227-230 
    Details
    ISSN: 0887-624X
    Keywords: IPN ; frontal polymerization ; propagating front ; SIN ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report a new method for the preparation of a simultaneous interpenetrating polymer network (SIN) using a thermal propagating front of two independent and noninterfering polymerization mechanisms. The system consists of the free radical crosslinking of triethylene glycol dimethacrylate (TGDMA) and the amine/BCl3 · amine curing of diglycidyl ether of bisphenol A (DGEBA). The front velocity dependence on the percentage of each monomer shows a minimum at 45% TGDMA. Temperature profile measurements indicate that a single reaction front propagates. A colored opaque material is produced, but SEM and TEM analysis were inconclusive whether phase separation occurred. Samples as large as 5 cm in diameter were prepared with this method. We conclude that this method should be especially suited for preparing large samples of IPNs in which significant phase separation occurs. © 1997 John Wiley & Sons, Inc.
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  • 4
    Electronic Resource
    Electronic Resource
    Controlled synthesis of amphiphilic block copolymers with pendant glucose residues by living cationic polymerization (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 255-261 
    Details
    ISSN: 0887-624X
    Keywords: controlled synthesis ; amphiphilic block copolymer ; pendant glucose residues ; vinyl ether ; living cationic polymerization ; microphase-separated structure ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Amphiphilic block copolymers of vinyl ethers (VEs) of the type  - [CH2CH(OCH2CH2OR)]m - [CH2CH(OiBu)]n - were synthesized by living cationic polymerization, where R is a D-glucose residue, and m and n are the degrees of polymerization (m = 20-50; n = 11-89). To obtain them, sequential living block copolymerization of isobutyl vinyl ether (IBVE) and the vinyl ether carrying 1,2:5,6-diisopropylidene-D-glucose residue was conducted by using the HCl adduct of IBVE, CH3CH(OiBu)Cl, as initiator in conjunction with zinc iodide. These precursor block copolymers had a narrow molecular weight distribution (M̄w/M̄n ∼ 1.1) and a controlled composition. Treatment of them with a trifluoroacetic acid/water mixture led to the target amphiphiles. The solubility of the amphiphilic block copolymers in various solvents depended strongly on composition or the m/n ratio. Their solvent-cast thin films were observed, under a transmission electron microscope, to exhibit various microphase-separated surface morphologies such as spheres, cylinders, and lamellae, depending on composition. © 1997 John Wiley & Sons, Inc.
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  • 5
    Electronic Resource
    Electronic Resource
    Transient species in pulse-irradiated poly(methyl methacrylate) pure and doped with pyrene (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 299-305 
    Details
    ISSN: 0887-624X
    Keywords: poly(methylmethacrylate) ; pulse radiolysis ; charge scavenging ; doped polymers ; excited states ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The spectra of transients produced in pulse-irradiated pure poly(methyl methacrylate), PMMA, at room and ∼ 130 K temperatures were measured. The intermediates were identified as PMMA radicals and radical anions. In the pulse-irradiated PMMA-pyrene (Py) system the solute excited states and radical ions were produced. Scavenging of negative charges by Py was directly observed at 130 K in a µs time scale. Py fluorescence was found to be produced mainly as a result of Čerenkov photoexcitation. At room temperature, some contribution of ionic mechanism to Py fluorescence formation was found. © 1997 John Wiley & Sons, Inc.
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  • 6
    Electronic Resource
    Electronic Resource
    Noncoordinating anions in carbocationic polymerization (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 329-344 
    Details
    ISSN: 0887-624X
    Keywords: carbocationic polymerization ; isobutylene ; metallocenes ; noncoordinating anions ; styrenics ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The initiation and catalysis of isobutylene polymerization from several new metallocene and nonmetallocene initiator-catalysts that contain the noncoordinating anions (NCA), B(C6F5)4- and RB(C6F6)3-, is reported. Application of these initiator-catalysts is extended to styrenics and vinyl ethers. The NCA does not contribute to termination and can be used in low concentrations compared with conventional Lewis acids. These qualities provide for isobutylene polymerizations that yield low Mn oligomers or high Mn polymer, dependent upon the initiator and polymerization conditions. Mechanistic aspects of initiation, transfer and termination as well as the participation of adventitious water are considered for each class of initiator-catalyst. The influence of the NCA on the stereoregularity of cationic styrene polymerization is also considered. NCAs do not cause the stereospecific carbocationic polymerization of styrene. We suggest that under conditions not conducive to carbocationic polymerization, NCA/metallocenes mediate the coordination polymerization of styrene. © 1997 John Wiley & Sons, Inc.
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  • 7
    Electronic Resource
    Electronic Resource
    Synthesis and enzymatic hydrolysis of lactic acid-depsipeptide copolymers with functionalized pendant groups (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 377-383 
    Details
    ISSN: 0887-624X
    Keywords: biodegradability ; poly(lactic acid) ; lactic acid-depsipeptide copolymer ; ring-opening copolymerization ; functionalized side-group ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Since poly(lactic acid) is the biodegradable polyester having low immunogenicity and good biocompatibility, it is utilized as a medical material. However, poly(lactic acid) is a water-insoluble crystalline polymer having no reactive side-chain group. Thus, the use of poly(lactic acid) is limited. To modify the properties of poly(lactic acid) and to introduce the functionalized pendant groups to poly(lactic acid), we synthesized two kinds of lactic acid-depsipeptide copolymers with reactive pendant groups, namely poly[LA-(Glc-Lys)] and poly[LA-(Glc-Asp)]. This was done through ring-opening copolymerizations of L-lactide with the corresponding protected cyclodepsipeptides, cyclo[Glc-Lys(Z)] and cyclo[Glc-Asp(OBzl)], and subsequent deprotection of benzyloxycarbonyl and benzyl groups, respectively. By changing the mole fraction of the corresponding depsipeptide units, the solubility, thermal transition and degradation behavior of the modified poly(lactic acid) could be varied. © 1997 John Wiley & Sons, Inc.
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  • 8
    Electronic Resource
    Electronic Resource
    Fluorescence lifetime of terbium(III) as a probe for the ion-binding properties of tactic poly(methacrylic acids) (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 393-398 
    Details
    ISSN: 0887-624X
    Keywords: terbium ; poly(methacrylic acid) ; tacticity ; fluorescence ; lifetime ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The binding properties of trivalent metal ions to polyelectrolytes were investigated through the use of terbium [Tb(III)] in fluorescence studies. The fluorescence intensity and lifetimes of the lanthanide ions are directly dependent upon the number of water molecules bound to their inner coordination sphere. The more efficiently a ligand coordinates to a lanthanide ion, the more water molecules are expelled and consequently, the greater the fluorescence intensity and lifetime. This effect was used to probe for differences in the complexation behavior of tactic polymers. Aqueous solutions of isotactic and syndiotactic poly(methacrylic acid) (PMA) were neutralized and complexed with Tb(III) ions. The fluorescence intensity of the 286 nm hypersensitive excitation band was monitored and the lifetimes were measured using several excitation wavelengths. It was found that the isotactic PMA/Tb(III) complex exhibited a six times greater fluorescence intensity than the syndiotactic PMA complex. Lifetime measurements gave the number of water molecules coordinated by Tb(III) in the isotactic complex to be 2.4 while 3.4 waters remained bound to the Tb(III) ion in the syndiotactic PMA complex. These results indicate that isotactic PMA has the greater binding affinity towards Tb(III) ions. © 1997 John Wiley & Sons, Inc.
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  • 9
    Electronic Resource
    Electronic Resource
    Ring opening during the cationic polymerization of 2-methylene-1,3-dioxepane: Cyclic ketene acetal initiation with sulfuric acid supported on carbonSee Reference 1. (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 485-491 
    Details
    ISSN: 0887-624X
    Keywords: 2-methylene-1,3-dioxepane ; cyclic ketene acetal ; cationic polymerization ; activated carbon black ; 1,2-polymerization ; ring-opening polymerization ; ring-retained polymerization ; initiator ; catalyst ; heterogeneous catalysis ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The stable cyclic ketene acetal, 2-methylene-1,3-dioxepane, 7, has been polymerized cationically in pentane, CH2Cl2 and THF at 25°C to form a polymer which is composed of both ring-opened (40-50%) and ring-retained (50-60%) structures. Initiation was catalyzed by using H2SO4-supported on activated carbon black. This unique outcome differs significantly from the cationic polymerization of several other five- and six-membered ring cyclic ketene acetals which gave 100% 1,2-vinylpolymerization under these conditions. As the polymerization temperature increased in cationic polymerization of 7 the ring-opened content increased and the molecular weight of the polymers decreased in such solvents as cyclohexane, 1,2-dichloroethane, dimethoxyethane, and bis-(2-methoxyethyl) ether. The mechanism of this polymerization is discussed. This research also illustrated the ability to initiate the cationic polymerization of cyclic ketene acetals by acidified carbon black while avoiding subsequent polymer decomposition. © 1997 John Wiley & Sons, Inc.
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  • 10
    Electronic Resource
    Electronic Resource
    An application of arene-iron chemistry in the synthesis of a novel liquid crystalline polymer (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 447-453 
    Details
    ISSN: 0887-624X
    Keywords: polyether ; polyester ; aryl ether ; nucleophilic substitution ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Arene-iron chemistry was applied in the synthesis of a novel liquid crystalline polymer. The chemistry, which is based on iron cyclopentadienyl (FeCp) arene complexes, allows sequential nucleophilic substitution of the chlorides from 1,3-dichlorobenzene-FeCp complex and photolytic decomplexation of the products to afford asymmetrical aryl ethers. This methodology provides easy access to novel polyether-esters, and is potentially useful in the synthesis of various functional polyarylates. © 1997 John Wiley & Sons, Inc.
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