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  • Physics  (46)
  • Atomic, Molecular and Optical Physics  (28)
  • Wiley-Blackwell  (74)
  • National Academy of Sciences
  • 1995-1999  (74)
  • 1
    Electronic Resource
    Electronic Resource
    Open spaces in the subsurface region of polyethylene probed by monoenergetic positron beams (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 2597-2605 
    Details
    ISSN: 0887-6266
    Keywords: positron annihilation ; open space ; relaxation ; subsurface ; polyethylene ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Open spaces in the subsurface region (〈10 μm) of very low density polyethylene were probed by a monoenergetic positron beam. From measurements of Doppler broadening spectra measurements of the annihilation radiation and the lifetime spectra of positrons as a function of incident positron energy, the size of the open spaces in the region of 0-3 μm was found to be larger than that in the deeper region. This was attributed to the cooperative motion of large segments of molecules which is activated near the surface. After the freezing in of such motions (below 230 K), although the lineshape parameter S in the bulk was almost constant, S in the region of 0-3 μm decreased with decreasing temperature. This discrepancy was associated with the presence of the open spaces with an excess content and the resultant contraction of amorphous structure near the surface. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2597-2605, 1998
    Additional Material: 5 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    Effect of permeation temperature on permeation and separation of a benzene/cyclohexane mixture through liquid-crystalline polymer membranes (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 281-288 
    Details
    ISSN: 0887-6266
    Keywords: side-chain liquid-crystalline polymer membrane ; pervaporation ; benzene/cyclohexane mixture ; state-transformation ; benzene permselectivity ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: When a benzene/cyclohexane mixture of 10 wt % benzene was permeated through side-chain liquid-crystalline polymer (LCP) membranes by pervaporation at various temperatures, the permeation rate increased with increasing permeation temperature. The LCP membranes also exhibited a benzene permselectivity. The permselectivity for the benzene/cyclohexane mixture through the LCP membrane was different in the glassy, liquid-crystalline, and isotropic states. The LCP membrane had different apparent activation energies for permeation at each state. LCP membrane in the liquid-crystalline state had the highest apparent activation energy of the three states. Results suggest that the benzene permselectivity was influenced by changes in the LCP membrane structure, i.e., a state-transformation. It was found that a balance of the orientation of mesogenic groups and the flexibility of the siloxane chains was very important for benzene permselectivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 281-288, 1998
    Additional Material: 7 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    Viscoelastic and morphological behavior of hybrid styryl-based polyhedral oligomeric silsesquioxane (POSS) copolymers (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 1857-1872 
    Details
    ISSN: 0887-6266
    Keywords: hybrid polymers ; rheology ; reptation ; silsesquioxane polymers ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: We report on the viscoelastic behavior of linear thermoplastic nonpolar hybrid inorganic-organic polymers. These materials have been synthesized through copolymerization of an oligomeric inorganic macromer with 4-methylstyrene where the inorganic portion of the material is a well-defined polyhedral oligosilsesquioxane (POSS), R7(Si8O12)(CH2CH2C6H4C=CH2), with R = c-C6H11 or c-C5H9. A series of 4-methyl styrene copolymers with approximately 4, 8, and 16 mol % POSS macromer incorporation were investigated. Rheological measurements show that the polymer dynamics are profoundly affected as the percent of POSS increases. In particular, a high-temperature rubbery plateau develops (where a terminal zone is not observed), despite the fact that the parent poly 4-methylstyrene is unentangled. It is also observed that the thermal properties are influenced as the percent of POSS incorporation increases, with increases in the glass and decomposition temperatures. The results suggest that interchain interactions between the massive inorganic groups are responsible for the retardation of polymer chain motion, a mechanism similar to the “sticky reptation” model conceived for hydrogen-bonded elastomers and developed by Leibler et al. [Macromolecules, 24, 4701 (1991)]. Control over the interchain interactions would also give rise to the observed increases in glass transition and the establishment of a rubbery plateau. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1857-1872, 1998
    Additional Material: 14 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    Transition and relaxation processes of polyethylene, polypropylene, and polystyrene studied by positron annihilation (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 35 (1997), S. 1601-1609 
    Details
    ISSN: 0887-6266
    Keywords: positron annihilation ; polyethylene ; polypropylene ; polystyrene ; glass transition ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transition and relaxation processes of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were studied by the positron annihilation technique. From measurements of lifetime spectra of positrons as a function of temperature, the lifetime of ortho-positronium, τ3, and its intensity, I3, were found to increase above 260 K for PP. This fact was attributed to a cooperative motion of large segments of molecules above the glass transition temperature, Tg. For PE, above Tg (140 K), the value of τ3 increased, but the temperature coefficient of I3 was negative below 230 K. From this fact, for PE, the molecular motions that cause the glass transition were associated with a rearrangement of molecules by local motions such as kink motions. The discrepancy between the results for PE and PP was attributed to the presence of methyl groups in PP and the resultant suppression of the local motions. For PS (Tg = 340 K), the molecular motions were found to start above 260 K, but those were suppressed by an interphenyl correlation. Detailed annihilation characteristics of positrons in polymers were also discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1601-1609, 1997
    Additional Material: 12 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    Permeation and separation of benzene/cyclohexane mixtures through liquid-crystalline polymer membranes (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 35 (1997), S. 699-707 
    Details
    ISSN: 0887-6266
    Keywords: side-chain liquid-crystalline polymer ; pervaporation ; benzene/cyclohexane mixtures ; state transformation ; permselectivity ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The side-chain liquid-crystalline polymer (LCP) was synthesized by the addition of the mesogenic monomer to poly(methylsiloxane) with Pt catalyst. When the benzene/cyclohexane mixtures were permeated through the LCP membranes by pervaporation at various temperatures, the permeation rate increased with increasing benzene concentration in the feed solution and permeation temperature. Though the LCP membranes exhibited a benzene permselectivity, a mechanism of the permeation and separation for the benzene/cyclohexane mixtures was different in the glassy, liquid-crystalline and isotropic state of the LCP membranes. These results suggested that the permselectivity was fairly influenced by the change of the LCP membrane structure, that is, a state transformation. It was found that a balance of the orientation of mesogenic groups and flexibility of siloxane chains is very important for the permeability and selectivity. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 699-707, 1997
    Additional Material: 5 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Characterization of epoxy-surfactant interactions (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 2781-2792 
    Details
    ISSN: 0887-6266
    Keywords: waterborne epoxy ; surfactant ; cooperativity ; Fox equation ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: We have investigated epoxies based on the diglycidyl ether of bisphenol A (DGEBA) cured with 2-ethyl-4-methylimidazole (EMI-24) in the presence of the nonionic surfactant Triton X-100. A goal was to determine if the viscoelastic properties of the epoxy-surfactant system differed when prepared in bulk form, as opposed to being cast as a thin film on the surface of E-glass cloth. Such a combination of materials has generated great interest for potential use in the construction of laminated circuit boards. Using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and atomic force microscopy (AFM), it was determined that the surfactant acts as a plasticizer and is miscible with the epoxy system in concentrations up to 15% by weight. The glass transition temperature (Tg) depression of the epoxy due to the surfactant was accurately described by the Fox equation. DMA master curves were constructed in the frequency domain. The temperature dependence of the shift factors was used to determine the fragility of each of the samples studied. It was found that the fragility (cooperativity) of the epoxy decreased as the concentration of surfactant increased, presumably due to a reduction of intermolecular constraints. The fragility of the combined epoxy-surfactant system increased when cast on the surface of the E-glass cloth. Results for our model epoxy-surfactant resin were in excellent agreement with those obtained using a commercially available aqueous waterborne epoxy resin. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2781-2792, 1998
    Additional Material: 12 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    The periodic table in flatland (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 53-61 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The D-dimensional Coulomb system serves as a starting point for generating generalized atomic shells. These shells are ordered according to a generalized Madelung rule in D dimensions. This rule together with an Aufbau Prinzip is applied to produce a D-dimensional periodic table. A model is developed to rationalize the ordering of the shells predicted by the generalized Madelung rule. This model is based on the introduction of a Hamiltonian, invariant under the q-deformed algebra Uq(so(D)), that breaks down the SO(D + 1) dynamical symmetry of the hydrogen atom in D dimensions. The D = 2 case (Flatland) is investigated in some detail. It is shown that the neutral atoms and the (moderately) positive ions correspond to the values q = 0.8 and q = 1, respectively, of the deformation parameter q. © John Wiley & Sons, Inc.
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  • 8
    Electronic Resource
    Electronic Resource
    Electronic structure of periodically Si-δ-doped GaAs (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 203-206 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculation of periodically Si-δ-doped GaAs was performed in the regime of high-donor concentration. The electronic confinement exhibit a quasi-2D behavior even for small periods (p = 45 Å). The electronic properties of the clustering SiGa—SAs also was studied. © 1995 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/qua.560560821
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  • 9
    Electronic Resource
    Electronic Resource
    Parallel computational chemistry made easier: The development of NWChem (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 475-483 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We describe the design philosophy, structure, and supporting tool kits of the NWChem computational chemistry package. The primary purpose of this effort was to develop efficient parallel algorithms for a broad range of methods commonly used in computational chemistry. To facilitate this, we developed a shared nonuniform access memory model which simplifies parallel programming while at the same time providing for portability across both distributed- and shared-memory machines. In addition to this specific focus on parallelization, a substantial effort has been made to simplify the general problem of large-scale software development, which is common to many research groups. We find that this simplification can be achieved through judicious use of ideas from the computer science field of software engineering in the design and implementation of the program with minimal extra effort on the part of the chemist. © 1995 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560560851
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  • 10
    Electronic Resource
    Electronic Resource
    A theoretical study of lithium ion and aromatic organic cation graphite intercalates (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 533-539 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A theoretical study was performed to design a method for predicting the intercalation behavior of lithium ion and aromatic organic cations into a graphite lattice. The purpose of these predictive capabilities is to provide fundamental information on the molecular interactions in graphite intercalates for optimization of electrodes in rechargeable batteries. The graphite sheet system was modeled on a workstation using MOPAC 93 to observe possible structural changes in the graphite following an intercalation process as well as any molecular orbital interactions between the graphite and the cation. Initial modeling of the graphite sheets used lithium intercalated graphite (LIG) as a benchmark. The results indicate that a complex graphite system can be modeled simply and effectively using semiempirical methods for the study of the intercalation-deintercalation behavior of ions. © 1995 John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/qua.560560856
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