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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Macromolecules 24 (1991), S. 6332-6334 
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7726-7740 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The bond fluctuation model on the simple cubic lattice is studied by Monte Carlo simulations on a multitransputer array, for polymer volume fractions φ in the range 0.025≤φ≤0.500 and chain lengths N in the range 20≤N≤200. Extensive data are presented on the dynamics of monomer displacements, center-of-gravity displacements, and relaxation times. This study is complementary to previous work, in which the crossover scaling properties of the chain linear dimensions, structure factor, and self-diffusion constant were tested for the same athermal model. The simulation technique takes both excluded volume interactions and entanglement constraints into account, but ignores hydrodynamic forces. Our results describe the crossover from Rouse behavior of swollen chains (τ∼N1+2ν, ν being the exponent describing the radius R of the chains, R∼Nν ) to reptation, τ∼N3. Since the excluded volume screening length is found to be smaller than the tube diameter by a factor of about 3, the rescaled times Wτ/N1+2ν decrease first as a function of the scaled chain length N˜∼Nφ1/(3ν−1), before they increase due to the onset of reptation. Additional evidence for reptative behavior is found by identifying the several successive crossovers in the time-dependent displacements predicted by de Gennes.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 4751-4755 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We make a detailed comparison of the incoherent dynamic structure factors Sinc(q,ω) and Sinc(q,t) for n-C100H202 melts, as determined from time-of-flight (TOF) neutron scattering measurements, with results from atomistic molecular dynamics simulations. Quantitative agreement is seen between simulations and experiments over the dynamic range accessible by the TOF measurements. From detailed analysis of the molecular dynamics trajectories, it is determined that the decay of Sinc(q,t) that occurs between 0.1 and 10 ps is the result of a combination of torsional librations and jumps, with the latter becoming more important with decreasing q. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 6526-6539 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model for a multichain polymer system in three-dimensional continuous space is studied by link cell Monte Carlo methods, using systems up to chain length N=64 and up to 16 384 monomers. The chains consist of beads with a hard core connected by rather stiff harmonic bonds, with a repulsive Lennard-Jones-type interaction between beads chosen such that chains cannot cross each other during their random motions. On RISC workstations the model performs only about a factor of 4 slower than the bond fluctuation lattice model, the qualitative behavior of the time-dependent mean-square displacements and relaxation functions being rather similar to the latter. For the model without obstacles, it is shown that the present continuum model can be approximately mapped on the lattice bond fluctuation model by a suitable rescaling of chain length and volume fraction. But the distinctive advantage of the present model is that it can be applied easily to random media (described by randomly placed rigid obstacles), without the severe ergodicity problems (locked-in configurations) arising in lattice models for such systems. It is shown that static properties of the chains stay nearly unaffected by the obstacles, while the chain motions are considerably slowed down in this frozen environment.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4786-4798 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently introduced bead-spring type model of polymer chains with purely repulsive interactions between the beads is modified to allow for attractive forces at intermediate distances. This new model is suitable for the study of thermal properties of three-dimensional polymer solutions and melts and can be efficiently simulated with Link–Cell Monte Carlo methods. As a first step, the single chain lengths up to N=128 beads are studied and the theta temperature θ is located. It is shown that the data are compatible with the theoretically predicted crossover scaling behavior, and that the properties of collapsed chains can be studied for reasonably low temperatures T〈θ. In addition to static properties (chain radii, polymer density inside the coil, internal energy, specific heat) also dynamic properties are obtained, namely various mean square displacements and relaxation times and the self diffusion constant. While for T=θ (and long enough chains), there is reasonable agreement with the Rouse model, a different behavior occurs for collapsed chains. As a second step, the behavior of the polymer solution at T=θ is studied as function of concentration. There is a clear increase of the chain gyration radii with concentration φ, although the values of 〈Rg2〉T=aitch-theta in the highly concentrated regime (melts) are still smaller than their counterparts for the corresponding "athermal'' model (with purely repulsive interactions) at melt densities. Thus one cannot identify the chain linear dimensions at the θ point with the chain dimensions in melts quantitatively. For the chain lengths studied (N≤64) the dynamic properties are compatible with Rouse behavior at all concentrations.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1702-1709 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present here an optimized united atom model that is able to reproduce properties of melts of n-alkane chains of varying molecular weights. This model differs from previous models in that the Lennard-Jones well depth for the terminal methyl group (0.2264 kcal/mol) differs from that of the methylene units (0.093 kcal/mol). The position of the minimum is at 4.5 A(ring) for both units. Properties of n-C44H90 melts from this model are compared with experiments and those from an explicit atom model. Good agreement with experiment is obtained for static properties of the melt, specifically P–V–T behavior, chain conformations, and x-ray scattering profiles. The large-scale dynamics, as measured by self-diffusion, are found to agree reasonably well with experimental results, being about 30% faster with our best united atom force field. Analysis of the end-to-end vector orientation autocorrelation function in terms of the Rouse model yields a monomer friction coefficient somewhat greater than that determined from the rate of self-diffusion, reflecting the fact that the n-C44H90 chains are not sufficiently long to behave as Gaussian coils. Detailed local chain dynamics for n-C44H90 melts, as measured by the P1(t) and P2(t) orientation autocorrelation functions for C–H vectors, are found to agree reasonably well with results from simulations using an explicit atom model, and yield spin-lattice relaxation times T1 and nuclear Overhauser enhancement values in reasonable agreement with experimental 13C NMR measurements. As with large scale dynamics, local dynamics are faster in general (about 20%) than experimental results. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Materials Research 32 (2002), S. 401-436 
    ISSN: 1531-7331
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Polymers offer a wide spectrum of possibilities for materials applications, in part because of the chemical complexity and variability of the constituent molecules, and in part because they can be blended together with other organic as well as inorganic components. The majority of applications of polymeric materials is based on their excellent mechanical properties, which arise from the long-chain nature of the constituents. Microscopically, this means that polymeric materials are able to respond to external forces in a broad frequency range, i.e., with a broad range of relaxation processes. Computer simulation methods are ideally suited to help to understand these processes and the structural properties that lead to them and to further our ability to predict materials properties and behavior. However, the broad range of timescales and underlying structure prohibits any one single simulation method from capturing all of these processes. This manuscript provides an overview of some of the more popular computational models and methods used today in the field of molecular and mesoscale simulation of polymeric materials, ranging from molecular models and methods that treat electronic degrees of freedom to mesoscopic field theoretic methods.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Theory of computing systems 10 (1976), S. 239-251 
    ISSN: 1433-0490
    Keywords: Pebble game ; Register allocation ; Space bounds ; Turing machines
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract We study a one-person game played by placing pebbles, according to certain rules, on the vertices of a directed graph. In [3] it was shown that for each graph withn vertices and maximum in-degreed, there is a pebbling strategy which requires at mostc(d) n/logn pebbles. Here we show that this bound is tight to within a constant factor. We also analyze a variety of pebbling algorithms, including one which achieves the 0(n/logn) bound.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Naturwissenschaften 31 (1943), S. 419-419 
    ISSN: 1432-1904
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 124 (1947), S. 121-128 
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Zusammenfassung Aus der experimentell gefundenen Verschiebung und dem Aufspaltungsbild von Multiplett-Termen im elektrischen Feld wird die Oszillatorenstärke von übergängen zu benachbarten Zuständen bestimmt. Es ergeben sich für dief-Werte eine obere und unter Zuhilfenahme des f-Summensatzes auch eine untere Grenze. Das Verfahren wird auf den 3P 1/2, 3/2 Term des Natriums angewandt. Es ergibt sich für den übergang 3P 1/2, 3/2 − 3D 3/2, 5/2 0,85 〈f 3D〈 0,93 und für den übergang 3P 1/2, 3/2 − 4S 1/2 0,14 〈f 4S 〈 0,17.
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