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  • Articles  (234)
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  • 1
    Electronic Resource
    Electronic Resource
    Dynamics of polymer solutions and melts. Reptation predictions and scaling of relaxation times (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7726-7740 
    Details
    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
    URL: http://dx.doi.org/10.1063/1.461346
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  • 2
    Electronic Resource
    Electronic Resource
    Dynamics of entangled linear polymer melts: A molecular-dynamics simulation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5057-5086 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an extensive molecular-dynamics simulation for a bead spring model of a melt of linear polymers. The number of monomers N covers the range from N=5 to N=400. Since the entanglement length Ne is found to be approximately 35, our chains cover the crossover from the nonentangled to the entangled regime. The Rouse model provides an excellent description for short chains N〈Ne, while the dynamics of the long chains can be described by the reptation model. By mapping the model chains onto chemical species we give estimates of the times and distances of onset of the slowing down in motion due to reptation. Comparison to neutron spin-echo data confirm our mapping procedure, resolving a discrepancy between various experiments. By considering the primitive chain we are able to directly visualize the confinement to a tube. Analyzing the Rouse mode relaxation allows us to exclude the generalized Rouse models, while the original reptation prediction gives a good description of the data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458541
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  • 3
    Electronic Resource
    Electronic Resource
    Glass transition of polymer melts: A two-dimensional Monte Carlo study in the framework of the bond fluctuation method (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 6291-6306 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The glass transition of a two-dimensional melt of lattice polymers is discussed. The glass temperature Tg is determined by the free volume approach, by the isothermal compressibility as well as by other methods. The decay of the bond autocorrelation function displays typical glasslike behavior. The time temperature superposition law detected in the dynamical polymer structure factor and in orientational autocorrelation functions can be explained by simple dynamical scaling arguments. Mean square displacements observed over more than five decades in time yield very precise diffusion constants as a function of temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462620
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  • 4
    Electronic Resource
    Electronic Resource
    Phase diagram and dynamics of Yukawa systems (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3286-3312 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The phase diagram and dynamical properties of systems of particles interacting through a repulsive screened Coulomb (Yukawa) potential have been calculated using molecular and lattice dynamics techniques. The phase diagram contains both a melting transition and a transition from fcc to bcc crystalline phases. These phase transitions have been studied as a function of potential shape (screening length) and compared to phenomenological criteria for transition temperatures such as those of Lindemann and of Hansen and Verlet. The transition from fcc to bcc with increasing temperature is shown to result from a higher entropy in the bcc phase because of its softer shear modes. Even when the stable solid phase below the melting temperature is fcc, bcc-like local order is found in the liquid phase. This may substantially slow crystallization. The calculated phase diagram and shear modulus are in good agreement with experiments on colloidal suspensions of polystyrene spheres. The single particle dynamics of Yukawa systems show several unusual features. There is a pronounced subdiffusive regime in liquids near and below the melting temperature. This regime reflects the existence of two time scales: a typical phonon period, and the time for a particle to feel a new environment. The second time scale becomes longer as the temperature is lowered or the range of interaction (screening length) increases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453924
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  • 5
    Electronic Resource
    Electronic Resource
    Erratum: Dynamics of entangled polymer melts: A molecular-dynamics simulation [J.Chem. Phys. 92, 5057 (1990)] (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4103-4103 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460746
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  • 6
    Electronic Resource
    Electronic Resource
    A comparison between integral equation theory and molecular dynamics simulations of dense, flexible polymer liquids (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1357-1364 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently we (J.G.C. and K.S.S.) formulated a tractable "reference interaction site model'' (RISM) integral equation theory of flexible polymer liquids. The purpose of this paper is to compare the results of the theory with recent molecular dynamics simulations (G.S.G. and K.K.) on dense chain liquids of degree of polymerization N=50 and 200. Specific comparisons were made between theory and simulation for the intramolecular structure factor ωˆ(k) and the intermolecular radial distribution function g(r) in the liquid. In particular it was possible to independently test the assumptions inherent in the RISM theory and the additional ideality approximation that was made in the initial application of the theory. This comparison was accomplished by calculating the intermolecular g(r) using the simulated intramolecular structure factor, as well as, ωˆ(k) derived from a freely jointed chain model.The RISM theory results, using the simulated ωˆ(k), were found to be in excellent agreement, over all length scales, with the g(r) from molecular dynamics simulations. The theoretical predictions using the "ideal'' intramolecular structure factor tended to underestimate g(r) near contact, indicating local intramolecular expansion of the chains. This local expansion can be incorporated into the theory self consistently by including the effects of the "medium induced'' potential on the intramolecular structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457159
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  • 7
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulation of a polymer chain in solution (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6983-6997 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results of a molecular dynamics simulation of a single polymer chain in a good solvent are presented. The latter is modeled explicitly as a bath of particles. This system provides a first-principles microscopic test of the hydrodynamic Kirkwood–Zimm theory of the chain's Brownian motion. A 30 monomer chain is studied in 4066 solvent particles as well as 40/4056 and 60/7940 systems. The density was chosen rather high, in order to come close to the ideal situation of incompressible flow, and to ensure that diffusive momentum transport is much faster than particle motions. In order to cope with the numerical instability of microcanonical algorithms, we generate starting states by a Langevin simulation that includes a coupling to a heat bath, which is switched off for the analysis of the dynamics. The long range of the hydrodynamic interaction induces a large effect of finite box size on the diffusive properties, which is observable for the diffusion constants of both the chain and the solvent particles. The Kirkwood theory of the diffusion constant, as well as the Akcasu et al. theory of the initial decay rate in dynamic light scattering are generalized for the finite box case, replacing the Oseen tensor by the corresponding Ewald sum. In leading order, the finite-size corrections are inversely proportional to the linear box dimensions. With this modification of the theory taken into account, the Kirkwood formula for the diffusion constant is verified. Moreover, the monomer motions exhibit a scaling that is much closer to Zimm than to Rouse exponents (t2/3 law in the mean square displacement; decay rate of the dynamic structure factor ∝k3). However, the prefactors are not consistent with the theory, indicating that (on the involved short length scales) the dynamics is more complex than the simple hydrodynamic description suggests.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465445
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  • 8
    Electronic Resource
    Electronic Resource
    Statics and dynamics of polymeric melts: a numerical analysis (1983)
    s.l. : American Chemical Society
    Macromolecules 16 (1983), S. 1632-1638 
    Details
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ma00244a015
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  • 9
    Electronic Resource
    Electronic Resource
    Structure and relaxation of end-linked polymer networks (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8169-8192 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The kinetics of end crosslinking a linear polymer melt and the dynamics of the resulting network is studied using molecular dynamics (MD) simulations. Starting from equilibrated melts of linear chains of length ranging from 1/3Ne to 3Ne, where Ne is the entanglement length, tetrafunctional crosslinkers are attached to a fraction x of the chain ends. When a free end comes within a short capture distance rx from an unsaturated crosslinker, the chain ends are attached. With a stoichmetric number x of crosslinkers, the long time kinetics for the number of free ends and the number of unsaturated crosslinkers decays as a power law in time t−a, with a≈0.5 for the present range of chain lengths. The resulting networks are then used to study the effect of entanglements on the motion of the crosslinks and the modulus of the network. Using a cluster search algorithm, the microscopic characteristics of the networks are determined. This allows us to compare our simulated networks to theoretical models of rubber elasticity without any adjustable parameters. While our results for the modulus are close to the phantom network model for the shortest chains, for the longer chains our results clearly support the Edwards tube model. Already for the presently employed largest chains the modulus is about 1.9 times larger than predicted from the affine model. Asymptotically, within an affine network model, about 2.2 entanglement lengths are needed to contribute to the modulus in the same way as one elastically active chemical crosslink. The data agree very well with recent experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468202
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  • 10
    Electronic Resource
    Electronic Resource
    The nature of flexible linear polyelectrolytes in salt free solution: A molecular dynamics study (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1669-1690 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present results of molecular dynamics simulations of linear polyelectrolytes in solution. The fundamental model for polyelectrolytes in solution is studied. Specifically, simulations are performed for multichain systems of a flexible chain model of charged polymers. The full Coulomb interactions of the monomers and counterions are treated explicitly. Experimental measurements of the osmotic pressure and the structure factor are reproduced. The simulations reveal a new picture of the chain structure based on calculations of the structure factor, persistence length, end-to-end distance, etc. We present a detailed discussion of the chain structure and a comparison with present theories. In contrast to the predicted dilute limit of rodlike chains, we find that the chains have significant bending at very low densities. Furthermore, the chains contract significantly before they overlap. We also show that counterion condensation dramatically alters the chain structure. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470698
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