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  • 1995-1999  (4)
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Year
  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3813-3820 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamic heterogeneities of glass-forming materials are elucidated for a simple model glass, for which we choose a generalized version of the well-known facilitated kinetic Ising model, originally proposed by Fredrickson and Andersen. The fluctuations within the heterogeneous distribution of reorientation rates are quantified in terms of a rate memory parameter by comparing appropriately chosen two-time and four-time correlation functions. Qualitatively, this parameter expresses after how many relaxation processes a typical relaxator has forgotten its dynamic history. In agreement with most experimental observations the rate memory parameter is close to its minimum value, indicating that the fluctuations within the heterogeneous distribution are on the same time scale as the relaxation processes itself. Upon increasing the "cooperativity parameter" in the glass model the rate memory parameter decreases. The same tendency is observed for decreasing temperature. The apparent dependence of the rate memory parameter on cooperativity is rationalized. Furthermore the results are compared with previous simulations of the facilitated kinetic Ising model. © 1997 American Institute of 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 107 (1997), S. 8686-8696 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Simulations on a Lennard-Jones computer glass are performed to study effects arising from defects in glasses at low temperatures. The numerical analysis reveals that already a low concentration of defects may dramatically change the low temperature properties by giving rise to extrinsic double-well potentials (DWP's). The main characteristics of these extrinsic DWP's are (i) high barrier heights, (ii) high probability that a defect is indeed connected with an extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv) smaller deformation potential coupling to phonons. Designing an extension of the standard tunneling model (STM) which parametrizes this picture and comparing with ultrasound experiments on the wet network glass a-B2O3 shows that effects of OH-impurities are accurately accounted for. This model is then applied to organic polymer glasses and proteins. It is suggested that side groups may act similarly like doped impurities inasmuch as extrinsic DWP's are induced, which possess a distribution of barriers peaked around a high barrier height. This compares with the structurlessly distributed barrier heights of the intrinsic DWP's, which are associated with the backbone dynamics. It is shown that this picture is consistent with elastic measurements on polymers, and can explain anomalous nonlogarithmic line broadening recently observed in hole burning experiments in polymethylmethacrylate. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Macromolecular Theory and Simulations 7 (1998), S. 665-674 
    ISSN: 1022-1344
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In the rotational isomeric state (RIS) approach the conformational statistics of a polymer chain can be conveniently described in terms of the statistical matrix. In this work it is shown from a conceptual point of view to which degree the projection onto a few states, inherently present in any RIS approach, may lead to systematic deviations for estimation of the a priori probabilities. It is shown that these deviations scale with some power of the inverse temperature. Furthermore an algorithm is presented, deriving the statistical matrix from computer simulations. This algorithm is applied to polyethylene. Comparison with the conformational statistics as obtained from Monte Carlo simulations allows the quality of the statistical matrix elements to be estimated. It turns out that the systematic deviation does not hamper practical applications. Questions like the relevance of the anharmonicity of the energy landscape or consideration of more than nearest-neighbor correlations in the RIS approach are discussed.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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