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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9563-9577 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neutron Spin Echo and Dynamic Light Scattering techniques are used for an extensive investigation of the bicontinuous phase in water/decane microemulsions. The dynamical behavior of different surfactant systems, decyl polyglycol ether (C10E4), C10E4 mixed with polyethylenepropylene/polyethyleneoxide amphiphilic block-copolymers-(PEPx/PEOy), and sodium-bisethylhexylsulfosuccinate (AOT) is investigated under comparable conditions. At scattering wave numbers q large compared to the inverse of the structure length scale, q0=2π/d, always stretched exponential relaxations ∝e−(Γqt)β with Γq∝q3 are found, as predicted theoretically. The relaxation rate increases almost linearly as function of the bicontinuous structure correlation scale—ξ(similar, equals)d/2. The apparent bare bending modulus κ determined by fitting theoretical predictions to the experimental high-q data yields values of about 1.3kBT—as inferred from previous small angle neutron scattering (SANS) studies and from other methods. The effect of increasing rigidity of the surfactant layers by anchoring amphiphilic block-copolymers, predicted theoretically and revealed experimentally in structural investigations, could not be clearly resolved due to its small influence on the dynamics. At structural length scales, the relaxation rate in water–oil contrast shows a minimum corresponding to the maximum of the static structure factor. At length scales much larger than the typical structure length the relaxation is single-exponential with a q2 dependent rate. In this regime we find indications of the additional membrane interaction due to the presence of block-copolymers. © 2001 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 580-600 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of amphiphilic diblock copolymers of several molecular weights on the structure and phase behavior of ternary amphiphilic systems (water, oil, and nonionic surfactant) is investigated. Small amounts of amphiphilic block copolymer polyethyleneoxide–polyethylpropylene lead to a dramatic decrease of the amount of total surfactant needed to solubilize given equal volumes of water and oil in a bicontinuous microemulsion. Neutron scattering experiments employing a high-precision two-dimensional contrast variation technique demonstrate that the polymer is distributed uniformly on the surfactant membrane. Based on these observations, we propose a mechanism for the enhancement of swelling behavior, which is due to the variation of the membrane curvature elasticity by polymer mushrooms anchored to the interface. © 2001 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6107-6120 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigated the single chain motions of monodisperse polyisobutylene chains in the melt by neutron spin echo spectroscopy. Thereby a wide range in momentum space over a large dynamic range was covered. Motional processes from the center of mass diffusion, the Rouse dynamics to the more local relaxation processes which limit the validity of the standard Rouse model, were elucidated. The observed dynamic structure factors were analyzed in terms of relevant theoretical approaches addressing the limiting factors of the Rouse model. We found that other than claimed in the literature effects of local chain stiffness—they were treated in terms of the all rotational states model and a bending force model—cannot account for the experimental observations. It appears that additional damping effects related to an internal viscosity of the chain have to be involved, in order to explain the experimental results. © 1999 American Institute of Physics.
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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. 5653-5661 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The crystal structure of α-phase toluene at 5 K has been refined from high resolution neutron powder diffraction data and the charge distribution in the molecule calculated ab initio. From these data, methyl rotational potentials of the two inequivalent methyl groups have been calculated for various parametrizations of transferable pair potentials and compared with potentials derived from tunneling and librational transitions. The results demonstrate that the inclusion of Coulomb interactions is necessary to reproduce the equilibrium orientations of the methyl groups. However, none of the model pair potentials yield the very strong sixfold contribution of the rotational potential of methyl group 1. The best approximation to the experimentally determined values is obtained with the pair potential parameters of Kitaigorodskii [Acta Crystallogr. 18, 585 (1965)]. However, by the inclusion of existing results on p-xylene, the average agreement obtained with the parametrization of Claverie [Acta Crystallogr. Part A 31, 448 (1975)] is equally good. Overall, it was found that general improvements can be made if the repulsive parts of the model pair potentials are softened. The presence of two inequivalent methyl groups in toluene which are influenced in different ways by the various contributions to the interaction provides a rigorous test of the modeling techniques.
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