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  • 1
    Electronic Resource
    Electronic Resource
    Computer simulation studies of diffusion in gels: Model structures (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9221-9233 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated particle diffusion through different obstacle geometries by computer simulations. The model structures used in this work — randomly placed point obstacles and cage-like structures — were chosen with the aim of represent a broad range of geometrical structures similar to gels and in order to be compared with our previous simulations of particle diffusion through polyacrylamide gels. The diffusion behavior was studied as a function of tracer size and obstacle concentration. The isomorphism between the diffusion of finite-sized tracers and the diffusion of point tracers in the presence of expanded obstacles was applied. Only hard-sphere interactions of the tracer with the immobile obstacles were considered and the theoretical description was made in terms of theory of the obstruction effect. In the case of randomly placed point obstacles an analytical expression for the dependence of the diffusion coefficient on tracer radius and obstacle concentration, applying the model of spherical cells, could be deduced. The same description was applied numerically to the other model systems. Up to moderatly high fractions of excluded volume this description was found to be successful. For very high fractions of excluded volume — higher concentrations or larger tracers — the validity of Fick's second equation for describing diffusion breaks down and anomalous diffusion was found. The anomalous diffusion exponent diverges as the tracer size becomes comparable to the size of the pores. Analysis of the trajectory of tracers in the cases where an anomalous diffusion takes place shows a Levy-flight-like characteristic. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475214
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  • 2
    Electronic Resource
    Electronic Resource
    Computer simulation studies of anomalous diffusion in gels: Structural properties and probe-size dependence (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 9074-9082 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out simulations of particle diffusion through polyacrylamide gel networks. The model structures were built on a diamond lattice, in a simulation box with periodic boundary conditions. The method of structure generation consists of a random distribution of knots on the lattice and interconnection between randomly chosen pairs of knots. The structures generated by this procedure approximate the topology of real polymer gels. Parameters that control the distance between knots and the degree of stretching of the chain permit us to simulate a polyacrylamide system in which the concentration of species as well as the degree of crosslinking can be compared to realistic gels as prepared by the available experimental procedures. These structures were geometrically characterized by the analysis of the pore size distribution and excluded volume. The structures thus generated are used as model networks for Monte Carlo studies of the diffusion of hard spheres in the restricted geometry. Modeling the deviations from the normal diffusion behavior as a purely geometrical phenomenon, these simulations can give us insights into the factors which lead to anomalous diffusion in gel-like systems. In these simulations a sphere of variable radius is allowed to perform an off-lattice random walk through the space between the generated structures, which are assumed to be rigid. It will be shown that the study of the influence of tracer size on diffusion is complementary to the study of the influence of obstacle concentration on the diffusion of monodispersed particles. While the latter method can give us information about the fractal nature of the internetwork space the former method provides information about the fractal nature of the network and the connectivity of the internetwork space. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470018
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  • 3
    Electronic Resource
    Electronic Resource
    Rayleigh-Brillouin spectra of poly[(phenylmethyl)siloxane] under high pressures (1985)
    s.l. : American Chemical Society
    Macromolecules 18 (1985), S. 993-996 
    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/ma00147a031
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  • 4
    Electronic Resource
    Electronic Resource
    Pressure- and temperature-dependent photon correlation study of bulk poly(ethyl acrylate) above the glass transition temperature (1982)
    s.l. : American Chemical Society
    Macromolecules 15 (1982), S. 870-874 
    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/ma00231a034
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  • 5
    Electronic Resource
    Electronic Resource
    Light beating spectroscopy of branched polypropylene glycol (1983)
    s.l. : American Chemical Society
    Macromolecules 16 (1983), S. 68-71 
    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/ma00235a013
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  • 6
    Electronic Resource
    Electronic Resource
    Photon correlation study of the relaxation processes in bulk poly(n-butyl methacrylate) (1984)
    s.l. : American Chemical Society
    Macromolecules 17 (1984), S. 957-961 
    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/ma00134a071
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  • 7
    Electronic Resource
    Electronic Resource
    A light scattering study of reorientational motions in neat liquids. Pressure dependence (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 391-396 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reorientational correlation times of liquid diphenyl methane and of diphenyl ether have been measured by depolarized light scattering as a function of temperature and pressure. The data yield activation energies and activation volumes. The viscosity dependence of the correlation times is well described by an extended Stokes–Einstein–Debye equation. The data indicate that the reorientational motion observed can to a good approximation be described in terms of a single-molecule motion. The two sets of data obtained from the two liquids are in many respects very similar. However, the more polar diphenyl ether displays larger values of the activation parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452576
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  • 8
    Electronic Resource
    Electronic Resource
    Translational correlation function of interaction-induced dipoles in dense media (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 383-390 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time-correlation functions associated with the translational motions of the dipole—induced by permanent multipoles of an arbitrary order 1=1,2,3,...—in a dense fluid have been calculated analytically assuming the isotropic diffusion mechanism. The correlation functions obtained from this model have been compared with the available results of the molecular dynamics simulations in liquid N2 (T=75.5 K, ρ=0.814 g cm−3) and CS2(T=298 K, ρ=1.26 g cm−3) and appear to reflect the proper time scale of the relaxation process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452575
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  • 9
    Electronic Resource
    Electronic Resource
    Ab initio Calculation of the Conformations and Vibrational Spectra of 2-Phenylbutane (1996)
    Springer
    Journal of molecular modeling 2 (1996), S. 373-375 
    Details
    ISSN: 0948-5023
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Molecules with internal degrees of rotation are of particular interest to understand the behavior of synthetic or natural polymers. With the extremely rapid increase in floating point performance of modern computers we are able to calculate a subspace of the Born-Oppenheimer hypersphere for quite large molecules with ab initio methods. We choose 2-Phenylbutane as an elementary model for Polystyrene (PS), because it possesses two internal degrees of freedom, that are highly relevant for PS. These are the rotation of the benzene ring and the chain like motion of the ethylgroup.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s0089460020373
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  • 10
    Electronic Resource
    Electronic Resource
    Determination of the optical anisotropy of atactic polystyrene using a grating monochromator (1996)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 197 (1996), S. 1945-1957 
    Details
    ISSN: 1022-1352
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: To extract the intrinsic molecular scattering from unwanted collision-induced contributions as much as possible we propose a simple useful method recording the depolarized Rayleigh spectra using a double grating monochromator and correcting the spectra for the high-frequency wings. For this purpose we tested the system polystyrene/CCl4 for which an asymptotic value of the optical anisotropy 〈γ2〉/x ≅ 30 Å6 with degree of polymerization x 〉 20 was found. The depolarized Rayleigh intensity of 2,4-diphenylpentane and anionically polymerized atactic polystyrene standards was also measured in dilute solutions in the θ-solvent bicyclohexyl (θ = 61°C). From the absolute depolarized Rayleigh ratio we obtained the configurational average optical anisotropy per number of monomer units, 〈γ2〉/x. We have chosen cumene as a model compound estimating that its optical anisotropy is similar to that of a quasi monomer unit. No significant temperature dependence of 〈γ2〉/x and of the static internal pair correlation in the temperature range from 25°C up to 80°C was found.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1996.021970614
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