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  • 1
    Electronic Resource
    Electronic Resource
    Modeling polyethylene with the bond fluctuation model (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 738-748 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This work presents an application of recently developed ideas about how to map real polymer systems onto abstract models. In our case the abstract model is the bond fluctuation model with a Monte Carlo dynamics. We study the temperature dependence of chain dimensions and of the self-diffusion behavior in the melt from high temperatures down to 200 K. The chain conformations are equilibrated over the whole temperature range, which is possible for the abstract type of model we use. The size of the chains as measured by the characteristic ratio is within 25% of experimental data. The simulated values of the chain self-diffusion coefficient have to be matched to experimental information at one temperature to obtain a scaling for the Monte Carlo time step. The melt viscosity from the simulations as determined by applying the Rouse model is then in good qualitative agreement with experimental data over the experimentally available temperature range. The activation energy as extracted from an Arrhenius fit is different because the simulations are done at constant volume. Both experimental data and the simulation, which covers a far greater temperature range, show Arrhenius behavior for the viscosity and no indication of a finite nonzero Vogel–Fulcher temperature. For one temperature (T=509 K) various time-dependent mean-square displacements are available from atomistic molecular-dynamics simulations, and are shown to be in excellent agreement with the results from the coarse-grained model. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473162
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  • 2
    Electronic Resource
    Electronic Resource
    Symmetric diblock copolymers in thin films. I. Phase stability in self-consistent field calculations and Monte Carlo simulations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5241-5250 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate the phase behavior of symmetric AB diblock copolymers confined into a thin film. The film boundaries are parallel, impenetrable and attract the A component of the diblock copolymer. Using a self-consistent field technique [M. W. Matsen, J. Chem. Phys. 106, 7781 (1997)], we study the ordered phases as a function of incompatibility χ and film thickness in the framework of the Gaussian chain model. For large film thickness and small incompatibility, we find first order transitions between phases with different number of lamellae which are parallel oriented to the film boundaries. At high incompatibility or small film thickness, transitions between parallel oriented and perpendicular oriented lamellae occur. We compare the self-consistent field calculations to Monte Carlo simulations of the bond fluctuation model for chain length N=32. In the simulations we quench several systems from χN=0 to χN=30 and monitor the morphology into which the diblock copolymers assemble. Three film thicknesses are investigated, corresponding to parallel oriented lamellae with 2 and 4 interfaces and a perpendicular oriented morphology. Good agreement between self-consistent field calculations and Monte Carlo simulations is found. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479778
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  • 3
    Electronic Resource
    Electronic Resource
    Symmetric diblock copolymers in thin films. II. Comparison of profiles between self-consistent field calculations and Monte Carlo simulations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5251-5258 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of lamellar phases of symmetric AB diblock copolymers in a thin film is investigated. We quantitatively compare the composition profiles and profiles of individual segments in self-consistent field calculations with Monte Carlo simulations in the bond fluctuation model for chain length N=32 and χN=30. Three film thicknesses are investigated, corresponding to parallel oriented lamellae with 2 and 4 interfaces and a perpendicular oriented morphology. Taking account of capillary waves, we find good quantitative agreement between the Monte Carlo simulations and the self-consistent field calculations. However, the fluctuations of the local interfacial position are strongly suppressed by confinement and mutual interactions between lamellae. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479823
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  • 4
    Electronic Resource
    Electronic Resource
    Effect of long-range forces on the interfacial profiles in thin binary polymer films (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 1221-1229 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the effect of surface fields on the interfacial properties of a binary polymer melt confined between two parallel walls. Each wall attracts a different component of the blend by a nonretarded van der Waals potential. An interface which runs parallel to the surfaces is stabilized in the center of the film. Using extensive Monte Carlo simulations we study the interfacial properties as a function of the film thickness, the strength of the surface forces, and the lateral size over which the profiles across the film are averaged. We find evidence for capillary wave broadening of the apparent interfacial profiles. However, the apparent interfacial width cannot be described quantitatively by a simple logarithmic dependence on the film thickness. The Monte Carlo simulations reveal that the surface fields give rise to an additional reduction of the intrinsic interfacial width and an increase of the effective interfacial tension upon decreasing the film thickness. These modifications of the intrinsic interfacial properties are confirmed by self-consistent field calculations. Taking account of the thickness dependence of the intrinsic interfacial properties and the capillary wave broadening, we can describe our simulation results quantitatively. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478164
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  • 5
    Electronic Resource
    Electronic Resource
    Interfacial tension of phase-separated polymer solutions and relation to their equation of state (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 3809-3819 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using an effective (coarse-grained) thermodynamic potential describing the excess free energy of mixing of a polymer solution and fitting its parameters to measured critical point data, we obtain the "hump'' ε(τ) of this potential in the two-phase region (τ being the reduced distance from the critical temperature T of unmixing). For 30 different systems (varying the degree of polymerization r as well as choosing different polymer–solvent pairs) it is shown that the data are reasonably well represented by a power law, ε(τ)=εˆττζ. While mean field theory implies ζ=5/2 and scaling theory ζ=3ν+β≈2.22 (using Ising model exponents ν≈0.63,β≈0.325), the "effective'' exponent extracted from the data mostly falls in between these limits (ζeff≈2.4). Since the interfacial tension satisfies a similar power law, σ(τ)=σˆττμ (with μ=3/2 in mean field theory or μ=2ν≈1.26 in scaling theory), we also consider a relation between interfacial tension and free energy hump, σ(ε)=σˆεεcursive-phi. While mean-field theory implies cursive-phi=3/5 and scaling theory cursive-phi=2/(3+β/ν)≈0.57, the empirical exponent lies in the range 0.5(approximately-less-than)cursive-phieff(approximately-less-than)0.6. We present estimates of molecular weight dependencies of critical amplitude prefactors εˆτ,σˆτ,σˆε and of related quantities for many different systems. We also discuss whether the critical amplitude combination (εˆτ/Bˆτ)2/3/σˆ, where Bˆτ describes the coexistence curve {φcoex(2)−φcoex(1)=Bˆττβ} is universal. Contrary to some theoretical expectations, our data imply that this combination is not universal, and hence it cannot be used to predict interfacial tensions from equation of state data. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470059
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  • 6
    Electronic Resource
    Electronic Resource
    Dynamics of star polymers in a good solvent: A Kramers potential treatment (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4452-4460 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The "effective'' relaxation time τ of isolated star polymers with excluded volume interactions in the Rouse model limit (i.e., disregarding hydrodynamic interactions present in real solvents) is studied varying both the number of arms f and the number of monomers per arm l. Here τ is defined from the response of the gyration radius of the star polymer to a Kramers potential that describes the effect of shear flow in lowest order in the shear rate. Monte Carlo simulations are performed with two different techniques (simple sampling with enrichment or dynamic Monte Carlo, respectively) for two different models (simple self-avoiding walks with an extended core or the bond fluctuation model, respectively). It is proposed that the ratio τ(f,l)/τ(1,l) should be a universal function of f for large l, and the Monte Carlo data are used to test this hypothesis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467429
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  • 7
    Electronic Resource
    Electronic Resource
    On the equation of state for thermal polymer solutions and melts (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 7166-7179 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We measure the chemical potential in a polymer melt with a modification of Widom's insertion method. Our method is based on partial insertion of a testchain by controlling the interaction between the testchain and the melt by means of a control parameter. The chemical potential can be obtained from a single run in a multicanonical-like simulation over a wide range of temperatures and densities. From the chemical potential the osmotic pressure and the entropy are calculated. We study a variant of the bond fluctuation model of a polymer melt where an energy is gained when the bond vector is taken from the set (±3,0,0) lattice spacings (or permutations thereof). From extensive previous studies it is known that the model exhibits a glass-like freezing at low temperatures. We show that the configurational entropy decreases strongly when one cools the system through this transition, but it remains distinctly nonzero in the glass phase. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470345
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  • 8
    Electronic Resource
    Electronic Resource
    Layers of semiflexible chain molecules endgrafted at interfaces: An off-lattice Monte Carlo simulation (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2960-2969 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A coarse-grained model for surfactant chain molecules at interfaces in the high density regime is studied using an off-lattice Monte Carlo technique. The surfactant molecules are modeled as chains consisting of a small number (e.g., seven) of effective monomers. For the modeling of lipid monolayers, each effective monomer is thought to represent several CH2 groups of the alkane chain, but applications of the model to other polymers end grafted at solid surfaces also should be possible. The head segments are restricted to move in the adsorption plane, but otherwise do not differ from the effective monomers, which all interact with Lennard-Jones potentials. Bond angle and bond length potentials take into account chain connectivity and chain stiffness. The advantage of this crude model is that its phase diagram can be studied in detail. Temperature scans show two phase transitions, a tilting transition at low temperatures between a tilted and an untilted phase, and a melting transition at high temperatures where the lattice of head groups loses its crystalline order. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468604
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  • 9
    Electronic Resource
    Electronic Resource
    Microphase separation in thin films of the symmetric diblock-copolymer melt (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3367-3377 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By means of Monte Carlo simulations of a lattice model, microphase separation transition of symmetric diblock-copolymer melts confined in the thin-film geometry between parallel hard walls is studied. We impose a surface field which acts repulsively only on one of the two species, to stabilize lamellar order parallel to the surfaces. However, interplay between two characteristic lengths, that is, the natural thickness of the lamellar l and the thickness of the film D, causes complicated behavior. In case that the two lengths are compatible with each other, clear lamellar order parallel to the surfaces is observed at temperatures lower than the bulk transition temperature, as expected. On the other hand, tilted or deformed lamellar structure, or even coexistence of lamellae in different orientations are found in cases of strong conflict. In both cases, lamellae are fully established. Even at temperature higher than the bulk transition temperature, weak order is induced by the surface field, and a gradual transition between such surface-induced order and bulklike order is observed. Film thickness and temperature dependence of the ordered structure is discussed, as well as a density reduction near the walls and in the interfaces between the segregated regions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467584
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  • 10
    Electronic Resource
    Electronic Resource
    Monte Carlo simulation of models for single polyethylene coils (1992)
    s.l. : American Chemical Society
    Macromolecules 25 (1992), S. 3117-3124 
    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/ma00038a015
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