Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular density-functional theory is extended to address the crystallization of chemically realistic polymers. The polymer (RISM) reference interaction site model integral-equation approach is employed to calculate the liquid-state structural information required as "input'' into our density-functional theory. The single-chain structure is described by the rotational isomeric state model, and the accuracy of both the theoretically calculated single-chain and liquid structures have been verified by direct comparison with Monte Carlo simulation and x-ray scattering, respectively. The driving forces for the crystallization of polymers are found to be completely different from those in monatomic systems and can be understood in terms of an effective "chain-straightening force'' (which results from chain packing) combined with a background attractive potential. Remarkably, the predicted melting temperatures for polyethylene and polytetrafluoroethylene at atmospheric pressure are within a few degrees of the experimental values, and the density–temperature phase diagrams are also in good agreement with experiment. Chemically unrealistic, coarse-grained models of polymer structure appear to be inadequate for the crystallization phenomenon, which is found to be quantitatively sensitive to interchain attractive forces and melt compressibility. The aspect ratios in polyethylene and polytetrafluoroethylene melts at the phase transition are predicted to be virtually identical.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4281-4289 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The density functional (DF) theory of freezing is extended to predict the equilibrium concentration of vacancies in the crystal. The vacancy density in the crystal at phase coexistence is predicted to be approximately 5×10−5 for the Lennard-Jones model system near the triple point, which is comparable to concentrations measured in real systems such as argon and krypton. The dependence of the vacancy density on the temperature and pressure of the system is investigated using the new DF formalism. The concentration of vacancies in the hard sphere crystal at coexistence is also calculated, and it is found to be very small. The perfect crystal approximation, used earlier by us and others, is found to be very accurate for both the hard sphere and Lennard-Jones systems.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3034-3039 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the details of a theory which predicts the freezing of quantum liquids, such as helium [J. Chem. Phys. 90, 4622 (1989)]. The freezing of a wide variety of classical liquids has been described by the density functional (DF) theory of statistical mechanics. By choosing a new ideal system, we construct a new DF theory which addresses directly many of the unusual features of the freezing of liquid helium, such as the weakly modulated liquid pair correlation function g(r) at freezing densities. The theory combines DF techniques with the Feynman path integral formulation of quantum mechanics, to include correctly dispersion effects. In classical DF theories, the density and external field of the ideal system are connected by a Boltzmann relation ρ(r)∝exp[−βV(r)]. In our quantum DF theory, we relate the density and external field of the ideal system through the Feynman path integral representation, in which the quantum particle is represented by a classical "ring polymer'' of P beads. In practical applications, the DF perturbation expansion is truncated at second order, and classical DF theories fail for helium because they employ an ideal system which is too far removed from the interacting system. For a certain simplified problem, the density path integral of the ideal system can be performed in closed form, leading to a compact, physically descriptive theory. In a companion paper, the full theory is applied to the freezing of helium-4 and yields good results.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3900-3909 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density functional theory, studied recently by us [J. Chem. Phys. 87, 5449 (1987)] is used to study the freezing of hard disks and hard spheres into crystals with hexagonal symmetry. Two different numerical techniques are used, namely a Gaussian approximation to the crystal density and a more general Fourier expansion of the crystal density. The results from these methods are compared with each other, more approximate versions of density functional theory, and computer simulations. In addition, we compare density functional theory with Landau theories of first order transitions, in which the free energy is expanded as a power series, usually in just one order parameter. We find that traditional Landau theory has little validity when applied to the freezing transition.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5449-5456 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The density functional theory of freezing is used to study the liquid to crystal phase transition in the hard-sphere and Lennard-Jones systems. An important step in the calculation is the parametrization of the solid phase average single particle density ρ(r). In this work two popular parametrizations are compared. The first method is a general Fourier decomposition of the periodic solid density in which the amplitude of each (non-symmetry-related) Fourier component is treated as an independent parameter. The second parametrization, which is more restrictive but easier to implement, approximates the solid density as a sum of Gaussian peaks centered at the sites of a periodic lattice. The two methods give essentially identical results for the phase diagrams for the two systems studied, but the crystal density predicted by the Fourier method exhibits significant anisotropies which are excluded from the Gaussian representation by construction.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 4853-4858 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the first implementation of our density functional theory [J. Chem. Phys. 85, 5971, 5977 (1986)] to investigate a fluid–solid phase transition. In this theory, designed specifically for polyatomic systems, the entropy functional with bonding constraints is treated exactly, and approximations are generated by truncating expansions of the intermolecular interaction part of the free-energy density functional. We examine the theory resulting from the quadratic truncation of the interaction free energy, and determine the resulting phase diagram for hard dumbbell molecules. The results for short bond lengths are in accord with known trends from experiment and simulation. However, the theory predicts no plastic crystal transition for hard dumbbells with a bond length that might characterize nitrogen, for which the experimental β phase is a plastic crystal. Reasons for this behavior are discussed.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2975-2986 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Athermal, tethered chains are modeled with density functional (DFT) theory for both the explicit solvent and continuum solvent cases. The structure of DFT is shown to reduce to self-consistent-field theory in the incompressible limit where there is symmetry between solvent and monomer, and to single-chain-mean-field (SCMF) theory in the continuum solvent limit. We show that by careful selection of the reference and ideal systems in DFT theory, self-consistent numerical solutions can be obtained, thereby avoiding the single chain Monte Carlo simulation in SCMF theory. On long length scales, excellent agreement is seen between the simplified DFT theory and molecular dynamics simulations of both continuum solvents and explicit-molecule solvents. In order to describe the structure of the polymer and solvent near the surface it is necessary to include compressibility effects and the nonlocality of the field. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 4289-4295 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density functional theory is applied to hard site chains between hard walls. The wall separation is varied and the normal pressures, surface excesses, and surface tensions are recorded. At low bulk density, a density depletion near the wall dominates the thermodynamic behavior. At high bulk density, the thermodynamic properties develop a damped oscillatory behavior with a period of approximately one site diameter and a range of roughly six site diameters. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 2021-2024 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a previous study of tangent hard-site chains near a surface, the inhomogeneous density profiles were found through density functional theory. In the current study, the surface tensions of these systems are found from the results of the previous study through a thermodynamic integration. The calculated surface tensions are then compared to those found directly through computer simulation. Both the surface tension and surface excess for polymeric systems are shown to differ qualitatively from those of atomic systems, although certain similarities are seen at high densities. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3090-3093 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Previous applications of density functional (DF) theory required a single chain Monte Carlo simulation to be performed within a self-consistent loop. In the current work, a methodology is developed which permits the simulation to be taken out of the iterative loop. Consequently, the calculation of the self-consistent, medium-induced potential, or field, is decoupled from the simulation. This approach permits different densities, different forms of UM(r), and different wall–polymer interactions to be investigated from a single Monte Carlo simulation. The increase in computational efficiency is immense. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    Signatur Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...