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  • 1
    Electronic Resource
    Electronic Resource
    Normal mode analysis of the conformational fluctuation of polymethylene chains in their nearly extended states (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7717-7725 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The conformational motion of a nearly stretched polymethylene molecule is studied by a normal mode analysis of the dihedral angle fluctuations. It is assumed that the conformation can be described by the dihedral angles only, the bond length, and the bond angle being fixed. The simplified model Hamiltonian consists of the intramolecular energy due to the dihedral angle distortions of the C–C bonds and the effective intermolecular energy which favors the stretched chain conformations. The Hamiltonian is expanded, around the transplanar state, in a quadratic form of the dihedral angles {τi}, and the normal modes of the dihedral angle fluctuations are obtained through diagonalization of the Hamiltonian. The normal modes are found to be classified into the in-phase and the out-of-phase modes; in the former modes the adjacent bonds make concerted rotations in the same direction resulting in the twisted chains, while in the latter modes they make counterrotations which give rise to the large out-of-plane bending of the chain. The power spectra of the dihedral angle fluctuations are calculated. They are found to be composed of the in-phase and the out-of-phase components; the spectra qualitatively reproduce those obtained in our previous computer simulation. A new variable, the curvature of the chain contour, is devised and is used throughout the discussion; it is found to be very useful in the description of the conformational fluctuations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461345
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  • 2
    Electronic Resource
    Electronic Resource
    Computer simulation study of the conformational statistics of long chain hydrocarbons in cylindrical potentials (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5990-5997 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Monte Carlo method is applied to the study of disordered conformations of polymethylene chains confined in cylindrical mean-field potentials. It is assumed that the molecule, which is composed of 30 united atoms (methylene groups), has fixed bond length and bond angle, and makes quasicontinuous bond rotations. Various statistical properties of the molecule, such as dihedral angle distributions, dihedral angle pair correlations, transverse fluctuations, etc., are calculated vs strength of the mean-field potential. The dihedral angle distributions calculated exhibit the marked reduction of the gauche peaks with increasing potential; it implies the increasing inaccuracy of the usual rotational isomeric model. The dihedral angle pair correlations reveal novel characteristics of the dihedral angle fluctuation: the fluctuation has approximate period of four bonds with marked tendency for the next nearest bonds to counter-rotate. The characteristics are more conspicuous under weaker potential constraint. There are large transverse fluctuations of the chain, the average linear form of the chain being still maintained. These characteristic dihedral angle fluctuation and the transverse deviation of the chain are found to be well understood by a small scale kink model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459484
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  • 3
    Electronic Resource
    Electronic Resource
    X-ray and thermal studies on the rotator phases of normal higher alcohols C17H35OH, C18H37OH, and their mixtures (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 631-641 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of the rotator phase and the phase transitions in n-alcohols C17, C18, and their mixtures are studied by differential scanning calorimetry (DSC) and by x-ray diffraction. Both the heating and cooling phase diagrams of the binary system are determined. During cooling, a new intermediate phase α' is observed under a temperature region of the usual rotator phase α. These α and α' phases are found to correspond to the rotator-II phase and the rotator-I phase, respectively, of n-paraffins. The dynamical crystal structure in the α phase is revealed by investigation of the diffuse scattering from the single crystal. The characteristic diffuse scattering observed originates from molecular fluctuations due to the translation along and the rotation around the molecular axes. Both modes of the molecular motions are studied separately from the diffuse scattering on the first and on the second layer lines. The translational disorder is described by the averaged relative translation between the nearest neighbor molecules; it is found to be as much as 20% of the repeat period 2.55 A of the polymethylene chain. On the other hand, it is found that the rotational disorder of the molecules is much smaller than the translational. The molecules are found to have a tendency to make their molecular planes parallel. It is suggested that the crystal in the rotator phase consists of domains, within which the molecules are ordered with respect to the orientations of their molecular planes but are making fairly active translations. Such structural model of the rotator phase is found to be quite consistent with that obtained in our recent computer simulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458414
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular-dynamics simulation of polymethylene chain confined in cylindrical potentials. I. Nature of the conformational defects (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5163-5167 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The conformational motion of a polymethylene molecule constrained by a cylindrical potential is simulated up to 100 ps. The molecule consists of 60 CH2 groups and has variable bond lengths, bond angles, and dihedral angles. Our main concern here is the excitation and the dynamics of the conformational defects: kinks, jogs, etc. Under weaker constraint a number of gauche bonds are excited; they mostly form pairs such as gtg¯ kinks or gtttg¯ jogs. These conformational defects show no continuous drift in space. Instead they often annihilate and then recreate at different sites showing apparently random positional changes. The conformational defects produce characteristic strain fields around them. It seems that the conformational defects interact attractively through these strain fields. This is evidenced by remarkably correlated spatial distributions of the gauche bonds.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463814
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  • 5
    Electronic Resource
    Electronic Resource
    Monte Carlo simulation of the crystal structure of the rotator phase of n-paraffins. II. Effects of rotation and translation of the rigid molecules (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2356-2365 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Structures and molecular motions in the rotator phases of n-paraffins are simulated by use of the Monte Carlo method—both rotational and translational degrees of freedom of the molecules being taken into account. Molecules of rigid trans-planar structure are assumed to be placed in a two-dimensional orthorhombic or orthohexagonal lattice with lattice parameters a and b. A model of the nearly continuous molecular motion is adopted in this work; each molecule is assumed to have 36 discrete orientations (in 10° intervals) and 10 discrete translations (in 0.128 A(ring) intervals) around and along the molecular axis, respectively. Molecular packing modes in various lattice systems of different a/b values are examined ranging from that of the ordinary orthorhombic phase (a/b=1.50) to that of the rotator-II (R-II) phase (a/b=31/2). It is found that the crystals in the rotator phases are generally composed of many domains within which molecules are packed more or less in order. In the rotator-I (R-I) phase, fairly large structural changes are observed with the change in a/b value; the structure of the R-I phase reported in our previous MC work should be slightly modified. The structure of the R-II phase, on the other hand, is found to be essentially the same as that of our previous MC work. Modes of the molecular motion in the R-II phase are studied by monitoring the time evolution of the rotational and translational states of all molecules in the system. It is found that the molecules are making fairly active rotational oscillation around one of the six equivalent directions connecting nearest neighbor molecular pairs. The molecules are also making sporadic large rotations to the different directions. The translational motion of the molecules is found to be very active and evidently independent of the rotational motion. The overall molecular motions in the rotator phase are found to be described in terms of the movement of the boundaries of the ordered domains.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455079
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  • 6
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulation of a polymethylene chain confined in cylindrical potentials. II. Dynamics of the trans planar segments (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6126-6134 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We here simulate, up to 100 ps, the conformational motion of a polymethylene molecule constrained by a cylindrical potential; the molecule is here made of 60 CH2 groups and has variable bond lengths, bond angles, and dihedral angles. We investigate the dynamical structure of the trans planar segments, especially the dihedral angle dynamics and the related twisting and out-of-plane bending motions, as a function of temperature and constraint, and we find the following: Under strong constraint, the twisting motions are dominant in the trans planar segments; the out-of-plane bending motions, however, increase with decreasing constraint resulting in a frequent generation of the gauche bonds. The dihedral angle waves propagate along the trans segments with a velocity of about 5000 m/s in two distinct series over even and odd bonds, which is a reflection of weak interactions between the adjacent dihedral angles. The overall twisting of the chain is made of local sharp twists at gauche bonds and broad gradual twists in the trans segments; the former twists are stationary, while the latter can propagate along the chain. The velocity of the latter twist is about 5000 m/s and it is almost independent of constraint. The out-of-plane bending motions, on the other hand, reflect the constraint rather sensitively. With decreasing constraint, the high frequency bending motions are greatly diminished and the velocity of the bending wave shows an appreciable slowdown. The apparently coherent annihilation and creation of the gauche bonds are often observed; the annihilation and creation occur as if they are mutually related through a dihedral angle wave.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465908
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  • 7
    Electronic Resource
    Electronic Resource
    Long-term stability of a one-dimensional current-driven double layer (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 2135-2143 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Long-term (〉an electron transit time over the system) stability of a one-dimensional current-driven double layer is studied by numerical experiments using particles. In these experiments, the potential difference across the system is self-consistently determined by the space charge distributions inside the system. Each boundary of the system supplies a nondrifting half-Maxwellian plasma. The current density is increased by increasing the number density of the source plasma at the injection (right) boundary. A double layer can be developed by injection of a sufficiently high current density. For a fixed level of current injection, plasmas carrying no current with various densities (n ˆ0) are loaded on the left side of the system. Whether or not the generated double layer can maintain its potential drop for a long period depends on the density (n ˆ0) relative to the initial density (n@B|0) near the injection boundary: (1) the double layer is found to grow when n ˆ0=n*0; (2) the steady double layer is seen for a long period when n ˆ0(approximately-greater-than)n@B|0; (3) the double layer is found to decay when n ˆ0 is even higher than n*0. A new concept of the current polarizability Pc=J/n# is introduced for understanding these results, where J is the current density flowing through the double layer and n# is the plasma density at the injection front, i.e., the low-potential edge of the double layer. Here Pc represents the potential of negatively polarizing a plasma. The different long-term features of the double layers in results 1–3 above can be explained by analyzing physical processes to control the current polarizability Pc.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866613
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  • 8
    Electronic Resource
    Electronic Resource
    Convective generation of a vortex street in plasma shear flow (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 2152-2164 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: By numerical calculation, it is shown that vortices are sequentially generated on the background charged layer forming parallel shear flows in a two-dimensional low beta incompressible plasma when the charged layer is locally disturbed by a small amount of additional charge. Numerical experiments are performed in a rectangular domain with 0〈x〈Lx and −Ly/2〈y〈Ly/2 and the background charged layers are distributed to form the plasma flow U in the x direction with a velocity shear of ∂U/∂y≠0. The boundaries at x=0 and Lx are to supply plasmas identical to the background plasma initially loaded inside the domain. The plasma particles reaching the boundaries are freely allowed to leave the experimental domain. A vortex (named the mother vortex) first develops the local charge disturbance initially given, and subsequently new vortices (named the daughter vortices) are generated on the charged layer. This can be thought of as a result of propagation of the initial disturbance: A relative plasma flow as seen from the mother vortex can carry downstream the disturbances originating from the vortex, which can trigger growth of the daughter vortex. The numerical experiments in our nonperiodic system show that the intervals between the vortices are relatively close to the wavelength of the most unstable mode predicted by the linear analysis for periodic vortices. It is also shown that the temporal growth rate γ of the vortex in its early growing period is comparable to the corresponding linear growth rate of the unstable periodic vortices with the same wavelength.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866615
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  • 9
    Electronic Resource
    Electronic Resource
    Solid–solid phase transitions in n-alkanes C23H48 and C25H52: X-ray power diffraction study on new layer stacking in phase V (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5762-5766 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The solid–solid phase transitions and the crystal structures of odd n-alkanes, n-C23H48, and n-C25H52, were investigated by x-ray powder diffraction. The phase transitions, I→V→RI for n-C23H48 and I→V→IV→RI for n-C25H52, were observed, where phases I, V, IV are the low-temperature ordered phases, and the phase RI is the rotator phase. The crystal structure of phase V was determined by the Rietveld profile refinement method. Phase V is orthorhombic (Pbnm). New molecular layer stacking appears in phase V, while the lamellar structure and the lateral packing of molecules are same as those in phase I and phase IV. The solid–solid phase transitions below the rotator phase transition of odd n-alkanes found to be characterized by the changes in molecular layer stacking are considered to be caused by the increased disorder in the layer surface. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470456
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  • 10
    Electronic Resource
    Electronic Resource
    Melting and crystallization in thin film of n-alkanes: A molecular dynamics simulation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 3351-3359 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent discovery of the anomalous crystallization in n-alkanes and n-alcohols at the free surfaces of melts has revealed the necessity for much deeper understanding of the molecular mechanism of crystallization. With the aid of large-scale molecular simulation, and by use of a simplified molecular model of bead spring, the molecular processes of melting and crystallization in a thin film of n-alkane are directly observed. It is found that melting of the thin film is greatly influenced by its surface state. The film is more stable when the surface chains are lying perpendicular to the surface, on which the chain ends have marked preference. Crystallization by both rapid and slow cooling of the melt is shown to give rise to a formation of monolayers on the free surfaces. By slow stepwise cooling, the layer-by-layer growth of stacked lamellae is clearly reproduced. Detailed inspection of the molecular processes involved shows that there are considerable differences in the molecular mechanisms of monolayer formation and 3D crystallization. Furthermore, the molecular mobility and diffusivity in the surface monolayer are discussed. It is shown that longitudinal displacements of the chains along their axes are much larger in the monolayer than in the crystals, and that the calculated rate of transverse diffusion in the monolayer shows good comparison with experimental values. The interchanges of the molecules between the melt and the surface monolayers are observed directly; dynamical stability of the surface monolayer is clearly demonstrated. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1287062
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