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  • 1
    Electronic Resource
    Electronic Resource
    Molecular arrangements and conformations of liquid n-tridecane chains confined between two hard walls (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 779-786 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Liquid n-tridecane (C13H28) chains confined between two parallel hard walls separated by 5.0 nm have been studied by Monte Carlo simulations, employing united CH2 atoms linked with fixed bond lengths and angles and continuously varying torsional angles subject to appropriate interaction energies. Molecules located in the central region are in remarkable agreement with ideal unperturbed chains. Significant influences of walls persist over a distance of ∼1.5 nm, exhibiting progressively less-dense and less-pronounced segmental layers of ∼0.4 nm thickness. Segmental orientations preferentially aligned along the surface are found only in the first layer, accompanied by little perturbations in the fraction of trans-conformations. Furthermore, nearly all the chain units in the first segmental layer adjacent to the walls belong to two-dimensional chains, and exhibit considerable orientational correlations between neighboring segments of different chains. The terminal portions of some of the molecules in the succeeding layer are also located in the first layer, thus increasing the fraction of methyl chain ends in contact with the walls. These findings match very closely the experimental results of surface forces and viscosities of liquid n-alkanes confined between two mica plates, measured as a function of the plate separation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459531
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  • 2
    Electronic Resource
    Electronic Resource
    Molecular arrangements and conformations of the alkyl chains in spherical micelles and droplets (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 757-767 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have modeled two systems of spherical micelles and of n-alkane droplets, each consisting of 52 molecules comprised by 13 units each in a spherical region of approximately 1.6 nm radius, by the Monte Carlo simulation method. The simulated model micelle exhibits the distribution of chain segments in good agreement with the results of neutron scattering and NMR experiments on micelles having similar compositions. Both the micelle and the droplet models show that the segments in the interior of the chains tend to stay away from the interfaces, resulting in enhanced chain-end densities at the surface region. Moreover, chain segments in contact with the interface tend to orient along the interface,thus exhibiting negative order parameters with respect to the radius vector. Despite this local anisotropic orientation, however, the average conformation of the alkyl groups is only slightly different from that of the unperturbed random-coil model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458429
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  • 3
    Electronic Resource
    Electronic Resource
    Off-lattice Monte Carlo simulations of polymer melts confined between two plates (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5206-5215 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we present the results of Monte Carlo simulations of the static properties of polymer melts confined between hard walls. The simulations are conducted in the canonical ensemble with a method that is a combination of reptation and crankshaft motions. 1250 polymer chains each comprising of 100 connected beads are placed in a box which allows for the simulation of a typical polymer melt confined between two hard plates at a separation of 51 bead diameters. Noncovalently bonded beads are assumed to interact with an empirical 6-12 Lennard-Jones potential which has parameters chosen to simulate a polyethylene melt at 400 K. From the analysis of the simulation results we show the existence of two relevant length scales in the problem. Single-chain statistics are perturbed by the wall, and this effect is screened out only after one proceeds to a distance comparable to twice the unperturbed radius of gyration of the polymer chain. However, many-chain statistics, i.e., packing and orientation of chain segments, are screened out as soon as one proceeds about three times the bead size from the wall. The simulation also allows for the study of the conformations of chains near the wall, and we observe that chains near the surface are flattened into nearly two-dimensional structures. The interface therefore corresponds to a region where chain configurations gradually evolve from this nearly two-dimensional structure to the unperturbed, three-dimensional Gaussian configurations in the bulk.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455611
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  • 4
    Electronic Resource
    Electronic Resource
    A comparison of a united atom and an explicit atom model in simulations of polymethylene (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 10037-10043 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out stochastic dynamics and molecular dynamics simulations of n-tridecane (C13H28) as isolated chains, in bulk melts, and in confined melts between solid surfaces, employing both a united atom (UA) model and an explicit atom (EA) model, in order to compare chain conformations, packing, orientational correlations, and self-diffusion predicted by the UA and EA models. The EA model, which explicitly takes into account all hydrogens, exhibits nearly identical results for chain conformations to those from the UA model. However, only the EA model, which shows considerably enhanced interchain packing and orientational correlations in the melts over those for the UA model, reproduces very closely the height and the width of the interchain peak in the experimental x-ray scattering profile. Dynamically, inclusion of explicit hydrogens decreases the self-diffusion constants in the melts by a factor of 6–8, resulting in reasonably good agreement with the experimental value. Moreover, in the melts confined between solid surfaces, the presence of explicit hydrogens leads to much more pronounced layering of both the monomer segments and the entire molecules, which are strongly oriented along the solid surfaces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464436
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  • 5
    Electronic Resource
    Electronic Resource
    Study of local stress, morphology, and liquid-crystal alignment on buffed polyimide surfaces (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 6811-6817 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnitude of local stress applied to a polymer surface in the buffing process, which is a critical method to uniformly align liquid crystals for displays, has been evaluated from the polarizing optical microscopic investigation of nematic liquid crystals in contact with polyimide surfaces which were suitably buffed in order to visualize the individual tracks of the buffing rayon fibers. The estimated magnitude of local stress is most likely to be far greater than the yield stress of the polymer film, thereby causing permanent molecular orientation along the buffing direction. The surfaces of poly(amic acid) and cured polyimide films, subjected to different buffing and cure profiles, have been examined by atomic force microscopy. The ability of these buffed polymer surfaces to align nematic and chiral smectic C (ferroelectric) liquid crystals were also investigated. These studies show that the presence of microgrooves on buffed polymer surfaces is not necessary for alignment of liquid crystals. The contribution of polymer molecules from the buffing fibers, deposited and oriented on the polyimide surfaces during buffing, can also be ruled out. Rather, it is concluded that the liquid-crystal alignment is mainly caused by the anisotropic interactions between liquid-crystal molecules and the polymer segments oriented through plastic deformation, with little contribution from frictional heating. For the alignment of smectics, both the degree of order and mechanical properties of polyimide films are found to be important factors. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361502
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  • 6
    Electronic Resource
    Electronic Resource
    Surface diffusion of thin perfluoropolyalkylether films : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5788-5790 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spreading characteristics of thin polymeric liquid films of perfluoropolyalkylethers (PFPEs) Fomblin Z15 and Fomblin Zdol (hydroxyl terminated PFPE) on silica surfaces have been measured by scanning microellipsometry (SME). We estimated the surface diffusion coefficients and propose a modified diffusion equation to interpret the spreading phenomenon from film thickness profiles measured with SME. We investigated the spreading of Z15 as a function of binary blend ratio of monodisperse Z15 fractions and found that the surface diffusion coefficients of the blends do not obey a simple linear mixing rule. A summary of thin PFPE film spreading characteristics as a function of molecular weight, film thickness, chain-end functionality, temperature, and humidity is presented. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362189
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  • 7
    Electronic Resource
    Electronic Resource
    Configurational characteristics of thermotropic polymers comprising rigid groups connected by polymethylene spacers (1985)
    s.l. : American Chemical Society
    Macromolecules 18 (1985), S. 651-657 
    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/ma00146a013
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  • 8
    Electronic Resource
    Electronic Resource
    NMR study of nematic order of semiflexible thermotropic polymers (1985)
    s.l. : American Chemical Society
    Macromolecules 18 (1985), S. 2709-2713 
    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/ma00154a057
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  • 9
    Electronic Resource
    Electronic Resource
    Conformational Characteristics of Poly(oxymethylene) Based upon ab Initio Electronic Structure Calculations (1994)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 9078-9082 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100087a041
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  • 10
    Electronic Resource
    Electronic Resource
    Equilibrium and dynamic properties of polymethylene melts from molecular dynamics simulations. I. n-Tridecane (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 649-658 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Employing an explicit atom (EA) model of polymethylene, we have carried out molecular dynamics simulations of n-tridecane (C13H28) melts at experimental densities to compute both equilibrium and dynamic properties. The calibrated EA model reproduces quite well the experimental results of pressure, x-ray diffraction patterns, and self-diffusion constants at different temperatures. A united atom (UA) model that reproduces the experimental pressures also yields good agreement with experimental x-ray diffraction patterns and self-diffusion data, and the calculated degree of intermolecular orientational correlation is in good agreement with predictions of the EA model. However, the UA model yields significantly more extended chain dimensions than a previously investigated model, and, most importantly, significantly enhanced local chain dynamics compared to the EA model, as monitored by the chain vector reorientation and local torsional dynamics. The EA model simulations yield C–H vector orientational correlation times associated with each carbon of n-tridecane, in excellent agreement with experimental values deduced from C13-NMR T1 spin–lattice relaxation times. The C–H vector reorientation was found to be closely related to conformational jumps. These jumps, whose rates closely follow torsional correlation times, appear to occur mostly as unconcerted individual transitions for these short chains.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466929
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