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  • 1
    Electronic Resource
    Electronic Resource
    The origin of "glassy'' dynamics at solid–oligomer interfaces (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5520-5527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present an equilibrium molecular dynamics study of oligomer (5-mer) films at liquid densities confined between planar simple-cubic surfaces. The systems studied were wide enough to develop a bulklike region in the middle half of the film and two well-separated interfaces. Our work focused on the effect of surface–segment attraction (εws) on chain dynamics. We studied separately the dynamics of "free'' and "adsorbed'' chains. "Free'' chains have no segments inside the first layer next to each surface, while "adsorbed'' chains have one or more of their segments inside these layers. "Free'' chains exhibited bulk dynamics, irrespectively, of the value of εws and their proximity to a surface. "Adsorbed'' chains exhibited two distinctly different dynamical patterns depending on the strength of surface segment attraction (εws). Chains "adsorbed'' on weakly physisorbing surfaces (εws≤1.0 kT) were moderately affected by the surface. Their relaxation time was at most 10 times higher than bulk for collapsed chains and became gradually bulklike with decreasing number of surface–segment contacts. In sharp contrast, chains "adsorbed'' on strongly physisorbing surfaces (εws=2.0, 3.0) experienced a dramatic increase of their relaxation times (by a factor of ∼1500 for εws=3.0) and a severe decrease of their mobility. Furthermore, chain relaxation was quite insensitive to the exact number of surface–segment contacts. Our findings indicate that the extremely sluggish dynamics of strongly physisorbed chains result from the severe slowdown of segmental motions inside the first layer. This slowdown is caused by the very high density of the first layer, rather than the bare surface–segment attraction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465970
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  • 2
    Electronic Resource
    Electronic Resource
    A lattice Monte Carlo study of long chain conformations at solid–polymer melt interfaces (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 3100-3111 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present a comprehensive lattice Monte Carlo study of long chain conformations at solid–polymer melt interfaces. Segmental scale interfacial features, like the bond orientational distribution were found to be independent of surface–segment energetics, and statistically identical with Helfand's predictions for the full-occupancy, infinite chain length limit. Conformational statistics of chains longer than 5–6 statistical segments followed the predictions of the Scheutjens–Fleer theory and the same power laws as a single ideal chain at the critical value of the surface–segment adsorption free-energy. Our simulations tested the predictions of random walk next to a "reflective'' surface statistics for the spatial variations of chain dimensions and chain center of mass density. It was found that these statistics furnish the correct long chain limit, independently of surface–segment energetics. The random walk next to a "reflective'' boundary predictions for the "adsorbed'' amount and the distributions of tail, loop, and train number, as well as tail and loop size were in quantitative agreement with the simulation data. The correspondence between random walks and real (or simulated) chains required the knowledge of a single, microscopic parameter, the number of chemical segments per statistical segment, a∞. This quantity was very close to the average length of adsorbed sequences (trains), in the long chain limit. Our simulations tested thoroughly and established firmly the validity of "reflective'' boundary statistics in the melt. The inevitability of these statistics has broad implications on "desorption'' kinetics, chain mobility, and chain relaxation, which are currently under study.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465163
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  • 3
    Electronic Resource
    Electronic Resource
    Microscopic dynamics of flow in molecularly narrow pores (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3427-3431 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed and is due to the inability of fluid layers to undergo the gliding motion of planar flow. This effect is partially responsible for the strong viscosity increases observed experimentally in thin films that still maintain their fluidity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458823
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  • 4
    Electronic Resource
    Electronic Resource
    Structural transitions at solid-liquid interfaces (1996)
    Springer
    Adsorption 2 (1996), S. 69-76 
    Details
    ISSN: 1572-8757
    Keywords: ultra thin films ; molecular dynamics ; n-alkanes and phase transitions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract In this paper we present the findings of our investigations using molecular dynamics, on molecularly thin films of n-octane confined between topographically smooth solid surfaces. We focus on the effect of increasing solid surface-methylene unit energetic affinity and the effect of increasing pressure (normal load) of the film in inducing liquid-solid phase transitions. We observed an abrupt transition in the structural features of the film at a critical value of the characteristic energy that quantified the affinity between solid surfaces and methylene units. This energetically driven transition was evident from the discontinuous increase of intermolecular order, a precipitous extension of the octane molecules and freezing of molecular migration and rotation. Increasing pressure had a similar effect in inducing a liquid-solid phase transition. The characteristics of the transition showed that it is a mild first order transition from a highly ordered liquid to a poorly organized solid. These findings demonstrate that the solidification of nanoscopically thin films of linear alkanes is a general phenomenon (driven either energetically or by increasing pressure), and does not require the aid of commensurate surface topography.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00127100
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  • 5
    Electronic Resource
    Electronic Resource
    Energetically and pressure driven phase transitions in molecularly thin films of n-alkanes (1995)
    Springer
    Tribology letters 1 (1995), S. 177-190 
    Details
    ISSN: 1573-2711
    Keywords: molecular simulations ; thin films ; phase transitions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Molecular dynamics simulations were carried out on molecularly thin films of n-octane confined between topographically smooth solid surfaces. We focused on determining the effect of increasing solid surface-methylene unit energetic affinity and the effect of increasing pressure (normal load) of the film in inducing liquid-solid phase transitions. Simulations of films wide enough to accomodate three segmental layers showed an abrupt transition in the structural features at a critical value of the characteristic energy that quantified the affinity between solid surfaces and methylene units. This energetically driven transition was evident from the discontinuous increase of intermolecular order, a precipitous extension of the octane molecules and freezing of molecular migration and rotation. Increasing pressure had a similar effect in inducing a liquid-solid phase transition. The characteristics of the transition showed that it is a mild first-order transition from a highly ordered liquid to a poorly organized solid. These findings demonstrate that the solidification of nanoscopically thin films of linear alkanes is a general phenomenon (driven either energetically or by increasing pressure), and does not require the aid of commensurate surface topography. Our findings on relatively wider films (5 segmental diameters wide) show that the interfacial layer undergoes a similar first-order phase transition with increasing solid-methylene unit energetic affinity. This energy threshold is significantly higher than the one observed in thin film simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00209772
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