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  • 1
    Electronic Resource
    Electronic Resource
    Dynamics of entangled linear polymer melts: A molecular-dynamics simulation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5057-5086 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an extensive molecular-dynamics simulation for a bead spring model of a melt of linear polymers. The number of monomers N covers the range from N=5 to N=400. Since the entanglement length Ne is found to be approximately 35, our chains cover the crossover from the nonentangled to the entangled regime. The Rouse model provides an excellent description for short chains N〈Ne, while the dynamics of the long chains can be described by the reptation model. By mapping the model chains onto chemical species we give estimates of the times and distances of onset of the slowing down in motion due to reptation. Comparison to neutron spin-echo data confirm our mapping procedure, resolving a discrepancy between various experiments. By considering the primitive chain we are able to directly visualize the confinement to a tube. Analyzing the Rouse mode relaxation allows us to exclude the generalized Rouse models, while the original reptation prediction gives a good description of the data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458541
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  • 2
    Electronic Resource
    Electronic Resource
    A comparison between integral equation theory and molecular dynamics simulations of dense, flexible polymer liquids (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1357-1364 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently we (J.G.C. and K.S.S.) formulated a tractable "reference interaction site model'' (RISM) integral equation theory of flexible polymer liquids. The purpose of this paper is to compare the results of the theory with recent molecular dynamics simulations (G.S.G. and K.K.) on dense chain liquids of degree of polymerization N=50 and 200. Specific comparisons were made between theory and simulation for the intramolecular structure factor ωˆ(k) and the intermolecular radial distribution function g(r) in the liquid. In particular it was possible to independently test the assumptions inherent in the RISM theory and the additional ideality approximation that was made in the initial application of the theory. This comparison was accomplished by calculating the intermolecular g(r) using the simulated intramolecular structure factor, as well as, ωˆ(k) derived from a freely jointed chain model.The RISM theory results, using the simulated ωˆ(k), were found to be in excellent agreement, over all length scales, with the g(r) from molecular dynamics simulations. The theoretical predictions using the "ideal'' intramolecular structure factor tended to underestimate g(r) near contact, indicating local intramolecular expansion of the chains. This local expansion can be incorporated into the theory self consistently by including the effects of the "medium induced'' potential on the intramolecular structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457159
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  • 3
    Electronic Resource
    Electronic Resource
    Phase diagram and dynamics of Yukawa systems (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3286-3312 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The phase diagram and dynamical properties of systems of particles interacting through a repulsive screened Coulomb (Yukawa) potential have been calculated using molecular and lattice dynamics techniques. The phase diagram contains both a melting transition and a transition from fcc to bcc crystalline phases. These phase transitions have been studied as a function of potential shape (screening length) and compared to phenomenological criteria for transition temperatures such as those of Lindemann and of Hansen and Verlet. The transition from fcc to bcc with increasing temperature is shown to result from a higher entropy in the bcc phase because of its softer shear modes. Even when the stable solid phase below the melting temperature is fcc, bcc-like local order is found in the liquid phase. This may substantially slow crystallization. The calculated phase diagram and shear modulus are in good agreement with experiments on colloidal suspensions of polystyrene spheres. The single particle dynamics of Yukawa systems show several unusual features. There is a pronounced subdiffusive regime in liquids near and below the melting temperature. This regime reflects the existence of two time scales: a typical phonon period, and the time for a particle to feel a new environment. The second time scale becomes longer as the temperature is lowered or the range of interaction (screening length) increases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453924
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  • 4
    Electronic Resource
    Electronic Resource
    Critical exponents and scaling functions of a self-avoiding walk interacting with a defect plane (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5882-5887 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We analyze the behavior of a single self-avoiding walk (single polymer in good solvent) in the presence of a penetrable (d−1) dimensional defect layer by the use of Monte Carlo simulations. The layer can be either neutral to the bonds or can be attractive or repulsive. We analyze the data by means of a scaling picture and show that there is an excellent agreement with the deGennes, Bray, and Moore conjecture cursive-phi=1−ν for the crossover exponent. This corrects a recent estimate of cursive-phi made by a series analysis of enumeration data of short chains.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449618
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  • 5
    Electronic Resource
    Electronic Resource
    Like-charge colloid–polyelectrolyte complexation (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2947-2960 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate the complexation of a highly charged sphere with a long flexible polyelectrolyte, both negatively charged in a salt-free environment. Electroneutrality is insured by the presence of divalent counterions. Using molecular dynamics within the framework of the primitive model, we consider different Coulomb coupling regimes. At strong Coulomb coupling we find that the adsorbed chain is always confined to the colloidal surface but forms different conformations that depend on the linear charge density of the chain. A mechanism involving the polyelectrolyte overcharging is proposed to explain these structures. At intermediate Coulomb coupling, the chain conformation starts to become three-dimensional, and we observe multilayering of the highly charged chain while for lower charge density the chain wraps around the colloid. At weak Coulomb coupling, corresponding to an aqueous solvent, we still find like-charge complexation. In this latter case the chain conformation exhibits loops. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1490595
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  • 6
    Electronic Resource
    Electronic Resource
    The effect of bond length on the structure of dense bead–spring polymer melts (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2776-2785 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We explored the effect of the ratio of bond length to excluded volume bead diameter, l0/d0, of simple bead–spring homopolymer chains on the resulting liquid structure of dense, monodisperse polymer melts. We conducted NVT molecular dynamics simulations of both bulk and planar confined polymer liquids. We find that both pressure and surface tension increase with l0/d0. We attribute this sensitivity to the reduction in short range liquid ordering, classically observed for simple fluids, when l0/d0〈1, and its persistence and even enhancement when l0/d0〉1. We explain this by examining specific correlations between bond orientations and monomer density. This leads to a better understanding of how intramolecular constraints influence intermolecular structure in molecular liquids. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1385791
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  • 7
    Electronic Resource
    Electronic Resource
    Effects of excluded volume and bond length on the dynamics of dense bead-spring polymer melts (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3162-3165 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We consider the effects of the ratio of average bond length, l0, to excluded volume bead diameter, d0, on the dynamics of dense liquids of freely jointed bead-spring polymer chains. Monodisperse (N=50) systems for which 0.73〈l0/d0〈1.34 were simulated using molecular dynamics in the NVT ensemble at a monomer number density ρ=0.85 σ−3. The monomer friction coefficient, ζ, increased by a factor of ∼20 for a factor ∼2 increase in l0/d0. This is due to the enhancement of sphere-packing homogeneity in the melts for l0/d0〉1, which in turn increases the volume density and optimizes packing. Friction coefficients depend on local density (i.e., density of nearest neighbors around any monomer) in a manner akin to the empirical relationship between diffusivity and total number density in simple Lennard-Jones fluids [Levesque and Verlet, Phys. Rev. A 2, 2514 (1970)]. This work demonstrates the degree of interplay between structure and dynamics in macromolecular liquids. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1445107
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  • 8
    Electronic Resource
    Electronic Resource
    Structure and relaxation of end-linked polymer networks (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8169-8192 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The kinetics of end crosslinking a linear polymer melt and the dynamics of the resulting network is studied using molecular dynamics (MD) simulations. Starting from equilibrated melts of linear chains of length ranging from 1/3Ne to 3Ne, where Ne is the entanglement length, tetrafunctional crosslinkers are attached to a fraction x of the chain ends. When a free end comes within a short capture distance rx from an unsaturated crosslinker, the chain ends are attached. With a stoichmetric number x of crosslinkers, the long time kinetics for the number of free ends and the number of unsaturated crosslinkers decays as a power law in time t−a, with a≈0.5 for the present range of chain lengths. The resulting networks are then used to study the effect of entanglements on the motion of the crosslinks and the modulus of the network. Using a cluster search algorithm, the microscopic characteristics of the networks are determined. This allows us to compare our simulated networks to theoretical models of rubber elasticity without any adjustable parameters. While our results for the modulus are close to the phantom network model for the shortest chains, for the longer chains our results clearly support the Edwards tube model. Already for the presently employed largest chains the modulus is about 1.9 times larger than predicted from the affine model. Asymptotically, within an affine network model, about 2.2 entanglement lengths are needed to contribute to the modulus in the same way as one elastically active chemical crosslink. The data agree very well with recent experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468202
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  • 9
    Electronic Resource
    Electronic Resource
    Time–temperature and time–density superposition in the simulation of rheological properties of polymers (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3288-3293 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Applying a periodical elongational strain in a computer simulation of polymer melts and networks, we are able for the first time to employ a time–temperature and time–density superposition in a numerical experiment for the study of the dynamic moduli. The simulation results can be analyzed and understood within the semiempirical free volume concept. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479608
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  • 10
    Electronic Resource
    Electronic Resource
    Dynamics of polymer solutions and melts. Reptation predictions and scaling of relaxation times (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7726-7740 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The bond fluctuation model on the simple cubic lattice is studied by Monte Carlo simulations on a multitransputer array, for polymer volume fractions φ in the range 0.025≤φ≤0.500 and chain lengths N in the range 20≤N≤200. Extensive data are presented on the dynamics of monomer displacements, center-of-gravity displacements, and relaxation times. This study is complementary to previous work, in which the crossover scaling properties of the chain linear dimensions, structure factor, and self-diffusion constant were tested for the same athermal model. The simulation technique takes both excluded volume interactions and entanglement constraints into account, but ignores hydrodynamic forces. Our results describe the crossover from Rouse behavior of swollen chains (τ∼N1+2ν, ν being the exponent describing the radius R of the chains, R∼Nν ) to reptation, τ∼N3. Since the excluded volume screening length is found to be smaller than the tube diameter by a factor of about 3, the rescaled times Wτ/N1+2ν decrease first as a function of the scaled chain length N˜∼Nφ1/(3ν−1), before they increase due to the onset of reptation. Additional evidence for reptative behavior is found by identifying the several successive crossovers in the time-dependent displacements predicted by de Gennes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461346
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