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Large scale molecular dynamics and grand canonical Monte Carlo simulation techniques are used to study the behavior of the interdiffusion of a solvent into an entangled polymer matrix as the state of the polymer changes from a melt to a glass. The weight gain by the polymer increases with time t as t1/2 in agreement with Fickian diffusion for all cases studied, although the diffusivity is found to be strongly concentration dependent especially as one approaches the glass transition temperature of the polymer. The diffusivity as a function of solvent concentration determined using the one-dimensional Fick’s model of the diffusion equation is compared to the diffusivity calculated using the Darken equation from simulations of equilibrated solvent-polymer solutions. The diffusivity calculated using these two different approaches are in good agreement. The behavior of the diffusivity strongly depends on the state of the polymer and is related to the shape of the solvent concentration profile.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effective diffusivity in a polymer matrix modified with inclusions is usually calculated based on Kalnin-Kotomin's model [J. R. Kalnin, E. A. Kotomin, and J. Maier, J. Phys. Chem. Solids 63, 449-456 (2002)], which extends the well known Maxwell-Garnett formula. Kalnin-Kotomin's model correctly predicts effective diffusivity for stationary diffusion or for infinite media. In the present paper diffusion in composite media is studied for finite systems under transient conditions. The process of diffusion is modeled numerically and effective diffusion coefficient in the transient state is estimated, which, under certain conditions, is different from the predictions of the Kalnin-Kotomin's model. An analytical model is proposed to explain deviations of the transient effective coefficient of diffusion from the stationary case. [ABSTRACT FROM AUTHOR]

Through a detailed Langevin dynamics simulation study, the role of memory effects during unbiased translocation is explored. Tests are devised to uncover the presence of memory effects by directly measuring forward/backward-correlated motion as well as the associated change in the dynamics. Conducting these tests at low and high viscosities, a range of behaviours across different time scales is revealed: short-time forward correlations at all viscosities, quasi-static behaviour at low viscosity, and long-time backward correlations at high viscosity. By applying these tests at different portions of the translocation process, these memory effects are also shown to vary as translocation proceeds. Combining this information with standard measurements, a physical picture of unbiased translocation as the diffusion of a local minimum is proposed. [ABSTRACT FROM AUTHOR]

The effect of crowded environment with static obstacles on the translocation of a three-dimensional self-avoiding polymer through a small pore is studied using dynamic Monte Carlo simulation. The translocation time τ is dependent on polymer-obstacle interaction and obstacle concentration. The influence of obstacles on the polymer translocation is explained qualitatively by the free energy landscape. There exists a special polymer-obstacle interaction at which the translocation time is roughly independent of the obstacle concentration at low obstacle concentration, and the strength of the special interaction is roughly independent of chain length N. Scaling relation τ ∼ N1.25 is observed for strong driving translocations. The diffusion property of polymer chain is also influenced by obstacles. Normal diffusion is only observed in dilute solution without obstacles or in a crowded environment with weak polymer-obstacle attraction. Otherwise, subdiffusion behavior of polymer is observed. [ABSTRACT FROM AUTHOR]

Brownian diffusion of rod-like polymers in the presence of randomly distributed spherical obstacles is studied using molecular dynamics simulations. It is observed that dependence of the reduced diffusion coefficient of these macromolecules on the available volume fraction can be described reasonably by a power law function. Despite the case of obstructed diffusion of flexible polymers in which reduced diffusion coefficient has a weak dependence on the polymer length, this dependence is noticeably strong in the case of rod-like polymers. Diffusion of these macromolecules in the presence of obstacles is observed that is anomalous at short time scales and normal at long times. Duration time of the anomalous diffusion regime is found that increases very rapidly with increasing both the polymer length and the obstructed volume fraction. Dynamics of diffusion of these polymers is observed that crosses over from Rouse to reptation type with increasing the density of obstacles. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations on the Kremer–Grest bead-spring model of polymer melts are used to study the effect of spherical nanoparticles on chain diffusion. We find that chain diffusivity is enhanced relative to its bulk value when polymer-particle interactions are repulsive and is reduced when polymer-particle interactions are strongly attractive. In both cases chain diffusivity assumes its bulk value when the chain center of mass is about one radius of gyration Rg away from the particle surface. This behavior echoes the behavior of polymer melts confined between two flat surfaces, except in the limit of severe confinement where the surface influence on polymer mobility is more pronounced for flat surfaces. A particularly interesting fact is that, even though chain motion is strongly speeded up in the presence of repulsive boundaries, this effect can be reversed by pinning one isolated monomer onto the surface. This result strongly stresses the importance of properly specifying boundary conditions when the near surface dynamics of chains are studied. [ABSTRACT FROM AUTHOR]

Within Kirkwood theory, we study the translational diffusion coefficient of a single polymer chain in dilute solution, and focus on the small difference between the short-time Kirkwood value D[SUPK] and the asymptotic long-time value D. We calculate this correction term by highly accurate large-scale Brownian dynamics simulations, and show that it is in perfect agreement with the rigorous variational result D

The mean first passage time for the close approach of the ends of a polymer is investigated by computer simulation of the Brownian dynamics of a Rouse model. There are N bonds, of length b, and the contact distance is a. The results for N=50, 75, and 100 are compared with theoretical predictions by Wilemski and Fixman (WF), and by Szabo, Schulten, and Schulten (SSS). Theory differs from simulation by a factor of 1.3 to 3 when a is comparable to b but improves considerably when a is reduced. Closest agreement is obtained at a/b=0.1 for WF with a δ function sink, where theory is within the statistical error (≊5%) of the simulation for each case. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations of the diffusion coefficient of systems of polydisperse chains are presented. Each system consists of two lengths of chain of chemically identical flexible polymers. The mean square displacement of the center of mass of each species is measured as a function its length and volume fraction in the blend. The polymer lengths range from N=10 monomers per chain to N=90, about three times the entanglement length. The polymer species that comprises the bulk of the melt shows little change in behavior regardless of the length of polymer which makes up the remainder. By contrast, when a species is the minority component, its motion is significantly affected by the length of the matrix chains. When a chain is immersed in a matrix of longer chains, its diffusion coefficient is smaller than its monodisperse value; conversely when a chain is in a blend of shorter chain its diffusion coefficient increases compared to a monodisperse melt. For chains shorter than the entanglement length, the diffusion coefficient compares well to theoretical predictions. The scaling exponent of the mean square displacement of the longest polymer is found to be sublinear, unless blended with very short polymers. The scaling exponent seems to be a measurement of the entanglements that the long polymers experience. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]