Albert Johner

Frictional forces between strongly compressed, nonentangled polymer brushes: molecular dynamics simulations and scaling theory.

By means of molecular dynamics simulations and scaling theory we study the response of opposing polymer brushes to constant shear motion under good solvent conditions. Model systems that contain explicit solvent molecules (Lennard-Jones dimers) are compared to solvent-free systems while varying of the distance between the grafted layers and their molecular parameters, chain length and grafting density. Our study reveals a power-law dependence of macroscopic transport properties on the Weissenberg number, W, beyond linear response. For instance, we find that the kinetic friction constant scales as mu approximately W(0.57) for large values of W. We develop a scaling theory that describes our data and previous numerical data including recent experiments.

Influence of bulk diffusion on the adsorption of hard spheres on a flat surface

Irreversible adsorption of hard spheres onto a solid surface is analyzed by using a generalization of the ‘‘random sequential adsorption’’ (RSA) model: ‘‘diffusion random sequential adsorption’’ (DRSA). In addition to the irreversible nature of the adsorption process and exclusion effects of the adsorbed configurations of hard spheres, the new model also considers the interactions between adsorbed particles and particles from the bulk, diffusing toward the surface. It is shown, in particular, that this affects the structure of adsorbed configurations for coverages different from the jamming limit coverage θ∞. Surprisingly, θ∞ appears to be identical for configurations generated by RSA and DRSA algorithms. Moreover, the structures of the configurations, as characterized by the radial distribution function g(r), are also identical at the jamming limit, whereas they differ for lower coverages. The coupling between the bulk diffusion process and the ‘‘adsorption process’’ is also analyzed as a function of the...

Polymer-brush lubrication in the limit of strong compression

Abstract.By means of molecular dynamics simulations we demonstrate power laws for macroscopic transport properties of strongly compressed polymer-brush bilayers to stationary shear motion beyond the Newtonian response. The corresponding exponents are derived from a recently developed scaling theory, where the interpenetration between the brushes is taken as the relevant length scale. This allows to predict the dependence of the critical shear rate, which separates linear and non-linear behavior, on compression and molecular parameters of the bilayer. We present scaling plots for chain extension (R , viscosity (

Single chain structure in thin polymer films: corrections to Flory's and Silberberg's hypotheses

\eta

Scale-Free Static and Dynamical Correlations in Melts of Monodisperse and Flory-Distributed Homopolymers

, and shear force (F over a wide range of Weissenberg numbers, W . In agreement with our theory, the simulation reveals simple power laws, R ∼ W0.53 ,

Adsorption of polymeric brushes: Bridging

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Conformation of weakly charged polyelectrolytes at a solid–liquid interface

∼ W-0.46 , and F ∼ W0.54 , for the non-Newtonian regime.

Polymer Brushes under High Load

Conformational properties of polymer melts confined between two hard structureless walls are investigated by Monte Carlo simulation of the bond fluctuation model. Parallel and perpendicular components of chain extension, bond–bond correlation function and structure factor are computed and compared with recent theoretical approaches attempting to go beyond Florys and Silberbergs hypotheses. We demonstrate that for ultrathin films where the thickness, H, is smaller than the excluded volume screening length (blob size), ξ, the chain size parallel to the walls diverges logarithmically, R2/2N≈b2+clog(N) with c~1/H. The corresponding bond–bond correlation function decreases like a power law, C(s) = d/sω with s being the curvilinear distance between bonds and ω = 1. Upon increasing the film thickness, H, we find—in contrast to Florys hypothesis—the bulk exponent ω = 3/2 and, more importantly, a decreasing d(H) that gives direct evidence for an enhanced self-interaction of chain segments reflected at the walls. Systematic deviations from the Kratky plateau as a function of H are found for the single chain form factor parallel to the walls in agreement with the non-monotonic behaviour predicted by theory. This structure in the Kratky plateau might give rise to an erroneous estimation of the chain extension from scattering experiments. For large H the deviations are linear with the wavevector, q, but are very weak. In contrast, for ultrathin films, H

Scale-free center-of-mass displacement correlations in polymer melts without topological constraints and momentum conservation: A bond-fluctuation model study

It has been assumed until very recently that all long-range correlations are screened in three-dimensional melts of linear homopolymers on distances beyond the correlation length ξ characterizing the decay of the density fluctuations. Summarizing simulation results obtained by means of a variant of the bond-fluctuation model with finite monomer excluded volume interactions and topology violating local and global Monte Carlo moves, we show that due to an interplay of the chain connectivity and the incompressibility constraint, both static and dynamical correlations arise on distances r≫ξ. These correlations are scale-free and, surprisingly, do not depend explicitly on the compressibility of the solution. Both monodisperse and (essentially) Flory-distributed equilibrium polymers are considered.

Dynamics of polymeric brushes: End exchange and bridging kinetics

We study the adsorption of grafted polymer layers on a planar surface parallel to the grafting surface. The layer consists of two types of chains: nonadsorbed chains with a free end and adsorbed chains forming bridges between the two plates. In the limit of strong adsorption a dead zone exists in the vicinity of the adsorbing plate; its size increases with the adsorption strength. Two adsorption mechanisms are possible: adsorption of the last monomer only and adsorption of all the monomers. In both cases the adsorption regimes at equilibrium (when no external force acts on the plates) are discussed within the framework of the self‐consistent mean‐field theory. We also give scaling laws taking into account excluded volume correlations. Finally, we consider situations where a finite external force, either tangential or normal to the plates, is applied on the adsorbing plate. Pulling and tangential forces both reduce the fraction of bridges and eventually lead to rupture, whereas compressional forces favor bridging. For normal forces, force vs distance profiles between planes and crossed cylinders are given.We study the adsorption of grafted polymer layers on a planar surface parallel to the grafting surface. The layer consists of two types of chains: nonadsorbed chains with a free end and adsorbed chains forming bridges between the two plates. In the limit of strong adsorption a dead zone exists in the vicinity of the adsorbing plate; its size increases with the adsorption strength. Two adsorption mechanisms are possible: adsorption of the last monomer only and adsorption of all the monomers. In both cases the adsorption regimes at equilibrium (when no external force acts on the plates) are discussed within the framework of the self‐consistent mean‐field theory. We also give scaling laws taking into account excluded volume correlations. Finally, we consider situations where a finite external force, either tangential or normal to the plates, is applied on the adsorbing plate. Pulling and tangential forces both reduce the fraction of bridges and eventually lead to rupture, whereas compressional forces favor b...