Dynamics of a Polymer in the Presence of Permeable Membranes

TL;DR

This paper investigates how linear polymers diffuse in the presence of permeable membranes, revealing different dynamic regimes and scaling behaviors depending on membrane spacing and polymer length.

Contribution

It provides a combined scaling theory and Monte Carlo simulation analysis of polymer diffusion across permeable membranes, identifying three distinct dynamic regimes.

Findings

Crossing time scales as N^{5/2} for a single membrane.

Diffusion is Rouse-like with D ~ N^{-1} in certain regimes.

Diffusion becomes isotropic and Rouse-like at large membrane spacing.

Abstract

We study the diffusion of a linear polymer in the presence of permeable membranes without excluded volume interactions, using scaling theory and Monte Carlo simulations. We find that the average time it takes for a chain with polymerization index~$N$ to cross a single isolated membrane varies with~$N$ as~% $N^{5/2}$, giving its permeability proportional to~$N^2$. When the membranes are stacked with uniform spacing~$d$ in the unit of the monomer size, the dynamics of a polymer is shown to have three different regimes. In the limit of small~\mbox{$d \ll N^{1/2}$}, the chain diffuses through reptation and \mbox{$D\sim N^{-2}$}. When $d$ is comparable to~$N^{1/2}$ the diffusion coefficients parallel and perpendicular to the membranes become different from each other. While the diffusion becomes Rouse-like, i.e.~\mbox{$D\sim N^{-1}$}, in the parallel direction, the motion in the perpendicular direction is still hindered by the two-dimensional networks. The diffusion eventually becomes isotropic and Rouse-like for large~\mbox{$d \gg N$}.