Electrophoresis of an end-labeled polymer chain: a molecular dynamics study.

We study the conformational and the dynamic properties of an end-labeled (telechelic) polymer chain embedded in a porous medium made of randomly distributed immobile spherical obstacles using a stochastic molecular dynamics (MD) simulation method for several obstacle densities rho(imp) and for various field intensities Fx applied only to one end of the chain. For F(x)=0, the chain initially shrinks with increasing density of the obstacles rho(imp). In general, for small Fx and low rho(imp), the chain elongates along the direction of the force and shrinks in the transverse direction whereby this effect becomes more pronounced at larger chain lengths. However, we notice that for moderate values of rho(imp) and Fx, the conformational properties exhibit extrema before reaching a saturation at larger values of rho(imp) and Fx. Likewise, we also find that the drift velocity V(d) of the center of mass of the chain is a nonmonotonic function of the field intensity in the sense that V(d) also exhibits a maximum at a critical value of the field intensity F(crit)(x) beyond which it decreases. Our MD results indicate that for large rho(imp) the chain still can be described by a self-avoiding random walk, which contradicts the prediction of variational calculation using the replica trick, but supports a more recent analytical result using the optimal fluctuation method, as well as a Monte Carlo simulation result for a slightly different disordered medium.