Log-periodic oscillations for biased diffusion of a polymer chain in a porous medium

A coarse-grained off-lattice bead-spring model is used to reveal the comple x dynamics of a polymer chain in a quenched porous medium in the presence o f an external field B. The behavior of the mean square displacement (MSD) o f the center chain bead and that of the center of mass of the chain as a fu nction of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important infor mation on the way in which chains move between obstacles and overcome them is gained from the MSD vs. time analysis in the directions parallel and per pendicular to the flow. Instead of a steady approach to uniform drift-like motion at low C, for sufficiently strong held B we observe logarithmic osci llations in the effective exponents describing the time dependence of the M SD along and perpendicular to field. A common nature of this phenomenon wit h oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested.