Kink motion in the Kramers problem for a chain molecule

TL;DR

This paper extends the Kramers escape problem to long chain molecules, revealing a kink-based mechanism with non-Arrhenius rate dependence and size-dependent crossing times.

Contribution

It introduces a kink motion framework for chain molecules crossing energy barriers, showing non-Arrhenius behavior and explicit scaling laws for crossing times.

Findings

Activation free energy scales with the square root of temperature.

Kink motion governs the crossing process in the chain.

Crossing time scales as N^2/T^{3/2} without free energy difference.

Abstract

We consider the generalization of the Kramers escape over a barrier problem to the case of a long chain molecule. It involves the motion of chain molecule of N segments across a region where the free energy per segment is higher, so that it has to cross a barrier. We use the Rouse model and find that the free energy of activation has a square root dependence on the temperature leading to a non-Arrhenius form for the rate. We also show that there is a special time dependent solution of the model, which corresponds to a kink in the chain, confined to the region of the barrier. The polymer goes from one side to the other by the motion of the kink in the reverse direction. If there is no free energy difference between the two sides of the barrier, then the kink moves by diffusion and the time of crossing t_{cross}~ N^2/T^{3/2}. If there is a free energy difference, then the kink moves with a non-zero velocity from the lower free energy side to the otherleading to t_{cross} ~ N/sqrt{T}.