Conservation of polymer winding states: a combinatoric approach

Abstract

The work in this article is inspired by a classical problem: the statistical physical properties of a closed polymer loop that is wound around a rod. Historically the preserved topology of this system has been addressed through identification of similarities with magnetic systems. We treat the topological invariance in terms of a set of rules that describe all augmentations by additional arcs of some fundamental basic loop of a given winding number. These augmentations satisfy the Reidemeister move relevant for the polymer with respect to the rod. The topologically constrained polymer partition function is now constructed using the combinatorics of allowed arc additions and their appropriate statistical weights. We illustrate how, for winding number 1, we can formally derive expressions for lower and upper bounds on the partition function. Using the lower bound approximation we investigate a flexible polymer loop wound between two slits, calculating the force on the slit as well as the average numbers of arc types in dependence of slit width and separation. Results may be extended to higher winding numbers. The intuitive nature of this combinatoric scheme allows the development of a variety of approximations and generalisations.