Stretching of a polymer below the Theta point

TL;DR

This paper investigates how polymers unfold under external force below the Theta point, revealing stepwise unfolding in finite chains and differences in phase behavior between two and three dimensions.

Contribution

It provides a detailed analysis of polymer unfolding below the Theta point, including phase diagrams and transition order changes between 2D and 3D.

Findings

Globule unfolds via multiple steps in finite chains.

Intermediate minima affect phase behavior mainly in 2D.

Transition order changes from 2D to 3D.

Abstract

The unfolding of a polymer below the $\theta$ point when pulled by an external force is studied both in d=2 on the lattice and in $d=3$ off lattice. A ground state analysis of finite length chains shows that the globule unfolds via multiple steps, corresponding to transitions between different minima, in both cases. In the infinite length limit, these intermediate minima have a qualitative effect only in $d=2$. The phase diagram in d=2 is determined using transfer matrix techniques. Energy-entropy and renormalization group arguments are given which predict a qualitatively correct phase diagram and a change of the order of the transition from d=2 to d=3.