The competition of hydrogen-like and isotropic interactions on polymer collapse

TL;DR

This paper explores how competing hydrogen-like and isotropic interactions influence polymer collapse, revealing complex phase behavior including globule, folded, and crystal states through Monte Carlo simulations.

Contribution

It introduces a lattice model with variable interaction types and maps out the phase diagram, highlighting the impact of interaction competition on polymer phase transitions.

Findings

Identification of a second phase transition from globule to crystal

Demonstration of continuous globule-crystal transition in 2D

First-order globule-crystal transition in 3D

Abstract

We investigate a lattice model of polymers where the nearest-neighbour monomer-monomer interaction strengths differ according to whether the local configurations have so-called ``hydrogen-like'' formations or not. If the interaction strengths are all the same then the classical $\theta$-point collapse transition occurs on lowering the temperature, and the polymer enters the isotropic liquid-drop phase known as the collapsed globule. On the other hand, strongly favouring the hydrogen-like interactions give rise to an anisotropic folded (solid-like) phase on lowering the temperature. We use Monte Carlo simulations up to a length of 256 to map out the phase diagram in the plane of parameters and determine the order of the associated phase transitions. We discuss the connections to semi-flexible polymers and other polymer models. Importantly, we demonstrate that for a range of energy parameters two phase transitions occur on lowering the temperature, the second being a transition from the globule state to the crystal state. We argue from our data that this globule-to-crystal transition is continuous in two dimensions in accord with field-theory arguments concerning Hamiltonian walks, but is first order in three dimensions.