Folding mechanism of a polymer chain with short-range attractions

TL;DR

This study uses transition path sampling to analyze the crystallization process of a flexible homopolymer chain with short-range attractions, identifying key features of transition states and proposing improved reaction coordinates.

Contribution

The paper introduces a new polymer-specific shooting move and develops an improved reaction coordinate for studying polymer crystallization transitions.

Findings

Transition states often feature a crystalline nucleus with attached chain fragments.

Number of particles in the crystalline core is not an effective reaction coordinate.

An improved reaction coordinate incorporating potential energy and crystallinity is proposed.

Abstract

We investigate the crystallization of a single, flexible homopolymer chain using transition path sampling (TPS). The chain consists of N identical spherical monomers evolved according to Langevin dynamics. While neighboring monomers are coupled via harmonic springs, the non-neighboring monomers interact via a hard core and a short-ranged attractive potential. For a sufficiently small interaction range {\lambda}, the system undergoes a first-order freezing transition from an expanded, disordered phase to a compact crystalline state. Using a new shooting move tailored to polymers combined with a committor analysis, we study the transition state ensemble of an N=128 chain and search for possible reaction coordinates based on likelihood maximization. We find that typical transition states consist of a crystalline nucleus with one or more chain fragments attached to it. Furthermore, we show that the number of particles in the crystalline core is not well suited as a reaction coordinate. We then present an improved reaction coordinate, which includes information from the potential energy and the overall crystallinity of the polymer.