Equilibration of Long Chain Polymer Melts in Computer Simulations

TL;DR

This paper compares methods for equilibrating long-chain polymer melts in simulations, highlighting issues with standard approaches and proposing two improved techniques that reduce local chain deformation.

Contribution

It introduces two novel methods for better equilibration of long-chain polymers in simulations, addressing limitations of the standard approach.

Findings

Standard method causes local chain deformation that relaxes slowly.

Pre-packing Gaussian chains reduces density fluctuations.

Double-pivot algorithm effectively creates new chain configurations.

Abstract

Several methods for preparing well equilibrated melts of long chains polymers are studied. We show that the standard method in which one starts with an ensemble of chains with the correct end-to-end distance arranged randomly in the simulation cell and introduces the excluded volume rapidly, leads to deformation on short length scales. This deformation is strongest for long chains and relaxes only after the chains have moved their own size. Two methods are shown to overcome this local deformation of the chains. One method is to first pre-pack the Gaussian chains, which reduces the density fluctuations in the system, followed by a gradual introduction of the excluded volume. The second method is a double-pivot algorithm in which new bonds are formed across a pair of chains, creating two new chains each substantially different from the original. We demonstrate the effectiveness of these methods for a linear bead spring polymer model with both zero and nonzero bending stiffness, however the methods are applicable to more complex architectures such as branched and star polymer.