Polymers as compressible soft spheres

TL;DR

This paper introduces a simplified coarse-grained model representing polymers as fluctuating soft spheres, accurately capturing the radius of gyration in dilute conditions but offering limited thermodynamic advantages over fixed-radius models.

Contribution

The study develops a single-sphere coarse-grained model with fluctuating radii, accurately derived from full-monomer simulations, and compares its effectiveness to fixed-radius models.

Findings

The model reproduces the density dependence of the radius of gyration.

Fluctuating sphere sizes do not improve thermodynamic predictions over fixed radii.

The model is accurate in dilute regimes but offers limited benefits for thermodynamics.

Abstract

We consider a coarse-grained model in which polymers under good-solvent conditions are represented by soft spheres whose radii, which should be identified with the polymer radii of gyrations, are allowed to fluctuate. The corresponding pair potential depends on the sphere radii. This model is a single-sphere version of the one proposed in Vettorel et al., Soft Matter 6, 2282 (2010), and it is sufficiently simple to allow us to determine all potentials accurately from full-monomer simulations of two isolated polymers (zero-density potentials). We find that in the dilute regime (which is the expected validity range of single-sphere coarse-grained models based on zero-density potentials) this model correctly reproduces the density dependence of the radius of gyration. However, for the thermodynamics and the intermolecular structure, the model is largely equivalent to the simpler one in which the sphere radii are fixed to the average value of the radius of gyration and radiiindependent potentials are used: for the thermodynamics there is no advantage in considering a fluctuating sphere size.