Influence of solvent quality on effective pair potentials between polymers in solution

TL;DR

This study investigates how solvent quality influences the effective pair potentials between polymers in solution by extending a coarse-graining method to variable solvent conditions, revealing temperature and concentration-dependent interactions.

Contribution

The paper extends a coarse-graining approach to account for solvent quality effects on polymer interactions, highlighting the complex dependence on temperature and concentration.

Findings

Effective pair potentials vary strongly with temperature and concentration.

At low concentration, interactions become more attractive as temperature decreases.

Higher polymer concentration restores predominantly repulsive interactions.

Abstract

Solutions of interacting linear polymers are mapped onto a system of ``soft'' spherical particles interacting via an effective pair potential. This coarse-graining reduces the individual monomer-level description to a problem involving only the centers of mass (CM) of the polymer coils. The effective pair potentials are derived by inverting the CM pair distribution function, generated in Monte Carlo simulations, using the hypernetted chain (HNC) closure. The method, previously devised for the self-avoiding walk model of polymers in good solvent, is extended to the case of polymers in solvents of variable quality by adding a finite nearest-neighbor monomer-monomer attraction to the previous model and varying the temperature. The resulting effective pair potential is found to depend strongly on temperature and polymer concentration. At low concentration the effective interaction becomes increasingly attractive as the temperature decreases, eventually violating thermodynamic stability criteria. However, as polymer concentration is increased at fixed temperature, the effective interaction reverts to mostly repulsive behavior. These issues help illustrate some fundamental difficulties encountered when coarse-graining complex systems via effective pair potentials.