Phase Diagram of Flexible Polymers with Quenched Disordered Charged Monomers

TL;DR

This study uses advanced simulation techniques to explore how charged monomers affect the structural phase transitions of flexible polymers, revealing new phases like pearl-necklace and helical structures at high charge concentrations.

Contribution

It introduces a bead-spring polymer model with randomly distributed charges and maps its phase diagram using the REWL method, highlighting the impact of electrostatic interactions on polymer transitions.

Findings

Second-order coil-globule transition across all concentrations.

First-order globular-solid transition across all concentrations.

High charge concentration leads to pearl-necklace and helical structures.

Abstract

Recent advances in Generalized Ensemble simulations and microcanonical analysis allowed the investigation of structural transitions in polymer models over a broad range of local bending and torsion strengths. It is reasonable to argue that electrostatic interactions play a significant role in stabilizing and mediating structural transitions in polymers. We propose a bead-spring polymer model with randomly distributed charged monomers interacting via a screened Coulomb potential. By combining the Replica Exchange Wang-Landau (REWL) method with energy-dependent monomer updates, we constructed the hyperphase diagram as a function of temperature ($T$) and charged monomer concentration ($\eta$). The coil-globule and globular-solid transitions are respectively second and first order for the entire concentration range. However, above a concentration threshold of $\eta=80\%$, electrostatic repulsion hinders the formation of solid and liquid globules, and the interplay between enthalpic and entropic interactions leads to the formation of liquid pearl-necklace ad solid helical structures. The probability distribution, $P(E,T)$, indicates that at high $\eta$, the pearl-necklace liquid phase freezes into a stable solid helix-like structure with a free energy barrier higher than the freezing globule transition at low $\eta$.