Chain Conformations and Phase Separation in Polymer Solutions with Varying Solvent Quality

TL;DR

This study uses molecular dynamics simulations to explore how varying solvent quality affects polymer chain conformations and phase separation, revealing unexpected collapse behavior at very strong polymer-solvent attractions.

Contribution

It uncovers the counterintuitive collapse and phase separation phenomena at high polymer-solvent interaction strengths, differing from traditional poor solvent effects.

Findings

Polymer chains collapse in poor solvents due to unfavorable interactions.

Chains swell and become ideal with increasing attractive interactions.

Strong polymer-solvent attractions lead to chain collapse and phase separation.

Abstract

Molecular dynamics simulations are used to investigate the conformations of a single polymer chain, represented by the Kremer-Grest bead-spring model, in a solution with a Lennard-Jones liquid as the solvent when the interaction strength between the polymer and solvent is varied. Results show that when the polymer-solvent interaction is unfavorable, the chain collapses as one would expect in a poor solvent. For more attractive polymer-solvent interactions, the solvent quality improves and the chain is increasingly solvated and exhibits ideal and then swollen conformations. However, as the polymer-solvent interaction strength is increased further to be more than about twice of the strength of the polymer-polymer and solvent-solvent interactions, the chain exhibits an unexpected collapsing behavior. Correspondingly, for strong polymer-solvent attractions, phase separation is observed in the solutions of multiple chains. These results indicate that the solvent becomes effectively poor again with very attractive polymer-solvent interactions. Nonetheless, the mechanism of chain collapsing and phase separation in this limit differs from the case with a poor solvent rendered by unfavorable polymer-solvent interactions. In the latter, the solvent is excluded from the domain of the collapsed chains while in the former, the solvent is still present in the pervaded volume of a collapsed chain or in the polymer-rich domain that phase separates from the pure solvent. In the limit of strong polymer-solvent attractions, the solvent behaves as a glue to stick monomers together, causing a single chain to collapse and multiple chains to aggregate and phase separate.