Spherically symmetric solvent is sufficient to explain lower critical solution temperature in polymer solutions

TL;DR

This study demonstrates that a spherically symmetric solvent model can adequately explain the lower critical solution temperature (LCST) behavior in polymer solutions, emphasizing the role of mean interaction energies over detailed solvent structure.

Contribution

It shows that simple, spherically symmetric solvent models are sufficient to capture LCST phenomena, simplifying understanding of solvent-polymer interactions.

Findings

LCST behavior results from competition between energy difference and entropy loss.

Solvent unbinding causes polymer collapse at high temperatures.

Mean interaction energies suffice to explain LCST, reducing need for detailed solvent structure.

Abstract

We study the lower critical solution temperature (LCST) in thermoresponsive polymer solutions by means of a coarse grained single polymer chain simulation and a theoretical approach. The simulation model includes solvent explicitly and thus accounts for solvent interactions and entropy directly. The theoretical model consists of a single chain polymer in an implicit solvent where the effect of solvent is included through the intra-polymer solvophobic potential proposed by Kolomeisky and Widom. Our results indicate that the LCST behavior is determined by the competition between the mean energy difference between the bulk and bound solvent, and the entropy loss due to the bound solvent. At low temperatures, solvent molecules are bound to the polymer and the solvophobicity of the polymer is screened, resulting in a coiled state. At high temperatures the entropy loss due to bound solvent offsets the energy gain due to binding which causes the solvent molecules to unbind, leading to the collapse of the polymer chain to a globular state. Furthermore, the coarse grained nature of these models indicates that mean interaction energies are sufficient to explain LCST in comparison to specific solvent structural arrangements.