Co-non-solvency: Mean-field polymer theory does not describe polymer collapse transition in a mixture of two competing good solvents

TL;DR

This paper explains the co-non-solvency phenomenon where polymers collapse in mixtures of two good solvents, showing that mean-field theories fail to describe this behavior, which is driven by local concentration fluctuations.

Contribution

The study extends previous analysis to demonstrate that co-non-solvency is a generic thermodynamic effect caused by competitive solvent displacement, not chemical specifics.

Findings

Co-non-solvency results from local concentration fluctuations.

Mean-field theories cannot capture the collapse transition.

Polymer collapse can occur even as solvent quality improves.

Abstract

Smart polymers are a modern class of polymeric materials that often exhibit unpredictable behavior in mixtures of solvents. One such phenomenon is co-non-solvency. Co-non-solvency occurs when two (perfectly) miscible and competing good solvents, for a given polymer, are mixed together. As a result, the same polymer collapses into a compact globule within intermediate mixing ratios. More interestingly, polymer collapses when the solvent quality remains good and even gets increasingly better by the addition of the better cosolvent. This is a puzzling phenomenon that is driven by strong local concentration fluctuations. Because of the discrete particle based nature of the interactions, Flory-Huggins type mean field arguments become unsuitable. In this work, we extend the analysis of the co-non-solvency effect presented earlier [Nature Communications 5, 4882 (2014)]. We explain why co-non-solvency is a generic phenomenon that can be understood by the thermodynamic treatment of the competitive displacement of (co)solvent components. This competition can result in a polymer collapse upon improvement of the solvent quality. Specific chemical details are not required to understand these complex conformational transitions. Therefore, a broad range of polymers are expected to exhibit similar reentrant coil-globule-coil transitions in competing good solvents.