Invitation to submit a manuscript to the Soft Matter 20th Anniversary themed collection A Novel Microscopic Origin of Co-Nonsolvency

Abstract

Co-nonsolvency presents a fundamental paradox in polymer physics where macromolecules undergo collapse or precipitation in mixed good solvents. Through investigations combining simulations of various binary good solvent systems of polymer, including single-chain and multi-chains of homopolymer and block copolymer, and ternary Flory-Huggins theoretical validation, we reveal that the competition between the enthalpy of the system and the mixing entropy of binary solvents results in the liquid-liquid phase separation (LLPS) of the better solvent (S-solvent) and co-nonsolvency phenomenon. To lower the enthalpy, the polymer and S-solvent tend to mix together to maximize their contact, which however, is entropically unfavorable due to the localization of S-solvent in the polymer domain. The LLPS of S-solvent, where different chain segments sharing the localized S-solvent molecules, simultaneously lowers the enthalpy and reduces the loss of the mixing entropy. This sharing leads to that the chain in single-chain systems is in curling conformation of a size being much smaller than that of the ideal chain. In multi-chain systems, however, the sharing can be among segments from different chains, which causes chain condensation and hence a larger average chain size. Our study provides a novel mechanism for co-nonsolvency, and may shed insights into the LLPS in other soft matter systems.