Simulated Osmotic Equation of State for Poly(ethylene Oxide) Solutions Predicts Tension-Induced Phase Separation

Poly(ethylene oxide) (PEO) is soluble in water, but its solubility is sensitive to changes in temperature, pressure, and stress. Notably, phase separation can occur when PEO solutions are subjected to strong flow. To explore these phenomena, we use molecular dynamics simulations to examine the phase behavior of PEO under tension in aqueous solution by pulling on the ends of each chain. At the same time, we impose a harmonic potential on the chains, resulting in a nonuniform concentration profile. We analyze this concentration profile to obtain the osmotic pressure versus concentration. Comparing this result to the osmotic pressure prediction from the Flory-Huggins free energy, we obtain the chi parameter χ for PEO in water as a function of volume fraction φ and chain tension. We observe an increase in χ(φ) as the tension increases and a moderate increase in χ as the polymer volume fraction φ increases, consistent with experimental observations.