Coil-Globule Collapse of Polystyrene Chains in Tetrahydrofuran-Water Mixtures

We study the coil and globule states of a single polymer chain in solution by performing molecular dynamics simulations with a united atom model. Specifically, we characterize the structural properties of atactic polystyrene chains with N = 20-150 monomers in tetrahydrofuran-water mixtures at varying mixing ratios. We find that the hydrophobic polymers form rather open coils when the mole fraction of water, X_W, is roughly below 0.25, whereas the chains collapse into globules when X_W ≳ 0.75. We confirm the theoretically expected scaling laws for the radius of gyration, R_g, in these regimes, i.e., R_g ∝ N^3/5 and R_g ∝ N^1/3 for good and poor solvent conditions, respectively. For poor solvent conditions with X_W = 0.75, we find a sizable fraction of residual tetrahydrofuran trapped inside the collapsed polymer chains with an excess amount located at the globule surface, acting as a protective layer between the hydrophobic polystyrene and the surrounding water-rich mixture. These findings have important implications for nanoparticle fabrication techniques where solvent exchange is exploited to drive polymer aggregation, since residual solvent can significantly influence the physical properties of the precipitated nanoparticles.