Sedimentation in the semidilute concentration range of polystyrene in a marginal solvent

Abstract

Sedimentation velocity measurements on polystyrene in butan-2-one, a marginal solvent, over an extended concentration region are reported. The exponent α in the apparent scaling law describing the concentration dependence of the sedimentation coefficient (s∼C^–α) in the semidilute regime is observed to be concentration dependent. However, a power-law fit to data for the highest molecular weight (M= 20.6 × 10⁶) and in the concentration region c < 5 kg m^–3 yields a value of α= 0.68. This value is significantly larger than that observed at good solvent conditions for the same molecular weight and in the same concentrations interval. This difference as well as the observed curvature in the plot of log₁₀S against log₁₀c at higher concentrations are discussed. The cross-over concentration, c*, separating the dilute and semidilute concentration regime, is well-defined and located at c= 1/[η]. c* is observed to vary with the molecular weight as M^{–0.58 ± 0.04}.