Direct measurements of polymer depletion layers by neutron reflectivity

We present results of direct measurements of polymer depletion layers at the liquid/air interface using neutron reflectivity. The systems studied are semidilute solutions of polystyrene in toluene using polymers of different molecular weights. The thickness of the depletion layer d is found to be of the order of the correlation length ξ of the polymer in the bulk solution. We show, for the first time, the existence of a depletion layer at the free surface of a polymer solution when ${\gamma }_{\mathit{p}}$>${\gamma }_{\mathit{s}}$, where ${\gamma }_{\mathit{p}}$ and ${\gamma }_{\mathit{s}}$ are surface tensions of pure polymer and pure solvent, respectively. We show also that the thickness of the depletion layer scales with the bulk concentration ${\mathrm{\phi }}_{\mathit{b}}$ as d∼${\mathrm{\phi }}_{\mathit{b}}^{\mathrm{-}0.75}$.