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 γₚ>γₛ, where γₚ and γₛ 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 φb as d{~}φb^-0.75.