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-xi 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-p > gamma-s, where gamma-p and gamma-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 phi-b as d approximately phi-b-0.75.