NEUTRON REFLECTIVITY STUDIES OF POLYMER CONCENTRATION PROFILES AT THE LIQUID AIR INTERFACE

The concentration profile of a linear flexible polymer adsorbed from good solvent at the liquid/air interface has been studied by neutron reflectivity. Two different isotopic compositions, deuterated polymer in protonated solvent and protonated polymer in deuterated solvent, have been considered; both systems furnish the same information concerning the physical properties of the adsorbed layer. The experimental results are interpreted in terms of predictions of scaling theory. In close proximity to the surface, there exists a monomer-rich zone, D, which is determined to be 10-15 angstrom thick and independent of polymer molecular weight and bulk concentration. Beyond D, the profile decreases with a self-similar structure with the monomer density scaling as phi(z) approximately z-4/3, where z is the distance from the surface. The reflectivity results are consistent with surface tension data and support a strong adsorption and weak coupling case for the system.