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Profiles of Adsorbed Polymer Layers Observed by Neutron Reflection

We have measured neutron reflectivities at the liquid-air interface of polydimethyl-siloxane solutions, and we have observed remarkable properties. We report, for the first time to our knowledge, a dramatic effect of the isotopic composition on the reflectivity curve of such interfaces. This is discussed in terms of the propagation equation. The results obtained with deuterated solvent and nondeuterated polymers agree with the kinematic approximation. These data show the existence of a polymer concentration profile extending far away from the surface and support recent scaling predictions.