Principles of neutron reflection, as applied to polymer studies

Neutron reflectivity is rapidly coming of age as a very important tool to study the composition of thin polymer films, with over a dozen instruments worldwide already on line or in construction at the major nuclear centers. Neutron reflectivity provides information similar to SANS; while the latter technique gives the density of spherical particles as a function of their radius, reflectivity gives the density of the layers as a function of the distance from a reference surface. As for SANS, neutron reflectivity is particularly adapt to study organic material in view of the contrast between the neutron cross section of the two stable isotopes, which allows the 'coloring' of chemically identical materials. A review is given of the success of neutron reflectivity in determining depth profiles in problems ranging from the interdiffusion of polymers to phenomena of surface enrichment and of precipitation