We have used neutron reflectivity and complementary techniques of ion beam analysis and contact angle to understand the adsorption behaviour of multi-fluoro end functional polyethylenes in polyethylene matrices. We have recently come across a surprising result - that our additives, which are highly surface active in model LLDPE matrix have little apparent surface activity in higher density polyethylene blends. We believe that this is likely due to temperature gradient in cooling films that tends to favour the higher melting component to crystallise at the film surface. In order to test this, and establish a rationale for more viable additives, we must measure the surface segregation of our additives in a series of blends over which the melting/crystallisation temperature of the matrix is systematically varied.