We have established multi-end functional polymers as potent additives for spontaneous surface modification of blends. Since semi-crystalline polymers such as PE, PP, PET could most benefit from this technology, we are now studying a model semi-crystalline PE-based system. Surface modification requires the additive to diffuse to the blend surface during processing, whilst the polymer is molten. During cooling & recrystallisation the surface composition may be further altered. These final states will be followed by complementary nuclear reaction analysis (NRA), which is good for crystalline polymers, but cannot be applied to the melt. It is essential therefore to determine the surface activity of additive as a function of structure and process conditions in the melt state by NR, since these are the necessary process conditions, and will establish the benchmark for crystallisation studies.