Clear Evidence of Reptation in Polyethylene from Neutron Spin-Echo Spectroscopy

The dynamic structure factor $S(q,t)$ of polyethylene (PEB-2) was measured by neutron spin echo in the Fourier time range of $t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.3--175$ nsec and for momentum transfers $q$ between 0.05 and $0.145{\AA{}}^{\ensuremath{-}1}$ to test the validity of competing phenomenological theories of relaxation in polymer melts. Previous spin-echo experiments limited to $t<25$ nsec were equally well described by a variety of models. This ambiguity has now been lifted, and the experiment clearly favors the reptation model, showing that the dominant relaxation mechanism in entangled linear polymers is via reptation.