Anomalous segment diffusion in polydimethylsiloxane melts

Segmental diffusion in long-chain polymer melts has been measured by field-gradient NMR. An enlarged set of data and an improved data analysis leads to a modification of our previously published (Macromolecules 1994, 27, 4274) view. We now come to the following conclusions: In monodisperse polydimethylsiloxane (PDMS) melts, the self part of the intermediate scattering function stays almost Gaussian, that is S(Q,t) similar to exp[-Q(2)D(app)(t)t] far t > 5 x 10(-3) s with an apparent self-diffusion coefficient whose time dependence is, within experimental accuracy, in accord with the reptation model. This is also the ease for the molecular weight dependence of the long-time self-diffusion coefficient D-infinity. However, the observed cross-over fi om anomalous to long time diffusion cannot be understood quantitatively within the strict reptation model.