Direct Observation of Two Distinct Diffusive Modes for Polymer Rings in Linear Polymer Matrices by Pulsed Field Gradient (PFG) NMR

Using high resolution PFG NMR spectroscopy, we have studied the diffusion of well-characterized polymer rings in linear host matrices at various observation times Delta, varying both ring and host molecular weights. For the first time, to our knowledge, for higher M-w, rings in entangled melts it was possible to directly distinguish two different diffusive modes: (i) fast diffusion that scales inversely with the host chain length and (ii) much slower diffusion depending much more strongly on the host molecular weight. Furthermore, we studied the diffusion of the linear chains in the host melts. The diffusion data were analyzed in terms of existing theories and compared to simulations. The two-mode structure directly verifies the hypothesis of qualitatively different mechanisms for ring diffusion in linear melts. The fast mode quantitatively agrees with the assertion of a special diffusion channel for once threaded rings, while the suggested diffusion of unthreaded rings was not found. The slow mode scales more weakly with the host chain length than predicted by the constraint release (CR) mechanism. However, considering an interdependence of constraints (Macromolecules 1986, 19, 105), the slow mode was quantitatively related to tube renewal processes. Using this concept also the molecular weight dependence of the matrix diffusion is described naturally. In contrast to the explicit observation of fast and slow diffusive modes simulations reveal broadly distributed heterogeneities leading to a prevailing CR mechanism only. The strong size dependent ring diffusion in an entangled matrix remains unintelligible. Finally, even though we have distinguished two well-defined significantly different diffusive modes with characteristic times in the millisecond range that would be expected to interchange, a detailed analysis in terms of a two-state diffusion model allowing for state changes reveals that within the experimental sensitivity no such exchanges take place.