Segment diffusion and flip-flop spin diffusion in entangled polyethyleneoxide melts: A field-gradient NMR diffusometry study

Chain dynamics in melts of entangled polyethyleneoxide melts has been investigated using fringe field nuclear magnetic resonance diffusometry. As already demonstrated in our previous work, intermolecular flip-flop spin diffusion strongly influences spin echo attenuation for long diffusion times and high molecular weights. The experimental data have been evaluated taking this phenomenon quantitatively into account. Predictions of the reptation model for the correspondingly modified time and molecular weight dependences of the effective segment diffusion coefficient are presented and compared with experimental results. While the ordinary Rouse model totally fails to explain the experimental data, a satisfactory qualitative description is provided on the basis of the tube/reptation model. However, the fitted parameter values turned out to be inconsistent with known properties of this polymer. This in particular refers to the mean squared chain end-to-end distance divided by the molecular weight, for which neutron-scattering values are available in the literature. Relative to those results, the value evaluated from our NMR diffusometry data on the basis of the tube/reptation model turned out to be much too large.