Spin diffusion in melts of entangled polymers

Based on theoretical considerations [N. F. Fatkullin, Sov. Phys. JETP 72, 563 (1991)], immaterial spin diffusion mediated by flip–flop transitions of dipolar coupled spins on different macromolecules was predicted to influence the diffusion coefficient measured in nuclear magnetic resonance field-gradient experiments. In order to test this hypothesis, we have carried out supercon fringe field proton magnetic resonance diffusometry experiments with polyethylene oxide melts ${\mathrm{(M}}_w\text{=438\hspace{0.17em}000)}$ using field gradients of up to 60 T/m. The polymer chains were dispersed in a matrix of deuterated chains of an equivalent molecular mass. The time-dependent segment diffusion coefficients measured in the diluted and undiluted polymer coincided for diffusion times below about 200 ms. However, increasing the diffusion time up to 1 s leads to a reduction of the diffusion coefficient in the deuterated matrix by a factor of about 2 relative to the undeuterated system. The long-time diffusion coefficient measured with long polymer chains, which are subject to interchain spin couplings, is obviously strongly influenced by spin diffusion mediated by flip–flop transitions of dipolar coupled spin pairs. This holds true in spite of the relatively long flip–flop time, which is estimated to be of the order 0.1 s.