NMR self‐diffusion study of polyethylene and paraffin melts

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Abstract
The self‐diffusion coefficient D of paraffin and polyethylene melts—covering the range between N = 19 and 103 where N is the number of monomeric units—was measured by the pulsed‐magnetic‐field‐gradient NMR method for diffusion times between 3 ms and 1 s. For the paraffins, D is proportional to N−2 though the molecular weights are smaller than the critical molecular weight for entanglement. In polyethylene, melts a strong dependence of the diffusion coefficient on the diffusion time is observed, whereas no such dependence is found in paraffin melts. A mathematical formalism for describing spin‐echo attenuation in terms of a velocity autocorrelation function is shown to yield qualitative agreement with the experimental results.