The small‐angle neutron scattering (SANS) method for measuring the self‐diffusion coefficient D has been analyzed for effects of polydispersity in degree of polymerization for the case of linear polymers diffusing by reptation. Polydispersities corresponding to Mw/Mn = 1.0−10 were considered. It is shown that in all cases a meaningful effective diffusion coefficient De can be obtained from the short time recovery of the SANS intensity. This quantity De ≤ 1.3 D(Mw), where D(Mw) is the diffusion coefficient of a monodisperse polymer having molecular weight M = Mw. The method relies on SANS intensities extrapolated to zero scattering angle; realistic extrapolation is shown to give rise to quite acceptable errors on the order of 0.05 De.
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