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Description:
A comprehensive study on the molecular conformation and dynamics of very large polyethylene-oxide (PEO) rings in the melt is reported. For all rings, independent of ring size, by SANS we observe a cross over, from a strong Q-dependence at intermediate Q to a Q^(-2) dependence at higher Q. Constructing a generic model including a cross over from Gaussian statistics at short distances to more compact structures at larger distances, we find the cross over at a distance along the ring of N_(e,0)=45±2.5 monomers close to the entanglement distance in the linear counterpart. This finding is clear evidence for the predicted elementary loops building the ring conformation. The radius of gyration R_g (N) follows quantitatively the result of numerous simulations. However, other than claimed, the cross over to mass fractal statistics does occur around N≅10N_(e,0), but up to N≅4N_(e,0) the relation R_g (N) ~ N^0.39 holds. The self-similar ring dynamics was accessed by PFG-NMR and NSE: We find three dynamic regimes for center of mass diffusion starting (i) with a strongly sub-diffusive domain 〈r_com^2 (t)〉 ~ t^α (0.4≤α≤0.65) (ii) a second sub-diffusive region 〈r_com^2 (t)〉 ~ t^0.75 that (iii) finally crosses over to Fickian diffusion. The internal dynamics at scales below the elementary loop size is well described by ring Rouse motion. At larger scales the dynamics is self-similar and follows very well the predictions of scaling models with preference for the fractal loopy globule (FLG) model.