Structure and dynamics of large ring polymers

Abstract

We report a comprehensive study on the molecular conformation and dynamics of very large poly(ethylene oxide) rings in the melt: (i) for all rings, independent of the ring size, by small angle neutron scattering we observe a crossover from a strong Q-dependence at intermediate Q to a Q−2 dependence at higher Q. Constructing a generic model including a crossover from Gaussian statistics at short distances to more compact structures at larger distances, we find the crossover at a distance along the ring of Ne,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. (ii) The radius of gyration Rg(N) follows quantitatively the result of numerous simulations. However, other than claimed, the crossover to mass fractal statistics does occur around N≅10Ne,0, but up to N≅44Ne,0, the relation Rg(N)∼N0.39 holds. The self-similar ring dynamics was accessed by pulsed field gradient-NMR and neutron spin echo spectroscopy: we find three dynamic regimes for center of mass diffusion starting (i) with a strong subdiffusive domain ⟨rcom2(t)⟩∼tα(0.4≤α≤0.65), (ii) a second subdiffusive region ⟨rcom2(t)⟩∼t0.75 that (iii) finally, crosses over to Fickian diffusion. The internal dynamics at scales below the elementary loop size is well described by the ring Rouse motion. At larger scales, the dynamics is self-similar and follows very well the predictions of scaling models with a preference for the fractal loopy globule model. Finally, we note that the key results were previously published in the form of two letters [Kruteva et al., ACS Macro Lett. 9, 507–511 (2020) and Kruteva et al., Phys. Rev. Lett. 125, 238004 (2020)].