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)].
Highlights
In melts of linear chains, the mutual interaction is screened and the chains perform an undisturbed Gaussian random walk [1,2]
We present small angle neutron scattering (SANS) results on the ring conformations followed by the results of dynamic studies that were achieved by neutron spin echo (NSE) spectroscopy and pulsed field gradient (PFG) NMR
The double folded lattice animal (DFLA) model [13] propounded an analogy to the randomly branched polymer—the lattice tree, where relaxation occurs by the retraction of double folded strands leading to a terminal relaxation time τd N3 with N being the number of monomers, fractal dimension of df 1⁄4 4, and a center of mass diffusion D NÀ2
Summary
In melts of linear chains, the mutual interaction is screened and the chains perform an undisturbed Gaussian random walk [1,2]. In terms of entanglement distances Ne in the corresponding linear melt, we covered sizes from N % 5 Ne up to N % 44 Ne. In terms of entanglement distances Ne in the corresponding linear melt, we covered sizes from N % 5 Ne up to N % 44 Ne This very large range is well beyond what experiments have so far achieved and facilitated the direct and clear experimental observation of small-scale, ring size independent substructures with Gaussian conformation. We take these substructures as the signature of elementary loops being the basis. Wrapping up the results, we conclude about the state of the art
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